WO2025012865A1

WO2025012865A1 - Broadband interferometry

Info

Publication number: WO2025012865A1
Application number: PCT/IB2024/056788
Authority: WO
Inventors: Gilad Barak
Original assignee: Nova Ltd
Current assignee: Nova Ltd
Priority date: 2023-07-12
Filing date: 2024-07-12
Publication date: 2025-01-16
Anticipated expiration: 2026-01-12

Abstract

A broadband interferometry system that includes (i) an interferometer that includes a tilted reference mirror that is oriented by a tilt angle in relation to a normal to an optical axis of a reference beam thereby introducing a range of optical path differences along a first axis while guaranteeing a formation of an interference pattern on a sensor, (ii) a movement unit for introducing a relative movement between the sample and the interferometer, along a second axis that is oriented to the first axis thereby virtually scanning the range of optical path differences, (iii) a processing circuit for receiving interference detection signals and reconstructing interferograms for different points of illumination on the sample. The measurement beam forms an elongated spot on the sample, having a longitudinal axis that is oriented to the first axis and the second axis.

Description

BROADBAND INTERFEROMETRY

CROSS REFERENCE

[001] This application claims priority from US provisional patent serial number 63/513,342 filing date July 12, 2023 which is incorporated herein in its entirety. BACKGROUND OF THE INVENTION

[002] Many scanning optical metrology solutions are available, such as bright- field\dark-field imaging, chromatic confocal scanning microscopy, various shearing interferometry solutions and many more. While these offer extensive metrology value on the measured samples, WLIs are known to offer significantly richer information and unique sensitivities to topography \ minute spatial variations, often reaching sub-nm accuracies. [003] The common approach to covering large spatial extensts in iWLI is based on a move-stop-scan protocol. Here, the measurement head is placed above one measured region, the interferometer mirror is scanned to obtain the WLI image and then the measurement site is changed to a new location. The FOV of each such acquisitions is limited by the extent of the interferometer mirror and associated optics, but high-end systems have demonstrated coverage of a few cm in one measurement. However, no solution of this kind is used in full coverage of large areas, as the involved acquisition times are impractical (especially when considering high- volume, high-TPT metrology).

[004] Another deficiency of current iWLI solutions involves the delicate control over the interferometer mirror. This includes the requirement for well controlled mirror motion as well as fine maintenance of its tip\tilt during motion.

[005] In the current invention, a static (non-motorized) interferometer mirror is used, with no required control or feedback on its location and orientation. While some implementations may also include motion degrees of freedom for the mirror, it is not at all required in the proposed method.

[006] There is a growing need to provide a cost efficient measurement method for measuring high-frequency response of a sample.

SUMMARY

[007] A system, a non-transitory computer readable medium and a method as illustrated in the specification and/or claims and/or drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[008] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

[009] FIG. 1A illustrates an example of a system;

[0010] FIG. IB illustrates an example of a method;

[0011] FIG. 2 illustrates an interferogram;

[0012] FIG. 3 illustrates different beams; and

[0013] FIG. 4 illustrates an example of different OPD values associated with a same location inspected during different points in time.

DETAILED DESCRIPTION OF THE DRAWINGS

[0014] Figure 1A illustrates an example of broadband interferometry system 10 for evaluating a sample 99, the broadband interferometry system 10 includes optics, a processing circuit 30 and a movement unit 40.

[0015] The optics include a source 12 that is configured to provide an input beam 81 of broadband radiation, and interferometer 20.

[0016] Interferometer 20 includes beam splitter 22, measurement arm 24, reference arm 26, and sensing unit 28.

[0017] Beam splitter 22 is configured to split the input beam 81 to a measurement beam 82 and reference beam 83.

[0018] Reference arm 26 includes tilted reference mirror 27 that is oriented by a tilt angle in relation to a normal to an optical axis 15 of the reference beam thereby introducing a range of optical path differences along a first axis 16 while guaranteeing a formation of an interference pattern on a sensor 29 of the sensing unit 28, the interference pattern is formed between a reflected measurement beam 84 reflected from the sample and a reflected reference beam 85.

[0019] Movement unit 40 is configured to introduce a relative movement between the sample 99 and the interferometer 20, during a measurement period, and along a second axis

17 that is oriented to the first axis thereby virtually scanning the range of optical path differences.

[0020] Processing circuit 30 is configured to receive from the sensing unit interference detection signals and to reconstruct interferograms for different points of illumination on the sample.

[0021] The measurement beam 82 forms a spot 89 on the sample, a length of the spot exceeds by at least a factor of five a width of the spot, and the spot has a longitudinal axis

18 that is oriented to the first axis and the second axis. [0022] The elongated spot increases the throughput of the system 10 as it provides a significantly long field of view normal to the second axis.

[0023] According to an embodiment, the length of the spot is of an order of a centimeter and the width of the spot is of an order of tens of microns.

[0024] According to an embodiment, the longitudinal axis, the first axis and the second axis are perpendicular to each other.

[0025] According to an embodiment, the tilt angle is smaller than half of a numerical aperture of the interferometer.

[0026] According to an embodiment, the broadband radiation is white light.

[0027] According to an embodiment, the reference mirror maintains still during the measurement period.

[0028] Maintaining the reference mirror still while introducing only a movement between the interferometer and the sample - along a single axis - greatly increases the accuracy and robustness of the measurements (which are sensitive to relative movements between different optical elements of the interferometer) and also eliminates using highly accurate reference mirror movement units.

[0029] According to an embodiment, the size of the range of optical path differences is between one and twenty microns.

[0030] According to an embodiment, the size of the range of optical path differences is between five and ten microns.

[0031] According to an embodiment, a length of the spot exceeds by at least a factor of ten a width of the spot.

[0032] Figure IB illustrates an example of method 100 for broadband interferometry. [0033] According to an embodiment, method 100 starts by step 110 of providing, by a source, an input beam of broadband radiation to an interferometer.

[0034] According to an embodiment, step 110 is followed by step 120 of splitting, by a beam splitter of the interferometer, the input beam to a measurement beam and a reference beam. The reference arm includes a tilted reference mirror that is oriented by a tilt angle in relation to a normal to an optical axis of the reference beam thereby introducing a range of optical path differences along a first axis while guaranteeing a formation of an interference pattern on a sensor of the sensing unit between a reflected measurement beam reflected from the sample and a reflected reference beam.

[0035] According to an embodiment step 120 is followed by step 130 of enabling a formation of interference patterns on the sensor. [0036] Step 130 includes: a. Directing the measurement beam to the sample to form a spot on the sample. A length of the spot exceeds by at least a factor of five a width of the spot. The spot has a longitudinal axis that is oriented to the first axis and to a second axis. b. Obtaining a reflected measurement beam reflected from the sample. c. Directing the reflected measurement beam to the sensor. d. Directing the reference beam to the tilted reference mirror. e. Obtaining the reflected reference beam. f. Directing the reflected reference beam to the sensor.

[0037] According to an embodiment step 130 is followed by step 140 of receiving interference detection signals from the sensing unit and by a processing circuit.

[0038] Multiple iterations of steps 110, 120, 130 and 140 are performed during a measurement period.

[0039] According to an embodiment, method 100 also includes step 150 of introducing, by a movement unit, a relative movement between the sample and the interferometer, during the measurement period. The movement is made along a second axis that is oriented to the first axis thereby virtually scanning the range of optical path differences.

[0040] According to an embodiments, method 100 also includes step 160 of reconstructing, based on interference detection signals and by the processing circuit, interferograms for different points of illumination on the sample. The reconstruction may be executed based on the interference detection signals obtained during the measurement period.

[0041] According to an embodiment, the length of the spot is of an order of a centimeter and the width of the spot is of an order of tens of microns.

[0042] According to an embodiment, the longitudinal axis, the first axis and the second axis are perpendicular to each other.

[0043] According to an embodiment, the tilt angle is smaller than half of a numerical aperture of the interferometer.

[0044] According to an embodiment, the broadband radiation is white light.

[0045] According to an embodiment, the method includes maintaining the reference mirror still during the measurement period. [0046] According to an embodiment, the size of the range of optical path differences is between one and twenty microns.

[0047] According to an embodiment, the size of the range of optical path differences is between five and ten microns.

[0048] According to an embodiment, a length of the spot exceeds by at least a factor of ten a width of the spot.

[0049] The suggested solution is much faster than prior art white light interferometry systems in which multiple seconds were required for a single image acquisition and were limited to small regions of the sample.

[0050] The suggested solution maintains the reference mirror still while introducing only a movement between the interferometer and the sample - along a single axis - which greatly increases the accuracy and robustness of the measurements (which are sensitive to relative movements between different optical elements of the interferometer) and also eliminates using highly accurate reference mirror movement units. The movement along the single movement increases the throughput as there is no need to perform additional movements along different axis - each movement may require a stabilization and calibration process.

[0051] The suggested solution further provides: a. Obtaining high-quality optical metrology information. The obtained data is equivalent to the interferograms obtained by white light interferometry. This information is significantly richer and offers improved sensitivity compared to other areal metrology solutions. b. Obtaining high spatial resolution - based at least in part on maintaining the different optical elements of the interferometer at a fixed spatial relationship. c. Large Field of View (FOV) coverage is provided using a long elongated spot formed on the sample. d. Being mechanically robust, accurate and simple - using a single-axis motion at constant speed.

[0052] Figure 2 illustrates an interferogram generated by a prior art interferometer. The collected signal (intensity - y-axis) as a function of reference mirror position (x-axis) - expressed as the Optical Path Difference (OPD) between the reference arm and the measurement arm - which is changes as the reference mirror position is changed. The interferogram is denoted fmeas.(^z)’ with ^x> y denoting the pixel and z the reference mirror position. The reference mirror is scanned across a span of z values depending on the application characteristics and measurement considerations.

[0053] Typically, the mirror should be scanned across (roughly) the vertical extent which requires characterization on the sample, added several times the longest wavelength used. Consider for example front-end semiconductor devices several hundred nm tall, measured by a WLI utilizing wavelengths in the range of 200-1000nm.

[0054] Figure 3 clarifies the imaging functionality of an interferometer. Light paths are presented for two imaged points (Pi and P2), each interfering with a corresponding point on the interferometer mirror (Qi and Q2) and collected at conjugate points on the camera (Ci and C2). This imaging layout allows concurrent WLI collection of an entire field, during which multiple images are measured while the mirror is scanned.

[0055] Referring back to figure 1A - an illumination module creates an elongated beam, extended in one direction (‘x’ in Fig. 1A). Implementation of such an illumination scheme is commonplace in many imaging applications. Such optical elements as light bars, cylindrical lenses or other beam shaping elements are used.

[0056] This beam passes through a beam splitter and focused on the sample and a reference mirror, creating long and narrow illuminated regions. Typical extents for the illuminated areas should be larger than ~lcm in the long dimension (‘x’ in Fig. 1A) - to exploit the large coverage opportunity offered by this method. For reasons explained below, the spot extent in the narrow dimension (‘y’ in Fig. 1 A) should preferably have an extent of at least a few hundred pm.

[0057] Light reflected from the sample and mirror is then collected (in a similar manner to standard iWLI) and imaged onto an elongated camera.

[0058] The illuminated area should preferably be somewhat larger than the imaged field of view, to guarantee homogeneous illumination intensity.

[0059] However, the imaged area should nevertheless cover a span as described above: few\many cm in the long dimention (‘x’) and at least a few hundred microns in the narrow dimension (‘y’).

[0060] The interferometer mirror is intentionally set at a predetermined angle (0 in Fig. 1A) compared to the optical axis:

[0061] In a standard implementation of iWLI, the mirror is aligned perpendicular to the optical axis. This guarantees the measured WLI interferograms on each pixel are not offset with respect to each other, allowing straightforward comparison and interpretation of the 2D image.

[0062] In the proposed approach, the mirror tilt intentionally creates an OPD difference across the measured field. Specifically, in Fig. 1A a tilt of angle 0 is introduced (when in a standard WLI 0=90°). Consequently, a point distanced by Az' from the mirror center of rotation receive an offset Az' = Ay' cos 9 along the optical path.

[0063] Such implementation is used in off-axis holography, where a narrow-band illumination is used. Here, we propose combining such a tilted mirror with a white-light interferometric measurement.

[0064] Note that coordinates for the mirror segment are marked (x’,y’,z’) in correspondence with coordinates at the sample segment (x,y,z) (see Fig. 1A).

[0065] Importantly, the induced tilt 0 must be small enough so as to guarantee the beam reflected from the sample and the beam reflected from the mirror interfere at the camera.

NA

This requirement implies 9 « with NA the used numerical aperture value.

[0066] In the proposed approach, the sample is scanned in the y direction while images are repeatedly acquired. During this time, the sample is scanned at speed v, a point (x₀, y₀) on the sample (in the sample coordinates) resides at (x₀, y₀ + vt) on the image, with t representing time.

[0067] Assume for simplicity the mirror and sample are OPD-matched for points along y₀ = 0. Here, OPD-matching meaning that the optical path between BS and mirror equals the optical path between the sample and mirror (this assumption is just taken for simplicity of description, and not important for the overall proposed method). By Eq. 1, points along this line interfere with points on the mirror situated at OPD offset of Ay' = vt cos 9. By acquiring multiple images during the sample motion, point (x₀, y₀) is measured with the range of OPD values spanned by the mirror tilt.

[0068] This idea is illustrated in Fig. 3, where for simplicity only the interferometer segment is presented, with just the chief-ray marked. Consider one point on the sample (marked by “*’). For an image taken when this point resides at the left side of the FOV, the OPD between sample and mirror is at an extreme value (-A in Fig. 4 example A). As the sample is scanned, this point traces a path along the FOV with the OPD correspondingly varying; the OPD reaches OPD-matching conditions (Fig. 4 example B, where the distance between mirror and BS is equal to the distance between sample and BS) and later positive OPD mismatch (Fig. 4 example C). Of course, an opposite tilt of the mirror would lead to a reversed OPD acquisition order, but can be used in exactly the same way. By collecting the reflection from this point as it is scanned, one essentially collects the its white-light interferogram with the reference mirror scanning the OPD span of -A to A

[0069] In order to allow such a metrology scheme, few requirements must be answered: [0070] As explained, the mirror tilt is limited by the requirement for pupil-plane overlap and must be significantly smaller than half the system NA. Commonly, this would limit the tilt value to a few degrees at most.

[0071] The OPD span across FOV (2A in Fig. 4) is preferably a few microns, to allow acquisition of a wide interferogram.

[0072] Together, these constraints require the FOV to span at least a few hundred microns.

[0073] Of course, the described acquisition applies for all points in the imaged FOV. The set of acquired images - fmeas.C (with x, y representing the different locations and t time) is then converted into a corresponding set of interferograms by collecting - for each imaged point on the sample its interferometric reflectivity from all images where this point is present. By associating the correct OPD mismatch corresponding to each image, one can obtain the White-Light Interferogram I meets. (^z)- This operation can be implemented for a wide area, scanned across long paths, providing a fast and simple means for WLI measurements.

[0074] Many optical considerations must be taken into account when implementing such a device, as dictated by the stringent requirements from interferometric tools.

[0075] As mentioned, the span on OPD values is (to some degree) sample-dependent. For ultra- thin samples, the required OPD span can be significantly smaller than for thick samples. To allow such flexibility, the interferometer mirror can be motorized, allowing its rotation for optimized metrology depending on the measured sample thickness.

[0076] Conversely, highly rigid and robust implementations are of high value for industrial applications. In such cases, a monolithic implementation may be preferred, with the mirror set at a predetermined angled and fixed to the BS module.

[0077] In the foregoing detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. [0078] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings.

[0079] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

[0080] Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

[0081] Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method and/or should be applied mutatis mutandis to a non-transitory computer readable medium that stores instructions for implanting the method.

[0082] Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system and/or should be applied mutatis mutandis to a non-transitory computer readable medium executable by the system.

[0083] Any reference in the specification to a non-transitory computer readable medium should be applied mutatis mutandis to a method implemented by executing instructions stored in the non-transitory computer readable medium and/or should be applied mutatis mutandis to a system capable to execute the instructions

[0084] In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims.

[0085] Moreover, the terms “front, ” “back, ” “top, ” “bottom, ” “over, ” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

[0086] Any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected, " or "operably coupled, " to each other to achieve the desired functionality.

[0087] Furthermore, those skilled in the art will recognize that boundaries between the above described operations are merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

[0088] Also, for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.

[0089] However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

[0090] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps than those listed in a claim. Furthermore, the terms “a” or “an, ” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first" and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

[0091] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

WE CLAIM

1. A broadband interferometry system for evaluating a sample, the broadband interferometry system comprises: optics; a processing circuit; and a movement unit; wherein the optics comprises: a. a source that is configured to provide an input beam of broadband radiation; b. an interferometer that comprises:

(ii) a beam splitter,

(iii) a measurement arm,

(iv) a reference arm, and

(v) a sensing unit, wherein the beam splitter is configured to split the input beam to a measurement beam and a reference beam; the reference arm comprises a tilted reference mirror that is oriented by a tilt angle in relation to a normal to an optical axis of the reference beam thereby introducing a range of optical path differences along a first axis while guaranteeing a formation of an interference pattern on a sensor of the sensing unit between a reflected measurement beam reflected from the sample and a reflected reference beam; wherein the movement unit is configured to introduce a relative movement between the sample and the interferometer, during a measurement period, and along a second axis that is oriented to the first axis thereby virtually scanning the range of optical path differences; a processing circuit the is configured to receive from the sensing unit interference detection signals and to reconstruct interferograms for different points of illumination on the sample; wherein the measurement beam forms a spot on the sample, a length of the spot exceeds by at least a factor of five a width of the spot, and the spot has a longitudinal axis that is oriented to the first axis and the second axis.

2. The broadband interferometry system according to claim 1 wherein the length of the spot is of an order of a centimeter and the width of the spot is of an order of tens of microns.

3. The broadband interferometry system according to claim 1, wherein the longitudinal axis, the first axis and the second axis are perpendicular to each other.

4. The broadband interferometry system according to claim 1 , wherein the tilt angle is smaller than half of a numerical aperture of the interferometer.

5. The broadband interferometry system according to claim 1, wherein the broadband radiation is white light.

6. The broadband interferometry system according to claim 1, wherein the reference mirror maintains still during the measurement period.

7. The broadband interferometry system according to claim 1, wherein the size of the range of optical path differences is between one and twenty microns.

8. The broadband interferometry system according to claim 1, wherein the size of the range of optical path differences is between five and ten microns.

9. The broadband interferometry system according to claim 1, wherein a length of the spot exceeds by at least a factor of ten a width of the spot.

10. A method for broadband interferometry, the method comprises: providing, by a source, an input beam of broadband radiation to an interferometer; splitting, by a beam splitter of the interferometer, the input beam to a measurement beam and a reference beam; wherein the reference arm comprises a tilted reference mirror that is oriented by a tilt angle in relation to a normal to an optical axis of the reference beam thereby introducing a range of optical path differences along a first axis while guaranteeing a formation of an interference pattern on a sensor of the sensing unit between a reflected measurement beam reflected from the sample and a reflected reference beam; introducing, by a movement unit, a relative movement between the sample and the interferometer, during a measurement period, and along a second axis that is oriented to the first axis thereby virtually scanning the range of optical path differences; receiving, by a processing circuit, from a sensing unit interference detection signals; reconstructing, by the processing circuit, interferograms for different points of illumination on the sample; wherein the measurement beam forms a spot on the sample, a length of the spot exceeds by at least a factor of five a width of the spot, and the spot has a longitudinal axis that is oriented to the first axis and the second axis

11. The method according to claim 10 wherein the length of the spot is of an order of a centimeter and the width of the spot is of an order of tens of microns.

12. The method according to claim 10, wherein the longitudinal axis, the first axis and the second axis are perpendicular to each other.

13. The method according to claim 10, wherein the tilt angle is smaller than half of a numerical aperture of the interferometer.

14. The method according to claim 10, wherein the broadband radiation is white light.

15. The method according to claim 10, comprising maintaining the reference mirror still during the measurement period.

16. The method according to claim 10, wherein the size of the range of optical path differences is between one and twenty microns.

17. The method according to claim 10, wherein the size of the range of optical path differences is between five and ten microns.

18. The method according to claim 10, wherein a length of the spot exceeds by at least a factor of ten a width of the spot.