Optical measurement system and optical measurement method

Abstract

An optical measurement system and an optical measurement method. The optical measurement system includes: a light source for generating measurement light; a light receiving portion that receives reflected light or transmitted light generated by irradiating a sample with the measurement light as observation light; an acquisition unit that acquires an observation light spectrum of a wavelength range included in the observation light, the wavelength range being set as a wavelength interval of the same phase at both ends; an initial value determination unit that determines an initial value of a film thickness of the sample based on a position of a peak appearing in a power spectrum obtained by Fourier transforming the observation light spectrum; a fitting unit that determines the film thickness of the sample by updating parameters of a model of the sample including the film thickness as a parameter in such a manner that an interference spectrum calculated from the model coincides with the observation light spectrum; and a determination unit that determines whether the determined film thickness satisfies a condition based on a previously acquired film thickness.

Description

TECHNICAL FIELD

[0001] The present application relates to an optical measurement system and an optical measurement method that utilize interference of light generated in a measurement target. BACKGROUND

[0002] In recent years, the progress of wafer thinning processes has been remarkable. By thinning wafers, high functionality of various devices can be achieved. For example, thinning of IC chips, multilayer integration of SiP (System in Package) used in mobile devices, high sensitivity of imaging elements such as CMOS image sensors, high efficiency of power devices, and the like can be achieved.

[0003] In mass production processes, a thinning process of 50 μm to several hundred μm needs to be established, and in the future, a thinning process of sub-μm needs to be established. In order to achieve this process, a measurement device that can measure the film thickness of a wafer with high precision and at high speed in a thinning process is indispensable.

[0004] As a background technique for measuring the film thickness of a wafer, (1) contact type, (2) electrostatic capacitance type, (3) OCT (Optical Coherence Tomography) method, and the like are known.

[0005] (1) As a related art document of the contact type, Japanese Patent Application Publication No. 2018-179672 discloses a method of measuring a film thickness using a contact type film thickness meter having a probe.

[0006] (2) As a related art document of the electrostatic capacitance type, Japanese Patent Application Publication No. 2009-109208 discloses a measurement device having a plurality of electrostatic capacitance displacement meters for calculating the electrostatic capacitance between a measurement target and a measurement head.

[0007] (3) As a related art document of the OCT method, Japanese Patent Application Publication No. 2013-205252 discloses a method of irradiating a coating film with light from a light source, and measuring the film thickness of the coating film by detecting the intensity of interference light containing reflected light from the coating film.

[0008] In general thinning processes, wafers are ground by an abrasive stone while being exposed to grinding processing water. Therefore, the film thickness of the wafer has to be measured in a non-contact and non-invasive manner, and a measurement method using light is applied. That is, (1) contact type and (2) electrostatic capacitance type, and the like cannot be adopted in the film thickness measurement of wafers in a thinning process. In addition, (3) the OCT method has difficulty in sufficiently securing the resolution in the film thickness direction, and in addition, there are problems such as system structure complication and large size.

[0009] A wafer represented by silicon is transparent to light having an energy band gap lower than that of the wafer (semiconductor) itself, and thus it is possible to measure a film thickness by utilizing light interference generated inside the wafer. In particular, the wafer as a measurement target is continuously moved as it is ground in a wafer thinning process, and a strong measurement method is required for the movement of such a measurement target. In this regard, since the light interference inside the wafer depends only on the film thickness and the refractive index of the wafer, the movement of the measurement target does not disturb the measurement and high-precision measurement can be achieved. SUMMARY

[0010] An object of the present application is to provide a method for measuring a film thickness with higher precision by utilizing light interference generated in a measurement target.

[0011] An optical measurement system according to an aspect of the present application includes: a light source configured to generate measurement light; a light receiving unit configured to receive reflected light or transmitted light generated by irradiating a sample with the measurement light as observation light; an acquisition unit configured to acquire an observation light spectrum of a wavelength range included in the observation light, the wavelength range being set to a wavelength interval in which phases at both ends are the same; an initial value determination unit configured to determine an initial value of a film thickness of the sample based on a position of a peak appearing in a power spectrum obtained by Fourier transforming the observation light spectrum; a fitting unit configured to determine the film thickness of the sample by updating parameters of a model of the sample so that an interference spectrum calculated from the model including the film thickness as a parameter coincides with the observation light spectrum; and a determination unit configured to determine whether the determined film thickness satisfies a condition based on a film thickness acquired previously.

[0012] The light receiving unit can include a spectrometer configured to output an intensity of the observation light at each wavelength for a prescribed wavelength range. The acquisition unit can search for two wavelength positions showing the same phase based on a detection result of the light receiving unit, and extract the observation light spectrum from information between the two wavelength positions searched for in the detection result.

[0013] The light receiving unit can include a light receiving element and a diffraction grating configured to be able to change a wavelength component of wavelength components included in the observation light that is incident on the light receiving element. The acquisition unit can sequentially change the wavelength component incident on the light receiving element of the light receiving unit, search for two wavelength positions showing the same phase, and sequentially cause the wavelength component between the two wavelength positions searched for to be incident on the light receiving element of the light receiving unit, thereby acquiring the observation light spectrum.

[0014] The condition can include a condition in which an error of the film thickness determined this time with respect to a film thickness acquired previously from the same sample or another sample is within a range determined in advance.

[0015] Also, the model of the sample can be an equation that simulates interference of light generated inside the sample, and includes a film thickness and optical constants.

[0016] Also, the optical measurement system can further include a correction unit that corrects the determined film thickness when it is determined that the determined film thickness does not satisfy the condition.

[0017] Also, the optical measurement system can further include a notification unit that notifies that the condition is not satisfied when it is determined that the determined film thickness does not satisfy the condition.

[0018] Another aspect of the present application relates to an optical measurement method including a step of irradiating a sample with measurement light from a light source and acquiring a spectrum of observation light that is reflected light or transmitted light generated from the sample. The spectrum of the observation light is a spectrum of a wavelength interval set to be the same phase at both ends in a wavelength range included in the observation light. The optical measurement method further includes a step of determining an initial value of a film thickness of the sample based on a position of a peak appearing in a power spectrum obtained by Fourier transforming the spectrum of the observation light, a step of determining the film thickness of the sample by updating parameters of a model of the sample in such a manner that an interference spectrum calculated from the model including the film thickness as a parameter coincides with the spectrum of the observation light, and a step of determining whether the determined film thickness satisfies a condition based on a film thickness acquired previously.

[0019] The above and other objects, features, aspects and advantages of the present application will become more apparent from the following detailed description of the present application when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a schematic view showing a structure example of an optical measurement system according to the present embodiment.

[0021] Figure 2 is a schematic view showing a schematic structure of a spectrometer used in the optical measurement system according to the present embodiment.

[0022] Figure 3 is a schematic view showing a structure example of a processing device included in the optical measurement system according to the present embodiment.

[0023] Figure 4 is a flowchart showing an outline of a processing procedure relating to film thickness measurement in the optical measurement system according to the present embodiment.

[0024] Figure 5 is a graph for explaining discontinuity of a waveform generated in a discrete Fourier transform.

[0025] Figure 6is a diagram for explaining a method of modifying an observation light spectrum in an optical measurement system according to the present embodiment.

[0026] Figure 7 is a diagram showing Figure 4 is a flowchart showing a more detailed procedure of the process of modifying the observation light spectrum (step S4) shown in

[0027] Figure 8 is a diagram showing an outline structure of a spectrometer used in a modification example of the optical measurement system according to the present embodiment.

[0028] Figure 9 is a flowchart showing a more detailed procedure of the process of acquiring the observation light spectrum in the modification example of the optical measurement system according to the present embodiment.

[0029] Figure 10 is a diagram showing an example of a power spectrum calculated from the observation light spectrum measured in the optical measurement system according to the present embodiment.

[0030] Figure 11 is a diagram showing Figure 10 is a diagram showing the region of interest shown in

[0031] Figure 12 is a diagram showing an example of a measurement result in a case where no preprocessing is performed.

[0032] Figure 13 is a diagram showing an example of a measurement result in a case where preprocessing is performed.

[0033] Figure 14 is a diagram for explaining a process related to a normal measurement condition performed by the optical measurement system according to the present embodiment.

[0034] Figure 15 is a diagram showing an example of an interface screen for setting a normal measurement condition in the optical measurement system according to the present embodiment.

[0035] Figure 16 is a diagram showing Figure 4 is a flowchart showing a more detailed procedure of the process related to the normal measurement condition (steps S22 and S24) shown in

[0036] Figure 17 is a diagram showing an example of a functional structure provided by the optical measurement system according to the present embodiment.

[0037] BRIEF DESCRIPTION OF REFERENCE NUMERALS

[0038] 1: optical measurement system; 2: sample; 4: Y-type optical fiber; 10: light source; 20, 20A: spectrometer; 22: diffraction grating; 24, 28: light receiving element; 26: interface circuit; 50: dummy sample; 51, 52: glass plate; 53: gap; 100: processing device; 102: processor; 104: main memory; 106: input section; 108: display section; 110: storage device; 112: operating system; 114: measurement program; 116: detection result; 118: measurement result; 120: communication interface; 122: network interface; 124: medium drive; 126: recording medium; 150: spectrum acquisition module; 152: buffer; 154: preprocessing module; 156: spectrometer control module; 160: Fourier transform module; 162: initial value decision module; 164: fitting module; 166: determination module; 168: correction module; 170: notification module; 202, 208: optical slit; 204, 206: concave mirror; 300: interface screen; 302: activation switch; 304: correction amount setting field; 306: determination change amount setting field; 308: determination upper and lower limit amount setting field. DETAILED DESCRIPTION

[0039] Embodiments of the present application are described in detail with reference to the accompanying drawings. In addition, for the same or equivalent portions in the drawings, the same reference numerals are assigned, and a description thereof is not repeated.

[0040] <A. Optical measurement system>

[0041] First, a configuration example of an optical measurement system 1 according to the present embodiment is described. The optical measurement system 1 is a film thickness measurement device of a spectrometer interference type. Hereinafter, an optical system (reflection light observation system) that irradiates a sample with light and observes reflected light thereof is mainly described, but of course, it can also be applied to an optical system (transmission light observation system) that irradiates a sample with light and observes transmitted light thereof.

[0042] In the present specification, "film thickness" refers to the thickness of a specific layer included in an arbitrary sample. However, in a case where a sample is configured of a uniform material such as a wafer, it is equivalent to a case where only a single layer is present, and the film thickness to be measured refers to the thickness of the sample. That is, in the present specification, "film thickness" does not refer only to the thickness of each layer included in a sample having a stacked structure, but also refers to the thickness of the sample itself.

[0043] Figure 1is a schematic view showing a configuration example of an optical measurement system 1 according to the present embodiment. The optical measurement system 1 includes a light source 10 that generates measurement light for irradiating a sample 2, a spectrometer 20 that receives observation light (reflected light or transmitted light) generated by irradiating the sample 2 with the measurement light, and a processing device 100 that is input with a detection result of the spectrometer 20. The processing device 100 calculates a measurement result (typically, a film thickness) relating to the sample 2 on the basis of the detection result of the spectrometer 20. The light source 10 and the spectrometer 20 are optically connected via a Y-type optical fiber 4 having an irradiation port toward the sample 2.

[0044] In the optical measurement system 1, the sample 2 is irradiated with the measurement light from the light source 10, and a film thickness or the like of the sample 2 is measured by observing light that occurs due to interference of light generated inside the sample 2.

[0045] The light source 10 generates measurement light having a prescribed wavelength range. The wavelength range of the measurement light is determined in accordance with a range of wavelength information that should be measured from the sample 2 or the like. The light source 10 uses, for example, a halogen lamp, a white LED, or the like.

[0046] The light source 10 can also generate measurement light containing a component of a near-infrared region. In this case, as the light source 10, an ASE (Amplified Spontaneous Emission) light source can also be employed.

[0047] Figure 2 is a schematic view showing a schematic configuration of the spectrometer 20 used in the optical measurement system 1 according to the present embodiment. Referring to Figure 2 , the spectrometer 20 outputs an intensity at each wavelength of the observation light with respect to a prescribed wavelength range.

[0048] More specifically, the spectrometer 20 includes a diffraction grating 22 for diffracting light incident via the Y-type optical fiber 4, a light-receiving element 24 having a plurality of channels arranged in correspondence with the diffraction grating 22, and an interface circuit 26 electrically connected to the light-receiving element 24 for outputting a detection result to the processing device 100. The light-receiving element 24 is constituted by a line sensor or a two-dimensional sensor or the like, and is capable of outputting an intensity at each frequency component as a detection result.

[0049] Figure 3 is a schematic view showing a configuration example of the processing device 100 included in the optical measurement system 1 according to the present embodiment. Referring to Figure 3 , the processing device 100 includes a processor 102, a main memory 104, an input section 106, a display section 108, a storage device 110, a communication interface 120, a network interface 122, and a medium drive 124.

[0050] The processor 102, which is typically a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), or the like, reads out one or more programs saved in the storage device 110 into the main memory 104 and executes the program. The main memory 104, which is a volatile memory such as a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory), functions as a work memory for the processor 102 to execute the program.

[0051] The input section 106 includes a keyboard, a mouse, or the like, and is used to accept operations from a user. The display section 108 outputs, to the user, an execution result of the program executed by the processor 102, or the like.

[0052] The storage device 110, which is constituted by a hard disk, a flash memory, or the like, is used to store various programs and data. More specifically, the storage device 110 is used to hold an operating system 112 (OS), a measurement program 114, a detection result 116, and a measurement result 118.

[0053] The operating system 112 provides an environment in which the processor 102 executes a program. The measurement program 114, by being executed by the processor 102, realizes an optical measurement method or the like according to the present embodiment. The detection result 116 contains data output from the spectrometer 20. The measurement result 118 contains a measurement result obtained by executing the measurement program 114.

[0054] The communication interface 120 is used to relay data transmission between the processing apparatus 100 and the spectrometer 20. The network interface 122 is used to relay data transmission between the processing apparatus 100 and an external server apparatus.

[0055] The media drive 124 reads out necessary data from a recording medium 126 (e.g., an optical disk or the like) that holds a program or the like executed by the processor 102, and saves it in the storage device 110. Further, the measurement program 114 or the like executed in the processing apparatus 100 can be installed via the recording medium 126 or the like, or downloaded from a server apparatus via the network interface 122 or the like.

[0056] The measurement program 114 can also be a program that performs processing as necessary among program modules provided as part of the operating system 112, called at a prescribed timing in a prescribed arrangement. In this case, a measurement program 114 that does not include this module is also within the technical scope of the present application. The measurement program 114 can also be provided as part of another program.

[0057] Furthermore, all or a part of the functions provided by execution of the program by the processor 102 of the processing device 100 can also be implemented by hardwired logic circuits (e.g., FPGA (field-programmable gate array), ASIC (application specific integrated circuit), etc.).

[0058] <B. SUMMARY>

[0059] The optical measurement system 1 according to the present embodiment is a spectroscopic interference type optical measurement device. The optical measurement system 1 irradiates a sample 2 with measurement light having a prescribed wavelength range, measures the spectrum of reflected light (reflected interference light) or transmitted light (transmitted interference light) from the sample 2 using a spectroscopic detector 20, and calculates the film thickness of the sample 2 by analyzing the spectrum. As a method of analyzing the spectrum, a method of analysis by fitting and a method based on frequency analysis can typically be cited.

[0060] For example, the optical measurement system 1 is used for in-situ measurement of film thickness in the manufacturing process of a wafer, etc. In order to measure the film thickness in real time while grinding a wafer as a sample, it is necessary to measure the light interference occurring inside the sample using a high-speed spectroscopic detector (spectroscopic detector 20 shown in the drawing). Figure 3 The light receiving element 24 of the spectroscopic detector 20 is configured in an array-like structure using a photoelectric device (CCD, NMOS, CMOS, etc.) of a Si substrate having 256 to 2048 channels, for example.

[0061] If the film thickness of the sample becomes large, the interference spectrum occurring inside the sample becomes dense, and the number of data points forming one interference waveform becomes small, so it is desirable to increase the number of channels as much as possible, but there is a limit to the increase in the number of channels. Thus, the measurement wavelength range is made narrow to make the number of channels per unit wavelength larger, thereby increasing the number of data points forming one interference waveform. However, if the measurement wavelength range is too narrow, the measurable range of the film thickness is limited, so there is also a limit to making the measurement wavelength range narrow.

[0062] Under the constraints of the number of channels and the measurement wavelength range as described above, the present inventors have conceived various improvements for improving the measurement accuracy as described below.

[0063] Figure 4 is a flowchart showing an outline of a process relating to film thickness measurement in the optical measurement system 1 according to the embodiment of the present application. With respect to the process shown in Figure 4 the process shown, it is assumed that multiple measurements are performed on the same sample 2 and / or measurements are performed on multiple samples 2 consecutively.

[0064] Referring to Figure 4 , the processing device 100 acquires a spectrum of observation light generated by irradiating the sample 2 disposed at the measurement position with measurement light (step S2). That is, the following process is performed: the sample 2 is irradiated with measurement light from the light source 10, and a spectrum of observation light, which is reflected light or transmitted light generated from the sample 2, is acquired.

[0065] The processing device 100 performs a pre-process as described later on the acquired spectrum of observation light to modify the spectrum of observation light (step S4). Then, the processing device 100 performs Fourier transform on the modified spectrum of observation light to calculate a power spectrum (measured value) (step S6), and determines an initial value of the film thickness used in the fitting described later based on a position of an inflection point (peak) appearing in the calculated power spectrum (step S8). In this way, the following process is performed: an initial value of the film thickness of the sample 2 is determined based on a position of a peak appearing in the power spectrum obtained by Fourier transform on the spectrum of observation light.

[0066] Next, on the assumption of a model having the configuration of the sample 2, parameters of the model that match the measured spectrum of observation light are determined by fitting.

[0067] More specifically, the processing device 100 defines a model of the sample 2 that includes the film thickness as a parameter (step S10). The processing device 100 calculates an interference spectrum of the defined model based on the currently set film thickness (step S12).

[0068] The processing device 100 judges whether the fitting has converged based on an error between the spectrum of observation light acquired in step S2 and the interference spectrum calculated in step S12 (step S14).

[0069] In the case where the fitting has not converged (NO in step S14), the processing device 100 updates the parameters of the model in accordance with the error between the spectrum of observation light and the interference spectrum (step S16), and repeats the process after step S12.

[0070] On the other hand, in the case where the fitting has converged (YES in step S14), the processing device 100 determines the film thickness corresponding to the current parameters of the model as a measurement result (step S18).

[0071] Next, the processing device 100 determines whether there is a previous measurement result related to the same specimen 2 or specimens 2 of the same type (step S20). If there is no previous measurement result related to the same specimen 2 or specimens 2 of the same type ( "No" in step S20), the processing device 100 skips the processing of steps S22 and S24.

[0072] If there is a previous measurement result related to the same specimen 2 or specimens 2 of the same type ( "Yes" in step S20), the processing device 100 determines whether the current measurement result satisfies predetermined normal measurement conditions (step S22).

[0073] If the current measurement result does not satisfy the normal measurement conditions ( "No" in step S22), the processing device 100 performs processing related to measurement anomalies (step S24). On the other hand, if the current measurement result satisfies the normal measurement conditions ( "Yes" in step S22), the processing device 100 skips the processing of step S24.

[0074] The processing device 100 determines whether an end of the measurement process has been indicated (step S26). If an end of the measurement process has not been indicated ( "No" in step S26), the processing device 100 repeats the processing after step S2.

[0075] If an end of the measurement process has been indicated ( "Yes" in step S26), the processing device 100 ends the measurement process.

[0076] Through the above-described processing procedure, the film thickness of the specimen 2 is measured in sequence. Next, Figure 4 details of several processes included in the illustrated processing procedure will be described.

[0077] <C. Pretreatment>

[0078] Next, Figure 4 the process of trimming the observation light spectrum in the illustrated processing procedure (step S4) will be described.

[0079] The optical measurement system 1 is an optical measurement system that utilizes light interference generated inside the specimen 2, so the observation light spectrum includes periodic components. The discrete Fourier transform (DFT) is mostly used to implement the Fourier transform for calculating the power spectrum. The mathematical Fourier transform assumes an infinitely long waveform, whereas the discrete Fourier transform assumes an infinitely repeated wavelength of a specified length (finite wavelength).

[0080] Therefore, in a case where the limited wavelength has a period that is an integer multiple of the original wavelength (i.e., a case where the phase at the start point of the limited wavelength is the same as the phase at the end point), the wavelength obtained by repeating the limited wavelength is substantially the same as the original wavelength. On the other hand, in a case where the phase at the start point of the limited wavelength is different from the phase at the end point, the continuity between the preceding limited wavelength and the subsequent limited wavelength is not maintained.

[0081] Figure 5 is a graph for explaining discontinuity of a waveform generated in a discrete Fourier transform. Figure 5 An example of a waveform that is a calculation target is shown in (A) of Figure 5 an arbitrary interval in the waveform shown in (A) of

[0082] Figure 5 A case where the phase at the start point of the extracted limited waveform is the same as the phase at the end point is shown in (B) of

[0083] On the contrary, Figure 5 A case where the phase at the start point of the extracted limited waveform is not the same as the phase at the end point is shown in (C) of Figure 5 If the limited waveform shown in (C) of Figure 5 the same result as the result of the Fourier transform of the waveform shown by the dotted line in (C) of

[0084] Therefore, the optical measurement system 1 according to the present embodiment acquires an observation light spectrum of a wavelength interval in a wavelength range included in the observation light, which is set so that the phase at the start point is the same as the phase at the end point. Typically, the spectrum detected by the spectrometer 20 is trimmed to be an observation light spectrum in which the phase at the start point is the same as the phase at the end point. The spectrum of the observation light thus acquired corresponds to a spectrum of a wavelength interval in a wavelength range included in the observation light, which is set so that the phase at the start point is the same as the phase at the end point.

[0085] Next, several methods for acquiring an observation light spectrum in which the phase at the start point is the same as the phase at the end point will be described.

[0086] (c1: trimming method of observation light spectrum)

[0087] Figure 6 is a graph for explaining a trimming method of an observation light spectrum in the optical measurement system 1 according to the present embodiment. Figure 6An example of an observation light spectrum observed from the sample 2 as a detection result of the spectrometer 20 is shown in (A) of FIG. 9. Further, the intensity of the observation light is output per channel (i.e., per wavelength) from the spectrometer 20. Figure 6 A continuous waveform obtained by interpolating the intensity of each channel is shown in (A) of FIG. 10.

[0088] In order to make the phase of the starting point coincide with the phase of the ending point, the processing device 100 searches for the position of the specific phase from both sides of the observation light spectrum as a detection result. The value of the phase at which the starting point coincides with the ending point can be an arbitrary value, but for example, the position at which the phases coincide can be decided based on the inflection point appearing in the observation light spectrum.

[0089] Figure 6 An example of deciding the interval to be extracted as the observation light spectrum based on the valley (the vertex at which the waveform is convex downward) appearing in the observation light spectrum is shown in (B) of FIG. 11. However, the peak (the vertex at which the waveform is convex upward) appearing in the observation light spectrum can also be used, and the intermediate intensity between the peak and the valley can also be used.

[0090] Generally, the spectrometer 20 is configured to output the intensity of 1024 channels or the like, which is a channel number suitable for a discrete Fourier transform, as a detection result. However, as shown in (B) of FIG. 12, in a case where a part of the observation light spectrum is extracted as the observation light spectrum to be subjected to the trimming, the spectrum becomes a spectrum having a channel number smaller than the original channel number. Therefore, as shown in (C) of FIG. 12, it is preferable to resample the extracted observation light spectrum at a channel number suitable for a discrete Fourier transform. Figure 6 Figure 6

[0091] As shown in (B) of FIG. 13, as the trimming process, the processing device 100 searches for two wavelength positions showing the same phase based on the detection result of the spectrometer 20, and extracts the observation light spectrum from information between the two wavelength positions searched for in the detection result. By performing such a trimming process, it is possible to reduce the measurement error. Figure 6

[0092] Figure 7 is a flowchart showing a more detailed process of the process (step S4) of trimming the observation light spectrum shown in Figure 4 As shown in (B) of FIG. 13, as the trimming process, the processing device 100 searches for two wavelength positions showing the same phase based on the detection result of the spectrometer 20, and extracts the observation light spectrum from information between the two wavelength positions searched for in the detection result. By performing such a trimming process, it is possible to reduce the measurement error. Figure 7 Referring to (A) of FIG. 14, the processing device 100 interpolates the intensity of each channel as a detection result of the spectrometer 20 to generate the observation light spectrum (step S41). Then, the processing device 100 searches for a first inflection point from the side of the smallest channel number in the generated observation light spectrum (step S42), and searches for a second inflection point from the side of the largest channel number in the generated observation light spectrum (step S43).

[0093] ​​​Further, the order of execution of step S42 and step S43 is not limited. In addition, it is assumed that the first inflection point and the second inflection point are inflection points of the same kind (peak or valley).

[0094] The processing device 100 extracts a spectrum of the section from the first inflection point to the second inflection point (step S44). Also, the processing device 100 resamples the extracted spectrum at a prescribed number of channels to generate a trimmed observation light spectrum (step S45).

[0095] (c2: Other configuration of acquiring observation light spectrum)

[0096] In Figure 6 and Figure 7 described an example of trimming the observation light spectrum based on the detection result of the spectrometer 20 that outputs a spectrum of a wavelength range determined in advance, but another optical system can be used to acquire an observation light spectrum in which the phase of the start point and the phase of the end point coincide.

[0097] Figure 8 is a schematic diagram showing a schematic configuration of a spectrometer 20A used in a variation of the optical measurement system 1 according to the present embodiment. Referring to Figure 8 , the spectrometer 20A includes two concave mirrors 204 and 206, a diffraction grating 22 configured to be rotatable, and a light-receiving element 28 having a single channel.

[0098] Light from the sample 2, after passing through the optical slit 202 of the spectrometer 20A, propagates in the order of the concave mirror 204, the diffraction grating 22, and the concave mirror 206, and enters the light-receiving element 28 through the optical slit 208.

[0099] The diffraction grating 22 reflects the incident light in a direction corresponding to the wavelength. For example, if the light incident on the diffraction grating 22 contains components of wavelengths f1, f2, and f3, the components of each of the wavelengths f1, f2, and f3 are reflected in a direction corresponding to each wavelength. Ultimately, only the components (wavelengths) that form an optical path through the optical slit 208 are detected by the light-receiving element 28.

[0100] In the spectrometer 20A, the diffraction grating 22 is configured to be rotatable, and the wavelength components incident on the light-receiving element 28 depend on the rotation angle of the diffraction grating 22. That is, by appropriately controlling the rotation angle of the diffraction grating 22, it is possible to sequentially change the wavelength components incident on the light-receiving element 28, that is, it is possible to achieve scanning along the wavelength. In this way, the diffraction grating 22 is configured to be able to change the wavelength components among the wavelength components contained in the observation light that are incident on the light-receiving element 28.

[0101] For example, the observation light spectrum is scanned to find positions (e.g., certain inflection points) showing a certain phase, and by setting any 2 points of these positions as a start point and an end point, the observation light spectrum that is the object of the discrete Fourier transform can be determined.

[0102] Figure 9 is a flowchart showing a more detailed procedure for acquiring the observation light spectrum in a variation of the optical measurement system 1 according to the present embodiment. Further, instead of the process of acquiring the observation light spectrum (step S2) and the process of trimming the observation light spectrum (step S4) shown in Figure 4 , the processes shown in Figure 9 are executed.

[0103] Referring to Figure 9 , the processing device 100 instructs the spectrometer detector 20A to perform scanning from an arbitrary lower limit wavelength (step S21), and searches for a wavelength at which an inflection point is generated in the detection result output from the spectrometer detector 20A (step S22). The processing device 100 determines the wavelength obtained by the search as a measurement start wavelength (step S23).

[0104] In addition, the processing device 100 instructs the spectrometer detector 20A to perform scanning from an arbitrary upper limit wavelength (step S24), and searches for a wavelength at which an inflection point is generated in the detection result output from the spectrometer detector 20A (step S25). The processing device 100 determines the wavelength obtained by the search as a measurement end wavelength (step S26).

[0105] Then, the processing device 100 instructs the spectrometer detector 20A to perform scanning from the measurement start wavelength determined in step S23 to the measurement end wavelength determined in step S26 (step S27), and associates the detection results output sequentially with the wavelengths and saves them as an observation light spectrum (step S28).

[0106] In this way, in a case where the spectrometer detector 20A as shown in Figure 8 is employed, the processing device 100 causes the wavelength components incident on the light-receiving elements 28 of the spectrometer detector 20A to be different sequentially, searches for two wavelength positions showing the same phase, and causes the wavelength components between the two wavelength positions searched for to be incident on the light-receiving elements 28 of the spectrometer detector 20A sequentially, thereby acquiring the observation light spectrum. By using such an observation light spectrum, it is possible to reduce measurement errors.

[0107] (c3: Measurement Result Example)

[0108] An example of a measurement result in a case where the observation light spectrum in which the phase of the start point and the phase of the end point are the same as described above is used will be described.

[0109] Figure 10is a graph showing an example of a power spectrum calculated from an observation light spectrum measured in the optical measurement system 1 according to the present embodiment. Figure 11 is a graph showing a region of interest shown in Figure 10 enlarged.

[0110] Referring to Figure 10 , the horizontal axis of the power spectrum indicates the film thickness of the sample 2, and the vertical axis indicates the power. In the power spectrum, a peak (an inflection point) is generated at a position corresponding to the film thickness of the sample 2 as a measurement target. By searching the power spectrum or performing fitting with a power spectrum (a theoretical value) calculated from a model, the position of the peak is determined, and thus the film thickness of the sample 2 can be calculated.

[0111] The resolution (the interval of the film thickness) of the horizontal axis of the power spectrum calculated by Fourier transform depends on the wavelength range incident on the light-receiving element 24 of the spectrometric detector 20 and the number of channels (the wavelength resolution) of the spectrometric detector 20. Generally, the resolution of the power spectrum is insufficient for the required measurement accuracy, and thus a process of interpolating data points in the vicinity of the peak of the power spectrum using a polynomial or the like is performed as shown in Figure 11 .

[0112] Referring to Figure 11 , without the preprocessing, the data points (□ points) constituting the power spectrum are not necessarily present at the original peak position, but are sometimes present in the vicinity of the peak position. Even if the distribution of such data points is interpolated, uncertainty about the position of the peak remains.

[0113] In contrast, by performing the preprocessing, the data points (◇ points) constituting the power spectrum are present at the original peak position. By obtaining such a distribution of data points, the resolution of the power spectrum can be improved by the interpolation process.

[0114] <D. Fitting>

[0115] Next, the fitting (steps S10 to S18) in the process shown in Figure 4 will be described. In the fitting, the parameters of the model are updated in such a manner that the interference spectrum calculated from the model of the sample 2 including the film thickness as a parameter coincides with the observation light spectrum, and the film thickness of the sample 2 is determined.

[0116] More specifically, a model having a film thickness dl and known optical constants (a refractive index nl and an extinction coefficient kl) is assumed to calculate the interference spectrum. Hereinafter, the reflectance interference spectrum obtained by the reflection light observation system will be described, but the transmittance interference spectrum obtained by the transmission light observation system can also be calculated by the same process.

[0117] First, assume a state in which a sample (complex refractive index N1) is disposed in air (complex refractive index N0). The complex refractive index N0 and the complex refractive index N1 can be expressed as the following (1-1) and (1-2).

[0118] N0 = n0... (1-1)

[0119] N1 = n1 - jk1... (1-2)

[0120] Here, n0 (=1) is the refractive index of air, n1 is the refractive index of the sample, k1 is the extinction coefficient of the sample, and j is an imaginary unit.

[0121] The amplitude reflectance r 01 and the phase factor β1 of the reflected light (air→sample→air) generated in the sample, taking into account multiple reflection inside the sample, are introduced.

[0122] As for the amplitude reflectance r 01 , the amplitude reflectance of s-polarized light and the amplitude reflectance of p-polarized light are calculated from the Fresnel coefficient. In the case where the measuring light is perpendicularly incident on the sample, since the difference between s-polarized light and p-polarized light disappears, both the s-polarized light and the p-polarized light can be handled as the same amplitude reflectance r 01 .

[0123] Here, if the optical constants of the sample are assumed, the extinction coefficient k1 is sufficiently small compared to the refractive index n1 (n1 » k1: as an example, n1 « 3 to 4, k1 « 10 -5 to 10 -4 ). Therefore, if the extinction coefficient k1 « 0 is approximated, an approximate expression such as the following (2) can be obtained. At this time, both the amplitude reflectance r 01 and the phase factor β1 are real numbers. The intensity reflectance R in (2) becomes a reflectance interference spectrum.

[0124]

[0125]

[0126]

[0127] In this way, the model of the sample adopts a formula including the film thickness and the optical constants, which simulates the light interference generated inside the sample.

[0128] As for the intensity reflectance R, the condition in which the reflected light from the surface of the sample and the reflected light from the back of the sample interfere with each other to weaken each other can be expressed as the following (3).

[0129]

[0130] where 2m+1 is the number of interference, m is the number of interference index (m = 1, 2, 3,...), and n1(λ) indicates that the refractive index n1 of the sample has wavelength dependency with respect to the wavelength λ.

[0131] The phase factor β1 includes the film thickness d1 of the sample as a variable, and in the fitting, after the film thickness d1 of the sample is set as an initial value (in the case of the measurement result of FIG. 8A, for example, d1 = 0.5 μm), the film thickness d1 is sequentially changed so that the calculated intensity reflectance R (reflectance interference spectrum) coincides with the measured observation light spectrum. For example, in the case where the least square method is adopted as the method of fitting, the film thickness d1 is sequentially changed so that the error (for example, residual sum of squares) between the intensity reflectance R and the observation light spectrum is minimized. Figure 4

[0132] However, in the fitting, the number of interference index m can also be a variable.

[0133] <E. Measurement Result Example>

[0134] Next, an example of the measurement result obtained by the optical measurement system 1 according to the present embodiment will be described. As an example of the sample, two glass plates were arranged in a non-parallel manner, and a pseudo sample in which the gap between the glass plates was changed obliquely was measured.

[0135] Figure 12 is a graph showing an example of the measurement result in the case where no pretreatment was performed. Figure 13 is a graph showing an example of the measurement result in the case where pretreatment was performed.

[0136] Figure 12 (A) of FIG. 8 and Figure 13 An example of the cross-sectional configuration of the pseudo sample 50 is shown in (A) of FIG. 8. The pseudo sample 50 includes a glass plate 51 arranged on the upper side and a glass plate 52 arranged on the lower side. A gap 53 exists between the glass plate 51 and the glass plate 52, and the optical measurement system 1 measures the film thickness of the gap 53. In Figure 12 (B) of FIG. 8 and Figure 13 An in-plane distribution (film thickness distribution) obtained by measuring the film thickness of 200 points along the lateral direction of the drawing is shown in the example of the measurement result shown in (B) of FIG. 8.

[0137] In the measurement result shown in (B) of FIG. 8, it is known that fluctuations are generated in the film thickness distribution that should have been linearly changed. The fluctuations generated in the film thickness distribution are caused by the fact that the resolution of the power spectrum is insufficient for the required measurement accuracy. As described above, the measurement result shown in (B) of FIG. 8 is the measurement result obtained in the case where the film thickness of the sample was not measured in the state where the sample was pretreated. Figure 12 Figure 11 ​​As shown, there is no data point (□) at the peak position of the power spectrum without the preprocessing. In the case where there is no data point at the peak position, the waveform of the power spectrum cannot be accurately reproduced, and thus the accuracy of the peak position read from the power spectrum is reduced. In addition, the read peak position deviates.

[0138] In contrast, in the measurement result shown in (B) of FIG. 6, the film thickness distribution changes linearly. This is because the observation light spectrum is modified by the preprocessing, and thus there is a data point (O) at the peak position of the power spectrum. Thus, in terms of the peak position, the waveform of the power spectrum can be more accurately reproduced, and the reading accuracy of the peak position read from the power spectrum can be improved. Figure 13

[0139] Further, the optical measurement system 1 according to the present embodiment does not directly use the film thickness determined based on the peak position read from the power spectrum as the measurement result, but further applies the fitting, and thus the measurement accuracy can be further improved. In addition, an incorrect number of interference can be determined in the fitting due to the deviation of the peak position read from the power spectrum. By performing the preprocessing as described above, the possibility of incorrectly determining the number of interference can be reduced.

[0140] <F. Normal measurement condition>

[0141] Next, monitoring of the normal measurement condition in the processing shown in (A) of FIG. 7 (steps S22 to S24) will be described. In the monitoring of the normal measurement condition, the following processing is performed: it is determined whether the film thickness determined by the fitting satisfies the normal measurement condition based on the previously acquired film thickness. Figure 4 As described above, in the processing of determining the film thickness of the sample 2 by the fitting, an incorrect number of interference can be determined. In this case, a value deviated from the original film thickness by an error of a predetermined size can be determined as the measurement result. Thus, in the continuous measurement, it can be determined whether the measurement is normally performed based on the previous measurement result.

[0142] Thus, by adopting the normal measurement condition based on the previously acquired measurement result, detection of the erroneous measurement (erroneous determination that the fitting has converged) and correction thereof can also be performed.

[0143]

[0144] is a diagram for explaining the processing related to the normal measurement condition performed by the optical measurement system 1 according to the present embodiment. It is assumed that the same sample 2 is measured multiple times, or the same kind of sample 2 is continuously measured. In such measurement, it is intended to output substantially the same measurement result. Figure 14 However, it is also possible that, as shown in (A) of FIG. 8, the film thickness distribution changes linearly in the measurement result of the sample 2 measured after the sample 2 shown in (A) of FIG. 7.

[0145] Figure 14 ​​occurs. In the optical measurement method employed in the optical measurement system 1 according to the present embodiment, there is a characteristic that an error occurs due to a deviation in the number of interference, and thus the error can be detected based on a previous measurement result. Therefore, as the normal measurement condition, a condition that an error of a film thickness determined this time with respect to a film thickness obtained from the same sample or another sample previously is within a range determined in advance can be included.

[0146] Further, in a case where a mis-measurement occurs, fitting can be performed again, but since the size of the error that occurs can be calculated in advance, correction can be made after the fact.

[0147] Figure 15 is a schematic view showing an example of an interface screen 300 for setting a normal measurement condition in the optical measurement system 1 according to the present embodiment. Referring to Figure 15 , the interface screen 300 includes an activation switch 302, a correction amount setting field 304, a determination change amount setting field 306, and a determination upper and lower limit amount setting field 308.

[0148] The activation switch 302 is for receiving a setting of making a determination based on a normal measurement condition valid / invalid. The correction amount setting field 304 is for receiving a setting of a correction amount when a mis-measurement is determined to have occurred. The determination change amount setting field 306 is for receiving a setting of a first threshold value for determining that a mis-measurement has occurred. The determination upper and lower limit amount setting field 308 is for receiving a setting of a second threshold value for determining that there is a problem with the measurement itself, not a mis-measurement.

[0149] Figure 16 is a flowchart showing a more detailed process of the processing (steps S22 and S24) related to the normal measurement condition shown in Figure 4

[0150] Referring to Figure 16 , the processing device 100 calculates an error of a measurement result this time with respect to a reference value set in advance (step S221). The processing device 100 determines whether an absolute value of the calculated error exceeds a second threshold value set in the determination upper and lower limit amount setting field 308 (refer to Figure 15 ). If the absolute value of the calculated error exceeds the second threshold value (YES in step S222), the processing device 100 determines that there is an abnormality in the measurement itself (step S223). Then, the processing device 100 attaches information of the measurement abnormality to the measurement result this time (step S241).

[0151] ​On the other hand, if the absolute value of the calculated error does not exceed the second threshold value (NO in step S222), the processing device 100 determines whether the absolute value of the calculated error exceeds the first threshold value set in the determination change amount setting field 306 (refer to Figure 15 ) (step S224). If the absolute value of the calculated error exceeds the first threshold value (YES in step S224), the processing device 100 determines that an erroneous measurement has occurred (step S225). Then, the processing device 100 calculates a corrected measurement result by subtracting or adding the correction amount set in the correction amount setting field 304 (refer to Figure 15 ) from or to the measurement result according to the direction (positive or negative) of the calculated error (step S242). In this way, the following processing can also be performed: when it is determined that the film thickness decided by fitting does not satisfy the normal measurement condition, the film thickness is corrected.

[0152] If the absolute value of the calculated error does not exceed the first threshold value (NO in step S224), the processing device 100 determines that no erroneous measurement has occurred (step S226). In this case, no processing can be performed. However, information that the measurement was normal can be added to the measurement result of this time.

[0153] Further, as for the reference value set in advance, either the measurement result of the last time can be directly used, or an average value calculated from a plurality of measurement results of the past that were normally measured can be used. Alternatively, a design value of the sample in advance can be used.

[0154] In addition, the correction amount set in the correction amount setting field 304 can be set in advance according to the film thickness decided as the initial value used in fitting. Alternatively, an average value of errors generated when an erroneous measurement has occurred can be set as the correction amount.

[0155] In addition, if the possibility that the number of times of interference is not only one deviation but two or more deviations is taken into account, the corrected measurement result can be calculated by subtracting or adding an integer multiple of the correction amount set in the correction amount setting field 304 from or to the measurement result according to the magnitude of the calculated error.

[0156] Further, in the above-described processing, as the processing related to the measurement abnormality (step S24), the processing of correcting the measurement result is exemplified, but is not limited thereto, and processing of notifying or recording that an erroneous measurement has occurred can also be included. That is, the following processing can also be performed: when it is determined that the film thickness decided by fitting does not satisfy the normal measurement condition, it is notified that the normal measurement condition is not satisfied.

[0157] Further, as the processing related to the measurement abnormality (step S24), instead of correcting the measurement result, fitting can also be performed again.

[0158] By the processing as above, even in a case where a mis-measurement occurs, it is possible to appropriately find and correct it.

[0159] < G. Functional Configuration >

[0160] Figure 17 is a diagram showing an example of a functional configuration of the optical measurement system 1 according to the present embodiment. Typically, Figure 17 Each function shown in the diagram can be realized by executing the measurement program 114 by the processor 102 of the processing device 100 of the optical measurement system 1. Further, as to the hardware for realizing the functional configuration shown in the diagram, an appropriate hardware is selected according to each era. Figure 17

[0161] Referring to Figure 17 , the processing device 100 includes a spectrum acquisition module 150, a Fourier transform module 160, an initial value determination module 162, a fitting module 164, a determination module 166, a correction module 168, and a notification module 170 as a functional configuration.

[0162] The spectrum acquisition module 150 acquires a spectrum of the observation light (reflected light or transmitted light) generated by irradiating the sample with the measurement light. The spectrum acquisition module 150 outputs, as the observation light spectrum, a spectrum of a wavelength range included in the observation light, which is set to a wavelength interval of the same phase at both ends.

[0163] More specifically, the spectrum acquisition module 150 includes a buffer 152, a preprocessing module 154, and a spectrometer control module 156. The buffer 152 is used to hold a detection result of the spectrometer 20. The preprocessing module 154 searches for two wavelength positions showing the same phase based on the detection result of the spectrometer 20 (spectrum of the wavelength range of the spectrometer 20) held in the buffer 152, and extracts the observation light spectrum from information between the two wavelength positions searched for in the detection result.

[0164] The spectrometer control module 156 gives an instruction to the spectrometer 20 as necessary. For example, in a case where the spectrometer 20A as shown in Figure 8 is employed, in order to acquire the observation light spectrum after the trimming, the spectrometer control module 156 gives an instruction or the like related to the wavelength as a detection target to the spectrometer 20A. More specifically, the spectrometer control module 156 makes the wavelength components incident to the light-receiving element 28 of the spectrometer 20A different sequentially, searches for two wavelength positions showing the same phase, and makes the wavelength components between the two wavelength positions searched for incident to the light-receiving element 28 of the spectrometer 20A sequentially, thereby acquiring the observation light spectrum. ​

[0165] The Fourier transform module 160 performs a Fourier transform on the observed light spectrum acquired by the spectrum acquisition module 150 to calculate the power spectrum. The initial value determination module 162 determines the initial value of the film thickness of the sample based on the positions of the peaks that appear in the power spectrum calculated by the Fourier transform module 160.

[0166] The fitting module 164 determines the film thickness of the sample by fitting the observed light spectrum acquired by the spectrum acquisition module 150. More specifically, the fitting module 164 updates the parameters of the model in such a way that the interference spectrum calculated according to the model including the film thickness as a parameter coincides with the observed light spectrum, and determines the film thickness of the sample.

[0167] The determination module 166 determines whether the film thickness determined by the fitting module 164 satisfies the normal measurement conditions based on the previously acquired measurement results.

[0168] When it is determined that the film thickness determined by the fitting module 164 does not satisfy the normal measurement conditions, the correction module 168 corrects the determined film thickness.

[0169] When it is determined that the film thickness determined by the fitting module 164 does not satisfy the normal measurement conditions, the notification module 170 notifies that the normal measurement conditions are not satisfied.

[0170] <H. Modified Example>

[0171] In the above description, an example is shown in which the power spectrum (measured value) calculated by performing a Fourier transform on the trimmed observed light spectrum is used to determine the initial value of the film thickness used in subsequent fitting. However, the film thickness determined based on the power spectrum (measured value) may also be directly output as the measurement result. Also in this case, in order to improve the measurement accuracy, the process of trimming the observed light spectrum so that the phase at the start point coincides with the phase at the end point is effective.

[0172] In addition, in the above description, a structural example in which the processing device 100 of the optical measurement system 1 performs necessary processing has been described. However, it is not limited to this. For example, the processing may be shared among multiple processing devices, or a part of the processing may be performed by the spectroscopic detector 20. Also, computing resources (so-called cloud) on a network not shown may be responsible for all or part of the necessary processing.

[0173] <I. Summary>

[0174] In the optical measurement apparatus according to the present embodiment, an initial value of the film thickness is determined based on a power spectrum calculated by Fourier transforming the observation light spectrum obtained from the sample, and the observation light spectrum is fitted to an interference spectrum calculated from a model of the sample using the determined initial value of the film thickness, whereby the film thickness of the sample is determined. By adopting the measurement method obtained by integrating the method using Fourier transform and the method using fitting, the measurement accuracy of the film thickness can be improved using the advantages of each method.

[0175] In the optical measurement apparatus according to the present embodiment, the observation light spectrum (the modified observation light spectrum) in which the wavelength intervals having the same phase at both ends are set is used to perform the process for calculating the film thickness, whereby the measurement accuracy of the film thickness can be improved. Further, a method in which the amplitudes at both ends of the analysis range are set to zero using a window function or the like is also assumed, but it is likely that an error is generated by using the window function or the like. In contrast to this, in the present embodiment, by adopting the method in which the specific wavelength intervals are extracted in a manner that the phases at both ends are the same, it is possible to prevent the error generated by using the window function.

[0176] In the optical measurement apparatus according to the present embodiment, the film thickness of the sample is determined by fitting the observation light spectrum to the interference spectrum calculated from the model of the sample, but if the number of interference increases, it is likely that the fitting is determined to converge with the wrong number of interference. In the optical measurement apparatus according to the present embodiment, by using the modified observation light spectrum, the determination accuracy of the initial value of the film thickness used in the fitting can be improved, and thus the possibility that the fitting is determined to converge with the wrong number of interference can be reduced.

[0177] In the optical measurement apparatus according to the present embodiment, it is determined whether the calculated film thickness satisfies the normal measurement condition, and if it is determined that the normal measurement condition is not satisfied, the measured film thickness is corrected as necessary. By adopting such a determination process for the normal measurement condition and a correction process for the film thickness, even in the case where the fitting is determined to converge with the wrong number of interference, it is possible to detect and correct it.

[0178] The embodiments of the present application have been described, but it should be considered that the embodiments disclosed this time are illustrative in all respects, but not restrictive. The scope of the present application is indicated by the claims, and intended to include all modifications equivalent within the meaning and range of the claims.

Claims

1. An optical measurement system comprising: a light source configured to generate measurement light; a light receiving unit configured to receive reflected light or transmitted light generated by irradiating a sample with the measurement light as observation light; an acquisition unit configured to acquire an observation light spectrum of a wavelength range included in the observation light, the wavelength range being set such that wavelength intervals having the same phase are present at both ends; an initial value determination unit configured to determine an initial value of a film thickness of the sample based on positions of peaks present in a power spectrum obtained by performing Fourier transform on the observation light spectrum; a fitting unit configured to determine the film thickness of the sample by updating parameters of a model of the sample including the film thickness as a parameter in such a manner that an interference spectrum calculated from the model coincides with the observation light spectrum; and a determination unit configured to determine whether the determined film thickness satisfies a condition based on a film thickness acquired previously.

2. The optical measurement system according to claim 1, wherein the light receiving unit includes a spectrometer configured to output an intensity of each wavelength of the observation light for a prescribed wavelength range, the acquisition unit searches for two wavelength positions showing the same phase based on a detection result of the light receiving unit, and extracts the observation light spectrum from information present between the two wavelength positions searched for in the detection result.

3. The optical measurement system according to claim 1, wherein the light receiving unit includes a light receiving element and a diffraction grating configured to be able to change a wavelength component of wavelength components included in the observation light that is incident on the light receiving element, the acquisition unit sequentially changes the wavelength component incident on the light receiving element of the light receiving unit, searches for two wavelength positions showing the same phase, and sequentially causes the wavelength component present between the two wavelength positions searched for to be incident on the light receiving element of the light receiving unit, thereby acquiring the observation light spectrum.

4. The optical measurement system according to any one of claims 1 to 3, wherein the condition includes a condition that an error of the film thickness determined this time with respect to a film thickness acquired previously from the same sample or another sample is within a range determined in advance.

5. The optical measurement system according to any one of claims 1 to 3, wherein the model of the sample is an equation including a film thickness and optical constants that simulates interference of light generated inside the sample.

6. The optical measurement system according to any one of claims 1 to 3, further comprising a correction unit configured to correct the film thickness determined when it is determined that the film thickness determined does not satisfy the condition.

7. The optical measurement system according to any one of claims 1 to 3, further comprising a notification unit configured to notify that the condition is not satisfied when it is determined that the film thickness determined does not satisfy the condition. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 8. An optical measurement method comprising a step of irradiating a measurement light from a light source to a sample and acquiring a spectrum of an observation light which is a reflected light or a transmitted light generated from the sample, the spectrum of the observation light being a spectrum of a wavelength range included in the observation light, set as a wavelength interval of the same phase at both ends, the optical measurement method further comprising steps of: deciding an initial value of a film thickness of the sample based on a position of a peak appearing in a power spectrum obtained by Fourier-transforming the spectrum of the observation light; deciding the film thickness of the sample by updating parameters of a model of the sample including the film thickness as a parameter in a manner that an interference spectrum calculated from the model coincides with the spectrum of the observation light; and judging whether the decided film thickness satisfies a condition based on a film thickness acquired previously.

9. The optical measurement method according to claim 8, wherein the condition based on the film thickness acquired previously is a condition that the film thickness acquired previously is within a predetermined range.

10. The optical measurement method according to claim 8 or 9, wherein the condition based on the film thickness acquired previously is a condition that the film thickness acquired previously is within a predetermined range.

11. The optical measurement method according to any one of claims 8 to 10, wherein the condition based on the film thickness acquired previously is a condition that the film thickness acquired previously is within a predetermined range.

12. The optical measurement method according to any one of claims 8 to 11, wherein the condition based on the film thickness acquired previously is a condition that the film thickness acquired previously is within a predetermined range.

13. The optical measurement method according to any one of claims 8 to 12, wherein the condition based on the film thickness acquired previously is a condition that the film thickness acquired previously is within a predetermined range.

14. The optical measurement method according to any one of claims 8 to 13, wherein the condition based on the film thickness acquired previously is a condition that the film thickness acquired previously is within a predetermined range.

15. The optical measurement method according to any one of claims

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