DE102024124298B3 - Lens and method for focusing multiple partial beams into multiple penetration depths of a sample - Google Patents

Abstract

The present application relates to a lens for focusing at least two partial rays of a light ray incident on the lens into at least one predetermined depth of an image space, comprising: (a) a focusing unit with at least one optical element; (b) at least one combined beam-splitting and aberration-compensating unit, which is configured to split the light ray entering the lens into at least two partial rays and to compensate for at least one aberration of the at least one optical element of the focusing unit in advance; and (c) wherein the at least one optical element is configured to focus the at least two partial rays leaving the combined beam-splitting and aberration-compensating unit into the at least one predetermined depth of the image space.

Description

1. Technical field

The present invention relates to a lens and a method for focusing several partial beams into several penetration depths of a sample. In particular, the present invention relates to a lens and a method for focusing at least two partial beams of a light beam incident on the lens into at least one predetermined depth of an image space whose refractive index is greater than one.

2. State of the art

Many applications of laser radiation require its focusing at various depths within a sample. Examples include the direct creation or writing of optical waveguides in optically transparent materials. In vitrography, or three-dimensional (3D) glass engraving, a vast number of light-scattering elements are created within a glass volume, producing a 3D image, such as a portrait. These light-scattering elements are generated by guiding a laser focus, exhibiting very high local intensity, through the glass volume in three dimensions. In a laser scanning microscope, objects within a 3D volume are imaged by scanning the sample with a laser focus. In 3D photopolymerization and photonic wire bonding, a laser spot is guided through a 3D volume of a liquid photopolymer to initiate local polymerization from the liquid phase.

The applicant has developed tools that enable the correction or repair of small remaining manufacturing defects in photolithographic masks. To correct transmissive photomasks, numerous small rotationally symmetric light-blocking elements can be introduced into the mask substrate using diffraction-limited short laser pulses, whereby the associated wavefronts in the mask substrate can represent nearly rotationally symmetric surfaces. This allows, for example, the transmission of the photomask to be influenced in such a way that the image on the wafer exhibits improved critical dimension uniformity (CDU). Furthermore, the applicant's tools can be configured to generate locally inducing microstress elements within a mask. For this purpose, laser foci are generally created with a non-rotationally symmetric wavefront in the mask substrate, resulting in non-rotationally symmetric intensity distributions within the foci. These local microstress-inducing elements can be used to selectively deform a photomask, enabling, for example, so-called "critical dimension control" (CDC). The local light-shading and microstress-inducing changes in a sample are collectively referred to as pixels. Non-rotationally symmetric wavefronts can be described, for example, using Zernike fringe polynomials where the 5th and 6th order Zernike fringe coefficients have non-zero values, resulting in astigmatism of the wavefront. The light distribution is then elliptical depending on the depth in the sample, thus modifying the sample material in an elliptical region.

When generating pixels in a sample that is optically transparent in the wavelength range of the pixel-generating laser radiation, the photons of an ultrashort laser pulse with very high optical intensity interact with the electrons of the sample in a nonlinear absorption process within an interaction zone in the region of the laser focus. As already indicated above, introducing a plurality or multitude of symmetric pixels results primarily in the shadowing or scattering of a portion of the radiation transmitting through the sample, whereas asymmetric pixels primarily lead to a locally variable material displacement and thus mainly to a local displacement of pattern elements of a photomask arranged on a surface of the mask.

The substrate of a transmissive photomask typically has a thickness of 6.35 mm. The corrective effect of pixels depends on the depth to which pixels are embedded or written into the mask substrate. Therefore, for some applications, it is advantageous to write pixels at different depths within the photomask. Furthermore, the distribution of optical intensity at the focus of the laser radiation influences the effect of the generated pixels. This distribution changes, however, when the optical radiation passes from typically air into the sample material, and especially when the focus is shifted to different depths within the sample. The effect of this wavefront change increases significantly with increasing refractive index of the sample. This means that the wavefront of laser radiation changes considerably when passing through sample material with a high refractive index, such as semiconducting materials.

In photolithography or microlithography, the object being processed is a photomask when correcting mask defects. However, in the production of semiconductor devices, the flatness of the wafer on which the devices are manufactured is also of great importance. This flatness is compromised by numerous individual, sequential process steps and requires repeated post-processing to restore the wafer's flatness for subsequent processes. US 2017 / 0 010 540 A1 This describes a device for generating a 3D contour of a wafer, which is based on the creation of 3D pixel arrangements within the wafer. This requires adjusting the laser focus to different depths within the wafer.

As the focus depth into the wafer, or generally into a target or sample, increases, the geometry of the wavefront of the penetrating laser beam or light beam in the medium between the objective and the sample must also change. This medium is referred to as the "upstream medium" because it is traversed by the beam path in front of the sample. This change in wavefront geometry is necessary to ensure that a pixel is generated with the same wavefront in the sample, regardless of the penetration depth. Thus, the pixels have the same geometry regardless of the penetration depth. Without such a correction, the focus quality would change with increasing penetration depth, which can be quantified as a decrease in Strehl ratio. This effect becomes more critical with increasing refractive index difference between the upstream medium and the sample, as well as with increasing aperture of the objective lens.

Generally, material processing requires lenses that create a focus with a large numerical aperture within the material being processed. Furthermore, to vary the depth or penetration depth of the focus within the material, the aberrations dependent on the penetration depth must be correctable. For example, a numerical aperture of 0.4 is used in the applicant's tool for photomask tuning. Larger apertures, such as 0.6 or 0.8, can be used for the application "creating wafer planarity." Generally speaking, the focus volume within which the sample material is modified becomes smaller as the numerical aperture increases. Typically, the numerical aperture is chosen to be higher the smaller the desired pixel volume. On the other hand, it may be desirable to cure larger volume areas, for example, when 3D printing photopolymers. In this case, it may be advantageous to use a small numerical aperture of 0.2 or 0.3 to cure a large sample volume simultaneously.

In the US 4 953 962 A Microscope objectives are described that can compensate for wavefront errors introduced by varying coverslip thicknesses in laser scanning microscopy. Two movable lenses and a variable distance of the last lens from the sample are proposed to compensate for the wavefront changes.

DE 10 2014 002 328 A1 Fluorescence scanning microscopes (1) with an observation beam path (A) from a measurement volume to an image plane (BE), a beam combiner (6) for coupling an illumination system (11), and an aperture (15) arranged in the image plane (BE) exhibit a slow image build-up due to sequential scanning and stress the sample (P) through inefficient use of the excitation light. An improved fluorescence scanning microscope is intended to simultaneously detect fluorescence from different focal planes in a quasi-confocal manner. This is achieved by an observation beam path (A) between the beam combiner (6) and the image plane (BE) comprising a first diffractive optics (7) for splitting light rays into beam bundles along different diffraction orders, which imposes a spherical phase on the light rays that differs from the other diffraction orders, a second diffractive optics (13) for compensating chromatic aberrations of the split beam bundles and a converging optics (8) for focusing the split beam bundles into the image plane (BE).

US 2023 / 0 036 386 A1 This describes a method for processing a workpiece, comprising the simultaneous alignment of a laser beam combination comprising a first beam and a second beam, wherein the first beam passes through a first impact surface of the transparent workpiece at a first impact point, and the second beam passes through a second impact surface of the transparent workpiece at a second impact point. The first beam forms a first laser beam focal line in the transparent workpiece and generates a first induced absorption to create a first defect segment within the transparent workpiece, wherein the first defect segment has a first edge angle. The second beam forms a second laser beam focal line in the transparent workpiece and generates a second induced absorption to create a second defect segment within the transparent workpiece. workpiece, wherein the second defect segment has a second edge angle, the second edge angle being different from the first edge angle.

US 2020 / 0 054 485 A1 This describes a refractive index writing system comprising a pulsed laser source, a lens for focusing the output of the pulsed laser source onto a focal spot in an optical material, and a scanner for moving the focal spot relative to the optical material within a scan range. A beam multiplexer splits the output of the laser source into at least two working beams, which are focused into the optical material with two differently shaped focal spots. A control unit controls at least a temporal and/or spatial shift between the focal spots of the working beams, together with the relative velocity and direction of the scanner, to maintain an energy profile in the optical material along the scan range above a nonlinear absorption threshold of the optical material and below a threshold for optical material breakdown.

In the patent US 7 733 564 B2 This section describes a microscope equipped with a wavefront modulator (WFM) for changing the penetration depth of foci within a sample. A WFM allows the penetration depth or focus position to be altered within a sample by varying the distance between the microscope's front lens and the sample. The WFM is positioned in the beam path between the microscope objective and an intermediate image plane. The WFM enables the laser focus to be shifted within a sample on the order of a few micrometers while maintaining an acceptable wavefront shift.

The US 2016 / 0 161 729 A1 describes a scanning light microscope with an LCOS (liquid crystal on silicon) element for structured illumination of a sample, where this element is used for aberration correction in addition to focusing.

Furthermore, the following are from the exemplary printed documents US 2005 / 0 207 003 A1, EP 2 498 116 A1 , DE 11 2013 006 111 T5 and US 2015 / 0 362 713 A1 Other microscope objectives with wavefront manipulators are known.

The authors M. Seeßelberg et al. describe in the article “Optical design of Zeiss For Tune photo mask tuning system: How to generate diffraction-limited laser foci in thick specimens”, Proc. of SPIE 10690, Optical Design and Engineering VII, 106900Y, 5 June 2018 A telecentric optical design concept involves a grid unit and an adaptive optical element remaining in conjugate planes within the pupil plane of a telecentric microscope objective, even when the focusing module is shifted. Telecentric means that the principal rays passing through the center of an aperture are parallel to each other in the image space. As explained in the article mentioned above, a telecentric objective is advantageous because the depth-of-penetration aberrations are then the same for all field bundles and can therefore be corrected particularly easily.

The US 2019 / 0 170 991 A1 and the US 2023 / 0 367 134 A1 These describe optical systems with a focusing unit that enable focusing through a thick sample. However, these focusing units have comparatively large dimensions and also provide relatively small field planes for processing.

In the article “ Multi-beam two-photon polymerization for fast large area 3D periodic structure fabrication for bioapplications", Scientific Reports (2020) 10:8740, https://doi.org/10.1038/ s41598-020-64955 , the authors C. Maibohm et al. describe a laser focus generation unit in which a diffractive optical element (DOE) splits a laser beam into several partial beams that are directed by a microscope objective onto a sample.

However, this laser focus generation unit cannot tune the partial beams to different depths within the sample. To accelerate sample processing, it is desirable to be able to process a sample simultaneously with multiple partial beams and to adjust the penetration depths of the foci within the sample.

The present invention therefore addresses the problem of providing a lens and a method that at least partially avoid the limitations discussed above.

3. Summary of the invention

According to one embodiment of the present invention, this problem is at least partially solved by the subject matter of the independent claims of the present application. Exemplary embodiments are described in the dependent claims.

A first embodiment relates to a lens for focusing at least two partial rays of a light ray incident on the lens into at least one predetermined depth of an image space, comprising: (a) a focusing unit with at least one optical element; (b) at least one combined beam-splitting and aberration-compensating unit, which is configured to split the light ray entering the lens into at least two partial rays and to pre-compensate at least one aberration of the at least one optical element of the focusing unit; and (c) wherein the at least one optical element is configured to focus the at least two partial rays exiting the at least one combined beam-splitting and aberration-compensating unit into the at least one predetermined depth of the image space.

By pre-compensating for the aberrations of the optical elements of the focusing unit of the object, it enables the simultaneous generation of multiple diffraction-limited foci at the same depth of a sample. It is also possible to perform the pre-compensation in such a way that each partial beam simultaneously generates a focus at different depths of the image space. Thus, an objective described herein allows for the generation of diffraction-limited foci in both 2D and 3D. Therefore, an objective according to the invention represents an important component for targeted material processing using laser beams. Currently, generating multiple foci in a plane perpendicular to the optical axis is the preferred application of the described objective.

By having a beam-splitting and aberration-compensating unit, in addition to splitting an incident light beam into two or more partial beams, also perform a beam-shaping function and thereby pre-compensate for the lens aberration(s), new degrees of freedom are opened up for the design of the lens's optical components. For example, the number of optical elements in the focusing unit, such as the number of lenses required for focusing, can be significantly reduced. This has a positive effect on the lens's weight and the space required to house the optical components. Furthermore, by allowing larger aberrations in the focusing unit's optical elements, which are pre-compensated by the combined beam-splitting and aberration-compensating unit, the usable area in the lens's image plane can be significantly increased. The usable area in the image plane represents the region in a plane perpendicular to an optical axis in which foci can be generated simultaneously.

The at least one combined beam-splitting and aberration-compensating unit can be arranged in a pupil plane of the objective lens.

Due to the dual function of the combined beam-splitting and aberration-compensating unit, a lens according to the invention can be designed such that the pupil plane located within the lens becomes accessible for the insertion of further optical elements, such as the combined beam-splitting and aberration-compensating unit. Furthermore, by simultaneously realizing two optical functions in the pupil of the lens, a lens according to the invention can be constructed simply and compactly. In particular, the creation of conjugate pupils outside the lens can be avoided. This significantly reduces the system complexity of a lens according to the invention. In other words, the use of a bulky relay optic to place the combined beam-splitting and aberration-compensating unit in a conjugate pupil plane located outside the lens within the beam path can be avoided. Naturally, it is also possible to position the combined beam-splitting and aberration-compensating unit outside the lens and to image it into the pupil plane of the lens using a relay.

Thus, a lightweight, compact lens according to the invention reduces the moving mass of a laser beam focus generation unit. A small, lightweight lens requires less force to move. Furthermore, less vibration and therefore fewer detrimental oscillations occur when moving the lens. This allows the material processing of samples based on short, intense laser pulses to be simultaneously accelerated and improved in precision. Moreover, the simultaneous Creating a large number of foci in a focal plane of the image space minimizes the adjustment effort required.

The advantages come at the cost of a limitation of the wavelength range in which a lens defined in this application can be used. However, this does not pose a limitation if a laser is used as the light source.

A portion of the image space, into which the at least two partial beams are focused, can have a refractive index n > 1. The image space can contain a sample with _n₂ > 1. A portion of the image space, particularly that between the objective lens outlet and the sample, can contain an immersion medium with a refractive index _nᵢ ; if the immersion medium is a gas, typically _nᵢ ≈ 1. Furthermore, the image space can include a sample holder with _n₁ > 1 and a sample with _n₂ > 1. Typically, _n₂ ≥ _n₁ . However, an objective according to the invention can also be designed for the case _n₂ < _n₁ .

A pupil or pupil plane can be considered an aperture of the objective lens, and a conjugate pupil of the objective lens can be viewed as an image of the pupil of the objective lens. An aperture can be understood as a defined limitation of the aperture of a light ray or photon beam in a plane perpendicular to the optical axis of the objective lens, caused, for example, by a beam-limiting element of the objective lens. This limitation can be essentially independent of the deflection of the light ray within the objective lens; that is, light beams belonging to different image points in the sample are limited almost exclusively by the aperture. Thus, the objective lens can be designed such that the light ray, or the generated partial beams, pass through the opening or diameter of the pupil. This also applies when individual optical elements are moved along the optical axis of the objective lens. In particular, it is also possible that the limit coincides with the limit of the combined beam-splitting and aberration-compensating unit – in this case, the combined beam-splitting and aberration-compensating unit also constitutes the aperture. The principal rays are those light rays that pass through the center of the aperture. If the aperture is positioned such that the principal rays between the objective and the sample are approximately parallel to each other, this is called a "telecentric objective." A telecentric objective is advantageous because the wavefronts belonging to different partial rays are then affected in the same way for different focal depths.

The lenses described in this application can be telecentric. This means that, with a planar sample, the principal beams in the medium in front of the sample are parallel and have the same angle of incidence into the sample. This implies that all partial beams undergo the same wavefront changes to create foci at a specific depth within the sample.

The lens pupil can be optically accessible, meaning that an optical component, such as a combined beam-splitting and aberration-compensating unit, can be placed within the pupil so that the function(s) of the optical component(s) are performed in the pupil plane. Typically, the functionality or performance of a lens is optimized when the combined beam-splitting and aberration-compensating unit is located within the lens's pupil plane. However, if the lens pupil lies within one of the optical elements, the pupil or the pupil plane cannot be used to introduce further optical components into the lens's beam path.

The lens can be configured so that at least one combined beam-splitting and aberration-compensating unit forms an aperture of the lens.

Regardless of whether the at least one combined beam-splitting and aberration-compensating unit forms the aperture of the lens, an optical component can be considered to be located in the pupil plane if a large portion of the light beam passes through the optical component, such as the combined beam-splitting and aberration-compensating unit. A large portion of the light beam comprises at least 80%, preferably at least 90%, more preferably at least 95%, and most preferably at least 99% of its optical intensity. For this purpose, the optical component can be arranged with a deviation from the pupil plane along the optical axis of the lens of less than ± 10 mm, preferably less than ± 5 mm, more preferably less than ± 2 mm, and most preferably less than ± 1 mm.

It is advantageous to use an objective whose image quality is essentially independent of the field point. Microscope objectives typically possess this property. Therefore, the microscopes described in this application can be implemented in the form of microscope objectives.

The at least one optical element can be set up based on at least one of: reflection, refraction or diffraction, and/or the at least one beam-splitting and aberration-compensating unit can be set up based on diffraction.

The optical element can comprise at least one of the following: a mirror, a lens, or a non-beam-splitting diffractive optical element. The focusing unit can comprise at least one of the following: a lens system of two or more lenses, a mirror system of two or more mirrors, two or more non-beam-splitting diffractive elements, or a combined system of lenses, mirrors, and non-beam-splitting diffractive elements.

The at least one combined beam-splitting and aberration-compensating unit can include a diffractive optical element (DOE). The diffractive optical element can include a beam-splitting diffractive optical element. The diffractive optical element can include a diffractive optical element that does not split the beam.

The at least one diffractive optical element can include at least one element of the group: an adaptive optical element, an active optical element, a hologram, or a spatial light modulator.

Adaptive optical elements with liquid crystal correction elements, such as LCOS (liquid crystal on silicon) or LCSLM (liquid crystal spatial light modulator), can vary wavefronts even in transmission and can therefore be used in a lens. Currently, their resolution and/or adjustment speed are not yet sufficient for high-precision applications in materials processing. This also applies to other types of electrically addressable spatial light modulators (EASLM).

The hologram can be a computer-generated hologram. The computer-generated hologram can be a multi-stage computer-generated hologram. The multi-stage computer-generated hologram can comprise 2^ ⁿ digitization stages, where preferably n ≥ 1, more preferably n ≥ 2, and most preferably n ≥ 3.

The lens is designed to accommodate at least one combined beam-splitting and aberration-compensating unit as an interchangeable component.

The ability to exchange a combined beam-splitting and aberration-compensating unit, or a DOE, allows for the construction of a modular lens. This greatly expands its range of applications. Details on this point are explained below.

The lens may incorporate a device for the controlled insertion and removal of a combined beam-splitting and aberration-compensating unit. Hereinafter, a combined beam-splitting and aberration-compensating unit is referred to as a combined unit. For example, a combined unit, such as a DOE, may be inserted, clamped, or screwed into a lens. The lens may include a magazine designed to hold a set of combined units and to insert and remove them from the lens's beam path as needed. Changing a combined unit may be configured to be manual, semi-automatic, or automatic. The magazine may be a rotating drum.

The lens can have an image-side numerical aperture (NA) greater than 0.3, preferably greater than 0.5, more preferably greater than 0.7, and most preferably greater than 0.9. As already explained above, the required NA of a lens depends on the specific application.

An important part of materials processing is writing pixels into a sample to create targeted local material changes. To generate pixels, optical radiation is concentrated in a very small volume, typically a laser focus. The size of the focus area in the beam direction can be described by the axial Rayleigh parameter: $d_R=2\cdot \frac{\lambda \cdot n_2}{N{A}^2},$ where λ is the exposure The wavelength is denoted by , the refractive index of the sample by _n , and the numerical aperture of the lens used to focus the laser beam. The size of the focus area in the beam direction can be significantly reduced by increasing the image-side NA of the lens. In other words, a large numerical aperture of the lens that focuses the laser beam to write pixels into a sample improves process control. This allows the depth at which a pixel is generated in the sample to be targeted with greater precision.

The image field of the lens can have a half-diameter greater than 0.1 mm, preferably greater than 0.5 mm, more preferably greater than 2 mm, and most preferably greater than 5 mm. The preferred diameter of the field area depends on the number of partial beams and the spacing of their foci within the field area. An upper limit to the number of partial beams depends, firstly, on the combined unit. Secondly, the power of the laser beam incident on the lens limits the energy density achievable at the foci of the partial beams. The maximum usable number of partial beams depends on the application.

By having the combined unit additionally correct the aberrations of one or more optical elements of the focusing unit, these elements can be selected with significantly greater flexibility, as far less consideration needs to be given to their aberrations when designing the focusing unit. The resulting design freedom can be used to increase the diameter of the usable image area by approximately one order of magnitude. In materials processing, this opens up the possibility of processing larger sample areas in a single adjustment operation, thereby significantly accelerating the processing time.

The at least one combined beam-splitting and aberration-compensating unit can further be configured to generate in the image space one of: rotationally symmetric wavefronts for rotationally symmetric foci or non-rotationally symmetric wavefronts for non-rotationally symmetric foci for the at least two partial beams.

The rotational symmetry of light rays or partial rays refers to their respective optical axes. Non-rotationally symmetric wavefronts result in aspherical foci for at least two partial rays. Aspherical foci can include at least one of the following: astigmatic foci or coma-affected foci. Furthermore, the combined beam-splitting and aberration-compensating unit can be configured to generate different focus shapes for the at least two partial rays. For example, a combined unit can be configured to produce a rotationally symmetric and a non-rotationally symmetric focus when focusing the at least two partial rays. A spherical focus exhibits rotational symmetry about its optical axis, whereas an aspherical focus lacks this symmetry property.

For example, by generating elliptical foci set in a defined manner, a lens according to the invention can be used to create pixels in a sample that deviate significantly from a circular shape in a plane perpendicular to the beam direction. This specific type of pixel can be used, for example, to correct placement errors of pattern elements on photomasks.

The at least one aberration of the at least one optical element of the focusing unit of the lens can include at least one of the following: spherical aberration, coma, astigmatism, or Petzval curvature.

A wavefront of a partial beam that deviates from a predefined reference wavefront of the partial beam results in an aberration of the lens. A reference wavefront of a partial beam can exhibit rotational symmetry about the optical axis of the partial beam. A reference wavefront may also lack this symmetry property.

The combined unit can be configured to generate at least two partial beams with the same optical intensity. Furthermore, the combined unit can be configured to set the distance between the foci of the at least two partial beams to a predetermined value.

Typically and preferably, the foci of at least two partial beams lie in a focal plane in image space, i.e., on or within a sample at an equidistant distance from the sample surface. However, it is also possible to design or configure a combined unit such that the A combined unit containing a lens generates foci for the different partial beams at different depths or penetration depths in a sample.

The at least one combined beam-splitting and aberration-compensating unit can comprise at least one first set of combined beam-splitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the first set of combined beam-splitting and aberration-compensating units is configured to completely or at least approximately correct the at least one aberration of the lens for a predetermined penetration depth of the foci of the at least two partial beams, wherein the predetermined penetration depth into the image space is different for each element of the first set.

The foci of at least two partial beams can be tuned with respect to their depth through a sample by adjusting the working distance of the objective for the respective penetration depth of the foci and inserting the combined unit designed for that penetration depth into the objective's beam path. The working distance is defined as the distance measured from the objective to the sample (more precisely, from the point on the objective closest to the sample), with the measurement taken along an optical axis. Tuning the foci through a sample does not need to be performed in equidistant steps; rather, combined units can be designed and manufactured for any desired penetration depth. Using a set of combined units eliminates the need for a wavefront modulator to tune the foci through a sample.

Each element of the first set of combined units can exist in at least two forms, wherein a first form generates spherical foci or rotationally symmetric wavefronts and a second form generates aspherical foci or non-rotationally symmetric wavefronts when the corresponding combined unit is inserted into the lens. A lens according to the invention can thus be used to generate different types of pixels at different depths of a sample.

The at least one combined beam-splitting and aberration-compensating unit can comprise at least one second set of combined beam-splitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the second set generates a predetermined number of partial beams, the predetermined number of partial beams being different for each element of the second set.

Combined units, such as DOEs, can be designed to split an incident light beam into a virtually any number of freely selectable partial beams. The arrangement of these partial beams is also adjustable. For example, two or more partial beams can be arranged as lying on a straight line or in the form of a two-dimensional (2D) geometric structure. Furthermore, it is possible to adjust the spacing between adjacent partial beams to accommodate potential limitations of the sample, such as the energy that can be applied per unit area.

The number of partial beams can be more than 2, preferably more than 10, more preferably more than 30, and most preferably more than 50 or even more than 150.

It is also possible to equip a first number of partial beams with spherical foci and a second number of partial beams with aspherical foci.

Firstly, the significantly increased size of the objective's usable image area enables diffraction-limited focusing of a large number of partial beams to a defined depth within a sample. Secondly, the objective's large image area allows for the simultaneous processing of a large sample section. The objective described here produces diffraction-limited foci if the wavefronts generated in the foci exhibit deviations from corresponding reference wavefronts that are smaller than a predefined threshold. This deviation can be defined, for example, as an RMS (root mean square) value. This RMS value can be related to the wavelength of the partial beams. The deviation of the wavefronts generated by the objective with respect to the corresponding reference wavefronts can be < 60 mλ, preferably < 40 mλ, more preferably < 20 mλ, and most preferably < 10 mλ. Here, mλ stands for milli-lambda, i.e., one part per thousand of the wavelength.

It is of course possible to create pixels in a sample by directing two or more successive laser pulses at a single point on the sample. It is also possible to create pixels at different depths.

Naturally, it is also possible to design a combined unit to correct the object's aberrations for only one light beam passing through the lens. That is, a combined unit can be designed to perform beam shaping or aberration compensation without beam splitting. This means a DOE can be designed to split or split beams.

The at least one combined beam-splitting and aberration-compensating unit may include a third set of combined beam-splitting and aberration-compensating units whose wavelength is tuned to the wavelength of an exposure source of the lens. The exposure source may include a laser beam source, in particular an ultra-short pulse-generating laser beam source.

Typically, a combined unit can be designed for illuminating the lens with a narrowband light source. However, the optical elements of the focusing unit, for example, a lens or lens system of the focusing unit of the lens, can be used in a broad spectral range of several hundred nm. By incorporating a combined unit adapted to the exposure source, a lens described in this application can be used in a broad spectral range, thus opening up a vast range of applications for a lens according to the invention. Naturally, it is also possible to adapt lenses according to the invention to the respective exposure source by permanently installing a combined unit within the lens.

An objective according to the invention thus opens up new degrees of freedom in various directions or dimensions, namely with regard to the exposure wavelength, the number of usable partial beams, the penetration depth of the foci into the sample and the usable image size.

The objective lens can be telecentrically configured. The objective lens can be telecentrically designed on the sample side. Due to the telecentricity of the objective lens, the partial beams exiting the combined beam-splitting and aberration-compensating unit are aligned parallel to an optical axis of the objective lens between the objective lens and the sample.

As stated above, the lens can be characterized by an RMS value between the reference wavefronts and the wavefronts generated for the at least two partial beams. This value can be, for example, less than 60 millilambda, preferably less than 40 millilambda, ideally less than 20 millilambda, and even more advantageously less than 10 millilambda. In the case of spherical wavefronts, this corresponds to Strehl ratios greater than 0.85, 0.93, 0.98, or 0.99, respectively. The Strehl ratio, as a dimensionless quantity for a spherical reference wavefront, indicates the ratio of the measured maximum intensity of a point light source in the image plane to the theoretically maximum intensity of a perfect optical system, such as a lens. According to this definition, the maximum Strehl ratio is 1.

The at least one optical element can comprise at least one first lens. The at least one first lens can comprise a lens group with three lenses, preferably two lenses, and most preferably one lens. The refractive index of the at least one first lens can be greater than 1.5, preferably greater than 1.7, and most preferably greater than 1.8. The exposure wavelength of the objective can range from 150 nm to 1700 nm. The exposure wavelength depends on the material of the sample to be irradiated. It is advantageous to select it such that the sample exhibits the greatest possible optical transparency at that wavelength. For semiconducting samples, this may mean using exposure wavelengths from the infrared region of the electromagnetic spectrum.

The lens may further comprise at least one non-rotationally symmetric optical element which is designed to generate non-rotationally symmetric foci for the at least two partial rays in the image space.

The at least one non-rotationally symmetric optical element can be rotatable about an optical axis. The lens can be configured to accommodate the at least one non-rotationally symmetric optical element in an interchangeable manner.

Both the rotatability and the interchangeability of the at least one non-rotationally symmetric element enable the creation of foci whose wavefronts deviate from a spherical wavefront in a defined manner. This allows a lens according to the invention to be flexibly configured for a wide range of applications.

The at least one non-rotationally symmetric optical element can comprise a lens having two different principal axis curvatures. Furthermore, the at least one non-rotationally symmetric optical element can comprise a cylindrical lens.

The at least one non-rotationally symmetric optical element can be arranged in the beam direction in front of or behind the at least one combined unit.

In addition to or as an alternative to generating two or more rotationally symmetric foci by means of a lens according to the invention with a combined unit, it is also possible to generate non-rotationally symmetric foci for two or more partial beams by introducing at least one non-rotationally symmetric optical element, for example a cylindrical lens, into the beam path of the lens.

At least one cylindrical lens can be designed to rotate around the optical axis of the objective. When the cylindrical lens is rotated, the principal axes of all elliptical foci rotate along with it. In this way, for example, micro-stress-generating pixels with different orientations can be written into the sample by rotating the cylindrical lens.

The lens can be configured to accommodate interchangeable or replaceable cylindrical lenses, allowing the generation of wavefronts with varying degrees of astigmatism. Generating pixels with an asymmetrical, astigmatic shape in a plane perpendicular to the beam direction is advantageous for efficiently correcting photomasks exhibiting placement errors of pattern elements. This also applies to correcting wafer planarity errors that occur during complex processing.

Astigmatic wavefronts of the focusing lens allow the generation of asymmetric pixels.

The lens may also include a unit that compensates for spherical aberrations in the image space and is arranged to be displaceable along an optical axis of the lens.

The spherical aberration compensation unit is configured to shift along the optical axis within the lens. Furthermore, the spherical aberration compensation unit can be configured to form an uncollimated light beam from a collimated light beam entering the lens. In particular, the spherical aberration compensation unit can form a convergent light beam. Thus, the spherical aberration compensation unit assists in focusing the at least two partial beams in the image space.

Furthermore, the unit that compensates for spherical aberrations in the image space can be located, at least in part, in front of the beam-splitting and aberration-compensating unit in the lens.

The inventors have discovered through extensive investigations that a lens can be designed such that the change in spherical aberrations caused by a change in the penetration depth of the foci of at least two partial beams into a sample can be essentially completely compensated by changing a single air space between the unit compensating for spherical aberrations in the image space and the combined beam-splitting and aberration-compensating unit, or between the unit compensating for spherical aberrations in the image space and the focusing unit of the lens—that is, by shifting the unit compensating for spherical aberrations in the image space within the lens. This allows a lens according to the invention to generate foci of very high quality for various penetration depths of the foci into the image space or into a sample. Foci of very high quality are diffraction-limited foci, which, for rotationally symmetric wavefronts, are characterized by a Strehl ratio close to 1. In general, the wavefronts of diffraction-limited foci exhibit a very small deviation (i.e., a small RMS value) from a reference wavefront.

The unit compensating for spherical aberrations in the image space can be arranged to be displaceable along the optical axis of the lens relative to the combined beam-splitting and aberration-compensating unit or the focusing unit of the lens.

Lenses that can be moved within a lens are known to those skilled in the art, for example from the patent specification. US 4 953 962 A . Possible embodiments for moving lenses or lens groups within a lens are described there.

The unit compensating for spherical aberrations in the image space can include at least one of the following: at least one lens, at least one mirror, or at least one lens and at least one mirror.

As explained above, diffraction-limited foci at various adjustable depths within a sample can be generated by a set of combined units tuned to different penetration depths. Moving a unit specifically configured for spherical aberration correction within the image space in a lens according to the invention offers a second possibility for placing diffraction-limited foci of the at least two partial beams at different depths within a sample. As already explained above, the wavefronts of diffraction-limited foci of the at least two partial beams exhibit a small deviation—expressed, for example, by a small RMS value—from corresponding reference wavefronts.

When a wavefront penetrates a sample with a refractive index difference | _n1 - _n2 | > 0, i.e., when the refractive indices of the sample and the medium between the sample and the objective lens do not match, the wavefront experiences an additional spherical aberration in the sample. This aberration increases sharply with increasing NA of the objective lens, where _n1 denotes the refractive index of the medium in front of the sample and _n2 denotes the refractive index of the sample. To generate diffraction-limited foci—which are essential for generating defined pixels—the spherical aberrations, which change with the penetration depth of the foci, must be corrected or compensated. This compensation can be achieved by shifting the unit that compensates for spherical aberrations in the image space within an objective lens according to the invention along its optical axis.

The displacement of the spherical aberrations compensating unit in the image space can encompass a distance of 0.01 mm, preferably 0.1 mm, more preferably 1.0 mm, and most preferably 5.0 mm.

The unit compensating for spherical aberrations in the image space can be set up to compensate for the sum of the spherical aberrations of the optical elements of the focusing unit and the at least two partial beams for a given penetration depth of the foci into the image space.

For example, the unit compensating for spherical aberrations in the image space can be configured to overcompensate spherically, while the focusing unit can be configured to undercompensate spherically. This allows the unit compensating for spherical aberrations in the image space to precisely compensate for the spherical aberrations of the focusing unit and the spherical aberrations of at least two partial beams for a given foci penetration depth into the image space or a sample.

The unit compensating for spherical aberrations in the image space can be configured to compensate for changes in the sum of the spherical aberrations of the focusing unit and changes in the spherical aberrations resulting from changes in the penetration depth of the foci of at least two partial rays into the image space by changing the position of the unit compensating for spherical aberrations in the image space along the optical axis of the lens.

With a greater penetration depth of at least two foci into the image space or the sample, the wavefronts of the at least two partial beams in the sample experience a greater proportion of spherical aberration. By appropriately shifting the unit in the objective lens that compensates for spherical aberrations in the image space, the increased aberration component resulting from deeper penetration depths of the foci into the sample can be compensated for in advance.

The same applies when reducing the penetration depth of the foci of the at least two partial beams into the sample. The lower proportion of spherical aberrations of the at least two partial beams in the sample can be precompensated by a corresponding shift of the unit compensating for spherical aberrations in the image space in the opposite direction.

The unit compensating for spherical aberrations in image space may comprise the combined unit. The lens may be configured to shift the unit compensating for spherical aberrations in image space and the at least one combined beam-splitting and aberration-compensating unit together with respect to the focusing unit along the optical axis of the lens.

This embodiment, in which the combined unit and the unit compensating for spherical aberrations in the image space are moved together along the optical axis of the lens, is superior to the embodiment discussed above, in which only the spherical aberrations in the image space are compensated. The movement of the compensating unit is preferred. By moving the combined unit and the unit compensating for spherical aberrations in the image space together, the optical distribution of the light beam entering the lens remains unchanged across the combined unit for a plane wavefront incident on the lens, thus preserving the functionality of the combined unit. Consequently, unwanted stray light is also independent of the position of the spherical aberration-compensating unit. This simplifies the design of the combined unit, which can be configured to simultaneously minimize unwanted stray light for all depth regions of the image space or a sample where foci are generated.

The unit compensating for spherical aberrations in the image space can compensate for a difference in optical path length (OP) caused by spherical aberration of less than 10 µm, preferably less than 50 µm, more preferably less than 200 µm, and most preferably less than 1000 µm by shifting it along the optical axis.

If foci are generated at different depths within a sample, the various components of the focused partial beams have different optical path lengths. The optical path length (OP) is defined as the product of the distance _dP traveled by a beam parallel to the optical axis in a sample with a refractive index _n₂ > 1 and the refractive index difference Δn = _n₂ - _n₁ between the sample _n₂ and an environment, which could be, for example, air with _n₁ ≈ 1. For a photomask with a quartz substrate ( _n₂ ≈ 1.5) and a thickness of 6.35 mm ( _dP ≈ 6 mm), the _OP is approximately 6000 µm·0.5 = 3000 µm. For a wafer with a thickness of 750 µm and a refractive index n ₂ ≈ 3.5, the corresponding OP _W ≈ 700 µm·2.5 = 1750 µm results, where in both cases n ₁ ≈ 1.

The unit compensating for spherical aberrations in the image space can include at least one diverging lens (concave lens) and at least one converging lens (convex lens), where the refractive power of the converging lens is greater than the refractive power of the diverging lens.

Setting up the spherical aberration-compensating unit can include at least one of the following: defining the refractive power of the at least one lens, defining the principal curvatures of the at least one lens, defining a distance between the at least two lenses, defining the deviation of the light ray exiting the spherical aberration-compensating unit from collimation, and defining a maximum displacement distance of the spherical aberration-compensating unit along the optical axis of the objective. The at least one lens can include an aspherical meniscus. Hereinafter, a spherical aberration-compensating unit is also referred to as the compensating unit.

The refractive index _n₂ of a sample can range from approximately 1.2 to 3.5. The refractive index n _₭ of an immersion medium should be as close as possible to the refractive index of the sample and ideally be the same. In the ideal case, _n₂ = n _₭ and therefore Δn = 0. In this case, the optical path length becomes independent of the penetration depth of the foci into the sample. Conversely, the greater the refractive index difference | _n₂ - n _₭ |, the greater the optical coefficient (OP) that needs to be compensated to avoid wavefront disturbances in the foci caused by the additional spherical aberrations introduced by the sample. The refractive index of the immersion medium can be, for example, less than 2.0, less than 1.5, less than 1.2, or less than 1.02.

A sample placed in the image space can comprise at least one of the following: a sample with a refractive index in the range of 1.5 and a thickness of at least 10 mm, or a sample with a refractive index greater than 3.2 and a thickness of at least 600 µm.

Two important examples of material processing with short laser pulses for pixel generation are, firstly, manufactured photomasks and, secondly, wafers during their processing. Photomasks can be transmissive or reflective. Photomasks to be processed can be any type of photomask, such as binary masks, phase-shifting masks, and/or masks for multiple exposures. Wafers can be any type, for example, elemental semiconductor wafers, such as silicon or germanium, or compound semiconductor wafers, such as gallium arsenide, indium phosphide, or gallium nitride wafers, to name just a few. The wafers can have a crystalline or an amorphous structure. However, these examples do not exhaust the scope of application of a lens according to the invention. Rather, it can be used in at least all of the application areas listed in the first part of the description.

The image space can comprise at least one sample and a sample holder, with the refractive index of the sample and the refractive index of the sample holder often being different. The image space can comprise a sample, a sample holder, a first medium between the objective lens, and a second medium between the sample holder and the sample. The image space can comprise a sample, a sample holder, and a first medium. Furthermore, the image space can comprise a sample and a medium between the objective lens and the sample, or a medium positioned in front of the sample. Preferably, the sample holder has the highest possible refractive index. This allows the beam diameter of the partial beams to be kept small, thus enabling a greater distance between the objective lens and the sample holder.

Both a combined unit, such as a DOE, and the compensating unit (i.e., the unit that compensates for spherical aberrations in image space) can be designed to compensate for, or pre-compensate, the spherical aberrations of both the sample holder and the sample itself. Furthermore, both the combined unit and the compensating unit can be designed to compensate for the spherical aberrations of varying sample holder thicknesses and varying foci of at least two partial beams into a sample.

The lens can be set up to change the penetration depth of the foci of at least two partial rays into the image space by shifting the unit that compensates for spherical aberrations in the image space.

The lens can be set up to change the penetration depth of the foci of at least two partial rays into the image space by shifting the unit that compensates for spherical aberrations in the image space.

In addition to compensating for spherical aberrations due to varying penetration depth, moving the compensating unit slightly changes the imaging behavior of the lens, since the light ray leaving the compensating unit is no longer compensated.

By shifting the compensating unit within the lens, the working distance between the lens exit and the sample being processed must be slightly varied to position the foci of at least two partial beams at a predetermined depth within the sample. In summary: The entire lens is moved relative to the image space to change the penetration depth of the foci into the sample. If the lens has a compensating unit, this is shifted relative to the focusing unit to compensate for the spherical aberration that arises in the image space due to the lens movement. These two movements each change the penetration depth, so both movements must be coordinated depending on the desired penetration depth.

The penetration depth of the foci into a sample can also be referred to as the longitudinal focus position. Both the penetration depth and the longitudinal focus position can be related to the surface of the sample through which the at least two partial beams penetrate the sample. The longitudinal focus position describes the focus position in the beam direction.

The objective lens can further comprise at least one lens arranged between the at least one combined beam-splitting and aberration-compensating unit and the focusing unit, and is configured for at least one of the following: moving the at least one lens along the optical axis together with the spherical aberrations-in-image-space compensating unit and the at least one combined beam-splitting and aberration-compensating unit, or moving the at least one lens along the optical axis independently of the spherical aberrations-in-image-space compensating unit, the focusing unit and the at least one combined beam-splitting and aberration-compensating unit.

The focusing unit of the objective can be simplified by the inclusion of at least one third lens. Furthermore, in an embodiment where the at least one lens is movable independently of the movement of the compensating unit, moving the at least one lens can be used to shift the foci of the at least two partial beams to different depths within a sample. By appropriately moving the compensating unit, the changes in spherical aberrations in the sample or within the image space caused by moving the at least one lens can be compensated.

At least one lens can be a converging lens.

In an embodiment where the at least one lens, the compensating unit, and the combined unit are moved together, the compensating effect with respect to spherical aberrations resulting from the displacement of these optical elements along the optical axis of the objective can be adjusted to the penetration depth or the longitudinal focus position. Furthermore, the joint displacement of the compensating unit, the at least one lens, and the combined unit enhances the focusing effect of the objective. This allows for a smaller change in the working distance between the objective's output and the sample for tuning the foci of the at least two partial beams through the sample.

In an embodiment where the at least one lens is movable along the optical axis independently of the compensating unit, the at least one lens can be designed such that the working distance between the sample and the objective does not need to be changed to shift the foci within the sample. This means that the objective with at least one lens has internal focusing. The mass that needs to be moved to change the penetration depth of the foci and to compensate for the resulting spherical aberrations can thus be minimized.

A second embodiment relates to a device for simultaneously processing a sample with at least two light beams at an adjustable depth of the sample, wherein the device comprises at least one lens according to one of the aspects described above for generating at least two partial beams from one light beam and for focusing the at least two partial beams.

The device may further include means for shifting the spherical aberrations compensating unit in the lens to compensate for spherical aberrations in the image space as a result of changing the penetration depth of the foci into the image space.

The device may further include a unit configured to analyze the sample. The analysis unit may comprise a particle beam for irradiating the sample and a detection unit for detecting particles emanating from the sample during irradiation. The particles of the particle beam may include charged particles, such as electrons and/or ions, and uncharged particles, such as photons and/or atoms or molecules.

Another embodiment relates to a method for focusing at least two partial beams into at least one predetermined depth of an image space, wherein the method comprises: adjusting a working distance of a lens according to one of the preceding aspects with respect to the image space to compensate for aberrations and to focus the at least two partial beams into the at least one predetermined depth of the image space.

The method can further include the step of shifting the at least two foci of the at least two partial beams in the image space. This can be achieved by changing the working distance of the objective to a sample, by shifting a compensating unit in the objective, by internal focusing of the objective, or a combination thereof.

Furthermore, the procedure can include the step of: moving the compensating unit in the lens, or changing the combined unit to compensate for the changes in the spherical aberrations of the image space caused by the changed focus position in the image space.

Changing the penetration depth of the foci of the at least two partial beams can include at least one of the following: changing the working distance between the lens and the image space and changing the at least one combined beam-splitting and aberration-compensating unit; changing the working distance between the lens and the image space and moving the spherical aberration-compensating unit along the optical axis of the lens; or moving the spherical aberration-compensating unit by a first distance and moving at least one lens by a second distance.

The first distance and the second distance can have different numerical values to change the penetration depth of the foci of at least two partial beams into the image space and to compensate for any resulting changes in spherical aberrations in the image space or in a sample.

Changing the penetration depth of the foci of the at least two partial beams into a sample can be carried out in at least four different embodiments: (I) The working distance of the objective can (1) The working distance of the object can be adjusted accordingly, and the resulting changes in spherical aberrations in the sample can be compensated or precompensated by removing a first combined unit from the beam path and by inserting a second combined unit, designed for the changed penetration depth, into the objective, preferably in the pupil plane of the object. (2) The working distance of the object can be adjusted to the new, changed penetration depth, and the resulting changes in the spherical aberrations of the sample can be precompensated by moving a compensating unit along the optical axis of the objective. (3) A changed penetration depth of the foci of the at least two partial beams can be set by a combined change in the working distance of the object and at least one movable lens of the objective, and the resulting change in the spherical aberrations in the image space can be precompensated by moving the compensating unit along the optical axis of the objective. (IV) Finally, a changed penetration depth of the foci of the at least two partial beams into the sample can be adjusted solely by moving the at least one lens within the objective, and the resulting changes in spherical aberrations in the image space or in the sample can be precompensated by moving the compensating unit along the optical axis of the objective.

Changing the working distance and moving the compensating unit can be done simultaneously, or they can be performed iteratively. In the first step, the working distance is adjusted to a predetermined new penetration depth of the foci of the at least two partial beams. Then, in the second step, the altered spherical aberrations are precompensated by moving the compensating unit and/or the at least one lens. Moving the compensating unit, or moving it and the at least one lens together, changes the penetration depth of the foci into the sample, at least slightly. In a second adjustment step, the changed longitudinal focus position can then be adjusted to the predetermined penetration depth, and the resulting changes in spherical aberrations can be precompensated as described in the first adjustment step. The adjustment process ends when both the penetration depth of the foci are within a predefined depth range and the remaining spherical aberrations are below a predefined threshold.

The movement of the compensating unit and the movement of the at least one lens can be performed simultaneously, or the movement of the compensating unit and the movement of the at least one lens can be performed in an iterative process.

Furthermore, a method according to the invention can comprise at least one of the following steps: changing the at least one combined beam-splitting and aberration-compensating unit to change the number of generated partial beams, changing the at least one combined beam-splitting and aberration-compensating unit to change the penetration depth of the foci of the at least two partial beams, or changing the at least one combined beam-splitting and aberration-compensating unit to change the exposure wavelength of the image space.

Changing the at least one combined beam-splitting and aberration-compensating unit may involve removing a first beam-splitting and aberration-compensating unit from the lens and inserting a second beam-splitting and aberration-compensating unit into the lens, particularly in its pupil plane.

In another embodiment, a computer program may include instructions that cause a computer system to execute the process steps according to one of the aspects described above.

The computer system can be part of a device that uses a lens according to the invention for splitting a laser beam into two or more partial beams, for compensating for imaging errors and for focusing the two or more partial beams at different depths of a sample.

The fabrication of an optical element, a photolithographic mask, a wafer, a template for nanoimprint lithography, a micro-electromechanical component, and/or a nano-electromechanical component may include a machining process and/or a repair process according to any of the aspects described above.

Furthermore, an optical element, a photolithographic mask, a wafer, a template for nanoimprint lithography, a micro-electromechanical component, and/or a nanomechanical component may include a machining process and/or a repair process according to any of the aspects described above.

4. Description of the drawings

• 1 ein Objektiv eines Laser-Fokus-Generators gemäß dem Stand der Technik wiedergibt;
• 2A ein Schnittbild durch den optischen Aufbau eines ersten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei Teilstrahlen zeigt;
• 2B in Tabelle 1 die Designdaten des Ausführungsbeispiels der 2A auflistet;
• 3A in den Teilbildern (a) bis (f) jeweils den Strahlengang eines von sechs Teilstrahlen wiedergibt, den ein diffraktives optisches Element (DOE) des Objektivs der 2A erzeugt, und die Fokussiereinheit dieses Objektivs in den Bildraum der 2A fokussiert;
• 3B eindimensionale (1D) Aberrationskurven in zwei zueinander senkrechten Richtungen für die sechs Laser-Foki der 3A wiedergibt;
• 4 in Tabelle 2 die Zernike-Fringe-Indices (ZFR) j und die zugehörigen Zernike-Fringe-Polynome P_j(R, A) für j = 1 bis 36 zusammenfasst;
• 5A ein erstes Ausführungsbeispiel einer zweidimensionalen (2D) Phasenverteilung eines diffraktiven optischen Elements (DOE) in Form eines Computer-generierten Hologramms (CGH) präsentiert;
• 5B einen vergrößerten 1D Schnitt der 2D Phasenverteilung des DOE der 5A entlang der x-Achse angibt;
• 5C einen vergrößerten 1D Schnitt der 2D Phasenverteilung des DOE der 5A entlang der y-Achse wiedergibt;
• 6 die 2D Phasenverteilung ΔW₁(x_CGH, y_CGH) des DOE der 3A zum Erzeugen des Laser-Fokus für den ersten Teilstrahl der 3A(a) auf der optischen Achse präsentiert;
• 7A die 2D Phasenverteilung ΔW₆(x_CGH, y_CGH) des DOE der 3A zum Erzeugen des Laser-Fokus für den sechsten, maximal ausgelenkten Teilstrahl der 3A(f) darstellt;
• 7B die 2D Phasenverteilung $\Delta {\text{W}}_6\left({\text{x}}_{\text{CGH}}{\text{,y}}_{\text{CGH}}\right)-\Delta {\text{W}}_1\left({\text{x}}_{\text{CGH}}{\text{,y}}_{\text{CGH}}\right)-Z_3^6\cdot P_3\left(R,A\right)$ des DOE der 3A(f) zeigt, d.h. von der 2D Phasenverteilung der 7A wurden die 2D Phasenverteilung der 6 und eine Kippung beschreibende Zernike-Funktion $Z_3^6\cdot P_3\left(R,A\right)$ subtrahiert;
• 8 die Zernike-Koeffizienten $Z_3^i,$ für Kippung, und $Z_4^i,$ für Defokussierung, für die sechs Teilstrahlen der 3A (d.h. i = 1 bis 6) als Funktion der Entfernung des jeweiligen Fokus von der optischen Achse darstellt;
• 9A einen schematischen Schnitt durch den optischen Aufbau eines zweiten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei Teilstrahlen zeigt;
• 9B in Tabelle 3 die Designdaten des Ausführungsbeispiels der 9A zusammenfasst;
• 10A die 2D Phasenverteilung des CGH des DOE der 9A wiedergibt;
• 10B einen vergrößerten 1D Schnitt der 2D Phasenverteilung des CGH des DOE der 9A entlang der x-Achse angibt;
• 10C einen vergrößerten 1D Schnitt der 2D Phasenverteilung des CGH des DOE der 9A entlang der y-Achse wiedergibt;
• 11A einen schematischen Schnitt durch den optischen Aufbau eines dritten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei Teilstrahlen zeigt;
• 11B in Tabelle 4 die Designdaten des Ausführungsbeispiels der 11A zusammenfasst;
• 12A die 2D Phasenverteilung des CGH des DOE der 11A präsentiert;
• 12B einen vergrößerten 1D Schnitt der 2D Phasenverteilung des CGH des DOE der 11A entlang der x-Achse angibt;
• 12C einen vergrößerten 1D Schnitt der 2D Phasenverteilung des CGH des DOE der 11A entlang der y-Achse wiedergibt;
• 13A einen schematischen Schnitt durch den optischen Aufbau eines vierten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei Teilstrahlen zeigt;
• 13B in Tabelle 5 die Designdaten des Ausführungsbeispiels der 13A zusammenstellt;
• 14 einen schematischen Schnitt durch den optischen Aufbau eines fünften Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei Teilstrahlen darstellt, wobei eine Referenz-Wellenfront in einer Probe einen Astigmatismus aufweist;
• 15 einen schematischen Schnitt durch den optischen Aufbau eines sechsten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei von sieben Teilstrahlen zeigt, wobei das Objektiv ein erstes Ausführungsbeispiel einer sphärische Aberrationen im Bildraum kompensierenden Einheit aufweist;
• 16A im oberen Teilbild eine erste Fokuslage eines der Teilstrahlen des Objektivs der 15 in einem Silizium-Wafer als Probe darstellt und das untere Teilbild sphärische Aberrationen der Foki des Objektivs der 15 in der ersten Fokuslage wiedergibt;
• 16B im oberen Teilbild eine zweite, geänderte Fokuslage eines der Teilstrahlen des Objektivs der 15 in einem Silizium-Wafer illustriert und das untere Teilbild Aberrationskurven der sieben Foki des Objektivs der 15 in der zweiten Fokuslage zeigt;
• 16C im oberen Teilbild das obere Teilbild der 16B reproduziert und im unteren Teilbild die Aberrationskurven der sieben Foki des Objektivs der 15 in der zweiten Fokuslage nach deren Kompensation durch Verschieben der sphärische Aberrationen im Bildraum kompensierenden Einheit in dem Objektiv der 15 präsentiert;
• 17 einen schematischen Schnitt durch den optischen Aufbau eines siebten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei von sieben Teilstrahlen wiedergibt, wobei das Objektiv ein zweites Ausführungsbeispiel einer sphärische Aberrationen im Bildraum kompensierenden Einheit mit entsprechend angepasster kombinierter Strahl-aufspaltenden und Abbildungsfehler-kompensierenden Einheit aufweist;
• 18 einen schematischen Schnitt durch den optischen Aufbau eines achten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei von sieben Teilstrahlen zeigt, dessen Linsen aus einem Material mit größerem Brechungsindex gefertigt werden (verglichen mit dem Objektiv des sechsten Ausführungsbeispiels), und ein drittes Ausführungsbeispiel einer sphärische Aberrationen im Bildraum kompensierenden Einheit aufweist;
• 19 einen schematischen Schnitt durch den optischen Aufbau eines neunten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei von sieben Teilstrahlen zeigt, das für eine Belichtungswellenlänge von 1550 nm ausgelegt ist, wobei dessen Linsen aus Silizium hergestellt sind, und das ein viertes Ausführungsbeispiel einer sphärische Aberrationen im Bildraum kompensierenden Einheit mit entsprechend angepasster kombinierter Strahl-aufspaltenden und Abbildungsfehler-kompensierenden Einheit beinhaltet;
• 20 einen schematischen Schnitt durch den optischen Aufbau eines zehnten Ausführungsbeispiels eines Objektivs mit nachfolgendem Bildraum samt Strahlengang für zwei von sieben Teilstrahlen zeigt, wobei das Objektiv eine NA von 0,8 aufweist (verglichen mit 0,6 im sechsten Ausführungsbeispiel) und ein fünftes Ausführungsbeispiel einer sphärische Aberrationen im Bildraum kompensierenden Einheit aufweist;
• 21 die Tabelle 6 beinhaltet, die die in den sechsten bis zehnten Ausführungsbeispielen ausgeführten Bewegungen der Objektive sowie der sphärische Aberrationen im Bildraum kompensierenden Einheiten innerhalb der Objektive zum Verschieben der Fokuslage der Teilstrahlen innerhalb eines Silizium-Wafers als Probe um 500 µm zusammenstellt; und
• 22 ein Flussdiagramm zum Fokussieren von zwei oder mehr Teilstrahlen eines auf ein hierin beschriebenes Objektiv auftreffenden Strahls in zumindest eine vorgegebene Tiefe des Bildraums wiedergibt.

In the following detailed description, currently preferred embodiments of the invention are described with reference to the drawings, wherein

• 1 reproduces a lens of a laser focus generator according to the state of the art;
• 2A a cross-sectional view through the optical structure of a first embodiment of a lens with subsequent image space including beam path for two partial beams;
• 2B Table 1 lists the design data of the exemplary embodiment of the 2A lists;
• 3A In the partial images (a) to (f) each shows the beam path of one of six partial beams, which a diffractive optical element (DOE) of the objective of the 2A generated, and the focusing unit of this lens into the image space of the 2A focused;
• 3B One-dimensional (1D) aberration curves in two mutually perpendicular directions for the six laser foci of the 3A reproduces;
• 4 Table 2 summarizes the Zernike Fringe indices (ZFR) j and the associated Zernike Fringe polynomials P _j (R, A) for j = 1 to 36;
• 5A a first embodiment of a two-dimensional (2D) phase distribution of a diffractive optical element (DOE) in the form of a computer-generated hologram (CGH) is presented;
• 5B an enlarged 1D section of the 2D phase distribution of the DOE of the 5A along the x-axis;
• 5C an enlarged 1D section of the 2D phase distribution of the DOE of the 5A represented along the y-axis;
• 6 the 2D phase distribution ΔW ₁ (x _CGH , y _CGH ) of the DOE of the 3A to generate the laser focus for the first partial beam of the 3A(a) presented on the optical axis;
• 7A the 2D phase distribution ΔW ₆ (x _CGH , y _CGH ) of the DOE of the 3A to generate the laser focus for the sixth, maximally deflected partial beam of the 3A(f) represents;
• 7B the 2D phase distribution $\Delta {\text{W}}_6\left({\text{x}}_{\text{CGH}}{\text{,y}}_{\text{CGH}}\right)-\Delta {\text{W}}_1\left({\text{x}}_{\text{CGH}}{\text{,y}}_{\text{CGH}}\right)-Z_3^6\cdot P_3\left(R,A\right)$ of the DOE 3A(f) shows, i.e., from the 2D phase distribution of the 7A The 2D phase distribution of the 6 and a Zernike function describing a tilt $Z_3^6\cdot P_3\left(R,A\right)$ subtracted;
• 8 the Zernike coefficients $Z_3^i,$ for tilting, and $Z_4^i,$ for defocusing, for the six partial beams of the 3A (i.e., i = 1 to 6) represents as a function of the distance of the respective focus from the optical axis;
• 9A a schematic section through the optical structure of a second embodiment of a lens with subsequent image space including beam path for two partial beams;
• 9B Table 3 contains the design data of the exemplary embodiment of the 9A summarizes;
• 10A the 2D phase distribution of CGH of the DOE 9A reproduces;
• 10B an enlarged 1D section of the 2D phase distribution of the CGH of the DOE 9A along the x-axis;
• 10C an enlarged 1D section of the 2D phase distribution of the CGH of the DOE 9A represented along the y-axis;
• 11A a schematic section through the optical structure of a third embodiment of a lens with subsequent image space including beam path for two partial beams;
• 11B Table 4 contains the design data of the exemplary embodiment of the 11A summarizes;
• 12A the 2D phase distribution of CGH of the DOE 11A presents;
• 12B an enlarged 1D section of the 2D phase distribution of the CGH of the DOE 11A along the x-axis;
• 12C an enlarged 1D section of the 2D phase distribution of the CGH of the DOE 11A represented along the y-axis;
• 13A a schematic section through the optical structure of a fourth embodiment of a lens with subsequent image space including beam path for two partial beams;
• 13B Table 5 contains the design data of the exemplary embodiment of the 13A compiles;
• 14 a schematic section through the optical structure of a fifth embodiment of a lens with subsequent image space including beam path for two partial beams, wherein a reference wavefront in a sample exhibits astigmatism;
• 15 a schematic section through the optical structure of a sixth embodiment of a lens with subsequent image space including beam path for two of seven partial beams, wherein the lens has a first embodiment of a unit compensating for spherical aberrations in the image space;
• 16A The upper part of the image shows the first focus position of one of the partial rays of the lens. 15 represented as a sample in a silicon wafer, and the lower part of the image shows spherical aberrations of the lens's foci. 15 reproduced in the first focus position;
• 16B In the upper part of the image, a second, changed focus position of one of the partial rays of the lens is visible. 15 illustrated in a silicon wafer, and the lower sub-image shows aberration curves of the seven foci of the lens. 15 in the second focus position;
• 16C in the upper part of the image, the upper part of the image 16B reproduced and in the lower part of the image the aberration curves of the seven foci of the lens of the 15 in the second focus position after their compensation by shifting the spherical aberrations in the image space compensating unit in the lens of the 15 presents;
• 17 a schematic section through the optical structure of a seventh embodiment of a lens with subsequent image space including beam path for two of seven partial beams, wherein the lens has a second embodiment of a spherical aberrations in the image space compensating unit with a correspondingly adapted combined beam splitting and aberration compensating unit;
• 18 Figure 1 shows a schematic section through the optical structure of an eighth embodiment of a lens with subsequent image space including beam path for two of seven partial beams, whose lenses are made of a material with a larger refractive index (compared to the lens of the sixth embodiment), and a third embodiment of a unit compensating for spherical aberrations in the image space;
• 19 a schematic section through the optical structure of a ninth embodiment of a lens with subsequent image space including beam path for two of seven partial beams, designed for an exposure wavelength of 1550 nm, its lenses being made of silicon, and which includes a fourth embodiment of a spherical aberrations in the image space compensating unit with appropriately adapted combined beam splitting and aberration compensating unit;
• 20 Figure 1 shows a schematic section through the optical structure of a tenth embodiment of a lens with subsequent image space including beam path for two of seven partial beams, wherein the lens has an NA of 0.8 (compared with 0.6 in the sixth embodiment) and a fifth embodiment of a unit compensating for spherical aberrations in the image space;
• 21 Table 6 contains the movements of the lenses and the units within the lenses that compensate for spherical aberrations in the image space, as carried out in the sixth to tenth embodiments, for shifting the focus position of the partial beams within a silicon wafer as a sample by 500 µm; and
• 22 a flowchart for focusing two or more partial rays of a ray incident on a lens described herein into at least a predetermined depth of the image space.

5. Detailed description of preferred embodiments

Preferred embodiments of the lenses and methods according to the invention are described below. Lenses according to the invention are discussed in detail using the example of generating foci for two or more partial beams in an image space of a laser beam incident on the lens. In the examples given, a lens according to the invention is part of a laser focus generator. However, the lenses described herein are not limited to use as part of a laser focus generator. Rather, they can be used to split the light beam of any narrowband light source into two or more partial beams and to focus them at a predetermined depth in a sample.

Furthermore, the functionality of lenses according to the invention is explained below using the example of generating pixels at different depths of a wafer or photomask. However, the use of lenses according to the invention is not limited to these applications. Rather, lenses according to the invention can be used in all fields of material processing where two or more partial beams are required or advantageous, and whose foci are to be placed at adjustable depths of a sample. Moreover, the materials that can be processed using a lens described herein are not limited to photomasks and wafers. All materials whose band gap between the valence band and the conduction band is greater than the energy of the photons of the laser beam used for processing can be processed using short, intense laser pulses. In addition, a lens according to the invention can be used for all applications listed in the first part of this description; in particular, the sample can also be in a liquid state and, for example, consist of photopolymers for 3D printing.

The following explains the function of a beam-splitting and aberration-compensating unit using the example of a diffractive optical element (DOE). However, this does not limit the use of beam-splitting and aberration-compensating units to DOEs.

The 1 Figure 1 schematically illustrates a side view of a lens according to the prior art. The lens has six lenses through which one non-displaced light beam (11) and two light beams ( _L2 and _L3 ) deflected relative to the optical axis pass and are focused at a depth within the sample. The principal rays of the two deflected light beams run parallel to the optical axis; the lens of the 1 It is therefore telecentric. The pupil of the lens lies within the first lens element, and this position is thus unavailable for the integration of any further optical components into the lens. The aperture, or diameter, of the pupil (DP) allows for the essentially complete passage of both the unexposed and the two deflected light rays.

Diagram 202 of the 2A shows a cross-sectional view through a first embodiment of a lens 200 according to the invention. In the 2A In the illustrated example, the focusing unit 210 is implemented by a lens group with three focusing lenses 213, 216, and 219. In this example—as in the following examples—the focusing unit 210 of the objective 200 uses lenses to focus the light rays into the image space. Alternatively, it is also possible to use one or more mirrors in the focusing unit 210 to focus the light rays into an image space. The image-side numerical aperture (NA) of the objective 200, or its focusing unit, is 0.6. The reference wavefronts of this embodiment for each focus 290 and 295 represent 280 spheres in the sample, so that the objective 200 produces rotationally symmetric foci.

The beam-splitting and aberration-compensating unit 220 is arranged in the pupil 230 of the objective 200, such that the objective 200 is telecentric. The objective 200 and the beam-splitting and aberration-compensating unit 220 can be designed so that the beam-splitting and aberration-compensating unit 220 can be inserted into and removed from the objective 200 from the outside (in the 2A (not shown). Changing the beam-splitting and aberration-compensating unit 220 can be done manually, semi-automatically, or automatically. Methods for interchangeably inserting and fixing a beam-splitting and aberration-compensating unit 220 into the objective 200 are known to those skilled in the art from the fields of microscopy or photography.

In the exemplary 200 lens of the 2A As in the following embodiments, the beam-splitting and aberration-correcting unit 220 is in the form of a diffractive optical element. The DOE 220 is implemented as a computer-generated hologram (CGH) and is applied to the left side, i.e., the light-entry side of the lens 200, onto a plane-parallel plate with a thickness of 1 mm. In the example discussed, the plane-parallel plate is a quartz plate. 2A The DOE 220 splits the light beam 240 entering the lens 200 into two partial beams 260 and 265. The two partial beams 260 and 265 have essentially the same intensity.

The pupil 230 represents an aperture or diaphragm for the parallel light ray 240 entering the lens 200. The diameter 235 of the pupil 230 is designed such that the incoming light ray 240, with its full diameter 245, can pass through the pupil 230 essentially unimpeded. In the exemplary lens 200 of the 2A The pupil 230 has a diameter 235 of 40 mm and limits the diameter 235 of the light beam 240 entering the lens 200 to this value. With the given focal length of the focusing unit 210, this dimensioning of the pupil 230 ensures that the NA is 0.6. If the application requires a smaller NA, a variable aperture iris diaphragm can be inserted directly in front of the pupil 230. For example, if the application requires an NA of 0.4, the diameter of the iris diaphragm can be reduced accordingly. In the example of the 2A As in the embodiments discussed below, the beam-splitting and aberration-compensating unit 220 is implemented as a diffractive optical element (DOE) 220. Alternative embodiments of a beam-splitting and aberration-compensating unit 220 are discussed above.

In the example of the 2A The DOE 220 splits the light beam 240 entering the objective 200 into two partial beams: an undeflected partial beam 260 and a deflected partial beam 265. The deflected main beam of partial beam 265, which passes through the center of the pupil, exits the objective 200 in the sample space almost parallel to the optical axis. The objective 200 is therefore telecentric. The aberration-compensating effect of the DOE 220, as an example of a beam-splitting and aberration-compensating unit 220, is described in the 2A It is shown, but barely recognizable. The light ray 240 entering the lens 200 shows, in the example of the 2A a wavelength of λ = 1064 nm.

The focusing unit 210 of the lens 200 generates foci 290, 295 in the image space 270, which lie in a plane oriented perpendicular to the optical axis 250 of the lens 200. In the example of the 2A The image space 270 comprises a working distance 285 between the output of the objective 200, for example the lens 219 of the objective 200, a sample holder 275, and a sample 280, in the illustrated example a silicon wafer 280. In the 2A The sample holder 275 comprises a quartz plate 275 with a diameter of 15 mm in the beam direction, which is located in the 2A The path runs from left to right. Typical wafer thicknesses are less than one millimeter. The working distance 285, or rather its medium, has a refractive index n ≈ 1, the refractive index of the sample holder 275 is approximately _nS ≈ 1.5, and for a silicon wafer it is approximately _nW ≈ 3.5.

A sample 280 is not limited to a silicon wafer 280. Rather, the objective 200 can focus two or more partial beams into all types of wafers, such as wafers made of elemental semiconductors or compound semiconductors, i.e., binary, ternary, or quaternary compound semiconductors. Furthermore, a sample 280 can include any type of photomask or a stamp for nanoimprint lithography. In the case of samples 280 in the form of photomasks, the partial beams 260 and 265 typically do not pass through a corresponding photomask holder. In general, the objective 200 can be used for material processing using focused laser radiation in all the areas mentioned above in this description.

The optical data for the 200 lens of the 2A are in Table 1 of the 2B In summary, the units for the radius, thickness, and half-diameter of the optical elements are given in millimeters (mm).

In diagram 300 of the 3A The DOE 220 of the objective 200 splits the incoming light beam 260 into six partial beams. For clarity, the beam paths in the objective 200 and their foci are shown in a sample, based on the 2A A wafer, reproduced individually. The deflection of the foci, or their lateral focus position, increases equidistantly from (a) to (f) with respect to the optical axis 250 (x _a , y _a ) = 0 mm, 0 mm): (x _b , y _b ) = 0 mm, 1 mm), (x _c , y _c ) = 0 mm, 2 mm), (x _d , Y _d ) = 0 mm, 3 mm), (x _e , y _e ) = 0 mm, 4 mm) and (x _f , y _f ) = 0 mm, 5 mm). The lateral focus position defines the lateral distance of a focus 290, 295 of a partial beam 260, 265 from the optical axis 250 of a lens 200.

Since this embodiment exhibits spherical wavefronts in the sample for all light beams, aberration curves representing the transverse aberrations are suitable for visualizing the focus quality. Diagram 350 of the 3B gives the aberration curves belonging to the foci of diagram 300 in the x-direction (i.e. pointing into the plane of the paper) and in the y-direction (i.e. pointing upwards in the plane of the paper). 3B reveals that the deviations of the wavefronts from the specified wavefront for all foci of the 3A They remain smaller than 1 µm and even mostly smaller than 0.1 µm. The corresponding Airy radius is $r_{Airy}=\mathrm{0,61}\cdot \frac{\lambda }{NA}=\mathrm{0,61}\cdot \frac{1.064\mu m}{\mathrm{0,6}}=\mathrm{1,09}\mu m$ and is larger than the lateral aberrations, so that the rotationally symmetric foci are diffraction-limited. It is also important that the DOE 220 can compensate for the aberrations of the focusing unit 210 of the objective 200 over a sample area of 5 mm. This means, compared to the prior art represented by the US 2023 / 0 367 134 A1 , an improvement of approximately a factor of 35. This means that the 200 lens opens up the possibility of processing large areas of a sample, such as the silicon wafer 280, simultaneously.

The six partial beams 310, 320, 330, 340, 350, and 360 can be described behind the DOE 220 by six (generally m) separate phase functions ΔW _i (x _CGH , y _CGH ). Each of the six phase functions can be described by its own set of Zernike-Fringe coefficients. $Z_j^i,\text{j}\ge \mathrm{1,}\text{i}=\mathrm{1,}\dots ,\text{m}$ $$ are described, where m denotes the number of partial beams generated by the DOE 220. The Zernike fringe coefficients are chosen such that the aberrations of the three lenses 213, 216, 219 of the focusing unit 210 are pre-compensated for the corresponding focus positions (x _i , y _i ), i = 1, ..., m. Due to the possible pre-compensation, in the example of the objective 200, three lenses 213, 216, 219 for the focusing unit 210 are sufficient to generate high-quality, i.e., essentially diffraction-limited, foci 290, 295, whereas in the objective of the 1 Six lenses are used for this purpose.

The following describes one way to calculate Zernike fringe coefficients. $Z_j^i,\text{j}\ge \mathrm{1,}\text{i}=\mathrm{1,}\dots ,\text{m}$ to determine. For the m foci, m lenses with an individual plane-parallel plate with a refractive index n are considered, where temporarily one side of the plate is an aspherical surface with a curvature. $$z_i\left(x,y\right)={\displaystyle \sum _j^{\square }{Z}_j^i}\cdot P_j\left(R,A\right)$$ is considered. To allow the later application of the so-called "Thin Element Approximation" to convert the area z _i (x, y) into a phase function, the refractive index n is chosen as a very large number, for example, n = 100 or n = 1000. This is possible because the material with this physically nonsensical refractive index n is only used temporarily for computational purposes; this material therefore does not occur in built systems. In this equation, P _j (R, A) is the j-th Zernike Fringe function, which in the 4 is reproduced with a normalized radius $$R\left(x,y\right)=\frac{{\sqrt{x^2+y^2}}^{\square }}{\sqrt{x_{max}^2+y_{max}^2{}_{{\square }_{\square }}}}$$ and angles $$A\left(x,y\right)=arctan2\left(\frac{y}{x}\right),$$ where arctan2() represents the four-quadrant arctangent. The optical design is executed simultaneously for all individual lenses, using the Zernike fringe coefficients. $Z_j^i$ They are optimized together with the lenses, which are the same for all microscopes. The phase functions ΔW _i are then given by: $$\Delta W_i\left(x,y\right)=\frac{2\pi }{\lambda }\cdot \left(n-1\right)\cdot z_i\left(x,y\right)-W_{in}\left(x,y\right).$$

In this equation, W _in (x, y) describes the phase of the wavefront incident on the plane-parallel plate, which is the same for all partial rays i = 1, ..., m. If a plane wavefront incident on the plane-parallel plate, then W _in (x, y) = const. If, for example, a convergent spherical wave with curvature and a wavelength λ incident on the plane-parallel plate, then: $$W_{in}x,y=\frac{2\pi }{\lambda }\cdot \frac{\varrho \cdot \sqrt{x^2+y^2}}{1+\sqrt{1-{\varrho }^2\cdot \left(x^2+y^2\right)}}.$$

The incident wave can also be described by any other phase functions W _in (x, y). This allows the determination of the phase functions ΔW _i , from which – as described below – the geometry of the DOE 220 is determined. The phase functions ΔW _i (x, y), i = 1, ..., m, are dimensionless quantities.

When designing the CGH to generate the DOE 220, the m individual phase functions of the m partial beams also take into account the wavefront of the light beam 240 incident on the objective 200. The CGH is now designed to generate an electric field of the following form from the light beam 240 after it has passed through: $$E\left(x_{CGH},y_{CGH}\right)={\displaystyle \sum {}_{i=1}^m}\sqrt{w_i}\cdot e^{-j2\pi \Delta W_i\left(x_{CGH},y_{CGH}\right)}.$$

Here, j in the exponent denotes the imaginary unit and should not be confused with the index j of the Zernike-Fringe coefficients. The individual weights w_{_i} are used to determine the individual intensities of the partial beams 310 to 360 of the 3A to adjust. If all partial beams are to have the same intensity, the weights can be assumed to be constant, i.e., w _i = const. A light beam 240 incident on the DOE 220, which is implemented as a CGH, can generally be split into m partial beams, for example, the six partial beams 310 to 360. The partial beams leaving the CGH exhibit high-quality foci in image space 270, in the example of the 2 especially in wafer 280. To a good approximation, the desired electric field can be generated by a phase modulation defined by: $$\phi \left(x_{CGH},y_{CGH}\right)=arctan2\left(\frac{Im\left\{x_{CGH},y_{CGH}\right\}}{Re\left\{x_{CGH},y_{CGH}\right\}}\right).$$

The surface description h(x _CGH , y _CGH ) of the CGH of the DOE 220 is given by: $$h\left(x_{CGH},y_{CGH}\right)=\frac{\lambda }{2\pi }\cdot \frac{\pi -\phi \left(x_{CGH},y_{CGH}\right)}{n_{CGH}-1},$$ where n _CGH is the refractive index of the medium in which the CGH is generated. In this case, this is a quartz plate with n _CGH = 1.449604.

Table 2 of the 4 lists the Zernike Fringe indices (ZFR) j and the associated Zernike Fringe polynomials P _j (R, A) for j = 1 to 36.

5A shows the design of the CGH of the DOE 220, the first embodiment of the lens 200. 2A . 5B provides a one-dimensional (1D) section of the CGH, i.e., the DOE 220 of the 5A along the x-axis, on, and 5C presents a 1D section along the y-axis of the 5A .

The focus of the partial beam 310 from the incoming light beam 240, which is not deflected by the DOE 220, is generated almost entirely by the three lenses 213, 216, and 219 of the focusing unit 210. The focus is corrected for spherical aberrations and to satisfy the sine condition. For an objective 200 with a numerical aperture (NA) of 0.6, the simple lens group of the focusing unit 210 would not be able to fulfill both conditions of the objective 200. Therefore, the partial beam 310 (i = 1) generated by the CGH of the DOE 220 is designed to compensate for, or precompensate for, the remaining spherical aberrations of the lenses 213, 216, and 219 of the focusing unit 210. Upon closer examination of the 3 It can be seen that the perfectly collimated incoming light beam 240 is modified by the CGH in such a way that its inner beam components are focused towards the optical axis 250, whereas the outer beam components are dispersed from the optical axis 250. Thus, as expected, the CGH introduces overcompensating spherical aberrations into the undeflected partial beam 310.

In the 6 The dimensionless phase _W1 (x _CGH , y _CGH ), added to the partial beam 310 focused on the optical axis 250 by the CGH of the DOE 220, is shown as contour plot 600. The rotationally symmetric shape of the phase function can be seen from diagram 600. This means that the phase added to the partial beam 310 by the CGH essentially comprises the Zernike fringe coefficient _Z9 and corrects spherical aberrations. Thus, the CGH of the DOE 220, in combination with the lenses 213, 216, and 219 of the focusing unit 210, enables the generation of a high-quality corrected focus for the partial beam 310 in image space 270, or on a silicon wafer 280 as an example sample.

For partial beams 320 to 360 of the 3A Pre-compensating for spherical aberrations proves somewhat more difficult. When looking at the 200mm lens... 2 and 3 It is noticeable that lenses with negative refractive power for correcting Petzval curvature are completely absent. Optical elements with negative refractive power can be omitted in the focusing unit 210 because the aberration correction, including Petzval curvature, is imposed on the CGH of the DOE 220. According to the equations presented above, the CGH contains, in addition to the phase component ΔW ₁ (x, y) for partial beam 310, phase components ΔW _i (x, y), i = 2, ..., 6 for partial beams 320 to 360. This phase function includes, besides Zernike terms for astigmatism and coma, a progressive focus term for the corresponding phase functions ΔW _i for i = 2 to 6 of partial beams 320 to 360.

For illustrative purposes, the complete phase function ΔW ₆ (x _CGH , y _CGH ) for the partial beam 360 corresponding to i = 6 and for h ₆ = 5 mm is shown in the 7A The contour plot 700 is represented as a contour plot. This plot is dominated by a tilt or skew, which can be described by the Zernike fringe coefficient _Z3 ; however, other Zernike coefficients are present besides the tilt because the contour plot 700 does not consist of equidistant straight lines. All Zernike fringe coefficients belonging to a phase function can be calculated using a Zernike decomposition, which is included in commercially available optical design software such as Code-V or OpticStudio.

The tilt is for the deflection of the partial beam 360° with i = 6 and _h⁶ = 5 mm in the direction of the upper end of the image area. 3A(f) responsible. The contour plot 750 of the 7B The phase ΔW ₆ (x _CGH , y _CGH ) of the focus of the sixth partial beam 360 is shown, where the phase W ₁ (x _CGH , y _CGH ) of the non-displaced partial beam 310 and the Zernike fringe coefficient Z ₃ have been subtracted. This leaves an essentially spherical phase, described by Z ₄ , along with slight astigmatism, represented by the Zernike fringe coefficient Z _5. This behavior reflects the fact that the CGH primarily adds a phase to partial beams 320 to 360, which is responsible for their focus and astigmatism, thereby correcting the Petzval curvature.

For a better understanding of the relationships of the phase functions of the partial beams 310 to 360, which the CGH adds to the partial beams 310 to 360, the Zernike fringe coefficient Z ₃ , which causes tilting, and the Zernike fringe coefficient Z ₄ , which is responsible for defocusing, are plotted as a function of the focus distance h _i from the optical axis 250 in diagram 800 of the 8 depicted. From the 8 It is clearly evident that the phase shift _Z3 increases linearly with the focus distance from the optical axis. The linear behavior of _Z3 is expected, since the angle of the i-th reference beam, which defines the focus distance _hi , is linear to _Z3 . The quadratic behavior of _Z4 reflects the knowledge known from textbooks that, due to Petzval curvature, the defocusing increases quadratically with the focus distance _hi . 8 This confirms that the CGH is responsible for the field tilt and corrects the Petzval curvature. This enables the realization of an objective 200 with a very simple focusing unit 210, which comprises only three lenses 213, 216 and 219.

After passing through the three lenses 213, 216, 219, the partial rays 260, 265, 310, 320, 330, 340, 350, 360 enter the image space 270. This space includes an air gap 285, which corresponds to the working distance 285 of the objective 200 from the sample holder 275 (chuck), and has a refractive index n ≈ 1. The sample holder 275 comprises, in the examples of 2A and 3A A plane-parallel quartz plate with a thickness of 15 mm, whose refractive index is given above. The material of the sample holder 275 can be freely chosen as long as it is optically transparent to the irradiation wavelength of the sample. Of course, the lenses 213, 216, 219 of the focusing unit 210 must be designed taking into account the material of the sample holder 275. After passing through the sample holder 275, the partial beams 260, 265, 310, 320, 330, 340, 350, 360 enter the sample 280, which in Figures 3A and 3B is a silicon wafer 280, and form corresponding foci 290, 295 at a predetermined depth. In the examples of 2A and 3B The sample is irradiated with a wavelength λ = 1064 nm.

Since the phase added to the m partial beams by the CGH of the DOE 220 compensates for the imaging errors or aberrations of the three lenses 213, 216, 219 of the focusing unit 210 for the respective lateral focus positions (x _i , y _i ) of the partial beams 260, 265, 310, 320, 330, 340, 350, 360, three lenses for the focusing unit 210 are sufficient to generate high-quality laser foci. The examples of 3 and 4 show that very large image fields $\sqrt{x_i^2+y_i^2}<5mm$ are possible. In contrast, the state of the art described above uses six lenses for an image field of $\sqrt{x_i^2+y_i^2}<\mathrm{0,14}mm$ required. The 200 lens is therefore very simple, compact, lightweight and inexpensive.

As explained above, the number of partial beams that a DOE 220 can generate is not limited to two or six; rather, a DOE 220 or a CGH can be designed to generate any number m of partial beams. Furthermore, the DOE 220 in the objective 200 can be replaced, allowing for flexible use of the objective 200.

In the 3A The lateral focus positions (x _i , y _i ) satisfy the condition: x _i = 0 mm, y _i = (i - 1) mm for all six partial beams i = 1, ... ,6 with reference symbols 310 to 360. By rotating the phase functions ΔW _i (x _CGH , y _CGH ) by an angle α _i , lateral focus positions (x _i , y _i ) ≈ (i - 1) mm·(sin α _i , cos α _i ) with x _i ≠ 0 mm can also be obtained for reasons of symmetry.

9A Figure 1 shows a second embodiment of a lens 900 according to the invention, which is designed for the same parameters as lens 200 (NA = 0.6, λ = 1064 nm, DP = 40 mm). The distance $h_i=\sqrt{x_i^2+y_i^2}$ The focal point 995 is greater than 1 mm from the optical axis 250, i.e., its lateral focus position, preferably greater than 2.5 mm, more preferably 4.5 mm, and most preferably 5.0 mm. The main difference from the lens 200 lies in the choice of a lens material with a higher refractive index. The quartz lenses 213, 216, and 219 of the lens 200 have a refractive index n ≈ 1.45, whereas the lenses 913 and 916 of the focusing unit 910 of the lens 900 are made of the material Schott N-SF66, which has a higher refractive index n ≈ 1.88. This allows for an even simpler design of the lens 900 compared to the lens 200. The focusing unit 910 of the lens 900 requires only two lenses, 913 and 916. The optical design of the 900 lens is shown in Table 3 of the 9B The image space 970 is unchanged compared to the image space 270 of the objective 200. This means that the sample holder 275 contains a plane-parallel quartz plate 275 and the sample 280 a silicon wafer 280.

Although the DOE 920, or rather its CGH, was modified compared to the DOE 220 to adapt to the 910 focusing unit, its fundamental properties are identical to the DOE 220 CGH discussed above. In the example of the 9A The CGH of the DOE 920 splits the light beam entering the lens 900 into 31 partial beams, i.e., m = 31, whose foci are at positions $\left({\text{x}}_{\text{i}},{\text{y}}_{\text{i}}\right)=\left(0\text{ mm}\text{,}\frac{i-1}{30}\cdot 2mm-1mm\right)$ The 31 individual foci are generated equidistantly along the y-axis within a range of ±1 mm. The maximum focal distance of the most distant partial beams, i.e., the maximum lateral focus position, is h _{_max} = ±1 mm.

The 2D contour plot 1000 of the CGH of the DOE 920 is in 10A reproduced. Diagrams 1030 and 1060 of the 10B and 10C show enlarged 1D sections of the CGH design of the 10A along the x-axis ( 10B) or the y-axis ( 10C ).

Diagram 1102 of the 11A Figure 1 describes a third embodiment of an objective 1100, which, with otherwise identical parameters, is designed for an exposure wavelength of 1550 nm, i.e., a wavelength from the infrared spectral range. At this wavelength, the semiconductor silicon is optically transparent and has a high refractive index of n ≈ 3.48. Therefore, in the image space 1170, a plane-parallel silicon plate is used as the sample holder 1175 instead of a plane-parallel quartz plate in the first and second embodiments. As in the first and second embodiments, the sample 1180 comprises a silicon wafer 1180. If the sample holder 1175 and the sample 1180, i.e., the silicon wafer, are planar, the wavefronts of the partial beams do not change significantly when transitioning from the sample holder 1175 to the sample 1180. The design data of the objective 1100 are given in Table 4 of the figure. 11B In summary, since silicon is optically transparent at 1550 nm, the lenses 1113 and 1116 of the focusing unit 1110 can be made of silicon.

The CGH of the DOE 1120 lens 1100 is designed to generate foci of 121 partial beams in the silicon wafer 1180. The design principles of the DOE 1120 CGH are explained above in the context of the first embodiment. The DOE 1120 CGH is designed such that the 121 partial beams generate 121 foci in the silicon wafer 1180 in a square grid at equidistant intervals. The side length of the square in the image area is 2 mm.

The 2D contour plot 1200 represents the phase distribution of the CGH of the DOE 1120 in 12A Diagrams 1230 and 1260 of the 12B and 12C enlarged 1D sections of the CGH design are shown. 12A along the x-axis ( 12B) or the y-axis ( 12C ).

Diagram 1302 of the 13A A fourth embodiment of a lens 1300 is presented, which, like the first two embodiments, is designed for an exposure wavelength of 1064 nm. Unlike in the first three embodiments, the sample 1380, again a silicon wafer 1380, is laterally fixed. This eliminates the need for a sample holder through which the partial beams 260 and 265 must pass to reach the sample 1380. The absence of an optically transparent sample holder in the beam path significantly simplifies the image area 1370. In particular, this reduces the requirements for the working distance 1385.

Like the optical design of the 13B As can be seen, the lenses 1313, 1316 and 1319 of the focusing unit 1310 - as in the first two embodiments - are again made of quartz glass.

The fifth embodiment of the 14 The fourth embodiment is further extended by the addition of a non-rotationally symmetric optical element in the form of a cylindrical lens 1430 to the objective lens for generating astigmatic wavefronts of the partial beams 1460 and 1495 in the laterally fixed sample 1480. The refractive power of the cylindrical lens 1430 is low, so that the path of the partial beam bundles 1460 and 1465 is only slightly altered by the objective lens 1400. The astigmatic wavefronts of partial beams 1460 and 1465 generate non-rotationally symmetric foci 1490 and 1495 in sample 1480. Incorporating the cylindrical lens 1430 into the objective 1400 represents a second possibility for generating non-rotationally symmetric foci 1490 and 1495, in addition to the option discussed above. By replacing the cylindrical lens 1430 with a cylindrical lens (not shown) of a different refractive power, it is possible to generate wavefronts with different astigmatism components, thus changing the geometry of the corresponding foci. If it is necessary to rotate the non-rotationally symmetric foci 1490 and 1495 in sample 1490, this can be achieved by rotating the cylindrical lens 1430. Thus, it is possible, for example, to write micro-stress-generating pixels into the sample 1480, which generate different stresses in various spatial directions. Furthermore, if it is necessary to obtain partial beams 1460 and 1465 with more complex wavefronts, the cylindrical lens 1430 can be replaced by a freeform lens to generate arbitrary aspherical wavefronts for the partial beams 1460 and 1465 and thus corresponding focal geometries.

The cylindrical lens 1430 is positioned along the beam direction immediately in front of the CGH of the DOE 1420 and thus near the pupil 1430 of the objective 1400. Alternatively, the CGH could also be positioned behind the CGH when viewed in the beam direction.

In all embodiments, the DOE, which in these examples is implemented as a CGH, can be replaced by a spatial modulator for light (SLM) if its resolution is sufficient. Since SLMs are switchable, lenses 200, 900, 1100, 1300, 1400 can be produced in which the number of partial beams 260, 265 and the positions of the foci 290, 295 generated by the partial beams 260, 265 in a sample 280 can be adjusted in real time.

The previous embodiments describe the focusing of any number of partial beams 260, 265, 310, 320, 330, 340, 350, 360, which are generated in a DOE 220, 920, 1120, 1320 by a focusing unit 210, 910, 1110, 1310 of a lens 200, 900, 1100, 1300, 1400 into predetermined positions of an image space 270, 970, 1170, 1370. The positions of the generated foci 290, 295, 1490, 1495 lie in a plane perpendicular to the optical axis 250 of the lens 200. In the embodiments described above, the foci 290, 295, 1490, 1495 lie at a predetermined depth of a silicon wafer 270, 970, 1170, 1370. For example, the silicon wafer has a thickness of 700 µm, and the predetermined depth of the foci 290, 295, 1490, 1495 is to be the mean depth of the wafer, i.e., the foci are to be generated at a depth of 350 µm, or their longitudinal focus positions are to be 350 µm. For this depth of field, the DOE 220, 920, 1120, 1320 of the 200, 900, 1100, 1300, 1400 lens compensates for the aberrations of all optical elements of the lens as well as the spherical aberrations. which the wavefronts of the partial beams experience within the wafer or a sample on their path within the sample. If the spherical aberrations generated by the sample 280 were not corrected, this would significantly degrade the quality of the generated foci 290, 295, especially with objectives 200 with a large NA (≥ 0.6). A focus quality, expressed as an RMS value of less than 50 mλ, preferably less than 20 mλ or less than 10 mλ, could not be achieved. As discussed with reference to the above embodiments, a DOE 220 can compensate not only for the aberrations of the optical elements 213, 216, 219 of an objective 200, but also for the spherical aberrations for a given longitudinal focus position or focus location in a sample 280.

By convention, the depth 0 µm is defined below as the surface of sample 280 facing the objective 200. The partial beams 260, 265 penetrate sample 280 through this surface.

In many material processing applications using short laser pulses, it is necessary to be able to adjust the penetration depth of foci 290, 295, 1490, 1495 or the longitudinal focus position across the sample depth. However, when the penetration depth of foci 290, 295, 1490, 1495 is changed within a sample 280, 1480, the wavefronts of the partial beams 260, 265, 1460, 1465 change, resulting in spherical aberration changes as they travel within the sample 280, 1480 to their respective longitudinal focus positions. If these changes in spherical aberration are not compensated for, the quality of the generated foci 290, 295, 1490, 1495 is significantly degraded. For the same change in penetration depth into a sample 280, 1480, the deterioration increases with a larger refractive index of the sample 280, 1480, i.e., for a silicon wafer 280, 1480 with n ≈ 3.5, the focus quality is reduced more than for a sample in the form of a quartz plate with n ≈ 1.45. Furthermore, larger NA values degrade the focus quality more than lenses with small image-side NA values (≤ 0.3).

One way to compensate for the change in spherical aberrations of sample 280, 1480 caused by a change in the penetration depth of foci 290, 295, 1490, 1495 is to insert a DOE 220, 1420, specifically designed for the new penetration depth or changed longitudinal focus position, into the objective 200, 1400. If necessary, the number m of generated partial beams 260, 265, 1460, 1465 and the resulting number of foci 290, 295, 1490, 1495 in sample 280, 1480 can be changed simultaneously. Furthermore, if necessary, the shapes of the wavefronts of the partial beams 260, 265, 1460, 1465, and thus the shape of the generated foci 290, 295, 1490, 1495, can be changed. This allows the generation of different types of pixels at different depths of a sample 280, 1480, or at different longitudinal focus positions within a sample 280, 1480.

The following describes exemplary embodiments of a second option for compensating changes in spherical aberrations when the penetration depth of the foci 290, 295, 1490, 1495 in a sample 280, 1480, such as a silicon wafer 280, 1480, is changed. Diagram 1502 of the 15 The sixth embodiment shows a lens 1500 mounted on the lens 200 of the 2 builds up. In addition to the DOE 1520 and the focusing unit 1510, the lens 1500 has a spherical aberrations compensation unit 1530 in the image space. In the example of the 15 The spherical aberration-compensating unit 1530, hereinafter also referred to as compensating unit 1530, comprises two lenses: a first diverging lens 1533 and a second, subsequent converging lens 1536, whose refractive power is greater than that of the diverging lens 1533. After the collimated light ray 1540, entering the objective 1500, passes through the compensating unit 1530, the resulting light ray is slightly convergent. In the example of the 15 Both the lenses 1513, 1516, 1519 of the focusing unit 1510 and the lenses 1533 and 1536 of the compensating unit 1530 are made of quartz glass.

The functions of the DOE 1520, which in turn was generated in the form of a CGH, and the focusing unit 1510 have already been discussed in detail in the above embodiments. Unlike in the previous embodiments, in the embodiment of the 15 A convergent light beam is directed onto the DOE 1520; therefore, the surface description h(x _CGH , y _CGH ) must be adjusted accordingly. The objective 1500 is nominally designed to create the image plane at a predetermined depth of the sample 1580 or the silicon wafer 1580. This can be the mean depth of 350 µm. It is, of course, also possible to place the image plane at a different depth of the sample 1580, for example, a depth of 600 µm, so that the partial beams 1560 and 1565 traverse a large portion of the sample 1580's thickness of 700 µm.

Now, is the penetration depth of foci 1590, 1595 of partial beams 1560, 1565 into the sample to be varied – in the example of the 15 If the longitudinal focus position is to be reduced from 600 µm to 100 µm, the working distance 1585 between the objective 1500 and the sample holder 1575, on which the sample 1580 is placed, will be increased. or the silicon wafer 1580 rests on, is altered. In the present example, it is enlarged. 15 This is symbolized by the left-pointing arrow Δ1. This can be achieved by moving the sample holder 1575 in the direction of the beam or by moving the objective 1500 against the direction of the beam. A combined movement of the sample holder 1575 and the objective 1500 is also possible, but is complex to implement.

Changing the image plane leads to a significant change in the spherical aberrations in sample 1580 and consequently to a deterioration in the quality of foci 1590 and 1595. The objective 1500 is designed to compensate for the variations in spherical aberrations in image space 1570 caused by changing the working distance 1585 by shifting the unit 1530, which compensates for spherical aberrations in image space 1570, along the optical axis 1550. In the example of the 15 This is achieved by shifting the compensating unit 1530 within the objective 1500 along the optical axis 250 against the beam direction. This increases the air space 1505 between the compensating unit 1530 and the DOE 1520. This is indicated by the left-pointing arrow Δ2 in 15 Illustrated. The enlargement of the airspace 1505 compensates for the changes in spherical aberrations in the image space 1570. By compensating for the changes in spherical aberrations in the image space 1570 through variation of the airspace 1505, the objective 1500 enables the generation of diffraction-limited foci 1590, 1595 at different depths of the silicon wafer 1580 as an exemplary sample 1580.

The compensating unit 1530 is designed such that, for the nominal longitudinal focus position of the lens 1500, its spherical aberrations, i.e., the sum of the spherical aberrations of the focusing unit 1510 and the image space 1570, are exactly compensated. For this purpose, the compensating unit 1530 changes the collimated light beam 1540 entering the lens 1500 into a non-collimated, preferably a convergent, light beam. This makes it possible to change the spherical aberrations of the lens 1500 by moving the compensating unit 1530 along the optical axis 1550 and the resulting variation of the air space 1505. This defined change in the spherical aberrations of the lens 1500 can be adjusted so that it exactly compensates for the change in spherical aberrations in the image space 1570 when varying the longitudinal focus position or the focus position within the sample 1580.

To illustrate the operation of the compensating unit 1530, the focusing unit 1510 is conceptually replaced by an ideal lens, and the entire image space 1570 by a plano-aspheric lens, i.e., a lens with one flat and one aspherical side. The extent of the spherical aberrations produced by the plano-aspheric lens depends on its illumination. The latter can be adjusted by moving the compensating unit 1530 based on the uncollimated light beam at the output of the compensating unit 1530. Moving the compensating unit 1530 along the optical axis 250 is equivalent to varying the airspace 1505 between the compensating unit 1530 and the DOE 1520. Since the airspace 1505 is used to tune or compensate for the spherical aberrations of the image space 1570, it is also referred to as the tuning airspace 1505.

If only the compensation of changes in the spherical aberrations of image space 1570 is considered, it is advantageous for the compensating unit 1530 to generate a highly convergent output beam. This allows the displacement of the compensating unit 1530 to be kept small. However, if the functions of the subsequent DOE 1520 are not to be unduly impaired by the displacement of the compensating unit 1530, a compromise must be found between these two conflicting requirements. Furthermore, the convergence of the light beam exiting the compensating unit 1530 to the focus position of the partial beams 1560 and 1565 in image space 1570 must be taken into account. This means that setting the working distance 1585 or Δ1 cannot be performed independently of the displacement of the compensating unit 1530 or Δ2.

For example, varying the penetration depth of the foci 1590, 1595 into a sample 1580 can be achieved by first modifying the working distance 1585, for instance by moving the entire objective 1500 against the beam direction. Then, inside the objective 1500, the tuning air space 1505 is modified by moving the compensating unit 1530. This, in turn, has a (minor) effect on the focus position or the longitudinal focus position in the image space 1570, i.e., the sample 1580. Repeating these two tuning steps iteratively leads to the desired result. Alternatively, it is also possible to change the working distance 1585 and the tuning air space 1505 simultaneously.

Only a single actuator is required to move the compensating unit 1530 (in the 15 (not shown). Manually moving the compensating unit 1530 is also conceivable.

Diagram 1605 of the 16A Figure 1615 schematically shows the focusing of the central partial beam 1560, located on the optical axis, into an image space 1570, which includes the sample holder 1575 and the sample 1580 in the form of a silicon wafer 1580, to a depth of 600 µm of the wafer 1580. The objective 1500 is designed for this penetration depth of the foci 1590 and 1595. 16A Diagram 1615 shows, at a high magnification, the meridional aberration curves of the field points or focal points of the seven partial beams produced by the DOE 1520. It can be seen that the 1500 lens produces very high-quality foci for the nominal longitudinal focus position, with the reference wavefronts being spherical waves.

Diagram 1635 of the 16B presents the focus position of the central partial beam 1630, after its penetration depth of 600 µm in 16A The difference was reduced to 100 µm by increasing the working distance 1585. Diagram 1635 of the 16B The image shows a high, unacceptable level of spherical aberrations in the seven partial beams. The quality of the generated foci 1630 is unacceptable for material processing.

Diagram 1665 of the 16C The central partial beam 1630 of diagram 1635 is shown after compensation of the spherical aberrations by shifting the spherical aberration-compensating unit 1530 within the lens 1500 in the image space. As can be seen from diagram 1675, after compensation of the spherical aberrations in the image space 1570 by shifting the compensating unit 1530 within the lens 1500, the lens 1500 produces high-quality foci 1690, i.e., with a Strehl ratio > 0.95 for rotationally symmetric partial beams and with a low RMS value relative to reference wavefronts for non-rotationally symmetric partial beams. The quality of the foci 1590 for the nominal longitudinal focus position ( 16A) and the changed penetration depth of the foci 1690 of the 16 C They do not differ noticeably. The compensation of the spherical aberrations of the image space 1570 can be achieved by tuning only one air space 1505 or tuning air space 1505, i.e., only by moving the compensating unit 1530 within the lens 1500 along the optical axis 1550.

The 1500 lens is very well telecentrically corrected. This is a necessary prerequisite for shifting the focus position of foci 1590, 1595 of two or more partial beams 1560, 1565 within a sample 1580. If the light beams of the partial beams 1560, 1565 were to enter the image space 1570 obliquely, defocusing would inevitably lead to undesirable astigmatism and coma of the foci 1590, 1595 of the partial beams 1560, 1565.

As above in the context of the 15 As discussed in the sixth embodiment, the compensating unit 1530 is moved relative to the DOE 1520, and the light beam exiting the compensating unit 1530 is convergent. When the compensating unit 1530 is moved along the optical axis 1550 of the lens 1500, the light distribution or illumination of the DOE 1520 changes. Although the modification of the scalar wavefront correction due to the change in illumination is inherently correctly accounted for when determining its phase distribution, the diffraction efficiencies of the DOE 1520 could change, and thus also the intensity of the unwanted stray light.

Furthermore, in the sixth embodiment of the 15 In some cases, large travel distances or displacements of up to 1 mm are necessary. This is due to the fact that the light beam leaves the compensating unit 1530 in a shape that deviates only slightly from the collimation. To achieve a significant corrective effect on the spherical aberrations of the image space 1570, the tuning air space 1505 must be significantly modified.

In diagram 1702 of the seventh in the 17 In the illustrated embodiment, a different optical design is therefore chosen for the lens 1700. In this example, the DOE 1720 is part of the compensating unit 1730 and is thus moved with it along the optical axis 250 of the lens 1700. The compensating unit 1730 has four lenses in this example, with the first lens 1733 and the second lens 1736 arranged in front of the DOE 1720, and the third lens 1738 and the fourth lens 1739 behind the DOE 1720. This arrangement ensures that the DOE 1720 is always illuminated the same way, regardless of the position of the compensating unit 1730 within the lens 1700, and the intensity of unwanted stray light can be minimized simultaneously for all longitudinal focus positions. The light rays leaving the compensating unit 1730 are already highly convergent. This has two advantages. Firstly, the partial rays 1760 and 1765 pass through the tuning air space 1705 in a shape that deviates significantly from the collimation path, thus enabling small travel or displacement distances of the compensating unit 1730 within the lens 1700. Secondly, the strong convergence of the partial rays 1760 and 1765 facilitates the task of the focusing unit 1710. This unit is used in the example of the 17 with two lenses 1713 and 1716.

A further change in the seventh embodiment compared to the sixth embodiment concerns the image space 1780. Unlike in the sixth embodiment of the 15 The partial beams 1760 and 1765 pass through in the seventh embodiment of the 17 The sample holder 1780 is not used; instead, the probes penetrate directly into the sample 1780 or the silicon wafer 1780. The sample 1780 is fixed laterally, analogous to the fourth embodiment, as explained above.

The specifications, such as the wavelength, the image-side NA of lens 1700, and the image field size, are the same as in the sixth embodiment. Lens 1700 has an additional, sixth lens element compared to lens 1500. Furthermore, the moving mass of the compensating unit 1730 is greater than that of the compensating unit 1530 of lens 1500. Conversely, to compensate for the spherical aberration changes in the image space 1780, the compensating unit 1730 only needs to be moved a short distance (see Table 6 of the [reference to table]). 21 ).

Since the partial rays 1760 and 1765 deviate significantly from collimation in the tuning air space 1705, this has a strong effect not only on the compensating effect on the spherical aberrations of the image space 1780, but also on the focus position in the image space 1780. Therefore, the variation of the longitudinal focus position or the penetration depth of the foci 1790 and 1795 and the compensation of the resulting spherical aberration changes in the image space 1780 can no longer be considered in isolation. Rather, the movements of the lens 1700 relative to the image space 1780, which are designated as Δ1, and the compensating unit 1730 within the lens 1700, which are described in 17 are illustrated by Δ2, to be determined in combination.

In diagram 1802 of the eighth embodiment, analogous to the second embodiment, the 9 - made use of the experience that optical elements with a higher refractive index generally simplify the optical design of a system. Otherwise, the parameters of the sixth and seventh embodiments were adopted. In the 1800 lens of the 18 The quartz lenses (n ≈ 1.45) of the sixth and seventh embodiments are made of higher refractive index N-SF66 material (n ≈ 1.88). As in all previous embodiments, the plane-parallel plate onto which the CGH was modulated to produce the DOE 1820 comprises synthetic quartz glass. However, it would also be possible to produce the DOE 1820 using a higher refractive index material.

Similar to the second embodiment of the 9 The focusing unit 1810 of the lens 1800 achieves similar imaging performance to the lens 1500. 15 with only two lenses 1813 and 1816. The compensating unit 1830 also requires two lenses 1833 and 1836. Compared to the objective 1500, the higher refractive index material thus allows one lens to be saved. The adjustment paths Δ1 for setting the working distance 1885 between the objective 1800 and the sample holder 1875, and Δ2 for adjusting the tuning air space 1805 by moving the compensating unit 1830, are shown in Table 6 of the 21 specified.

The diagram from 1902 of the 19 The ninth embodiment shows a lens 1900, which – similar to the lens 1100 of the third embodiment – 11 It is designed for a wavelength in the near-infrared range (λ = 1550 nm). In this wavelength range, silicon is optically transparent and can therefore be used as a high-refractive-index lens material. This allows for a significant simplification of the design of the 1900 lens, which is designed for this infrared wavelength. Its focusing unit 1910 requires only a single lens 1913. However, the compensating unit 1930 still requires two lenses, 1933 and 1936. The 1900 lens of the 19 This means that only three lenses are required in total. The plane-parallel plate of the DOE 1920 is still made of quartz glass. However, it is also possible to manufacture this component from silicon. In contrast, the sample holder 1975 is a plane-parallel silicon plate on which a silicon wafer 1980 rests as the sample 1980.

Due to the very high refractive index of the sample holder 1975 and the sample 1980, the entire objective 1900 only needs to be moved slightly (Δ1) to shift the working distance 1985 by 500 µm for a change in the longitudinal focus position in the image space 1970, i.e. in the silicon wafer 1980. On the other hand, compensating for the large spherical aberration changes in the image space 1970 requires a large displacement of the compensating unit 1930 (Δ2) within the lens 1900. The numerical values are given in Table 6 of the 21 listed.

Finally, the diagram shows the 2002 20 As a tenth embodiment, an objective 2000 is described, again designed for a wavelength λ = 1064 nm. Unlike the previous embodiments, however, the objective 2000 is designed for a numerical aperture of 0.8 (previously 0.6). The focusing unit 2010 requires three lenses 2013, 2016, and 2019 to focus the partial beams 2060 and 2065 in the image space 2070, which comprises a plane-parallel plate as a sample holder 2075 and a silicon wafer 2080 as an exemplary sample 2080. The compensating unit 2030 also has three lenses 2033, 2036, and 2039. The material of the lenses with negative refractive power is fused silica. Higher refractive index N-SF66 is used for the lenses with positive refractive power.

All embodiments described herein enable the generation of essentially diffraction-limited foci of any adjustable shape. As explained above, the beam-splitting and aberration-compensating unit 220, 920, 1120, 1320, 1420, 1520, 1720, 1820, 1920, 2020 can be designed to generate not only rotationally symmetric foci but also non-rotationally symmetric foci whose wavefronts exhibit a predefined aspherical wavefront. This allows, for example, the generation of pixels with a defined astigmatic shape in a plane perpendicular to the beam direction within a sample. To generate non-rotationally symmetric foci, non-rotationally symmetric optical elements, such as cylindrical lenses, can also be inserted into the optical beam path, for example, near the combined element.

Table 6 of the 21 summarizes the movements of the lens 1500, 1700, 1800, 1900, 2000 of the sixth to tenth embodiments of the 15 , 17 , 18 , 19 and 20 together, which are designated Δ1, to change the penetration depth of the foci 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095 by 500 µm, namely from the original 600 µm to 100 µm, in image spaces 1570, 1770, 1870, 1970, 2070. The change in the penetration depth of the foci, or their changed longitudinal focus position, is abbreviated by _Δ3 in Table 6. As in the 15 and 17-20 As specified, Δ2 describes the displacement distance of the respective compensating units 1530, 1730, 1830, 1930, 2030. The displacements Δ1, Δ2, and Δ3 have positive numerical values when moving in the direction of the beam and correspondingly negative numerical values when moving against the direction of the beam, according to the definition given above.

Finally, the flowchart 2200 presents the 22 A method for focusing at least two partial beams 260, 265, 1560, 1565 of a light beam 240, 1540 incident on a lens 200, 1500 according to the invention into at least a predetermined depth of an image space 270, 1570. The method begins at step 2210.

In the first step 2220, a working distance 285, 1585 of the lens 200, 1500 according to the invention is set with respect to an image space 270, 1570 for focusing the at least two partial beams 260, 265, 1560, 1565 into a predetermined depth of the image space 270, 1570. This can be done by moving the lens 200, 1500 and/or a sample 280, 1580 placed in the image space 270, 1570 along the optical axis 250 of the lens 200, 1500.

In step 2230, the foci 290, 295, 1590, 1595 of the at least two partial beams 260, 265, 1560, 1565 are shifted in the image space 270, 1570 along the optical axis 250 of the objective 200, 1500. For this purpose, the objective 200, 1500 and/or the sample 280, 1580 can again be moved along the optical axis 250 of the objective 200, 1500.

Then, in step 2240, a spherical aberration-compensating unit 1530 is moved within the lens 200, 1530, or the combined beam-splitting and aberration-compensating unit 220, 1520 is changed to compensate for the changes in spherical aberrations caused by the altered longitudinal focus position in the image space 270, 1570. Steps 2230 and 2240 are optional and are therefore omitted. 22 Marked by a dashed outline. Steps 2230 and 2240 are mutually dependent. They can be executed simultaneously or sequentially.

The process ends at step 2250.

A method according to the invention can further comprise the step of generating non-rotationally symmetric foci 1490, 1495 by introducing a non-rotationally symmetric element 1430 into the light beam 240 entering through a lens 200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000. The orientation of the non-rotationally symmetric foci 1490, 1495 can be adjusted by rotating the non-rotationally symmetric element 1430. In particular, the introduction and orientation of a non-rotationally symmetric element 1430 enables the writing of pixels into a sample 280, 1180, 1380, 1480, 1580, 1780, 1880, 1980, 2080, which generate a non-rotationally symmetric micro-stress distribution in the sample in a plane perpendicular to the beam direction of the at least two partial beams 1490, 1495.

Claims (19)

Lens (200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000) for focusing at least two partial rays (290, 295, 310, 320, 330, 340, 350, 360, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) of a light ray (240) incident on the lens into at least a predetermined depth of an image space (270, 970, 1170, 1370, 1470, 1570, 1770, 1870, 1970, 2070), comprising: a. a focusing unit (210, 910, 1110, 1310, 1410, 1510, 1710, 1810, 1910, 2010) with at least one optical element (213, 216, 219, 913, 916, 1113, 1116, 1313, 1316, 1319, 1513, 1516, 1519, 1713, 1716, 1813, 1816, 1913, 2013, 2016, 2019); b. at least one combined beam-splitting and aberration-compensating unit (220, 920, 1120, 1320, 1420, 1520, 1720, 1820, 1920, 2020) configured to split the light beam entering the lens into at least two partial beams and to pre-compensate at least one aberration of the at least one optical element of the focusing unit; c. wherein the at least one optical element is configured to focus the at least two partial beams exiting the combined beam-splitting and aberration-compensating unit into at least one predetermined depth of the image space; and d. wherein the lens is configured to incorporate the at least one beam-splitting and aberration-compensating unit as an interchangeable component. lens after Claim 1 , wherein the at least one combined beam-splitting and aberration-compensating unit is arranged in a pupil plane (230) of the objective. lens after Claim 1 or 2 , wherein the at least one optical element is set up based on at least one of: reflection, refraction or diffraction, and/or wherein the at least one beam-splitting and aberration-compensating unit is set up based on diffraction. Lens according to one of the preceding claims, wherein the combined beam-splitting and aberration-compensating unit comprises a diffractive optical element. Lens according to one of the preceding claims, wherein the at least one combined beam-splitting and aberration-compensating unit is further arranged to generate in the image space one of: rotationally symmetric foci (290, 295) or non-rotationally symmetric foci (1490, 1495). Objectively after one of the Claims 1 - 5 , wherein the at least one combined beam-splitting and aberration-compensating unit comprises at least one first set of combined beam-splitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the first set of combined beam-splitting and aberration-compensating units is configured to correct the at least one aberration of the lens for a predetermined penetration depth of the foci of the at least two partial beams, wherein the predetermined penetration depth into the image space is different for each unit of the first set. Objectively after one of the Claims 1 - 6 , wherein the at least one combined beam-splitting and aberration-compensating unit comprises at least one second set of combined beam-splitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the second set generates a predetermined number of partial beams, the predetermined number of partial beams being different for each unit of the second set. Lens according to one of the preceding claims, further comprising at least one non-rotationally symmetric optical element (1430) which is configured to generate non-rotationally symmetric foci for the at least two partial rays in the image space. Lens according to one of the preceding claims, further comprising a unit (1530, 1730, 1830, 1930, 2030) compensating for spherical aberrations in the image space, which is arranged to be displaceable along an optical axis (250) of the lens. Lens according to the preceding claim, wherein the spherical aberrations compensating unit in the image space comprises at least one: at least one lens (1533, 1536, 1733, 1736, 1833, 1836, 1933, 1936, 2033, 2036, 2039), at least one mirror, or at least one lens and at least one mirror. lens after Claim 9 or 10 , wherein the spherical aberrations in the image space compensating unit is set up, to compensate for a change in the sum of the spherical aberrations of the focusing unit and a change in the spherical aberration that occurs due to a change in the penetration depth of the foci of the at least two partial rays into the image space, by changing the position of the spherical aberrations in the image space compensating unit along the optical axis of the lens. Objectively after one of the Claims 9 - 11 , wherein the lens is configured to displace the spherical aberrations in the image space compensating unit (1730) and the at least one combined beam splitting and aberration compensating unit (1720) together with respect to the focusing unit (1710) along the optical axis (250) of the lens (1700). Objectively after one of the Claims 9 - 12 , wherein the lens is set up to change the penetration depth of the foci of at least two partial rays into the image space by shifting the unit compensating for spherical aberrations in the image space. Objectively after one of the Claims 9 - 13 , further comprising at least one lens (2013) arranged between the at least one combined beam-splitting and aberration-compensating unit (2020) and the focusing unit (2010), and configured for at least one of: moving the at least one lens along the optical axis together with the spherical aberrations-compensating unit and the at least one combined beam-splitting and aberration-compensating unit, or moving the at least one lens along the optical axis independently of the spherical aberrations-compensating unit, the focusing unit and the at least one combined beam-splitting and aberration-compensating unit. Device for simultaneously processing a sample (280, 1180, 1380, 1580, 1780, 1880, 1980, 2080) with at least two light beams (290, 295, 310, 320, 330, 340, 350, 360, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) at an adjustable depth of the sample, wherein the device has at least one objective (200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000) according to one of the Claims 1 until 14 used to generate at least two partial beams (290, 295, 310, 320, 330, 340, 350, 360, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) from a light beam (240) and to focus the at least two partial beams. Method (2200) for focusing at least two partial beams (290, 295, 310, 320, 330, 340, 350, 360, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) into at least a predetermined depth of an image space (270, 970, 1170, 1370, 1470, 1570, 1770, 1870, 1970, 2070), comprising: setting (2220) a working distance (285, 1585, 1785, 1885, 1985, 2085) of a lens (200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000) according to one of the Claims 1 until 14 with regard to the image space for focusing the at least two partial rays into the at least one predetermined depth of the image space. A method according to the preceding claim, wherein changing the penetration depth of the foci (290, 295, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) of the at least two partial beams comprises at least one of: changing a working distance between the lens and the image space and changing the at least one combined beam-splitting and aberration-compensating unit (220, 920, 1120, 1320, 1420, 1520, 1720, 1820, 1920, 2020), changing the working distance between the lens and the image space and shifting (2240) the spherical aberrations in the image space compensating unit (1520, 1730, 1830, 1930, 2030) along an optical axis (250) of the lens, or shifting the spherical aberrations compensating unit in the image space by a first distance and shifting at least one lens (2013) by a second distance. Procedure according to Claim 16 or 17 , furthermore comprising at least one of the following steps: changing the at least one combined beam-splitting and aberration-compensating unit to change the number of generated partial beams, changing the at least one combined beam-splitting and aberration-correcting unit to change the penetration depth of the foci of the at least two partial beams into the image space, or changing the at least one combined beam-splitting and aberration-compensating unit to change the exposure wavelength of the image space. Computer program containing instructions that cause a computer system to execute the process steps of the Claims 16 until 18 to execute.