System and method for use in depth-resolved inspection of multi-layer patterned structures

Abstract

A control system is presented for use in depth-resolved inspection of multi-layer structures. The control system comprises a computerized system capable of processing input measured data indicative of a light spectrum obtained from the structure by camera pixels of a broadband interferometer during variation of an optical path difference (OPD). A data processor of the computerized system comprises: first processing utility for processing the input measured data and extracting complex reflectivity data of the structure; second processing utility for processing the complex reflectivity data and determining a time-domain impulse response of the structure, for each of N lateral positions corresponding to N of said camera pixels, and obtaining a 3D map of the time-domain impulse responses of the structure for at least a wavelength range of interest from the range used in the broadband interferometer; and layers' alignment data extractor for directly extracting depth-resolved information from said 3D map data.

Description

[0001] SYSTEM AND METHOD FOR USE IN DEPTH-RESOLVED INSPECTION OF MULTI-LAYER PATTERNED STRUCTURES

[0002] TECHNOLOGICAL FIELD

[0003] The present disclosure is generally in the field of optical metrology and relates, in particular, to characterization of alignment errors between vertically-separated patterned layers in a multi-layer structure.

[0004] BACKGROUND

[0005] Optical metrology solutions play a critical role in modern process control for advanced semiconductor fabrication. An important family of metrology uses cases relate to the characterization of alignment errors between vertically-separated layers, in particular patterned layers. Examples for such imperfections are overlay errors, where patterned structures on different layers are offset laterally, as is often the result from imperfect lithography or etch. Another case relates to structural tilt, for example arising due to a non-vertical etch process.

[0006] Various solutions are available for high-end overlay metrology of semiconductor nanostructures. However, these become increasingly inaccurate and complex the taller the measured structure is. When the structure vertical extent exceeds several microns, many common overlay / tilt metrology techniques become ineffective, and others suffer from increasingly large errors. Such relatively tall structures are becoming increasingly important in modem semiconductor fabrication, resulting both from the significant increase in 3D design architectures (as in 3D-NAND and 3D-DRAM memory devices) and from the rising adoption of advanced packaging solutions.

[0007] Overlay and tilt solutions can be categorized in two main groups:

[0008] (1) Front-end metrology for nanometric structures: these represent the high-end market of overlay tools for optical lithography and nanometric devices. Common solutions are Diffraction-Based Overlay (DBO), Image-Based Overlay (IBO) as well as Scanning-Electron Microscopy (SEM). However, this market is unrelated to the current disclosure, addressing structures of significantly smaller dimensions, typically extending vertically to a few lOOnm at most.

[0009] (2) Large-scale metrology for multiple-micron dimension structures: the current disclosure addresses this category, where the most widely used techniques are microscopy and IR microscopy. In these solutions, an image of the measured structure is obtained, and overlay / tilt properties are extracted using image processing. IR microscopy is often employed, in case the measured structure is opaque to spectral range. The key deficiency of these methods is the inability to differentiate between reflections from the structure top and bottom regions, which can greatly confound interpretation. Other techniques - such as confocal microscopy - can be used to provide some ability to separate the contributions from different layers, but none offer a high-quality and fast solution.

[0010] GENERAL DESCRIPTION

[0011] There is a need in the art for a novel approach for alignment errors (overlay errors / tilt) detection suitable for effective measurements in relatively tall structures / samples, generally in structures of any vertical extent.

[0012] The present disclosure provides a novel technique for characterization of alignment errors indicative of a shift between z-axis distinct features in the structure, typically alignment errors between vertically separated layers of a multi-layer structure. This technique is particularly useful in metrology tools used for measurements in semiconductor wafers.

[0013] The technique of the present disclosure provides for measuring asymmetry (such as tilt and overlay) in tall structures, by effectively separating signals arising from reflections of different vertically- separated interfaces in a multi-layer structure / sample. With this method, the lateral location of each segment in the measured structure can be identified separately, and their relative position can be extracted in a straightforward and accurate manner. The technique of the present disclosure utilizes the principles of state-of-the art imaging white-light interferometer (generally, broadband illumination). In such interferometer, an interferogram is obtained for each lateral position (x,y) on the sample (each pixel in the camera image), while the reference mirror / sample is scanned (i.e., an optical path difference is varied) over the total vertical extent (z-axis) of the sample under investigation.

[0014] As mentioned above, the technique of the present disclosure is capable of resolving reflection signal arising from different optical path delay. This allows obtaining a clear view of separate features, materials and interfaces at a specific depth within the structure. The misalignment can be determined with a minimal interference of the underlying structure. The technique utilizes broadband illumination, which also enhances the ability to resolve the required features as it is possible to isolate a defined wavelength range to work with, during the measured data interpretation process.

[0015] The technique of the present disclosure utilizes a novel approach for measured data analysis enabling to obtain a 3D map of the structure’s reflectivity, in a wavelength range of interest. In such map, OPD value / range defines the depth of the feature (layer / interface) along the z-axis, whereas (x,y) values from the imaging camera define the lateral location for the respective OPD value, thereby providing information about relative alignment (lateral shift) of the features / layers.

[0016] More specifically, according to the technique of the present disclosure, data indicative of Fourier transformation (frequency domain) of each interferogram of the structure is analyzed to extract, from the Fourier transform of the interferogram, the complex reflectivity of the sample by utilizing the known (previously measured) reflectivity of the mirror and a known reference sample (a calibration target such as Si wafer).

[0017] Then, back-Fourier transformation of the complex reflectivity of the sample is performed to obtain the time domain impulse response, for each pixel of the camera (i.e., each lateral position (x,y)). By this, the 3D map of the sample’s time domain impulse response, in a wavelength range of interest, is obtained.

[0018] Thus, according to one broad aspect of the present disclosure, it provides a control system for use in depth-resolved inspection of a multi-layer structure. The control system comprises a computerized system configured and operable for data communication with a measured data provider and comprising data input and output utilities, memory, and a data processing system. The data processing system comprises: a first processing utility configured and operable to process input measured data being indicative of a light spectrum obtained by camera pixels of a broadband interferometer during a variation of an optical path difference (OPD) of the structure, and extract data indicative of complex reflectivity of the structure; a second processing utility configured and operable to process said data indicative of complex reflectivity of the structure and determine a time-domain impulse response of the structure, for each of lateral positions corresponding to N pixels of said camera pixels, and thereby obtain a 3D map of the time-domain impulse responses of the structure for at least a wavelength range of interest from a broadband range used in the broadband interferometer; and a layers’ alignment data extractor configured and operable to directly extract depth-resolved information from said 3D map of the time-domain impulse response of the structure.

[0019] The input measured data indicative of the light spectrum obtained by camera pixels of the broadband interferometer during the variation of the OPD is indicative of a plurality of A / broadband interferograms detected by the N pixels for a respective plurality of N lateral locations on the structure.

[0020] In some embodiments, the data processor system includes a pre-processor which is configured and operable to receive raw measured data and convert it to the A / respective interferograms of position in the sample plane: where rs, rmdenote a field reflectivity of the structure and reference mirror, and S (fc) is a power spectrum density reaching the camera with k being the wave- vector

[0021] In some embodiments, the first processor is configured and operable to extract the data indicative of complex reflectivity of the structure by performing Fourier transform of the input measured data and using known complex reflectivity of a reference sample to extract said data of the complex reflectivity of the structure. In some embodiments, the second processor is configured and operable to determine the time-domain impulse response of the structure by performing back-Fourier transform of the data indicative of complex reflectivity of the structure for each of N lateral positions corresponding to N camera pixels.

[0022] In some embodiments, the layers’ alignment data extractor is configured and operable to present the depth-resolved information from said 3D map of the time-domain impulse response of the structure in a form of the complex reflectivity of the structure as a function of optical path difference (OPD), for each of N lateral positions corresponding to N camera pixels.

[0023] In some embodiments, the depth-resolved information from said 3D map of the time-domain impulse response of the structure is presented in the form of a summed signal over certain OPD ranges to yield a map representing reflectivity from the corresponding optical depth in the sample.

[0024] In some embodiments, the depth-resolved information from said 3D map of the time-domain impulse response of the structure is presented in the form of an image proportional to an electric field reflected from each interface of the multi-layer structure comprising two or more different interfaces.

[0025] In some embodiments, said depth-resolved information is indicative of misalignment between patterns in the layers of the structure.

[0026] The input measured data may be indicative of the light spectrum in the infra-red spectral range and / or the light spectrum in the ultraviolet spectral range.

[0027] In some embodiments, the control system also comprises a correlation utility configured and operable to correlate said depth-resolved information with Optical Critical Dimension (OCD) measurements performed on said structure concurrently with collection of said measured data, to thereby enable correction of interpretation of said 3D map of the time-domain impulse response of the structure.

[0028] In some embodiments, the control system is configured and operable to determine layers' alignment data from the 3D map of the time-domain impulse response for each of a plurality of structures, and use this data as a train set for training a machine learning model, thereby enabling to use the model for direct detection of alignment errors of patterned structures from said measured data. According to another broad aspect of the present disclosure, it provides a system for depth-resolved inspection of a multi-layer structure, the system comprising: a measurement system comprising a broadband interferometer configured and operable to perform broadband interferometer measurements on the structure and generate said input measured data indicative of the light spectrum obtained by camera pixels of the broadband interferometer during a variation of an optical path difference (OPD); and the control system according to claim 1, configured and operable for data communication with said measurement system to receive and process said input measured data.

[0029] In some embodiments, the measurement system further comprises a spectrometer which is configured and operable to receive interference pattern of said broadband light returned from the structure during said broadband interferometer measurements and provide data indicative of Optical Critical Dimensions (OCD) of the structure. The control system may thus be configured and operable to utilize said data indicative of Optical Critical Dimensions (OCD) of the structure to calibrate said broadband interferometer measurements. For example, the control system performs said calibration by accurately determining z-axis positions during the broadband interferometer measurements.

[0030] According to yet further broad aspect of the present disclosure, it provides a method for use in depth-resolved inspection of a multi-layer structure, the method comprising: providing measured data indicative of a light spectrum of a structure obtained by camera pixels of a broadband interferometer during a variation of an optical path difference (OPD), the measured data being indicative of a plurality of N broadband interferograms detected by A pixels of said camera pixels, for a respective plurality of N lateral positions on the structure during said variation of the OPD; processing said measured data to convert it to N respective interferograms of position (x,y), Ix y(OPD), in the sample plane: where rs, rmdenote a field reflectivity of the structure and reference mirror, and S (fc) is a power spectrum density reaching the camera with k being the wave- vector. performing Fourier transform of each x,y data piece of the measured data, to frequency domain, thus obtaining a complex light spectrum of an interface in the structure being defined as: processing said complex light spectrum and determining complex reflectivity of the structure: performing back-Fourier transform of the complex reflectivity of the structure, rs(fc), to time domain, thereby obtaining the complex reflectivity of the structure as a function of OPD, for each pixel of the camera; and analyzing the function f (OPD) for all the camera pixels and determining data indicative of a 3D map of the structure’s reflection, thereby providing layers’ alignment data of the structure.

[0031] The processing of the complex light spectrum and determining complex reflectivity of the structure may utilize reference measured data for a reference sample with pre-known reflectivity and known mirror reflectivity rm to obtain said complex reflectivity of the structure.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

[0034] Fig. 1 is a block diagram of a control system of the present disclosure for use in alignment measurements in a multi-layer structure;

[0035] Fig. 2A shows schematically the basic parts of the state-of-the-art white-light interferometer;

[0036] Fig. 2B exemplifies a single WLI interferogram detectable by the white-light interferometer; Fig. 2C shows more specifically imaging capability of the white-light interferometer providing the input measured data to the control system of the present disclosure;

[0037] Fig. 3 exemplifies a flow diagram of the measured data analysis technique of the present disclosure;

[0038] Figs. 4A to 4D show more specifically the various steps in the measured data analysis technique of the present disclosure described in Fig. 3;

[0039] Figs. 5A to 5C exemplify results obtainable by applying the technique of the present disclosure on a simple structure, wherein Fig. 5A shows the structure including two non-opaque materials; Fig. 5B shows (absolute value of) the complex reflectivity in time domain of said structure for two different lateral locations (pixels) on the surface of the structure; and Fig. 5C shows surface maps of the respective interfaces in the structure; and

[0040] Fig. 6 shows an example of a measurement system suitable to implement the technique of the present disclosure, in which a scatterometric OCD tool is combined with the WLI interferometer.

[0041] DETAILED DESCRIPTION OF EMBODIMENTS

[0042] The technique of the present disclosure is aimed at and provides for measuring asymmetry (such as tilt and overlay) in patterns made in multiple (generally at least two) layers of a structure. This technique is not limited to the structure’s height and can advantageously be used for measurements in tall structures, by effectively separating signals arising from reflections of different vertically- separated features (interfaces) within the structure. The technique of the present disclosure allows for identifying separately the lateral location of each segment in the measured structure, and extracting the relative position of each segment in a straightforward and accurate manner.

[0043] Referring to Fig. 1, there is schematically illustrated, by way of a block diagram, a control system 10 of the present disclosure. The control system 10 is configured as a computerized system including inter alia such functional parts as data input and output utilities 12 and 14, memory utility 16, and data processor system 18. These parts of the control system are typically formed by suitable hardware and / or software elements. The control system 10 is configured and operable for data communication with a measured data provider 20 and receives therefrom input measured data MD. The measured data provider may be constituted by a separate storage device 30 which receives and stores the measured data provided by a measurement system 40, or may be constituted by an internal memory of the measurement system 40. The control system 10 may be a stand-alone system or may be integral with the measurement system 40. Thus, generally, the control system 10 is connectable (via wired connection or wireless communication of any known suitable type) to the measured data provider 20. This is exemplified in the figure by provision of suitable communication utilities 22 and 24 in the control system 10 and measured data provider 20, respectively.

[0044] The input measured data MD being received by the control system 10 includes or is indicative of image data obtained by a broadband interferometry measurement system 40. Such measured data is indicative of a plurality of N broadband interferograms for a respective plurality of N lateral positions on the structure during a variation of an optical path difference (OPD) which actually corresponds to z-scan of the structure. More specifically, each interferogram corresponds to data detected by each pixel in the camera image of the structure being measured and thus corresponds to each lateral position (x,y) on the structure, while a reference mirror or the structure is scanned over the total vertical extent (z-axis) of the structure.

[0045] The measurement system 40 utilizing an imaging broadband interferometer (typically termed “white-light interferometer”) may be of any known suitable configuration including such main constructional parts as: broad band light source 42, camera 44, reference mirror 46, z-scanner 48, and possibly also x-y scanner 49.

[0046] It should be noted that the configuration and operation of the imaging white-light interferometer are known per se and do not form part of the present disclosure, and therefore need not be described in detail. However, since the imaging capability of such system provides the measured data whose analysis forms the basis of the technique of the present disclosure, the general structural and operational principles of such measurement system are described herein.

[0047] Such a system is generally sketched in Fig. 2A. Other layouts may be used for the purpose of the technique of the present disclosure, i.e., to obtain multiple interferograms, each corresponding to a pixel in the camera image and thus to lateral position (x,y) on the structure, while a reference mirror / structure is scanned over the total vertical extent (z- axis) of the structure, while all such layouts utilize the following structural and operation scheme:

[0048] A broadband light source is used to produce illumination light beam. This light beam is focused using an objective lens (OL). Interferometric beam splitter / combiner (BS) splits the light beam in two light components. One light component propagates to illuminate the sample, and the other light component is directed to a reference mirror, while the sample undergoes z-scan. Generally, it is possible to perform such a scan via scan of the reference mirror or the sample itself, i.e., via controllable displacement of the reference mirror or of the sample along the z-axis. After being reflected from the sample and the reference mirror, the reflected light components are recombined by passing again through the beam splitter / combiner BS. An imaging lens collects the combined light and focuses the collected light onto a camera. The camera takes pictures of the resulting interfered light while the position of the reference mirror / sample is scanned. This yields a series of images with varying optic path difference (OPD) between the BS and the reference mirror / sample.

[0049] It should be noted, although not specifically described here, that various technical parameters of the optic / mechanic layout are properly selected / controlled. Such parameters include inter alia focus, spot size / resolution, magnification, pixel size, optical aberrations, etc.

[0050] The resulting measured data obtained from the white light interferometer scan is an interferogram depicting the intensity of the interfered collected light fields reflected from the mirror and the sample, as a function of mirror / sample position along z-axis. For simplicity in the description below the z-scan is described as being obtained by mirror scan. However, it should be understood and is also described above that, generally, it is possible to scan either the reference mirror or the sample itself.

[0051] As the mirror is scanned across the z-axis, the optical path difference (OPD) between the two light paths (from the beam splitter / combiner BS to sample and back, and from the beams splitter / combiner BS to the reference mirror and back) changes. This OPD determines the manner by which the two light components interfere, creating the interferogram described above.

[0052] Fig. 2B presents an exemplary interferogram. Such an interferogram is recorded for each camera pixel of the image. For a typical WLI measurement, the z-scan (e.g., of the mirror) is to be performed over the total vertical extent of the sample under investigation, with an added scanning extent of several times the longest wavelength used. Fig. 2B shows an interferogram obtained from a single interface.

[0053] Fig. 2C demonstrates the imaging capability of the imaging WLI setup described above (exemplified in Fig. 2A), showing beam path for two different points on the sample Light from each of these points interferes with light reflected from points and R2on the reference mirror, to be collected by points and C2on the camera, correspondingly. Thus, with such imaging setup, such interferogram is collected by each of the pixels of the camera - indexed by i, j - to result in a corresponding measured light intensity This position on the camera corresponds to the collected light from the position (x,y) on the sample. Mapping the interferogram over i,j, and z- axis, can thus be converted to an interferogram of position (x,y), Ix y(OPDy in the sample plane.

[0054] Turning back to Fig. 1, the control system 10 of the present disclosure is configured and operable to receive and analyze the measured data MD indicative of the N interferograms obtained on a structure / sample and extract layers' alignment data. The data processor system 18 includes a first processor 50 configured and operable as a complex reflectivity extractor, which processes the data indicative of the interferograms, to extract therefrom the complex reflectivity of the structure:

[0055] Further provided in the data processor system 18 is a second processor 52 which is configured and operable to process this complex reflectivity of the structure, and extract therefrom a time-domain impulse response of the structure, f (OPDy for each of the N lateral positions corresponding to N camera pixels, and thereby obtain a 3D map of the time-domain impulse responses of the structure for at least a wavelength range of interest from a broadband range used in the broadband interferometer. This 3D map data is analyzed by a layers' alignment data extractor 54 which generates output alignment data.

[0056] As exemplified in the figure, the data processor system 18 can also include a pre- processor 56 which receives (from the measured data provider) raw measured data MD and translates it into the interferogram data, i.e., N respective interferograms of position in the sample plane of reference,

[0057] Alternatively, such pre-processing can be implemented at the measurement system, i.e., at the system camera.

[0058] Reference is made to Fig. 3 which exemplifies more specifically, by way of a flow diagram 100, the measured data analysis method of the present disclosure.

[0059] In step 110, input measured data is provided as described above, being indicative of a light spectrum obtained by camera pixels of a broadband interferometer during a z- scan of the structure. As noted above, the measured data may be raw data, or may be pre- processed data in the form of interferograms. Thus, generally, the measured data is indicative of a plurality of N broadband interferograms for a respective plurality of A lateral positions on the structure during a z-scan of the structure.

[0060] The light intensity as measured by the camera, includes a DC term which is independent of the mirror position along the z-axis, and an oscillating AC term. The DC term is of no interest to the needs of the technique of the present disclosure and can be omitted. The AC term, having the shape of an interferogram, shown in Fig. 4A, corresponds to the interference between light reflected from the sample / structure and light reflected from the reference mirror.

[0061] It should be noted that plots illustrated in Figs. 4A-4D are synthesized, for visualization purposes only. The plot in Fig. 4A corresponds to raw measured data.

[0062] The data corresponding to the term AC (i.e., broadband interferogram data) forarespective plurality of N lateral positions on the structure during the z-scan of the structure, is processed (step 112) to be converted to N respective interferograms of position (x,y), Ix y(0PD), in the sample plane (plane of reference). Each Ix y(0PD) is equal to: where rs, rmdenote the (complex) field reflectivity of the sample / structure and reference mirror, and is the power spectrum density reaching the camera with k the wave- vector.

[0063] In step 114, each x,y data piece of the measured data, Ix y(OPD), is Fourier transformed to frequency domain, thus obtaining a complex light spectrum of the respective interface in the structure being defined as:

[0064] An example of data corresponding to such complex light spectrum is shown in Fig. 4B.

[0065] Then, the complex light spectrum is processed by the structure’s complex reflectivity extractor (50 in Fig. 1) and the complex reflectivity of the structure is determined (step 120):

[0066] To this end, reference measured data is provided [Ix yOPD)ref] - step 116, e.g., is measured on a reference sample with pre -known reflectivity and known mirror reflectivity rmis provided (step 118) to obtain, in step 120, the complex reflectivity of the structure. An example of the resulting (calibrated) complex reflectivity of the structure is shown in Fig. 4C.

[0067] It should be noted that at this stage, when the measured data is in the spectral domain, it is possible to select a desired wavelength window to work with, to enhance sensitivity to certain features of the measured structure, or to keep working with the entire spectral data.

[0068] In step 122, the processor / extractor 52 (Fig. 1) operates to perform back-Fourier transform of the complex reflectivity of the structure, to time domain, providing the complex reflectivity of the structure as a function of OPD, for each pixel of the camera. An example of the resulting complex reflectivity (the absolute value is shown) in time domain (i.e., as a function of OPD) is shown in Fig. 4D. In the last step 124, the layers’ alignment data extractor 54 (Fig. 1) analyzes the function f (OPD) for all the camera pixels and determines data indicative of a 3D map of the structure’s reflection (e.g., in the required wavelength range) and provides the layers’ alignment data.

[0069] Reference is made to Figs. 5A to 5C exemplifying the results of applying the technique of the present disclosure on a simple application (i.e., simple patterned) sample.

[0070] Fig. 5A shows a sample consisting of two non-opaque materials, Material A and Material B. Such sample has different reflectivity spectra from these different materials, and therefore different time-domain plots, when measured in points marked by A and B. It is noted that such sample is characterized by at least two interfaces for incident broadband light interaction with the sample from the top down (along z axis). The first interface IF1 is interface between the sample and air, i.e., either Material A-Air or Material B-Air interface. The second interface, IF2, Material B - Material A interface, is located further down the sample.

[0071] For samples consisting of two or more different interfaces, separated far enough from each other, it is possible to obtain an image proportional to the electric field reflected from each interface. Separation is to be larger than the resolving capabilities of the scan process; a thin interface presents a peak in the order of a few times the longest wavelength of the light source.

[0072] Fig. 5B shows the (absolute value of) complex reflectivity in time domain of the sample of Fig. 5A for two different lateral locations (pixels) on the sample's surface, indicated by curves A and B points. The data is shown as a function of the optical path delay (OPD), corresponding to depth within the sample, up to index of refraction. Working in the time domain, the inventors are able to separate features according to their z-position. The curve A (corresponding to reflection from point A in Fig. 5A) shows a reflection at "0" OPD obtained from the top surface (interface IF1 being Material A - Air interface), whereas no more reflections are obtained at higher OPDs, since no further interfaces exist as the incident light propagates down the z direction. The curve B (corresponding to reflection from point B in Fig. 5A) shows two peaks at two distinct values of OPD. The reflection observed at "0" OPD comes from the first interface IF1 (Material B - Air). The reflection at some arbitrary value, e.g., OPD=20, comes from the second interface IF2 (Material B - Material A interface).

[0073] The peak values shown in the curve B, corresponding to the reflection from the various interfaces, are arbitrary, and are chosen for the purpose of demonstration of the principles of the technique of the present disclosure.

[0074] Fig. 5C shows surface maps of the respective interfaces, obtained by summing the signal over certain OPD ranges (highlighted areas in Fig. 5B) for each pixel on the surface. The resulting maps represent reflectivity from the corresponding optical depth in the sample. For the sample considered here (Fig. 5A), a tilt of the Material B lines in the x-direction is observed as misalignment of the top and bottom layer maps.

[0075] In some embodiments, methods such as center-of-mass calculation, or specific function fitting may allow determining x-y position of optically resolved features within sub-pixel accuracy. If the sample exhibit tilt / misalignment / overlay errors between its layers, it is manifested in measurable differences in such features’ position.

[0076] The combination of the technique of the present disclosure with a scatterometric OCD tool can further enhance accuracy, as correction / calibration of the WLI information can be done according to the OCD results. Some samples may possess complex relation between actual overlay and measured results. OCD data can be used, along with theoretical or experimental analysis to provide ability for correct interpretation of the WLI results.

[0077] Fig. 6 shows an example of a measurement system in which a scatterometric OCD tool (spectrometer) is combined with the WLI interferometer. The system is configured generally similar to the system of Fig. 2C and further includes an additional beam splitter / combiner BM' in the optical path of combined light beam propagation towards the camera. This beam splitter / combiner BM' splits the combined light and allows propagation of a part thereof to the camera and the other part - to the spectrometer.

[0078] The implementation of WLI in OCD tool also allows use of the spectrometer measurements during the z-axis scan (e.g., reference mirror scan). It should be understood, although not specifically shown, that the control system of the present disclosure may utilize the spectral resolved interferograms collected by the spectrometer to accurately determine the mirror position. A pixel in the spectrometer sensor, corresponding to a specific wavelength, may experience an interferometric pattern with a well-known dependence in the mirror position / OPD. Collecting this data in parallel with the camera images provides accurate position of the mirror in each image frame taken.

[0079] It should also be noted that the control system may include a model-based processor utilizing a machine learning (ML) based model trained on the above-described measurements (on a few reference measurements) to allow accurate conversion of the measured information to quantitative overlay values.

[0080] In some embodiments, an IR sensitive camera, with the appropriate light source may be used to further extend the spectral range of the WLI. Near-IR wavelengths are of specific interest as they provide penetration of otherwise opaque layers.

[0081] In some embodiments, extension of the spectrum can be made towards the UV range, since shorter wavelengths used may provide better resolution in the vertical direction.

[0082] In some embodiments, polarized measurement may provide additional information and may therefore assist in optimizing sensitivity for some polarization- dependent samples.

[0083] In some embodiments, measurement scan may be performed over the top and bottom regions of the sample only, while jumping over the range in between, thus ignoring the non-relevant part in between. This may significantly reduce measurement scan time for thick enough samples.

[0084] It should also be noted that the measurement sequence can be optimized for each new sample / application. Such optimization stage can be used to determine the best working point in terms of polarization, spectral range, reference mirror scan range and speed, and other parameters.

[0085] Thus, the technique of the present disclosure is advantageously characterized by its ability to resolve reflection signal arising from different optical path delays, due to the novel approach for the measured data analysis. This technique allows a clear view of separate features, materials and interfaces at a specific depth within the sample. The misalignment between such elements can be determined with a minimal interference of the underlying structure. Working with a broadband light source, also enhances the ability to resolve the required features as it is possible to isolate a required wavelength range to work with, during the interpretation process.

Claims

CLAIMS:

1. A control system for use in depth-resolved inspection of a multi-layer structure, the control system comprising a computerized system configured and operable for data communication with a measured data provider and comprising data input and output utilities, memory, and a data processing system, wherein the data processing system comprises: a first processing utility configured and operable to process input measured data being indicative of a light spectrum obtained by camera pixels of a broadband interferometer during variation of an optical path difference (OPD), and extract data indicative of complex reflectivity of the structure; a second processing utility configured and operable to process said data indicative of complex reflectivity of the structure and determine a time-domain impulse response of the structure, for each of A lateral positions corresponding to A pixels of said camera pixels, and thereby obtain a 3D map of the time-domain impulse responses of the structure for at least a wavelength range of interest from a broadband range used in the broadband interferometer; and a layers’ alignment data extractor configured and operable to directly extract depth-resolved information from said 3D map of the time-domain impulse response of the structure.

2. The control system of claim 1, wherein said input measured data being indicative of the light spectrum obtained by camera pixels of the broadband interferometer during the variation of the OPD is indicative of a plurality ofbroadband interferograms detected by said N pixels for a respective plurality of lateral locationson the structure.

3. The control system of claim 1, wherein said input measured data being indicative of the light spectrum obtained by camera pixels of the broadband interferometer during the variation of the OPD is obtained by performing a z-axis movement of a reference mirror with respect to said structure or performing a z-axis movement of the structure with respect to a reference mirror.

4. The control system of claim 1, wherein said first processor is configured and operable to extract the data indicative of complex reflectivity of the structure byperforming Fourier transform of the input measured data and using known complex reflectivity of a reference sample to extract said data of the complex reflectivity of the structure.

5. The control system of claim 1, wherein said second processor is configured and operable to determine the time-domain impulse response of the structure by performing back-Fourier transform of the data indicative of complex reflectivity of the structure for each of N lateral positions corresponding to N camera pixels.

6. The control system of claim 1, wherein said layers’ alignment data extractor is configured and operable to present the depth-resolved information from said 3D map of the time-domain impulse response of the structure in a form of the complex reflectivity of the structure as a function of the optical path difference (OPD), for each of N lateral positions corresponding to N camera pixels.

7. The control system of claim 1, wherein said layers’ alignment data extractor is configured and operable to present said depth-resolved information from said 3D map of the time-domain impulse response of the structure in a form of a summed signal over certain OPD ranges to yield a map representing reflectivity from the corresponding optical depth in the sample.

8. The control system of claim 1, wherein said layers’ alignment data extractor is configured and operable to present said depth-resolved information from said 3D map of the time-domain impulse response of the structure in a form of an image proportional to an electric field reflected from each interface of the multi-layer structure comprising two or more different interfaces.

9. The control system of claim 1, wherein said depth-resolved information is indicative of misalignment between patterns in the layers of the structure.

10. The control system of claim 1, wherein said input measured data is indicative of the light spectrum in the infra-red spectral range.

11. The control system of claim 1, wherein said input measured data is indicative of the light spectrum in the ultraviolet spectral range.

12. The control system of claim 1, further comprising a correlation utility configured and operable to correlate said depth-resolved information with Optical Critical Dimension(OCD) measurements performed on said structure concurrently with collection of said measured data, to thereby enable correction of interpretation of said 3D map of the time- domain impulse response of the structure.

13. The control system of claim 1, configured and operable to determine layers' alignment data from the 3D map of the time-domain impulse response for each of a plurality of structures, and use this data as train set for training a machine learning model, thereby enabling to use the model for direct detection of alignment errors of patterned structures from said measured data.

14. A system for depth-resolved inspection of a multi-layer structure, the system comprising: a measurement system comprising a broadband interferometer configured and operable to perform broadband interferometer measurements on the structure and generate said input measured data indicative of the light spectrum obtained by camera pixels of the broadband interferometer during a variation of an optical path difference (OPD); and the control system according to claim 1, configured and operable for data communication with said measurement system to receive and process said input measured data.

15. The system of claim 14, wherein the measurement system further comprises a spectrometer which is configured and operable to receive interference pattern of said broadband light returned from the structure during said broadband interferometer measurements and provide data indicative of Optical Critical Dimensions (OCD) of the structure.

16. The system of claim 15, wherein the control system is configured and operable to utilize said data indicative of Optical Critical Dimensions (OCD) of the structure to calibrate said broadband interferometer measurements.

17. The system of claim 16, wherein the control system performs said calibration by accurately determining z-axis positions during the broadband interferometer measurements.

18. A method for use in depth-resolved inspection of a multi-layer structure, the method comprising:providing measured data indicative of a light spectrum of a structure obtained by camera pixels of a broadband interferometer during variation of an optical path difference (OPD), the measured data being indicative of a plurality of N broadband interferograms detected by N pixels of said camera pixels, for a respectiveplurality of N lateral positions on the structure during variation of anoptical path difference (OPD); processing said measured data to convert it to N respective interferograms of position (x,y), Ix y(OPD), in the sample plane:where rs, rmdenote a field reflectivity of the structure and reference mirror, and S (k) is a power spectrum density reaching the camera with k being the wave- vector. performing Fourier transform of each x,y data piece of the measured data, to frequency domain, thus obtaining a complex light spectrum of aninterface in the structure being defined as:processing said complex light spectrum and determining complex reflectivity of the structure:performing back-Fourier transform of the complex reflectivity of the structure, to time domain, thereby obtaining the complex reflectivity ofthe structure as a function of OPD, for each pixel of the camera; and analyzing the function f OPD) for all the camera pixels and determining data indicative of a 3D map of the structure’s reflection, thereby providing layers’ alignment data of the structure.

19. The method of claim 18, wherein said processing of the complex light spectrum and determining complex reflectivity of the structure comprises utilizing reference measured data for a reference sample with pre-known reflectivityand known mirror reflectivity rmto obtain said complex reflectivity of thestructure.

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