Optical measurement device and optical measurement method

Abstract

An optical measurement device according to the present invention includes: a first optical frequency comb light source that generates a first optical frequency comb at a first repetition frequency; a second optical frequency comb light source that generates a second optical frequency comb at a second repetition frequency; a superposition unit that spatially superposes reflected light generated when the first optical frequency comb is incident on a sample and the second optical frequency comb; and a processing unit that calculates a reflectance spectrum of the sample on the basis of a third optical frequency comb generated by spatially superposing the reflected light and the second optical frequency comb.

Description

Optical measurement device and optical measurement method

[0001] The present invention relates to an optical measurement device and an optical measurement method.

[0002] Conventionally, methods for optically measuring various samples have been proposed. For example, Japanese Patent Application Laid-Open No. 2009-092454 (Patent Document 1) discloses a multilayer film analysis device that can measure the film thickness of a wavelength-dependent multilayer film sample with high accuracy.

[0003] Furthermore, Japanese Patent Application Laid-Open Publication No. 2021-156828 (Patent Document 2) discloses a technology that uses dual-comb spectroscopy to solve the problem of phase ambiguity without processing the sample and accurately measure the real and imaginary parts of the complex refractive index and thickness of an object to be measured. Dual-comb spectroscopy is considered to be a highly accurate laser spectroscopic measurement technology based on optical frequency comb technology.

[0004] Japanese Patent Publication No. 2009-092454 Japanese Patent Publication No. 2021-156828

[0005] One object of the present invention is to provide a new optical measurement device that can also be used to measure the film thickness of a sample.

[0006] An optical measurement device according to one embodiment of the present invention includes a first optical frequency comb light source that generates a first optical frequency comb at a first repetition rate, a second optical frequency comb light source that generates a second optical frequency comb at a second repetition rate, a superposition unit that spatially superposes the second optical frequency comb on reflected light generated when the first optical frequency comb is incident on a sample, and a processing unit that calculates the reflectance spectrum of the sample based on a third optical frequency comb generated by the spatial superposition of the reflected light and the second optical frequency comb.

[0007] The processing unit may calculate the thickness of one or more layers included in the sample based on the reflectance spectrum of the sample.

[0008] The optical measurement device may further include a first optical bandpass filter that transmits light in a first spectral band, a second optical bandpass filter that transmits light in a second spectral band, and a separator that guides a portion of light in which the reflected light and the second optical frequency comb are spatially superimposed to the first optical bandpass filter and another portion to the second optical bandpass filter. The processing unit may calculate a reflectance spectrum of the sample based on the light that has passed through the first optical bandpass filter and the light that has passed through the second optical bandpass filter.

[0009] The optical measurement device may further include a first photodetector associated with the first optical bandpass filter and a second photodetector associated with the second optical bandpass filter, and the processing unit may calculate a reflectance spectrum of the sample for each spectral band based on the time waveform detected by the first photodetector and the time waveform detected by the second photodetector.

[0010] The optical measurement device may further include a first variable ND (Neutral Density) filter disposed between the first optical bandpass filter and the first photodetector, and a second variable ND filter disposed between the second optical bandpass filter and the second photodetector.

[0011] An optical measurement method according to another embodiment of the present invention includes the steps of generating a first optical frequency comb with a first repetition rate using a first optical frequency comb light source, generating a second optical frequency comb with a second repetition rate using a second optical frequency comb light source, spatially superimposing, using a superposition unit, reflected light generated when the first optical frequency comb is incident on a sample and the second optical frequency comb, and calculating a reflectance spectrum of the sample based on a third optical frequency comb generated by the spatial superposition of the reflected light and the second optical frequency comb.

[0012] According to an embodiment of the present invention, a new optical measurement device that can be applied to measurement of film thickness of a sample can be realized.

[0013] FIG. 2 is a schematic diagram showing an example of the device configuration of an optical measurement device according to the present embodiment. FIG. 3 is a schematic diagram showing an example of the hardware configuration of a processing unit shown in FIG. 1. FIG. 4 is a schematic diagram showing an example of the functional configuration of the processing unit shown in FIG. 1. FIG. 5 is a schematic diagram showing an example of the device configuration of an optical measurement device according to the present embodiment. FIG. 6 is a diagram for explaining spectral filtering according to the present embodiment. FIG. 7 is a schematic diagram showing an example of the device configuration of an optical measurement device according to the present embodiment. FIG. 8 is a flowchart showing a procedure for measuring a sample using the optical measurement device according to the present embodiment. FIG. 9 is a graph showing an example of measurement results by the optical measurement device according to the present embodiment.

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the accompanying drawings, in which the same or corresponding parts are designated by the same reference numerals and will not be described repeatedly.

[0015] <A. Device Configuration Example> An example of the device configuration of the optical measurement device 1 according to the present embodiment will be described. The optical measurement device 1 measures a sample using an optical frequency comb light source. More specifically, the optical measurement device 1 measures the sample using dual comb spectroscopy.

[0016] 1 is a schematic diagram showing an example of the device configuration of an optical measurement device 1 according to the present embodiment. Referring to FIG. 1, the optical measurement device 1 includes optical frequency comb light sources (hereinafter also simply referred to as "optical comb light sources") 10 and 20, a beam splitter 30, a mirror 40, focusing lenses 50 and 60, a photodetector 70, and a processing unit 100.

[0017] The optical comb light sources 10 and 20 are laser light sources that generate light having a comb-like optical intensity spectrum with equally spaced pulses (hereinafter also referred to as an "optical comb" or "optical frequency comb"). In the following description, the frequency interval in the optical intensity spectrum is also referred to as the "repetition frequency." The light generated by the optical comb light sources 10 and 20 is an optical pulse train in the time domain.

[0018] The optical frequency comb source 10 (first optical frequency comb source) has a repetition frequency f ｒｅｐ１ More specifically, the optical frequency comb source 10 generates an optical frequency comb 12 (first optical frequency comb) having a first repetition frequency fｏｆｆｓｅｔ + n × f ｒｅｐ１ (n≧0) ｏｆｆｓｅｔ is the offset frequency.

[0019] The optical frequency comb source 20 (second optical frequency comb source) has a repetition frequency f ｒｅｐ２ More specifically, the optical comb source 20 generates an optical comb 22 (second optical frequency comb) having a second repetition frequency f ｏｆｆｓｅｔ + n × f ｒｅｐ２ (n≧0). The offset frequency f ｏｆｆｓｅｔ are identical.

[0020] The repetition frequency f of the optical comb light source 10 ｒｅｐ１ and the repetition frequency f of the optical comb light source 20 ｒｅｐ２ That is, the repetition frequency difference Δf ｒｅｐ＝ |f ｒｅｐ１ -f ｒｅｐ２ | is set. Note that the repetition frequency difference Δf ｒｅｐ Since is infinitesimal, in the following, the repetition frequency f ｒｅｐ１ and repetition frequency f ｒｅｐ２ "Repetition frequency f ｒｅｐ They are sometimes collectively referred to as ".

[0021] The optical comb 12 generated by the optical comb source 10 passes through the beam splitter 30 and is reflected by the mirror 40. The optical comb 12 passes through the focusing lens 50 and is incident on the sample S. The mirror 40 is positioned so that the optical comb 12 is incident perpendicularly on the sample S. The reflected light 14 generated when the optical comb 12 is reflected by the sample S passes through the focusing lens 50 and the mirror 40 and reaches the beam splitter 30.

[0022] When the optical comb 22 generated by the optical comb source 20 reaches the beam splitter 30, it interferes with the reflected light 14. The beam splitter 30 spatially overlaps the reflected light 14 and the optical comb 22. In other words, the beam splitter 30 corresponds to an overlapping unit that spatially overlaps the reflected light 14, which is generated when the optical comb 12 is incident on the sample S, with the optical comb 22.

[0023] Repetition frequency f ｒｅｐ The slight difference in (Δf ｒｅｐ ), the reflected light 14 and the optical comb 22 interfere with each other, generating a secondary optical comb, an RF (Radio Frequency) comb 16, in the RF domain. In this specification, the term "RF domain" refers to a frequency domain in which the secondary optical comb exists, resulting from the interference of two optical combs. The RF domain may include, for example, a range from 0 to several hundred MHz.

[0024] The RF comb 16 generated by the interference between the reflected light 14 and the optical comb 22 passes through the condenser lens 60 and enters the photodetector 70 .

[0025] The RF comb 16 is generated in response to the beat between the reflected light 14 (optical comb) and the optical comb 22. The spectrum of the RF comb 16 corresponds to the light intensity spectrum of the reflected light 14 shifted to the RF region. At this time, the spectrum of the RF comb 16 maintains the spectral shape of the reflected light 14 while the repetition frequency is increased to f ｒｅｐ From Δf ｒｅｐ It will be converted into

[0026] The photodetector 70 detects the optical intensity of the incident light. From one perspective, the RF comb 16 can be said to be light observed by the photodetector 70 when the spatially superimposed reflected light 14 and the optical comb 22 are incident on the photodetector 70.

[0027] The processing unit 100 calculates the reflectance spectrum of the sample S, the film thickness of the sample S, and the like, based on the signal waveform (time waveform of light intensity) of the RF comb 16 detected by the photodetector 70. The processing unit 100 calculates the reflectance spectrum of the sample S based on the RF comb 16 (third optical frequency comb) generated by the spatial superposition of the reflected light 14 and the optical comb 22. The processing unit 100 also calculates the film thickness of one or more layers included in the sample S based on the reflectance spectrum of the sample S. The measurement results output by the processing unit 100 include the reflectance spectrum of the sample S, the film thickness of the sample S, and the like. Details of the processing by the processing unit 100 will be described later.

[0028] 1, due to layout considerations, the mirror 40 is used to change the propagation direction of the optical frequency comb 12, but the mirror 40 may be omitted. In the configuration example shown in FIG. 1, the focusing lens 50 may be omitted. In a configuration in which the focusing lens 50 is omitted, the optical frequency comb 12 is incident on the sample S without being focused.

[0029] <B. Processing Unit 100> Next, the processing unit 100 of the optical measurement device 1 according to the present embodiment will be described.

[0030] Fig. 2 is a schematic diagram showing an example of the hardware configuration of the processing unit 100 shown in Fig. 1. Referring to Fig. 2, the processing unit 100 includes, as main hardware components, a processor 102, a main memory 104, a storage 106, an input unit 112, an output unit 114, and an interface circuit 116.

[0031] The processor 102 is a processing circuit such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit), and reads programs stored in the storage 106 into the main memory 104 and executes them. The main memory 104 is a volatile storage device such as a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory), and functions as a working memory for the processor 102 to execute the programs.

[0032] The storage 106 is a non-volatile storage device such as a hard disk drive or a solid state drive (SSD) that stores programs and data. The storage 106 stores, for example, a system program 108, a measurement program 110, and measurement results 120.

[0033] The system program 108 includes computer-readable instructions for executing basic processing in the processing unit 100. The measurement program 110 includes computer-readable instructions for executing the optical measurement method according to the present embodiment. The measurement result 120 includes the reflectance spectrum and film thickness of the sample S.

[0034] The programs stored in storage 106 (system program 108 and measurement program 110) may be installed via any recording medium (e.g., an optical disk, etc.), or may be downloaded from a server via a network interface (not shown).

[0035] The measurement program 110 may execute processing by calling necessary software modules from among the software modules provided as part of the system program 108. Therefore, the measurement program 110 may not include software modules included in the system program 108.

[0036] The input unit 112 is a device that accepts user operations, such as a keyboard or a mouse, etc. The input unit 112 may be an interface for connecting to a device that accepts user operations.

[0037] The output unit 114 is a device that outputs the measurement results, etc., such as a display, a printer, etc. The output unit 114 may be an interface for connecting to a device that outputs the measurement results, etc.

[0038] The interface circuit 116 acquires the detection result of the photodetector 70. In this specification, the term "processor" includes a CPU, a GPU, a DSP (Digital Signal Processor), an ISP (Image Signal Processor), and the like, as well as a hardwired logic circuit (for example, an FPGA (Field-Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit)).

[0039] As used herein, the term “memory” includes main memory 104 and storage 106 .

[0040] The processing unit 100 may employ a SoC (System on Chip).

[0041] The configuration is not limited to one in which the processing unit 100 performs all processing; multiple processing devices may share the processing, or computing resources on a network (so-called cloud) not shown may be responsible for all or part of the required processing.

[0042] Fig. 3 is a schematic diagram showing an example of the functional configuration of processing unit 100 shown in Fig. 1. Referring to Fig. 3, processing unit 100 includes, as main functional components, a buffer 150, a frequency conversion unit 152, a frequency calibration unit 154, a reference storage unit 156, a reflectance calculation unit 158, and a film thickness calculation unit 160.

[0043] Buffer 150 stores time-series data of the optical intensity of RF comb 16 detected by photodetector 70 (and photodetector 72, described below). Buffer 150 also stores the signal waveform (time waveform of optical intensity) of RF comb 16.

[0044] When the time series data of the light intensity of the RF comb detected by the photodetector 70 and the time series data of the light intensity of the RF comb detected by the photodetector 72 are acquired simultaneously, areas for storing each of the time series data may be provided in the buffer 150.

[0045] The frequency converter 152 converts the signal waveform of the RF comb 16 stored in the buffer 150 from the time domain to the frequency domain. The conversion to the frequency domain may be performed using, for example, a fast Fourier transform (FFT). The frequency converter 152 outputs the spectrum of the RF comb 16.

[0046] The frequency calibration unit 154 calibrates the electrical frequency (RF domain) of the RF comb 16 to an optical frequency (optical domain). ｒｅｐ and the repetition frequency difference Δf ｒｅｐ More specifically, the frequency calibration unit 154 calculates a conversion ratio (f ｒｅｐ / Δf ｒｅｐ ) and the offset frequency f ｏｆｆｓｅｔ The repetition rate is then converted to Δf ｒｅｐFrom f ｒｅｐ The frequency calibration unit 154 outputs the optical intensity spectrum of the optical comb (the reflected light or transmitted light from the sample S).

[0047] The reference storage unit 156 stores the light intensity spectrum obtained by the reference measurement (hereinafter also referred to as the "reference spectrum").

[0048] The reflectance calculation unit 158 ​​calculates the ratio between the light intensity spectrum obtained by signal measurement of the sample S (hereinafter also referred to as the "signal spectrum") and the reference spectrum stored in the reference storage unit 156. The calculated ratio of the light intensity spectra becomes the reflectance spectrum.

[0049] The reflectance calculation unit 158 ​​may calculate a transmittance spectrum instead of a reflectance spectrum.

[0050] The film thickness calculation unit 160 calculates the film thickness of the sample S based on the reflectance spectrum (or transmittance spectrum). The film thickness calculation may employ a method of performing spectral fitting using a nonlinear least squares method on the reflectance spectrum (or transmittance spectrum). Alternatively, the film thickness calculation may employ a method of performing a simulation based on a model corresponding to the structure of the sample S so as to match the reflectance spectrum (or transmittance spectrum). Either the method using the nonlinear least squares method or the method performing a simulation based on a model can calculate the film thickness of each layer included in the multilayer film sample S.

[0051] The reflectance spectrum (or transmittance spectrum) calculated by the reflectance calculation unit 158 ​​and / or the film thickness calculated by the film thickness calculation unit 160 are output as measurement results.

[0052] <C. Transmission Optical System> Although FIG. 1 shows an example of the device configuration of a reflection optical system, a transmission optical system may also be employed.

[0053] 4 is a schematic diagram showing an example of the device configuration of an optical measurement device 1A according to the present embodiment. Referring to FIG. 4, the optical measurement device 1A includes optical frequency comb light sources 10 and 20, a beam splitter 32, a focusing lens 60, a photodetector 70, and a processing unit 100.

[0054] The optical frequency comb 12 generated by the optical frequency comb source 10 is incident on the sample S. The transmitted light 18 generated when the optical frequency comb 12 passes through the sample S is incident on the beam splitter 32.

[0055] The optical frequency comb 22 generated by the optical frequency comb source 20 interferes with the transmitted light 18 when it reaches the beam splitter 32. The beam splitter 32 spatially overlaps the transmitted light 18 and the optical frequency comb 22. The RF frequency comb 24 generated by the interference between the transmitted light 18 and the optical frequency comb 22 passes through the focusing lens 60 and enters the photodetector 70. The RF frequency comb 24 is generated by the beat between the transmitted light 18 (optical frequency comb) and the optical frequency comb 22.

[0056] The processing unit 100 calculates the reflectance spectrum of the sample S, the film thickness of the sample S, and the like, based on the signal waveform of the RF comb 24 detected by the photodetector 70 .

[0057] A condenser lens 50 may be disposed between the optical frequency comb source 10 and the sample S. By employing a transmission optical system as shown in FIG. 4, the internal structure of the sample S can also be measured.

[0058] <D. Spectral Filtering> When measuring the film thickness of a sample, it is necessary to acquire a light intensity spectrum (signal spectrum) over a sufficiently wide measurement wavelength range, but it is not always necessary to acquire the entire light intensity spectrum; in many cases, it is sufficient to selectively acquire a portion of the light intensity spectrum. The acquired light intensity spectrum may be discrete.

[0059] A new measurement method using spectral filtering will be described below. This new measurement method allows selective acquisition of a portion of the light intensity spectrum. This measurement method allows selective acquisition of a spectrum over a wider measurement wavelength range without reducing the measurement rate (a parameter indicating the number of measurements that can be made per unit time) or the signal intensity per wavelength.

[0060] Fig. 5 is a schematic diagram showing an example of the device configuration of an optical measurement device 1B according to this embodiment. The optical measurement device 1B shown in Fig. 5 additionally includes a beam splitter 80, optical bandpass filters 86 and 88, variable ND (Neutral Density) filters 82 and 84, a condenser lens 62, and a photodetector 72 in addition to the optical measurement device 1 shown in Fig. 1.

[0061] 1, the optical comb 12 generated by the optical comb source 10 passes through the beam splitter 30, the mirror 40, and the condenser lens 50 before being incident on the sample S. The reflected light 14 generated when the optical comb 12 is reflected by the sample S passes through the condenser lens 50 and the mirror 40 before reaching the beam splitter 30.

[0062] When the optical comb 22 generated by the optical comb source 20 reaches the beam splitter 30, it interferes with the reflected light 14 to generate the RF comb 16. When the RF comb 16 enters the beam splitter 80, it is split into two optical paths.

[0063] The beam splitter 80 corresponds to a separation section that directs a portion of the light resulting from the spatial superposition of the reflected light 14 and the optical comb 22 to an optical bandpass filter 86 (first optical bandpass filter) and directs another portion to an optical bandpass filter 88 (second optical bandpass filter).

[0064] The RF comb 16 split into one optical path (light resulting from the spatial superposition of the reflected light 14 and the optical comb 22) passes through an optical bandpass filter 86, a tunable ND filter 82, and a condenser lens 60 before reaching the photodetector 70. The RF comb 16 split into the other optical path passes through an optical bandpass filter 88, a tunable ND filter 84, and a condenser lens 62 before reaching the photodetector 72.

[0065] Each of the optical bandpass filters 86 and 88 transmits only light in a particular spectral band (wavelength range). The optical bandpass filter 86 transmits light in a first spectral band. The optical bandpass filter 88 transmits light in a second spectral band.

[0066] The variable ND filter 82 (first variable ND filter) is disposed between the optical bandpass filter 86 and the photodetector 70. The variable ND filter 84 (second variable ND filter) is disposed between the optical bandpass filter 88 and the photodetector 72. Each of the variable ND filters 82, 84 adjusts (reduces) the intensity of light that it transmits. The variable ND filters 82, 84 are configured to be able to adjust the degree to which they reduce the light intensity. The light attenuation rates of the variable ND filters 82, 84 are set so that the detection results from the photodetectors 70, 72 are not saturated.

[0067] The photodetector 70 (first photodetector) is associated with an optical bandpass filter 86. The photodetector 70 detects the optical intensity of the light from the RF comb 16 that has passed through the optical bandpass filter 86.

[0068] The photodetector 72 (second photodetector) is associated with the optical bandpass filter 88. The photodetector 72 detects the optical intensity of the light from the RF comb 16 that has passed through the optical bandpass filter 88.

[0069] The processing unit 100 calculates the reflectance spectrum of the sample S, the film thickness of the sample S, and the like, based on the signal waveform of the RF comb 16 detected by the photodetector 70 (after passing through the optical bandpass filter 86) and the signal waveform of the RF comb 16 detected by the photodetector 72 (after passing through the optical bandpass filter 88). The processing unit 100 may also calculate the reflectance spectrum of the sample S in the spectral bands corresponding to the multiple photodetectors based on the time waveforms detected by each of the multiple photodetectors. That is, the processing unit 100 may calculate the reflectance spectrum of the sample S for each spectral band.

[0070] In this way, the processing unit 100 calculates the reflectance spectrum of the sample S based on the light that has passed through the optical bandpass filter 86 and the light that has passed through the optical bandpass filter 88. Furthermore, the processing unit 100 calculates the film thickness of one or more layers included in the sample S based on the reflectance spectrum of the sample S.

[0071] 6 is a diagram for explaining the spectral filtering according to the present embodiment. The optical comb (and RF comb) has a repetition frequency f ｒｅｐ (and Δf ｒｅｐ ) apart, but for convenience of explanation, it is represented as a continuous spectrum (envelope) in FIG. 6.

[0072] The influence of aliasing that occurs in general dual-comb spectroscopy will be described with reference to FIG.

[0073] The upper part of Fig. 6A shows an example of the optical intensity spectrum (optical domain) of an optical frequency comb generated by an optical frequency comb light source, and the lower part of Fig. 6A shows an example of the spectrum (RF domain) of an RF frequency comb generated by interference of the optical frequency comb. Fig. 6A shows the optical domain where the spectrum of the optical domain exists and the RF domain where the spectrum appears. For convenience, the negative electrical frequency domain of the RF frequency comb spectrum is also shown.

[0074] In the optical domain shown in FIG. 6A, there is a position (position C) corresponding to a reference offset frequency included in the optical comb, and a position f ｒｅｐ ２ / 2Δf ｒｅｐ (≒f ｒｅｐ１ f ｒｅｐ２ / 2Δf ｒｅｐ ) (positions A and B and positions D and E) are shown.

[0075] In dual-comb spectroscopy, the optical intensity spectrum of an RF comb is reproduced by calibrating the frequency axis of the RF comb (RF domain) to the optical frequency (optical domain). Here, the spectrum of the RF comb ranges from 0 to f ｒｅｐ The spectrum of the RF comb must be within the range of 0 to f ｒｅｐ Aliasing occurs when the RF comb frequency is greater than 0 to f / 2. ｒｅｐ If it is outside the range of 0 to f ｒｅｐ / 2 and signals in the range 0 to f ｒｅｐ This is because it is not possible to distinguish between signals outside the range of 1 / 2. Thus, aliasing causes errors in the measurement results.

[0076] To avoid the effects of aliasing, the optical intensity spectrum of the optical comb (reflected light 14 or transmitted light 18) in the optical domain must exist in only one of the bands A-B, B-C, C-D, and D-E. Therefore, the maximum spectral band of the optical comb is adjusted to fit within one band. That is, in general dual-comb spectroscopy, the band of the optical intensity spectrum of the optical comb is at most the band C-D (f ｒｅｐ ２ / 2Δf ｒｅｐ ) is limited to

[0077] To avoid aliasing effects, the repetition rate difference Δf ｒｅｐ The measurement conditions such as the repetition frequency difference Δf must be set appropriately. ｒｅｐIf it is not possible to set appropriate measurement conditions such as the above, it is necessary to avoid the effects of aliasing by limiting the spectral bandwidth of the optical comb using an optical bandpass filter or the like.

[0078] As described above, the measurement wavelength range Δν, which is the spectral band of the optical comb, and the repetition frequency f ｒｅｐ and the repetition frequency difference Δf ｒｅｐ There is the following relationship between

[0079] Δν=f ｒｅｐ ２ / 2Δf ｒｅｐ The measurement rate is the repetition frequency difference Δf ｒｅｐ Since it is proportional to, the above relationship can be expressed as follows:

[0080] Δν × measurement rate ∝ f ｒｅｐ ２ / 2 Repetition frequency f ｒｅｐ Since it is difficult to significantly change the repetition frequency f, the above formula shows that the demand to widen the measurement wavelength range Δν and the demand to increase the measurement rate are mutually contradictory. ｒｅｐ Therefore, in order to ensure a predetermined measurement wavelength range Δν, the repetition frequency difference Δf ｒｅｐ is forced to be small, which results in a decrease in the measurement rate, i.e., the measurement rate is sacrificed to avoid the effects of aliasing.

[0081] Furthermore, when measuring over a wide wavelength range, the absolute light intensity of the optical comb increases as the measurement wavelength range is widened. However, the maximum light intensity that a photodetector can detect is predetermined. Therefore, if the measurement wavelength range is widened and more wavelengths are incident on the photodetector, the light intensity of each wavelength is limited to fit within the predetermined maximum light intensity, which may result in a decrease in the signal intensity per wavelength.

[0082] When measuring the film thickness of a sample, it is necessary to acquire light intensity spectra (signal spectrum and reference spectrum) over a sufficiently wide wavelength range. However, with typical dual-comb spectroscopy, problems such as a slow measurement rate and / or a low signal intensity per wavelength can occur.

[0083] Spectral filtering according to the present invention provides a solution to such problems.

[0084] Referring to FIG. 6B, spectral filtering in optical measurement device 1B according to the present embodiment will be described.

[0085] In the optical measurement device 1B according to this embodiment, the optical frequency comb 12 generated by the optical frequency comb light source 10 is set to, for example, a band A to E. In this example, the spectral band of the optical frequency comb 12 is 4×f ｒｅｐ ２ / 2Δf ｒｅｐ This becomes:

[0086] 6B, the optical bandpass filter 86 transmits only light in a specific spectral band set to bands A-B (spectrum 26), and the optical bandpass filter 88 transmits only light in a specific spectral band set to bands D-E (spectrum 28).

[0087] The light transmitted through the optical bandpass filter 86 includes components that transmit within a set spectral band from the reflected light 14 from the sample S and the optical comb 22 from the optical comb light source 20. Therefore, the RF comb 16 incident on the photodetector 70 also has a spectrum that corresponds to the spectral band that the optical bandpass filter 86 transmits.

[0088] Similarly, the light transmitted through the optical bandpass filter 88 contains components that transmit within a set spectral band from the reflected light 14 from the sample S and the optical comb 22 from the optical comb light source 20. Therefore, the RF comb 16 incident on the photodetector 72 also has a spectrum that corresponds to the spectral band that the optical bandpass filter 88 transmits.

[0089] As a result, in the RF domain, spectrum 90 corresponding to spectrum 26 appears in band A-B. Due to aliasing, spectrum 91 appears at a position where spectrum 90 is folded back at position B. Furthermore, spectrum 92 appears at a position where spectrum 91 is folded back at position C.

[0090] Similarly, in the RF domain, spectrum 95 corresponding to spectrum 28 appears in bands D-E. Due to aliasing, spectrum 96 appears at a position where spectrum 95 is folded back at position D. Furthermore, spectrum 97 appears at a position where spectrum 96 is folded back at position C. Furthermore, spectrum 98 appears at a position where spectrum 97 is folded back at position B.

[0091] By setting the spectral bands that the optical bandpass filters 86 and 88 transmit as shown in Figure 6B, the optical intensity spectrum of the optical comb that enters the photodetector 70 corresponds to spectrum 26, and the optical intensity spectrum of the optical comb that enters the photodetector 72 corresponds to spectrum 28. ｒｅｐ Within the range of .beta. / 2 (band CD), there is a spectrum 92 corresponding to spectrum 26, and there is also a spectrum 96 corresponding to spectrum 28.

[0092] As a result, the optical intensity spectrum of the optical comb in each spectral band can be reconstructed from the signal waveforms of the RF comb 16 detected by the photodetectors 70 and 72 .

[0093] In the optical measurement device 1B according to the present embodiment, the spectral band of the optical comb is set to f ｒｅｐ ２ / 2Δf ｒｅｐ Therefore, when widening the measurement wavelength range, the repetition frequency difference Δf ｒｅｐ This allows for an increase in the measurement wavelength range without sacrificing the measurement rate. In addition, the measurement wavelength range can be expanded to selectively acquire only the required spectral band, allowing the signal strength per wavelength to be maintained high.

[0094] The spectral bands that each of the optical bandpass filters 86 and 88 transmit can be set arbitrarily depending on the material and structure of the sample S. The spectral bands that each of the optical bandpass filters 86 and 88 transmit may also be set depending on the magnitude of noise, the spectral bandwidth of the optical comb, etc.

[0095] From the viewpoint of widening the spectral band to be selectively observed as much as possible, the spectral band (first spectral band) transmitted by the optical bandpass filter 86 and the spectral band (second spectral band) transmitted by the optical bandpass filter 88 may be set so as not to overlap with each other. However, the spectral bands transmitted by the optical bandpass filters 86 and 88 are set so as not to overlap with each other. ｒｅｐ ２ / 2Δf ｒｅｐ It is set as follows:

[0096] When measuring the film thickness of the sample S, it is advantageous if the spectral bands transmitted by the optical bandpass filter 86 and the optical bandpass filter 88 are as far apart as possible.

[0097] When a configuration is adopted in which the signal waveforms of the RF combs detected by photodetector 70 and photodetector 72 are processed independently, the spectral bands transmitted by optical bandpass filter 86 and optical bandpass filter 88 may overlap with each other.

[0098] The spectral bandwidth of the optical comb 12 is defined as the maximum spectral bandwidth f ｒｅｐ ２ / 2Δf ｒｅｐ However, the spectrum band can be set arbitrarily.

[0099] In addition to the above-described reflective optical system, a transmissive optical system may also be employed. Fig. 7 is a schematic diagram showing an example of the device configuration of an optical measurement device 1C according to this embodiment. The optical measurement device 1C shown in Fig. 7 is obtained by changing the optical measurement device 1B shown in Fig. 5 to a transmissive optical system. The optical measurement device 1C shown in Fig. 7 has a beam splitter 32 instead of the beam splitter 30, mirror 40, and condenser lens 50 of the optical measurement device 1B shown in Fig. 5.

[0100] The optical frequency comb 12 generated by the optical frequency comb source 10 is incident on the sample S. The transmitted light 18 generated when the optical frequency comb 12 passes through the sample S is incident on the beam splitter 32.

[0101] When the optical comb 22 generated by the optical comb light source 20 reaches the beam splitter 32, it interferes with the transmitted light 18, generating an RF comb 24. When the RF comb 24 enters the beam splitter 80, it is split into two optical paths. The rest of the process is the same as that of the optical measurement device 1B shown in FIG. 5.

[0102] 5 and 7 show configuration examples using two optical bandpass filters, three or more optical bandpass filters may be used. In this case, the spectral bands transmitted by each bandpass filter may be selected so that the spectra of light transmitted through each optical bandpass filter do not overlap. Conversely, a single optical bandpass filter may be used.

[0103] Although FIGS. 5 and 7 show examples of configurations in which the optical bandpass filters are fixedly arranged, a system in which the optical bandpass filters are mechanically switched may also be employed.

[0104] <E. Processing Procedure> Next, an example of a measurement procedure for the sample S using the optical measurement device according to the present embodiment will be described.

[0105] FIG. 8 is a flowchart showing a procedure for measuring a sample S using the optical measurement device according to this embodiment. Referring to FIG. 8, a user places a reference as a measurement target for the optical measurement device (step S2). The reference is a substance with known optical properties. For example, the reference may be an Au mirror. The processing unit 100 acquires a light intensity spectrum (reference spectrum) observed when the reference is placed (step S4). The processing unit 100 stores the acquired reference spectrum.

[0106] Next, the user places the sample S, which is the original measurement target, instead of the reference (step S6). The processing unit 100 acquires the light intensity spectrum (signal spectrum) observed when the sample S is placed (step S8). The processing unit 100 calculates the ratio between the signal spectrum and the reference spectrum (step S10). The calculated ratio of the light intensity spectra becomes the reflectance spectrum.

[0107] The processing unit 100 calculates the film thickness of the sample S from the reflectance spectrum (step S12).

[0108] The reference measurement in steps S2 to S4 and the signal measurement in steps S6 to S10 may be performed at different times. After the reference measurement in steps S2 to S4 is performed once, the signal measurement in steps S6 to S10 may be repeated.

[0109] Furthermore, when performing measurements using the optical measurement device 1B, before executing step S1, the user selects the spectral bands to be transmitted by the optical bandpass filters 86 and 88. The user also adjusts the light attenuation rates of the variable ND filters 82 and 84 so that the light intensity of the light incident on the photodetectors 70 and 72 is maximized within the detectable range.

[0110] <F. Example of Measurement Result> Next, an example of a measurement result obtained by optical measurement device 1B according to the present embodiment will be described.

[0111] 9 is a graph showing an example of measurement results obtained by optical measurement device 1B according to the present embodiment. Fig. 9 shows an example of the results obtained by measuring a Si wafer (film thickness d = 775 ± 20 μm) using optical measurement device 1B. An Au mirror was used as a reference.

[0112] Figure 9(A) shows the measured reflectance spectrum (solid line) and the theoretical reflectance spectrum (dashed line) at film thickness d, and Figure 9(B) shows an enlarged view of the 1559-1561 nm wavelength portion of the graph shown in Figure 9(A).

[0113] Referring to FIG. 9B, it can be seen that the shape of the reflectance spectrum can be measured with high wavelength resolution.

[0114] 9(C) shows the change in root mean squared error (RMSE) versus film thickness, calculated by performing spectral fitting using the nonlinear least squares method on the measured reflectance spectrum shown in FIG. 9(A). In the graph shown in FIG. 9(C), the film thickness at which the RMSE was minimized was searched for, resulting in 774.501 μm. This value of 774.501 μm was determined as the film thickness of the wafer.

[0115] <G. Summary> The optical measurement device according to the present embodiment uses dual-comb spectroscopy to acquire a reflectance spectrum from the sample S. By using dual-comb spectroscopy, it is possible to ensure a wide measurement wavelength range and higher wavelength resolution.

[0116] Because all wavelengths of an optical frequency comb light source are phase-locked to an atomic clock and have the same frequency (wavelength) accuracy as an atomic clock, it can achieve significantly more accurate measurements than film thickness measurements using a spectrometer. In film thickness measurements using a spectrometer that combines a diffraction grating and an image sensor, the narrow measurement wavelength range can result in the measured reflectance spectrum fitting to a spectrum corresponding to an incorrect film thickness value. Expanding the measurement wavelength range reduces wavelength resolution as long as the number of image sensor channels remains constant. As a result, the sample's reflectance spectrum cannot be measured with a sufficient number of sampling points, resulting in reduced measurement accuracy. Thus, it is difficult to ensure both a sufficient measurement wavelength range and wavelength resolution in film thickness measurements using a spectrometer. In contrast, the optical measurement device according to this embodiment can ensure both a sufficient measurement wavelength range and wavelength resolution.

[0117] The optical measurement device according to this embodiment can selectively acquire a spectrum over a wider measurement wavelength range by using spectral filtering without reducing the measurement rate or the signal strength per wavelength, and can measure the film thickness of a sample based on the selectively acquired spectrum.

[0118] The optical measurement device according to this embodiment calculates the film thickness of the sample by performing spectral fitting using the nonlinear least squares method on the acquired reflectance spectrum. The optical measurement device according to this embodiment also calculates the film thickness of the sample by comparing the acquired reflectance spectrum with the simulation results based on a model corresponding to the structure of the sample S. Either method can measure the film thickness of not only single-layer films but also multilayer films. This also allows for analysis of multilayer films.

[0119] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0120] 1, 1A, 1B, 1C Optical measurement device, 10, 20 Optical comb light source (optical frequency comb light source), 12, 22 Optical comb (optical frequency comb), 14 Reflected light, 16, 24 RF comb, 18 Transmitted light, 26, 28, 90, 91, 92, 95, 96, 97, 98 Spectrum, 30, 32, 80 Beam splitter, 40 Mirror, 50, 60, 62 Condenser lens, 70, 72 Photodetector, 82, 84 Tunable ND filter, 86, 88 Optical bandpass filter, 100 Processing unit, 102 Processor, 104 Main memory, 106 Storage, 108 System program, 110 Measurement program, 112 Input unit, 114 Output unit, 116 Interface circuit, 120 Measurement result, 150 Buffer, 152 Frequency conversion unit, 154 Frequency calibration unit, 156 reference storage unit, 158 reflectance calculation unit, 160 film thickness calculation unit, S sample.

Claims

1. An optical measurement device comprising: a first optical frequency comb light source that generates a first optical frequency comb with a first repetition frequency; a second optical frequency comb light source that generates a second optical frequency comb with a second repetition frequency; a superposition unit that spatially superposes the second optical frequency comb on reflected light generated when the first optical frequency comb is incident on a sample; and a processing unit that calculates the reflectance spectrum of the sample based on a third optical frequency comb generated by the spatial superposition of the reflected light and the second optical frequency comb.

2. The optical measurement device according to claim 1, wherein the processing unit calculates the film thickness of one or more layers included in the sample based on the reflectance spectrum of the sample.

3. An optical measurement device as described in claim 1 or 2, further comprising: a first optical bandpass filter that transmits light in a first spectral band; a second optical bandpass filter that transmits light in a second spectral band; and a separation unit that guides a portion of the light in which the reflected light and the second optical frequency comb are spatially superimposed to the first optical bandpass filter and another portion to the second optical bandpass filter, wherein the processing unit calculates the reflectance spectrum of the sample based on the light that has passed through the first optical bandpass filter and the light that has passed through the second optical bandpass filter.

4. The optical measurement device according to claim 3, further comprising: a first photodetector associated with the first optical bandpass filter; and a second photodetector associated with the second optical bandpass filter, wherein the processing unit calculates the reflectance spectrum of the sample for each spectral band based on the time waveform detected by the first photodetector and the time waveform detected by the second photodetector.

5. The optical measurement device of claim 4, further comprising: a first variable ND (Neutral Density) filter disposed between the first optical bandpass filter and the first photodetector; and a second variable ND filter disposed between the second optical bandpass filter and the second photodetector.

6. An optical measurement method comprising the steps of: generating a first optical frequency comb with a first repetition rate using a first optical frequency comb light source; generating a second optical frequency comb with a second repetition rate using a second optical frequency comb light source; spatially superimposing the second optical frequency comb on reflected light generated when the first optical frequency comb is incident on a sample using a superposition unit; and calculating a reflectance spectrum of the sample based on a third optical frequency comb generated by the spatial superposition of the reflected light and the second optical frequency comb.

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