DE102022208009A1 - Method for determining a change in an optical property and/or an absorption property of an optical sample - Google Patents

Abstract

The invention relates to a method for determining a change (dn) caused by absorption of electromagnetic heating radiation (3) in at least one optical property (n) of an optical sample (5) and/or at least one absorption property (k) of the optical sample (5). the absorption of useful electromagnetic radiation (73). The method comprises the steps: introducing the optical sample (5) into an optical resonator (7) which has two mirrors (9,9'), irradiating the electromagnetic heating radiation (3) in the form of an electromagnetic heating beam (19) into the optical Sample (5), irradiation of an electromagnetic measuring beam (23) into the optical resonator (7), the electromagnetic measuring beam (23) passing through the optical sample (5) many times as a result of multiple reflections on the mirrors (9,9') and/ or reflected multiple times by the optical sample (5), spatially resolved measurement of an intensity distribution (I) of the electromagnetic measuring beam (23') emerging from the optical resonator (7), determination of a phase shift distribution (φ) from the intensity distribution (I) of the electromagnetic measuring beam (23') emerging from the optical resonator (5), determining a change (dφ) in the phase shift distribution (φ) by comparing the phase shift distribution (φ) with a reference (φR), and determining the change (dn ) the at least one optical property (s) of the optical sample (5) and/or the at least one absorption property (k) of the optical sample (5) from the change (dφ) in the phase shift distribution (φ). The invention also relates to a device (1) for carrying out the method and an optical system.

Description

Background of the Invention

The invention relates to a method for determining a change caused by the absorption of electromagnetic heating radiation in at least one optical property of an optical sample and/or at least one absorption property of the optical sample for the absorption of useful electromagnetic radiation. The invention also relates to an optical system, in particular a lithography system.

The electromagnetic radiation used in lithography systems, in particular in projection exposure systems or projection lenses, which typically has a wavelength in the UV range (less than 400 nm), is partially absorbed during operation on the surfaces and in the volume of the optical elements of the lithography systems. As a result, the optical elements are initially heated locally by the absorbed radiation. Due to this local heating, there is a deformation of the surfaces of the optical elements and a location-dependent change in the refractive index. This is accompanied by a change in the imaging properties of the optical elements, which is also referred to as "lens heating" or "mirror heating", depending on whether the optical element concerned is a transmissive optical element, for example a lens, or a reflective optical element , for example a mirror. This change in imaging properties is typically undesirable; the absorption of the electromagnetic radiation therefore leads to imaging errors and the intended resolution may no longer be achieved.

This problem is particularly pronounced in lithography systems, since particularly high intensities are typically used here in order to enable a high throughput, ie the processing of as many wafers as possible per unit of time.

As a result of the high intensities, the absorption is typically non-linear, i.e. dependent on the intensity or the fluence of the electromagnetic radiation used. If only one-photon absorption and two-photon absorption are taken into account, the absorption properties of the optical elements can be quantified via a one-photon absorption coefficient and a two-photon absorption coefficient, which are generally location-dependent. Furthermore, a distinction is made between volume absorption coefficients and surface absorption coefficients for absorption in the volume and on the surface of the optical elements.

In order to minimize aberrations, i.e. the effects of lens or mirror heating, the materials, layers and processing processes used in the manufacture of optical elements can be optimized. Such an optimization requires a characterization of the absorption properties of the optical elements or the change in their imaging properties as a result of the absorption, ie the lens or mirror heating effects. Alternatively or additionally, the aberrations in lithography systems can be actively compensated (via appropriate correction optics and manipulators). In this case, too, the absorption or the aberration must be characterized.

Methods for determining the absorption of an optical sample, for example in the form of a blank for the production of an optical element, are variously described in the prior art.

On the one hand, so-called LID (laser-induced deflection) methods are known. A corresponding arrangement for the optical determination of the absorption experienced by a light beam when passing through a transparent medium as a result of the conversion of part of its energy into heat is, for example, in DE 101 399 06 A1 described. Here, a laser measurement beam runs transversely, ie at an angle of approx. 90°, to the light beam and passes through the transparent medium outside of the cross-section of the light beam. The refractive index gradient caused by the increase in temperature in the medium leads to a deflection of the laser measurement beam, which can be detected by a position-sensitive detector.

On the other hand, there are methods that are based on determining the change in the wavefront of a measuring light beam. For example, such a method and a corresponding device from the article "A novel photo-thermal setup for determination of absorptance losses and wavefront deformations in DUV optics" by K. Mann et al. proc. SPIE 6924, Optical Microlithography XXI, 69242P (2008). There, the measurement on cylindrical samples made of quartz glass with a diameter of approx. 25 mm and a length of approx. 45 mm is described. A thin heating light beam in the form of an excimer laser beam (wavelength approx. 193 nm) penetrates these in the longitudinal direction and is heated in the process. An expanded measuring light beam from a diode laser (wavelength approx. 639 nm), which runs at a small angle to the heating light beam, radiates through the entire cross-section of the sample. The heating causes thermal expansion and a change in the refractive index, which in turn changes the wavefront of the measurement that has passed through the sample affect the light beam. This wavefront is measured using a Hartmann-Shack sensor. Finally, the absorption properties of the sample can be determined from the change in the wavefront by calibrating using an analytical solution of the heat conduction equation.

A similar method and an associated device are described in the article "Photo-thermal measurement of absorptance losses, temperature induced wavefront deformation and compaction in DUV-optics" by B. Schäfer et al., Opt. Express 17, 23025 (2009). Two variants of the calibration are discussed in the article, on the one hand a comparison with a numerical simulation, on the other hand a comparison with a measurement of the change in the wavefront on a sample that is heated by means of electrical resistances.

Finally, from the article "Absolute measurement of surface and bulk absorption in DUV optics from temperature induced wavefront deformation", by B. Schäfer et al., Opt. Express 18, 21534 (2010) disclose a device and a method for the quantitative determination of both the surface area and the volume area of the absorption of a sample. In this case, the sample is cuboid. A heating light beam and a measuring light beam intersect at an angle of 90° within the sample volume, with a Hartmann-Shack sensor being used to analyze the wavefront of the measuring light beam that has passed through the sample. The calibration is carried out here by comparing the change in the wavefront with a numerical simulation.

A major disadvantage of the known methods, which are based on measuring the wavefront using a Hartmann-Shack sensor, is their limited resolution. In lithography in particular, due to the advanced optimization of materials, layers and surfaces, the absorption values to be measured have meanwhile become so small that they cannot be reliably determined with the known methods.

Furthermore, with the known methods, the measurement is particularly susceptible to changes in the wave front of the measuring light beam before and after the sample.

In addition, these methods have an almost linear dynamic. Small changes in the wavefront correspond to a small geometric deviation in the position of the focus points in the Hartmann-Shack sensor. The total dynamic range that can be measured with the Hartmann-Shack sensor is typically much larger than the measurement signal itself. This is necessary because the wavefront of the measurement light beam is typically not flat and may be wider in the case of a non-flat yoke of the sample is curved. The sensor must have a large dynamic range in order to be able to measure both this unevenness in the wavefront and small changes in the wavefront after exposure to the heating light beam.

object of the invention

In contrast, the object of the invention is to provide a method for characterizing optical properties, in particular lens heating or mirror heating effects, in an optical sample and/or for determining at least one absorption property of the optical sample for the absorption of useful electromagnetic radiation, which is characterized by a high resolution in the Determination of the property (s) of the sample distinguished.

subject of the invention

According to a first aspect, this object is achieved by a method for determining a change caused by the absorption of electromagnetic heating radiation in at least one optical property of an optical sample and/or at least one absorption property of the optical sample for the absorption of useful electromagnetic radiation, comprising the steps of: introducing the optical sample in an optical resonator, which has two mirrors, with preferably at least one of the two mirrors being partially transparent, irradiating the electromagnetic heating radiation in the form of an electromagnetic heating beam into the optical sample, irradiating an electromagnetic measuring beam into the optical resonator, whereby multiple reflections the mirrors, the electromagnetic measuring beam passes through the optical sample multiple times and/or is reflected multiple times by the optical sample, spatially resolved measurement of an intensity distribution of the electromagnetic measuring beam emerging from the optical resonator, determining a phase shift distribution from the intensity distribution of the electromagnetic measuring beam emerging from the optical resonator measuring beam, determining a change in the phase shift distribution by comparing the phase shift distribution with a reference, and determining the change in the at least one optical property of the optical sample and/or the at least one absorption property of the optical sample from the change in the phase shift distribution.

The optical sample is preferably an optical sample for use in lithography, in particular in lithography with wavelengths of the used electromagnetic radiation in the DUV range or EUV range. The DUV range refers to the wavelength range between about 190 nm and 250 nm, the EUV range the wavelength range between about 5 nm and about 30 nm exposed to useful electromagnetic radiation, with structures of masks (also called reticles) being imaged onto the wafer with a resolution of typically a few nanometers, or be provided in mask inspection systems.

The optical property of the optical sample can be, for example, the refractive index or a shape of a surface of the optical sample. Alternatively, it can also be at least one optical path length of the optical sample for a specific irradiation direction. The heating of the optical sample as a result of the absorption of the electromagnetic heating radiation results in a change in the at least one optical property. This change, which can be determined using the method according to the invention, is in particular a measure of lens heating effects or mirror heating effects that occur or are to be expected. Under certain circumstances, the optical property can vary spatially within the optical sample. This can be the case in particular with the refractive index. In such cases, the value of the optical property determined using the method according to the invention typically corresponds to a spatial averaging along the propagation path of the optical measurement beam through the optical sample. Perpendicular to this, however, the spatial progression of the optical property can be determined, for example if the optical sample is scanned with the electromagnetic measuring beam (see below).

The at least one absorption property of the optical sample can be, for example, at least one absorption coefficient, for example a one-photon absorption coefficient and/or a two-photon absorption coefficient. The at least one absorption coefficient can be a volume absorption coefficient and/or a surface absorption coefficient. The absorption coefficients can also generally be location-dependent, ie they can vary within the optical sample or along the surface of the optical sample. In this case, the absorption coefficient determined using the method according to the invention corresponds to a spatial averaging along the propagation path of the optical measuring beam through the optical sample. On the other hand, the course of the absorption coefficient can be determined perpendicular to this, for example if the optical sample is scanned with the electromagnetic measuring beam (see below).

In order to be able to reliably determine the at least one absorption property of the optical sample for the absorption of the useful electromagnetic radiation from the intensity distribution, which was measured when the electromagnetic heating radiation was radiated in, the electromagnetic heating radiation should be sufficiently similar to the useful electromagnetic radiation. In particular, the wavelength of the electromagnetic heating radiation should not deviate significantly, in particular by no more than 20% or no more than 10%, preferably by no more than 1%, from the wavelength of the useful electromagnetic radiation (e.g. of the lithography system). The wavelength of the electromagnetic heating radiation particularly preferably corresponds to the wavelength of the useful electromagnetic radiation. For example, both the wavelength of the electromagnetic heating radiation and the wavelength of the useful electromagnetic radiation can be 193 nm and thus in the DUV range. This applies analogously to the transfer of the change in the optical property through the absorption of the electromagnetic heating radiation to a corresponding change in the optical property through the absorption of the useful electromagnetic radiation. The electromagnetic heating beam is preferably a laser beam, in particular a laser beam of an excimer laser. However, the electromagnetic heating radiation does not necessarily have to be coherent.

Within the optical sample, the electromagnetic measuring beam typically has a lower power density, preferably a power density lower by more than three orders of magnitude, than the electromagnetic heating beam. The electromagnetic measuring beam can be a laser beam, for example. The electromagnetic measuring beam preferably has a wavelength at which the electromagnetic measuring beam can be easily generated and detected. For example, the electromagnetic measuring beam can have a wavelength of around 632 nm. Suitable beam sources include helium-neon lasers and diode lasers.

In principle, the angle between the electromagnetic measuring beam and the electromagnetic heating beam can be arbitrary. An advantage of a small angle, especially an angle of 30° or less, is a very compact design. In this case, it must be taken into account that the two mirrors of the optical resonator are not affected by the electromagnetic heating beam should heat up. In order to ensure this, the two mirrors can have openings, for example, through which the electromagnetic heating beam passes. However, the measuring range is reduced in this case. If the electromagnetic measuring beam and the electromagnetic heating beam intersect at a larger angle, in particular at a right angle, this problem is avoided. However, in this case the structure is usually less compact.

Both mirrors of the optical resonator are particularly preferably partially transparent. Each of the partially transparent mirrors can have, for example, a substrate on which a thin metal layer or a layer stack made up of a number of layers, in particular a number of dielectric layers, is applied to a side surface. A desired degree of reflection can be set in a targeted manner for the wavelength of the electromagnetic measuring beam via the selection of the metal and the thickness of the metal layer or via the selection of the materials and parameters of the layer stack. In principle, it is favorable for the method according to the invention if the degree of reflection is close to one. For example, the reflectivity of the mirrors can be 0.94 or higher. In this case, the transmittance of the mirror is around or below 0.06.

The mirrored side surfaces of the partially transparent mirrors preferably face one another. If the mirrored side faces are also flat, they are preferably aligned parallel to one another. On the other hand, the side surfaces of the partially transparent mirrors that face away from one another preferably have a wedge angle of, for example, a few arc minutes to the mirrored side surfaces.

If the mirrored side surfaces of the two partially transparent mirrors are flat and parallel to one another, then the optical resonator is a Fabry-Perot interferometer. A central variable in the description of a Fabry-Perot interferometer is the so-called finesse. The finesse corresponds to the ratio of the free spectral range (the distance between the transmission maxima) and the half-width of a single maximum and results from the degree of reflection of the mirror. The greater the degree of reflection, the greater the finesse and thus the resolution.

The electromagnetic measuring beam can be radiated into the optical resonator, for example, through a first of the two partially transparent mirrors. In this case, only a portion of the electromagnetic measuring beam corresponding to the transmittance of the partially transparent mirror passes through the latter. Within the optical resonator, the electromagnetic measuring beam is reflected back and forth between the two mirrors many times. Depending on its transmittance, partial beams emerge from the second partially transparent mirror, which have a phase difference to one another that results from the optical path that the electromagnetic measuring beam follows between the reflection on the second mirror during propagation through the optical resonator and the reflection on the first mirror and re-propagation through the optical resonator. The emerging partial beams interfere and together form the electromagnetic measuring beam emerging from the optical resonator. By adjusting the distance between the two partially transparent mirrors (for example by moving one of the two mirrors), the optical path of the electromagnetic measuring beam can be adjusted, for example, so that the partial beams interfere constructively, but this is not absolutely necessary.

Deviating from the configuration just explained, according to the invention only one of the two mirrors can also be partially transparent, which can possibly reduce the costs. Further details on multi-beam interference in general and on Fabry-Perot interferometers can be found in the book “Optik” by E. Hecht, 4th edition, Oldenbourg Verlag, in Chapter 9.6.

The electromagnetic measuring beam emerging from the optical resonator has an intensity distribution that is measured in a spatially resolved manner. The intensity distribution can be measured in one dimension or preferably in two dimensions with spatial resolution. A suitable detector device, for example based on a CCD sensor or CMOS sensor, can be used for this purpose.

In accordance with the intensity distribution, the phase shift distribution is typically a distribution along one or two spatial axes. The phase shifts are those which the electromagnetic measuring beam experiences during propagation through the optical resonator along both directions, including two reflections at the optical sample and/or passing through the optical sample twice. Each phase shift of the phase shift distribution thus corresponds to the local phase difference between two successive partial beams emerging from the optical resonator. The local intensity is the result of the multi-beam interference of all exiting sub-beams.

In principle, from the distribution of the phase shift distribution, the waves front of the electromagnetic measuring beam can be determined. However, this may require several measurements at different distances between the two mirrors.

The reference is the phase shift distribution that is present without the influence of the electromagnetic heating radiation. To determine the reference, the intensity distribution of the electromagnetic measuring beam emerging from the optical resonator can therefore be measured before the electromagnetic heating beam is irradiated or long enough after it is irradiated (typically more than 10 s, preferably more than 30 s). Alternatively, the reference can also be determined analytically or numerically. The phase shift distribution is preferably compared with the reference by forming the difference.

Calibration may be necessary in order to be able to determine the change in the at least one optical property and/or the at least one absorption property from the change in the phase shift distribution. For example, analytical calculations or numerical simulations come into question, with which relationships between the change in the phase shift distribution and the change in the at least one optical property and/or the at least one absorption property can be derived. For this purpose, other parameters typically have to be known, for example the geometry of the optical sample and the dependence of the refractive index on the temperature.

The calibration can also be carried out using a calibration sample. For example, the calibration sample can have a comparatively high absorption coefficient, which is determined using a different method. The change in the phase shift distribution is then determined for the calibration sample. The relationship thus determined between the change in the phase shift distribution and the absorption coefficient can now be extrapolated towards smaller absorption coefficients, for example assuming a linear dependency.

In principle, the calibration can be carried out largely analogously to the procedure described in the article "Photo-thermal measurement of absorptance losses, temperature induced wavefront deformation and compaction in DUV-optics" by B. Schäfer et al., Opt. Express 17, 23025 (2009), the content of which is hereby fully incorporated into this application.

Like photothermia, laser deflectometry and laser calorimetry, the method according to the invention uses the conversion of absorbed radiation into thermal energy, ie, unlike transflex methods, the absorption in the material is used for the measurement. The method is therefore system-related for the measurement of lens and mirror heating effects.

In contrast to the known methods, which are based on determining the change in a wavefront using a Hartmann-Shack sensor, the electromagnetic measuring beam passes through the optical sample many times due to the multiple reflection at the two mirrors or is reflected many times by the optical sample . As a result, even small changes in the optical properties are clearly reflected in the measured intensity distribution. The resolution of the method is therefore significantly higher, typically several orders of magnitude higher, than that of photothermia and similar laser deflectometric methods and even higher in comparison to transflex methods and laser calorimetric methods. The measuring range is therefore extended to significantly smaller absorption coefficients.

The higher resolution is also accompanied by the ability to measure at lower power densities of the electromagnetic heating beam. The required energy densities of the heating beam are typically lower by a factor of 100 than with photothermal processes. Suitable radiation sources for emitting the electromagnetic heating beam are therefore significantly more compact and cheaper to purchase and maintain. Furthermore, the measurement can take place at fluences as used in the optical system in which the optical sample is used or is intended to be used.

The at least one absorption property of the optical sample, which is determined using the method according to the invention, can also characterize a processed surface of the optical sample. Thus, different surface treatment methods (e.g. polishing or treatment with an ion beam) can be compared with each other with regard to the resulting absorption of the surface. For example, one can use it to determine the effect of the choice of polishing agent for polishing the surface.

In a variant of the method, a coherence length of the electromagnetic measuring beam is at least three times, preferably at least five times, particularly preferably at least ten times, in particular at least fifty times as large as an optical path length of the electromagnetic measuring beam in the optical resonator. At the The coherence length is the maximum path length difference at which there is still a stable interference pattern. The optical path length here corresponds to a simple passage of the electromagnetic measuring beam through the optical resonator and results from integration over the refractive index, d _opt =∫n(s) ds . A larger coherence length leads to sharper maxima and thus a higher resolution. Also, the larger the coherence length, the larger the optical sample can be.

In a further variant of this method, the optical sample is a material sample or a blank for the production of an optical element, with the electromagnetic heating beam preferably being radiated into the optical sample perpendicularly or collinearly to the electromagnetic measuring beam. The determination of at least one absorption property of a material sample can be used, for example, for iterative optimization of the material, for example with regard to the lowest possible absorption. A measurement on a blank, for example by determining an absorption property of the blank, can be used, among other things, to decide whether it is suitable for further processing to produce an optical element.

In a further variant of this method, the optical sample has at least one surface and at least one layer, in particular a layer stack, on the surface, with an angle between the electromagnetic measuring beam and the electromagnetic heating beam preferably being between 0° and 20°. The layer stack can be used, for example, to achieve the highest possible degree of reflection at a specific wavelength. The method can be used to characterize the at least one layer or the layer stack. In particular, at least one absorption property of the at least one layer or of the layer stack can be determined in this way. Consequently, the method enables, among other things, an iterative optimization of the parameters of the layer or the layer stack, including the materials of the layer stack and their layer thicknesses.

In a further variant of this method, the optical sample is an optical element or an optical arrangement with a plurality of optical elements, with the electromagnetic heating beam preferably passing through the optical sample collinear to an intended propagation direction of useful electromagnetic radiation and with preference the electromagnetic measuring beam passes through the optical sample perpendicularly or collinearly to the intended propagation direction of the useful electromagnetic radiation. In the case of an optical arrangement with a plurality of optical elements, all optical surfaces of the optical arrangement can be characterized in a single step when the electromagnetic heating beam and the electromagnetic measuring beam pass through collinearly to the intended propagation direction of the useful electromagnetic radiation. Since an optical arrangement is typically relatively large compared to a single optical element, a large coherence length of the electromagnetic measurement beam is particularly important here. Corrective optics can preferably be used in order to achieve passage of the electromagnetic heating beam collinear to the intended propagation direction of the useful electromagnetic radiation. Correction optics can also be used to adjust the wavefronts of the electromagnetic measuring beam, for example to ensure that it impinges on the two mirrors of the resonator with flat or approximately flat wavefronts.

In the case of optical samples in the form of optical elements or optical arrangements in particular, the high resolution of the method has the advantage that the measurement can be carried out with an energy of the electromagnetic heating beam that is equal to the system energy of the useful electromagnetic radiation when the optical element or optical arrangement is used in an optical system.

The optical samples can also be coated, ie for example have a surface with a layer stack. In such cases, the characterization of the coating and the base body of the optical sample can be carried out together.

In a further variant of this method, the electromagnetic measuring beam passes through the optical sample multiple times and a distance between the two partially transparent mirrors is specified or set, in which a distance covered by the electromagnetic measuring beam in the optical resonator outside the optical sample is at most 100% , preferably less than 50%, particularly preferably less than 25% of the path covered by the electromagnetic measuring beam in the optical sample. Reducing the path length of the electromagnetic measuring beam within the optical resonator, which is outside the optical sample, leads to a reduction in streaks that form, for example, due to the heating of air or another gas in the optical resonator. Unlike the methods from the prior art, in which such interference on the entire path of the measuring beam, only the distance of the electromagnetic measuring beam in the optical resonator and outside the optical sample is affected, which can be kept as small as possible.

In a further variant of this method, the irradiation of the electromagnetic heating beam and the electromagnetic measuring beam is carried out in a gas atmosphere containing an inert gas, preferably nitrogen, and/or a noble gas, preferably argon, and/or the irradiation of the electromagnetic heating beam and the electromagnetic Measuring beam takes place with an electromagnetic heating beam in the UV range or in the DUV range at a pressure of less than 0.1 mbar and with an electromagnetic heating beam in the EUV range at a pressure of less than 10 ^-4 mbar. The gas atmosphere should preferably not absorb the electromagnetic heating beam, or absorb it only slightly. This is the case with nitrogen at a wavelength of 193 nm for the electromagnetic heating beam. The lower absorption leads to less heating and thus a reduction in the formation of streaks, which is a significant influence of noise. For the same reason, it is also advantageous to choose a gas atmosphere which has a low dependence of the refractive index on density and temperature. Avoiding heat transfer into the gas atmosphere is also advantageous. Argon and even heavier gases are particularly suitable for this due to the small heat transfer coefficients. Density fluctuations and the associated formation of streaks can also be reduced by carrying out the measurement in the specified pressure ranges.

In a further variant of this method, at least one of the two mirrors is a curved mirror, in particular a spherical mirror. The curvature or the curvatures are preferably selected in such a way that the optical resonator is optically stable. In principle, at least one curved mirror can be used for optical samples of any type, including material samples, blanks, coated samples, optical elements and optical arrangements with a plurality of optical elements. The curvature of the mirrors and the optical path length in the optical resonator are preferably matched to one another. Since the optical path length in the optical resonator changes when the optical sample is introduced, the optical path length may have to be adjusted accordingly before the optical sample is introduced. Furthermore, both mirrors are preferably curved and optimized for the resonance of a Gaussian beam. The at least one curved mirror results in an increase in finesse and thus a higher resolution capacity due to sharper maxima in the transmission spectrum. In addition, this makes it possible to construct larger or longer optical resonators. Furthermore, the measurement area for the two-dimensional spatially resolved measurement of the intensity distribution is larger.

In a further variant of this method, an expansion of the electromagnetic measuring beam perpendicular to the beam direction in the optical sample is at least three times greater than an expansion of the electromagnetic heating beam perpendicular to the beam direction. If the electromagnetic measuring beam is expanded as stated, this has the advantage that the spatially resolved measurement of the intensity distribution (and thus the determination of the phase shift distribution) can be carried out in a single step, i.e. simultaneously, using a spatially resolving detector device.

In a further variant of this method, the optical sample is scanned with the electromagnetic measuring beam. The extent of the electromagnetic measuring beam perpendicular to the beam direction is preferably smaller than the extent of the electromagnetic heating beam perpendicular to the beam direction. Such a procedure has the advantage that the spatially resolved measurement can be carried out using a detector device that is not spatially resolving. Instead, the spatial resolution results here from the scanning of the optical sample. When measuring the intensity distribution, the local intensities for each position are measured one after the other.

Another aspect of the invention relates to a device for determining a change caused by the absorption of electromagnetic heating radiation in at least one optical property of an optical sample and/or at least one absorption property of the optical sample for the absorption of useful electromagnetic radiation, comprising: an optical resonator in which the optical sample can be introduced and which comprises two mirrors, with at least one of the two mirrors preferably being partially transparent, a heating beam source for emitting electromagnetic heating radiation in the form of an electromagnetic heating beam, which is designed to radiate the electromagnetic heating beam into the optical sample, a measuring beam source for emission an electromagnetic measuring beam, which is designed to radiate the electromagnetic measuring beam into the optical resonator, with multiple reflections on the mirrors causing the electromagnetic measuring beam to pass through the optical element multiple times and/or to be reflected multiple times by the optical sample, a detector device for spatially resolved measurement of an intensity distribution of the electromagnetic measuring beam emerging from the optical resonator and an evaluation device for determining a phase shift distribution from the intensity distribution of the electromagnetic measuring beam emerging from the optical resonator, for determining a change in the phase shift distribution by comparing the phase shift distribution with a reference, and for determining the change in the at least one optical property of the optical sample and/or the at least one absorption property of the optical sample from the change in the phase shift distribution.

The device is used to carry out the method according to the invention. A correspondingly adapted device can be used to carry out the variants of the method. With regard to further details and the advantages achieved with the device, reference is made to the above statements on the method and its variants.

Another aspect of the invention relates to an optical system, in particular a lithography system, comprising a plurality of optical elements and at least one optical resonator in which at least one of the optical elements is arranged, the optical resonator having two mirrors, with preferably at least one of two mirrors is partially transparent, at least one measuring beam source for emitting an electromagnetic measuring beam, which is designed to radiate the electromagnetic measuring beam into the optical resonator, with multiple reflections on the mirrors causing the electromagnetic measuring beam to pass through the optical element multiple times and/or multiple times from the optical element element is reflected, at least one detector device for spatially resolved measurement of an intensity distribution of the electromagnetic measuring beam emerging from the optical resonator and at least one evaluation device for determining a phase shift distribution from the intensity distribution of the electromagnetic measuring beam emerging from the optical resonator.

The lithography system can in particular be a projection exposure system or a mask inspection system. The optical elements can be, for example, transmissive optical elements, for example lenses, and/or reflective optical elements, for example mirrors. With regard to the optical resonator, the two mirrors, the measuring beam source for emitting the electromagnetic measuring beam, the detector device for spatially resolved measurement of the intensity distribution of the electromagnetic measuring beam exiting the optical resonator and the evaluation device for determining the phase shift distribution from the intensity distribution of the electromagnetic measuring beam exiting the optical resonator electromagnetic measuring beam, reference is made to the above statements on the method according to the invention and the device according to the invention.

The phase shift distribution determined by the evaluation device makes it possible to make statements about the state of the at least one optical element that is arranged in the at least one optical resonator and/or the state of the entire optical system. Depending on the phase shift distribution, certain measures, such as replacing an optical element, can be taken.

Only one optical element is preferably arranged in the optical resonator or in each of the optical resonators. However, a plurality of optical elements can also be arranged in the optical resonator or in each of the optical resonators.

In principle, but not necessarily, each of the optical elements of the optical system can be arranged in the at least one optical resonator. However, preferably only those optical elements are arranged in the at least one optical resonator that are of particular interest, for example because their condition is particularly important for the resolution of the optical system.

In one embodiment, the measuring beam source is designed to emit an electromagnetic measuring beam that has a coherence length that is at least three times, preferably at least five times, particularly preferably at least ten times, in particular at least fifty times as large as an optical path length of the electromagnetic measuring beam im optical resonator. A large coherence length has the advantage of a higher resolution for determining the phase shift distribution from the intensity distribution, measured by the detector device, of the electromagnetic measuring beam emerging from the optical resonator.

In a further embodiment, the optical system comprises a device for setting a distance between the two mirrors, which preferably has at least one piezo actuator. By changing the distance between the two mirrors, the range in which the phase shift distribution from the intensity of the electromag netic measuring beam with high resolution or can be determined at all, are adjusted. Thus, compared to a fixed mirror spacing, the means for adjusting the spacing between the two mirrors allows this range to be extended almost at will.

In a further embodiment, the optical system comprises a useful radiation source for emitting useful electromagnetic radiation, which passes through the at least one optical element along a predetermined propagation direction. If the optical system is a projection exposure system, the useful electromagnetic radiation emitted by the useful radiation source can be used, for example, to expose wafers.

In a further embodiment, the optical system comprises at least one monitoring device for monitoring the at least one optical element using at least one monitoring parameter determined from the phase shift distribution. The value of the at least one monitoring parameter or a change in the at least one monitoring parameter over time can, for example, be a measure of the degradation of the at least one optical element, an indicator of imminent failure of the at least one optical element, or a malfunction of the optical system Clues. The degradation of the at least one optical element is typically a consequence of the high radiation intensities.

In principle, two types of monitoring parameters are possible: on the one hand, those that can be determined without the useful electromagnetic radiation, and on the other hand, those that can only be determined through the useful electromagnetic radiation, which in this case, in addition to its actual purpose, also plays the role of played the electromagnetic heating radiation in the method described above or the device described above.

For example, optical path lengths that can be determined directly from the phase shift distribution fall into the first category. The second category includes, for example, a change in an optical path length under the influence of the useful electromagnetic radiation compared to the corresponding optical path length without the influence of the useful electromagnetic radiation and absorption properties of the at least one optical element. Degradation of the at least one optical element over time is typically expressed both in a change in monitoring parameters of the first category and of the second category.

If the monitoring parameter is one of the second category, in particular an absorption property, the monitoring device is preferably designed, in accordance with the method according to the invention, to determine a change in the phase shift distribution by comparing the phase shift distribution with a reference and from the change to determine the at least one monitoring parameter from the phase shift distribution.

The high resolution achieved by means of the optical resonator typically only allows the determination of monitoring parameters of the second category. In many lithography systems, the useful electromagnetic radiation is pulsed. This results in short time intervals, typically on the order of 1 ms, in which at least one specific area of the at least one optical element is not exposed to the useful electromagnetic radiation. A reference measurement can now be made within this period. Due to the high resolution, the short time interval (within which no equilibrium can be established) between the actual measurement and the reference measurement is sufficient to determine an absorption property, for example.

In a further development of this embodiment, the monitoring device is designed to compare the monitoring parameter with at least one reference value, in particular at least one value of the monitoring parameter at at least one point in time in the past, and preferably to send a notification via an output device depending on a deviation of the monitoring parameter from the at least one reference value output a user. The past point in time is preferably weeks or months ago.

The comparison of the monitoring parameter with the at least one reference value can be used, for example, to detect a failure of the at least one optical element at an early stage. If an impending failure is detected in good time, the at least one optical element can be replaced before it fails, for example. This avoids a complete standstill of the optical system and the associated high costs. An imminent failure can preferably also be recognized from a service life model that describes the expected change in the at least one monitoring parameter over time. Typi Typically, the course of the at least one monitoring parameter is initially relatively flat for a long time and then breaks off abruptly.

In a further embodiment, the optical system comprises at least one control device for influencing a wave front of the useful electromagnetic radiation as a function of the phase shift distribution determined by means of the evaluation device. For this purpose, the control device preferably includes at least one active optical element. With such a control device it is possible, for example, to compensate for the effects of aging optical elements.

Further features and advantages of the invention result from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be realized individually or together in any combination in a variant of the invention.

character list

• 1 eine schematische Darstellung einer Vorrichtung zur Durchführung eines Verfahrens zur Bestimmung einer durch Absorption von elektromagnetischer Heizstrahlung bedingten Veränderung mindestens einer optischen Eigenschaft einer optischen Probe und/oder mindestens einer Absorptionseigenschaft der optischen Probe, wobei es sich bei der optischen Probe um eine Materialprobe handelt,
• 2 eine schematische Darstellung einer alternativen Ausführungsform der in der 1 gezeigten Vorrichtung, wobei die optische Probe eine Oberfläche mit einem Schichtstapel aufweist,
• 3 eine schematische Darstellung einer alternativen Ausführungsform der in der 1 gezeigten Vorrichtung, wobei es sich bei der optischen Probe um ein optisches Element handelt,
• 4 eine schematische Darstellung einer alternativen Ausführungsform der in der 1 gezeigten Vorrichtung, wobei es sich bei der optischen Probe um eine optische Anordnung mit einer Mehrzahl von optischen Elementen handelt,
• 5 eine schematische Darstellung einer alternativen Ausführungsform der in der 1 gezeigten Vorrichtung, wobei die Messung an der optischen Probe in Reflexion erfolgt,
• 6 eine schematische Darstellung eines optischen Systems in Form eines Lithographiesystems mit einer Mehrzahl von optischen Elementen, wobei zwei der optischen Elemente in jeweils einem optischen Resonator angeordnet sind und mittels einer Überwachungseinrichtung überwacht werden, sowie
• 7 einen Zusammenhang zwischen dem Transmissionsgrad eines optischen Resonators in Form eines Fabry-Perot-Interferometers, in dem eine optische Probe angeordnet ist, und einem Absorptionskoeffizienten der optischen Probe.

Exemplary embodiments are shown in the schematic drawing and are explained in the following description. It shows

• 1 a schematic representation of a device for carrying out a method for determining a change caused by the absorption of electromagnetic heating radiation in at least one optical property of an optical sample and/or at least one absorption property of the optical sample, the optical sample being a material sample,
• 2 a schematic representation of an alternative embodiment in the 1 shown device, wherein the optical sample has a surface with a layer stack,
• 3 a schematic representation of an alternative embodiment in the 1 device shown, wherein the optical sample is an optical element,
• 4 a schematic representation of an alternative embodiment in the 1 shown device, wherein the optical sample is an optical arrangement with a plurality of optical elements,
• 5 a schematic representation of an alternative embodiment in the 1 shown device, whereby the measurement on the optical sample takes place in reflection,
• 6 a schematic representation of an optical system in the form of a lithography system with a plurality of optical elements, wherein two of the optical elements are each arranged in an optical resonator and are monitored by a monitoring device, and
• 7 a relationship between the transmittance of an optical resonator in the form of a Fabry-Perot interferometer in which an optical sample is placed and an absorption coefficient of the optical sample.

In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

The 1 shows a schematic representation of an exemplary device 1 for carrying out a method for determining a change caused by the absorption of electromagnetic heating radiation 3 of at least one optical property of an optical sample 5 and/or at least one absorption property of the optical sample 5. The optical sample 5 is here, for example, a material sample, but it can also be a blank for the production of a transmitting optical element. Act. In the example shown, the optical property is the refractive index n of the sample, and the change dn is the change in the refractive index n due to the absorption of the electromagnetic heating radiation 3. In the example shown, the absorption property is the in -Photon absorption coefficients k in the volume of the optical sample 5.

The device 1 comprises an optical resonator 7 into which the optical sample 5 is introduced. The resonator 7 comprises two mirrors 9, 9', both of which are partially transparent here, by way of example but not necessarily. To achieve the partially transparent mirror coating, a coating 15,15' in the form of a metal layer is applied to the mutually facing side surfaces 11,11' of the base body 13,13' of the two mirrors 9,9'. However, the partially transparent mirror coating can also be implemented differently.

Furthermore, the device 1 comprises a heat ray source 17 for emitting electromagnetic heat radiation 3 in the form of an electromagnetic heat ray 19 , which is designed to radiate the electromagnetic heat ray 19 into the optical sample 5 . The device 1 includes also a measuring beam source 21 for emitting an electromagnetic measuring beam 23, which is designed to radiate the electromagnetic measuring beam 23 into the optical resonator 7, with the electromagnetic measuring beam 23 passing through the optical sample 5 multiple times as a result of multiple reflections on the mirrors 9, 9`, as well as a detector device 25 for spatially resolved measurement of an intensity distribution I of the electromagnetic measuring beam 23′ emerging from the optical resonator 7.

The device 1 also includes an evaluation device 27 for determining a phase shift distribution φ from the intensity distribution I of the electromagnetic measuring beam 23' emerging from the optical resonator, for determining a change dφ in the phase shift distribution φ by comparing the phase shift distribution φ with a reference φ _R , and for determining the change dn in the at least one optical property n of the optical sample 5 and/or the at least one absorption property k of the optical sample from the change dφ in the phase shift distribution φ. The evaluation device 27 is suitably programmed hardware and/or software.

The heating beam source 17 is, for example, an excimer laser with a wavelength of 193 nm. However, the heating beam source 17 can also be designed differently and have a different wavelength, in particular 248 nm or 365 nm, for example.

In particular, the heating beam source 17 does not have to be a laser.

The heating beam source 17 is also arranged in such a way that the electromagnetic heating beam 19 is radiated into the optical sample 5 perpendicularly to the electromagnetic measuring beam 23 . Deviating from this, the electromagnetic heating beam 19 can also be irradiated at a different angle. In particular, the electromagnetic heating beam 19 can also be radiated into the optical sample 5 collinear to the electromagnetic measuring beam 23 .

A beam trap 29 is used here, for example, to absorb the portion of the electromagnetic heating beam 19 that is not absorbed in the optical sample 5.

The measuring beam source 21 is, for example, a helium-neon laser, and the electromagnetic measuring beam 23 has a wavelength of approximately 633 nm. Furthermore, the electromagnetic measuring beam 23 has a coherence length I _c that is approximately fifty times greater than the optical path length d _opt of the electromagnetic measuring beam 23 in the optical resonator 7.

In general, however, the wavelength of the electromagnetic measuring beam 23 can also deviate from the stated value and the measuring beam source 21 can be designed differently. Furthermore, the electromagnetic measuring beam 23 can also have a shorter or longer coherence length. In particular, the coherence length I _c of the electromagnetic measuring beam 23 can be at least three times, preferably at least five times, particularly preferably at least ten times as large as the optical path length d _opt of the electromagnetic measuring beam 23 in the optical resonator 7.

In addition, here, by way of example but not necessarily, an extent a _M of the electromagnetic measuring beam 23 perpendicular to the beam direction is at least three times greater than an extent of the electromagnetic heating beam a _H perpendicular to the beam direction.

The detector device 25 shown comprises a CCD sensor with a two-dimensional pixel array and thus enables a two-dimensional spatially resolved measurement of the intensity distribution I of the electromagnetic measuring beam 23′ emerging from the optical resonator 7.

The distance d between the two partially transparent mirrors 9, 9' can, in contrast to what is shown here, also be specified or adjusted in such a way that the distance I _a that the electromagnetic measuring beam 23 covers in the optical resonator 7 outside the optical sample 5 is at most 100%, preferably less than 50%, particularly preferably less than 25% of the distance I _p that the electromagnetic measuring beam 23 travels in the optical sample 5.

To adjust the distance d between the two mirrors 9, 9', which can also be used to achieve a larger measuring range, a suitable device (not shown here), for example based on a piezo actuator, can be used, which is designed to accommodate one of the two mirrors 9 ,9' or, if necessary, to move both mirrors 9,9'.

In the 2 is a schematic representation of an alternative embodiment in the 1 shown device 1 shown. Different from the 1 In this case, the optical sample 5 is not a material sample but an optical sample 5 with a surface 31 to which a layer stack 33 is applied. The layer stack 33 includes, by way of example, a multiplicity of dielectric layers which are not illustrated here. Deviating from this, the surface of the opti For example, the optical sample 31 can also have only one layer, or a stack of layers or a layer can be applied to a plurality of surfaces of the optical sample 5 . The layer or layer stack 33 can be applied to the surface 31 using a conventional coating method for depositing thin layers.

In the example shown, an angle α between the electromagnetic measuring beam 23 and the electromagnetic heating beam 19 is approximately 23°. Deviating from this, however, the angle α between the electromagnetic measuring beam 23 and the electromagnetic heating beam 19 can also be larger or smaller and in particular can be between 0° and 20°. In order to allow the electromagnetic heating beam 19 to pass through, the two mirrors 9, 9' each have an opening 35, 35'.

The irradiation of the electromagnetic heating beam 19 and the electromagnetic measuring beam 23 is carried out in a gas atmosphere G containing an inert gas IG and an inert gas EG. For this purpose, the optical resonator 7 is arranged in a gas-tight chamber 37 which has openings 39, 39' or windows for the electromagnetic heating beam 19 and the electromagnetic measuring beam 23, 23'.

The inert gas IG or the noble gas EG is here, for example, nitrogen or argon, but it can also be other inert gases or noble gases. It goes without saying that a noble gas EG and an inert gas IG do not necessarily have to be present in the gas atmosphere G.

Alternatively or in addition to the presence of a noble gas EG and/or an inert gas in the gas atmosphere G, the electromagnetic heating beam 19 and the electromagnetic measuring beam 23 can be irradiated with an electromagnetic heating beam 19 in the UV range or DUV range at a pressure of less than 0.1 mbar and with an electromagnetic heating beam 19 in the EUV range at a pressure of less than 10 ^-4 mbar.

After all, here is different than in the 1 the electromagnetic measuring beam 23 is not expanded. For spatially resolved measurement of the intensity distribution I, the optical sample 5 is scanned with the electromagnetic measuring beam 23 instead. For this purpose, the measuring beam source 21 and the detector device 25 and/or the optical sample 5 can be shifted. It is also possible for the measuring beam source 21 to have a scanner device in order to enable the optical sample 5 to be scanned.

Deviating from the representation, an expanded measuring beam 23 can also be used in the case of an optical sample 5 with a layer stack 33 .

In the 3 is a schematic representation of a further alternative embodiment in the 1 shown device 1 shown. Unlike in the 1 here the optical sample 5 is an optical element in the form of a lens. The lens shape is not limited to the shape shown, but is basically arbitrary.

The two side surfaces 41, 41' of the optical sample 5 are polished. Furthermore, these two side surfaces 41, 41' are aligned parallel to one another and perpendicular to the electromagnetic measuring beam 23, but this does not necessarily have to be the case.

Unlike in the 1 and the 2 In addition, the two mirrors 9.9' are curved mirrors, which leads, among other things, to greater finesse. Deviating from this, for example, only one or none of the mirrors 9, 9' can be curved. After all, is different than in the 1 and as in the 2 the electromagnetic measuring beam 23 is not expanded.

Also in the 4 is a schematic representation of a further alternative embodiment in the 1 shown device 1 shown. The optical sample 5 is an optical arrangement with a plurality of optical elements 43. More precisely, the optical arrangement is an objective and the optical elements are lenses. Deviating from this, the optical sample 5 can also be a different optical arrangement.

The electromagnetic heating beam 19 and the electromagnetic measuring beam 23 occur collinear to the intended propagation direction 45 from in 4 non-illustrated useful electromagnetic radiation through the optical sample 5 through. In order to achieve the collinear propagation of the electromagnetic heating beam 19, corrective optics 47, 47' are arranged between the optical sample 5 and the two mirrors 9, 9'. Instead of the correction optics 47, 47', the two mirrors 9, 9' can also each have an opening through which the electromagnetic heating beam 19 passes.

Basically, the electromagnetic radiant heat 19 and the electromagnetic Measurement beam 23 does not pass through the optical sample 5 collinear to the intended propagation direction 45 of the useful electromagnetic radiation. For example, the electromagnetic measuring beam 23 can also pass through the optical sample perpendicularly to the intended propagation direction 45 of the useful electromagnetic radiation.

The method can now be used, for example, in the adjustment process of the optical arrangement, in order to correct lens heating effects determined in this way.

Finally, also in the 5 is a schematic representation of a further alternative embodiment in the 1 shown device 1 shown. Unlike in the 1 until 4 Here, the electromagnetic measuring beam 23 does not pass through the optical sample 5 many times, but is reflected by the optical sample 5 many times. For example, the optical sample 5 is a reflective optical element in the form of a substrate of a mirror, on the surface 31 of which the measuring beam 23 is reflected. Other than this in 5 As shown, the optical sample 5 may be a mirror having a reflective coating applied to the surface 31 in the form of a layer or stack of layers, as discussed in connection with FIG 2 is described. In principle, however, such measurements can be carried out on any optical sample on which at least part of the electromagnetic measuring beam 23 is reflected.

In the 6 is a schematic representation of an optical system 49 in the form of a lithography system, which includes a plurality of optical elements 51,51′,51″ In the example shown, the lithography system is a lithography system for DUV lithography and the optical elements 51, 51′, 51″ are transmissive optical elements in the form of lenses, but they can also be other optical elements . For example, if the optical system 49 is in the form of an EUV lithography system, the optical elements 51, 51′, 51″ are typically EUV mirrors.

The optical system 49 has two optical resonators 53, 53', in each of which one of the optical elements 51, 51' is arranged. The optical elements 51, 51' arranged in the optical resonators 53, 53' have polished side surfaces 55, 55'. The optical resonators 53, 53' each have two mirrors 57, 57', both of which are partially transparent here by way of example, but not necessarily. One of the optical elements 51'' is not arranged in an optical resonator here, for example.

The optical system 49 also includes two measuring beam sources 59, 59', two detector devices 61, 61' and two evaluation devices 63, 63'.

The two measurement beam sources 59, 59' each serve to emit an electromagnetic measurement beam 65, 65' and are designed to radiate the respective electromagnetic measurement beam 65, 65' into one of the optical resonators 53, 53', with multiple reflections on the mirrors 57 57′ the respective electromagnetic measuring beam 65,65′ passes through the respective optical element 51,51′ multiple times.

The two detector devices 61, 61' are used for spatially resolved measurement of the respective intensity distribution I of the electromagnetic measuring beam 67, 67' emerging from the respective optical resonator 53, 53'.

The two evaluation devices 63, 63' each serve to determine a phase shift distribution φ from the respective intensity distribution I of the electromagnetic measuring beam 67, 67' emerging from the respective optical resonator 53, 53'.

In contrast to what is shown here, the optical resonator 53, 53', the measuring beam source 59, 59', the detector device 61, 61' and the evaluation device 63, 63' or parts thereof can also be integrated in sockets or holders of the respective optical element 51, 51' be. The optical resonator 53, 53', the measuring beam source 59, 59', the detector device 61, 61' and the evaluation device 63, 63' or parts thereof preferably form a device 1 for use with the optical system 49, in particular the lithography system. The two mirrors 57 of the optical resonator 53 can also be applied directly to the polished side surfaces 55 (ie without an air gap). Furthermore, the optical system can also have a different number of optical elements, optical resonators, measuring beam sources, detector devices and evaluation devices. Furthermore, due to multiple reflection at the mirrors 57, 57', the respective electromagnetic measuring beam 65, 65' can be reflected multiple times by the optical element 51, 51' instead of passing through multiple times. More than two optical elements 51, 51' of the optical system 49 can also be arranged in one optical resonator 53, 53' each. More than one optical element 51, 51' can also be arranged in each of the optical resonators 53, 53'. The optical elements are preferably arranged in an optical resonator whose condition is of particular interest. For example, the measuring system can measure absorption changes at strategic positions in the system with high precision over the lifetime.

The two measurement beam sources 59,59' of the optical system 49 are each designed to emit an electromagnetic measurement beam 65,65' which has a coherence length I _c that is fifty times the optical path length d _opt of the electromagnetic measurement beam 65,65 `in the respective optical resonator 53,53'. Deviating from this, however, the coherence length I _c of the electromagnetic measuring beams 65, 65' can also be larger or smaller.

The optical system 49 also includes, by way of example but not necessarily, a device 69 for setting the distance d between the two mirrors 57 of the first optical resonator 53, which has a piezo actuator (not shown here) and is designed to move one of the two mirrors 57. Deviating from this, the device 69 for setting the distance d between the two mirrors 57 can also be designed differently. Furthermore, a corresponding device can also be provided for several, all or none of the optical resonators 53, 53'.

The optical system further includes a useful radiation source 71 for emitting useful electromagnetic radiation 73, which passes through the optical elements 51,51',51" along a propagation direction 75. The useful electromagnetic radiation 73 is used in the example described here, which is the optical system is a lithography system, for exposure of a wafer 77, but this is not absolutely necessary.

In addition, the optical system 49 includes a monitoring device 79 for monitoring the optical element 51 arranged in the first optical resonator 53 using a monitoring parameter U derived from the phase shift distribution φ. The monitoring parameter U here is a monitoring parameter in the form of an absorption property, more precisely said a volume absorption coefficient k of the optical element 51 for the one-photon absorption, but in principle it can also be another monitoring parameter, for example an optical path length or its change. As described above, the monitoring parameter U is determined by comparing the phase shift distribution φ with a reference, more precisely with a reference phase shift distribution φ _R .

The monitoring device 79 is designed to compare the monitoring parameter U with a reference value U _R . The reference value U _R is a threshold value here. A notification is sent to a user via an output device 81 if the monitoring parameter U exceeds the reference value U _R . This is used here, for example, to give the user an early warning of an impending failure of the optical element 51 . Alternatively, other deviations of the monitoring parameter U from the reference value U _R can also result in the user being notified.

In addition, the monitoring device 79 can also be designed to compare the monitoring parameter U with more than one reference value U _R . The at least one reference value U _R can in particular also be at least one value of the monitoring parameter U at at least one point in time in the past. Furthermore, the monitoring by the monitoring device 79 can also take place using a plurality of monitoring parameters U. The monitoring device 79 can also be designed to monitor more than one optical element. The optical system 49 can also have more than one monitoring device.

Alternatively or additionally, the optical system 49 can also have at least one control device (not shown here) for influencing a wave front of the useful electromagnetic radiation 73 as a function of the phase shift distribution φ determined by the evaluation device 63, 63'.

In the 7 is a relationship between the transmittance I _t /I ₀ of an optical resonator 7 in the form of a Fabry-Perot interferometer, in which an optical sample 5 is arranged, and an absorption coefficient k, more precisely the volume absorption coefficient for the one-photon absorption , the optical sample 5 shown schematically. The transmittance I _t /I ₀ is entered on the ordinate and the absorption coefficient k on the abscissa.

Such a relationship can be used to determine the absorption coefficient k of the optical sample 5 from the intensity distribution I of the electromagnetic measuring beam 23 ′ emerging from the optical resonator 5 .

wobei I₀ die eingestrahlte Intensität, I_t die transmittierte Intensität, T der Transmissionsgrad jedes Spiegels 9,9`, R der Reflexionsgrad jedes Spiegels 9,9` und φ der Phasenunterschied je zweier aufeinanderfolgender aus dem optischen Resonator 7 austretender Teilstrahlen ist. Dabei gilt T + R + A = 1, wobei A die Absorption jedes Spiegels 9,9` ist.As described, for example, in the book “Optik” by E. Hecht, 4th edition, Oldenbourg Verlag, Chapter 9.6, which is incorporated into the content of this application by reference, the transmittance I _t/ I ₀ of an optical resonator 7 results in the form of a Fabry-Perot interferometer from the equation $$\frac{I_t}{I_0}={\left(\frac{\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="DE102022208009A1\_0003.png,DE102022208009A1\_0004.png"\; state="\{\{state\}\}">T</figure-callout>}}{1-R}\right)}^2\frac{1}{1+\frac{4R}{{\left(1-R\right)}^2}{\left(\text{sin}\left(\frac{\mathrm{\phi }}{2}\right)\right)}^2}$$

where I ₀ is the incident intensity, I _t is the transmitted intensity, T is the transmittance of each mirror 9.9`, R is the reflectance of each mirror 9.9` and φ is the phase difference between two successive partial beams emerging from the optical resonator 7. Here T + R + A = 1, where A is the absorption of each mirror 9.9`.

The transmittance 1 _t /I ₀ consequently passes through maxima when the phase difference φ is a multiple of 2π, ie when φ=2π·m with m=1,2,3,.... applies.

The half width γ of the maxima is given by $$g=\frac{2\left(1-R\right)}{\sqrt{R}}$$

The FWHM, y, decreases as the reflectance, R, approaches unity. This means that the maxima become sharper the higher the reflectance R of the mirror.

If one now considers an optical sample 5 with plane-parallel side surfaces, which is aligned perpendicularly to the electromagnetic measuring beam 23 and which is arranged in the optical resonator 7 in the form of a Fabry-Perot interferometer, as is the case, for example, in 1 is shown, with a known refractive index n of the optical sample 5, with a known dependence dn/dT of the refractive index n on the temperature T, with a known material of the optical sample 5 (with known thermal conductivity and thermal capacity, and possibly known thermal expansion coefficients). known thickness I _p of the optical sample 5, with a known distance d and a known degree of reflection R of the mirrors 9,9` in the optical resonator 7, with a known power and wavelength of the electromagnetic heating beam 19 and the wavelength of the electromagnetic measuring beam 23, there is a relationship between the absorption coefficient k and the phase difference φ of two successive partial beams emerging from the optical resonator 7 . This relationship can be determined by a calculation or with the aid of a calibration. This results in equation (1) in the 7 shown relationship between the transmittance I _t / I ₀ and the absorption coefficient k. In the example shown, the optical resonator 7 was dimensioned such that a maximum intensity I ₀ is measured without the irradiation of the electromagnetic heating beam 23, but this is not absolutely necessary.

For this context, too, a half-value width k _1/2 of the maxima can be specified, which is 7 is shown. A range D results from the half-value width k _1/2 in which the absorption coefficient k can be determined from the intensity of the electromagnetic measuring beam 23 exiting the optical resonator 7 with sufficient resolution. Under conservative assumptions, this range corresponds to approximately D ≈ 3 k _1/2 . By changing the distance d between the two mirrors 9, 9', this area D can be expanded in almost any way. Analogous to 7 a connection between the half-value width k _1/2 and the degree of reflection R of the two mirrors 9.9' can also be established.

The resolving power can also be determined from the half-width k _1/2 . Under the conservative assumption of measuring the intensity with 10 bits or approx. 1000 steps, the resolution when determining the absorption is approx. Δk ≈ k _1/2/500 .

In the following, specific numerical values are given as examples that apply to an optical sample 5 made of quartz glass: The refractive index n of the optical sample 5 is assumed to be n≈1.46008, for the dependency of the refractive index on the temperature applies: dn/dT≈1.02 · 10 ^-5 K ^-1 , the degree of reflection R of each mirror 9, 9' is R=0.98, the distance d between the two mirrors 9.9' is d=0.05 m and one dimension of the optical sample 5 perpendicular to the electromagnetic measuring beam 23 is at I _p =0.04 m. The electromagnetic heating beam 19 is irradiated with a power of 1 W at a wavelength of 193 nm and a wavelength of the electromagnetic measuring beam 23 is 546.07 nm.

With the stated numerical values one obtains: k _1/2 ≈ 6 · 10 ^-5 cm ^-1 ; D ≈ 1.8 10 ^-4 cm ^-1 and Δk ≈ 1.2 10 ^-7 cm ^-1 . In this example, the resolving power is more than a factor of 1000 better than the typical resolving power of photothermal processes (approx. 2 · 10 ^-4 cm ^-1 ).

The absorption coefficient k is determined for a location x, y of the intensity distribution I in the example described above. The associated phase shift φ therefore relates to the phase shift distribution φ(x,y) at the associated location x,y of the intensity distribution I(x,y). To simplify the presentation, the dependency of the phase difference was in the above description exercise distribution φ and the intensity distribution I from the respective location x,y not specified.

As described above, the optical sample 5 can be scanned with the electromagnetic measuring beam 23 in order to determine the intensity distribution I and thus also the phase shift distribution φ at different locations x,y of the intensity distribution I. In this way, the absorption coefficient k can also be determined at different locations of the intensity distribution I and thus at different positions at which the electromagnetic measuring beam 23 passes through the optical sample 5 .

As described above, the phase shift distribution φ at a respective location is compared with a reference phase shift φ _R without the influence of the electromagnetic measuring beam 23 in order to detect a change over time, in the example shown a difference dφ = φ - φ _R of the phase shift -Distribution φ to determine the irradiation of the electromagnetic measuring beam 23. The change dn in the refractive index n of the optical sample 5 over time can be analogous to the change in the absorption coefficient k over time described above.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 10139906 A1 [0007]

Non-patent Literature Cited

• B. Schäfer et al., Opt. Express 17 , 23025 [0009, 0033]
• B. Schäfer et al., Opt. Express 18, 21534 [0010]

Claims (18)

Method for determining a change (dn) caused by absorption of electromagnetic heating radiation (3) in at least one optical property (n) of an optical sample (5) and/or at least one absorption property (k) of the optical sample (5) for the absorption of electromagnetic Useful radiation (73), comprising the steps: - introducing the optical sample (5) into an optical resonator (7) which has two mirrors (9,9'), with at least one of the two mirrors (9,9') preferably being partially transparent - irradiation of the electromagnetic heating radiation (3) in the form of an electromagnetic heating beam (19) into the optical sample (5), - irradiation of an electromagnetic measuring beam (23) into the optical resonator (7), with multiple reflection at the mirrors ( 9,9') the electromagnetic measuring beam (23) passes through the optical sample (5) multiple times and/or is reflected multiple times by the optical sample (5), - spatially resolved measurement of an intensity distribution (I) of the optical resonator (5) exiting electromagnetic measuring beam (23'), - determining a phase shift distribution (φ) from the intensity distribution (I) of the optical resonator (5) exiting electromagnetic measuring beam (23'), - determining a change (dφ) in the phase shift distribution (φ) by comparing the phase shift distribution (cp) with a reference (φ _R ), and - determining the change (dn) in the at least one optical property (n) of the optical sample (5) and/or the at least one absorption property ( k) the optical sample (5) from the change (dφ) of the phase shift distribution (φ). procedure after claim 1 , characterized in that a coherence length (I _c ) of the electromagnetic measuring beam (23) is at least three times, preferably at least five times, particularly preferably at least ten times, in particular at least fifty times as large as an optical path length (d _opt ) of the electromagnetic measuring beam (23) in the optical resonator (7). procedure after claim 1 or 2 , characterized in that the optical sample (5) is a material sample or a blank for the production of an optical element, the electromagnetic heating beam (19) preferably entering the optical sample perpendicularly or collinearly to the electromagnetic measuring beam (23). (5) is irradiated. Method according to one of the preceding claims, characterized in that the optical sample (5) has at least one surface (31) and at least one layer (33), in particular a layer stack, on the surface (31), with an angle (α) preferably between the electromagnetic measuring beam (23) and the electromagnetic heating beam (19) is between 0° and 20°. Procedure according to one of Claims 1 until 3 , characterized in that the optical sample (5) is an optical element or an optical arrangement with a plurality of optical elements (43), the electromagnetic heating beam (19) preferably being collinear with an intended propagation direction (45) of useful electromagnetic radiation (73) passes through the optical sample (5) and the electromagnetic measuring beam (23) preferably passes through the optical sample (5) perpendicularly or collinearly to the intended propagation direction (45) of the useful electromagnetic radiation (73). Method according to one of the preceding claims, characterized in that the electromagnetic measuring beam (23) passes through the optical sample (5) many times and a distance (d) between the two partially transparent mirrors (9,9`) is specified or set in which a distance (I <sub>a</sub> ) covered by the electromagnetic measuring beam (23) in the optical resonator (7) outside the optical sample (5) is at most 100%, preferably less than 50%, particularly preferably less than 25% of the distance (I <sub>p</sub> ) that the electromagnetic measuring beam (23) travels in the optical sample (5). Method according to one of the preceding claims, characterized in that the irradiation of the electromagnetic heating beam (19) and the electromagnetic measuring beam (23) is carried out in a gas atmosphere (G) which is an inert gas (IG), preferably nitrogen, and/or an inert gas (EG), preferably argon, and/or in which the irradiation of the electromagnetic heating beam (19) and the electromagnetic measuring beam (23) with an electromagnetic heating beam (19) in the UV range or in the DUV range at a pressure of less than 0.1 mbar and with an electromagnetic heating beam (19) in the EUV range at a pressure of less than 10 <sup>-4</sup> mbar. Method according to one of the preceding claims, characterized in that at least one of the two mirrors (9, 9') is a curved mirror, in particular a spherical mirror. Method according to one of the preceding claims, characterized in that in the optical sample (5) an extent (a <sub>M</sub> ) of the electromagnetic measuring beam (23) perpendicular to the beam direction is at least three times greater than an extent (a <sub>H</sub> ) of the electromagnetic heating beam ( 19) perpendicular to the beam direction. Procedure according to one of Claims 1 until 8th , characterized in that the optical sample (5) is scanned with the electromagnetic measuring beam (23). Device (1) for determining a change (dn) caused by absorption of electromagnetic heating radiation (3) in at least one optical property (n) of an optical sample (5) and/or at least one absorption property (k) of the optical sample (5) for the Absorption of useful electromagnetic radiation (73), comprising: - an optical resonator (7) into which the optical sample (5) can be introduced and which comprises two mirrors (9, 9'), with preferably at least one of the two mirrors (9, 9') is partially transparent, - a heating beam source (17) for emitting electromagnetic heating radiation (3) in the form of an electromagnetic heating beam (19), which is designed to radiate the electromagnetic heating beam (19) into the optical sample (5), - a Measuring beam source (21) for emitting an electromagnetic measuring beam (23), which is designed to radiate the electromagnetic measuring beam (23) into the optical resonator (7), the electromagnetic measuring beam (23 ) passes through the optical sample (5) multiple times and/or is reflected multiple times by the optical sample (5), - a detector device (25) for the spatially resolved measurement of an intensity distribution (I) of the electromagnetic measuring beam ( 23') and - an evaluation device (27) for determining a phase shift distribution (φ) from the intensity distribution (I) of the optical resonator (7) exiting electromagnetic measuring beam (23), for determining a change (dφ) of the phase shift Distribution (φ) by comparing the phase shift distribution (φ) with a reference (φ <sub>R</sub> ), and for determining the change (dn) in the at least one optical property (n) of the optical sample (5) and/or the at least one absorption property (k) the optical sample (5) from the change (dφ) of the phase shift distribution (φ <sub>R</sub> ). Optical system (49), in particular lithography system, comprising: a plurality of optical elements (51,51',51''), characterized by a device (1), having - at least one optical resonator (53,53'), in which at least one of the optical elements (51, 51`) is arranged, with the optical resonator (53, 53') having two mirrors (57, 57'), with at least one of the two mirrors (57, 57') preferably being partially transparent, - At least one measuring beam source (59,59') for emitting an electromagnetic measuring beam (65,65'), which is designed to radiate the electromagnetic measuring beam (65,65') into the optical resonator (53,53'), with multiple Reflection on the mirrors (57,57') the electromagnetic measuring beam (65,65') passes through the optical element (51,51') many times and/or is reflected many times by the optical element (51,51'), - at least a detector device (61,61`) for spatially resolved measurement of an intensity distribution (I) of the electromagnetic measuring beam (67,67') emerging from the optical resonator (53,53') and - at least one evaluation device (63,63') for determining a Phase shift distribution (φ) from the intensity distribution (I) of the electromagnetic measuring beam (67,67`) emerging from the optical resonator (53,53'). Optical system (49) according to claim 12 , characterized in that the measuring beam source (59,59') is designed to emit an electromagnetic measuring beam (65,65') which has a coherence length (I _c ) which is at least three times, preferably at least five times, particularly preferably at least is ten times, in particular at least fifty times, as long as an optical path length (d _opt ) of the electromagnetic measuring beam (65,65') in the optical resonator (53,53'). Optical system (49) according to claim 12 or 13 , further comprising a device (69) for setting a distance (d) between the two mirrors (57), which preferably has at least one piezoelectric actuator. Optical system (49) according to one of Claims 12 until 14 , further comprising a useful radiation source (71) for emitting useful electromagnetic radiation (73) which passes through the at least one optical element (51,51',51'') along a predetermined propagation direction (75). Optical system (49) according to one of Claims 12 until 15 , characterized by at least one monitoring device (79) for monitoring the at least one optical element (51) using at least one monitoring parameter (U) determined from the phase shift distribution (φ). Optical system (49) according to Claim 16 , characterized in that the monitoring device (79) is designed to compare the monitoring parameter (U) with at least one reference value (U _R ), in particular at least one value of the monitoring parameter (U) at at least one point in time in the past, and preferably as a function of a deviation of the monitoring parameter (U) of the at least one reference value (U _R ) to issue a notification to a user via an output device (81). Optical system (49) according to claim 15 , characterized by at least one control device for influencing a wave front of the useful electromagnetic radiation (73) as a function of the phase shift distribution (φ) determined by means of the evaluation device (63, 63').

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