Optical metrology system and method

Abstract

A measurement system for use in optical metrology measurements is provided, the measurement system including a control system that processes raw measurement data indicative of a spectral interference signal measured on a sample in response to an illuminating electromagnetic field incident on the top of the sample and including at least one spectral range in which the sample does not substantially absorb, the processing including extracting from the raw measurement data a portion of the spectral interference signal that describes variations in signal intensity with changes in optical path difference during the interferometric measurement, the extracted signal portion being independent of the interference signal returned from the bottom of the sample in response to the illuminating electromagnetic field, and directly determining from the extracted portion both the spectral amplitude and spectral phase of the reflection of the electromagnetic field from the top of the sample, thereby determining a measured spectral signature that characterizes the top of the sample.

Description

[Technical Field]

[0001] The invention is in the field of optical measurement of patterned structures such as semiconductor wafers, and is particularly useful for metrology measurements. [Background technology]

[0002] Optical measurement methods widely used in semiconductor metrology and process control traditionally rely on spectral reflectometry and / or spectral ellipsometry. However, the electromagnetic field scattered from structures also contains spectral phase information, which is highly useful for extracting more accurate information from measurements, but is not directly accessible at optical frequencies, so spectral interferometry is used.

[0003] Some spectral interference techniques are described, for example, in the following applications assigned to the assignee of the present application: [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 10,739,277 [Patent Document 2] U.S. Patent No. 10,161,885 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a need in the art for novel techniques to perform infrared (IR)-based spectral interferometry on patterned structures and to be able to separate contributions from the backside of the structure based on the limited temporal coherence of the interferometer.

[0006] Extending the wavelength range of scatterometry measurements into the IR range offers several advantages, particularly the ability to penetrate opaque materials in the Vis-UV range, inherent sensitivity (e.g., to material properties), and advantages in simulation simplicity. More specifically, when measuring structures that are partially or completely transparent to some or all of the optical system's operating wavelengths (e.g., structures fabricated on (standard) Si wafers), the incident light is not absorbed within the structure but rather continues to propagate through it, some of which is reflected back from the bottom (backside) and emerges through the top of the structure, resulting in a parasitic signal that must be separated from the top reflection. At wavelengths above approximately 1.1 μm, the Si substrate becomes transparent, and more significant contamination can occur due to reflections from the backside of the wafer. The backside reflection of the wafer is not stable due to the low repeatability of the wafer pickup surface.

[0007] In general, it is desirable to use normal-incidence metrology, which has several advantages when compared to oblique-incidence operation, such as those related to the simplicity of the hardware (illumination and collection optics are common over most of the optical path), the compactness of the dimensions, allowing the integration of solutions (e.g., integrated metrology) into small footprint implementations, and even the simplicity of the simulation of light-matter interactions.

[0008] However, in normal incidence mode with IR illumination, it is inevitable that reflections from the top of the structure (desired data) and the bottom of the structure (contamination) are collected. When high spectral quality is required, as is typical in current metrology requirements, this contamination can detrimentally hinder the accurate interpretation of the measurement data. Therefore, it is desirable to avoid or precisely remove this contamination while maintaining other system parameters such as spot size and throughput.

[0009] Several approaches are available to mitigate this problem. For example, wafer back-surface reflections can be removed through a confocal optical layout (implementation of a confocal layout allows for a significant reduction of out-of-focus contributions). However, confocal mode presents a problem for scatterometry in that it introduces strong sensitivity to small focus errors. Following algorithmic removal of wafer back-surface reflections, the signal contribution arising from wafer back-surface reflections is estimated and algorithmically subtracted from the measurement signal. However, back-surface reflections cannot be considered a "constant" contribution because they can vary between different positions on the wafer (depending on the roughness or properties of the underlying stage holding the wafer), and measurement structures (fabricated on top of the wafer) affect the signal reflected from the back surface, and changes in these structures affect the overall light transmission through it. The expected spectral errors associated with back-reflections are on the order of a few percent (or a fraction of a percent for some optical layouts), and a reasonable quantitative interpretation of the error may still be possible. However, for high-end scatterometry addressing advanced applications, such errors are significantly unacceptable, and therefore the back-surface contribution cannot be simply ignored. Therefore, IR scatterometry for semiconductor metrology is generally based on oblique-incidence measurements. [Means for solving the problem]

[0010] The technique of the present invention is based on a novel approach, which analyzes raw measurement data, showing the spectral interference signals measured on the sample for different optical path differences between the sample and the reference arm of the interferometer, utilizing operating wavelengths that include wavelengths that the sample does not substantially absorb. The invention allows for the direct extraction of both the spectral reflectance and the spectral phase of the top of the sample, regardless of whether the raw measurement data also includes reflections from interfaces other than the top of the sample (e.g., bottom reflections).

[0011] This is achieved in the present invention by extracting from the raw measurement data a portion of the interference signal describing the variation in signal intensity with changes in the optical path difference (OPD) between the sample and reference arms over several (at least four) different values ​​of OPD. This portion of the measured spectral interference signal is independent of the interference signal returned from one or more internal interfaces of the sample, and therefore the spectral amplitude and spectral phase of the signal returned from the top of the sample can be determined directly from that portion. In the following description, this signal portion describing the intensity (amplitude) variation will be referred to as the "AC portion of the signal" measured over variations in optical path difference.

[0012] As described below, the inventors have found that the so-called "DC portion" of the measured signal, i.e., the portion independent of the OPD, is the only signal portion related to reflection from the back surface (or other internal interface) of the sample, while the AC portion of the signal is related only to the top reflection. This condition holds under the assumption that the spectral bandwidth of a single pixel is generally very narrow (typically about 1 nm to a few nm), so that the light field reflections from the interferometer mirror (generally the optical path difference introducing unit), the bottom surface (generally an internal interface of the sample), and the top surface do not change over the spectral bandwidth of the pixel.

[0013] While the invention is particularly useful for optical metrology of silicon structures (semiconductor wafers) where normal incidence measurements using the IR spectral range are desired, it should be noted that the principles of the invention are not limited to either the type of sample or the spectral range of operating wavelengths. The invention is advantageously useful for spectral interferometric measurements of samples of this type where the top response (amplitude and phase) of the sample to illumination applied to the top is determined, and the illumination includes wavelengths that are not substantially absorbed by the sample material.

[0014] Thus, according to one broad aspect of the present invention, there is provided a measurement system for use in optical metrology measurements, the measurement system including a control system configured as a computer system including a data input utility, a memory, and a data processor, the control system configured to be in data communication with a measurement data provider for receiving raw measurement data indicative of a measured spectral interference signal returned from the sample in response to an illuminating electromagnetic field incident on the top of the sample and including at least one spectral range in which the sample does not substantially absorb, the data processor including an analyzer utility configured and operative to:

[0015] Extracting from the raw measurement data a portion of spectral interferometric signals describing the variation of signal intensity with changes in optical path difference OPD during the interferometric measurement, said portion of the spectral interferometric signals being independent of the interferometric signal returned from the bottom of the sample in response to said irradiating electromagnetic field.

[0016] From the extracted portion of the spectral interference signal, both the spectral amplitude and spectral phase of the reflection of the irradiating electromagnetic field from the top of the sample can be directly determined, thereby determining the measured spectral signature that characterizes the top of the sample.

[0017] The control system may further comprise a fitting utility configured and operable to apply model-based processing to the measured spectral signature to determine one or more parameters of the sample being measured.

[0018] The technique of the present invention is essentially useful for monitoring measurements of samples / structures made of silicon materials using the IR spectrum or a combination of visible and IR spectra.

[0019] In some embodiments, the raw measurement data includes data indicative of spectral interference signals returned from the top and bottom of a sample made of silicon material (and returned from one or more internal interfaces of the sample) in response to an irradiating electromagnetic field that includes an IR spectrum.

[0020] In some embodiments, the measurement system includes a measurement unit configured as a measurement data provider to generate and provide said raw measurement data, the measurement unit configured and operable to perform spectral interferometric measurements of the sample using an operating wavelength that includes at least one spectral range in which the sample does not substantially absorb.

[0021] For example, a spectral interferometric signal has a signal intensity profile that is determined as follows:

[0022]

number

[0023] Here, E top (λ i ) is the spectral amplitude of the electromagnetic field reflected from the top, and E bot. (λ i ) is the spectral amplitude of the electromagnetic field reflected from the bottom of the sample, and E m (λ i ) is the electromagnetic field reflection from the interference mirror, and λ i is the operating wavelength, z is the position of the interference mirror, and φ(λ i) is the spectral phase of the measured spectral interference signal.

[0024] The extracted signal parts are:

[0025]

number

[0026] This describes the variation of the signal intensity with the change of the position z of the interference mirror, independent of the interference signal returned from the bottom of the sample, allowing the direct determination of the spectral amplitude and spectral phase of the top electromagnetic field reflection.

[0027] According to another broad aspect of the invention, there is provided a method for use in optical metrology measurements, the method comprising: providing raw measurement data indicative of a measured spectral interference signal returned from the sample in response to an illuminating electromagnetic field incident on the top of the sample, the measured spectral interference signal including at least one spectral range in which the sample does not substantially absorb; The raw measurement data is processed, and the processing is extracting from the raw measurement data a portion of the spectral interference signal that describes the variation of signal intensity with changes in optical path difference OPD during the interferometric measurement, said portion of the spectral interference signal being independent of the interference signal returned from the bottom of the sample in response to said irradiating electromagnetic field; directly determining from the extracted portion of the spectral interference signal both the spectral amplitude and the spectral phase of the reflection of the irradiating electromagnetic field from the top of the sample, thereby determining a measured spectral signature that characterizes the top of the sample; Includes:

[0028] According to yet another aspect of the present invention, a measurement unit configured and operable to perform normal incidence spectral interferometry measurements on the sample and generate raw measurement data indicative of measured spectral interferometry signals on the sample for a number of different optical path differences (OPDs) between the sample arm and the reference arm using operating wavelengths that include at least one spectral range in which the sample does not substantially absorb; a control system configured and operable to: extract from the raw measurement data a portion of a spectral interference signal describing variations in signal intensity with changes in optical path difference OPD during the interferometric measurement, said portion of the spectral interference signal being independent of an interference signal returned from the bottom of the sample in response to said irradiating electromagnetic field; and from the extracted portion of the spectral interference signal, directly determine both the spectral amplitude and spectral phase of the reflection of the irradiating electromagnetic field from the top of the sample, thereby being able to determine a measured spectral signature characterizing the top of the sample; A measurement system is provided that includes:

[0029] In order to better understand the subject matter disclosed in this specification and to illustrate how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 2 is a block diagram of the control system of the present invention associated with the spectral interferometry unit. [Figure 2] FIG. 1 is a flow diagram of a method of the present invention used for data interpretation of spectral interferometry data to directly extract the amplitude and phase of a sample's response to illumination. [Figure 3] This illustrates the principles underlying the present invention. [Figure 4] This illustrates the principles underlying the present invention. [Figure 5] 1 is a specific example of a measurement system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] 1, a block diagram of a measurement system 10 of the present invention is shown. The system includes a measurement unit 12 and a control system 14.

[0032] The measurement unit 12 is configured as a spectral interferometer system and is operable to measure the spectral phase of light returned (reflected and / or scattered) from a structure / sample S (e.g., a semiconductor wafer) and can determine one or more parameters of the structure (e.g., pattern parameters). The measurement unit 12 may be based on a typical spectral reflectometer configuration in which sample reflectivity is precisely measured and corrected for spectral interferometry.

[0033] For the purposes of this invention, the spectral signature of the top of the sample is measured using normal incidence measurements applied to the top of the sample, with the operating wavelengths including at least one spectral range in which the sample does not substantially absorb. Thus, in addition to the useful signal (reflection / scattering at the top of the sample), the raw measurement data inevitably includes "parasitic" signal components related to the internal interfaces of the sample, e.g., reflection / scattering at the bottom of the sample.

[0034] Therefore, the measurement unit 12 receives the broadband input light L in The optical fiber 10 includes a nominal-incidence interferometer assembly including a light source system 16 providing a wavelength (e.g., including in the IR spectral range), a detection system 18 (including a spectrometer), and light steering optics 20. The light steering optics 20 includes beam splitters / combiners 22 and 24, an objective lens unit 26 (one or more lenses), and an optical path difference steering unit 28 (e.g., an interference mirror).

[0035] The beam splitter / combiner 22 splits the input light beam L inThe input light L in is propagated (reflected) toward the objective lens unit 26, which then converts the input light L in The light is focused on the measurement surface MP where the structure S is located, and the light L is returned from the measurement surface. com The beam splitter 24 is located between the objective lens unit 26 and the measurement plane MP, and transmits the input light L in The sample light component L sam and the reference light component L ref and propagates along different optical paths toward the measurement surface (sample surface) and the optical path difference guiding unit 28, and the reflection of the sample and the reflection of the mirror 28 are combined into a composite light L com This light is combined into a single beam and guided to the objective lens unit 26, which focuses the light on the detection surface to form an interference pattern.

[0036] The control system 14 is configured here as a computer system including a data input / output utility 14A, a memory 14B, and a data processor 14C. The control system also includes a controller 14D associated with the mirror 28, which controls the mirror position and thus a change in the optical path difference between the sample and reference arms. The reflectivity E of the mirror 28 for the wavelength range of interest is m (λ i ) is measured once and stored in memory 14B.

[0037] The data processor 14C is configured and operable to analyze raw measured data generated by the detection system 18, including data indicative of a measured spectral interference signal. According to the invention, the data processor 14C includes an analyzer 30, which includes a signal portion extraction utility 15 for extracting from the raw measured data a portion of the spectral interference signal describing the variation in signal strength (amplitude) due to changes in several (at least four) different OPD values, and a top characterization utility 17. The latter utilizes a predetermined (single measured) reflectivity of the mirror 28 to derive from the extracted portion of the interference signal the spectral reflectance E of the top of the sample. top (λ i ) and the spectral phase φ(λ i ) directly.

[0038] The apex characterization utility 17 is further configured to utilize the spectral reflectance and phase of the apex to generate a spectral signature that characterizes the apex of the sample.

[0039] The control system 14C also includes a fitting utility 32 configured and operable to apply model-based processing to the spectral signature of the top of the sample to determine one or more parameters of the sample.

[0040] The control system 14 may or may not be integrated with the measurement unit 12. In general, the control system 14 may be a stand-alone system configured to communicate data with the measurement unit 12 itself or a measured data provider, which may be a storage device on which the measurement data is stored.

[0041] According to this invention, both the spectral reflectance and the spectral phase of the top of the sample can be directly determined (extracted) from the raw measured spectral interference signal, regardless of whether the raw measured data contains any contamination components related to the reflectance of intermediate layers / interfaces in the sample.

[0042] An optical path difference-inducing element (e.g., an interferometer mirror) does not suffer from problems associated with reflections from its back surface. This can be avoided by making the mirror 28 from a material that is opaque in the spectral range to be measured (e.g., a specific IR range by using a metal or metal-coated mirror) or by using a mirror that is thick enough to guarantee no loss of time coherence between its back surface and other reflections in the system. More specifically, the mirror is made from a different material composition and has a different thickness than the sample to be measured (typically Si in the case of a semiconductor wafer), eliminating or at least significantly reducing the interfering contribution to the measurement signal of coherent reflections from the sample and the bottom surface of the mirror.

[0043] The principle of the present invention will be explained below. In the following explanation, the top portion may be referred to as the "top surface."

[0044] Considering the case of interference reflectometry where only the top surface reflection of the sample is detected (as in the case of visible wavelengths), the single wavelength λ interference equation becomes:

[0045]

number

[0046] where, E m is the electromagnetic reflectivity of the mirror 28 (|E m | 2 can be measured when the structure S is outside the field of view of the objective lens unit 26). E w is the electromagnetic field reflectivity of the sample S (|Ew | 2 can be measured at a tilt position where the mirror 28 is tilted so that the reflected light from the mirror propagates along an axis that does not intersect the objective lens unit). k=2π / λ ·z is the optical path difference between the mirror 28 surface and the sample surface MP.

[0047]

number

[0048] The above φ is the interference phase.

[0049] Here, we consider interference reflectance measurements for the resolution of a spectrometer. Each pixel in the spectrometer has a finite spectral width. For simplicity, we consider each pixel to be averaged over the following range:

[0050]

number

[0051] The interference equation is as follows:

[0052]

number

[0053] For the optical path difference (OPD) condition OPD<<λ, the interference equation is:

[0054]

number

[0055] If OPD>>λ, then:

[0056]

number

[0057] Thus, in general, for interference reflectance measurements where only the top surface reflection of the sample is detected (as at visible wavelengths), the electromagnetic field reflected from the sample, E w can be extracted from the baseline or "DC" component of the measured intensity of the interference signal, and the phase data can be extracted from the fluctuation or "AC" component of the measured data. To this end, a "baseline" measurement of the sample's reflectance is performed without the contribution of the mirror, and then multiple (e.g., four) interferometric measurements are performed while varying the optical path difference (e.g., by moving mirror 28).

[0058] The inventors have shown that when working with broadband interferometric spectra, the "desired" component of the sample reflection (i.e., top surface reflection) can be extracted from the fluctuating component, which is the profile of the measured amplitude that varies with the optical path difference over several interferometric measurements (e.g., at least four such measurements). This method can be used for visible channel spectral interferometry. However, the advantages of this technique are even more significant when using IR spectra, where the technique allows for the isolation of contributions from the back surface of the sample based on the limited time coherence of the interferometer.

[0059] The following is a description of the technique of the present invention for IR spectral interferometry. For simplicity, some assumptions have been made regarding the reflective properties of the sample and interferometer mirrors. However, it should be understood that the principles of the present invention do not depend on these assumptions, and they are used only to simplify the explanation.

[0060] The temporal coherence of a spectrometer is dictated by its spectral resolution. For this purpose, λ i -δ<λ<λ i Consider a particular pixel on a spectrometer that measures the spectral range of +δ, where λ i is the central wavelength read by this pixel, and δ determines its spectral bandwidth.

[0061] λ i The measured intensity of a pixel for is given by the following equation:

[0062]

number

[0063] The measured interference intensity is measured by the interferometer mirror E m (λ) and the field reflected from the sample E w It is defined by the interference between the field reflected from (λ) and the field reflected from (λ).

[0064] The material composition of the interferometer mirrors is chosen so that their reflectance does not have abrupt spectral changes. Because the spectral bandwidth of a single pixel is generally very narrow (typically about 1 nm to a few nm), the mirror reflectance in this range is constant, and E m (λ)≒E m (λ i ) can be assumed as follows.

[0065] The sample reflectance over this spectral range is similarly nearly constant (typical samples rarely exhibit significant spectral variation over such a narrow spectral range), except that it is composed of two contributions:

[0066]

number

[0067] Item 1 E top (λ) relates to the reflection from the top of the sample. Under the simplifying assumption of a reflectance that is (roughly) invariant over the spectral bandwidth of the pixel, E top (λ)≒E top (λ i ) can be used.

[0068] 2nd term

number

number

[0069] where 2h is twice the total sample thickness and is the path the light takes as it goes down to the bottom of the sample and back to the top. In reality, the light may undergo multiple reflections between the two sample sides, but for simplicity, these terms are ignored in the analysis.

[0070] Similarly, E bot. (λ)≒E bot. (λ i ) can be assumed. However, the vibration term

number

[0071] The measured intensity is determined by the interference of the electromagnetic fields reflected from the sample and the mirror. The position z of the mirror 28 is controllably scanned through multiple values, providing multiple interference spectra from which both the reflected phase and amplitude can be derived.

[0072] For a particular mirror position z, the intensity for a particular wavelength λ is given by equation (4) below:

[0073]

number

[0074] λi The intensity measured at a pixel for is given by:

[0075]

number

[0076] Under the above assumptions, this strength is given by:

[0077]

number

[0078] Here, it is defined as follows:

[0079] I0(λ i )≡|E top (λ i )| 2 +|E bot. (λ i )| 2 +|E m (λ i )| 2 (6)

[0080] This is independent of the mirror position z and is the E bot represents the signal contribution related to reflections from

[0081] Therefore, unlike measurements using the UV-VIS spectrum, when working in the IR spectrum, the DC part of the measurement signal I0(λ i ) is the top surface reflection E top It cannot be used to extract E bot This is because it is connected to

[0082]

number

[0083] It involves the interference between light reflected from a mirror and light reflected from the top of a sample, which is the actual structure (ie, the component of interest) that is the subject of the interferometric measurement.

[0084] The above equation (7) can be written differently as follows:

[0085]

number

[0086] Here, φ(λ i ) is the spectral phase between the electromagnetic field reflected from the mirror and the top surface of the sample. The measurement characteristics of the spectral interferometer are |E top (λ i )| and φ(λ i )

[0087]

number

[0088] This is related to the interference between the reflection from the bottom surface of the sample and the mirror.

[0089] It should be noted that the mirror position (z) is typically on the order of the measurement wavelength, since it is used to measure the effect of changing the phase between the sample and the mirror. It is therefore typical for a few hundred nanometers or at most a few microns, and in either case it is significantly smaller than the optical path length represented by the reflection from the back surface of the sample, i.e., 2nh.

[0090] term

number

number

number

number

[0091] I B (λ i The integral term in ) is related to temporal decoherence, reflecting the fact that interference from the back surface of the sample produces a highly oscillating signal that is averaged over the pixel.

[0092] Specifically, for n~4, h~700 μm and a typical spectral response of the spectrometer (e.g., δ~1 nm spectral bandwidth) and a measurement wavelength of ~2000 nm, this term is <10 of the signal. -10 This represents a decay of the order of , and this contribution is effectively completely removed from the measurement.

[0093] Therefore, the spectral interferometric signal can be expressed as follows:

[0094]

number

[0095] Here, the term I0(λ i ) includes the effect of reflection from the rear surface of the sample, but is independent of the interferometer mirror position z. |E top (λ i )| and φ(λ i ) are the reflected field amplitude and phase, which are properties measurable by the spectral interference unit, and |E m (λ i )| is the mirror reflectivity and z is the mirror position.

[0096] The z-independent term I0(λ i) is affected by the reflection from the back surface of the sample, but it has been shown that there is no such influence on the z-dependent term

Number

[0097] Refer to FIGS. 2, 3 and 4 which illustrate the measurement scheme according to this invention. As shown in the flow diagram 100 of the method of this invention in FIG. 2, once the measured mirror reflectance E m (λ i ) and a plurality of measured values of I(λ i , z) obtained at various mirror positions z are provided.

[0098] FIG. 3 shows the variation of the measurement intensity I(λ i ) with the change of the mirror position z. This graph has an AC component I AC , that is, an intensity variation profile during the z-scan. FIG. 4 illustrates similar spectral measurements for two wavelengths λ1 and λ2 (for example, those in the IR spectrum).

[0099] According to the technology of this invention, this part of the raw measurement data (that is, the interference signal part describing the intensity / amplitude variation with the change of the OPD value) is extracted and used to directly determine the spectral reflection and phase at the top of the sample. For this purpose, four or more measurements I1 to I4 with different optical path difference values (z values) can be performed.

[0100] As described above (the above formula (10)), the only part of the spectral interference measurement signal related to the bottom reflection of the sample is the z-independent term. Therefore, the reflectance E top (λ i ) and the phase φ(λ i) can be extracted from the amplitude-varying signal portion only, i.e., the intensity profile of the signal measured over the z-variation. Ignoring the intensity average in the interpretation of the measured data means completely eliminating the contribution related to the (incoherent) rear surface reflection of the sample.

[0101] Equation (10) can be expressed as follows:

[0102]

number

[0103] where A is a z-independent constant (i.e., the term I0(λ i ) is independent of the interferometer mirror position z, although it does include some contribution from the reflection from the back surface of the sample, and the parameter B (i.e., 2|E top (λ i )||E m (λ i )|) and the parameter C (i.e.,

number

[0104] For example, the spectral amplitude and spectral phase can be determined as follows:

[0105]

number

[0106] Returning to Figure 1, the control system 14 receives raw measurement data, e.g., from the spectral interferometry measurement unit 12, indicative of measured spectral interference signals for at least four different values ​​of the optical path difference between the sample arm and the reference arm, e.g., different z positions of the mirror 28. The data processor 14C (its analyzer 30) extracts from the raw measurement data a portion of the interference signal with varying amplitude over varying z values ​​and calculates from this signal portion the top surface reflection E top (λ i ) and phase φ(λ i ) directly.

[0107] It should be understood that the present invention is suitable for use with any spectral interferometry system to directly filter out reflection components from sample interfaces other than the top from data analysis, but is particularly useful for interpretation of IR measurements, which otherwise does not allow direct extraction of the top portion response.

[0108] Reference is now made to Figure 5, which illustrates a schematic representation of a particular, non-limiting example of a measurement system 200 embodying the present invention. System 200 is configured generally similarly to the above-described system 10 of Figure 1, i.e., it includes a spectral interferometry measurement unit 12 and a control system 14. The same reference numerals are used to indicate system components common to both examples.

[0109] System 200 includes a light source system 216, which in this example provides broadband input light in both the visible and IR spectrum, a detection system 218, which in this example includes a spectrometer (spectrophotometer) 18 that generates spectral data of the light incident thereon and an imaging detector 19 for navigating to a measurement site on the structure and / or collecting an interference fringe pattern, and an optical system 220 configured as a light guide to direct light from light source 216 towards an optical path difference guide mechanism 28 (e.g., a planar reference mirror in this non-limiting example) towards a sample / structure S under measurement disposed on a sample support, and to direct returning light towards detection system 216. The output of detection unit 216 is communicated (via wired or wireless signal communication) to control system 14.

[0110] The optical system 20 receives input light L from a light source 216. in The illumination channel for propagating the measured light L com and a detection channel for propagating the input light L towards the detection system 218. in is directed (reflected) by beam splitter / combiner 22 to objective lens unit 26, which directs this light to beam splitter / combiner 24, which converts the input light L in is the sample beam L sam and the reference beam L ref These light beams L sam and L ref interact with the sample S and the optical path difference inducing unit 28, respectively, and the respective reflections (scattering) L' sam and L' ref is combined by the splitter / combiner 24 into a light beam L com The latter is directed by a lens unit 26 and a beam splitter / combiner 22 to the detection units 18 and 19 of the detection system 218. For this purpose, the optical system 20 directs the return light beam L com The light portion (Lcom )1 and (L com )2.

[0111] In this non-limiting example, the light directing device 20 directs input light L propagating from the light source 216 towards the beam splitter / combiner 22. in and a tube lens 23 in the detection channel in the optical path of the measurement light propagating towards the detection unit 18.

[0112] Furthermore, in this non-limiting example, the optical system 20 includes polarizers 32 and 34 located in the illumination and detection channels, respectively. More specifically, the input light L from the light source 216 in The input light passes through a polarizer 32, and the input light of a particular polarization (e.g., linear polarization) is directed by a beam splitter / combiner 22 to an objective lens 26, which in turn is directed to a beam splitter / combiner 24. The latter converts the polarized input light into a sample polarized beam L. sam and the reference polarized beam L ref and directs them to the structure S and the reference mirror 28, respectively. Reflections from the structure and the mirror are converted by the beam splitter / combiner 24 into a combined light beam L having the particular polarization. com The combined light beam L com The combined polarized beam passes through the objective lens 26 and the beam splitter / combiner 24 to the polarizer 34, which allows only that particular polarization of light to propagate to the detection system. This combined polarized beam is split by the beam splitter 29 into a light portion (L com )1 and (L com ) is split into two and directed respectively to an imaging detector 19 and a spectrometer 18. The spectrometer 18 measures the intensity of each wavelength separately, and thus the measurement data generated by the spectrometer corresponds to a spectral interference pattern (which can also be detected by the imaging detector 19).

[0113] System 200 also includes a drive unit 33 associated with either or both of mirror 28 and the sample support, which controllably moves mirror 28 and the sample support along the optical axis, i.e., the z-axis, thereby inducing an optical path difference that results in a time-varying spectral interference pattern. Note that, although not specifically shown in Figure 1, measurement unit 12 of Figure 1 may also include a similar drive unit.

[0114] It should be appreciated that the use of polarizers 32 and 34 housed and oriented as described above actually provides a cross-polarized light scheme, resulting in a dark-field measurement mode. It should also be appreciated that when mirror 28 is not used (i.e., moved out of the path of the incident light or deactivated by use of an appropriate shutter), measurement unit 12 can operate as a spectral reflectometer. Thus, the same measurement unit 12 can be shifted between two different modes of operation, such as a spectral interferometer and a spectral reflectometer.

[0115] Control system 14 is typically a computer system configured and operable as described above with reference to FIGS.

[0116] For both the examples in Figures 1 and 5, note that although refractive optics are shown in these examples, partially or fully reflective optics (mirror based) can be used. For the broad IR range, reflective optics may be preferable.

Claims

1. a control system configured as a computer system including a data entry utility, a memory, and a data processor, the control system configured to communicate data with a measurement data provider that receives raw measurement data indicative of a measured spectral interference signal returned from a sample in response to an illuminating electromagnetic field incident on the top of the sample, the measured spectral interference signal including a spectral range reflected at the top of the sample and a spectral range that is transmitted substantially without absorption by the sample; The data processor: extracting from the raw measurement data a portion of a spectral interference signal that describes the variation in signal intensity with changes in optical path difference OPD during the interferometric measurement, the portion being independent of the interference signal returned from the bottom of the sample in response to the irradiating electromagnetic field; determining directly from the extracted portion of the spectral interference signal both the spectral amplitude and the spectral phase of the reflection of the illuminating electromagnetic field from the top of the sample, thereby determining a measured spectral signature characterizing the top of the sample. an analyzer utility configured and operable to: further comprising a measurement unit configured as the measurement data provider to generate and provide the raw measurement data; the measurement unit is configured and operable to perform spectral interferometric measurements of the sample using a light source that outputs light at operating wavelengths that include a spectral range that is reflected at the top of the sample and a spectral range that is transmitted substantially without absorption by the sample; A measurement system for use in optical metrology measurements.

2. 2. The measurement system of claim 1, wherein the raw measurement data comprises data indicative of the spectral interference signals returned from the top and bottom of the sample of silicon material in response to the irradiating electromagnetic field comprising an IR spectrum.

3. 2. The measurement system of claim 1, wherein the raw measurement data comprises data indicative of the spectral interference signals returned from the top and bottom and one or more internal interfaces of the sample in response to the irradiating electromagnetic field, the data comprising an IR spectrum.

4. 10. The measurement system of claim 1, wherein the operating wavelengths include the IR spectrum.

5. 10. The measurement system of claim 1, wherein the control system further comprises a fitting utility configured and operable to apply model-based processing to the measured spectral signature to determine one or more parameters of the sample being measured.

6. 10. The measurement system of claim 1, wherein the control system further comprises a fitting utility configured and operable to apply model-based processing to the measured spectral signature to determine one or more parameters of the sample being measured.

7. providing raw measurement data indicative of a measured spectral interference signal returned from the sample in response to an illuminating electromagnetic field incident on the top of the sample, the measured spectral interference signal including a spectral range reflected at the top of the sample and a spectral range transmitted substantially without absorption by the sample; Processing the raw measurement data, The above process is extracting from the raw measurement data a portion of a spectral interference signal that describes the variation in signal intensity with changes in optical path difference OPD during the interferometric measurement, the portion being independent of the interference signal returned from the bottom of the sample in response to the irradiating electromagnetic field; determining directly from the extracted portion of the spectral interference signal both the spectral amplitude and the spectral phase of the reflection of the illuminating electromagnetic field from the top of the sample, thereby determining a measured spectral signature characterizing the top of the sample; further comprising performing a spectral interferometric measurement of the sample using a light source that outputs light at operating wavelengths that include a spectral range that is reflected at the top of the sample and a spectral range that is transmitted substantially without absorption by the sample. Method for use in optical metrology measurements.

8. 8. The method of claim 7, wherein the raw measurement data comprises data indicative of the spectral interference signals returned from the top and bottom of the sample of silicon material in response to the irradiating electromagnetic field comprising an IR spectrum.

9. 8. The method of claim 7, wherein the raw measurement data comprises data indicative of the spectral interference signals returned from the top and bottom surfaces and one or more internal interfaces of the sample in response to the irradiating electromagnetic field, the data comprising an IR spectrum.

10. The method of claim 7 , wherein the processing further comprises applying a model-based fitting to the measured spectral signature to determine one or more parameters of the sample being measured.

11. The method of claim 7 wherein the operating wavelengths include the IR spectrum.

12. The method of claim 7 , wherein the sample is a silicon structure.

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