JP6677407B2 - Observation method of fault structure, observation device, and computer program - Google Patents

Description

  The present invention relates to a method for observing a fault structure, an observation apparatus, and a computer program.

  Non-Patent Document 1 discloses that a layer thickness measurement and tomographic imaging of a transparent resist thin film applied to a semiconductor substrate surface are obtained by optical coherence tomography (OCT) using a visible light source. I have. By using OCT, a transparent resist thin film can be observed in a non-destructive and non-contact manner.

  In OCT, the layer thickness is obtained as the optical path length. The optical path length corresponds to the actual thickness of the layer multiplied by the refractive index of the material of the layer. Therefore, in order to obtain the actual layer thickness of the resist thin film, the optical path length between both interfaces of the transparent resist thin film obtained by OCT may be divided by the refractive index of the resist thin film.

Takeshi Nishi, Nobuhiko Ozaki, Hirotaka Osato, Eiichiro Watanabe, Naoki Ikeda, Yoshimasa Sugimoto, "Application of μm-OCT to Semiconductor Microfabrication Process Using Visible Light Broadband Light Source", 62nd JSAP Spring Meeting, Japan Society of Applied Physics, March 2015

  The semiconductor substrate under the transparent resist thin film described in Non-Patent Document 1 is opaque, and it has been considered that OCT is not suitable for observing a tomographic structure inside the semiconductor substrate. However, the present inventors have found that OCT is capable of observing not only a transparent resist thin film applied on a semiconductor substrate surface but also an opaque semiconductor layer under the transparent resist thin film, and completed the present invention. did.

  The present invention viewed from one viewpoint is an observation method for observing a tomographic structure by optical coherence tomography (OCT). The tomographic structure to be observed has a light-absorbing layer made of a light-absorbing substance that absorbs light of at least part of the wavelength of the light source spectrum. Measurement light is emitted from the light source to the tomographic structure. The light source spectrum is preferably broadband. In the light absorbing substance, light having at least a part of the wavelength of the irradiated light is absorbed, so that the spectrum of the light changes as the irradiated light travels through the light absorbing substance. When the spectrum of light changes, the refractive index for the light also changes due to the dispersion characteristics of the light absorbing material. In addition, since the amount of light absorption increases according to the traveling distance of light in the substance, the change in the refractive index also changes according to the traveling distance of light. As a result, it is difficult to determine the thickness of the light absorbing layer made of the light absorbing material by dividing the optical path length by a single refractive index. It is necessary to derive the relationship between the length and the actual film thickness.

  Therefore, the observation method according to the present invention includes a step of generating a reflected light profile of the tomographic structure based on interference light between the measurement light reflected from the tomographic structure and the reference light; A step of comparing a plurality of light propagation simulation results with a plurality of simulated tomographic structures having simulated light absorption layers having different thicknesses, and a simulation target for obtaining a light propagation simulation result that most closely matches a reflected light profile Determining the layer thickness of the simulated light absorbing layer as the layer thickness of the light absorbing layer of the tomographic structure to be observed, and the light propagation simulation in the simulated tomographic structure by reflecting the reflected light profile of the tomographic structure By comparison with the result, the layer thickness of the light absorbing layer can be easily determined. The above observation method can be applied whether or not the measurement light is absorbed in the light absorbing layer.

  Note that the measurement light preferably includes light having a wavelength that is not absorbed by the light-absorbing substance, but when reflected light is obtained without being absorbed even at a wavelength that is absorbed because the light-absorbing layer is thin, The measurement light may be only light having a wavelength that is absorbed by the light absorbing substance. For example, when the light absorbing material absorbs visible light, the measuring light may be visible light if the light absorbing layer is sufficiently thin.

  Another aspect of the present invention is an observation apparatus that observes a tomographic structure of an observation target by optical coherence tomography. The observation device includes a light source that irradiates light to the tomographic structure, a detecting unit that detects interference light between the reflected light from the tomographic structure and the reference light and outputs a detection signal, and a processing unit that processes the detection signal. Prepare. The tomographic structure has a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum.

  The processing unit generates a reflected light profile of the tomographic structure based on the detection signal, and generates a plurality of light beams in the plurality of simulated tomographic structures each having a simulated light absorption layer having a different thickness from the reflected light profile. The propagation simulation results are compared with the thickness of the simulated light absorption layer that is the object of the simulation for obtaining the light propagation simulation results that best match the reflected light profile, and the tomographic structure that is the observation target Is determined as a layer thickness of the light absorbing layer.

  Another aspect of the present invention is a computer program for causing a computer to function as the processing unit.

  According to another aspect of the present invention, a tomographic structure to be observed is irradiated with measurement light substantially not including light having a wavelength absorbed by a light absorbing substance. Even if the tomographic structure has a light absorbing layer, the measurement light is not absorbed, so that a change in the light spectrum can be prevented. Therefore, a constant refractive index of the light absorbing layer can be used for calculating the thickness of the light absorbing layer. The light absorbing material here absorbs light of at least a part of the wavelength, for example, absorbs light of at least a part of the wavelength of the visible light spectrum. The measurement light substantially not containing the light of the wavelength absorbed by the light-absorbing substance is the light that does not contain the light of the wavelength that is absorbed by the light-absorbing substance at all, or the light of the wavelength that is absorbed by the light-absorbing substance Contains only negligible amounts. The term “negligibly negligible in the wavelength of light absorbed by the light-absorbing substance” means, for example, that the intensity of light of the wavelength absorbed by the light-absorbing substance is 10% or less of the peak intensity of the measurement light. . Further, the constant refractive index means a constant refractive index that does not change depending on the distance over which light propagates. If the measurement light is absorbed in the light absorption layer, the thickness of the light absorption layer cannot be calculated with a constant refractive index, but if the measurement light is not absorbed in the light absorption layer, a constant value corresponding to the material of the light absorption material is obtained. Can be used to calculate the layer thickness.

  According to yet another aspect of the present invention, there is provided a method for observing a tomographic structure having a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength of a light source spectrum by optical coherence tomography. Irradiating the target tomographic structure with measurement light including light having a wavelength not absorbed by the light-absorbing substance from a light source, and measuring light based on interference light between the reflected light reflected from the tomographic structure and the reference light. Acquiring a tomographic image of the absorption layer.

  According to yet another aspect of the present invention, there is provided a method for observing a tomographic structure having a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength of a light source spectrum by optical coherence tomography. Irradiating the target tomographic structure with measurement light from a light source, and acquiring a tomographic image of the light absorption layer based on interference light between the measurement light reflected from the tomographic structure and the reference light. The step of obtaining a tomographic structure image of the light absorbing layer includes obtaining a layer thickness of the light absorbing layer based on an optical path length between both interfaces of the light absorbing layer, and calculating a film thickness of the light absorbing layer in the tomographic structure image. This is an observation method including setting the film thickness to the determined value.

  Another aspect of the present invention is an observation apparatus for observing a tomographic structure to be observed by optical coherence tomography, wherein a light source that irradiates light to the tomographic structure, a light reflected from the tomographic structure and a reference light are provided. A detection unit that detects the interference light and outputs a detection signal, and a processing unit that processes the detection signal, wherein the tomographic structure includes a light absorption layer made of a light absorption material that absorbs light of at least a part of the wavelength. The light source is configured to output measurement light including light having a wavelength not absorbed by the light absorbing substance, and the processing unit acquires a tomographic structure image of the light absorption layer based on the detection signal. An observation device configured to perform a process including the following.

  Another aspect of the present invention is an observation apparatus for observing a tomographic structure to be observed by optical coherence tomography, wherein a light source that irradiates light to the tomographic structure, a light reflected from the tomographic structure and a reference light are provided. A detection unit that detects the interference light and outputs a detection signal; and a processing unit that processes the detection signal, wherein the tomographic structure is formed of a light absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum. Having an absorption layer, the processing unit is configured to execute a process including: acquiring a tomographic structure image of the light absorption layer based on the detection signal; and acquiring the tomographic structure image of the light absorption layer. An observing apparatus comprising: determining a layer thickness of the light absorbing layer based on an optical path length between both interfaces of the light absorbing layer, and setting the thickness of the light absorbing layer in the tomographic structure image to the determined thickness. It is.

  Another aspect of the present invention is a computer program for causing a computer to function as the processing unit.

It is a block diagram of an observation device. It is explanatory drawing of the tomographic structure of the observation object, and the light which advances in a tomographic structure. It is a flowchart which shows an observation procedure. It is a figure which shows the example of the observation pattern of a reflected light intensity distribution map. FIG. 9 is an explanatory diagram of a simulation result table. It is a two-dimensional tomographic image of an observation target. It is a light propagation simulation result of an observation object. It is a light propagation simulation result of an observation object. It is a light propagation simulation result of an observation object.

[1. Overview of Embodiment]

(1) The observation method according to the embodiment is a method of observing a tomographic structure to be observed by optical coherence tomography (OCT). OCT may be, for example, any of Spectral Domain (SD) -OCT, Time Domain (TD) -OCT, and Swept Source (SS) -OCT.

  The tomographic structure to be observed is, for example, a tomographic structure of a semiconductor device. The semiconductor device to be observed may be a finished product or an unfinished product in a manufacturing process. The semiconductor device is, for example, a semiconductor device used as a light emitting device. The fault structure may be a single-layer structure or a multilayer structure. The tomographic structure has a light absorbing layer. One or a plurality of light absorbing layers may be included in the tomographic structure. The tomographic structure may have a layer that does not absorb light. The layers included in the tomographic structure (including the light-absorbing layer and / or the layer that does not absorb light) are preferably made of a material having a uniform material constant (dielectric constant / magnetic permeability) and a uniform refractive index. , A crystalline material, an amorphous material, or a polymer material is preferable.

The light absorbing layer is made of a light absorbing material that absorbs light of at least a part of the wavelength. The light absorbing material is, for example, a semiconductor. The light here includes visible light as well as ultraviolet light and infrared light. When the light absorbing layer is made of a light absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum, the measurement light is absorbed in the light absorbing layer. The light absorbing substance may be, for example, a substance that absorbs light of at least a part of the wavelength of the visible light spectrum. The light-absorbing substance may be, for example, a substance having a band gap energy for absorbing light of at least a part of the wavelength of visible light. Since semiconductors have a band gap, many semiconductors absorb at least some wavelengths of visible light.

  “Band gap energy” is the energy difference from the valence band to the conduction band in a crystalline structure such as a semiconductor. "Band gap energy for absorbing light" is a band gap energy equal to or lower than the energy of light. "Absorb light of at least a part of wavelength of visible light" means to absorb all light in the visible light wavelength range or to absorb light of some wavelength in the visible light wavelength range. It is.

  The energy of light corresponding to the wavelength range of visible light is about 1.7 [eV] to about 3 [eV]. Therefore, a substance having a band gap energy of less than about 3 [eV] absorbs light of at least a part of the wavelength of visible light, and a substance having a band gap energy of less than about 1.7 [eV] is visible light. Absorbs all light in the wavelength range Many semiconductors have a band gap energy of less than about 3 [eV] and absorb at least a portion of visible light. For example, the band gap energy of gallium arsenide (GaAs) at room temperature is 1.42 [eV], and gallium arsenide (GaAs) at room temperature absorbs all light in the visible light wavelength range and transmits visible light. do not do.

  The observation method according to the embodiment may include irradiating a tomographic structure to be observed with measurement light including light having a wavelength not absorbed by the light absorbing substance. The measurement light may include light having a wavelength that is absorbed by the light absorbing material. When a substance that absorbs a part of visible light is irradiated with visible light in a wide band, light of a part of the wavelength of the visible light is absorbed, but light of the remaining wavelength is not absorbed. When a substance that absorbs all light in the wavelength region of visible light but does not absorb infrared light is irradiated with visible light including infrared light, visible light can be absorbed but infrared light can be transmitted without being absorbed.

When the measurement light is a broadband visible light, the optical axis resolution of OCT can be increased. Assuming that the optical axis resolution of the OCT is Δz, the measurement light preferably has a center wavelength λ ₀ and a bandwidth Δλ such that a high resolution in which Δz is smaller than 1 μm is obtained. The optical axis resolution Δz of the OCT is determined by the expression Δz = 0.44λ ₀ ² / Δλ. In this equation, the bandwidth Δλ is a full width at half maximum (FWHM).

  Of the measurement light, light having a wavelength that is not absorbed by the light-absorbing substance passes through the light-absorbing layer in the tomographic structure, is reflected at the interface of the light-absorbing layer, and becomes reflected light. Based on the interference light between the reflected light and the reference light, a reflected light profile of the tomographic structure including the interface of the light absorbing layer is generated. Also, of the measurement light, light having a wavelength that is absorbed by the light-absorbing substance is attenuated by light absorption, but if the light-absorbing layer is thin, it is not completely absorbed and may be reflected light.

  The observation method according to the embodiment includes a step of comparing a reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures. The plurality of simulated tomographic structures to be subjected to the light propagation simulation respectively simulate the tomographic structure to be observed, and each has a simulated light absorption layer having a different layer thickness.

  The simulated tomographic structure subjected to the simulation to obtain the light propagation simulation result that best matches the reflected light profile will most closely match the tomographic structure to be observed. Therefore, it is possible to determine the layer thickness of the simulated light absorption layer that has been the target of the simulation for obtaining the simulation result that best matches the thickness of the light absorption layer of the tomographic structure that is the observation target.

(2) The comparison between the reflected light profile and the plurality of light propagation simulation results in the plurality of simulated tomographic structures can be performed, for example, by comparing patterns. In this case, the reflected light profile includes, for example, an observation pattern indicating the reflected light intensity distribution in the depth direction of the tomographic structure to be observed, and the light propagation simulation result includes, for example, the reflected light in the depth direction of the simulated tomographic structure. A prediction pattern indicating an intensity distribution is included.

  By comparing the observed pattern of the reflected light intensity distribution with the predicted pattern indicating the reflected light intensity distribution predicted by the light propagation simulation by using a method such as pattern matching, the observed pattern is best matched from the plurality of predicted patterns. Prediction patterns to be extracted can be extracted. The layer thickness of the simulated light absorption layer in the simulated tomographic structure subjected to the simulation for obtaining the predicted pattern that best matches the observation pattern is determined as the layer thickness of the light absorption layer of the tomographic structure to be observed.

(3) The comparison between the reflected light profile and the plurality of light propagation simulation results in the plurality of simulated tomographic structures may include a comparison of the optical path length instead of or in addition to the pattern comparison. In this case, the reflected light profile includes the observed optical path length between both interfaces of the light absorption layer of the tomographic structure to be observed, and the light propagation simulation result includes the predicted optical path length between both interfaces of the simulated light absorption layer. Shall be considered.

  By comparing the observed optical path length with the predicted optical path length, a predicted optical path length that best matches the observed optical path length can be extracted from the plurality of predicted optical path lengths. The thickness of the simulated light absorption layer in the simulated tomographic structure that was the object of the simulation to obtain the predicted optical path length that best matches the observed optical path length is determined as the thickness of the light absorption layer of the tomographic structure to be observed. You.

(4) The observation device according to the embodiment is for observing a tomographic structure to be observed by optical coherence tomography (OCT). The observation device includes a light source that irradiates light to the tomographic structure, a detecting unit that detects interference light between the reflected light from the tomographic structure and the reference light and outputs a detection signal, and a processing unit that processes the detection signal. Prepare.

  The tomographic structure to be observed has a light-absorbing layer made of a light-absorbing substance that absorbs light of at least part of the wavelength of the light source spectrum. Preferably, the light source is configured to output measurement light including light having a wavelength absorbed by the light absorbing substance and light having a wavelength not absorbed by the light absorbing substance. The light source includes, for example, a halogen lamp, an LED light source, or an SLD light source.

  The processing unit generates a reflected light profile of the tomographic structure based on the detection signal. The processing unit compares the reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures each having a simulated light absorption layer having a different layer thickness. The processing unit determines the thickness of the simulated light absorption layer of the simulated tomographic structure that has been subjected to the simulation to obtain the light propagation simulation result that best matches the reflected light profile, and the light absorption layer of the tomographic structure that is the observation target. Is determined as the layer thickness.

(5) The processing unit includes, for example, a computer including a CPU and a memory. The computer can function as a processing unit by executing a computer program stored in the memory. A computer program for causing a computer to function as a processing unit can be stored in a computer-readable recording medium.

(6) An observation method according to another embodiment includes irradiating a tomographic structure to be observed with measurement light substantially not including light having a wavelength absorbed by the light absorbing substance. Based on the interference light between the measurement light reflected from the tomographic structure and the reference light, the optical path length of the light absorbing layer of the tomographic structure is determined, and based on the optical path length and the refractive index of the light absorbing layer, The layer thickness of the absorbing layer can be determined.

(7) The light source of the observation device according to another embodiment is configured to output measurement light substantially not including light having a wavelength absorbed by the light absorbing substance. The processing unit performs a process of obtaining an optical path length of the light absorption layer. The processing unit performs a process of determining a layer thickness of the light absorption layer based on the obtained optical path length and the refractive index of the light absorption layer. The light source may be constituted by a light emitting device that outputs only light substantially not containing light having a wavelength absorbed by the light absorbing substance, or may output light containing light having a wavelength absorbed by the light absorbing substance. And a filter that cuts light having a wavelength that is absorbed by the light-absorbing substance in the light output from the light-emitting device.

(8) The observation method according to the embodiment is a method for observing, by optical coherence tomography, a tomographic structure having a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum. Irradiating the tomographic structure with measurement light including light having a wavelength that is not absorbed by the light absorbing substance from a light source; and measuring the light based on interference light between the reflected light reflected from the tomographic structure and the reference light. Obtaining a tomographic image of the layer. The tomographic structure image may be a two-dimensional image or a three-dimensional image.

(9) The observation method according to the embodiment is a method for observing, by optical coherence tomography, a tomographic structure having a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum, Irradiating measurement light from a light source to the tomographic structure to be observed, and acquiring a tomographic image of the light absorption layer based on interference light between the measurement light reflected from the tomographic structure and the reference light And obtaining the tomographic structure image of the light absorbing layer includes obtaining a layer thickness of the light absorbing layer based on an optical path length between both interfaces of the light absorbing layer, and obtaining a film thickness of the light absorbing layer in the tomographic structure image. May be an observation method including setting the film thickness to the determined film thickness. In this case, a tomographic image showing the film thickness more accurately is obtained.

(10) The observation device according to the embodiment is an observation device that observes a tomographic structure to be observed by optical coherence tomography, and includes a light source that irradiates light to the tomographic structure, light reflected from the tomographic structure, and reference light. A detection unit that detects the interference light and outputs a detection signal, and a processing unit that processes the detection signal, wherein the tomographic structure is made of a light-absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum. A light absorbing layer, the light source is configured to output measurement light including light having a wavelength not absorbed by the light absorbing substance, and the processing unit acquires a tomographic structure image of the light absorbing layer based on the detection signal. The observation device may be configured to execute a process including:

(11) An observation device according to an embodiment is an observation device that observes a tomographic structure to be observed by optical coherence tomography, and includes a light source that irradiates light to the tomographic structure, light reflected from the tomographic structure, and reference light. A detection unit that detects the interference light and outputs a detection signal, and a processing unit that processes the detection signal, wherein the tomographic structure is made of a light-absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum. Having a light absorbing layer, the processing unit is configured to perform processing including obtaining a tomographic structure image of the light absorbing layer based on the detection signal, and obtaining a tomographic structure image of the light absorbing layer. Is an observation apparatus that includes determining a layer thickness of the light absorbing layer based on an optical path length between both interfaces of the light absorbing layer, and setting the thickness of the light absorbing layer in the tomographic structure image to the determined film thickness. You may.

(12) As described above, the processing unit includes, for example, a computer including a CPU and a memory. The computer can function as a processing unit by executing a computer program stored in the memory. A computer program for causing a computer to function as a processing unit can be stored in a computer-readable recording medium.

[2. Details of Embodiment]
Hereinafter, embodiments will be described in detail with reference to the drawings.

[2.1 First Embodiment]
FIG. 1 shows an observation device 100 according to the first embodiment. The observation apparatus 100 is used for non-destructive and non-contact observation of the tomographic structure of the semiconductor device that is the observation target 30 during or after a semiconductor device manufacturing process, for example. The semiconductor device to be observed 30 is, for example, an optical device such as an optical integrated circuit.

FIG. 2A shows a cross-sectional structure of a semiconductor substrate for an optical device as an observation target 30. The observation target 30 is an Al _0.35 Ga _0.65 AS / GaAs substrate coated with a photoresist. The Al _0.35 Ga _0.65 AS / GaAs substrate has an Al _0.35 Ga _0.65 AS layer 32 immediately below the photoresist layer 31 and a GaAs layer 33 thereunder. In the tomographic structure of the observation target 30, the thickness of the photoresist layer 31 is about 2 μm, and the thickness of the Al _0.35 Ga _0.65 AS layer 32 is about 0.5 μm. The observation device 30 measures the thickness of the photoresist layer 31 and the Al _0.35 Ga _0.65 AS layer 33 and performs tomographic imaging of the observation target 30.

The observation device 100 is configured as an SD-OCT, and includes an optical interference measurement unit 10 and a processing unit 20, as shown in FIG. The optical interference measurement unit 10 includes a light source 11, lenses 12, 13, a splitter 14, a reference mirror 15, a lens 16, and a detection unit 17. The light source 11 is constituted by a halogen lamp. Halogen lamps can output visible light in a wide band. The halogen lamp used was one that outputs a wide-band visible light having a center wavelength λ ₀ : 625 nm and a bandwidth Δλ (FWHM): 260 nm). Since the light source 11 outputs broadband visible light, high-resolution observation of the observation target 30 is possible.

  In the optical interference measurement unit 10, the light output from the light source 11 is split by the splitter 14 into measurement light and reference light. The measurement light is applied to the observation target 30 and becomes reflected light from the observation target 30. The reference light is reflected by the reference mirror 15 and merges with the reflected light. Interference light is generated by the merge of the reflected light and the reference light. The interference light (interference signal) is detected by the detection unit 17. The detection unit 17 outputs an electric signal corresponding to the interference light as a detection signal. The detection signal is provided to the processing unit 20. The detection unit 17 of the embodiment is configured as a spectrometer, and outputs an interference light spectrum as a detection signal.

  The processing unit 20 is configured to include a computer. The computer performs a function as the processing unit 20 by executing a computer program for causing the computer to perform the function of the processing unit 20. The processing unit 20 performs the process of measuring the layer thickness of the observation target and tomographic imaging based on the detection signal.

  FIG. 3 shows an observation procedure using the observation device 100. First, in step S1, the optical interference measurement unit 10 performs optical interference measurement using visible light broadband light to obtain an interference light spectrum. In step S1, the light source 11 irradiates measurement light to a tomographic structure of a semiconductor device to be observed. The detection unit 17 outputs a detection signal (interference light spectrum) based on the interference light and supplies the detection signal to the processing unit 20.

  In step S2, the processing unit 20 converts the interference light spectrum indicated by the detection signal into a wave number domain, and performs a Fourier transform to generate a reflected light profile of the tomographic structure that is the observation target 30. The reflected light profile is, for example, a reflected light intensity profile in the optical axis depth direction. FIG. 4 shows a reflected light intensity profile of the observation target 30 in the optical axis depth direction. This profile is a reflected light intensity distribution pattern in which the horizontal axis represents depth and the vertical axis represents the intensity of reflected light. Hereinafter, the pattern in FIG. 4 is referred to as an “observation pattern”. It should be noted that the reflected light profile only needs to be an analysis result on the reflected light.

As shown in FIG. 2A, reflected light from the observation target 30 is generated at the first interface 30a, the second interface 30b, and the third interface 30c of the observation target 30. That is, each interface 30a, 30b, 30c is a reflection surface. The first interface 30a is the surface of the photoresist layer 33, the second interface 30b is the interface between the photoresist layer 31 and the Al _0.35 Ga _0.65 AS layer 32, and the third interface 30c is , Al _0.35 Ga _0.65 The interface between the AS layer 32 and the GaAs layer 33. Hereinafter, light reflected from the first interface 30a is referred to as first reflected light, light reflected from the second interface 30b is referred to as second reflected light, and light reflected from the third interface 30b is referred to as third reflected light. That.

  In the observation pattern shown in FIG. 4, a peak of the reflected light intensity appears at a depth position corresponding to each of the interfaces 30a, 30b, and 30c. The processing unit 20 can detect the position of each interface 30a, 30b, 30c by detecting the peak of the reflected light intensity in the observation pattern.

  In the observation pattern of FIG. 4, a first peak occurs at a position at a depth of about 19 μm, which corresponds to the first reflected light from the first interface 30a. Similarly, a second peak is generated at a position at a depth of about 23 μm, which corresponds to the second reflected light from the second interface 30b. Similarly, a third peak occurs at a position between the depth of 25 μm and 26 μm, and this corresponds to the third reflected light from the third interface 30c.

  The processing unit 20 obtains an interval between the first peak and the second peak based on the observation pattern of FIG. 4 to obtain the thickness of the photoresist layer 31. In the observation pattern of FIG. 4, the distance between the first peak and the second peak is 3.77 μm. The distance between the first peak and the second peak determined from the observation pattern in FIG. 4 is the optical path length of the photoresist layer 31 (the optical path length from the first interface 30a to the second interface 30b). It is necessary to convert to the actual thickness of the resist layer 31.

Since the photoresist layer 31 is transparent and does not absorb light, the refractive index of the photoresist layer 31 may be used to determine the actual layer thickness from the optical path length. Specifically, the processing unit 20 divides the optical path length of the photoresist layer 31 of 3.77 μm by the refractive index of the photoresist layer 31 of 1.62, and the thickness of the photoresist layer 31 is about 2.33 μm. Is determined. The refractive index 1.62 of the photoresist layer 31 is a value in the case where the light has a center wavelength λ ₀ = 625 nm.

The processing unit 20 may obtain the optical path length of the Al _0.35 Ga _0.65 AS layer 32 (the optical path length from the second interface 30b to the third interface 30c) from the distance between the second peak and the third peak. it can. In the observation pattern of FIG. 4, the optical path length of the Al _0.35 Ga _0.65 AS layer 32 is 2.54 μm. _However, Al _{0.35 Ga 0.65} AS layer 32 are the light-absorbing layer which absorbs most of the visible light, simply by using the refractive _index, Al _{0.35 Ga 0.65} AS layer An exact layer thickness of 32 is not obtained.

Here, the band gap energy of Al _0.35 Ga _0.65 AS is 1.86 [eV], and the wavelength (optical absorption edge) corresponding to this band gap energy = 1.86 [eV] is 667 nm. is there. Al _0.35 Ga _0.65 AS absorbs light having a wavelength of 667 nm or less and transmits light having a wavelength of 667 nm or less without absorbing it. As described above, Al _0.35 Ga _0.65 AS is an opaque substance that absorbs most of the light in the wavelength region of visible light, and Al _0.35 Ga _0.65 AS layer 32 is a light absorbing layer. is there.

Al _0.35 Ga _0.65 AS absorbs most of the visible light, but the measurement light output from the light source 11 according to the embodiment is a broadband light, and as shown in FIG. Also, light having a wavelength larger than 667 nm, which is the optical absorption edge of Al _0.35 Ga _0.65 AS, is included. The measurement light having a wavelength longer than 667 nm is reflected by the third interface 30c without being absorbed by the Al _0.35 Ga _0.65 AS layer 32, and becomes the third reflected light.

On the other hand, the measurement light having a wavelength smaller than 667 nm is absorbed by the Al _0.35 Ga _0.65 AS layer 32. Therefore, as shown in FIG. 2C, the spectrum of the measurement light traveling in the Al _0.35 Ga _0.65 AS layer 32 has a light intensity attenuated in a wavelength range smaller than 667 nm. Since the absorption of the measurement light gradually occurs during the passage through the Al _0.35 Ga _0.65 AS layer 32, the attenuation of the light intensity as shown in FIG. The spectrum of the measurement light (particularly, the peak wavelength of the light) changes during passage through the Al _0.35 Ga _0.65 AS layer 32. As a result, it is difficult to determine the refractive index of the Al _0.35 Ga _0.65 AS layer 32 in a state where the layer thickness is unknown.

The optical path length T from the position _{x 1} to the position _{x 2} of the third interface 30c of the second interface 30b can be expressed by the following equation.
Here, n g _(x) is the group refractive index at any position from position x ₁ to the position x _2, x is the position of the optical axis depth direction. It is clear from the above equation that it is difficult to determine the refractive index of the Al _0.35 Ga _0.65 AS layer 32 when the layer thickness is unknown.

As described above, the layer thickness of the Al _0.35 Ga _0.65 AS layer 32 is not determined in the same manner as the layer thickness of the photoresist layer 31. Therefore, in order to obtain the layer thickness of the Al _0.35 Ga _0.65 AS layer 32, the processing unit 20 of the present embodiment compares the reflected light profile with the light propagation simulation result in step S3.

The light propagation simulation predicts light propagation in a simulated tomographic structure simulating the tomographic structure of the observation target 30. In this light propagation simulation, the same light as the measurement light output from the light source 11 (center wavelength λ ₀ : 625 nm, bandwidth Δλ (FWHM): 260 nm) is irradiated to the simulated tomographic structure. Similar to the tomographic structure of the observation target 30, the simulated tomographic structure includes a layer simulating the photoresist layer 31, a layer simulating the Al _0.35 Ga _0.65 AS layer 32, and a layer simulating the GaAs layer 33. Have. The layer simulating the Al _0.35 Ga _0.65 AS layer 32 as the light absorbing layer is a simulated light absorbing layer.

At the time of the light propagation simulation, the thickness of each simulation layer is set to a predetermined value. For example, the layer thickness simulating the photoresist layer 31 is set to 2 μm, and the layer thickness simulating the Al _0.35 Ga _0.65 AS layer 32 is set to 500 nm. The light propagation simulation is performed a plurality of times while changing the thickness of the simulated light absorption layer simulating the Al _0.35 Ga _0.65 AS layer 32. That is, there are a plurality of simulated tomographic structures to be subjected to the light propagation simulation, and each of the plurality of simulated tomographic structures has a simulated light absorption layer having a different layer thickness. In the light propagation simulation for a plurality of times, the simulation light absorption layer is changed in thickness, for example, by 10 nm.

  The light propagation simulation is performed by, for example, an electromagnetic field analysis using a time domain difference method (FDTD method). By the light propagation simulation for each of the plurality of simulated tomographic structures, a plurality of light propagation simulation results are obtained for each of the plurality of simulated tomographic structures.

  By the light propagation simulation, a predicted pattern indicating the reflected light intensity distribution in the depth direction of the simulated tomographic structure can be obtained as a light propagation simulation result. The prediction pattern is a prediction of a reflected light intensity distribution (observation pattern) obtained when a tomographic structure having a light absorption layer of a known thickness is observed by OCT. Since the intensity of the reflected light from each of the interfaces 30a, 30b, and 30c is determined by the material and the thickness of each of the layers 31, 32, and 33, the intensity of the reflected light from each of the interfaces 30a, 30b, and 20c is different for each of the interfaces 30a, 30b, and 30c. Different. Therefore, the reflected light intensity distribution has a unique pattern corresponding to the thickness of each layer. Therefore, by performing a light propagation simulation on a tomographic structure having a light absorption layer with a known layer thickness and obtaining a predicted pattern of the reflected light intensity distribution in the depth direction of the tomographic structure, from the predicted pattern, It is possible to predict at what depth position in the reflected light intensity distribution and at what intensity the reflected light is generated. Therefore, by comparing the actually observed reflected light intensity distribution (observed pattern) with the predicted pattern, it becomes easy to specify the intensity peak due to the reflected light from each interface (particularly, the third interface 30c).

The processing unit 20 stores a simulation result table 21 illustrated in FIG. 5 in a memory of the processing unit 20. In the table 21 of FIG. 5, a plurality of prediction patterns as a plurality of light propagation simulation results, and an Al _0.35 Ga _0.65 AS layer in the simulated tomographic structure subjected to the light propagation simulation for obtaining each prediction pattern The layer thickness of the (simulated light absorption layer) is associated with the layer thickness.

  The processing unit 20 compares each of the plurality of prediction patterns stored in the table 21 with the observation pattern, for example, by pattern matching, and extracts a prediction pattern that best matches the observation pattern from the plurality of prediction patterns. (Step S3 in FIG. 3). The prediction pattern stored in the table 21 only needs to have information necessary for comparison with the observation pattern, and may be image data of the reflected light intensity distribution, or may be a feature amount of the reflected light intensity distribution. May be indicated.

The processing unit 20 calculates the layer thickness of the Al _0.35 Ga _0.65 AS layer (simulated light absorption layer) in the simulated tomographic structure subjected to the simulation for obtaining the predicted pattern that most closely matches the observed pattern, using the table 21. It is obtained by referring to. The processing unit 20 determines the layer thickness obtained by referring to the table 21 as the layer thickness of the Al _0.35 Ga _0.65 AS layer 32 of the tomographic structure that is the observation target 30 (Step S4 in FIG. 3). .

The processing unit 20 outputs the thicknesses of the Al _0.35 Ga _0.65 AS layer 32 and the photoresist layer 31 determined as described above. The layer thickness may be output as a numerical value, or a two-dimensional or three-dimensional tomographic structure image having the determined layer thickness may be output. FIG. 6 shows a two-dimensional image of the tomographic structure of the observation target 30 obtained by scanning the measurement light in the plane direction of the observation target 30. In the two-dimensional image of the tomographic structure, three interfaces 30a, 30b, and 30c are imaged with three white lines, and the space between the white lines indicates the thickness of each layer. The two-dimensional image of the tomographic structure in FIG. 6 is different from the actual layer thickness because the depth direction is indicated by the optical path length, but the tomographic structure image is obtained by using the layer thickness determined as described above. By correcting each layer thickness in the inside (the position in the depth direction of the white line indicating the interface 30a, 30b, 30c), a tomographic structure indicating the actual layer thickness can be imaged. Qualitative tomographic structure verification, such as confirmation of uniformity of each layer, can be performed even with a tomographic structure image in which the depth direction is indicated by the optical path length. Verification can also be performed.

[2.2 Modification of First Embodiment]
The simulation result table 21 in FIG. 5 has a predicted pattern of the reflected light intensity distribution as a light propagation simulation result, but instead of or in addition to the predicted pattern, a predicted value (predicted value) of the optical path length of the simulated light absorption layer. (Optical path length) may be included as a light propagation simulation result. In this case, in the table 21, the predicted optical path lengths as a plurality of light propagation simulation results are associated with the layer thickness of the simulated light absorption layer which is the target of the light propagation simulation for obtaining the predicted optical path length. Will be. Note that the optical path length of the simulated light absorption layer can be predicted by calculation according to the above-described equation of the optical path length T.

The processing unit 20 calculates a plurality of predicted optical path lengths stored in the table 21 and an optical path length of the light absorption layer (Al _0.35 Ga _0.65 AS layer 32) obtained from the observed pattern (observed optical path length = 2. 54 μm), and a predicted optical path length that best matches the observed optical path length is extracted from a plurality of predicted optical path lengths.

The processing unit 20 refers to the table 21 for the layer thickness of the Al _0.35 Ga _0.65 AS layer (simulated light absorption layer) that has been subjected to the simulation for obtaining the predicted optical path length that most closely matches the observed optical path length. You get by doing. The processing unit 20 determines the layer thickness obtained by referring to the table 21 as the layer thickness of the Al _0.35 Ga _0.65 AS layer 32 of the tomographic structure that is the observation target 30.

[2.3 Consideration on Multiple Reflected Light in Photoresist Layer]
In the observation pattern of FIG. 4, a peak (fourth peak) is recognized at a position 0.69 μm away from the third peak toward the longer wavelength side. The fourth peak is a peak due to the double reflection of the second reflected light on the photoresist layer 31, as shown in FIG. The distance from the second peak to the fourth peak is 3.23 μm, which is almost the same as the distance from the first peak to the second peak 3.77 μm. Therefore, the fourth peak is a double reflection of the second reflected light. It can be seen that the peak is due to

  The observation of the peak due to the double reflection of the second reflected light on the photoresist layer 31 means that the double reflected light is generated on the photoresist layer in the light propagation simulation results shown in FIGS. It is clear from.

7 to 9 show simulation results for the simulated tomographic structure. As described above, the simulated tomographic structure is a layer simulating the photoresist layer 31 and a layer simulating the Al _0.35 Ga _0.65 AS layer 32. , And a layer simulating the GaAs layer 33. 7 shows a case where the layer simulating the Al _0.35 Ga _0.65 AS layer 32 has a layer thickness of 500 nm, and FIG. 8 shows a layer having a layer thickness simulating the Al _0.35 Ga _0.65 AS layer 32 of 1000 nm. FIG. 9 shows a simulation result when the thickness of the layer simulating the Al _0.35 Ga _0.65 AS layer 32 is 1500 nm. In each of the simulated tomographic structures, the layer simulating the photoresist layer 31 had a uniform thickness of 2 μm.

7 to 9, the horizontal axis represents the light propagation length, and the vertical axis represents the optical signal intensity. 7 to 9 show the intensity of the optical signal (the intensity of the attenuated light during propagation) at each position of the light propagation path, not the light reflected from the interface. As shown in FIGS. 7 to 9, the distance between the light at the second interface and the light at the third interface changes according to the difference in the layer thickness of the layer simulating the Al _0.35 Ga _0.65 AS layer 32. On the other hand, the double reflected light in the photoresist layer always occurs at the same position.

  As shown in FIG. 9, when the propagation length of light at the third interface and the propagation length of double reflected light at the photoresist layer are significantly different, when reflected light is observed, The three reflected light and the double reflected light in the photoresist layer are observed at different depth positions. On the other hand, as shown in FIG. 8, when the propagation length of light at the third interface and the propagation length of double reflected light in the photoresist layer substantially match, or as shown in FIG. When the difference between the propagation length of the light and the propagation length of the double reflected light in the photoresist layer is small, as shown in FIG. 4, when the reflected light is observed, the distance from the third interface 30c from the third interface 30c is increased. The three reflected light (third peak) and the double reflected light (peak due to double reflection of the second reflected light) in the photoresist layer are observed overlapping at different depth positions. In this case, it may be difficult to accurately determine the depth position of the third peak due to the influence of the peak due to the double reflection of the second reflected light.

If it is difficult to obtain the depth position of the third peak with high accuracy, the observation optical path length from the second peak to the third peak cannot be obtained with high accuracy, and “2.2 Modification of First Embodiment” , The accuracy of the thickness of the Al _0.35 Ga _0.65 AS layer 32 is reduced.

  On the other hand, as shown in FIG. 5, in the case of the method of comparing the observed pattern and the predicted pattern, the predicted pattern is generated by a light propagation simulation in which the multiple reflected light is considered. By comparing the observation pattern including the reflected light with the predicted pattern including the multiple reflected light, the film thickness can be accurately determined even if there is the multiple reflected light without obtaining the optical path length. When the influence of the multiple reflection light is small, the film thickness can be accurately determined even from the optical path length as in “2.2 Modification of First Embodiment”. In addition, in the observation pattern, not only peaks due to reflected light from each interface but also peaks due to multiple reflected light and noise due to noise as described above may occur. Therefore, it may be necessary to determine a peak due to light reflected from each interface. In this regard, in the predicted pattern, since the depth position and the intensity of the reflected light from each of the interfaces 30a, 30b, and 30c (particularly, the third interface 30c) are obtained, the reflected light from each of the interfaces is observed in the observation pattern. The peak closest to the predicted position of the peak (the predicted depth position and the predicted intensity) can be easily determined as the peak due to the reflected light from each interface.

[2.4 Second Embodiment]
The observation device 100 according to the second embodiment is the same as the observation device 100 according to the first embodiment except for the processing contents of the light source 11 and the processing unit 20. The measurement light output from the light source 11 does not substantially include light having a wavelength absorbed by the Al _0.35 Ga _0.65 AS layer 32. Since the optical absorption edge of Al _0.35 Ga _0.65 AS is 667 nm, the measurement light does not substantially include light having a wavelength of 667 nm or less and is light having a wavelength larger than 667 nm. The light source 11 outputs, for example, infrared light. More specifically, the light source 11 has a center wavelength λ ₀ = 900 nm and a bandwidth Δλ = 500 nm. In this case, the irradiation light output from the light source 11 includes light having a wavelength smaller than 667 nm, but is very small. Even if the light is absorbed by the Al _0.35 Ga _0.65 AS layer 32, the irradiation light is irradiated. The spectrum of the light hardly changes.

Therefore, the processing unit 20 _obtains the optical path length of the Al _0.35 Ga _0.65 AS layer 32 from the observation pattern, and calculates the obtained optical path length as the refractive index of the measurement light in the Al _0.35 Ga _0.65 AS layer 32. By division, the thickness of the Al _0.35 Ga _0.65 AS layer 32 can be determined.

Note that, even when the light source 11 that outputs measurement light substantially not including light having a wavelength absorbed by the Al _0.35 Ga _0.65 AS layer 32 is used, the film thickness is determined by comparing the observed pattern with the predicted pattern. May be determined.

[3. Appendix]
The present invention is not limited to the above embodiment, and various modifications are possible.

Reference Signs List 10 Optical interference measuring unit 11 Light source 17 Detecting unit 20 Processing unit 30 Observation target 100 Observation device

Claims (9)

A method of observing a fault structure to be observed by optical coherence tomography,
Irradiating measurement light from a light source to the tomographic structure having a light absorbing layer made of a light absorbing material absorbing light of at least a part of the wavelength of the light source spectrum,
Based on interference light between the measurement light and the reference light and the reflected light reflected from the tomographic structure, generating a reflected light profile of the tomographic structure,
Comparing the reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures each having a simulated light absorption layer having a different layer thickness,
The layer thickness of the simulated light absorption layer that was the object of the simulation to obtain the light propagation simulation result that best matches the reflected light profile, as the layer thickness of the light absorption layer that the tomographic structure that is the observation target has Deciding;
Only including,
The reflected light profile is a reflected light intensity distribution in a depth direction of the tomographic structure that is the observation target, and an observation pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the light absorbing layer. Including
The light propagation simulation result is a reflected light intensity distribution in the depth direction of the simulated tomographic structure and includes a predicted pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the simulated light absorption layer,
The observing method , wherein the comparing step includes comparing the observed pattern and the predicted pattern . The reflected light profile includes an observation optical path length between both interfaces of the light absorption layer of the tomographic structure to be observed,
The light propagation simulation result includes a predicted optical path length between both interfaces of the simulated light absorption layer,
The observation method according to claim 1, wherein the comparing step includes comparing the observed optical path length with the predicted optical path length. An observation device that observes a fault structure of an observation target by optical coherence tomography,
A light source for irradiating light to the tomographic structure;
A detection unit that detects a reflected light from the tomographic structure and interference light between the reference light and outputs a detection signal,
A processing unit that processes the detection signal;
With
The tomographic structure has a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum,
The processing unit includes:
Based on the detection signal, generating a reflected light profile of the tomographic structure,
Comparing the reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures each having a simulated light absorption layer having a different layer thickness,
The layer thickness of the simulated light absorption layer that was the object of the simulation to obtain the light propagation simulation result that best matches the reflected light profile, as the layer thickness of the light absorption layer that the tomographic structure that is the observation target has To decide,
It is configured to perform processing including,
The reflected light profile is a reflected light intensity distribution in a depth direction of the tomographic structure that is the observation target, and an observation pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the light absorbing layer. Including
The light propagation simulation result is a reflected light intensity distribution in the depth direction of the simulated tomographic structure and includes a predicted pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the simulated light absorption layer,
An observation device , wherein comparing the reflected light profile with the light propagation simulation result includes comparing the observed pattern with the predicted pattern . A computer program for causing a computer to function as the processing unit according to claim 3 .   A method of observing a fault structure to be observed by optical coherence tomography,
  Irradiating measurement light from a light source to the tomographic structure having a light absorbing layer made of a light absorbing material absorbing light of at least a part of the wavelength of the light source spectrum,
  Based on interference light between the measurement light and the reference light and the reflected light reflected from the tomographic structure, generating a reflected light profile of the tomographic structure,
  Comparing the reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures each having a simulated light absorption layer having a different layer thickness,
  The layer thickness of the simulated light absorption layer that was the object of the simulation to obtain the light propagation simulation result that best matches the reflected light profile, as the layer thickness of the light absorption layer that the tomographic structure that is the observation target has Deciding;
  Including
  The reflected light profile includes an observation optical path length between both interfaces of the light absorption layer of the tomographic structure to be observed,
  The light propagation simulation result includes a predicted optical path length between both interfaces of the simulated light absorption layer,
  The step of comparing includes comparing the observed optical path length with the predicted optical path length.
  Observation method. An observation device that observes a fault structure of an observation target by optical coherence tomography,
A light source for irradiating light to the tomographic structure;
A detection unit that detects a reflected light from the tomographic structure and interference light between the reference light and outputs a detection signal,
A processing unit that processes the detection signal;
With
The tomographic structure has a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength of the light source spectrum,
The processing unit includes:
Based on the detection signal, generating a reflected light profile of the tomographic structure,
Comparing the reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures each having a simulated light absorption layer having a different layer thickness,
The layer thickness of the simulated light absorption layer that was the object of the simulation to obtain the light propagation simulation result that best matches the reflected light profile, as the layer thickness of the light absorption layer that the tomographic structure that is the observation target has To decide,
Is configured to perform a process including
The reflected light profile includes an observation optical path length between both interfaces of the light absorption layer of the tomographic structure to be observed,
The light propagation simulation result includes a predicted optical path length between both interfaces of the simulated light absorption layer,
An observation device, wherein comparing the reflected light profile with the result of the light propagation simulation includes comparing the observed optical path length with the predicted optical path length .   A computer program for causing a computer to function as the processing unit according to claim 6.
M A method of observing a tomographic structure having a light absorbing layer made of a light absorbing material that absorbs light of at least a part of the wavelength by optical coherence tomography,
Irradiating the tomographic structure to be observed with measurement light substantially not including light having a wavelength absorbed by the light absorbing substance,
Based on interference light between the measurement light and the reference light and the reflected light reflected from the tomographic structure, generating a reflected light profile of the tomographic structure,
Comparing the reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures each having a simulated light absorption layer having a different layer thickness,
The layer thickness of the simulated light absorption layer that was the object of the simulation to obtain the light propagation simulation result that best matches the reflected light profile, as the layer thickness of the light absorption layer that the tomographic structure that is the observation target has Deciding;
Including
The reflected light profile is a reflected light intensity distribution in a depth direction of the tomographic structure that is the observation target, and an observation pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the light absorbing layer. Including
The light propagation simulation result is a reflected light intensity distribution in the depth direction of the simulated tomographic structure and includes a predicted pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the simulated light absorption layer,
The observing method , wherein the comparing step includes comparing the observed pattern and the predicted pattern . An observation device that observes a fault structure of an observation target by optical coherence tomography,
A light source for irradiating light to the tomographic structure;
A detection unit that detects a reflected light from the tomographic structure and interference light between the reference light and outputs a detection signal,
A processing unit that processes the detection signal;
With
The tomographic structure has a light-absorbing layer made of a light-absorbing material that absorbs at least a part of the wavelength,
The light source is configured to output measurement light substantially free of light having a wavelength absorbed by the light absorbing material,
The processing unit includes:
Based on the detection signal, generating a reflected light profile of the tomographic structure,
Comparing the reflected light profile with a plurality of light propagation simulation results in a plurality of simulated tomographic structures each having a simulated light absorption layer having a different layer thickness,
The layer thickness of the simulated light absorption layer that was the object of the simulation to obtain the light propagation simulation result that best matches the reflected light profile, as the layer thickness of the light absorption layer that the tomographic structure that is the observation target has To decide,
Is configured to perform a process including
The reflected light profile is a reflected light intensity distribution in a depth direction of the tomographic structure that is the observation target, and an observation pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the light absorbing layer. Including
The light propagation simulation result is a reflected light intensity distribution in the depth direction of the simulated tomographic structure and includes a predicted pattern indicating a reflected light intensity distribution including peaks of reflected light from both interfaces of the simulated light absorption layer,
An observation device , wherein comparing the reflected light profile with the light propagation simulation result includes comparing the observed pattern with the predicted pattern .