An interferometry technique

Abstract

An interferometry system for evaluating a sample having a region of interest of a given depth, the interferometry system includes a memory configured to store interference pattern measurement results indicative of interference patterns associated with different measurement conditions and with different locations of the sample; the interference pattern measurement results are associated with a coherence length that is smaller than the given depth; and an analysis unit comprising a processing circuit that is configured to: extract, when operating in a first mode, from the interference pattern measurement results, coherent interference data; and determine, when operating in the first mode, one or more attributes of the region of interest based on the coherent interference data.

Description

9943-PC AN INTERFEROMETRY TECHNIQUE CROSS REFERENCE

[0001] This application claims priority from US provisional patent serial number 63 / 513,345 filing date July 12, 2023 which is incorporated herein in its entirety.

[0002] BACKGROUND OF THE INVENTION

[0003] Scatterometry methods such as Spectral Reflectometry (SR), Spectral Ellipsometry (SE) and Spectral Interferometry (SI) are extensively used in semiconductor process control. These techniques provide valuable information on the measured layers and nanostructures, characterizing their dimensions and material properties.

[0004] All these methods face a critical challenge when measuring thick, transparent structures: the reflected spectrum from such structures typically includes extremely fast oscillations, meaning – reflection is significantly changed for extremely small wavelength differences. In order to resolve these spectral features, an extremely high spectral resolution is required for the measurement apparatus (e.g. spectrometer), which is technically challenging, has various negative effects on other measurement attributes (SNR, cost, complexity) and for very thick stacks – simply not feasible.

[0005] In such situations, the measured spectrum is ‘smeared’ with spectral features unresolved by the measurement. Such situation leads to loss of sensitivity and applicability of the metrology solution.

[0006] Another contributor to the decoherence and the loss of sensitivity is the NA smearing. The reflected spectrum from thick transparent structures also features strong oscillatory and / or sharp-featured dependence of the reflectivity on the angle of incidence and, if it is not cylindrically symmetric, on the azimuth. Since SR, SE and SI essentially incoherently integrate over the pupil plane, these oscillations and sharp features are smeared in the pupil plane, yielding a result different from the ideal single-ray measurement. One may think of it as lack of angular resolution, which complements the lack of appropriate spectral resolution.

[0007] As stated, increasing the spectral resolution can help resolve the fast spectral oscillations, at the expense of performance, cost and complexity. Moreover, such solutions are not scalable – as semiconductor applications become thicker, high-end spectrometers cannot keep up with the required spectral resolution.9943-PC

[0008] Another possible approach involves using longer wavelengths, i.e. IR and MIR (Infra-Red and Mid-Infra-Red). Very roughly, the frequency of the spectral oscillations is proportional to 1 / λ (with λ the wavelength), leading to slower oscillations at longer wavelengths. Such mitigation has multiple negative aspects – in terms of lost sensitivity (UV and Vis wavelength ranges hold various unique sensitivities to attributes of the measured structure) as well as system complexity, measurement times (due to lower brightness light sources and lower efficiency detectors) and measurement spot size (due to diffraction of the longer wavelengths).

[0009] A straightforward solution to the spectral resolution challenge is given by monochromator-based solutions, where a scanning element measures the scattered light at a specific wavelength at any given instant. Extremely high spectral resolutions are attainable in such approaches, but at the expense of very long measurement times – commonly unsuitable for process control and high-TPT metrology.

[0010] Another alternative for increased spectral resolution is provided by Fourier-based methods. In such methods an integral over a broad spectral range is measured, but using a scanning element (typically a mirror) different measurement instances capture different weighted-sums of the signal. Methods in this category are Fourier-Transform IR (FTIR) and White Light Interferometry (WLI). Typically, the eventual spectral resolution is proportional to the range across which the scanning element is swept, allowing very high spectral resolutions. However, as before, high resolutions come at the direct expense of measurement time.

[0011] The insufficient angular resolution can be addressed by reducing the collection NA to a size over which the angular dependence of the spectrum is weak, so the measurement is closer to an ideal measurement of a single ray. The drawback of this approach is that less light is collected per unit time, so either the measurement time has to be increased in order to gather more light at the expense of throughput or the measurement time is kept the same resulting in a noisier spectrum. If the small NA is insufficient, one could use a moving pinhole, which slows the measurement even further and introduces additional complexity to the measurement system.

[0012] There is a growing need to provide a cost efficient measurement method for measuring high-frequency response of a sample.9943-PC SUMMARY

[0013] A system, a non-transitory computer readable medium and a method as illustrated in the specification and / or claims and / or drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

[0015] FIG.1 illustrates an example of an interferometry system; and

[0016] FIG.2 illustrates an example of method. DETAILED DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 illustrates an example of an interferometry system 10 for evaluating a sample having a region of interest of a given depth, the interferometry system includes: a. A memory unit 12 configured to store interference pattern measurement results indicative of interference patterns associated with different measurement conditions and with different locations of the sample. The interference pattern measurement results are associated with a coherence length that is smaller than the given depth. The memory unit includes at least one integrated circuit or be included in one or more integrated circuits. The memory unit may be a dynamic memory unit or a static memory unit. b. An analysis unit 16 includes a processing circuit that is configured to: (i) extract, when operating in a first mode, from the interference pattern measurement results, coherent interference data (such as|^^^^^|^; and (ii) determine, when operating in the first mode, one ormore attributes of the region of interest based on the coherent interference data. The processing circuit may include at least one integrated circuit or be included in one or more integrated circuits. c. A communication unit 20 used for communicating between units of the system and / or for communicating with other computerized systems.

[0018] According to an embodiment, the interferometry system includes an interferometer 18 used to provide the multiple interference pattern information units. An example of an interferometer is illustrated in US patent 10,161,885 which is incorporated9943-PC herein by reference. Figure 1 illustrates the interferometer as including an illumination unit (radiation source), a beam splitter, a reference arm having a movable reference mirror, a measurement arm, a sensing unit and another beam splitter. The sample is illustrated as including two reflecting surfaces – a top surface and a second surface that define the region of interest. This is a simplified sample and more complex samples may be evaluated.

[0019] According to an embodiment the interferometer is not included in the interferometry system.

[0020] According to an embodiment, the different locations of the sample correspond to different sensing elements of a sensing sensor, or to different locations of a spot.

[0021] According to an embodiment, the extraction of the coherent interference data includes compensating for a decoherence associated with the interference pattern measurement results.

[0022] According to an embodiment, the extraction of the coherent interference data includes compensating for interference pattern measurement results that are uninfluenced by the different measurement conditions.

[0023] According to an embodiment, the extraction of the coherent interference data is based, at least in part, or measured or simulated optical properties of one or more optical components involved in the generation of the interference pattern measurement results.

[0024] According to an embodiment, the different measurement conditions include different optical path differences between a measurement arm of an interferometer and a reference arm of the interferometer, the interferometer being used for obtaining interference pattern measurement results.

[0025] According to an embodiment, the different measurement conditions include different locations of a reference mirror of a reference arm of an interferometer and a reference arm of the interferometer, the interferometer being used for obtaining interference pattern measurement results.

[0026] According to an embodiment, the different locations of the reference mirror correspond to surfaces located at different depths of the region of interest, wherein a depth difference between adjacent surfaces is smaller than the coherence length.

[0027] According to an embodiment, the analysis unit is configured to apply a vertical travelling scatterometry on the coherent interference data to determine one or more attributes of the region of interest. An example of vertical travelling scatterometry is illustrated in US patent application 2024 / 0085805 which is incorporated herein by reference.9943-PC

[0028] According to an embodiment, the different measurement conditions include different focus conditions.

[0029] According to an embodiment, the processing circuit is configured to operate in a second mode when it is determined that interference pattern measurement results are indicative of summed interferences of different wave-numbers.

[0030] According to an embodiment, when operating in the second mode, the processing circuit is configured to separate between summed interferences to determine a contribution of individual interference patterns associated with different wave-numbers.

[0031] According to an embodiment, the processing circuit is configured to separate between summed interferences by solving a quadratic optimization problem.

[0032] According to an embodiment, the different measurement conditions include different locations of a reference mirror of a reference arm of an interferometer, the interferometer being used for obtaining interference pattern measurement results, wherein the different locations of the reference mirror correspond to surfaces located at different depths of the region of interest, wherein a number of the different locations is at least a sum of a number of measured wave-numbers (^^) and twice a number of higher spectral resolution wave-numbers (^^^).

[0033] According to an embodiment, the different measurement conditions include different focus conditions.

[0034] Figure 2 illustrates an example of method 100 for evaluating a sample having a region of interest of a given depth.

[0035] According to an embodiment, method 100 starts by step 110 of obtaining interference pattern measurement results indicative of interference patterns associated with different measurement conditions and with different locations of the sample; the interference pattern measurement results are associated with a coherence length that is smaller than the given depth.

[0036] Obtaining may include generating or receiving.

[0037] According to an embodiment, step 110 includes using an interferometer to provide the multiple interference pattern information units. This may include providing radiation to the interferometer that in turn provides an interference pattern indicative of the sample.

[0038] According to an embodiment, the different measurement conditions include different optical path differences between a measurement arm of an interferometer and a9943-PC reference arm of the interferometer, the interferometer being used for obtaining interference pattern measurement results.

[0039] According to an embodiment, the different measurement conditions include different locations of a reference mirror of a reference arm an interferometer and a reference arm of the interferometer, the interferometer being used for obtaining interference pattern measurement results.

[0040] According to an embodiment, the different locations of the reference mirror correspond to surfaces located at different depths of the region of interest, wherein a depth difference between adjacent surfaces is smaller than the coherence length.

[0041] According to an embodiment, the different locations of the sample correspond to different sensing elements of a sensing sensor, or to different locations of a spot.

[0042] According to an embodiment, the different measurement conditions include different focus conditions.

[0043] According to an embodiment, the different measurement conditions include different locations of a reference mirror of a reference arm of an interferometer, the interferometer being used for obtaining interference pattern measurement results, wherein the different locations of the reference mirror correspond to surfaces located at different depths of the region of interest, wherein a number of the different locations is at least a sum of a number of measured wave-numbers and twice a number of higher spectral resolution wave-numbers.

[0044] According to an embodiment, step 110 is followed by step 120 of storing the multiple interference pattern information units.

[0045] According to an embodiment, step 120 is followed by step 130 of determining one or more attributes of the region of interest based on the multiple interference pattern information units.

[0046] According to an embodiment, step 130 includes at least one of step 131 of determining, by the processing circuit, when operating in a first mode, the one or more attributes of the region of interest based on the coherent interference data, and step 132 of determining, by the processing circuit, when operating in a second mode, the one or more attributes of the region of interest based on the coherent interference data.

[0047] According to an embodiment, the selection between step 131 and 132 is made based on whether the interference pattern measurement results are indicative of summed interferences of different wave-numbers. The different wave-numbers represent9943-PC wavelengths that are far enough than each other so that the interferences of different waves significantly differ from each other.

[0048] The selection between step 131 and 132 may be based on measurements or simulation or any other estimate regarding whether the interference pattern measurement results are indicative of summed interferences of different wave-numbers.

[0049] According to an embodiment, step 131 includes at least one of: a. Extracting by a processing circuit from the interference pattern measurement results, coherent interference data and. b. Determining, by the processing circuit one or more attributes of the region of interest based on the coherent interference data. c. Compensating for a decoherence associated with the interference pattern measurement results. d. Compensating for interference pattern measurement results that are uninfluenced by the different measurement conditions. e. Extracting the coherent interference data based, at least in part, or measured or simulated optical properties of one or more optical components involved in the generation of the interference pattern measurement results. f. Applying a vertical travelling scatterometry on the coherent interference data to determine one or more attributes of the region of interest. An example of vertical travelling scatterometry is illustrated in US patent application 2024 / 0085805 which is incorporated herein by reference.

[0050] According to an embodiment, step 132 includes at least one of: a. Separating, by the processing circuit, between summed interferences to determine a contribution of individual interference patterns associated with different wave-numbers. b. Separating, by the processing circuit, between summed interferences by solving a quadratic optimization problem.

[0051] Step 140 may be followed by storing information about one or more attributes, transmitting the information about one or more attributes, changing one or more illumination and / or collection and / or interferometer parameter based on the information about one or more attributes, granting access to a remote user to the information about one9943-PC or more attributes, generating a manufacturing process alert, changing a manufacturing parameter of the sample, and the like.

[0052] According to an embodiment there is provided a non-transitory computer readable medium for evaluating a sample having a region of interest of a given depth, the non-transitory computer readable medium stores instructions executable by a processing circuit for: obtaining interference pattern measurement results indicative of interference patterns associated with different measurement conditions and with different locations of the sample; the interference pattern measurement results are associated with a coherence length that is smaller than the given depth; storing the multiple interference pattern information units; extracting by a processing circuit, when operating in a first mode, from the interference pattern measurement results, coherent interference data; and determining, by the processing circuit, when operating in the first mode, one or more attributes of the region of interest based on the coherent interference data

[0053] According to an embodiment, there is provided a new measurement sequence and supporting algorithmic approach, removing the concern of fast spectral oscillations described above. According to an embodiment, the interferometry system includes an interferometer and additional hardware components such as those illustrated in US patent 10,161,885 which is incorporated herein by reference.

[0054] In an SI measurement, the incident light is split by a beam-splitter (BS) into two parts, one part reflected from the sample and another from a reference mirror. Light is reflected from the sample and mirror, re-combined by the BS and the resulting interference measured by a spectrometer. The core of the invention relies on the concept of coherence loss of the interference signal and on the dependence of the interferometric signal on both the wavelength and the reference mirror location. We can describe the measured signal in terms of the interference between light reflected from the sample and the reference mirror. A simplified description of the collected interference signal at wavelength ^ can be ^^ ^expressed as: ^^ = ^^^^^ ^ ^+ ^^^

[0055] of the mirror and ^^the field reflectivity of the sample. ^ represents the path-length difference between the two light paths (see Fig.1) – from BS to the mirror and back (path ‘A’ in the sketch) and to the sample top and back (path ‘B’).

[0056] Clearly, this description is grossly simplified; it does not account for the different transmissions of the two light paths, the overall system transmission, light source intensity,9943-PC detector collection efficiency etc. Here and in the discussion below all elements not important for the discussed invention are omitted for simplification (and can be accounted for by standard methods and calibrations).

[0057] We can define several technical terms: a. Optical Path Difference (OPD) matching: the Optical Path Difference is the difference in optical path length between two light paths. The optical path length of each path is the product of the geometric length and the refractive index of the material through which the light is propagating (inside a structure each electromagnetic eigenmode has a different phase velocity and hence a different effective refractive index and a different OPD). In interferometric measurements, the OPD plays an important role in determining the character of the measured signal. OPD matching is the practice of tuning a system to a specific value, or range of values, of the OPD. The OPD matching may have a crucial effect on the measurement, e.g., if the OPD is too large the coherence may be lost, as explained below. b. Coherence: two electromagnetic waves are said to be coherent if they are correlated during the measurement time and hence produce an interference pattern in time or in space. The intensity of the interference of two fields^^^^^^^^^^^^ and ^^^^^^^^^^^^ contains the contribution of the intensities ofthe twothe interference termwhere 〈⋅〉 denotes time-averaging. If the timeand &^is completely uncorrelated, the average will vanish. Similarly, if '^and '^are are too different (compared to the inverse of the averaging, the interference term will be suppressed. The coherence ( is defined as the ratio between the measured interference term (which can be measured, for example, by moving one of the mirrors in a Michaelson interferometer and measuring the amplitude of the resulting oscillations, as described in the US patent 10,161,885 which is incorporated herein by reference, and the interference expected for completely coherent fields.

[0058] One source of decoherence is the finite spectral resolution of the spectrometer. Consider a light wave leaving the light source and being split into two beams which travel optical paths )^and )^, respectively, which are then recombined in the spectrometer. If the9943-PC spectrometer’s effective resolution (which is determined by e.g., pixel width, point-spread- function (PSF) of the spectrometer, finite spot size, etc.) collects fields between wave- vectors + and + + Δ+, the measured interference term is -^^0^^ .+ ^^∗^+^^^^+^^^^^ / ^# / ^^^^^#^^(for simplicity we assume the integration time is are completely incoherent and do not interfere, and+. However, these assumptions are not essential for the idea).

[0059] If )^and )^differ by too much, the integrand will be highly oscillatory and the integral will be highly suppressed, so the interference term will be small and the coherence(will be low.

[0060] For the coherence to be high, one should require Δ+^)^− )^^ ≪ 23, so the coherence length—the OPD above which the interference is significantly suppressed—is inversely proportional to the spectral resolution Δ+. Hence, in order to have reasonable coherence, the OPD between the two beams, must be small enough, which is termed OPD matching. In a thick enough measured sample, it is impossible to have both reflection from the top and the reflection from the bottom OPD-matched with the beam passing in the other arm (the reference beam) of the interferometer. Thus, at least one of them will suffer from significant decoherence and hence its phase relative to the reference beam cannot be measured, and in fact is ill-defined.

[0061] For simplicity, let us describe the invention as applied to a sample with two reflecting interfaces. The idea can be applied to any thick sample, but the explanation is simpler in this case.

[0062] The reflection from a sample with two reflecting interfaces can be approximated (this approximation is inessential for the invention, it just simplifies the explanation) as ^ ^^^^^ = ^^45^^^ + ^64^^4^^^^^^ 78 ^9:

[0063] where ^^45 =top interface,^64^^4^ = |^64^^4^|^^^?=<<=@ is the field-reflectivity from the bottom, A is the geometricdifference between the interfaces, and B is the refractive index. The exponential factor ^^^^ ^ ^9:represents the phase that the light accumulates as it travels through the sample.

[0064] An ideal reflected signal would be = ;^^45^^^;^+ + 2;^^45^^^^64^^4^^^^;cos ^&^45− &64^^4^9943-PC

[0065] However, the interference between the top and the bottom is in general not fully coherent. This is due to several physical causes, e.g., finite spectrometer resolution etc., as explained above. Under such conditions, the measured signal can be approximated by ^ ^^| ^ ^|^^ ^#^L^M9^:^^FGHIJFK= ;^^45^ ; + + ^64^^4^^ + 2;^^45^ ^64^^4^^^^;^ cos N&^45−. ^^ ^^ ^would be ^ =^^^^^ ^ ^+ ^^45^^^ + ^64^^4^^^^^^ ^ ^9:^which, after taking decoherence into

[0067] ^^FGHIJFK= ;^^45^^^; +|^64^^4^^^^|^+|^^^^^|^+ term(^^45^64^^4^) depends only on the sample itself, the decoherence of the other terms (^^45^^and ^64^^4^^^) depends on the OPD (through ^). Thus, the OPD matching has anwhich terms will be coherent and which will suffer more significantly from decoherence. If the matching will be to the top surface, namely |^| ≪P#^^ , the interference of the top and the mirror will be coherent. If, on the other hand, thematching will be to the bottom, namely |2AB − ^| ≪ P^#^, the bottom will coherently interfere with the mirror. (If AB ≪ P#^,i.e., thein the sample is smaller the coherence length, both conditions can be fulfilled.)

[0069] Without loss of generality, let us describe the invention for the case of OPD matching to the top surface. The same ideas, with required modifications, apply to the coherent interference with any reflecting surface.

[0070] While for AB > P^#^the total measured signal is not coherent, one can take the coherent part of the signal. Consider W = ^^FGHIJFK−|^^|^−^FGHIJFK. This entity is accessible through direct measurements of the mirror and the sample.9943-PC Q^ ^

[0071] From the above we get W = 2;^^45^^^^^^^^;^# MR^cosN&^45− &^−^78 ^O+

[0073] One can write W as a coherent interference of the mirror with a field ^^= Q^MR^ Q^^^STUR^^ ^^;^^45^^^;^# ^^^^<=>+ |^64^^4^^^^|^# M^^^N^?=<<=@^ ^ ^9:O. related to ^ is|^^| = ;^^45^^^; + +|^64^^4^^^^|^^#[LM 9: +|^^|^the reflectionthe interface that is not OPD-matched to the mirror is attenuated, and only the coherent part of the reflection remains.

[0076] The suggested solution used|^^^^^|, the coherent part of the reflection, as the amplitude of the complex SI signal, instead of \^FGHIJFK.

[0077] In SI measurements, the field-reflectivity is extracted from a set of measurements ^^^, ^^^ =^^^^^^^ ^^ ^]+ ^^^.

[0078] As discussedseveral decoherence processes. Thus, the interference term W = ^^FGHIJFK− |^^|^−^FGHIJFKis written as2|^^^^|( cos N&^− &^−^78 ^^O, where (^^^ is a factor describing all decoherence effects.

[0079] From a set of such measurements, with prior knowledge of|^^|^and &^, the phase &^is extracted. However, as explained above, the phase &^is related to the field ^^rather than to ^^. But, by comparing the above analysis to the expression for W, ^^is nothing but (^^. This is true for the decoherence causes described above. However, there may be additional decoherence mechanisms. Let us denote the decoherence due to imperfections of the apparatus as (H^H.

[0080] Then W = 2|^^|(H^H|^^| cos N&^− &^−^7^^O. As long as ( is constant and does notout.9943-PC

[0081] So, taking the pre-factor of the cosine and dividing by 2|^^|(H^H, the coherent amplitude is obtained.

[0082] Since the amplitude is now taken from the interference term, it is sensitive to any imperfection in the measurement that introduces additional decoherence, e.g., vibrations of the mirror. Such imperfections may not be constant, and their effect on the interferometric signal can be written in a general functional form, which may include unknown coefficients. Since these unknown coefficients may vary between measurements, comparison or any other processing of the interferometric signal can be done only after accounting for this factor. This can be done in several ways. One approach is to fit these coefficients by optimizing some objective function (e.g., maximizing signal explained by the model) or fulfilling other known requirements, for example, matching between two spectra. Another option is to remove from all spectra every component that has that functional form. One way of doing this is by projecting the spectrum onto the orthogonal complement of these components.

[0083] Having the coherent signal instead of a mixed signal is a qualitative improvement. Among its implications are the ability to apply a vertical travelling scatterometry (VTS) algorithm on arbitrarily thick stacks (as disclosed in PCT Patent Application No. PCT / IB2022 / 050774 and US patent application US2024 / 0085805, incorporated herein by reference.

[0084] Since the signal is coherent, the VTS is well-defined and does not contain unphysical components (e.g., causality-breaking parts). This enables using spectral regions that suffer from smearing, thus improving vertical resolution (which depends on the spectral range).

[0085] Another implication is the ability to remove non-coherent contributions that limit measurement stability.

[0086] This also enables gaining more information on the bottom, or any other surface, of thick stacks – if the mirror is OPD-matched to the bottom, the main coherent reflection will be from the bottom, similarly, by OPD matching to another surface, the coherent reflection will be from that surface. Now VTS can separate the bottom (or any other surface) from other parts.

[0087] The mentioned above process is applied when operating in a first mode.

[0088] The above procedure is limited by the system coherence length which is dictated by the system’s effective spectral resolution as explained above. If the OPD is matched to a9943-PC certain surface in the sample, then the SI measurement captures several coherence lengths to either side of the surface, anything further will be incoherent and unusable for VTS.

[0089] One possible solution to this problem is to make multiple SI measurements, each of which OPD-matched to a different surface in the sample and the phase and amplitude be extracted separately for each one of them. The time-domain spectra obtained from the VTS algorithm for each of these measurements can then be combined into a single larger time- domain spectrum based on the locations of the mirror in each measurements. This approach has the drawback that each segment of the time-domain spectrum is based on the results of a single SI measurement instead of taking advantage of data from all the SI measurements and sewing them together may result in a discontinuous time-domain since any two segments may assign different coherence values to the same ^.

[0090] A better method would be to utilize all the raw interferometric measurements together to generate a frequency-domain spectrum whose resolution is larger than the resolution of the ordinary SI measurement. The higher spectral resolution implies a coherence length which is larger than the one of an ordinary spectral measurement. This approach has the advantage of better scalability: in theory the spectral resolution can be made arbitrarily at the expense of having to make more measurements, costing more time.

[0091] The salient idea is that when multiple wavelengths contribute to the same effective pixel, the weights of their contribution change in different ways as the mirror is moved. Measuring the intensity at the pixel at different mirror positions gives different linear combinations of the fields of the different wavelengths, which can be solved for the contribution of each field.

[0092] For example, if the _^`effective pixel (which can be a physical pixel, the spot size on the spectrometer, etc.), whose nominal wave-number is +G=^78a, gets contribution from B wavelengths, whose wave-numbers are +^Gb, theintensity in that effective pixel is ^^+G, ^^=∑GbdGGbe;^^f+^ ^Gbg; + ;^^f+^ ^Gbg; +wavelength to the _^`pixel (note that a wavelength may contribute to more than one pixel, e.g., if the effective pixel is determined by the spot size in the spectrometer.9943-PC

[0094] It follows that ^^+G, ^^ is of the form ^^+G, ^^ = i^+G^ +2 ∑GbdGGb;^^f+^Gbg;fjf+^Gbg cos 2+^Gb^ + kf+^Gbg sin 2+^Gb^g where j^+^ =cos^&^^+^ − &^^+^^ and k^+^ = sin^&^^+^ − &^^+^^.on z

[0096] By measuring at sufficiently many mirror locations ^ one can solve (in a least- squares sense, that is solving the optimization problemom,pi,nq∑G,^;^^+G, ^^^− i^+G^− 2∑GbdGGb;^^f+^Gbg;fjf+^Gbg cos +^Gb^^+ kf+^Gbg sin +^Gb^^^g ; which is quadratic and simple to spectral resolution which is higher

[0097] According to an embodiment, the mentioned above equation is dependent on z and an angle of incidence of the radiation on the sample.

[0098] That higher spectral resolution leads in turn to a longer coherence length.

[0099] In order to obtain jf+^Gbg, kf+^Gbg and i^+G^, there should be at least 2^^^+ ^^mirror positions (more are increase the resilience to noise), where^^^ is the number of higher spectral resolution wave-numbers +^Gband ^^is the number of measured wave-numbers. Obviously, the mirror locations should be chosen such that the formula above is not “blind” to any linear combination of the jf+^Gbg, kf+^Gbg and i^+G^ (or in other words, the corresponding matrix should be non-minimum. Furthermore, for each surface of interest in there should be at least two mirror locations whose OPD with that surface is roughly one coherence length or less; otherwise, no measurement ^^+G, ^^^ will contain any coherent signal from that surface and therefore neither will jf+^Gbgand kf+^Gbghave any contribution from that surface.

[0100] A similar approach can be used to increase the angular resolution. Defocusing introduced a phase that changes radially across the pupil thus making pupil points at different radii interfere with an extra radial phase depending on the defocus when imaged onto the spectrometer. By measuring at different defocus values, known different linear combinations of the angular average field at different radii on the pupil are measured, The angular average fields on the pupil can then be solved for in a similar fashion to what was done above for the spectrometer effective pixels. One downside for this approach is that it increases the illumination and collection spots.9943-PC

[0101] Specifically, if there is spearing on the pupil and no spectral smearing, themeasured interferometric intensity is ^^+, ^^ = - .r cos r s|t^^+, r^|^ + |t^^+, r^|^ +2|t^^+, r^|^j^+, r^ cos^2+^ cos r^ + k^+, r^ sin^2+^ cos r^^u where r is the angle ofincidence. Hence, we can find can discretize the angles of incidence, measure multiplemirror locations ^^, and find j^+G, rv^ and k^+G, rv^ by the optimization problemo m,pi,nq ∑wM,wRGx^,^x^ ;^^+G, ^^^ − 2 ∑v dv|t^^+G, rv^|fj^+G, rv^ cos^2+G^^ cos rv^ +the mirror and thus different pupil points interfere (assuming they are in the same coherent patch). In this case the measured interferometric intensity is

[0104] ^^+G, ^, y^ = - .+.^+ ^ ^zdG^+, {z, y^||t^^+, {z^| + |t^^+, {z + y^| +a pupil vector. ^̂f^^, +^Gb, {z, yvgis half of the optical path to the tilted mirror and back. dG^+, {z, y^ is a weight that takes into account the spectral smearing and the coherence between different points on the pupil as well as and wavelength and pupil dependent properties of the system.

[0105] By taking several measurements at different tilt angles in two directions, the reflected field in different regions in the pupil plane can then be extracted by the optimization problem wM,wR,w^^

[0106] mode.

[0107] According to an embodiment, the suggested solution provides at least the following benefits.9943-PC a. The coherent signal rather than the entire signal is measured even for thick structures. This enables the use of algorithms such as VTS without any acausal artifact. b. The additional spectral and / or angular resolution may provide access to spectral and angular features sensitive to various properties of interest in the measured sample. c. The invention provides scalability: there is no hard barrier to increasing the spectral and angular resolution and the resulting coherence length. d. The price paid in measurement time is lower than in alternative methods.

[0108] In the foregoing detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

[0109] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings.

[0110] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

[0111] Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

[0112] Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method and / or should be applied mutatis mutandis to a non-transitory computer readable medium that stores instructions for implanting the method.9943-PC

[0113] Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system and / or should be applied mutatis mutandis to a non-transitory computer readable medium executable by the system.

[0114] Any reference in the specification to a non-transitory computer readable medium should be applied mutatis mutandis to a method implemented by executing instructions stored in the non-transitory computer readable medium and / or should be applied mutatis mutandis to a system capable to execute the instructions.

[0115] In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims.

[0116] Moreover, the terms “front, ” “back, ” “top, ” “bottom, ” “over, ” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

[0117] Any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected, " or "operably coupled, " to each other to achieve the desired functionality.

[0118] Furthermore, those skilled in the art will recognize that boundaries between the above described operations are merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

[0119] Also, for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.9943-PC

[0120] However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

[0121] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps than those listed in a claim. Furthermore, the terms “a” or “an, ” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first" and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

[0122] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

9943-PC WE CLAIM 1. An interferometry system for evaluating a sample having a region of interest of a given depth, the interferometry system comprises: a memory unit configured to store interference pattern measurement results indicative of interference patterns associated with different measurement conditions and with different locations of the sample; the interference pattern measurement results are associated with a coherence length that is smaller than the given depth; and an analysis unit comprising a processing circuit that is configured to: extract, when operating in a first mode, from the interference pattern measurement results, coherent interference data; and determine, when operating in the first mode, one or more attributes of the region of interest based on the coherent interference data.

2. The interferometry system according to claim 1, wherein the extraction of the coherent interference data comprises compensating for a decoherence associated with the interference pattern measurement results.

3. The interferometry system according to claim 1, wherein the extraction of the coherent interference data comprises compensating for interference pattern measurement results that are uninfluenced by the different measurement conditions.

4. The interferometry system according to claim 1, wherein the extraction of the coherent interference data is based, at least in part, or measured or simulated optical properties of one or more optical components involved in the generation of the interference pattern measurement results.

5. The interferometry system according to claim 1, wherein the different measurement conditions comprise different optical path differences between a measurement arm of an interferometer and a reference arm of the interferometer, the interferometer being used for obtaining interference pattern measurement results.

6. The interferometry system according to claim 1, wherein the different measurement conditions comprise different locations of a reference mirror of a reference arm an interferometer and a reference arm of the interferometer, the interferometer being used for obtaining interference pattern measurement results.

7. The interferometry system according to claim 6, wherein the different locations of the reference mirror correspond to surfaces located at different depths of the region of interest, wherein a depth difference between adjacent surfaces is smaller than the coherence length.9943-PC 8. The interferometry analysis unit is configured to apply a vertical travelling scatterometry on the coherent interference data to determine one or more attributes of the region of interest.

9. The interferometry system according to claim 1, wherein the different measurement conditions comprise different focus conditions.

10. The interferometry system according to claim 1, wherein the processing circuit is configured to operate in a second mode when it is determined that interference pattern measurement results are indicative of summed interferences of different wave- numbers.

11. The interferometry system according to claim 10, wherein operating in the second mode, the processing circuit is configured to separate between summed interferences to determine a contribution of individual interference patterns associated with different wave-numbers.

12. The interferometry system according to claim 11, the processing circuit is configured to separate between summed interferences by solving a quadratic optimization problem.

13. The interferometry system according to claim 11, wherein the different measurement conditions comprise different locations of a reference mirror of a reference arm of an interferometer, the interferometer being used for obtaining interference pattern measurement results, wherein the different locations of the reference mirror correspond to surfaces located at different depths of the region of interest, wherein a number of the different locations is at least a sum of a number of measured wave-numbers and twice a number of higher spectral resolution wave- numbers.

14. The interferometry system according to claim 11, wherein the different measurement conditions comprise different focus conditions.

15. The interferometry system according to claim 11, wherein the different measurement conditions comprise different reference mirror tilt conditions.

16. The interferometry system according to claim 1, comprising an interferometer being used for obtaining interference pattern measurement results.

17. A method for evaluating a sample having a region of interest of a given depth, the method comprising: obtaining interference pattern measurement results indicative of interference patterns associated with different measurement conditions and with different locations9943-PC of the sample; the interference pattern measurement results are associated with a coherence length that is smaller than the given depth; storing the multiple interference pattern information units; extracting by a processing circuit, when operating in a first mode, from the interference pattern measurement results, coherent interference data; and determining, by the processing circuit, when operating in the first mode, one or more attributes of the region of interest based on the coherent interference data.

18. The method according to claim 17 comprising operating the processing circuit in a second mode when it is determined that interference pattern measurement results are indicative of summed interferences of different wave-numbers, wherein the operating in the second mode comprises separating between summed interferences to determine a contribution of individual interference patterns associated with different wave-numbers.

19. A non-transitory computer readable medium for evaluating a sample having a region of interest of a given depth, the non-transitory computer readable medium stores instructions executable by a processing circuit for: obtaining interference pattern measurement results indicative of interference patterns associated with different measurement conditions and with different locations of the sample; the interference pattern measurement results are associated with a coherence length that is smaller than the given depth; storing the multiple interference pattern information units; extracting by a processing circuit, when operating in a first mode, from the interference pattern measurement results, coherent interference data; and determining, by the processing circuit, when operating in the first mode, one or more attributes of the region of interest based on the coherent interference data 20. The non-transitory computer readable medium according to claim 18 that further stores instructions executable by the processing circuit for operating in a second mode when it is determined that interference pattern measurement results are indicative of summed interferences of different wave-numbers, wherein the operating in the second mode comprises separating between summed interferences to determine a contribution of individual interference patterns associated with different wave- numbers.

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