Calibration method for optical thickness gauges

Abstract

Provided is a calibration method for an optical thickness gauge that can improve the absolute accuracy of an optical substrate thickness gauge. 【Solution means】 The calibration method of the optical thickness gauge uses the ultrasonic propagation velocity. The calibration method of the optical thickness gauge includes an optical path length L measured by using an optical method for a substrate to be measured o and a provisional refractive index n of the substrate to be measured temp to obtain a provisional substrate thickness L temp in a provisional substrate thickness calculation step (step S7), and uses the ultrasonic propagation velocity to obtain a true substrate thickness L true of the substrate to be measured in a true thickness calculation step (step S9), and uses the provisional substrate thickness L temp obtained in the provisional substrate thickness calculation step, the true substrate thickness L true obtained in the true thickness calculation step, and the provisional refractive index n temp to obtain a true refractive index n true of the material to be measured in a refractive index calculation step (step S10).

Description

[Technical Field]

[0001] The present invention relates to a calibration method for an optical thickness gauge, and more particularly to a calibration method for an optical thickness gauge using ultrasonic propagation velocity. [Background technology]

[0002] In recent years, with the rapid development of communication technologies such as mobile phones, there has been a demand for elastic wave devices usable in high-frequency bands. Known elastic wave devices include SAW (Surface Acoustic Wave) filters, which propagate elastic waves horizontally by forming an electrode pattern on a piezoelectric substrate, and BAW (Bulk Acoustic Wave) filters, which propagate elastic waves vertically by placing electrodes above and below a piezoelectric substrate.

[0003] BAW filters used in the high-frequency band include the FBAR (Film Bulk Acoustic Resonator) type, which allows the piezoelectric film to vibrate freely by creating a cavity in the lower part of the device, and the SMR (Solidly Mounted Resonator) type, which has both a piezoelectric film and an acoustic multilayer film.

[0004] A known method for manufacturing elastic wave devices involves bonding a piezoelectric substrate and a support substrate. For example, Patent Documents 1 and 2 disclose a method for manufacturing a composite substrate usable as an elastic wave device. By bonding a piezoelectric substrate and a support substrate and polishing one side of the piezoelectric substrate surface, a composite substrate with a reduced piezoelectric substrate thickness is formed. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 7378723 [Patent Document 2] Japanese Patent Publication No. 2024-95689 [Overview of the project] [Problems that the invention aims to solve]

[0006] In SAW and BAW filters, which have a structure in which a piezoelectric substrate and a support substrate are joined together, the thickness of the piezoelectric substrate needs to be reduced to accommodate high frequencies. Furthermore, uniformity of the piezoelectric substrate thickness is required to improve filter performance and manufacturing yield. When performing thinning polishing on a piezoelectric substrate after bonding, it is necessary to know the original thickness of the piezoelectric substrate beforehand in order to calculate the target amount of polishing required. Furthermore, in order to make the thickness of the piezoelectric substrate uniform across the entire wafer, it is important to prepare a substrate with as small a thickness distribution as possible for the original piezoelectric substrate and the support substrate. However, to achieve this, it is necessary to accurately understand the thickness distribution of the substrate and the thickness distribution of the piezoelectric substrate layer (thin film) after thinning and optimize the polishing conditions.

[0007] Generally, optical methods are used to measure substrate thickness distribution and film thickness distribution. For example, one method for measuring substrate thickness involves irradiating the substrate surface from one side using infrared monochromatic light and detecting the reflected peak light from the substrate surface and back surface to determine the optical thickness L. o Measure the optical thickness L o By dividing by the refractive index n in the substrate, the physical substrate thickness L is obtained. p There is a method that involves converting to [a certain value] to obtain the answer. Another method for measuring film thickness involves irradiating a thin film formed on a substrate with light (mainly visible light), and then measuring the reflected light that interferes due to multiple reflections occurring within the film by varying the wavelength of the light. The film thickness can then be determined from the periodicity of the resulting reflection spectrum. This periodicity of the reflection spectrum depends on the wavelength of the light and the optical path length (= the distance the light travels back and forth within the thin film × the refractive index of the film).

[0008] The two optical methods described above for measuring substrate thickness or film thickness depend on the refractive index of the substrate or film; therefore, the absolute value of the thickness cannot be determined unless the refractive index is accurately known. Generally, the refractive index of a material changes with the wavelength of light, and in many cases, only data measured at specific wavelengths is published. For this reason, the measurement wavelength of the refractive index published in the literature does not necessarily match the wavelength of light used by the device used to measure the thickness. This results in measurements using literature values ​​with slight differences in refractive index, which carries the risk of the final obtained absolute thickness being inaccurate. Furthermore, since the properties of a material change depending heavily on its manufacturing process, even materials said to be the same may exhibit different properties depending on the manufacturer due to slight variations in composition or trace impurities. Therefore, the actual refractive index of the material may differ from the refractive index of the literature value, which again poses a problem as it can lead to discrepancies in the final obtained absolute thickness.

[0009] In realizing high-frequency elastic wave devices using dissimilar material bonded substrates, it is crucial to accurately understand the thickness distribution of the piezoelectric substrate and support substrate before bonding, as well as the film thickness distribution of the thinned piezoelectric substrate after bonding. However, commonly used optical methods calculate thickness using the refractive index of the substrate material or film material being measured, which presents a challenge in that accurate thickness cannot be calculated unless the refractive index is known for the material.

[0010] The present invention has been made in view of the above problems, and the object of the present invention is to provide a calibration method for an optical thickness gauge that can improve the absolute accuracy of an optical substrate thickness gauge. [Means for solving the problem]

[0011] The aforementioned problem is solved by a calibration method for an optical thickness gauge using ultrasonic propagation velocity, which includes: a provisional substrate thickness calculation step of determining a provisional substrate thickness using a predetermined value measured on the substrate to be measured using an optical method and a provisional refractive index of the substrate to be measured; a true thickness value calculation step of determining the true thickness of the substrate to be measured using ultrasonic propagation velocity; and a refractive index calculation step of determining the true refractive index of the substrate to be measured using the provisional substrate thickness obtained in the provisional substrate thickness calculation step, the true thickness obtained in the true thickness value calculation step, and the provisional refractive index.

[0012] In this way, by determining the true value of the refractive index based on the true value of the thickness calculated using a provisional substrate thickness and ultrasonic propagation velocity, and a provisional refractive index, the refractive index required when measuring substrate thickness can be calibrated with high accuracy. Therefore, the accurate substrate thickness or film thickness can be determined.

[0013] At this time, the process includes: a sample substrate preparation step of preparing a substrate that will serve as a standard sample made of the same material and with the same crystal orientation as the substrate to be measured; a standard propagation time measurement step of measuring the standard ultrasonic round-trip propagation time in the thickness direction of the standard sample by ultrasonic measurement; a standard sample thickness measurement step of measuring the standard sample thickness of the standard sample; a reference value calculation step of calculating a reference value for ultrasonic propagation velocity using the standard ultrasonic round-trip propagation time obtained in the standard propagation time measurement step and the standard sample thickness obtained in the standard sample thickness measurement step; and the substrate to be measured The method includes a substrate propagation time measurement step, in which the round-trip ultrasonic propagation time in the substrate thickness direction is measured on a plate by ultrasonic measurement, wherein the provisional substrate thickness calculation step determines the provisional substrate thickness using the optical path length measured on the substrate using an optical method and the provisional refractive index of the substrate, and the true thickness calculation step determines the true thickness of the substrate using the round-trip ultrasonic propagation time of the substrate obtained in the substrate propagation time measurement step and the ultrasonic propagation velocity reference value obtained in the reference value calculation step. Using the method described above, the refractive index can be calibrated with high accuracy using a simple method with standard samples.

[0014] In this case, it is preferable to prepare the standard sample having a thickness of 1 mm or more in the sample substrate preparation process. By using the method described above, the accuracy can be further improved by making the thickness of the standard sample 1 mm or more.

[0015] In this case, the process includes: a constant determination sample substrate manufacturing step of manufacturing a substrate to be used as a constant determination sample using the same material as the substrate to be measured; a constant determination step of determining material constants by performing sound velocity measurement, dielectric constant measurement, and density measurement on the manufactured constant determination sample; a propagation speed calculation step of calculating the propagation speed of ultrasonic waves propagating in the thickness direction of the substrate to be measured using the material constants obtained in the constant determination step; and a substrate propagation time measurement step of measuring the round-trip ultrasonic wave propagation time in the substrate thickness direction using ultrasonic measurement on the substrate to be measured. In the provisional substrate thickness calculation step, the provisional substrate thickness is determined using the optical path length measured using an optical method on the substrate to be measured and the provisional refractive index of the substrate to be measured. In the true thickness calculation step, the true thickness of the substrate to be measured is determined using the round-trip ultrasonic wave propagation time of the substrate to be measured obtained in the substrate propagation time measurement step and the calculated propagation speed obtained in the propagation speed calculation step. By determining the material constants using the method described above, the propagation velocity for any crystal substrate orientation can be easily calculated even when the orientation of the substrate being measured is changed, and the true value of the refractive index can be easily calculated.

[0016] In this case, the substrate to be measured is a substrate with a thin film, and in the provisional substrate thickness calculation step, the provisional thin film thickness is determined using the reflection spectrum measured on the substrate to be measured using an optical method and the provisional refractive index of the substrate to be measured. In the true thickness calculation step, the true thickness of the thin film on the substrate to be measured is determined from the frequency dependence of the total ultrasonic propagation speed obtained by bulk wave ultrasonic measurement on the substrate to be measured. In the refractive index calculation step, the true refractive index of the substrate to be measured is determined using the provisional thin film thickness obtained in the provisional substrate thickness calculation step, the true thickness of the thin film obtained in the true thickness calculation step, and the provisional refractive index. Using the method described above, the refractive index of a thin film can be determined even for substrates with a thin film attached, by employing bulk wave ultrasonic measurement.

[0017] In this case, in the temporary substrate thickness calculation step, it is preferable to determine the wavelength range of light effective for analysis from the reflection spectrum, perform a fast Fourier transform to assume the temporary refractive index, and then determine the temporary thin film thickness from the peak position of the analysis spectrum of the fast Fourier transform. Using the above method, the solution for film thickness can be easily determined without having to examine fine details, by using the peak positions of the fast Fourier transform analysis spectrum.

[0018] In this case, the substrate to be measured is a substrate with a thin film, and the measurement process involves: a frequency dependence measurement step of measuring the frequency dependence of the LSAW velocity in a predetermined propagation direction relative to the substrate to be measured; a calculation value calculation step of calculating the fH dependence of the LSAW velocity in the same propagation direction with the same structure as the substrate to be measured used in the frequency dependence measurement step; a measurement value calculation step of calculating the measured value of the fH dependence of the LSAW velocity by assuming a provisional thickness for the frequency dependence obtained in the frequency dependence measurement step; and the calculated value of the fH dependence obtained in the calculation value calculation step and the measured value of the fH dependence obtained in the measurement value calculation step. The process includes a determination step to determine whether the speed difference has been minimized. In the provisional substrate thickness calculation step, a provisional thin film thickness is determined using the reflection spectrum measured on the substrate under test using an optical method and the provisional refractive index of the substrate under test. In the true thickness calculation step, the provisional thin film thickness when the speed difference is minimized is determined as the true thickness of the thin film on the substrate under test. In the refractive index calculation step, the true refractive index of the substrate under test is determined using the provisional thin film thickness obtained in the provisional substrate thickness calculation step, the true thickness of the thin film obtained in the true thickness calculation step, and the provisional refractive index. By using the above method, the difference between the calculated and measured values ​​of the fH dependence of the LSAW velocity can be used to determine the film quality of the substrate with the thin film being measured, along with the calibration of the refractive index.

[0019] In this case, the substrate to be measured is a substrate with a thin film, and the measurement process includes: a speed measurement step of measuring the LSAW speed in a predetermined propagation direction at a specific frequency with respect to the substrate to be measured; a calculation value calculation step of calculating the fH dependence of the LSAW speed in the same propagation direction with the same structure as the substrate to be measured used in the speed measurement step; and an fH value calculation step of calculating an fH value from the calculated fH dependence obtained in the calculation value calculation step that matches the LSAW speed measurement value measured in the speed measurement step, and the temporary substrate thickness calculation step is performed on the substrate to be measured In the step to calculate the true thickness of the thin film, the fH value obtained in the step to calculate the fH value and the frequency used in the step to calculate the velocity are used to calculate the true thickness of the thin film on the substrate. In the step to calculate the refractive index, the provisional thin film thickness obtained in the provisional substrate thickness calculation step, the true thickness of the thin film obtained in the true thickness calculation step, and the provisional refractive index are used to calculate the true refractive index of the substrate. By calculating the fH dependence at a specific frequency using the method described above, the film thickness distribution of the substrate with the thin film under test can be determined with high accuracy.

[0020] In this case, it is preferable to include a step of obtaining the true value of the substrate thickness distribution of the substrate to be measured using the result of mapping the optical path length measured at an arbitrary measurement point on the substrate surface of the substrate to be measured and the true value of the refractive index obtained in the refractive index calculation step. By using the above method, the accurate distribution of the substrate thickness of the substrate under measurement can be determined by using the results of the optical path length mapping and the true value of the refractive index.

[0021] At this time, it is preferable to include a step of obtaining the true value of the thin film thickness at the measurement point by analyzing the reflection spectrum measured at an arbitrary measurement point on the substrate surface of the substrate under the same measurement conditions as the provisional substrate thickness calculation step, and replacing the true value of the refractive index obtained in the refractive index calculation step with the provisional refractive index. By using the above method, the accurate film thickness distribution of the substrate with the thin film under test can be determined using the reflection spectrum and the true value of the refractive index.

[0022] In this case, the temporary substrate thickness calculation step is to place the sample and the optical thickness measuring device in a constant temperature chamber and perform the measurement using an optical method in an environment controlled at a constant temperature. By using the method described above, by placing not only the sample to be measured but also the optical thickness gauge measuring device itself in a temperature-controlled chamber, it is possible to suppress fluctuations in measurement values ​​caused not only by changes in the thermal expansion of the sample itself, but also by the wavelength of the light source of the measuring device and changes in the thermal expansion of the internal mechanical parts. [Effects of the Invention]

[0023] According to the calibration method for optical thickness gauges of the present invention, the absolute accuracy of optical substrate thickness gauges can be improved. [Brief explanation of the drawing]

[0024] [Figure 1] A process diagram showing the refractive index calibration method when preparing a standard sample according to the first embodiment. [Figure 2] A process diagram showing a refractive index calibration method when using calculated values ​​based on the material constants of the second embodiment. [Figure 3] A process diagram illustrating a refractive index calibration method using measured bulk wave propagation velocity for a thin film / substrate structure of the third embodiment. [Figure 4A] A diagram illustrating the concept of a measurement system for bulk wave propagation velocity for a thin film / substrate structure according to the third embodiment. [Figure 4B] This figure shows an example of Vtotal calculation obtained for a 42°Y-LiTaO3 thin film / (100)Si substrate structure sample, compared to the thin film / substrate structure in Figure 4A. [Figure 5] A process diagram showing a refractive index calibration method using measured values ​​of the frequency dependence of the LSAW speed for a thin film / substrate structure in the fourth embodiment. [Figure 6] A process diagram showing a refractive index calibration method using LSAW velocity measurements at one frequency for a thin film / substrate structure of the fifth embodiment. [Figure 7] A process diagram showing a refractive index calibration method when the number of steps in the sixth embodiment is simplified. [Figure 8] A schematic diagram of a system in which an optical thickness gauge and a sample are placed in a constant temperature chamber and the temperature is controlled for measurement. [Figure 9] This figure shows an example of measuring the substrate thickness distribution of a (100)Si substrate according to the first embodiment. [Figure 10] This figure shows an example of measuring the substrate thickness distribution of a 42°Y-LiTaO3 substrate according to the second embodiment. [Figure 11A] This figure shows an example of the reflection spectrum results measured by an optical film thickness meter for a 42° Y-LiTaO3 thin film / (100)Si substrate structure sample of the third embodiment. [Figure 11B] This figure shows an example of the spectral result obtained by performing an FFT analysis on the reflection spectrum of Figure 11A in the third embodiment. [Figure 11C] This figure shows the results of measuring the diametrical thin film thickness distribution of the 42°Y-LiTaO3 thin film / (100)Si substrate structure of the third embodiment. [Figure 12A] This figure shows the measurement results of the frequency dependence of the LSAW speed for the 36°Y-LiTaO3 thin film / AT-cut quartz substrate structure of the fourth embodiment. [Figure 12B] This figure shows an example of estimating the film thickness from the fH dependence of the LSAW rate for a 36°Y-LiTaO3 thin film / AT-cut quartz substrate structure of the fourth embodiment. [Figure 12C] This figure shows the measurement results of the diametrical thin film thickness distribution of the 36°Y-LiTaO3 thin film / AT-cut quartz substrate structure of the fourth embodiment. [Figure 13A] This figure shows the measurement results (ultrasonic frequency 225 MHz) corresponding to the diametrical distribution of LSAW velocity for the 36°Y-LiTaO3 thin film / AT-cut quartz substrate structure of the fifth embodiment. [Figure 13B] This figure shows the calculation results of the fH dependence of the LSAW rate for the 36°Y-LiTaO3 thin film / AT-cut quartz substrate structure of the fifth embodiment. [Figure 13C] This figure shows the diametrical fH distribution converted from the velocity distribution results in Figure 13A of the fifth embodiment using the relationship in Figure 13B. [Figure 13D] This figure shows the results of the diametrical film thickness distribution, calculated from the results in Figure 13C of the fifth embodiment. [Figure 14A] This diagram shows the temperature fluctuations when the entire optical thickness gauge and the sample are placed in a constant temperature chamber and the temperature is controlled. [Figure 14B] Figure 14A shows the temporal variation of film thickness measurements taken with an optical thickness gauge in a temperature-controlled environment. [Figure 14C] A diagram showing temperature fluctuations in a constant temperature chamber and when the indoor air conditioning is not temperature-controlled. [Figure 14D] Figure 14C shows the temporal variation of film thickness measurements taken with an optical thickness gauge in an environment without temperature control. [Modes for carrying out the invention]

[0025] The calibration method for an optical thickness gauge according to one embodiment of the present invention (hereinafter referred to as "this embodiment") will be described below with reference to Figures 1 to 14D.

[0026] <First Embodiment> The calibration method for an optical thickness gauge using ultrasonic propagation velocity in this embodiment includes, as shown in Figure 1, a step of preparing a standard sample from the material to be measured (steps S2 to S5) and a step of performing calculations through measurements on an actual substrate to be measured (steps S6 to S10).

[0027] More specifically, the calibration method for an optical thickness gauge involves a step of selecting the material to be measured (step S1), a sample substrate preparation step of creating a standard sample substrate of the same material and crystal orientation as the substrate to be measured (step S2), and measuring the standard ultrasonic wave propagation time T in the thickness direction of the standard sample using ultrasonic measurement. std The standard propagation time measurement process (step S3) is performed to measure the standard sample thickness L of the standard sample. std The standard sample thickness measurement step (step S4) and the standard ultrasonic round-trip propagation time T obtained in the standard propagation time measurement step are used to measure the standard sample thickness. std And the standard sample thickness L obtained in the standard sample thickness measurement process. stdand, using, the ultrasonic propagation speed reference value V std a reference value calculation step (step S5) for calculating, and includes.

[0028] Also, the calibration method of the optical thickness meter includes a substrate production step (step S6) of producing a measurement substrate from a measurement material, and the optical path length L measured using an optical method for the measurement substrate o and the provisional refractive index n of the measurement substrate temp and, using, a provisional substrate thickness calculation step (step S7) for obtaining the provisional substrate thickness L temp a substrate propagation time measurement step (step S8) of measuring the ultrasonic reciprocating propagation time T in the substrate thickness direction of the measurement substrate by ultrasonic measurement for the measurement substrate, the ultrasonic reciprocating propagation time T of the measurement substrate obtained in the substrate propagation time measurement step, and the ultrasonic propagation speed reference value V obtained in the reference value calculation step std and, using, a true thickness calculation step (step S9) for obtaining the true value L of the thickness of the measurement substrate true the provisional substrate thickness L obtained in the provisional substrate thickness calculation step temp the true value L of the thickness obtained in the true thickness calculation step true and the provisional refractive index n temp and, using, a refractive index calculation step (step S10) for obtaining the true value n of the refractive index of the measurement substrate true and includes. Note that the provisional refractive index n temp is a known refractive index of the substrate material or film material to be measured, for example, the refractive index described in technical literature or the like.

[0029] Furthermore, the calibration method of the optical thickness meter includes a step of obtaining the true value of the substrate thickness distribution of the measurement substrate using the result of the mapping of the optical path length L o measured at an arbitrary measurement point in the substrate plane of the measurement substrate and the true value n of the refractive index obtained in the refractive index calculation step true . The details will be described below. The details will be described below.

[0030] First, select the material to be measured (Step S1). Prepare a substrate to serve as a standard sample using the selected material (Step S2). The thickness of this standard sample is preferably as thick as possible, preferably 1 mm or more. More preferably about 10 mm. If the material to be measured is crystalline, the crystal plane orientation of the standard sample should be the same as the crystal plane orientation of the substrate to be measured prepared in Step S6, described below. The standard ultrasonic wave propagation time T in the thickness direction of the prepared standard sample was measured using ultrasonic measurement. std The measurement is performed (Step S3). In this case, the ultrasound is a bulk wave, and it is possible to select between longitudinal and transverse wave modes. The ultrasonic measurement method is preferably a high-precision measurement using ultrasonic microspectroscopy (UMS) technology as described in References 1 and 2, etc. ([Reference 1] J. Kushibiki and M. Arakawa, “Diffraction effects on bulk-wave ultrasonic velocity and attenuation measurements”, J. Acoust. Soc. Am. vol. 108, no. 2, pp. 564-573 (2000). [Reference 2] J. Kushibiki et al., “Development of the Line-Focus-Beam Ultrasonic Material Characterization System”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 49, no. 1, pp. 99-113 (2002).)

[0031] Next, the standard sample thickness L of the standard sample. std Measure the thickness (step S4). For thickness measurement here, it is preferable to use a contact-type thickness gauge with a built-in linear encoder. The standard ultrasonic round-trip propagation time T obtained in step S3. std and the standard sample thickness L obtained in step S4 std Using the ultrasonic propagation velocity reference value V stdCalculate (Step S5). More specifically, the ultrasonic propagation velocity reference value V std This is calculated from the following formula (1). V std = 2 × L std / T std ...(1)

[0032] Next, the substrate to be measured, fabricated from the material selected in step S1, is prepared (step S6). The optical path length L is measured using an optical method on the substrate to be measured. o And, a hypothetical refractive index n temp Using this, a temporary substrate thickness L temp We determine (step S7). More specifically, the temporary substrate thickness L temp This is calculated from the following formula (2). L temp =L o / n temp ...(2)

[0033] Step S8 measures the ultrasonic round-trip propagation time T in the substrate thickness direction using ultrasonic measurement on the substrate under test. The ultrasonic round-trip propagation time T obtained in Step S8 and the ultrasonic propagation velocity reference value V obtained in Step S5 are used. std Using this method, the true value L of the thickness of the substrate being measured is obtained. true We find the true value L of the thickness (step S9). More specifically, the true value L of the thickness. true This is calculated from the following formula (3). L true =T × V std / twenty three)

[0034] The provisional substrate thickness L obtained in step S7 temp And the true value L of the substrate thickness obtained in step S9. true And, a hypothetical refractive index n temp The true value of the refractive index n using true We find the true value of the refractive index n (step S10). More specifically, the true value of the refractive index n true This is calculated from the following formula (4). n true =L temp / L true ×n temp ...(4)

[0035] Then, at any point within the surface of the substrate being measured, the optical path length L o The results of the measured mapping and the true value n of the refractive index obtained in step S10 true By using this method, the true value of the thickness distribution of the substrate being measured can be obtained.

[0036] In this embodiment, for the same material that has been measured once, the steps of preparing and measuring the standard sample (steps S2 to S4) in the process diagram of Figure 1 can be omitted.

[0037] In this way, the true value of the refractive index n is obtained using the ultrasonic propagation speed. true By determining this, the absolute accuracy of optical substrate thickness gauges can be improved. Specifically, a hypothetical substrate thickness L temp And the true value of thickness L calculated using the ultrasonic propagation velocity. true And, a hypothetical refractive index n temp Based on this, the true value of the refractive index n true By determining this, the refractive index required when measuring substrate thickness can be calibrated with high precision. Therefore, this optical method, which allows for short-time, non-contact, and wide-range measurement of substrate thickness, can accurately obtain the absolute value of the refractive index of the substrate material, which is essential for substrate thickness measurement. This enables efficient and accurate determination of the absolute value of the substrate thickness and its distribution. Furthermore, even for materials with birefringence, the refractive index necessary for thickness determination can be accurately determined for substrates with any crystal orientation. In addition, the correct substrate thickness can be calculated even if the wavelength of the light used in the measurement is unknown.

[0038] <Second Embodiment> The calibration method for the optical thickness gauge of this embodiment, as shown in Figure 2, includes the steps of preparing a sample for constant determination from the material to be measured, determining the material constants, and calculating the ultrasonic propagation speed using those material constants (steps S12 to S14), and the steps of performing calculations through measurements on the actual substrate to be measured (steps S15 to S19).

[0039] More specifically, the calibration method for an optical thickness gauge involves: a step of selecting the material to be measured (step S11); a step of preparing a sample substrate for constant determination using the same material as the substrate to be measured (step S12); a step of determining material constants by measuring the sound velocity, dielectric constant, and density of the prepared sample for constant determination (step S13); and a step of calculating the propagation velocity V of ultrasonic waves propagating in the thickness direction of the substrate to be measured using the material constants obtained in the constant determination step. calc This includes a step (step S14) for calculating the propagation speed to obtain the value.

[0040] Furthermore, the calibration method for the optical thickness gauge consists of a substrate manufacturing process (step S15) in which a substrate to be measured is made from the material to be measured, and the optical path length L measured using an optical method on the substrate to be measured. o And the provisional refractive index n of the material being measured temp Using this, a temporary substrate thickness L temp Step S16 is a preliminary substrate thickness calculation step to determine the following: Step S17 is a substrate propagation time measurement step in which the ultrasonic round-trip propagation time T in the substrate thickness direction is measured on the substrate to be measured by ultrasonic measurement; and Step S17 is a calculation step in which the ultrasonic round-trip propagation time T of the substrate to be measured obtained in the substrate propagation time measurement step and the propagation velocity calculated value V obtained in the propagation velocity calculation step are used. calc Using and , the true value L of the thickness of the substrate to be measured is obtained. true The true thickness calculation step (step S18) determines the true thickness, and the temporary substrate thickness L obtained in the temporary substrate thickness calculation step temp The true value L of thickness obtained in the true thickness calculation process. true , and a hypothetical refractive index n temp Using this method, the true value n of the refractive index of the substrate under test is obtained. true The process includes a step (step S19) for calculating the refractive index to determine the value of the value.

[0041] Furthermore, the calibration method for the optical thickness gauge involves measuring the optical path length L at any measurement point on the substrate surface of the substrate being measured. o The mapping results and the true value n of the refractive index obtained in the refractive index calculation process. true The process includes obtaining the true value of the substrate thickness distribution of the substrate to be measured using the above method. The details are explained below.

[0042] First, the material to be measured is selected (Step S11). A substrate to be used as a sample for determining the material constants is prepared from the selected material (Step S12). The material constants referred to here are the elastic constant, piezoelectric constant, dielectric constant, and density involved in the propagation of ultrasound. The attenuation coefficient and temperature coefficient of each parameter are also included as needed. The constants are determined for the prepared sample for constant determination according to the constant determination method described in References 3 and 4, etc. ([Reference 3] J. Kushibiki et al., “High-Accuracy Standard Specimens for the Line-Focus-Beam Ultrasonic Material Characterization System”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 49, no. 6, pp. 827-835 (2002). [Reference 4] J. Kushibiki et al., “Accurate Measurements of the Acoustical Physical Constants of LiNbO3 and LiTaO3 Single Crystals”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 46, no. 5, pp. 1315-1323 (1999).) (Step S13). The constant determination procedure differs depending on the crystal system and point group to which the material under test belongs, as well as the number of samples prepared for constant determination, their surface orientations, and the determination procedure itself. Using the material constants determined in step S13, the propagation velocity V of ultrasonic waves propagating in an orientation corresponding to the thickness direction of the substrate under test, which is prepared in step S15 described below, is calculated. calc This is calculated (Step S14).

[0043] Next, the substrate to be measured, which was fabricated from the material selected in step S11, is prepared (step S15). The optical path length L is measured on the substrate using an optical method. o And, a hypothetical refractive index n temp Using this, a temporary substrate thickness Ltemp We determine (step S16). More specifically, the temporary substrate thickness L temp This is calculated from equation (2) above.

[0044] Step S17 measures the ultrasonic round-trip propagation time T in the thickness direction of the substrate by ultrasonic measurement on the substrate under test. The ultrasonic round-trip propagation time T obtained in step S17 and the calculated ultrasonic propagation velocity V obtained in step S14 are used. calc Using this method, the true value L of the thickness of the substrate being measured is obtained. true We find the true value L of the thickness (step S18). More specifically, the true value L of the thickness. true This is calculated from the following formula (5). L true =T × V calc / twenty five)

[0045] The provisional substrate thickness L obtained in step S16 temp And the true value L of the substrate thickness obtained in step S18. true And, a hypothetical refractive index n temp The true value of the refractive index n using true We find the true value of the refractive index n (step S19). More specifically, the true value of the refractive index n true This is calculated from equation (4) above.

[0046] The calibration method for the optical thickness gauge involves measuring the optical path length L at an arbitrary measurement point on the substrate surface of the substrate being measured. o The mapping results and the true value n of the refractive index obtained in step S19 true Using this method, the true value of the substrate thickness distribution of the substrate under test can be obtained.

[0047] In this embodiment, for materials for which the material constants have already been determined, steps S12 and S13 in Figure 2 can be omitted.

[0048] In this way, the true value of the refractive index n is obtained using the ultrasonic propagation speed. true By determining this, the absolute accuracy of optical substrate thickness gauges can be improved. Specifically, a hypothetical substrate thickness L temp And the true value of thickness L calculated using the ultrasonic propagation velocity.true and a provisional refractive index n temp Based on this and, the true value n of the refractive index true By obtaining this, the refractive index required for measuring the substrate thickness can be calibrated with high precision when measuring the substrate thickness. Therefore, it is possible to obtain the absolute value of the refractive index of the substrate material required for substrate thickness measurement by an optical method that enables measurement over a wide range in a short time and non-contact, and it is possible to efficiently and accurately obtain the absolute value and distribution of the substrate thickness.

[0049] <Third Embodiment> As shown in FIG. 3, the calibration method of the optical thickness meter of this embodiment includes a step of obtaining a nominal refractive index from measurement of a substrate with a measured thin film. In the first and second embodiments, the substrate to be measured is the "substrate", and the "provisional substrate thickness L temp ", and the "true value L of the thickness of the substrate to be measured true " were described as being obtained. However, in the third embodiment, the substrate to be measured is a "substrate with a thin film" (that is, a substrate with a measured thin film), and the "provisional thin film thickness L temp ", and the "true value L of the thickness of the thin film true " will be described as being obtained. Also, in the first and second embodiments, the "true value n of the refractive index true " was described as being obtained using the ultrasonic propagation speed. However, in the third embodiment, the "true value of the nominal refractive index (specifically, the true value of the nominal refractive index n nominal )" will be described as being obtained using the ultrasonic propagation speed. Here, the reason for expressing it as the nominal true value is that since the refractive index originally has dispersion, when the refractive index is treated as a constant value, for example, the calculated thin film thickness fluctuates depending on the method of selecting the wavelength range used for FFT analysis. Therefore, the refractive index determined here is not necessarily physically correct, but from the viewpoint that the correct thin film thickness can be obtained as the final value experimentally.

[0050] More specifically, the calibration method for the optical thickness gauge involves a substrate fabrication step (step S21) in which a substrate with a thin film to be measured is fabricated from the material to be measured, and a reflection spectrum is measured on the substrate with the thin film to be measured using an optical method, and a provisional refractive index n is calculated from that reflection spectrum. temp Using a temporary thin film thickness L temp Step S22 is a preliminary substrate thickness calculation step to determine the true value L of the thin film thickness from the frequency dependence of the total ultrasonic propagation velocity obtained by bulk wave ultrasonic measurement on the substrate with the thin film to be measured. true The true thickness calculation step (step S23) determines the true value, and the temporary thin film thickness L obtained in the temporary substrate thickness calculation step temp The true value L of the thin film thickness obtained in the true thickness calculation process. true , and a hypothetical refractive index n temp Using this, the nominal refractive index true value n nominal The process includes a refractive index calculation step (step S24) to determine the refractive index.

[0051] Furthermore, the calibration method for the optical thickness gauge involves measuring the reflection spectrum at an arbitrary measurement point on the substrate surface with the thin film to be measured, under the same measurement conditions as the provisional substrate thickness calculation step, and then determining the nominal refractive index true value n obtained in the refractive index calculation step. nominal Let n be a hypothetical refractive index temp The process includes a step of obtaining the true value of the thin film thickness at the measurement point by replacing it with the other value and performing the analysis. The details are explained below.

[0052] First, prepare the thin-film substrate to be measured (step S21). Measure the reflection spectrum (wavelength dependence of reflected light intensity) of the thin-film substrate using an optical method (mainly in the wavelength region near visible light), determine the wavelength range of light effective for analysis from the reflection spectrum, and perform a Fast Fourier Transform (FFT) to obtain a provisional refractive index n. temp Assuming this, the hypothetical thin film thickness L is obtained from the peak position of the FFT analysis spectrum. temp We find this (step S22). Note that the process in step S22 may be calculated not only by FFT analysis but also by a fitting analysis method. Also, the provisional refractive index n here temp It is acceptable to assume that this value is constant across all wavelength ranges.

[0053] Here, FFT (Fast Fourier Transform) analysis is a method for determining film thickness using the peak position of the spectrum as a representative value. For example, in the embodiment shown in Figure 11A, the horizontal axis displays the reflection spectrum as a function of the wavelength λ of light, and the period of the peaks and valleys is not constant. When this horizontal axis is displayed as a function of frequency f, a waveform with a nearly constant period is obtained. In this case, the relationship f = c / (nλ) holds, and it is proportional to the reciprocal of the wavelength λ. Here, c represents the speed of light and n represents the refractive index. FFT analysis is actually performed on a graph where the horizontal axis is converted to frequency. If it is a sinusoidal waveform with a constant period, the spectrum obtained from the FFT analysis will show only a single thin peak. However, in reality, the refractive index changes with frequency, and light attenuation also occurs with frequency, so it does not appear as a clean sinusoidal waveform, but as shown in Figure 11B, it appears as a spectrum with some width. In FFT analysis, the peak position of this spectrum is used as a representative value to determine the film thickness. In this case, it is usually assumed that the refractive index n is constant at all wavelengths.

[0054] On the other hand, the fitting method is a technique in which the refractive index is defined as a function of the wavelength of light (including cases where it is a constant value), and the intensity change of the reflection spectrum is obtained as a function of the wavelength of light by calculations that take into account the phase rotation due to the back-and-forth movement within the film at each wavelength, and the transmission coefficient and reflection coefficient at each boundary, and the film thickness is determined so that the difference is minimized when compared with experimental values. In the fitting method, the peak heights and periods of the reflectance spectrum and the peaks and valleys often do not strictly match between experimental and calculated values, making it difficult to determine which is the optimal solution. Therefore, it is more suitable for measuring small numbers of samples at the laboratory level than for measuring many samples on a mass production line. Thus, the fitting method is difficult to master because it determines the film thickness while considering fine characteristics, including refractive index dispersion. However, by using FFT analysis, the film thickness solution can be easily determined without having to examine such fine characteristics.

[0055] Next, the ultrasonic wave propagation velocity in the thickness direction of the thin film is measured on the substrate with the thin film attached using ultrasonic measurement. In this case, since the object of measurement is a substrate with a thin film attached, the measured value of the bulk wave propagation velocity within the thin film is obtained from the bulk wave velocity measurement method for thin film attached substrates described in Reference 5. meas A method for determining this is used ([Reference 5] Junichi Kushibiki, Yuji Ohashi, et al., "Method for Measuring Thin Film Acoustic Properties using UMS Technology and Application Examples", Proceedings of the 14th Workshop of the Acoustic Wave Device Technology Consortium, pp. 1-5 (2024.3.18).).

[0056] Figure 4A shows a conceptual diagram of a bulk wave velocity measurement system for a thin-film coated substrate, and the measurement method is outlined below. An RF pulse is applied to a transducer and converted into ultrasound, which propagates through the buffer rod as planar ultrasound. The planar ultrasound is incident on the sample substrate via an acoustic coupler. At this time, reflected waves are generated at each interface. V1 is the reflected wave from the buffer rod end face, V2 is the reflected wave from the sample surface, and V4 is the reflected wave from the back surface of the sample. The amplitude and phase of the time-separated V2 and V4 signals are measured as functions of the ultrasound frequency, and the phase difference φ between the two is determined at each frequency. The total bulk wave velocity V of the film-coated substrate is then calculated from equation (6) below. total You can obtain this.

[0057] TIFF2026089556000002.tif16149

[0058] Here, ω is the angular frequency, expressed as ω = 2πf using the ultrasonic frequency f. In this case, V4 is affected by multiple reflections within the thin film on the back surface of the sample, so when calculating it, the effect of multiple reflections within the thin film should be considered in the changes in the amplitude and phase of V4. Figure 4A shows the case where the film surface is placed on the back side for measurement, but the same method can be used to determine it even if the film surface is on the front side. Figure 4B shows the V for a 42°Y-LiTaO3 thin film / (100)Si substrate (2mm thick) structure sample. total In an example of calculations regarding the frequency dependence, it can be seen that different frequency characteristics result from varying the thickness of the 42°Y-LiTaO3 thin film.

[0059] Measured V total The frequency-dependent profile is fitted with calculations performed by changing the film thickness value, and the film thickness that minimizes the velocity difference is determined by the true value L of the thin film thickness. true This is determined (step S23). Here, the propagation speed and substrate thickness of the substrate used are assumed to be known in advance, and the true value V of the bulk wave propagation speed in the thin film used in the calculation is determined. true This is the ultrasonic propagation velocity reference value V obtained by following the procedure in steps S2 to S5 in Figure 1. std , or the calculated propagation velocity V obtained by following the steps S12 to S14 in Figure 2. calc We can assume that this matches. Finally, the provisional thin film thickness L obtained above temp , a hypothetical refractive index n temp , true value L of thin film thickness true Using this, the nominal true value of the refractive index n becomes the nominal true value. nominal Determine the true value of the nominal refractive index n (step S24). More specifically, the true value of the nominal refractive index n nominal This is calculated from the following formula (7). n nominal =L temp / L true ×n temp ...(7)

[0060] Subsequently, the nominal refractive index true value n is obtained from the reflection spectrum measured under the same measurement conditions as in step S22 for any other measurement point or other identical sample. nominal Only the provisional refractive index n temp By swapping the values ​​and keeping all other conditions the same, the true value of the thin film thickness at each measurement point can be obtained.

[0061] In this way, the nominal refractive index n is obtained using the ultrasonic propagation speed. nominal By determining this, the absolute accuracy of optical substrate thickness gauges can be improved. Specifically, a hypothetical thin film thickness L temp And the true value L of the thin film thickness calculated using the ultrasonic propagation velocity. true And, a hypothetical refractive index n temp Based on this, the nominal refractive index true value n nominalBy determining this, the refractive index required when measuring film thickness can be calibrated with high precision. Therefore, the absolute value of the refractive index of the substrate material, which is essential for substrate thickness measurement using an optical method that allows for short-time, non-contact, and wide-area measurement, can be obtained with high precision, and the absolute value of the film thickness and its distribution can be obtained efficiently and accurately.

[0062] <Fourth Embodiment> The calibration method for the optical thickness gauge of this embodiment, as shown in Figure 5, consists of a step of obtaining the nominal refractive index from measuring the frequency dependence of the LSAW speed on a substrate with a thin film to be measured. In the fourth embodiment, similar to the third embodiment, the substrate to be measured is a "substrate with a thin film" (i.e., a substrate with a thin film to be measured), and the "provisional thin film thickness L" temp "The true value of the thickness of the thin film L" true This will be explained as a method for determining the "true value of the nominal refractive index (specifically, the true value of the nominal refractive index n)" using the ultrasonic propagation speed. nominal It is explained as something that seeks ")".

[0063] More specifically, the calibration method for the optical thickness gauge involves a substrate fabrication step (step S31) in which a substrate with a thin film to be measured is fabricated from the material to be measured, and a reflection spectrum is measured on the substrate with the thin film to be measured using an optical method, and a provisional refractive index n is calculated from that reflection spectrum. temp Using a temporary thin film thickness L tempThe method includes a temporary substrate thickness calculation step (step S32) to determine the temporary substrate thickness, a frequency dependence measurement step (step S33) to measure the frequency dependence of the LSAW speed in a predetermined propagation direction for the substrate with the thin film to be measured, a calculation value calculation step (step S34) to calculate the calculated value of the fH dependence of the LSAW speed in the same propagation direction with the same structure as the substrate with the thin film to be measured used in the frequency dependence measurement step, a measurement value calculation step (step S35) to calculate the measured value of the fH dependence of the LSAW speed by assuming a temporary thickness (i.e., temporary film thickness H) for the frequency dependence obtained in the frequency dependence measurement step, and a determination step (step S36) to determine whether the speed difference between the calculated value of the fH dependence obtained in the calculation value calculation step and the measured value of the fH dependence obtained in the measurement value calculation step has been minimized. Furthermore, the calibration method for the optical thickness gauge determines the temporary film thickness H when the speed difference is minimized, and sets the determined temporary film thickness H to the true value L of the thickness of the substrate with the thin film to be measured. true The true thickness calculation step (step S37) and the provisional thin film thickness L obtained in the provisional substrate thickness calculation step temp The true value L of the thin film thickness obtained in the true thickness calculation process. true , and a hypothetical refractive index n temp Using this, the nominal refractive index true value n nominal This includes a refractive index calculation step (step S38) to determine the refractive index.

[0064] Furthermore, the calibration method for the optical thickness gauge involves measuring the reflection spectrum at an arbitrary measurement point on the substrate surface with the thin film to be measured, under the same measurement conditions as the provisional substrate thickness calculation step, and then determining the nominal refractive index true value n obtained in the refractive index calculation step. nominal Let n be a hypothetical refractive index temp The process includes a step of obtaining the true value of the thin film thickness at the measurement point by replacing it with the other value and performing the analysis. The details are explained below.

[0065] First, prepare the thin-film substrate to be measured (step S31). Measure the reflection spectrum (wavelength dependence of reflected light intensity) of the thin-film substrate using an optical method (mainly in the wavelength region near visible light), determine the wavelength range of light effective for analysis from the reflection spectrum, perform a Fast Fourier Transform (FFT), and determine a provisional refractive index n tempAssuming this, the hypothetical thin film thickness L is obtained from the peak position of the FFT analysis spectrum. temp The following is calculated (step S32). The process in step S32 may be calculated using a fitting analysis method, not just FFT analysis. Also, the provisional refractive index n here temp It is acceptable to assume that this value is constant across all wavelength ranges.

[0066] Next, the frequency dependence of the LSAW velocity is measured by selecting a predetermined propagation direction using a linear focused beam ultrasonic material analysis (LFB-UMC) system, one of the UMS technologies, on the substrate with the thin film to be measured (step S33). LSAW is a leakage surface wave (LSAW) that propagates on the surface of a water-loaded sample. LSAW is a mode in which energy is concentrated and propagates at a depth of about one wavelength below the sample surface, and its propagation velocity changes depending on the relationship between the thickness of the thin film on the sample surface and its wavelength (frequency). The principle of measuring the LSAW velocity using the LFB-UMC system is as described in reference 2 above.

[0067] Calculated value of the fH dependence of the LSAW velocity in the propagation direction selected in step S33 for the substrate structure with the thin film under measurement (calculated propagation velocity V) calc ) is calculated (step S34). In this calculation, known material constants for the thin film material and substrate material may be used, or constants determined as shown in steps S12 and S13 of Figure 2 may be used. Then, assuming a hypothetical film thickness H, the measured value V of the LSAW velocity's fH dependence is obtained for the frequency dependence of the LSAW velocity obtained in step S33. meas The following is calculated (step S35). In other words, the frequency dependence of the LSAW velocity obtained in step S33 is converted into a relationship between the product fH of frequency f and the hypothetical film thickness H, assuming a hypothetical film thickness H, and the LSAW velocity.

[0068] Then, the calculated propagation speed V in step S34 calc and the measured value V in step S35 measIt is determined whether the speed difference has been minimized (step S36). In step S36, the provisional film thickness H is determined when the speed difference is minimized, and this is used as the true value L of the thin film thickness. true This is determined (step S37).

[0069] Finally, the provisional thin film thickness L obtained above temp , a hypothetical refractive index n temp The true value L of the thin film thickness true Using this, the nominal true value of the refractive index n becomes the nominal true value. nominal Determine the true value of the nominal refractive index n (step S38). More specifically, the true value of the nominal refractive index n nominal This is calculated from equation (7) above.

[0070] Subsequently, the nominal refractive index true value n is obtained from the reflection spectrum measured under the same measurement conditions as in step S32 for any other measurement point or other identical sample. nominal Only the provisional refractive index n temp By swapping the values ​​and keeping all other conditions the same, the true thickness of the thin film at each measurement point can be obtained.

[0071] In this way, the nominal refractive index n is obtained using the ultrasonic propagation speed. nominal By determining this, the absolute accuracy of optical substrate thickness gauges can be improved. Specifically, a hypothetical thin film thickness L temp And the true value L of the thin film thickness calculated using the ultrasonic propagation velocity. true And, a hypothetical refractive index n temp Based on this, the nominal refractive index true value n nominal By determining this, the refractive index required when measuring film thickness can be calibrated with high precision. Therefore, the absolute value of the refractive index of the substrate material, which is essential for substrate thickness measurement using an optical method that allows for short-time, non-contact, and wide-area measurement, can be obtained with high precision, and the absolute value of the film thickness and its distribution can be obtained efficiently and accurately.

[0072] <Fifth Embodiment> The calibration method for the optical thickness gauge of this embodiment, as shown in Figure 6, consists of a step of obtaining the nominal refractive index from a single-frequency LSAW velocity measurement on a substrate with a thin film to be measured. In the fifth embodiment, similar to the third and fourth embodiments, the substrate to be measured is a "substrate with a thin film" (i.e., a substrate with a thin film to be measured), and the "provisional thin film thickness L" temp "The true value of the thickness of the thin film L" true This will be explained as a method for determining the "true value of the nominal refractive index (specifically, the true value of the nominal refractive index n)" using the ultrasonic propagation speed. nominal It is explained as something that seeks ")".

[0073] More specifically, the calibration method for the optical thickness gauge involves a substrate fabrication step (step S41) in which a substrate with a thin film to be measured is fabricated from the material to be measured, and a reflection spectrum is measured on the substrate with the thin film to be measured using an optical method, and a provisional refractive index n is calculated from that reflection spectrum. temp Using a temporary thin film thickness L temp The method includes a preliminary substrate thickness calculation step (step S42) to determine the true value L of the thin film thickness, a velocity measurement step (step S43) to measure the LSAW velocity in a predetermined propagation direction for the substrate to be measured at a specific frequency, a calculation value calculation step (step S44) to calculate the calculated value of the fH dependence of the LSAW velocity in the same propagation direction with the same structure as the substrate with the thin film to be measured used in the frequency dependence measurement step, and an fH value calculation step (step S45) to calculate an fH value from the calculated fH dependence values ​​obtained in the calculation value calculation step that matches the measured LSAW velocity value obtained in the velocity measurement step. Furthermore, the calibration method for the optical thickness gauge uses the fH value obtained in the fH value calculation step and the frequency used in the velocity measurement step to determine the true value L of the thin film thickness. true The true thickness calculation step (step S46) determines the temporary thin film thickness L obtained in the temporary substrate thickness calculation step. temp The true value L of the thin film thickness obtained in the true thickness calculation process. true , and a hypothetical refractive index n temp Using this, the nominal refractive index true value n nominal The process includes a refractive index calculation step (step S47) to determine the refractive index.

[0074] Furthermore, the calibration method for the optical thickness gauge involves measuring the reflection spectrum at an arbitrary measurement point on the substrate surface with the thin film to be measured, under the same measurement conditions as the provisional substrate thickness calculation step, and then determining the nominal refractive index true value n obtained in the refractive index calculation step. nominal Let n be a hypothetical refractive index temp The process includes a step of obtaining the true value of the thin film thickness at the measurement point by replacing it with the other value and performing the analysis. The details are explained below.

[0075] First, prepare the thin-film substrate to be measured (step S41). Measure the reflection spectrum (wavelength dependence of reflected light intensity) of the thin-film substrate using an optical method (mainly in the wavelength region near visible light), determine the wavelength range of light effective for analysis from the reflection spectrum, perform a Fast Fourier Transform (FFT), and determine a provisional refractive index n temp Assuming this, the hypothetical thin film thickness L is obtained from the peak position of the FFT analysis spectrum. temp The following is determined (step S42). The process in step S42 may be calculated using a fitting analysis method, not just FFT analysis. Also, the provisional refractive index n here temp It is acceptable to assume that this value is constant across all wavelength ranges.

[0076] Next, the LSAW velocity is measured at a frequency fm by selecting a predetermined propagation direction using a linear focused beam ultrasonic material analysis (LFB-UMC) system, which is one of the UMS technologies, on the substrate with the thin film under test (step S43). The fH dependence of the LSAW velocity in the propagation direction selected in step S43 for the substrate structure with the thin film under test is calculated (step S44). In this calculation, known material constants for the thin film material and substrate material may be used, or constants determined as shown in steps S12 and S13 of Figure 2 may be used. Then, among the calculated values ​​obtained in step S44, the LSAW speed that matches the measured value obtained in step S43 is searched for, and the fH value at that time is calculated (step S45). Using the fH value obtained in step S45 and the frequency fm used for measurement in step S43, the true value L of the thin film thickness is calculated. true The true value L of the thin film thickness is determined (step S46). Specifically, the true value L of the thin film thickness is determined.true This is calculated from the following formula (8). L true = fH / fm ···(8)

[0077] Finally, the provisional thin film thickness L obtained above temp , a hypothetical refractive index n temp , true value L of thin film thickness true Using this, the nominal true value of the refractive index n becomes the nominal true value. nominal Determine the true value of the nominal refractive index n (step S47). More specifically, the true value of the nominal refractive index n nominal This is calculated from equation (7) above.

[0078] Subsequently, the nominal refractive index true value n is obtained from the reflection spectrum measured under the same measurement conditions as in step S42 for any other measurement point or other identical sample. nominal Only the provisional refractive index n temp By swapping the values ​​and keeping all other conditions the same, the true value of the thin film thickness at each measurement point can be obtained.

[0079] In this way, the nominal refractive index n is obtained using the ultrasonic propagation speed. nominal By determining this, the absolute accuracy of optical substrate thickness gauges can be improved. Specifically, a hypothetical thin film thickness L temp And the true value L of the thin film thickness calculated using the ultrasonic propagation velocity. true And, a hypothetical refractive index n temp Based on this, the nominal refractive index true value n nominal By determining this, the refractive index required when measuring film thickness can be calibrated with high precision. Therefore, the absolute value of the refractive index of the substrate material, which is essential for substrate thickness measurement using an optical method that allows for short-time, non-contact, and wide-area measurement, can be obtained with high precision, and the absolute value of the film thickness and its distribution can be obtained efficiently and accurately.

[0080] <Other Embodiments> The calibration method for the optical thickness gauge of this embodiment consists of a process of directly performing physical thickness measurement and optical thickness measurement on a substrate to be measured, which is made from the material to be measured as shown in Figure 7, and calibrating the refractive index. The details are described below.

[0081] First, the material to be measured is selected (Step S51). A substrate to be measured is fabricated using the selected material (Step S52). The optical path length L is measured using an optical method on the fabricated substrate. o And, a hypothetical refractive index n temp Using this, a temporary substrate thickness L temp (Step S53) Determine the true value L of the physical thickness of the substrate using a contact-type thickness gauge with a built-in linear encoder or the like. true Measure the (step S54). The provisional substrate thickness L obtained above temp And the true value L of the substrate thickness true And, a hypothetical refractive index n temp The true value of the refractive index n using true We find the answer (step S55).

[0082] Then, at any point within the surface of the substrate being measured, the optical path length L o The results of the measured mapping and the true value n of the refractive index obtained in step S55 true By using this method, the true value of the thickness distribution of the substrate being measured can be obtained.

[0083] This embodiment uses the ultrasonic propagation speed to determine the true value n of the refractive index. true This does not aim to determine the true value of the refractive index n through physical measurement using a contact thickness gauge, etc. true This is what we are looking for. While this method is more efficient than the first embodiment shown in Figure 1 because it eliminates the need to prepare standard samples, commercially available wafers used as substrates for measurement are often thin, typically less than 0.5 mm thick. Therefore, the measurement error by the contact-type thickness gauge becomes relatively large relative to the substrate thickness, reducing the absolute accuracy of the true substrate thickness and ultimately lowering the absolute accuracy of the refractive index that is calibrated. Consequently, it is best to select a calibration method appropriately, taking into account the required accuracy.

[0084] <Regarding temperature environment> In measuring substrate thickness or film thickness using the optical thickness gauge described above, the following temperature environment is important in order to improve the reproducibility of the measured thickness. Figure 8 shows a schematic diagram of a system in which a sample T and an optical thickness measuring device 2 are placed in a constant temperature chamber 1, and the temperature is controlled by a spot air conditioner 3 to measure the thickness. A HEPA filter box 4 is installed on top of the constant temperature chamber 1. An XYZ stage 6 is placed on the stage base 5. The sample T is placed on the XYZ stage 6 for measurement. The temperature-controlled air from the spot air conditioner 3 is introduced into the constant temperature chamber 1 through a HEPA filter and returns to the spot air conditioner 3 from the bottom of the constant temperature chamber 1. Then, the temperature-controlled air from the spot air conditioner 3 is sent out again. In this way, the system is a circulating mechanism in which temperature-controlled air is circulated.

[0085] In other words, in the temporary substrate thickness calculation step of each embodiment, the sample T and the optical thickness measuring device 2 are placed in the constant temperature chamber 1, and measurements are taken using an optical method in an environment controlled at a constant temperature. Note that TA, TB, and TC in Figure 8 are thermocouple measurement points. TA is the point where the air near the sample inside the constant temperature chamber 1 is measured. TB is the point where the temperature of the stage base 5 near the HEPA filter outlet inside the constant temperature chamber 1 is measured. TC is the room temperature outside the constant temperature chamber 1, and is measured by a thermocouple attached to the outer wall of the constant temperature chamber 1.

[0086] Since the thickness of the sample T to be measured fluctuates due to thermal expansion caused by temperature changes, it is important to place the sample T in a constant temperature chamber 1 to ensure temperature stability. Furthermore, since the optical thickness measuring device 2 itself is also affected by temperature changes and the measured values ​​fluctuate, it is important to place the optical thickness measuring device 2 in a constant temperature chamber 1 as well. If the temperature environment in which the optical thickness gauge measuring device is installed is insufficient, the reproducibility of the measurement values ​​themselves may deteriorate. However, as in this embodiment, by installing not only the sample T to be measured but also the optical thickness measuring device 2 itself in a temperature-controlled constant temperature chamber 1, it is possible to suppress fluctuations in the measurement values ​​caused not only by changes in the thermal expansion of the sample T itself, but also by changes in the wavelength of the light source of the optical thickness measuring device 2 and changes in the thermal expansion of the internal mechanical parts.

[0087] In this embodiment, the calibration method for the optical thickness gauge according to the present invention was mainly described. However, the embodiments described above are merely examples to facilitate understanding of the present invention and do not limit it. The present invention can be modified and improved without departing from its spirit, and of course, equivalents thereof are included. Furthermore, although the above embodiment shows an example where only the refractive index is assumed (the extinction coefficient is assumed to be zero), calibration can be performed similarly when the extinction coefficient (attenuation coefficient of light) is set along with the refractive index. [Examples]

[0088] <Example 1> An example of measurement for a Si single crystal material used as a support substrate will be described below, following the process shown in Figure 1, which was described in the first embodiment.

[0089] A (100)Si single crystal substrate was prepared as a standard sample (Step S2). The standard ultrasonic round-trip propagation time T was measured for this (100)Si standard sample using ultrasonic measurement. std The following measurements were taken. The measurement method used was the Planar Ultrasonic Material Analysis (PW-UMC) system (reference 1 mentioned above), which is one of the constituent systems of UMS technology, and the ultrasonic propagation mode was longitudinal wave. The measurement results showed that the standard ultrasonic round-trip propagation time T std =1992.19ns was obtained (Step S3).

[0090] Next, the thickness of the standard sample was measured using a contact-type thickness gauge with a built-in linear encoder. The result was L std= 8399.50 μm (Step S4). The reproducibility of the measurement was approximately ±0.01 μm. Furthermore, this substrate thickness was corrected considering the difference in the amount of strain that occurs when the contactor of the thickness gauge contacts the sample and the sample stage. The correction method follows the above-mentioned reference 3. Based on the above results, the reference value V of the ultrasonic propagation velocity of the standard sample was calculated. std = 8432.41 m / s was obtained (Step S5).

[0091] Next, a (100)Si substrate with a diameter of approximately 4 inches and a thickness of approximately 0.5 mm was prepared (Step S6). The substrate thickness distribution in the diametrical direction of this (100)Si substrate was measured using an optical method with infrared light. The measurement results are shown in Figure 9. The provisional refractive index of the Si substrate was set to n temp The result of the provisional substrate thickness distribution obtained by setting =3.67 is shown by the dotted line (L) in Figure 9. temp ). From this result, the provisional substrate thickness at the center point of the substrate is L temp The value was 474.638 μm (Step S7).

[0092] At the center point of the substrate under test, the longitudinal wave ultrasonic round-trip propagation time T was measured using ultrasonic measurement with a PW-UMC system. The result was T = 112.1807 ns (Step S8). Using this result, the true value of the thickness at the center point of the substrate under test was determined to be L true =472.977μm was obtained (Step S9). Based on these results, the true value of the refractive index of the substrate under test was determined to be n true =3.6828 was obtained (Step S10). Using this true refractive index, the true value of the thickness distribution of the substrate under measurement was recalculated, and the result is shown by the solid line (L) in Figure 9. true )

[0093] <Example 2> A measurement example for a LiTaO3 single crystal material (hereinafter referred to as LT) used as a piezoelectric substrate will be described below, following the process shown in Figure 2, which was described in the second embodiment.

[0094] As samples for determining the constants, LT substrates with seven different crystal plane orientations (X, Y, Z, 32.98°Y, 58.35°Y, 107.13°Y, 147.02°Y) were prepared (Step S12). Through measurements on these samples for determining the constants, the material constants were determined according to the procedure described in Reference 4 above (Step S13). Using the obtained material constants, the propagation velocity of longitudinal waves propagating through the LT substrate was calculated. Since the substrate plane orientation prepared in Step S15 described later is 42°Y axis direction, the propagation direction of the longitudinal waves is 42°Y axis propagation. Calculated value V of longitudinal wave propagation velocity in the 42°Y axis direction calc V calc The value was 6354.65 m / s (Step S14).

[0095] Next, a 42° Y-cut LiTaO3 single crystal substrate (hereinafter referred to as 42Y-LT) was prepared as the substrate to be measured (Step S15). The size of this 42Y-LT substrate was approximately 4 inches in diameter and 0.2 mm in thickness. The substrate thickness distribution in the diametrical direction of this 42Y-LT substrate was measured using an optical method with infrared light. The measurement results are shown in Figure 10. The provisional refractive index of the 42Y-LT substrate was set to n temp The result of the provisional substrate thickness distribution obtained by setting =2.23 is shown by the dotted line (L) in Figure 10. temp ) . From this result, the provisional substrate thickness at the center point of the substrate is L temp The value was 187.838 μm (Step S16).

[0096] Next, the longitudinal wave ultrasonic round-trip propagation time T was measured at the center point of the substrate under test using ultrasonic measurement with the PW-UMC system. The result was T = 60.8153 ns (step S17). Using this result, the true value of the thickness at the center point of the substrate under test was determined to be L true = 193.320 μm was obtained (Step S18). Based on these results, the true value of the refractive index of the substrate under test was determined to be n true =2.1678 was obtained (Step S19). Using this true refractive index, the true value of the thickness distribution of the substrate under measurement was recalculated, and the result is shown by the solid line (L) in Figure 10. true )

[0097] <Example 3> A measurement example for a 42°Y-LiTaO3 thin film / (100)Si substrate structure sample will be described below, following the steps shown in Figure 3, which were described in the third embodiment.

[0098] A 42Y-LT thin film / (100)Si substrate structure sample was prepared as the sample with the thin film to be measured (Step S21). The sample size was 4 inches in diameter, the thickness of the 42Y-LT thin film was about 1 μm, and the thickness of the (100)Si substrate was about 0.5 mm. The reflection spectrum was measured on the above-mentioned substrate with the thin film to be measured using an optical method (mainly in the wavelength region near visible light). An example is shown in Figure 11A. From the reflection spectrum in Figure 11A, the wavelength range of light effective for analysis was set to 400 nm to 1000 nm, and FFT analysis was performed to determine a provisional refractive index n temp An example of an FFT analysis spectrum obtained assuming =2.23 is shown in Figure 11B. A provisional thin film thickness was obtained from the peak positions of the FFT analysis spectrum in Figure 11B. The result of the film thickness distribution in the diametrical direction is shown by the dotted line (L) in Figure 11C. temp The provisional thin film thickness at the center point of the sample is L. temp The value was 1037.290 nm (step S22).

[0099] Next, the longitudinal wave propagation velocity in the thickness direction of the thin film was measured on the substrate with the thin film attached using ultrasonic measurement. The method used to measure the longitudinal wave propagation velocity of the thin film was the method described in Reference 5 mentioned above. At this time, the true value V of the longitudinal wave propagation velocity in the thin film was measured. true This is the calculated propagation velocity V used in Figure 2. calc Assuming, the measured value V meas V is the true value of the longitudinal wave propagation velocity in a thin film. true The true value L of the thin film thickness is set to match. true The result at the center point of the sample was L true =1067.063nm was obtained (step S23).

[0100] Finally, the provisional thin film thickness L obtained above temp , a hypothetical refractive index n temp The true value L of the thin film thickness true Using this, the nominal true value of the refractive index n becomes the nominal true value.nominal When we decided on n nominal =2.1678 was obtained (step S24). The obtained n nominal The result of reanalyzing the true value of the thin film thickness distribution of the substrate with the thin film under measurement using this method is shown by the solid line (L) in Figure 11C. true )

[0101] <Example 4> A measurement example for a 36°Y-LiTaO3 thin film / AT-cut quartz substrate structure sample will be described below, following the steps shown in Figure 5, which were described in the fourth embodiment.

[0102] A 36Y-LT thin film / AT quartz substrate structure sample was prepared as the sample with the thin film to be measured (Step S31). This sample had a diameter of 4 inches, a 36Y-LT thin film thickness of approximately 1 μm, and an AT quartz substrate thickness of approximately 0.35 mm. The reflection spectrum was measured for the above-mentioned substrate with the thin film using optical methods (mainly in the wavelength region near visible light). A provisional refractive index n was used. temp Assuming = 2.23, the provisional thin film thickness at the sample center point obtained from the peak position of the FFT analysis spectrum is L temp = 1077.17 nm (Step S32).

[0103] Next, the frequency dependence of the LSAW velocity in the X-axis propagation was measured using the LFB-UMC system on the substrate with the thin film under test (step S33). The method for measuring the LSAW velocity using the LFB-UMC system was the method described in Reference 2 mentioned above. The results are shown in Figure 12A. The calculated value of the fH dependence of the LSAW velocity in the X-axis propagation for the 36Y-LT thin film / AT quartz substrate structure was calculated (step S34). The results are shown by the dotted line in Figure 12B. In Figure 12B, the dotted line shows the calculated value, and the solid line shows the fH dependence of the LSAW velocity when the film thickness is at its optimal value. The dashed-dotted line shows the fH dependence of the LSAW velocity when the film thickness is -10% from the optimal value, and the dashed-dotted line shows the fH dependence of the LSAW velocity when the film thickness is +10% from the optimal value. The material constants from the aforementioned references 4 and 6 (J. Kushibiki et al., “Accurate Measurements of the Acoustical Physical Constants of Synthetic α-Quartz for SAW Devices”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 49, no. 1, pp. 125-135 (2002)) were used in the calculations at this time. The frequency dependence of the LSAW velocity obtained in step S33 is converted to a relationship between the product fH of frequency f and hypothetical film thickness H and the LSAW velocity, assuming a hypothetical film thickness H (step S35). The propagation velocity calculation value V in step S34 is then used. calc and the measured value V in step S35 meas The determination of whether the speed difference had been minimized was repeated (step S36). In step S36, a provisional film thickness H of H = 972.518 nm was obtained when the speed difference was minimized. This was then used as the true value L of the thin film thickness. true This was decided (Step S37).

[0104] Finally, the provisional thin film thickness L obtained above temp , a hypothetical refractive index n temp The true value L of the thin film thickness true Using this, the nominal true value of the refractive index is n nominal = 2.4700 was determined (step S38). The obtained n nominal Using this method, the measurement result of the thin film thickness distribution of the substrate with the thin film under test is shown by the solid line (L) in Figure 12C. true )

[0105] <Example 5> A measurement example for a 36°Y-LiTaO3 thin film / AT-cut quartz substrate structure sample will be described below, following the steps shown in Figure 6, which were described in the fifth embodiment.

[0106] A 36Y-LT thin film / AT quartz substrate structure sample was prepared as the sample with the thin film to be measured (Step S41). The sample size was 4 inches in diameter, with a 36Y-LT thin film thickness of approximately 1 μm and an AT quartz substrate thickness of approximately 0.35 mm. The reflectance spectrum was measured for the above-mentioned substrate with the thin film using optical methods (mainly in the wavelength region near visible light). A provisional refractive index n was used. temp Assuming = 2.23, the provisional thin film thickness at the sample center point obtained from the peak position of the FFT analysis spectrum is L temp = 1077.17 nm (Step S42).

[0107] Next, the LSAW velocity of the X-axis propagation was measured along the diameter of the substrate at a frequency of 225 MHz using the LFB-UMC system on the substrate with the thin film under test (step S43). The method for measuring the LSAW velocity using the LFB-UMC system at this time was the method described in Reference 2 mentioned above. The results are shown in Figure 13A. The fH dependence of the LSAW velocity in the X-axis propagation was calculated for the 36Y-LT thin film / AT quartz substrate structure (step S44). The results are shown in Figure 13B. The material constants from references 4 and 6 mentioned above were used in the calculations. Among the calculated values ​​obtained in Figure 13B, the LSAW velocity that matched the measured value obtained in Figure 13A was searched for, and the fH value at that point was calculated (step S45). The results are shown in Figure 13C. By dividing the fH value obtained in Figure 13C by the frequency fm = 225 MHz used in the measurement in step S43, the true value L of the thin film thickness can be obtained. true This was calculated (step S46). The result is shown in Figure 13D.

[0108] Finally, the provisional thin film thickness L obtained above temp , a hypothetical refractive index n temp , true value L of thin film thickness true Using this, the nominal true value of the refractive index n becomes the nominal true value. nominalDetermine (step S47). If measurements are taken at multiple points as shown in Figure 13D, the calculated film thickness from the LSAW velocity measurement at the position corresponding to the position where the optical film thickness measurement was performed in step S42 is used. nominal The measurement results for the thin film thickness distribution of the substrate with the thin film under test using the same method as those shown in Figure 12C.

[0109] <Results of measured temperatures in temperature control> The results of the thermocouple temperature measurements for the system shown in Figure 8 above are explained below. Figure 14A shows the thermocouple temperature measurements at the measurement points (TA, TB, TC) when the sample and optical thickness measuring device are placed inside a constant temperature chamber and the temperature inside the chamber is controlled to a constant level. The solid line in Figure 14A represents the temperature measurement results at the TA. Near the TA, a gentle airflow is generated from top to bottom within the constant temperature chamber, causing temperature fluctuations of approximately ±0.04°C. However, the TA itself is maintained at a nearly constant temperature. The dashed line in Figure 14A shows the results of temperature measurements at TB. TB exhibits small and stable temperature fluctuations. The dashed line in Figure 14A represents the temperature measurement results at the temperature control center (TC). Near the TC, the temperature fluctuates periodically within a range of approximately 1°C due to control by the indoor air conditioner. Figure 14B shows the results of repeated measurements of the thin film thickness at a single central point of the thin film / substrate structure sample under these temperature conditions. The measured film thickness in Figure 14B was nearly constant, with a maximum variation of 0.330 nm.

[0110] Next, using the same sample and optical thickness measuring device setup as in Figure 8, the temperature control of the spot air conditioner was stopped, and then the temperature control of the indoor air conditioner was also stopped, and the film thickness was measured without temperature control. The results of the temperature change in this case are shown in Figure 14C. The results of the change in the measured film thickness in this case are shown in Figure 14D.

[0111] Figure 14C shows that the temperature fluctuations at all measurement points were affected by the ambient temperature fluctuations, resulting in a temperature fluctuation range of approximately 4°C. The results of the film thickness fluctuations measured under these temperature conditions (Figure 14D) show that the film thickness fluctuated gradually, with a maximum fluctuation range of 0.640 nm. This is about twice the fluctuation range compared to when the temperature is controlled. However, the peak time for the temperature fluctuation observed in Figure 14C does not coincide with the peak time for the film thickness fluctuation in Figure 14D, indicating that the profiles of the two do not necessarily match. This suggests that the film thickness fluctuation in Figure 14D does not simply represent fluctuations due to the thermal expansion of the sample, but is also influenced by the temperature environment dependence of the measurement device itself. Generally, such effects from temperature fluctuations are slight and tend to be overlooked. However, it has become clear that temperature stability is a crucial point for high-precision measurements to determine the absolute thickness of thin films. [Explanation of Symbols]

[0112] 1. Constant temperature chamber 2. Optical Thickness Measuring Device 3 Spot air conditioners 4 HEPA filter boxes 5 Stage base 6 XYZ Stages

Claims

1. A calibration method for an optical thickness gauge using ultrasonic propagation velocity, A provisional substrate thickness calculation step in which a provisional substrate thickness is determined using a predetermined value measured on the substrate under test using an optical method and a provisional refractive index of the substrate under test, A thickness true value calculation step that uses the ultrasonic propagation speed to determine the true thickness of the substrate to be measured, A calibration method for an optical thickness gauge, comprising: a refractive index calculation step, which determines the true refractive index of the substrate to be measured using the provisional substrate thickness obtained in the provisional substrate thickness calculation step, the true thickness obtained in the true thickness value calculation step, and the provisional refractive index.

2. A sample substrate preparation step is to prepare a standard sample substrate made of the same material and with the same crystal orientation as the substrate to be measured, A standard propagation time measurement step is performed on the standard sample by ultrasonic measurement to measure the standard ultrasonic round-trip propagation time in the thickness direction of the standard sample, A standard sample thickness measurement step for measuring the standard sample thickness of the aforementioned standard sample, A reference value calculation step for calculating a reference value for ultrasonic propagation velocity using the standard ultrasonic round-trip propagation time obtained in the standard propagation time measurement step and the standard sample thickness obtained in the standard sample thickness measurement step, The process includes a substrate propagation time measurement step, in which the ultrasonic wave propagation time in the thickness direction of the substrate is measured by ultrasonic measurement on the substrate to be measured, In the provisional substrate thickness calculation step, the provisional substrate thickness is determined using the optical path length measured using an optical method for the substrate to be measured and the provisional refractive index of the substrate to be measured. The calibration method for an optical thickness gauge according to claim 1, characterized in that the true thickness calculation step involves determining the true thickness of the substrate to be measured using the ultrasonic round-trip propagation time of the substrate to be measured obtained in the substrate propagation time measurement step and the ultrasonic propagation velocity reference value obtained in the reference value calculation step.

3. The calibration method for an optical thickness gauge according to claim 2, characterized in that the sample substrate preparation step involves preparing the standard sample having a thickness of 1 mm or more.

4. A constant determination sample substrate preparation step, which involves preparing a substrate for constant determination using the same material as the substrate to be measured, A constant determination step involves determining the material constants by performing sound velocity measurement, dielectric constant measurement, and density measurement on the prepared sample for constant determination. A propagation velocity calculation step is performed to determine the propagation velocity of ultrasonic waves propagating in the thickness direction of the substrate to be measured, using the material constants obtained in the constant determination step. The process includes a substrate propagation time measurement step, in which the ultrasonic wave propagation time in the thickness direction of the substrate is measured by ultrasonic measurement on the substrate to be measured, In the provisional substrate thickness calculation step, the provisional substrate thickness is determined using the optical path length measured using an optical method for the substrate to be measured and the provisional refractive index of the substrate to be measured. The calibration method for an optical thickness gauge according to claim 1, characterized in that the true thickness of the substrate to be measured is determined in the true thickness calculation step by using the ultrasonic round-trip propagation time of the substrate to be measured obtained in the substrate propagation time measurement step and the propagation velocity calculated value obtained in the propagation velocity calculation step.

5. The substrate to be measured is a substrate with a thin film, In the provisional substrate thickness calculation step, the provisional thin film thickness is determined using the reflection spectrum measured with respect to the substrate using an optical method and the provisional refractive index of the substrate. In the above thickness true value calculation step, the true value of the thickness of the thin film of the substrate to be measured is determined from the frequency dependence of the total ultrasonic propagation velocity obtained by bulk wave ultrasonic measurement on the substrate to be measured. The calibration method for an optical thickness gauge according to claim 1, characterized in that the refractive index calculation step determines the true refractive index of the substrate to be measured using the provisional thin film thickness obtained in the provisional substrate thickness calculation step, the true thickness of the thin film obtained in the true thickness value calculation step, and the provisional refractive index.

6. The calibration method for an optical thickness gauge according to claim 5, characterized in that, in the provisional substrate thickness calculation step, a wavelength range of light effective for analysis is determined from the reflection spectrum, a fast Fourier transform is performed to assume the provisional refractive index, and the provisional thin film thickness is determined from the peak position of the analysis spectrum of the fast Fourier transform.

7. The substrate to be measured is a substrate with a thin film, A frequency dependence measurement step is performed to measure the frequency dependence of the LSAW velocity in a predetermined propagation direction with respect to the substrate under measurement. A calculation step for calculating the fH dependence of the LSAW velocity in the same structure and propagation direction as the substrate used in the frequency dependence measurement step, A measurement value calculation step is performed to calculate the fH dependence of the LSAW velocity by assuming a hypothetical thickness for the frequency dependence obtained in the frequency dependence measurement step, The process includes a determination step to determine whether the difference in velocity between the calculated value of the fH dependence obtained in the calculation step and the measured value of the fH dependence obtained in the measurement step has been minimized. In the provisional substrate thickness calculation step, the provisional thin film thickness is determined using the reflection spectrum measured with respect to the substrate using an optical method and the provisional refractive index of the substrate. In the process of calculating the true thickness, the provisional thin film thickness when the speed difference is minimized is determined as the true thickness of the thin film on the substrate to be measured. The calibration method for an optical thickness gauge according to claim 1, characterized in that the refractive index calculation step determines the true refractive index of the substrate to be measured using the provisional thin film thickness obtained in the provisional substrate thickness calculation step, the true thickness of the thin film obtained in the true thickness value calculation step, and the provisional refractive index.

8. The substrate to be measured is a substrate with a thin film, A speed measurement step is performed to measure the LSAW speed in a predetermined propagation direction at a specific frequency for the substrate to be measured. A calculation step for calculating the fH dependence of the LSAW velocity in the same structure and propagation direction as the substrate used in the velocity measurement step, The process includes calculating an fH value among the calculated fH-dependent values ​​obtained in the calculation value calculation step that matches the LSAW speed measurement value measured in the speed measurement step, In the provisional substrate thickness calculation step, the provisional thin film thickness is determined using the reflection spectrum measured with respect to the substrate using an optical method and the provisional refractive index of the substrate. In the thickness true value calculation step, the fH value obtained in the fH value calculation step and the frequency used in the speed measurement step are used to determine the true value of the thickness of the thin film of the substrate to be measured. The calibration method for an optical thickness gauge according to claim 1, characterized in that the refractive index calculation step determines the true refractive index of the substrate to be measured using the provisional thin film thickness obtained in the provisional substrate thickness calculation step, the true thickness of the thin film obtained in the true thickness value calculation step, and the provisional refractive index.

9. A calibration method for an optical thickness gauge according to any one of claims 2 to 4, characterized in that it includes a step of obtaining the true value of the substrate thickness distribution of the substrate to be measured using the result of mapping the optical path length measured at an arbitrary measurement point on the substrate surface of the substrate to be measured and the true value of the refractive index obtained in the refractive index calculation step.

10. A calibration method for an optical thickness gauge according to any one of claims 5 to 8, characterized in that it includes a step of obtaining the true value of the thickness of a thin film at a measurement point by analyzing the reflection spectrum measured at an arbitrary measurement point on the substrate surface of the substrate under the same measurement conditions as the provisional substrate thickness calculation step, and replacing the true value of the refractive index obtained in the refractive index calculation step with the provisional refractive index.

11. The calibration method for an optical thickness gauge according to any one of claims 1 to 8, characterized in that the temporary substrate thickness calculation step involves placing the sample and the optical thickness measuring device in a constant temperature chamber and measuring using an optical method in an environment controlled at a constant temperature.

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