Vertically-resolved metrology with asymmetry-sensitive measurement

Abstract

A system and methods for OCD metrology are provided including setting an interferometry mirror of the system at each of multiple positions z, and measuring spectra of interferometer signals reflected from lower and higher surfaces while applying a first setting of an asymmetry-sensitive measurement parameter, thereby creating a set of multiple spectra. Subsequently, a function representing a coherent portion of the reflected signals is determined and transformed to a time domain to generate a first signal signature representing the coherent portion of the reflected signal. Subsequently, the process is repeated for a second setting of the asymmetry-sensitive measurement parameter, to generate a second signal signature. A measure of difference between the first and second signal signatures indicates a measure of asymmetry of the structure.

Description

VERTICALLY-RESOLVED METROLOGY WITH ASYMMETRY-SENSITIVE MEASUREMENTFIELD OF THE INVENTION

[0001] The present invention relates generally to the field of optical inspection of integrated circuit wafer patterns, and in particular to algorithms for measurement of wafer parameters.BACKGROUND

[0002] Integrated circuits (ICs) are produced on semiconductor wafers through multiple steps of depositing, altering, and removing thin layers, which build up into stacked structures on the wafers. These stacked structures, also referred to as “stacks” or “features,” have optical properties similar to diffraction gratings.

[0003] Optical critical dimension (OCD) metrology employs methods of scatterometry, measuring reflected light radiation from structures formed on a sample (i.e., a silicon wafer) during IC production. Common scatterometry methods include spectral reflectometry (SR), spectral ellipsometry (SE) and spectral interferometry (SI). Scatterometric measurements are commonly applied in OCD metrology during IC production to determine whether wafer patterns are being fabricated with correct parameters. Measurements may determine the extent of variation from design specifications. Manufacturing protocols may specify allowed deviations from mean values.

[0004] Scatterometry methods face a serious challenge when measuring thick structures, particularly structures with several reflective surfaces that are significantly separated from each other. One common application with these characteristics is “through-silicon via” (TSV) manufacturing, particular common for implementing, for example, chiplet technology.

[0005] TSV structures are etched circular holes, which, after metal filling, are used as interconnects. TSVs typically have depth ranging from several tens of microns to severalhundred microns. Being larger than 10pm, TSVs are considered relatively tall structures in semiconductor fabrication. Other examples of tall (or “thick”) structures include ultra-thick dielectrics, 3D-NAND memory devices, CMOS image sensors and various other structures used in advanced semiconductor packaging.

[0006] A set of technologies for addressing the difficulty of characterizing these structures has been described in several international patent applications in the name of Nova Measuring Instruments Ltd., in particular the following international patent applications: WO2022162617, “Time-domain optical metrology and inspection of semiconductor devices”; W02024003758, “Optical critical dimensions (OCD) metrology for thick stacks”; and WO2024100674, “Coherent spectroscopy for TSV,” all of which are incorporated herein by reference. The approaches described in these publications analyze signals reflected from vertically distinct regions in a structure by a process that includes filtering incoherent portions of the signals, then converting the filtered signals from the frequency domain to the time domain, to identify coherent signals from specific layers of the structure with less spectral smearing. Hereinbelow, such approaches and methods are referred to as “ Vertically - Resolved Metrology” (VRM). VRM techniques described previously may be used, for example, to characterize the total depth of a TSV.

[0007] A further metrology challenge for tall structures involves the characterization of lateral dimensions of TSVs, and especially of structural asymmetry, that is, deviations of such structures from their nominally-symmetric design specifications. Examples of asymmetries include structural tilts in overlay metrology, where asymmetry is introduced by relative offsets between different layers. An additional asymmetry is dimensional asymmetry, where, for example, a cross section of a cylindrical TSV becomes elliptical.

[0008] Several techniques exist for characterizing asymmetries in thin wafer structures that are typically a few nanometers in height. For example, overlay metrology, using imagebased overlay (IBO) or diffraction-based overlay (DBO), can identify some asymmetries. Ellipsometry-based metrology, using polarization changes induced by asymmetry, was described, for example, in US Patent 10,876,959 in the name of Nova Measuring Instruments Ltd.

[0009] However, while these methods can identify and quantify overall symmetrybreaking in a sample, they are not capable of identifying precise locations or layers of asymmetry, and, in particular, they are not designed to identify asymmetry appearing in tall (or “thick”) structures such as TSVs, as described above. Even with the advances of VRM, tall structure asymmetries, such as those described above, pose a significant metrology challenge, as the sensitivities to small structural disparities are typically overwhelmed by other sensitivities.SUMMARY

[0010] Embodiments of the present invention provide systems and methods for use in optical critical dimension (OCD) metrology, based on optical path difference (OPD)- matched interferometry, extending vertically-resolved metrology (VRM) methods with the application of asymmetry-sensitive measurement techniques. The process described herein allows deviations from symmetry to be resolved with improved signal-to-noise ratio (SNR), while also identifying specific parameters of symmetry-breaking. The invention permits characterization of systematic, tall structure asymmetries. This type of asymmetry may be particularly noticeable at the bottom of a deep structure, where a TSV, for example, may have different widths in different directions. Degradation of manufacturing elements in a wafer processing chamber can lead to such asymmetries, and timely identification of such problems prevents manufacturing waste and inefficiency.BRIEF DESCRIPTION OF DRAWINGS

[0011] For a better understanding of various embodiments of the invention and to show how the same may be carried into effect, reference is 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. In the figures:

[0012] Fig. 1 is a schematic diagram of a system for optical critical dimension (OCD) metrology, including adjustable elements for asymmetry-sensitive measurements, in accordance with embodiments of the present invention;

[0013] Figs. 2A and 2B show, respectively, a TSV structure with several layers and patterning near the TSV, and a time domain graph of coherent reflections from the TSV surfaces that is generated by optical path difference (OPD) matching of both the top layers and the bottom surface;

[0014] Figs. 3A and 3B show, respectively, tilt and elliptical asymmetries of TSVs, which may be caused by manufacturing aberrations;

[0015] Figs. 4A and 4B are schematic diagrams of a system for OCD metrology employing an adjustable polarization filter for making asymmetry-sensitive measurements;

[0016] Figs. 5A and 5B are schematic diagrams of a system for OCD metrology in which the interferometry mirror mount system has an adjustable tilt for performing asymmetry- sensitive measurements;

[0017] Figs. 6A and 6B are schematic diagrams of a system for OCD metrology employing an adjustable spectrometer aperture setting to make asymmetry-sensitive measurements;

[0018] Figs. 7A and 7B are schematic diagrams of a system for OCD metrology employing an adjustable light spot offset to make asymmetry- sensitive measurements;

[0019] Fig. 8 is a graph of a time-domain impulse response transform of spectrogram results, indicating a difference between results obtained with different settings of an asymmetry- sensitive measurement parameter;

[0020] Figs. 9 and 10 are schematic diagrams of a wafer, showing ratios of TSV elliptical asymmetry at locations across a wafer; and

[0021] Fig. 11 is a flow diagram depicting a process for characterizing OCD metrology, the process including OPD-matched interferometry and one or more asymmetry-sensitive measurements, in accordance with an embodiment of the present invention.DETAILED DESCRIPTION

[0022] Embodiments of the present invention provide systems and methods for use in optical critical dimension (OCD) metrology, based on optical path difference (OPD) matched interferometry, with one or more asymmetry- sensitive measurements.

[0023] Fig. 1 is a schematic diagram of a system 20 for spectral interferometry metrology of a wafer 22, for characterizing structural asymmetries particularly of tall structures, according to embodiments of the present invention. The system 20 may operate within a production line (not shown) for production and monitoring of structures of a wafer. Additional details of spectral interferometry (SI), and its underlying principles can be found in the US Patent No. 10,161,885 and PCT Patent No. PCT / IB2022 / 050774, both in the name of Nova Ltd. and incorporated herein by reference.

[0024] The wafer 22 typically has multiple surfaces, also referred to herein as “layers” or “interfaces.” In the example shown, there is a higher surface 24 and a lower surface 26, separated by a distance H. The system is shown configured to characterize a structure, suchas a “through-silicon via” (TSV) 28, extending from the higher surface to the lower surface 26.

[0025] Light is projected from a light source 30, typically of a predetermined frequency range, such as broadband illumination covering the range of visual light, or a greater range extending to infrared and / or ultraviolet. The light is directed by a first beam-splitter (BS) 32 towards the structure under measurement. Light is then split by a second beam-splitter 34, one portion directed towards the sample and another portion of the light directed towards an interferometer mirror 36.

[0026] Reflections of the two portions of the light, that is, from the sample and from the interferometer mirror, recombine at the second beam splitter 34 and are directed towards a spectrometer 40. Data from the spectrometer may then be processed by a processor 42, providing the calculations described herein to identify and characterize structure asymmetries.

[0027] The interferometer mirror may be moved linearly, along a linear axis 46, to positions that are be “matched” to reflective surfaces of the structure being measured, such that an optical path difference (OPD) between the light distance travelled to a target surface (i.e., layer) and the light travelled to the mirror is zero. For example, OPD-matching of the mirror with the higher surface involves setting the distance B (the distance of the mirror to the beam splitter 34) equal to the distance A (the distance from the beam splitter to the higher surface). OPD-matching of the bottom layer involves setting B to A+H.

[0028] The optical path length of each path traversed by the light is a function of the geometric length and the refractive index of the material through which the light is propagating, that is, the length is measured in terms of light propagation. Inside a structure, each electromagnetic eigenmode has a different phase velocity and hence a different effective refractive index and a different OPD.

[0029] The system 20 may be applied to implement vertically-resolved metrology (VRM) methods, and, in addition, to enhance such methods with the application of asymmetry- sensitive measurement techniques. For example, in some embodiments of the present invention, as described further hereinbelow, the mirror 36 may have, as an additional axis of movement, a tilt axis 48, around which the mirror may be rotated to make asymmetrysensitive measurements.

[0030] In additional, or alternative embodiments of the present invention, as described further hereinbelow, a polarizing filter, indicated as polarizer 50, may be positioned along the path of the light, typically between the first and second beam-splitters. The direction of polarization may also be adjusted to make an asymmetry- sensitive measurement. Light passing through the polarizer may be focused on the sample using an objective lens 52.

[0031] After reflection, light coming from the sample and from the mirror are combined by the second beam-splitter, as described above, and may be collimated by the objective lens before passing again through the polarizer and the first beam-splitter to reach the spectrometer. Typically, additional lenses are provided to direct the light (e.g., a tube lens, not shown). Typically, an aperture 60 also limits the light entering the spectrometer.

[0032] In further embodiments of the present invention, also described further hereinbelow, the aperture 60 may be adjusted to make an asymmetry- sensitive measurement.

[0033] In additional, or alternative embodiments, also described further hereinbelow, the objective lens, or an alternative lens of the system, such as a lens of the light source (not shown), may be adjusted to cause an offset of the light spot at the target surface of the wafer to make the asymmetry-sensitive measurements. Alternatively, the stage on which the wafer is placed can be moved to cause offsets of the light spot.

[0034] Additional components of system 20 may include imaging lenses, additional polarizing filters, variable aperture stops, and motors (not shown). Operation of many systemelements, such as the mirrors, beam splitters, filters, aperture stops, and motors is typically automated by computer controllers, such as processor 42, which may include I / O devices and which may also be configured to perform data processing tasks, such as generating scatterometry data from the received spectrometer signals. In typical OCD metrology, the range of light that is measured may cover the visible light spectrum and may also include wavelengths in ultraviolet and infrared regions. A typical spectrogram output for OCD metrology may have, for example, 245 data points covering a wavelength range of 200 to 970 nm.

[0035] Fig. 2A shows a TSV structure 28 of a wafer 22, as described above, with several layers (or “surfaces”) of structures 100 adjacent to the top of the TSV structure. A spot of light from the light source is simultaneously reflected from the multiple layers of the structure. Fig. 2B shows a time domain graph of coherent reflections from the structures shown in Fig. 2A, as determined by OPD-matching, according to VRM techniques described above (in the Background). That is, Fig. 2B is a function representing, in the time domain, the coherent portion of the reflected spectra of a structure such as the TSV. The function reveals distinct peaks, with the separation between top and bottom surfaces directly proportional to the stack height H.

[0036] Note that the x-axis may be in units of time or units of distance (such as the optical path length, OPL), given that the axis represents the distance light travels. As shown, there are multiple peaks appearing around the shorter optical path length of the top structures 100. An isolated peak is obtained for the TSV bottom surface.

[0037] Figs. 3 A and 3B show examples of tall structures with structural asymmetry faults, in particular, asymmetries of etched circular hole structures, such as may occur in a TSV or a 3D-NAND channel hole. Fig. 3A shows a structural tilt (which may be a consistent tilt through the extent of the hole, as shown, or an asymmetry induced near the structurebottom). Fig. 3B shows breaking of circular symmetry at the structure bottom, that is, the bottom has different via widths in different directions. An additional asymmetry not shown is an overlay error, by which a TSV is not centered on its target underlaying structure. Asymmetries such as these may be caused by manufacturing aberrations, such as uneven chamber distribution of etching plasma.

[0038] The present invention provides systems and methods for determining whether tall structures exhibit the types of asymmetries described above. The basic process includes making separate interferometer measurements while applying two or more different settings of asymmetry-sensitive parameters. For each type of asymmetry-sensitive parameter (also referred to herein as a “channel”), when the application of different settings causes different interferometer results (also referred to herein as “signal signatures”), the difference between the results is an indication of asymmetry.

[0039] The differences between results for different asymmetry-sensitive settings can also be compared to previously calibrated measures, such as thresholds, or “signature models,” to determine the type and extent of asymmetry. In some cases, measurements of two different settings for a given parameter (i.e., “channel”) are approximately equal when the sample structure is symmetric (e.g., the TSV has no tilt and / or the TSV bottom is round, etc.), and the measurements are different when the sample symmetry is broken (e.g., the TSV bottom is elliptical).

[0040] As described above, asymmetry-sensitive parameters may include: 1) polarization of the source and / or of the reflected light; 2) modification of the light angle-of- incidence, for example, by changing a tilt of the interferometer mirror; 3) modification of an angular span of light reaching the spectrometer aperture, for example by blocking part of the aperture; and 4) changing an offset of light incident on the sample. Methods for adjustingthese four types of asymmetry-sensitive parameters will now be described in more detail with respect to accompanying figures.

[0041] Figs. 4A and 4B are schematic diagrams of system 20 with an adjustable polarizer 50 for making asymmetry- sensitive measurements. When incident light is perpendicular to a sample surface, the reflected signal is approximately the same for all polarization planes of incident light when a symmetrical structure is being examined. However, when an aspect of the structure is asymmetrical, as when a TSV has an elliptical or tilted bottom critical dimension (BCD), the intensity (i.e., amplitude) of polarized, reflected light changes with the direction of polarization. As one example, light polarized in the plane of the major axis of an elliptical TSV bottom, is reflected with a different intensity than incident light polarized along the minor axis of an elliptical TSV bottom. Differences between signal signatures of the reflected signals from different polarizations, isolated by VRM techniques as described above, indicate the type and extent of the symmetry breaking. Typically, two polarizing settings may be made with orthogonal polarizations, as indicated by the different pattern of polarization of the polarization filter 50 in Figs. 4A and 4B. Additional measurements at different polarization orientations may be made to characterize asymmetry when a direction of possible asymmetry is not known. As described below, elliptical asymmetry of the BCD often appears with the elliptical axes aligned with radial and tangential directions of the wafer, in which case polarization in these orthogonal directions is an efficient method for identifying this type of asymmetry.

[0042] Figs. 5A and 5B are schematic diagrams of an angle-of-incidence (AOI) asymmetry- sensitive parameter, achieved by adjusting a tilt of the interferometry mirror 36 around a tilt axis 48. Although SI measurements are commonly conducted at normal incidence to the sample, small tilts can be introduced between the optical axis and the wafer surface. Two VRM measurements of a symmetric structure, taken at two different AOIs,typically provide the same result when the target structure is symmetrical. When sample symmetry is broken, i.e., a target structure is asymmetrical, these measurements become unequal.

[0043] Such a measurement scheme may be implemented using a mirror tilt, whereby the angular span of rays reflected from the mirror is different from that reflected from the sample (offset by twice the mirror angle). Upon interference, the two beam portions (from the sample and from the mirror) only interfere in a partial angular span. The coherent part of the collected signal thus originates from a tilted angular span.

[0044] Alternatively, the incidence angular span can be fixed at some non-perpendicular span, and the sample rotated such that different measurements are taken at different azimuths.

[0045] The tilt angle is typically on the order of 0.5 degrees or less, and in any case not more than an angle at which light from the mirror no longer reaches an aperture of the spectrometer. The results may be enhanced by applying polarization, for example polarization oriented at 45 degrees to the AOL

[0046] Figs. 6A and 6B are schematic diagrams of a system for OCD metrology in which a setting of the spectrometer aperture 60 is adjusted to make an asymmetry- sensitive measurement. An asymmetric pupil-plane introduces an asymmetry to the measured angular span, which can be achieved, for example, by blocking part of the system’s numerical aperture. Such a restriction can be implemented using a movable aperture and / or a shutter, under control by the processor. This asymmetry can be implemented at the illumination source or at the point of collection (i.e., the spectrogram aperture, as shown) or both.

[0047] Figs. 7A and 7B are schematic diagrams of a system for OCD metrology employing an adjustment of a light spot offset (under control by the processor) to make an asymmetry- sensitive measurement. Shifting a spot of the incident light by a small lateral shift (with or without an additional angular shift) with respect to the TSV changes themeasurement and the resulting signal signatures. Such shifts may be implemented by moving lenses of the system or by moving the wafer (as long as the movement maintains at least partial light coverage of the structure surface under examination, e.g., the TSV bottom).

[0048] Fig. 8 is a graph of a time-domain impulse response transform of spectrogram results indicating a difference between results obtained with different settings of an asymmetry- sensitive parameter. As described above, using principles of vertically-resolved metrology (VRM), which are explained in the publications cited above and with respect to the flow diagram described hereinbelow, a measured interferometry signal is converted to the time-dependent, impulse-response domain in order to isolate coherent signals, which are correlated with different heights in a stack of layers of a tall structure.

[0049] The results shown in Fig. 8 are for a structure with a single top surface and a single bottom surface, both indicated by individual peaks. The top surface peak is not affected by the setting of an asymmetry-sensitive parameter. However, the asymmetry of the bottom surface causes the peak of the signals acquired from the bottom surface to change according to the setting of the asymmetry-sensitive parameter.

[0050] When two measurements with different asymmetry-sensitive settings are compared, asymmetry-based differences are observed in the reflected signatures related to the bottom asymmetry. By this approach, a vertical position of a symmetry-breaking element can also be identified. This may be accomplished by converting the time at which the two measured spectra differ and converting the time difference to a measure of distance to identify the location in the structure stack. The differences between the signal signatures thus provide an indication of asymmetry. In some cases, the target for the difference is zero; that is, any deviation may warrant intervention in the manufacturing process.

[0051] As described above, differences between the signal signatures can also be analyzed by techniques of calibration, model-based interpretation, machine-learning analysisor any similar interpretation protocol to determine the target parameters of interest (POI), in particular, the type and magnitude, i.e., the dimensions, of a structure’s asymmetry. Such dimensions, also referred to herein as “measures of asymmetry” may include, for example, elliptical dimensions of a TSV base. Methods for modelling or calibrating the correlation of measures of difference with the measure of asymmetry may rely on initial analysis or “training” of models of asymmetry effects. The differences between signatures generated by different asymmetry-sensitive settings can also be quantified as a relative or absolute difference in peak intensity, amplitude, or pattern. The level of asymmetry can then be compared with a previously determined threshold or scale, to indicate dimensions of the asymmetry or other measures of asymmetry, such as a severity of asymmetry. Asymmetry greater than a preset level (or, in some cases, zero) may require, for example, intervention by a human operator and adjustment of a manufacturing process.

[0052] To calibrate or model the measures of difference against actual measures of asymmetry, measures of asymmetry may first be determined by other techniques. One technique that may be used applies high-energy e-beam tools to measure the bottom of the tall structure. An additional technique that is possible is Critical-Dimension Small Angle X- ray Scattering (CD SAXS). To measure the elliptical shape and tilt of a tall structure such as a TSV, after acquiring the measure of difference data to be correlated, the TSV can be filled, for example with copper (Cu), and then, after bonding the top wafer side, the back side of the wafer may be polished to expose the TSV bottom. The fill that is exposed can then be measured, for example with Critical Dimension Scanning Electron Microscope (CD SEM) techniques. After correlating the measures of difference with measures of asymmetry, the less expensive, more efficient interferometry method of measurement described herein may then be used in production.

[0053] Figs. 9 and 10 are schematic diagrams of a wafer 22, showing TSV BCD elliptical asymmetry at locations across a wafer. When the target parameter of interest (POI), such as the TSV bottom shape, has a known, expected behavior, significant sensitivity benefits can be achieved by setting the asymmetry- sensitive parameters accordingly. Fig. 9 shows the theoretical deviations of the bottom shape of a tall structure from a circular shape, due to etching problems. As indicated, the expected asymmetry is along the radial axis of the wafer.

[0054] To improve asymmetry sensitivity, the settings of an asymmetry-sensitive parameter, such as the polarization or AOI settings, can be set to match the parallel and perpendicular axes of the expected elliptical asymmetry. Similarly, measurements at suitable mirror tilts, angular span offsets or measurement position offsets can be optimized according to the expected structural tilt directions. Differences between reflection intensities for radial and tangential asymmetry- sensitive settings indicate that the anticipated, etch-related aberrations are present.

[0055] Fig. 10 shows results of testing for TSV asymmetry on a wafer manufactured with a multiple TSVs, where the polarizer was oriented in vertical and horizontal directions for each set of measurements at each measured location. Shading indicates relative intensities of the bottom peak reflection as a difference of intensity of the two polarization directions. As shown, the asymmetry is readily apparent at the extremities of the horizontal and vertical axes of the wafer. At 45 degree shifts, the asymmetry is not clear, given that the elliptical radius at plus and minus 45 degrees is equal. As discussed above, asymmetry-sensitive settings along the radial and tangential direction of the wafer, for any given location on the wafer, would be a better indicator of asymmetry, given the tendency for asymmetry along these axes due to etching aberrations.

[0056] Fig. 11 is a flow diagram depicting a process for characterizing asymmetries in OCD metrology, the process including OPD-matched interferometry with VRM filtering and one or more asymmetry- sensitive measurements.

[0057] The process includes, at a step 510, performing a set of OCD measurement with a first setting of an asymmetry-sensitive parameter. This involves measuring multiple interferometer spectra, by setting the interferometry mirror to different positions z around the OPD of a surface under inspection, while applying a first setting of an asymmetry-sensitive parameter. Alternatively, a combination of asymmetry-sensitive parameters may be set to a first combination of values (and then set to a second combination at the subsequent step 530 described below). For typical investigations of asymmetry, the relevant surface will be the bottom of a tall structure, such as a TSV. The asymmetry- sensitive parameter may be, for example a polarization direction, a mirror tilt, an aperture offset, or a light spot offset, or any combination of these parameters.

[0058] All OPD measurements should be within a full coherence range of the target surface, i.e., within the “coherence length” of the source light (such that no “partial coherence” factor is needed). For multiple, distinguishable height differences between surfaces, the different surfaces may each be measured by separate OPD-matching.

[0059] At a step 520, the multiple measured spectra are filtered by the VRM techniques described above, whereby the signals are fit to a mathematical equation for Imeasured , which may be defined, for example, as:

[0060] where, k is the wave number (= ^ ) at each interferometer measurement;A.Zi is the mirror position for each of the z measurements;(fc) is the wavelength-dependent reflected phase difference, for the respective pairs of reflections, tb (top-bottom), tm (top-mirror), and bm (bottom-mirror);H is the geometric difference between the interfaces; n is the refractive index of the structure; rm, rt, rbare reflectivities, responsively of the mirror, the top surface and the bottom surface, all being functions of k; y is the coherence for OPD 2nH ; y' is the coherenceR is the real function of a function with an imaginary part; andCoherence y is defined as the ratio between the measured interference term and the interference expected for completely coherent fields (which can be measured, for example, by moving one of the mirrors in a Michaelson interferometer and measuring the amplitude of the resulting oscillations).

[0061] The multiple measured spectra are then fit it to the expected functional form of ^measured (k>z)- The non- -dcpcndcnt terms can then be cancelled out, as common to all mirror positions, leaving functions that are dependent on z, for the top and bottom, respectively. Converting the z-dependent function to the impulse time domain, then permits identification of coherent reflection signals, i.e., a signal signature, for the surface under inspection, for the given setting of the asymmetry-sensitive parameter.

[0062] At a step 530, the above steps are then repeated for a second setting of the asymmetry- sensitive parameter (or parameters), to generate a second signal signature. Additional signatures may also be generated, repeating steps 510-520 for settings of the asymmetry- sensitive parameter, to provide additional specificity, and thereby overcoming, for example, the possible problem of an asymmetry orientation in an unknown direction.

[0063] At a step 540, a difference or set of differences are determined that distinguish between the measured signatures (i.e., the time domain spectra). The differences may be between maximum values of peaks, or between maximum values at a given time (i.e., value of the x-axis parameter). A further measure of difference may be the area under the signature curves, which may be calculated over a given time range. Other relative relationships between the signatures may also be used, such as a ratio between maximum values. The difference between the signatures may also be represented as a difference function, which may be compared to a previously generated difference model.

[0064] At a step 550, the difference calculated may be compared with a previously determined threshold or model for asymmetry, to generate an indicator of asymmetry. Such an indicator may then be output to operators, for example as an alert with respect to a manufacturing problem.

[0065] Advantages of the invention include:• High accuracy characterization of symmetry-breaking POIs buried in a measured structure;• Robustness against details and variations in other parts of the stack;• Identification of the vertical location of symmetry-breaking along the stack.

[0066] It is to be understood that processing elements shown or described herein are preferably implemented by one or more computers in computer hardware and / or in computer software embodied in a non-transitory, computer-readable medium in accordance with conventional techniques, such as employing a computer processor, a memory, I / O devices, and a network interface, coupled via a computer bus or alternate connection arrangement.

[0067] Unless otherwise described, the terms “processor” and “device” are intended to include any processing device, such as, for example, one that includes a CPU (central processing unit) and / or other processing circuitry (e.g., GPUs), and may refer to more thanone processing device. Various elements associated with a processing device may be shared by other processing devices.

[0068] The term “memory” as used herein is intended to include memory associated with a processor or CPU, such as, for example, RAM, ROM, a fixed memory device (e.g., hard drive), a removable memory device (e.g., diskette, tapes), flash memory, etc. Such memory may be considered a computer readable storage medium.

[0069] In addition, phrases “input / output devices” or “I / O devices” may include one or more input devices (e.g., keyboard, mouse, scanner, HUD, etc.) for entering data to the processing unit, and / or one or more output devices (e.g., speaker, display, printer, HUD, AR, VR, etc.) for presenting results associated with the processing unit.

[0070] Embodiments of the invention may include a system, a method, and / or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the invention.

[0071] The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non- exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), Blue-Ray, magnetic tape, Holographic Memory, a memory stick, a floppy disk, a mechanically encoded device such as punch-cardsor raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

[0072] Computer readable program instructions described herein can be downloaded to respective computing / processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and / or a wireless network. A network adapter card or network interface in each computing / processing device may receive computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing / processing device.

[0073] Computer readable program instructions for carrying out operations of the invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like, and conventional procedural programming languages, such as the "C" programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network,including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the invention.

[0074] Where aspects of the invention are described herein with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention, it will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer readable program instructions.

[0075] These computer readable program instructions may be provided to a processor of a general-purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks. These computer readable program instructions may also be stored in a non-transient, computer readable storage medium that can direct a computer, a programmable data processing apparatus, and / or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein may include an article of manufacture including instructions which implement aspects of the function / act specified in the flowchart and / or block diagram block or blocks.

[0076] The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0077] Any flowchart and block diagrams included herein illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which may include one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order shown herein. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and / or flowchart illustration, and combinations of blocks in the block diagrams and / or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

[0078] The descriptions of the various embodiments of the invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found inthe marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

[0079] EXAMPLES

[0080] The present invention may include the following configurations.Example 1 is a method for optical critical dimension (OCD) metrology, for characterizing a structure having at least one lower surface separated from a higher surface. Typically the structure is a tall structure, such as a TSV. The method includes: a) for each of multiple linear positions z of an interferometry mirror of the interferometry system, measuring spectra of interferometer signals reflected from the lower and higher surfaces while applying a first setting of an asymmetry-sensitive measurement parameter, to create a set of multiple spectra, wherein each linear position z is within a coherent, optical path difference (OPD)-matched range with the lower surface; b) from the set of multiple measured spectra, determining a function representing a coherent portion of the reflected signals, and applying a time domain transformation to the function to generate a first signal signature representing the coherent portion in the time domain; c) repeating steps a and b for a second setting of the asymmetry-sensitive measurement parameter to generate a second signal signature; d) calculating a measure of difference between the first and second signal signatures and outputting the measure of difference as an indicator of asymmetry of the structure.

[0081] An example 2 includes the above features and the light emitted at a light source of the interferometry system has a coherence length less than the distance separating the lower surface from the higher surfac.

[0082] An example 3 includes the features of either example 1 or 2, where the measure of difference is a function of a difference between the first and second signatures over the optical path difference (OPD)-matched range of the signatures.

[0083] An example 4 includes the features of any of the above examples, further including determining a severity of asymmetry by comparing the measure of difference to a previously calibrated threshold value.

[0084] In an example 5, the difference between the first and second signal signatures may be measured as a difference between areas under curves of the respective signal signatures.

[0085] In an example 6, the difference between the first and second signal signatures may be measured as a difference between signal amplitudes of the respective signal signatures.

[0086] An example 7 includes the features of any of the above examples, as well as determining one or more dimensions of asymmetry of the structure by comparing the measure of difference with a previously determined measure of difference correlated by a function or machine learning model to the one or more dimensions asymmetry.

[0087] An example 8 includes the features of any of the above examples, and the asymmetry- sensitive measurement parameter is one or more of: a polarization state of the reflected interferometer signal; an angle-of-incidence (AOI) achieved by a tilted orientation of the mirror, an aperture adjustment for partial blocking of light emitted at a light source or of light received at a spectrometer of the interferometry system; and an offset of a light spot focused by the interferometry system on the structure. In an example 9, the polarization is implemented by a polarizer positioned between the wafer and a spectrometer of the interferometry system. In an example 10, the first and second settings are orthogonalpolarizations of the reflected signals. In an example 11, the wafer is circular, and the first and second settings are orthogonal polarizations in radial and tangential directions of the wafer.

[0088] An example 12 includes the features of any of the above examples, and determining a function representing a coherent portion of the reflected signals, from each of the first and second sets of multiple measured spectra, includes fitting a mathematical equation of interferometry signals to the reflected signals.

[0089] An example 13 includes the features of any of the above examples, and the structure includes one of: a Through-Silicon Via (TSV), a DRAM capacitor, a 3D-NAND structure, a 3D DRAM structure, a CMOS image sensor, or an advanced packaging feature.

[0090] An example 14 includes the features of any of the above examples, and the functions representing coherent portions of the reflected signals are portions of a mathematical equation of interferometry that are dependent on z.

[0091] Further examples of the present invention include a metrology unit for optical critical dimension metrology that includes: a) an interferometer with adjustable mirror positioning; b) a polarization control system; c) a programmable aperture system; and d) a processor configured to execute the method of any of claims 1-14.

[0092] The metrology unit may also include a mirror mount system enabling controlled tilts of the interferometry mirror typically under the control of the processor.

[0093] Further examples of the present invention include a non-transitory computer readable medium that stores instructions for optical critical dimension (OCD) metrology, for characterizing a structure by interferometry, the structure having at least two reflective surfaces, including a lower (or bottom) surface separated by a distance greater than acoherence length from at least one higher (or top) reflective surface, the instructions configured to implement any of the methods of claims 1-14.

[0094] It is to be understood that a further example of the present invention is a metrology unit configured to implement any of the above method examples. In addition, a further example of the present invention is non-transitory computer readable medium storing instructions to implement any of the above method examples.

Claims

CLAIMS1. A method of optical critical dimension (OCD) metrology for characterizing a structure of a wafer by an interferometry system, wherein the structure has at least one lower surface separated from a higher surface, the method comprising steps of: a) for each of multiple linear positions z of an interferometry mirror of the interferometry system, measuring spectra of interferometer signals reflected from the lower and higher surfaces while applying a first setting of an asymmetry-sensitive measurement parameter, to create a set of multiple spectra, wherein each linear position z is within a coherent, optical path difference (OPD)-matched range with the lower surface; b) from the set of multiple measured spectra, determining a function representing a coherent portion of the reflected signals, and applying a time domain transformation to the function to generate a first signal signature representing the coherent portion in the time domain; c) repeating steps a and b for a second setting of the asymmetry-sensitive measurement parameter to generate a second signal signature; d) calculating a measure of difference between the first and second signal signatures and outputting the measure of difference as an indicator of asymmetry of the structure.

2. The method of claim 1, wherein light emitted at a light source of the interferometry system has a coherence length and the lower surface is separated from the higher surface by a distance greater than the coherence length.

3. The method of claim 1, wherein the measure of difference is a function of a difference between the first and second signatures over the optical path difference (OPD)-matched range of the signatures.

4. The method of claim 1, further comprising determining a severity of asymmetry by comparing the measure of difference to a previously calibrated threshold value.

5. The method of claim 1, wherein the difference between the first and second signal signatures is measured as a difference between areas under curves of the respective signal signatures.

6. The method of claim 1, wherein the difference between the first and second signal signatures is measured as a difference between signal amplitudes of the respective signal signatures.

7. The method of claim 1, further comprising determining one or more dimensions of asymmetry of the structure by comparing the measure of difference with a previously determined measure of difference correlated by a function or machine learning model to the one or more dimensions asymmetry.

8. The method of claim 1, wherein the asymmetry-sensitive measurement parameter is one or more of: a polarization state of the reflected interferometer signal; an angle-of-incidence (AOI) achieved by a tilted orientation of the mirror, an aperture adjustment for partial blocking of light emitted at a light source or of light received at a spectrometer of the interferometry system; and an offset of a light spot focused by the interferometry system on the structure.

9. The method of claim 8, wherein the polarization is implemented by a polarizer positioned between the wafer and a spectrometer of the interferometry system.

10. The method of claim 8, wherein the first and second settings are orthogonal polarizations of the reflected signals.

11. The method of claim 8, wherein the wafer is circular, and wherein the first and second settings are orthogonal polarizations in radial and tangential directions of the wafer.

12. The method of claim 1, wherein determining a function representing a coherent portion of the reflected signals, from each of the first and second sets of multiple measured spectra, comprises fitting a mathematical equation of interferometry signals to the reflected signals.

13. The method of claim 1, wherein the structure comprises one of: a Through-Silicon Via (TSV), a DRAM capacitor, 3D-NAND structure, a 3D DRAM structure, a CMOS image sensor, or an advanced packaging feature.

14. The method of claim 1, wherein the functions representing coherent portions of the reflected signals are portions of a mathematical equation of interferometry that are dependent on z.

15. A metrology unit for optical critical dimension metrology comprising: an interferometer with adjustable mirror positioning; a polarization control system; a programmable aperture system; and a processor configured to execute steps of: a) for each of multiple linear positions z of an interferometry mirror of the interferometry system, measuring spectra of interferometer signals reflected from the lower and higher surfaces while applying a first setting of an asymmetry-sensitive measurement parameter, to create a set of multiple spectra, wherein each linear position z is within a coherent, optical path difference (OPD)-matched range with the lower surface;b) from the set of multiple measured spectra, determining a function representing a coherent portion of the reflected signals, and applying a time domain transformation to the function to generate a first signal signature representing the coherent portion in the time domain; c) repeating steps a and b for a second setting of the asymmetry-sensitive measurement parameter to generate a second signal signature; d) calculating a measure of difference between the first and second signal signatures and outputting the measure of difference as an indicator of asymmetry of the structure.

16. The metrology unit of claim 15, further comprising a mirror mount system enabling controlled tilts.

17. A non-transitory computer readable medium that stores instructions for optical critical dimension (OCD) metrology, for characterizing a structure by interferometry, wherein the structure has at least two reflective surfaces, wherein a bottom surface is separated by a distance greater than a coherence length from at least one top reflective surface, the instructions configured to execute steps of: a) for each of multiple linear positions z of an interferometry mirror of the interferometry system, measuring spectra of interferometer signals reflected from the lower and higher surfaces while applying a first setting of an asymmetry-sensitive measurement parameter, to create a set of multiple spectra, wherein each linear position z is within a coherent, optical path difference (OPD)-matched range with the lower surface; b) from the set of multiple measured spectra, determining a function representing a coherent portion of the reflected signals, and applying a time domain transformation to the function to generate a first signal signature representing the coherent portion in the time domain;c) repeating steps a and b for a second setting of the asymmetry-sensitive measurement parameter to generate a second signal signature; d) calculating a measure of difference between the first and second signal signatures and outputting the measure of difference as an indicator of asymmetry of the structure.

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