Broadband interferometry

Abstract

A broadband interferometric measurement system comprising: (i) an interferometer comprising a tilted reference mirror oriented at a tilt angle with respect to a normal to an optical axis of a reference beam so as to introduce a range of optical path differences along a first axis while ensuring formation of an interference pattern on the sensor; (ii) a movement unit for introducing a relative movement between the sample and the interferometer along a second axis, the second axis being oriented to the first axis so as to virtually scan the range of optical path differences, (iii) processing circuitry for receiving the interference detection signal and reconstructing interferograms of different illumination points on the sample. The measurement beam forms an elongated spot on the sample, the elongated spot having a longitudinal axis oriented to a first axis and a second axis.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Provisional Patent No. 63 / 513,342, filed July 12, 2023, which is incorporated herein by reference in its entirety. Background Technology

[0003] Many scanning optical metrology solutions are available, such as bright-field / dark-field imaging, color confocal scanning microscopy, and various shear interferometry solutions. While these provide a wide range of metrological values ​​on the samples being measured, WLI is known to offer significantly richer information and unique sensitivity to topographic and minute spatial variations, typically achieving sub-nanometer accuracy.

[0004] A common approach to covering large spatial areas in iWLI is based on a move-stop-scan protocol. Here, the measurement head is placed over a measurement area, the interferometer mirror is scanned to obtain a WLI image, and then the measurement site is moved to the new location. The FOV of each such acquisition is limited by the range of the interferometer mirror and associated optics, but high-end systems have proven to cover several centimeters in a single measurement. However, this type of solution is not used for complete coverage of large areas due to the impractical acquisition time involved (especially when considering high volumetric, high TPT metrology).

[0005] Another shortcoming of the current iWLI solution involves the fine control of the interferometer mirrors. This includes the requirement for well-controlled mirror movement and the fine maintenance of their tilt / slant during movement.

[0006] In this invention, a static (non-motorized) interferometer mirror is used, eliminating the need for control or feedback on its position and orientation. While some implementations may include degrees of freedom for the mirror's motion, this proposed method does not require such control.

[0007] There is an increasing need for a cost-effective measurement method for measuring the high-frequency response of samples. Summary of the Invention

[0008] A system, a non-transitory computer-readable medium, and a method are shown in the specification and / or claims and / or drawings. Attached Figure Description

[0009] The subject matter considered to be the invention is specifically pointed out and clearly claimed in the concluding section of this specification. However, the invention, with respect to its organization and operation methods, as well as its objects, features, and advantages, can be best understood by referring to the following detailed description, in conjunction with the accompanying drawings: Figure 1AAn example of the system is shown; Figure 1B An example of the method is shown; Figure 2 An interferogram is shown; Figure 3 Different beams were shown; and Figure 4 Examples of different OPD values ​​associated with the same location examined at different time points are shown. Detailed Implementation

[0010] Figure 1A An example of a broadband interferometric measurement system 10 for evaluating sample 99 is shown. The broadband interferometric measurement system 10 includes optics, processing circuitry 30, and a movement unit 40.

[0011] The optical components include a source 12 and an interferometer 20 configured to provide an input beam 81 of broadband radiation.

[0012] The interferometer 20 includes a beam splitter 22, a measuring arm 24, a reference arm 26, and a sensing unit 28.

[0013] Beam splitter 22 is configured to divide the input beam 81 into a measurement beam 82 and a reference beam 83.

[0014] The reference arm 26 includes a tilted reference mirror 27 oriented at a tilt angle relative to the normal of the optical axis 15 of the reference beam, thereby introducing a range of optical path difference along the first axis 16, while ensuring that an interference pattern is formed on the sensor 29 of the sensing unit 28 between the reflected measurement beam 84 reflected from the sample and the reflected reference beam 85.

[0015] The moving unit 40 is configured to introduce relative movement between the sample 99 and the interferometer 20 during the measurement period and along the second axis 17, which is oriented to the first axis, thereby virtually scanning the range of optical path difference.

[0016] The processing circuit 30 is configured to receive the interference detection signal from the sensing unit and reconstruct the interferogram of different illumination points on the sample.

[0017] The measuring beam 82 forms a spot 89 on the sample, the length of the spot being at least 5 times the width of the spot, and the spot having a longitudinal axis 18 oriented to the first axis and the second axis.

[0018] The elongated light spot increases the throughput of system 10 because it provides a significantly longer field of view perpendicular to the second axis.

[0019] According to the implementation method, the length of the light spot is on the order of centimeters, and the width of the light spot is on the order of tens of micrometers.

[0020] According to the implementation method, the longitudinal axis, the first axis, and the second axis are perpendicular to each other.

[0021] According to the implementation method, the tilt angle is less than half of the numerical aperture of the interferometer.

[0022] According to the implementation method, broadband radiation is white light.

[0023] According to the implementation method, the reference mirror remains stationary during the measurement period.

[0024] By keeping the reference mirror stationary while introducing movement between the interferometer and the sample only along a single axis, the accuracy and robustness of the measurement are greatly increased (it is sensitive to the relative movement between different optical elements of the interferometer), and the use of a highly accurate reference mirror movement unit is also eliminated.

[0025] According to the implementation method, the optical path difference ranges from 1 micrometer to 20 micrometers.

[0026] According to the implementation method, the optical path difference ranges from 5 micrometers to 10 micrometers.

[0027] According to the implementation method, the length of the light spot is at least 10 times greater than the width of the light spot.

[0028] Figure 1B An example of a method 100 for broadband interferometry is shown.

[0029] According to an implementation, method 100 begins with step 110, in which an input beam of broadband radiation is provided to the interferometer from a source.

[0030] According to the implementation, step 110 is followed by step 120, which involves splitting the input beam into a measurement beam and a reference beam using the beam splitter of the interferometer. The reference arm includes a tilted reference mirror oriented at a tilt angle relative to the normal of the optical axis of the reference beam, thereby introducing a range of optical path difference along the first axis while ensuring that an interference pattern is formed on the sensor of the sensing unit between the reflected measurement beam reflected from the sample and the reflected reference beam.

[0031] According to the implementation, step 120 is followed by step 130, which enables the formation of an interference pattern on the sensor.

[0032] Step 130 includes: a. Guide the measurement beam to the sample to form a light spot on the sample. The length of the light spot is at least 5 times the width of the light spot. The light spot has a longitudinal axis oriented to the first axis and the second axis.

[0033] b. Obtain the reflected measurement beam from the sample.

[0034] c. Guide the reflected measurement beam to the sensor.

[0035] d. Guide the reference beam to the tilted reference mirror.

[0036] e. Obtain the reflected reference beam.

[0037] f. Guide the reflected reference beam to the sensor.

[0038] According to the implementation, step 130 is followed by step 140, which receives the interference detection signal from the sensing unit and through the processing circuit.

[0039] Multiple iterations of steps 110, 120, 130, and 140 are performed during the measurement cycle.

[0040] According to an embodiment, method 100 further includes step 150: introducing relative movement between the sample and the interferometer via a movement unit during the measurement period. This movement is performed along a second axis oriented to the first axis, thereby virtually scanning the range of the optical path difference.

[0041] According to an embodiment, method 100 further includes step 160: reconstructing interferograms of different illumination points on the sample based on the interference detection signal and through processing circuitry. The reconstruction can be performed based on the interference detection signal obtained during the measurement period.

[0042] According to the implementation method, the length of the light spot is on the order of centimeters, and the width of the light spot is on the order of tens of micrometers.

[0043] According to the implementation method, the longitudinal axis, the first axis, and the second axis are perpendicular to each other.

[0044] According to the implementation method, the tilt angle is less than half of the numerical aperture of the interferometer.

[0045] According to the implementation method, broadband radiation is white light.

[0046] According to one implementation, the method includes keeping the reference mirror stationary during the measurement period.

[0047] According to the implementation method, the optical path difference ranges from 1 micrometer to 20 micrometers.

[0048] According to the implementation method, the optical path difference ranges from 5 micrometers to 10 micrometers.

[0049] According to the implementation method, the length of the light spot is at least 10 times greater than the width of the light spot.

[0050] The proposed solution is much faster than existing white light interferometry systems, which require several seconds to acquire a single image and are limited to a small area of ​​the sample.

[0051] The proposed solution keeps the reference mirror stationary while introducing movement between the interferometer and the sample only along a single axis. This significantly increases the accuracy and robustness of the measurement (which is sensitive to relative movement between different optical elements of the interferometer) and also eliminates the need for a highly accurate reference mirror movement unit. Movement along a single axis increases throughput because additional movements along different axes are not required—each of which could potentially involve stabilization and calibration processes.

[0052] The proposed solution further provides: a. Obtain high-quality optical metrology information. The obtained data is equivalent to the interferogram obtained through white-light interferometry. Compared with other area measurement solutions, this information is significantly richer and provides improved sensitivity.

[0053] b. High spatial resolution is achieved, at least in part, by maintaining the different optical elements of the interferometer in a fixed spatial relationship.

[0054] c. Use long, elongated light spots formed on the sample to provide large field of view (FOV) coverage.

[0055] d. Mechanically robust, accurate, and simple - using a single axis of motion at a constant speed.

[0056] Figure 2 An interferogram generated by a prior art interferometer is shown. The collected signal (intensity - y-axis) varies with the position of the reference mirror (x-axis), and this signal is represented as the optical path difference (OPD) between the reference arm and the measuring arm, which varies with the position of the reference mirror. The interferogram is represented as follows. (z), where, This indicates the pixel and This indicates the position of the reference mirror. Depending on application characteristics and measurement considerations, the reference mirror may be located at... Scan across the value span.

[0057] Typically, the scanning mirror should be applied across (roughly) the vertical range of the sample to be characterized, adding several times the longest wavelength used. For example, consider a front-end semiconductor device several hundred nm high measured by WLI using wavelengths in the 200-1000 nm range.

[0058] Figure 3 The imaging function of the interferometer was explained. For two imaging points ( and The optical path is presented, and each imaging point is related to the interferometer mirror ( and Interference at corresponding points on the camera ( and Collection is performed at conjugate points on the mirror. This imaging configuration allows for parallel WLI collection across the entire field, during which multiple images are measured while scanning the mirror.

[0059] Return to reference Figure 1A The lighting module generates light in one direction ( Figure 1A A narrow beam of light extending from the "x" in the diagram. This illumination scheme is common in many imaging applications. It utilizes optical elements such as light bars, cylindrical lenses, or other beam-shaping components.

[0060] The beam passes through a beam splitter and is focused onto the sample and reference mirrors, thus producing a long and narrow illumination area. The typical range of the illumination area is in the long dimension ( Figure 1A The "x" in the figure should be greater than approximately 1 cm to take advantage of the large coverage opportunities offered by this method. For reasons explained below, narrow dimensions ( Figure 1A The range of the light spot in “y” should preferably have a range of at least several hundred μm.

[0061] The light reflected from the sample and the mirror is then collected (in a manner similar to that of a standard iWLI) and imaged onto a slender camera.

[0062] The illumination area should preferably be slightly larger than the imaging field of view to ensure uniform illumination intensity.

[0063] However, the imaging area should still cover the span described above: several centimeters or more on the long dimension (“x”) and at least several hundred micrometers on the narrow dimension (“y”).

[0064] Compared to the optical axis, the interferometer mirror is intentionally set at a predetermined angle ( Figure 1A In ): In the standard implementation of iWLI, the mirrors are aligned perpendicular to the optical axis. This ensures that the WLI interferograms measured at each pixel are not offset relative to each other, thus allowing for direct comparison and interpretation of 2D images.

[0065] In the proposed method, the mirror tilt intentionally generates an OPD difference across the measured field. Specifically, in Figure 1A In the middle, the angle introduced is The tilt (when in standard WLI) =90° (when). Therefore, the distance from the center of rotation of the reflector is... Point z' receives offset along the optical path z'= y' cos .

[0066] This implementation is used in off-axis holography, where narrowband illumination is employed. Here, we propose combining such a tilted mirror with white-light interferometry.

[0067] Note that the coordinates of the mirror segment (x', y', z') are labeled corresponding to the coordinates of the sample segment (x, y, z) (see...). Figure 1A ).

[0068] Importantly, it senses tilt. It must be small enough to ensure that the beam reflected from the sample and the beam reflected from the mirror interfere at the camera. This requirement means... Where NA is the numerical aperture value used.

[0069] In the proposed method, while repeatedly acquiring images, Scan samples in the direction. During this time period, at a speed Scan the sample, the points on the sample ( (in sample coordinates) residing on the image ( ) at, among which, Indicates time.

[0070] For simplicity, assume the mirror and sample are aligned along... The points are OPD matched. Here, OPD matching means that the optical path between the BS and the mirror is equal to the optical path between the sample and the mirror (this assumption is only made for simplicity of description and is not important to the overall proposed method). By Equation 1, the points along this line and located at the OPD offset Point interference on the mirror at the location. Points are measured by acquiring multiple images during sample movement, using the range of OPD values ​​spanned by the mirror tilt. ).

[0071] This idea is in Figure 3 The diagram shows, for simplicity, only the interferometer section is presented, and only the principal ray is marked. Consider a point on the sample (denoted by "..."). "Marked"). For images taken when this point is to the left of the FOV, the OPD between the sample and the mirror is at an extreme value ( Figure 4 In example A - As the sample is scanned, the point follows the FOV tracking path, and the OPD changes accordingly; the OPD reaches the OPD matching condition ( Figure 4 Example B, where the distance between the mirror and BS is equal to the distance between the sample and BS) and the subsequent positive OPD mismatch ( Figure 4Example C). Of course, the opposite tilt of the mirror will result in the opposite OPD acquisition order, but it can be used in exactly the same way. By collecting reflections from that point during scanning, one essentially collects its white-light interferogram, where the reference mirror scans the OPD span - arrive

[0072] In order to allow this metrology scheme, several requirements must be met: As explained, the tilt of the reflector is limited by the requirement of overlap of the pupil plane and must be significantly less than half of the system's NA. Typically, this limits the tilt value to a few degrees at most.

[0073] Cross FOV ( Figure 4 2 in The OPD range is preferably a few micrometers to allow for the acquisition of wide interferograms.

[0074] These constraints together require the FOV to span at least several hundred micrometers.

[0075] Of course, the described acquisition method applies to all points within the imaged field of view (FOV). Then, for each imaging point on the sample, its interferometric reflectance is collected from all images where that point is present, resulting in a set of acquired images. (Where x and y represent different positions, and t represents time) is converted into a corresponding set of interferograms. White light interferograms can be obtained by associating the correct OPD mismatches with each image. This operation can be implemented for wide areas across long path scans, thus providing a fast and simple means for WLI measurements.

[0076] When implementing such a device, many optical considerations must be taken into account, as indicated by the stringent requirements of interferometric instruments.

[0077] As mentioned, the range of OPD values ​​is (to some extent) sample-dependent. For ultrathin samples, the required OPD range can be significantly smaller than for thick samples. To allow for this flexibility, the interferometer mirrors can be motorized, allowing them to rotate for optimized metrology depending on the thickness of the sample being measured.

[0078] Conversely, highly rigid and robust implementations are of high value for industrial applications. In such cases, a monolithic implementation may be preferred, in which the reflector is positioned at a predetermined angle and fixed to the BS module.

[0079] In the foregoing detailed description, numerous specific details have been set forth in order to provide a thorough understanding of the invention. However, those skilled in the art will understand that the invention may be practiced without these specific details. In other instances, well-known methods, processes, and components have not been described in detail so as not to obscure the invention.

[0080] The subject matter considered to be the invention is specifically pointed out and clearly claimed in the concluding section of this specification. However, the organization and methods of operation, as well as its objectives, features, and advantages, can be best understood by referring to the following detailed description when read in conjunction with the accompanying drawings.

[0081] It will be understood that, for the sake of simplicity and clarity, the elements shown in the figures need not be drawn to scale. For example, the dimensions of some elements may be enlarged relative to others for clarity. Furthermore, reference numerals may be repeated in the figures where deemed appropriate to indicate corresponding or similar elements.

[0082] Because the embodiments shown in this invention can be implemented largely using electronic components and circuits known to those skilled in the art, the details will not be interpreted to the extent deemed necessary, as shown above, in order to understand and comprehend the basic concepts of this invention and in order not to obscure or distract from the teachings of this invention.

[0083] Any reference to a method in this specification shall be interpreted as applicable to systems capable of performing the method and / or to non-transitory computer-readable media storing instructions for implementing the method.

[0084] Any references to the system in this specification shall be interpreted as applicable to methods that can be performed by the system and / or to non-transitory computer-readable media that can be performed by the system.

[0085] Any reference in this specification to non-transitory computer-readable media shall be interpreted in accordance with the methods implemented by executing instructions stored in such non-transitory computer-readable media and / or with the systems capable of executing such instructions.

[0086] In the foregoing description, the invention has been described with reference to specific examples of embodiments thereof. However, it will be apparent that various modifications and changes can be made to the invention without departing from the broader spirit and scope of the invention as set forth in the appended claims.

[0087] Furthermore, the terms “before,” “after,” “on top of,” “at bottom of,” “above,” “below,” etc., as used in the specification and claims (if any) are for descriptive purposes and are not necessarily used to describe permanent relative positions. It should be understood that such terms are interchangeable where appropriate, such that embodiments of the invention described herein can operate, for example, on orientations other than those shown or otherwise described herein.

[0088] Any arrangement of components that perform the same function is effectively “associated” to achieve the desired function. Therefore, any two components combined in this paper to achieve a specific functionality can be considered “associated” with each other to achieve the desired function, regardless of the architecture or intermediate components. Similarly, any two such associated components can also be considered “operably connected” or “operably coupled” to each other to achieve the desired function.

[0089] Furthermore, those skilled in the art will recognize that the boundaries between the above operations are merely illustrative. Multiple operations can be combined into a single operation, a single operation can be distributed among additional operations, and operations can be performed at least partially overlapping in time. Moreover, alternative implementations may include multiple examples of a particular operation, and the order of operations may be varied in different other implementations.

[0090] Furthermore, for example, in one implementation, the illustrated example can be implemented as circuitry located on a single integrated circuit or within the same device. Alternatively, the example can be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.

[0091] However, other modifications, variations, and alternatives are also possible. Therefore, the specification and drawings are to be regarded as illustrative rather than restrictive.

[0092] In the claims, any reference signs placed between parentheses should not be construed as limiting the claims. The word “comprising” does not exclude the presence of other elements or steps besides those listed in the claims. Furthermore, the terms “a” or “an” as used herein are defined as one or more. Similarly, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed as limiting any particular claim containing such an introductory element to an invention containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and introductory articles such as “a” or “an.” The same applies to the use of definite articles. Unless otherwise specified, terms such as “first” and “second” are used to arbitrarily distinguish the elements described by such terms. Therefore, these terms are not necessarily intended to indicate the time or other priority of such elements. The fact that certain measures are recited in mutually different claims does not mean that combinations of such measures cannot be used advantageously.

[0093] While certain features of the invention have been shown and described herein, many modifications, substitutions, alterations, and equivalents will now occur to those skilled in the art. Therefore, it should be understood that the appended claims are intended to cover all such modifications and alterations falling within the true spirit of the invention.

Claims

1. A broadband interferometric measurement system for evaluating samples, the broadband interferometric measurement system comprising: Optical devices; Processing circuitry; as well as Mobile unit; The optical device includes: a. The source, configured to provide an input beam of broadband radiation; b. Interferometer, including: (ii) Beam splitter, (iii) Measuring arm, (iv) Reference arm, and (v) Sensing unit, The beam splitter is configured to divide the input beam into a measurement beam and a reference beam. The reference arm includes a tilted reference mirror oriented at a tilt angle relative to the normal of the optical axis of the reference beam, thereby introducing a range of optical path difference along the first axis, while ensuring that an interference pattern is formed on the sensor of the sensing unit between the reflected reference beam and the reflected measurement beam reflected from the sample. The moving unit is configured to introduce relative movement between the sample and the interferometer during the measurement period and along a second axis oriented to the first axis, thereby virtually scanning the range of the optical path difference; The processing circuit is configured to receive an interference detection signal from the sensing unit and reconstruct an interferogram of different illumination points on the sample; The measuring beam forms a spot on the sample, the length of the spot being at least 5 times the width of the spot, and the spot having a longitudinal axis oriented to the first axis and the second axis.

2. The broadband interferometric measurement system according to claim 1, wherein, The length of the light spot is on the order of centimeters, and the width of the light spot is on the order of tens of micrometers.

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

4. The broadband interferometric measurement system according to claim 1, wherein, The tilt angle is less than half the numerical aperture of the interferometer.

5. The broadband interferometric measurement system according to claim 1, wherein, The broadband radiation is white light.

6. The broadband interferometric measurement system according to claim 1, wherein, The reference mirror remains stationary during the measurement period.

7. The broadband interferometric measurement system according to claim 1, wherein, The optical path difference ranges from 1 micrometer to 20 micrometers.

8. The broadband interferometric measurement system according to claim 1, wherein, The optical path difference ranges from 5 micrometers to 10 micrometers.

9. The broadband interferometric measurement system according to claim 1, wherein, The length of the light spot is at least 10 times the width of the light spot.

10. A method for broadband interferometry, the method comprising: The source provides a broadband radiation input beam to the interferometer; The input beam is divided into a measurement beam and a reference beam by the beam splitter of the interferometer; wherein the reference arm includes a tilted reference mirror, which is oriented at a tilt angle relative to the normal of the optical axis of the reference beam, thereby introducing a range of optical path difference along the first axis, while ensuring that an interference pattern is formed on the sensor of the sensing unit between the reflected reference beam and the reflected measurement beam reflected from the sample. The range of the optical path difference is virtually scanned by introducing relative movement between the sample and the interferometer during the measurement period by a moving unit and along a second axis oriented to the first axis; The processing circuit receives the interference detection signal from the sensing unit. The processing circuit reconstructs the interference pattern of different illumination points on the sample; The measuring beam forms a spot on the sample, the length of the spot being at least 5 times the width of the spot, and the spot having a longitudinal axis oriented to the first axis and the second axis.

11. The method according to claim 10, wherein, The length of the light spot is on the order of centimeters, and the width of the light spot is on the order of tens of micrometers.

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 less than half the numerical aperture of the interferometer.

14. The method of claim 10, wherein, The broadband radiation is white light.

15. The method of claim 10, comprising: The reference mirror is kept stationary during the measurement period.

16. The method of claim 10, wherein, The optical path difference ranges from 1 micrometer to 20 micrometers.

17. The method according to claim 10, wherein, The optical path difference ranges from 5 micrometers to 10 micrometers.

18. The method according to claim 10, wherein, The length of the light spot is at least 10 times the width of the light spot.

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