Single-shot, multi-spectral interferometry

Abstract

An optical interferometry system and associated methods are provided for interferometric measurement of topography of a sample region having a first direction in which a topographic metric is relatively uniform compared with an orthogonal direction of higher variation. A sensor having a two-dimensional array of pixels receives radiation including interference between reflections from the sample and from a tilted reference mirror. The tilt introduces, at the sensor, a gradient direction of the optical path difference (OPD), such that lines of pixels along the gradient direction correspond to positions of the sample region aligned in the direction of relative constancy. From a single captured image, an interferogram of OPD versus intensity is generated for each such line of pixels. A processor then calculates, from the interferogram, the corresponding topographic metric, providing a topographic profile of the sample region without scanning motion of the mirror or sample.

Description

SINGLE-SHOT, MULTI-SPECTRAL INTERFEROMETRYFIELD OF THE INVENTION

[0001] The present invention relates generally to the field of optical inspection, and in particular to techniques of interferometry for structural metrology.BACKGROUND

[0002] Multi-Spectral Interferometry (MSI), including White Light Interferometry (WLI), is a flexible optical characterization technique, particularly useful for topography metrology. MSI provides extensive information about a measured structure and is commonly used to deduce both surface and beneath-surface topography, as well as structural details. WLI metrology is especially common in semiconductor process control to provide spatial characterization of wafers during processing.

[0003] A WLI measurement typically consists of acquiring multiple images of a region of a sample while varying the optical path difference (OPD) between a reference mirror and the sample plane. The OPD can be changed by moving either the reference mirror, the sample, or both. Based on multiple images, an interferogram is generated for each captured pixel, the interferogram being a graph of light intensity as a function of OPD. A common WLI protocol follows a move-stop-scan process, whereby a measurement head is placed above a region, the reference mirror is moved to obtain multiple images of the region with different OPDs, and then the measurement site is changed to focus on a new region.

[0004] The measured signal at any pixel on the interferometry sensor is given

[0005] The first two terms,correspond to the reflectivity of the sample and mirror, respectively, and are independent of the OPD. The third term represents the interference between the light from the mirror and from the sample and is used to determine the OPD, written in the equation as Az. The value of OPD is defined as where zmis the position of the reference mirror, and zsis the surfaceheight of the sample at the corresponding point. For example, when the mirror is fixed andthe sample height is changed, the OPD changes as a function of

[0006] is the field reflected from the sample from a given pointcorresponding to the pixel as a function of wavelength . Similarly is thefield reflected from the corresponding point of the reference mirror. The cosine term in the third term of the equation represents the interference modulation at each wavelength as the OPD changes.

[0007] The equation formay also be modified to account for system -relatedeffects, such as light source spectral power density, optical transmissions, and detector collection efficiency. These modifications are typically accounted for by known calibration methods.

[0008] In typical interferometry, a range of values of are acquired to generate aninterferogram for each point of a sample corresponding to all points of the sensor.Using a cosine transform (or similar data processing based on Fourier analysis) over Az, and by separate calibration of the mirror reflectivity the sample field reflectivitycan be obtained, so as to solve for the desired topography information.

[0009] A recent innovation in WLI measurement was described by the applicant of the present invention, Nova Ltd., in the international patent application PCT / IB2024 / 056788, published as WO2025012865A1, and titled “Broadband Interferometry.” The application describes using a tilted reference mirror, such that there is a gradient of OPD for any given region of a wafer captured as an image by the interferometry sensor. Given that a single image includes this OPD gradient, an interferogram can be generated for each point of the wafer by moving the wafer for each successive image, thereby exposing each point to a series of successive OPD values. The same number of images must be taken as for the move-stop-scan process described above, but the speed and resolution are improved because the reference mirror can be held at a fixed position, reducing variations caused by mechanical motion of the mirror.SUMMARY

[0010] The present invention provides a system and method for topographic characterization of a sample by interferometry, in particular for characterizing a sample region that has a first direction of topography that is relatively uniform and an orthogonalsecond direction of higher variation. As described below, the system is particularly relevant for characterizing wafer edges, which typically narrow towards the edge.

[0011] The system includes a sensor having a two-dimensional array of pixels, and a reference mirror oriented with a tilt angle to create a gradient direction of optical path difference (OPD) across the sensor, that is a variation of OPD along one axis. A processor is configured to analyze intensity data acquired by the sensor. The OPD gradient direction corresponds to the direction of lower variation of the sample region, such that a line of pixels of the sensor in the gradient direction corresponds to a line in the sample region that is substantially uniform for one or more topographic metrics of interest, such as surface height or other vertical metrics.

[0012] During operation, the sensor receives radiation from the sample region, the radiation including interference between light reflected from the sample and light reflected from the tilted reference mirror. The sensor generates an image of intensity at each pixel, which inherently encodes a range of OPD values along the gradient direction. The processor then generates, from the image, one or more interferograms of intensity versus OPD for respective linear sets of pixels extending in the gradient direction.

[0013] From these interferograms, the processor calculates the one or more metrics of topography, using standard interferometric analysis techniques. By generating multiple interferograms for parallel lines corresponding to adjacent positions along the direction of higher variation, the system produces a continuous topographic profile of the sample region. Because the required range of OPD values is captured in a single image, the measurement can be completed without moving the mirror or sample to modify OPD. The system thereby enables high-speed, high-precision, and vibration-robust characterization, suitable for in-line metrology applications such as wafer-edge inspection.

[0014] The system and methods provided by the present invention are of particular value in characterizing topography of wafer edges, which typically exhibit the phenomenon described above of having “slow” variations in one direction, parallel to the wafer circumference (the “tangential” direction), while having “fast” variations in the radial direction. For example, variations in the tangential direction may be significant only across multiple millimeters, whereas, in the radial direction, significant topography variations may be present across tens of microns or even less. Extremely detailed characterization isrequired for the most extreme few mm of a wafer edge, to guarantee that the topography complies with processing requirements, such as Advanced Packaging (AP) integration.BRIEF DESCRIPTION OF DRAWINGS

[0015] For a better understanding of various embodiments of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings. Structural details of the invention are shown to provide a fundamental understanding of the invention, the description, taken with the drawings, making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

[0016] In the accompanying drawings:

[0017] Fig. 1 is a schematic diagram of an interferometry measurement system for metrology of sample region having topography varying in one direction, such as a wafer edge, according to embodiments of the present invention;

[0018] Fig. 2A is a schematic diagram of a sample showing a region at a wafer edge under test and corresponding interferograms of linear cross-sections. Fig. 2B is a topographic profile representing a wafer surface at a wafer edge, for a cross section in the radial direction, as derived from the interferograms, according to embodiments of the present invention; and

[0019] Fig. 3 is a flow diagram of steps performed in the operation of an interferometry measurement system for metrology of a region with one dimensional topographic variation, such as a wafer edge, according to embodiments of the present invention.DETAILED DESCRIPTION

[0020] For a better understanding of the system and methods, reference is now made to the figures, which illustrate representative embodiments of the invention. Taken together, these figures demonstrate how the system architecture, optical conditioning, and coordinated process control work in combination to enable stable, accurate, and high- throughput optical metrology.

[0021] Fig. 1 illustrates an interferometry system 10 for inspection of a sample 20 (e.g., a semiconductor wafer), the sample being mounted on a sample stage 22.

[0022] A light source 30, typically a broadband, white light source, generates a beam of radiation that is focused by interferometer optics 32a towards a beam splitter 34. From the beam splitter, a portion of the beam is directed towards a region of sample 20 (the region indicated in the figure as P1-P2) and a second portion of the beam is directed towards a corresponding region of a reference mirror 36 (the corresponding region indicated as Ql- Q2).

[0023] Optics 32b of the interferometry system then direct reflected beams of radiation from the sample 20 and from the mirror 36 towards a sensor 40, the radiation covering a range at the sensor indicated in the figure by C1-C2.

[0024] The interference pattern formed by superposition of the reference and sample beams is captured as a two-dimensional intensity distribution by the sensor 40. Each pixel records interference between a defined pair of points — one on the sample region and one on the corresponding conjugate position of the reference mirror — ensuring spatially resolved interferometric sampling across the field.

[0025] In embodiments of the present invention, the reference mirror is tilted, such that the optical path difference (OPD) between the reference mirror and the sample plane changes across the illuminated area of the reference mirror, in turn causing a gradient of change in OPD at the sensor. The tilt is typically introduced about an axis corresponding to the direction of faster variation of the sample (e.g., a radial direction at a wafer edge), so that the resulting OPD gradient extends along the orthogonal direction of slower variation, that is, the direction of relatively uniform topography. Herein, the relative uniformity is understood to be relative to the orthogonal direction, and, more generally, means that the variation in that direction is relatively insignificant for determining subsequent manufacturing processes.

[0026] A processor 50 receives the intensity signals from the sensor 40 and executes instructions stored in memory to perform calibration, signal processing, and topographic analysis as described further hereinbelow with respect to Figs. 2-3. Data acquisition steps performed by the processor, as described below with reference to Fig. 3, may include coordinating illumination timing, frame capture, and interferogram computation. Such steps typically include communication with and control of system elements such as the light source 10, the sample stage 22, the reference mirror, and external computing systems fordata storage and visualization. During system setup, calibration may be performed by measuring mirror reflectivity Em( ), spectral response of the source, and transmission of optical elements, so that subsequent data can be normalized to remove system-dependent amplitude or phase effects.

[0027] In embodiments of the present invention, the interferometry system is configured to measure topography of a region of a sample that has a first direction of variation that is lower than an orthogonal direction of higher variation. An example of such a region is the edge of a manufactured silicon wafer, which is typically a few millimeters wide, or less. The edge is typically beveled, that is, it thins in the radial direction, towards the outer circumference. In this radial direction, height variations are typically significant across a few tens of microns or even less. By comparison, height variations in the orthogonal direction, parallel to the wafer circumference, are generally much smaller, with changes in the orthogonal direction typically being consequential only over lengths of multiple millimeters.

[0028] Characterization of the edge topography is becoming increasingly important, in particular for Advanced Packaging (AP) involving bonding of multiple wafers together. Fine control over the wafer edge topography is critical to ensure successful wafer bonding.

[0029] Fig. 2A shows an exemplary sample 20 under test, the sample shown as a typical semiconductor wafer, with the interferometer focus on a region 200 at an edge 202 of the wafer. The edge typically falls outside of a patterned region 204 of the wafer. Typically, the wafer edge is tested after deposition of multiple dielectric layers, indicated as layers 210 and 220.

[0030] The wafer edge typically has a radial direction x of high variation and a tangential direction y of low variation. For many situations, the low variation in the y direction means that the variations are negligible with respect to characterization requirements.

[0031] As described above, the reference mirror of the system is tilted to produce a gradient of optical path difference (OPD) along the direction y of slower variation. Each pixel of the sensor 22 receives interference radiation from a corresponding position on the sample region 200 and from an optically conjugate position on the reference mirror 30. The variation of OPD along the y direction allows a single captured image to contain intensitydata representing different OPD values for linear sets of pixels aligned in the y direction (i.e., all pixels having the same x value). Because the OPD varies continuously along the y-direction, each captured image inherently contains, within a single exposure, intensity data representing a sequence of OPD values, effectively substituting spatial position along y for the temporal OPD scan of conventional WLI. From the data provided by the image, the processor 20 then generates an interferogram of relative OPD versus intensity for linear sets of pixels extending in the y direction.

[0032] This alignment of linear sets of pixels thus provides corresponding interferograms 250, each interferogram corresponds to a fixed radial coordinate x (that is, a position along the direction of higher variation) and indicates an intensity variation along the direction y due to the OPD gradient. By analyzing multiple such interferograms obtained at successive x positions, the processor reconstructs the full topographic profile along the radial direction. Standard calculations may be used to extract desired topographic information, such as surface height, as well as other vertical, typographic details of surface and subsurface features. In short, a single image, obtained with elements of the system in fixed positions (e.g., the mirror, the sample, and the optical lenses) provides the data needed for full topographic characterization of the target region.

[0033] Mathematically, the process may be represented by revising the equation for intensity vs. OPD at pixels in the image. In its standard form, the equation for intensity for any given pixel (x,, yt) can be given by:

[0034] where is a pixel position in the sensor, and is the OPDbetween the mirror and sample at point Given the slow sample variability along the y direction,

[0035] The equation indicates that the role of Az as a variable that is “scanned” to provide different values during the standard WLI measurement is now played by the coordinate yi, with the added prefactor cos 0 . Because a single images includesmeasurements spanning a range of yia full interferogram can be extracted from a single image. The system thereby allows full characterization of the measurement structure - including vertically-resolved information, including characterization of buried underlayers, as for any WLI interferogram interpretation, typically by applying additional calculations, such as applying a cosine transform or Fourier-domain operation over the OPD in the equation above. Following the transform, and after applying a calibration of mirror reflectivity the processor can isolate the sample field reflectivityUsing the known source spectrum and the measured interferogram intensity theprocessor can form a spectral distribution By dividing by(and optionally by S(k) the complex sample field is recovered and can beresampled wavenumber. An inverse transform then yields a coherence envelope .4 (Az), whose peak identifies the OPD corresponding to the sample surface at that pixel set. The position of maximum correlation (the envelope peak) defines a surface OPD Azpeak, which the processor converts to a surface height:where neffis the effective refractive index of the medium. Each height value thus determined represents the topography of the sample at the corresponding radial coordinate x. By processing multiple parallel interferograms 250 at successive radial positions, the processor can also generate a full topographic profile 230 along the radial direction of higher variation.

[0036] Fig. 3 is a flow diagram illustrating an example process of measuring the topography of the sample region using the system described above. The process begins by providing broadband illumination to the optical system of Fig. 1 and proceeds through a sequence of steps performed by the processor 50 based on radiation intensity signals acquired from the sensor 40.

[0037] A first step 302 of the process includes acquiring interference radiation from the sample region. Broadband radiation from source 30 is divided by optics 32a and a beam splitter 34 into sample and reference beams that are directed respectively toward the sample 20 and the tilted reference mirror 34. The mirror is oriented with a tilt angle so as to introduce a varying OPD across the measured field. Specifically, a tilt of angle 0 isintroduced (as opposed to a standard WLI configuration with tilt angle 0 = 0°). A point distanced by y' from the mirror center of rotation (i.e., of tilt) receives an offset ofAz' = y' cos 0 along the optical path. The radiation collected at the sensor thus forms a two-dimensional array of interference intensities representing OPD variation along the y axis (the “gradient direction”). The two-dimensional array is raw data input for subsequent interferogram generation. In this configuration, the OPD variation produced by the mirror tilt substitutes for the scanning motion conventionally used to vary mirror position, allowing the system to capture a range of OPD values within a single frame. (It may be noted that by varying the system optics, and / or mirror surface, the mathematical relationship between OPD and the y extent can also be adjusted.)

[0038] It should be noted further that the tilt angle 0 must be small enough so as to guarantee the beam reflected from the sample and the beam reflected from the mirrorNA interfere at the camera. This requirement implies that cos 0 is significantly less than — ,NA being the numerical aperture of the imaging optics. Because both the tilt angle 0 andNA the numerical aperture (NA) are dimensionless, the inequality cos 0 < — expresses a geometric overlap requirement to set the reflected beams from the sample and the reference mirror within the collection cone of the imaging optics. That is, the tilt should be small enough to ensure that both the sample and mirror reflected beams have full-field overlap within the lens’s paraxial region.

[0039] Before or during step 302, the processor 50 may perform calibration of the system, including measurement of mirror reflectivity £’m(A), source spectral response, and optical transmission characteristics, so that subsequent data may be normalized. The sample or wafer stage may be positioned or translated so that different radial positions are brought into the field of view. Each pixel of the sensor detects interference intensity from an optically conjugate position of the reference mirror and produces corresponding intensity signals forming a two-dimensional data frame in which the OPD varies along the first axis.

[0040] A second step 304 of the process includes generating interferograms of OPD versus intensity from the acquired sensor data. The processor removes DC components corresponding to mirror-only and sample-only reflections and typically normalizes thedetected intensity to compensate for source spectrum and detector efficiency. Based on the known angle of the mirror tilt, the processor associates a linear set of pixels extending in the y direction with the known range of OPD values. For each such set, the processor generates an interferogram representing intensity variation with OPD. The data may be resampled to a uniform wavenumber grid and dispersion-compensated before analysis. The resulting interferograms represent the interference information for multiple radial positions of the sample region, as described above with respect to Fig. 2A.

[0041] A third step 306 of the process includes calculating topography metrics of the sample from the generated interferograms, using known methods of interferometry. The processor may apply a cosine transform or other Fourier-domain analysis over the range of each interferogram to determine values of OPD corresponding to wafer features, such as, for example, surface height, buried interface depth, film thickness, step height, slope, curvature, or combinations thereof, relative to a defined reference plane. Each topographic metric represents the topography of the sample at the corresponding radial coordinate. By repeating the calculation for multiple parallel linear sets of pixels at successive radial positions, and optionally applying spectral or phase-based refinement, the processor constructs a topographic profile along the radial direction of higher variation and generates a continuous two-dimensional topographic representation of the measured region.

[0042] Several additional considerations and optional enhancements may be applied in connection with the process of Fig. 3. A first consideration relates to selection of the mirror tilt. As derived above, the total OPD span measurable in a single image is given by Ay(cos 0), where Ay represents the total image length along the y-axis of slow variation. The tilt angle 0 should be sufficiently small to ensure that reflected beams from the sample and from the mirror continue to overlap and interfere at the sensor plane, yet large enough to provide the required OPD span for the target sample. The required span depends on the optical thickness of the sample under test. For thicker applications, a wider OPD range may be desired, in which case the system may acquire several snapshots at different OPD offsets, for example by translating the mirror or the sample vertically between frames. Nevertheless, even in such cases, the number of images remains far fewer than for conventional WLI scanning.

[0043] In some embodiments, the system may perform measurement while the sample is in motion. Because a complete interferogram can be obtained from a single capturedframe, the process allows acquisition during translation or rotation of the wafer. For example, the system may capture images continuously or intermittently during wafer rotation, thereby collecting topography data circumferentially around the entire wafer edge. This mode of operation enables high-throughput, in-line metrology without interrupting process flow. To further enhance such motion-enabled acquisitions, illumination may be strobed, reducing any residual image blur due to motion or vibration. Because each interferogram is derived from a single exposure rather than multiple synchronized frames, the strobe pulses do not require synchronization with mirror motion or OPD modulation.

[0044] The system may further permit adjustment or optimization of the OPD span and interferogram sampling density according to sample properties. The mirror tilt angle 0 directly affects the OPD gradient across the field. A larger tilt yields a wider OPD span per image, suitable for thicker or multilayered samples, while a smaller tilt increases sampling resolution across a narrower OPD range, improving signal -to-noise ratio and phase sensitivity for thin-film or shallow-structure measurements. The processor or control unit may therefore select or dynamically adjust the mirror tilt to match the intended measurement regime.

[0045] The proposed single-shot interferometry offers several technical advantages relative to conventional techniques. Measurement speed is substantially improved, with full interferometric data captured in milliseconds rather than seconds, thereby improving both throughput and sampling resolution of the wafer edge. The short acquisition time also confers robustness to environmental vibration, reducing the need for vibration isolation. Furthermore, the static mirror configuration simplifies system design by removing the need for dynamic mirror control or feedback to maintain OPD synchronization. Because all OPD values are simultaneously encoded in the captured image, interferometric spacing between pixels is determined with high precision by the known pixel pitch 6pix along the y direction and the cosine of the mirror tilt angle. Accurate OPD differentials are therefore determined without mechanical actuation or timing control.

[0046] The described steps are performed by instructions executed by processor 50 or by equivalent computing logic. With respect to the flow chart described above, it is to be understood that blocks may be implemented in a different order than shown, including concurrent or reversed execution, and may also be realized by special purpose hardware or by a combination of hardware and instructions. It also is to be understood that embodimentsof the invention may include a system, a method, and / or a computer program product. A computer program product may include a computer readable storage medium having instructions stored thereon for causing the control unit, or other processing elements, to carry out aspects of the invention.

[0047] In some embodiments, a non-transitory computer-readable medium stores instructions that, when executed by a processor, cause the system to generate interferograms and compute topographic metrics as described herein.

[0048] It is to be understood that the processing elements described herein may include the control unit as well as other processing elements operating in common. The processing elements may include one or more processors, memory, I / O devices, and a network interface, and may execute instructions stored in non-transitory computer readable memory in order to carry out aspects of the invention.

[0049] The memory of the control unit, or of other processing elements, may include RAM, ROM, fixed memory (e.g., hard drive), removable memory, or other forms of nontransient, computer readable storage medium capable of retaining instructions for execution. Instructions stored in memory of the processing elements, may include assembler code, ISA instructions, machine instructions, microcode, firmware, state-setting data, or source / object code in one or more languages. Execution may occur entirely within a local device, partly local and partly remote, or entirely remote via a LAN, WAN, or the Internet. In some embodiments, programmable logic such as FPGAs or PLAs may execute the instructions by configuring circuitry to perform aspects of the invention.

[0050] The foregoing descriptions are provided for illustration and are not limiting. Variations and modifications may be made without departing from the scope of the embodiments. Terminology has been selected to explain the principles, technical improvements, and practical applications, and to enable those skilled in the art to practice the invention.

[0051] Examples of the Invention

[0052] An example 1 of the present invention is a system for measuring topography of a region of a sample, where the sample region has a first direction over which a topographic metric is relatively uniform compared with an orthogonal direction of higher topographic variation. The system includes a sensor having a two-dimensional array of pixelsconfigured to receive radiation reflected from the sample region and from a reference mirror. The system further includes the reference mirror, oriented with a tilt angle to create a gradient direction of an optical path difference (OPD) between reflected radiation from the reference mirror and from the sample region. Lines of pixels oriented in the gradient direction correspond to positions of the sample region aligned in the first direction of the relatively uniform topographic metric. The system further includes a processor with associated memory storing instructions that, when executed, cause the processor to receive from the sensor an image of radiation intensity at each pixel of the sensor. The processor may generate from the image an interferogram of OPD versus intensity for a line of pixels along the gradient direction. The processor may then calculate, from the interferogram, the topographic metric for positions of the sample region aligned in the first direction corresponding to the line of pixels.

[0053] An example 2 of the present invention includes the features of example 1, and wherein the processor is configured to generate multiple interferograms of OPD versus intensity for multiple respective parallel lines of pixels of the sensor. The multiple lines correspond to multiple parallel lines in the first direction of the sample region, and the processor is configured to calculate the topographic metric, from each interferogram, to provide a topographic profile of the sample region along the orthogonal direction of higher variation.

[0054] An example 3 of the present invention includes the features of example 1 or of example 2 and further includes that the topographic metric is at least one of a surface height and a vertical depth of a sample structure.

[0055] An example 4 of the present invention includes the features of any one of examples 1-3 and further includes that the sample region is a region of a processed wafer no more than 5 mm from the wafer’s edge.

[0056] An example 5 of the present invention includes the features of any one of examples 1-4 and further includes a sample stage, configured to receive signals from the processor to move while images are acquired.

[0057] An example 6 of the present invention includes the features of any one of examples 1-5 and further includes imaging optics configured to direct reflected beams from the sample and from the reference mirror toward the sensor. The tilt angle is limited so thatthe reflected beams from the sample and from the reference mirror overlap fully within a paraxial region of the imaging optics.

[0058] An example 7 of the present invention includes the features of any one of examples 1-6 and further includes that the tilt angle is set to distribute a range of OPD values across the sensor to achieve a targeted interferogram span.

[0059] An example 8 of the present invention includes the features of any one of examples 1-7 and wherein the processor is configured to calculate the topographic metric by identifying, from the interferogram of a given linear set, an OPD at which an envelope of the interferogram attains a maximum, and converting that OPD to a measure of a surface position relative to a defined reference plane.

[0060] An example 9 of the present invention includes the features of any one of examples 1-8 and wheren the processor is configured to calculate the topographic metric by applying a Fourier transform to the detected intensity signals of the interferogram to generate a spectral distribution F(k) as a function of wavenumber k = 2TT / , and by determining the OPD corresponding to the sample surface from a peak in the spectral distribution.

[0061] An example 10 of the present invention is a method of optical interferometry for measuring topography of a region of a sample having a first direction over which a topographic metric is relatively uniform compared with an orthogonal direction of higher topographic variation. The method includes receiving, at a sensor having a two-dimensional array of pixels, radiation reflected from the sample region and from a reference mirror. The reference mirror is oriented with a tilt angle to create a gradient direction of optical path difference (OPD) between the reflected radiation from the reference mirror and from the sample region. Lines of pixels oriented in the gradient direction correspond to positions of the sample region aligned in the first direction having the relatively uniform topographic metric. The method also includes, at a processor with associated memory storing instructions, receiving from the sensor an image of radiation intensity at each pixel of the sensor. The method includes generating, from the image, an interferogram of OPD versus intensity for a line of pixels along the gradient direction, and calculating, from the interferogram, the topographic metric for positions of the sample region corresponding to the line of pixels.

[0062] An example 11 of the present invention is a method including the steps of example 10 and further includes generating, by the processor, multiple interferograms of OPD versus intensity for multiple respective parallel lines of pixels of the sensor. The multiple lines correspond to multiple parallel lines in the first direction of the sample region. The method includes calculating, from each interferogram, the topographic metric in order to provide a topographic profile of the sample region along the orthogonal direction of higher variation.

[0063] An example 12 of the present invention is a method including the steps of example 10 or example 11 and further includes that the topographic metric is at least one of a surface height and a vertical depth of a sample structure.

[0064] An example 13 of the present invention is a method including the steps of any one of examples 10-12 and further includes that the sample region is a region of a processed wafer no more than 5 mm from the wafer’s edge.

[0065] An example 14 of the present invention is a method including the steps of any one of examples 10-13 and further includes a sample stage. The processor may issue signals to move the sample stage while the image is acquired.

[0066] An example 15 of the present invention is a method including the steps of any one of examples 10-14 and further includes that the reflected beams from the sample and from the reference mirror are directed toward the sensor through imaging optics. The method further includes limiting the tilt angle so that the reflected beams from the sample and from the reference mirror overlap fully within a paraxial region of the imaging optics.

[0067] An example 16 of the present invention is a method including the steps of any one of examples 10-15 and further includes that the tilt angle is set to distribute a range of OPD values across the sensor to achieve a targeted interferogram span.

[0068] An example 17 of the present invention is a method including the steps of any one of examples 10-16 and further includes that the processor is configured to calculate the topographic metric by identifying, from the interferogram of a given linear set, an OPD at which an envelope of the interferogram attains a maximum, and converting that OPD to a measure of a surface position relative to a defined reference plane.

[0069] An example 18 of the present invention is a method including the steps of any one of examples 10-17 and further includes that the processor is configured to calculate thetopographic metric by applying a Fourier transform to the detected intensity signals of the interferogram to generate a spectral distribution F(k) as a function of wavenumber k = 2K / X, and by determining the OPD corresponding to the sample surface from a peak in the spectral distribution.

[0070] An example 19 of the present invention is a non-transitory computer readable medium that includes instructions for measuring topography of a region of a sample having a first direction over which a topographic metric is relatively uniform compared with an orthogonal direction of higher topographic variation. The instructions include receiving from a sensor having a two-dimensional array of pixels an image of intensity of radiation impinging on each pixel, wherein the radiation includes interference between radiation reflected from a corresponding position of the sample region and radiation reflected from an optically conjugate position of a reference mirror, where the reference mirror is oriented with a tilt angle to create a gradient direction of optical path difference (OPD) between reflected radiation from the reference mirror and from the sample region. Lines of pixels of the sensor oriented in the gradient direction correspond to positions of the sample region aligned in the first direction of the relatively uniform topographic metric. The instructions further include generating, from the image, an interferogram of OPD versus intensity for a line of pixels along the gradient direction, and calculating, from the interferogram, the topographic metric for positions of the sample region corresponding to the line of pixels.

Claims

CLAIMS1. An optical interferometry system for measuring topography of a region of a sample, the sample region having a first direction over which a topographic metric is relatively uniform compared with an orthogonal direction of higher topographic variation, the system comprising: a sensor comprising a two-dimensional array of pixels configured to receive radiation reflected from the sample region and from a reference mirror; the reference mirror, oriented with a tilt angle to create a gradient direction of an optical path difference (OPD) between reflected radiation from the reference mirror and from the sample region, wherein lines of pixels oriented in the gradient direction correspond to positions of the sample region aligned in the first direction of the relatively uniform topographic metric; and a processor with associated memory storing instructions that, when executed, cause the processor to: receive from the sensor an image of radiation intensity at each pixel of the sensor; generate from the image an interferogram of OPD versus intensity for a line of pixels along the gradient direction; and calculate, from the interferogram, the topographic metric for positions of the sample region aligned in the first direction corresponding to the line of pixels.

2. The system of claim 1, wherein the processor is configured to generate multiple interferograms of OPD versus intensity for multiple respective parallel lines of pixels of the sensor, the multiple lines corresponding to multiple parallel lines in the first direction of the sample region, and to calculate, from each interferogram, the topographic metric in order to provide a topographic profile of the sample region along the orthogonal direction of higher variation.

3. The system of claim 1, wherein the topographic metric is at least one of a surface height and a vertical depth of a sample structure.

4. The system of claim 1, wherein the sample region is a region of a processed wafer no more than 5 mm from the wafer’s edge.

5. The system of claim 1, further comprising a sample stage, wherein the processor is further configured to issue signals to move the sample stage while the image is acquired.

6. The system of claim 1, further comprising imaging optics configured to direct reflected beams from the sample and from the reference mirror toward the sensor, wherein the tilt angle is limited so that the reflected beams from the sample and from the reference mirror overlap fully within a paraxial region of the imaging optics.

7. The system of claim 1, wherein the tilt angle is set to distribute a range of OPD values across the sensor to achieve a targeted interferogram span.

8. The system of claim 1, wherein the processor is further configured to calculate the topographic metric by identifying, from the interferogram of a given linear set, an OPD at which an envelope of the interferogram attains a maximum, and converting that OPD to a measure of a surface position relative to a defined reference plane.

9. The system of claim 1, wherein the processor is further configured to calculate the topographic metric by applying a Fourier transform to the detected intensity signals of the interferogram to generate a spectral distribution F(fc)as a function of wavenumber k = and determining the OPD corresponding to the sample surface from a peak in thespectral distribution.

10. A method of optical interferometry for measuring topography of a region of a sample having a first direction over which a topographic metric is relatively uniform compared with an orthogonal direction of higher topographic variation, the method comprising: receiving, at a sensor comprising a two-dimensional array of pixels, radiation reflected from the sample region and from a reference mirror, wherein the reference mirror is oriented with a tilt angle to create a gradient direction of optical path difference (OPD) between the reflected radiation from the reference mirror and from the sample region, wherein lines of pixels oriented in the gradient direction correspond to positions of the sample region aligned in the first direction having the relatively uniform topographic metric; and at a processor with associated memory storing instructions: receiving from the sensor an image of radiation intensity at each pixel of the sensor;generating, from the image, an interferogram of OPD versus intensity for a line of pixels along the gradient direction; and calculating, from the interferogram, the topographic metric for positions of the sample region corresponding to the line of pixels.

11. The method of claim 10, further comprising generating, by the processor, multiple interferograms of OPD versus intensity for multiple respective parallel lines of pixels of the sensor, the multiple lines corresponding to multiple parallel lines in the first direction of the sample region, and calculating, from each interferogram, the topographic metric, in order to provide a topographic profile of the sample region along the orthogonal direction of higher variation.

12. The method of claim 10, wherein the topographic metric is at least one of a surface height and a vertical depth of a sample structure.

13. The method of claim 10, wherein the sample region is a region of a processed wafer no more than 5 mm from the wafer’s edge.

14. The method of claim 10, further comprising a sample stage, wherein the processor is further configured to issue signals to move the sample stage while the image is acquired.

15. The method of claim 10, wherein the reflected beams from the sample and from the reference mirror are directed toward the sensor through imaging optics, and wherein the method further comprises limiting the tilt angle so that the reflected beams from the sample and from the reference mirror overlap fully within a paraxial region of the imaging optics.

16. The method of claim 10, wherein the tilt angle is set to distribute a range of OPD values across the sensor to achieve a targeted interferogram span.

17. The method of claim 10, wherein the processor is further configured to calculate the topographic metric by identifying, from the interferogram of a given linear set, an OPD at which an envelope of the interferogram attains a maximum, and converting that OPD to a measure of a surface position relative to a defined reference plane.

18. The method of claim 10, wherein the processor is further configured to calculate the topographic metric by applying a Fourier transform to the detected intensity signals of the interferogram to generate a spectral distribution F(fc)as a function of wavenumber k = 2TT / , and determining the OPD corresponding to the sample surface from a peak in the spectral distribution.

19. A non-transitory computer readable medium that stores instructions for measuring topography of a region of a sample, wherein the region has a first direction over which a topographic metric is relatively uniform compared with an orthogonal direction of higher topographic variation, the instructions comprising: receiving from a sensor comprising a two-dimensional array of pixels, an image of intensity of radiation impinging on each pixel, wherein the radiation includes interference between radiation reflected from a corresponding position of the sample region and radiation reflected from an optically conjugate position of a reference mirror, wherein the reference mirror is oriented with a tilt angle to create, at the sensor, a gradient direction of an optical path difference (OPD) between reflected radiation from the reference mirror and from the sample region, wherein lines of pixels oriented in the gradient direction correspond to positions of the sample region aligned in the first direction of the relatively uniform topographic metric; generating, from the image, an interferogram of OPD versus intensity for a line of pixels along the gradient direction; and calculating, from the interferogram, the topographic metric for positions of the sample region corresponding to the line of pixels.

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