Optical measuring apparatus and optical measuring 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

Technical Field

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

Background Art

[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 capable of measuring the film thickness of a multilayer film sample having wavelength dependence with higher accuracy.

[0003] In addition, Japanese Patent Application Laid-Open No. 2021-156828 (Patent Document 2) discloses a technique for solving the phase indeterminacy problem without processing a sample and accurately measuring the real part, imaginary part, and thickness of the complex refractive index of a measurement object using dual-comb spectroscopy. Dual-comb spectroscopy is a high-precision laser spectroscopy measurement technique based on optical frequency comb technology.

Prior Art Documents

Patent Documents

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

[0008] The optical measuring 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 separation unit that directs a portion of the light obtained by spatially superimposing the reflected light and the second optical frequency comb to the first optical bandpass filter and another portion to the second optical bandpass filter. The processing unit may calculate the reflectance spectrum of the sample based on the light after it has passed through the first optical bandpass filter and the light after it has passed through the second optical bandpass filter.

[0009] The optical measuring device may further include a first photodetector associated with a first optical bandpass filter and a second photodetector associated with a second optical bandpass filter. The processing unit may calculate 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.

[0010] The optical measuring device may further include a first variable ND (Neutral Density) filter positioned between a first optical bandpass filter and a first photodetector, and a second variable ND filter positioned between a second optical bandpass filter and a 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 frequency using a first optical frequency comb light source; generating a second optical frequency comb with a second repetition frequency using a second optical frequency comb light source; spatially superimposing the reflected light generated when the first optical frequency comb is incident on a sample with the second optical frequency comb using an overlay unit; and calculating the reflectance spectrum of a sample based on a third optical frequency comb generated by the spatial overlay of the reflected light and the second optical frequency comb. [Effects of the Invention]

[0012] According to one embodiment of the present invention, a new optical measuring device can be realized that can also be applied to measuring the film thickness of a sample. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing an example of the device configuration of an optical measuring device according to this embodiment. [Figure 2] Figure 1 is a schematic diagram showing an example of the hardware configuration of the processing unit. [Figure 3] Figure 1 is a schematic diagram showing an example of the functional configuration of the processing unit. [Figure 4] This is a schematic diagram showing an example of the device configuration of an optical measuring device according to this embodiment. [Figure 5] This is a schematic diagram showing an example of the device configuration of an optical measuring device according to this embodiment. [Figure 6] This figure illustrates spectral filtering according to this embodiment. [Figure 7] This is a schematic diagram showing an example of the device configuration of an optical measuring device according to this embodiment. [Figure 8] This is a flowchart showing the procedure for measuring a sample using an optical measuring device according to this embodiment. [Figure 9] This graph shows an example of measurement results obtained using an optical measuring device according to this embodiment. [Modes for carrying out the invention]

[0014] Embodiments of the present invention will be described in detail with reference to the drawings. For the same or corresponding parts in the drawings, the same reference numerals are given and their descriptions will not be repeated.

[0015] <A. Example of device configuration> 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 a sample using dual-comb spectroscopy.

[0016] FIG. 1 is a schematic diagram showing an example of the device configuration of the 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, condenser 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 (hereinafter, also referred to as "optical comb" or "optical frequency comb") having a comb-shaped light intensity spectrum arranged at equal intervals. In the following description, the frequency interval in the light intensity spectrum is also referred to as the "repetition frequency". The light generated by the optical comb light sources 10 and 20 is a light pulse train in the time domain.

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

[0019] The optical comb light source 20 (the second optical frequency comb light source) generates an optical comb 22 (the second optical frequency comb) having a repetition frequency f rep2 (the second repetition frequency). More specifically, the optical comb light source 20 is foffset +n×f rep2 It includes a plurality of frequencies of (n≧0). The offset frequency f is the same between the optical comb light source 10 and the optical comb light source 20. offset It is the same.

[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 are slightly different. That is, the repetition frequency difference Δf rep= |f rep1 -f rep2 | is set. Since the repetition frequency difference Δf rep is very small, hereinafter, the repetition frequency f rep1 and the repetition frequency f rep2 may also be collectively referred to as "repetition frequency f rep ".

[0021] The optical comb 12 generated by the optical comb light source 10 is reflected by the mirror 40 after passing through the beam splitter 30. The optical comb 12 passes through the condenser lens 50 and is incident on the sample S. The mirror 40 is arranged so that the optical comb 12 is incident on the sample S perpendicularly. The reflected light 14 generated by the reflection of the optical comb 12 by the sample S reaches the beam splitter 30 through the condenser lens 50 and the mirror 40.

[0022] When the optical comb 22 generated by the optical comb light 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. That is, the beam splitter 30 corresponds to an overlapping portion that spatially overlaps the reflected light 14 generated by the incidence of the optical comb 12 on the sample S and the optical comb 22.

[0023] A slight difference (Δf rep of the repetition frequency f repDue to interference between the reflected light 14 and the optical comb 22, a secondary optical comb, the RF (Radio Frequency) comb 16, is generated in the RF region. In this specification, "RF region" means the frequency region in which the secondary optical comb generated by the interference of the two optical combs exists. The RF region 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 focusing lens 60 and is incident on 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 optical 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 f rep From Δf rep It will be converted to [this].

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

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

[0028] In the configuration example shown in FIG. 1, the propagation direction of the optical communication 12 is changed using the mirror 40 due to layout reasons, but the mirror 40 may be omitted. In the configuration example shown in FIG. 1, the condenser lens 50 may be omitted. In the configuration where the condenser lens 50 is omitted, the optical communication 12 enters the sample S without being condensed.

[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 a hardware configuration example 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 and executes the program stored in the storage 106 into the main memory 104. 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 program.

[0032] The storage 106 is a non-volatile storage device such as a hard disk drive or an SSD (Solid State Drive), and stores programs and data. The storage 106 stores, for example, the system program 108 and the measurement program 110, and the measurement result 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 an optical measurement method according to this embodiment. The measurement results 120 include 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., optical disc) or downloaded from a server via a network interface (not shown).

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

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

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

[0038] The interface circuit 116 acquires the detection result from the photodetector 70. In this specification, the term "processor" includes CPUs, GPUs, DSPs (Digital Signal Processors), and ISPs (Image Signal Processors), as well as hardwired logic circuits (e.g., FPGAs (field-programmable gate arrays) and ASICs (application-specific integrated circuits)).

[0039] In this specification, the term “memory” includes the main memory 104 and the storage 106.

[0040] A System on Chip (SoC) may be used in the processing unit 100.

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

[0042] Figure 3 is a schematic diagram showing an example of the functional configuration of the processing unit 100 shown in Figure 1. Referring to Figure 3, the processing unit 100 includes, as its 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 light intensity of the RF comb 16 detected by the photodetector 70 (and the photodetector 72 described later). Buffer 150 stores the signal waveform (time waveform of light intensity) of the RF comb 16.

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

[0045] The frequency conversion unit 152 converts the signal waveform of the RF comb 16 stored in the buffer 150 from the time domain to the frequency domain. For example, the conversion to the frequency domain may be performed using FFT (Fast Fourier Transform). The frequency conversion unit 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 the optical frequency (optical domain). For frequency calibration, the repetition frequency f rep and the repetition frequency difference Δf rep These are some of the references. More specifically, the frequency calibration unit 154 converts the frequency axis of the spectrum of the RF comb 16 to a conversion ratio (f rep / Δf rep ) is multiplied by the offset frequency f offset The spectrum of the optical comb is reconstructed by adding the values. The conversion gives the repetition frequency Δf rep from f rep It is converted to the optical intensity spectrum of the optical comb (reflected or transmitted light from the sample S). The frequency calibration unit 154 outputs the optical intensity spectrum of the optical comb.

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

[0048] The reflectance calculation unit 158 ​​calculates the ratio between the light intensity spectrum (hereinafter also referred to as the "signal spectrum") obtained by signal measurement of the sample S and the reference spectrum stored in the reference storage unit 156. The calculated ratio of 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). For the calculation of the film thickness, a method of performing spectral fitting using the non-linear least squares method on the reflectance spectrum (or transmittance spectrum) may be adopted. Alternatively, for the calculation of the film thickness, 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) may be adopted. Whichever method of using the non-linear least squares method or performing a simulation based on a model is used, the film thickness of each layer included in the multilayer film sample S can be calculated.

[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> FIG. 1 shows an example of the device configuration of the reflection optical system, but a transmission optical system may also be adopted.

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

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

[0055] 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. The beam splitter 32 spatially overlaps the transmitted light 18 and the optical comb 22. The RF comb 24 generated by the interference between the transmitted light 18 and the optical comb 22 passes through the condenser lens 60 and is incident on the photodetector 70. The RF comb 24 is generated by the beat between the transmitted light 18 (optical comb) and the optical comb 22.

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

[0057] Furthermore, a focusing lens 50 may be placed between the optical comb light source 10 and the sample S. By employing a transmission optical system as shown in Figure 4, it is possible to measure the internal structure of sample S.

[0058] <D.スペクトルフィルタリング> When measuring the film thickness of a sample, it is necessary to obtain an optical intensity spectrum (signal spectrum) over a sufficiently wide measurement wavelength range. However, it is not always necessary to obtain the entire optical intensity spectrum; in many cases, it is sufficient to selectively obtain a portion of the optical intensity spectrum. The obtained optical intensity spectrum may be discrete.

[0059] The following describes a new measurement method using spectral filtering. This new measurement method allows for the selective acquisition of a portion of the light intensity spectrum. This method allows for the selective acquisition of spectra over a wider measurement wavelength range without reducing the measurement rate (a parameter indicating the number of measurements that can be taken per unit time) or the signal intensity per wavelength.

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

[0061] Similar to the optical measurement apparatus 1 shown in Figure 1, the optical comb 12 generated by the optical comb light source 10 passes through the beam splitter 30, mirror 40, and focusing lens 50 before entering 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 mirror 40 before reaching the beam splitter 30.

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

[0063] The beam splitter 80 is a separation unit that directs a portion of the light, which is spatially superimposed by the reflected light 14 and the optical comb 22, to the optical bandpass filter 86 (first optical bandpass filter) and another portion to the optical bandpass filter 88 (second optical bandpass filter).

[0064] The RF comb 16 (light from which the reflected light 14 and the optical comb 22 are spatially superimposed), which is split into one optical path, passes through the optical bandpass filter 86, the variable ND filter 82, and the focusing lens 60 before entering the photodetector 70. The RF comb 16, which is split into the other optical path, passes through the optical bandpass filter 88, the variable ND filter 84, and the focusing lens 62 before entering the photodetector 72.

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

[0066] The variable ND filter 82 (first variable ND filter) is placed between the optical bandpass filter 86 and the photodetector 70. The variable ND filter 84 (second variable ND filter) is placed between the optical bandpass filter 88 and the photodetector 72. Each of the variable ND filters 82 and 84 adjusts (reduces) the intensity of light transmitted. The variable ND filters 82 and 84 are configured to allow adjustment of the degree to which they reduce the light intensity. The attenuation rate of the variable ND filters 82 and 84 is set so that the detection results from the photodetectors 70 and 72 do not saturate.

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

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

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

[0070] In this manner, the processing unit 100 calculates the reflectance spectrum of the sample S based on the light transmitted through the optical bandpass filter 86 and the light transmitted through the optical bandpass filter 88. The processing unit 100 also calculates the film thickness of one or more layers contained in the sample S based on the reflectance spectrum of the sample S.

[0071] Figure 6 is a diagram illustrating spectral filtering according to this embodiment. The optical comb (and RF comb) has a repetition frequency f rep (and Δf rep Although it is a set of multiple frequencies that exist discretely at positions separated by 1 / 2, for the sake of explanation, it is represented as a continuous spectrum (envelope) in Figure 6.

[0072] Refer to Figure 6(A) to explain the effects of aliasing that occur in typical dual-comb spectroscopy.

[0073] The upper part of Figure 6(A) shows an example of the optical intensity spectrum (optical region) of an optical comb generated by an optical comb light source, and the lower part of Figure 6(A) shows an example of the spectrum (RF region) of an RF comb produced by interference of the optical comb. Figure 6(A) shows the correspondence between the optical region where the spectrum of the optical region exists and the RF region in which that spectrum appears. For convenience, the negative region of electrical frequency is also shown for the RF comb spectrum.

[0074] In the optical region shown in Figure 6(A), the position (position C) corresponding to the reference offset frequency included in the optical comb, and the distance from the said reference offset frequency (reference frequency) to f rep 2 / 2Δf rep (≒f rep1 f rep2 / 2Δf rep The positions (positions A, B and positions D, E) are shown, separated by ) units.

[0075] In dual-comb spectroscopy, the optical intensity spectrum of an optical comb is reproduced by calibrating the frequency axis (RF domain) of the RF comb to the optical frequency (optical domain). Here, the spectrum of the RF comb ranges from 0 to f. rep The RF comb spectrum must be within the range of / 2. rep Aliasing occurs when the frequency of the RF comb extends beyond the range of / 2. rep If it exists outside the range of / 2, it is between 0 and f repSignals within the range of / 2, and from 0 to f rep This is because it is impossible to distinguish signals outside the / 2 range. Thus, aliasing causes errors in the measurement results.

[0076] To avoid aliasing, the optical intensity spectrum of an optical comb (reflected light 14 or transmitted light 18) in the optical region must reside in only one of the bands AB, BC, CD, or DE. Therefore, the optical comb is adjusted so that its maximum spectral band falls within a single band. In other words, in typical dual-comb spectroscopy, the optical intensity spectrum of the optical comb is at most within band CD(f rep 2 / 2Δf rep ) is restricted.

[0077] To avoid the effects of aliasing, the repetition frequency difference Δf rep Measurement conditions such as the repeating frequency difference Δf must be set appropriately. rep If it is not possible to properly set measurement conditions such as those mentioned 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 is the spectral bandwidth of the optical comb, and the repetition frequency f rep and the repetition frequency difference Δf rep The following relationship exists between them.

[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 shown as follows.

[0080] Δν × measurement rate ∝ f rep 2 / 2 Repetition frequency f repSince it is difficult to significantly change this, the above equation shows that the requirement to broaden the measurement wavelength range Δν and the requirement to increase the measurement rate are mutually contradictory. Repetition frequency f rep Since it is difficult to change it significantly, in order to secure the predetermined measurement wavelength range Δν, the repetition frequency difference Δf rep This necessitates reducing the size of the measurement, resulting in a decrease in the measurement rate. In other words, the measurement rate is sacrificed to avoid the effects of aliasing.

[0081] Furthermore, when performing measurements over a wide wavelength range, the absolute amount of light emitted by the optical comb increases proportionally to the widening of the measurement wavelength range. On the other hand, the maximum amount of light that a photodetector can detect is predetermined. Therefore, when the measurement wavelength range is widened and more wavelengths are incident on the photodetector, the light intensity of each wavelength is limited to stay within the predetermined maximum light intensity, which can reduce the signal intensity per wavelength.

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

[0083] The spectral filtering according to this embodiment provides a solution to such problems.

[0084] Referring to Figure 6(B), spectral filtering in the optical measuring device 1B according to this embodiment will be described.

[0085] In the optical measuring device 1B according to this embodiment, the optical intensity spectrum of the optical comb 12 generated by the optical comb light source 10 is set to, for example, a band AE. In this example, the spectral bandwidth of the optical comb 12 is 4 × f rep 2 / 2Δf rep This is the result.

[0086] In the example in Figure 6(B), the optical bandpass filter 86 transmits only light in a specific spectral band set to band AB (spectrum 26). The optical bandpass filter 88 transmits only light in a specific spectral band set to band DE (spectrum 28).

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

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

[0089] As a result, in the RF region, spectrum 90, which corresponds to spectrum 26, appears in band AB. Due to aliasing, spectrum 91 appears at the position where spectrum 90 is folded back at position B. Furthermore, spectrum 92 appears at the position where spectrum 91 is folded back at position C.

[0090] Similarly, in the RF region, spectrum 95, which corresponds to spectrum 28, appears in band DE. Due to aliasing, spectrum 96 appears at the position where spectrum 95 is folded at position D. Furthermore, spectrum 97 appears at the position where spectrum 96 is folded at position C. Furthermore, spectrum 98 appears at the position where spectrum 97 is folded at position B.

[0091] By setting the spectral bandwidth transmitted by the optical bandpass filters 86 and 88 as shown in Figure 6(B), the optical intensity spectrum of the optical comb incident on photodetector 70 has a spectrum corresponding to spectrum 26, and the optical intensity spectrum of the optical comb incident on photodetector 72 has a spectrum corresponding to spectrum 28. From 0 to f in the RF region. rep Within the range of / 2 (bandwidth CD), there is spectrum 92 corresponding to spectrum 26, and there is 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 respective signal waveforms of the RF comb 16 detected by the photodetectors 70 and 72.

[0093] In the optical measuring device 1B according to this embodiment, the spectral bandwidth of the optical comb is f rep 2 / 2Δf rep The following restrictions do not need to be applied, so when widening the measurement wavelength range, the repetition frequency difference Δf rep This allows for an increase in measurement rate without sacrificing performance. Furthermore, by broadening the measurement wavelength range and selectively acquiring only the required spectral band, high signal intensity per wavelength can be maintained.

[0094] Furthermore, the spectral bandwidth transmitted by each of the optical bandpass filters 86 and 88 can be arbitrarily set according to the material and structure of the sample S. Alternatively, the spectral bandwidth transmitted by each of the optical bandpass filters 86 and 88 may be set according to the noise level, the spectral bandwidth of the optical comb, and other factors.

[0095] From the viewpoint of making the selectively observed spectral band as wide as possible, the spectral band transmitted by the optical bandpass filter 86 (first spectral band) and the spectral band transmitted by the optical bandpass filter 88 (second spectral band) 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 f rep 2 / 2Δf rep The following settings will be applied.

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

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

[0098] The spectral bandwidth of optical comb 12 is defined as the maximum spectral bandwidth f in typical dual-comb spectroscopy. rep 2 / 2Δf rep I explained an example where it is set to four times the original value, but the spectral bandwidth can be set arbitrarily.

[0099] In addition to the reflective optical system described above, a transmissive optical system may also be employed. Figure 7 is a schematic diagram showing an example of the configuration of an optical measuring device 1C according to this embodiment. The optical measuring device 1C shown in Figure 7 is a modified version of the optical measuring device 1B shown in Figure 5, with the optical system changed to a transmissive optical system. Compared to the optical measuring device 1B shown in Figure 5, the optical measuring device 1C shown in Figure 7 has a beam splitter 32 instead of the beam splitter 30, mirror 40, and focusing lens 50.

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

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

[0102] In addition, FIGS. 5 and 7 show a configuration example using two optical band-pass filters, but three or more optical band-pass filters may be arranged. Also in this case, the spectral bands transmitted by each band-pass filter may be selected so that the spectra of the lights transmitted through each optical band-pass filter do not overlap with each other. Conversely, only one optical band-pass filter may be used.

[0103] In addition, FIGS. 5 and 7 show a configuration example in which the optical band-pass filters are fixedly arranged, but a method of mechanically switching the optical band filters may be adopted.

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

[0105] FIG. 8 is a flowchart showing the measurement procedure of the sample S using the optical measurement apparatus according to the present embodiment. Referring to FIG. 8, the user places a reference as a measurement target of the optical measurement apparatus (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 the light intensity spectrum (reference spectrum) observed when the reference is placed (step S4). The processing unit 100 stores the acquired reference spectrum.

[0106] Subsequently, 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] Note that the reference measurement in steps S2 to S4 and the signal measurement in steps S6 to S10 may be executed at different timings. After the reference measurement in steps S2 to S4 is executed once, the signal measurement in steps S6 to S10 may be repeated.

[0109] Also, when performing measurement using the optical measurement device 1B, before executing step S1, the user selects the spectral band transmitted by the optical band-pass filters 86 and 88. Also, the user adjusts the light attenuation rate of the variable ND filters 82 and 84 so as to maximize the light intensity of the light incident on the photodetectors 70 and 72 within the detectable range.

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

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

[0112] FIG. 9(A) shows the measured reflectance spectrum (solid line) and the theoretical reflectance spectrum (dashed line) at the film thickness d. FIG. 9(B) shows an enlarged view of the portion of the graph shown in FIG. 9(A) with wavelengths from 1559 to 1561 nm.

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

[0114] Figure 9(C) shows the change in the root mean squared error (RMSE) with respect to film thickness, calculated by spectral fitting using the nonlinear least squares method to the measured reflectance spectrum shown in Figure 9(A). In the graph shown in Figure 9(C), the film thickness at which the RMSE is minimized was found to be 774.501 μm. This 774.501 μm was determined to be the wafer film thickness.

[0115] <G.まとめ> The optical measuring device according to this embodiment acquires a reflectance spectrum from a sample S using dual-comb spectroscopy. By using dual-comb spectroscopy, a measurement wavelength range can be secured, and higher wavelength resolution can be ensured.

[0116] The optical frequency comb light source has all wavelengths phase-locked to an atomic clock and possesses frequency (wavelength) accuracy equivalent to that of an atomic clock, enabling significantly more accurate measurements compared to film thickness measurements using a spectrometer. In film thickness measurements using a spectrometer combining a diffraction grating and an image sensor, the measurement wavelength range is narrow, which can cause the measured reflectance spectrum to fit to a spectrum corresponding to an incorrect film thickness value. If the measurement wavelength range is widened, the wavelength resolution decreases as long as the number of channels in the image sensor remains constant. As a result, it becomes impossible to measure the reflectance spectrum of the sample with a sufficient number of sampling points, leading to a decrease in measurement accuracy. Thus, it is difficult to ensure both the measurement wavelength range and wavelength resolution when measuring film thickness using a spectrometer. In contrast, the optical measuring device according to this embodiment can ensure both the measurement wavelength range and wavelength resolution.

[0117] The optical measuring device according to this embodiment can selectively acquire spectra over a wider measurement wavelength range without reducing the measurement rate or the signal intensity per wavelength, by using spectral filtering. Based on the selectively acquired spectra, the film thickness of the sample can be measured.

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

[0119] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included. [Explanation of Symbols]

[0120] 1,1A,1B,1C Optical measuring 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 Focusing lens, 70,72 Photodetector, 82,84 Variable 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 results, 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. 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 section that spatially superimposes the reflected light generated when the first optical frequency comb is incident on the object to be measured with the second optical frequency comb, An optical measuring apparatus comprising: a processing unit that calculates the reflectance spectrum of a sample using the optical intensity spectrum of the reference, which is based on a third optical frequency comb generated by the spatial superposition of reflected light generated when the reference is placed as the object to be measured and the second optical frequency comb; and the optical intensity spectrum of the sample, which is based on a fourth optical frequency comb generated by the spatial superposition of reflected light generated when the sample is placed as the object to be measured and the second optical frequency comb.

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

3. 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, The system further includes a separation unit that guides a portion of the light obtained by spatially superimposing the reflected light and the second optical frequency comb to the first optical bandpass filter, and another portion to the second optical bandpass filter, The optical measuring apparatus according to claim 1 or 2, wherein the processing unit calculates the reflectance spectrum of the sample based on the light transmitted through the first optical bandpass filter and the light transmitted through the second optical bandpass filter.

4. A first optical detector associated with the first optical bandpass filter, The system further comprises a second photodetector associated with the second optical bandpass filter, The optical measuring apparatus according to claim 3, 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. A first variable ND (Neutral Density) filter is disposed between the first optical bandpass filter and the first photodetector, The optical measuring apparatus according to claim 4, further comprising a second variable ND filter disposed between the second optical bandpass filter and the second photodetector.

6. A first optical frequency comb is generated using a first optical frequency comb light source, A step of generating a second optical frequency comb with a second repetition frequency using a second optical frequency comb light source, A step of using an overlapping section to spatially superimpose the reflected light generated when the first optical frequency comb is incident on a reference with the second optical frequency comb to generate a third optical frequency comb, The step of using the superposition portion to spatially superimpose the reflected light generated when the first optical frequency comb is incident on the sample with the second optical frequency comb to generate a fourth optical frequency comb, An optical measurement method comprising the step of calculating the reflectance spectrum of a sample using the light intensity spectrum of the reference based on the third optical frequency comb and the light intensity spectrum of the sample based on the fourth optical frequency comb.

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