Bottom surface roughness measuring method

Abstract

The invention provides a method for measuring the roughness of a bottom surface, and the method comprises the steps: enabling detection light to be vertically incident from a second surface of a to-be-detected structure, obtaining reflected light, and enabling a part of a detection light beam to be incident to the bottom surface of a groove structure; dividing the spectrum of the reflected light into a first sub-spectrum and a second sub-spectrum; the first sub-spectrum is a spectrum corresponding to a first wave band, which does not enter the interior of the to-be-detected structure and is directly reflected by the second surface, in the probe light, and the second sub-spectrum is a spectrum corresponding to a second wave band, which enters the interior of the to-be-detected structure and is reflected by the interior of the to-be-detected structure, in the probe light; obtaining the roughness of the second surface according to the first sub-spectrum; obtaining the bottom thickness of the groove structure according to the second sub-spectrum, wherein the bottom thickness is the distance between the bottom surface of the groove structure and the second surface; and obtaining the bottom surface roughness of the groove structure according to the bottom thickness of the to-be-detected structure, the roughness of the second surface and the spectrum of the reflected light.

Description

Technical Field

[0001] This invention relates to the field of optical measurement, and in particular to a method for measuring the roughness of a bottom surface. Background Technology

[0002] In the semiconductor manufacturing field, the bottom roughness of deep silicon etched structures is a key parameter affecting device performance, and its importance is mainly reflected in three aspects: First, in terms of electrical performance, for example, the bottom roughness of capacitive MEMS devices (such as RF MEMS switches) will change the effective electrode spacing, affecting the capacitance value and signal integrity; Second, in terms of fluid properties, for example, in microfluidic chips, according to the Navier-Stokes equation, a 20% increase in bottom roughness can increase the microchannel flow resistance by 15-30%, which has a significant impact on the performance of microfluidic chips; Third, in terms of optical performance, for example, for optical MEMS (such as diffraction gratings), experiments show that when Ra>50nm, the diffraction efficiency of optical MEMS devices (such as diffraction gratings) can decrease by up to 40%.

[0003] At the process monitoring level, the bottom roughness in the DRIE process directly reflects the uniformity of ion bombardment (typically, Ra should be controlled below 30 nm). Simultaneously, the uniformity of ALD / PVD film coverage at the bottom of the deep trench is strongly correlated with the bottom roughness (for every 10 nm increase in roughness, the film porosity increases by approximately 5%). In terms of reliability, a rough surface can form microcrack initiation points. Finite element analysis shows that doubling the surface roughness reduces fatigue life by 35-50%. In devices such as micro-accelerometers, measured data indicates that when Ra > 40 nm, the probability of structural layer adhesion increases threefold.

[0004] Currently, the industry has strict standards for bottom roughness. Taking Bosch processes as an example, inertial devices require Ra < 20 nm, biosensors require Ra < 50 nm, and optical components require Ra < 10 nm. However, existing measurement technologies have significant shortcomings: probe technologies such as surface profilometers and atomic force microscopes (AFM) are physically limited and cannot reach the bottom of deep trenches; scanning electron microscopes (SEM) and transmission electron microscopes (TEM) require destructive sample preparation and cannot achieve non-destructive measurement; confocal microscopes and white light interferometric profilometers experience a sharp drop in signal-to-noise ratio in structures with an aspect ratio greater than 10:1; and while spectral ellipsometry and spectral reflectance techniques can measure some geometric parameters, they are difficult to obtain information on the roughness of the inner wall. These technical deficiencies severely restrict the quality control and performance optimization of deep silicon etching processes. Therefore, there is an urgent need to develop a new method that can achieve non-destructive, rapid, and accurate measurement of bottom roughness in high aspect ratio structures. Summary of the Invention

[0005] In view of this, embodiments of the present invention provide a method for measuring bottom surface roughness. The method is applicable to measuring the bottom surface roughness of a groove structure formed on a structure to be inspected. The groove structure extends from a first surface of the structure to be inspected to a second surface of the structure to be inspected, where the second surface is the surface of the structure to be inspected opposite to the first surface. The measurement method includes:

[0006] The probe light is incident perpendicularly from the second surface of the structure to be detected and reflected light is obtained. A portion of the probe light beam can be incident on the bottom surface of the groove structure.

[0007] The spectrum of the reflected light is divided into a first sub-spectrum and a second sub-spectrum; the first sub-spectrum is the spectrum corresponding to the first band of the probe light that does not enter the interior of the structure to be detected and is directly reflected by the second surface; the second sub-spectrum is the spectrum corresponding to the second band of the probe light that enters the interior of the structure to be detected and is reflected by the interior of the structure to be detected.

[0008] The roughness of the second surface is obtained based on the first sub-spectrum;

[0009] The bottom thickness of the groove structure is obtained from the second sub-spectrum, and the bottom thickness is the distance between the bottom surface of the groove structure and the second surface;

[0010] The bottom surface roughness of the groove structure is obtained based on the bottom thickness of the structure to be tested, the roughness of the second surface, and the spectrum of the reflected light.

[0011] According to an embodiment of the present invention, the bottom surface roughness of the groove structure is obtained based on the bottom thickness of the structure to be detected, the roughness of the second surface, and the spectrum of the reflected light, including:

[0012] The intensity of the reflected light is obtained from the spectrum of the reflected light.

[0013] The actual reflectivity of the structure to be detected to the probe light is obtained based on the intensity of the reflected light and the intensity of the incident light.

[0014] The bottom surface roughness of the groove structure is obtained based on the actual reflectivity, the bottom thickness of the structure to be tested, and the roughness of the second surface.

[0015] According to an embodiment of the present invention, the measurement method further includes:

[0016] The surface roughness of the bottom surface of the groove structure is evaluated.

[0017] According to an embodiment of the present invention, the roughness of the bottom surface of the groove structure is evaluated, including:

[0018] Based on the structure to be tested, multiple optical propagation models with different bottom surface roughness are established, and the simulated reflectivity of each of the multiple optical propagation models under the probe light is determined.

[0019] The simulated reflectance with the smallest deviation from the actual reflectance is selected as the target simulated reflectance.

[0020] The bottom surface roughness of the groove structure is evaluated based on the bottom surface roughness of the optical propagation model corresponding to the target simulated reflectivity.

[0021] According to an embodiment of the present invention, the material of the structure to be detected is silicon, and the wavelength range of the probe light is 350nm~1700nm.

[0022] According to an embodiment of the present invention, the light spot of the second band is located on the bottom surface of the groove structure, and the diameter of the light spot is smaller than the opening width of the groove structure.

[0023] According to an embodiment of the present invention, the method further includes downsampling the spectrum of the reflected light.

[0024] According to an embodiment of the present invention, obtaining the bottom thickness of the structure to be detected based on the second sub-spectrum includes:

[0025] A fast Fourier transform is performed on the second sub-spectrum, and the spectral oscillation frequency is extracted. Based on the spectral oscillation frequency and the refractive index dispersion relationship of the material to be tested, the bottom thickness of the structure to be tested is calculated.

[0026] According to an embodiment of the present invention, dividing the spectrum of the reflected light into a first sub-spectrum and a second sub-spectrum includes:

[0027] Based on the oscillation characteristics of the spectral curve of the reflected light, the cutoff wavelength that does not penetrate the structure to be detected is determined;

[0028] The spectrum of the reflected light is divided into a first sub-spectrum and a second sub-spectrum based on the cutoff wavelength.

[0029] According to an embodiment of the present invention, the material of the structure under test is a conductor, a semiconductor, or an insulator.

[0030] According to the detection method provided in this embodiment of the invention, the probe light is incident perpendicularly from the second surface of the structure to be tested. By utilizing the principle of perpendicular incident reflection, it replaces contact-based measurements such as AFM / profilometers, achieving non-contact measurement of the bottom surface roughness of deep groove structures. The detection method provided in this embodiment of the invention employs the principle of optical reflection, completing the measurement without damaging the sample, avoiding the need for slicing samples as required by traditional SEM / TEM, and maintaining the integrity of the structure to be tested. According to the embodiments of the invention, in conventional methods, the measurement method using a traditional confocal / white light interferometer is limited by the Abbe diffraction limit. When the aspect ratio is >10:1, multiple reflections of the incident light within the deep groove lead to energy attenuation of over 90%. The method of this embodiment of the invention avoids the signal attenuation problem of confocal / white light interferometers when the aspect ratio is >10:1, making it suitable for measuring the bottom surface roughness of groove structures with large aspect ratios (>10:1). Attached Figure Description

[0031] The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments of the invention with reference to the accompanying drawings, in which:

[0032] Figure 1 A cross-sectional view of the roughness of the structure to be tested according to an embodiment of the present invention is shown.

[0033] Figure 2 A flowchart of a roughness measurement method according to an embodiment of the present invention is shown.

[0034] Figure 3 The spectrum of reflected light according to an embodiment of the present invention is shown.

[0035] Figure 4 It shows Figure 3 The spectrum of the 350~750nm band.

[0036] Figure 5 It shows Figure 3 The spectrum of the 1150~1700nm band. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0038] The roughness measurement method of this invention is applicable to measuring the bottom surface roughness of a groove structure formed on the structure to be tested.

[0039] Figure 1 A cross-sectional view of the roughness of the structure to be tested according to an embodiment of the present invention is shown.

[0040] like Figure 1As shown, the groove structure 11 extends from the first surface 12 of the structure to be tested 1 to the second surface 13 of the structure to be tested, and the second surface 13 is the surface of the structure to be tested 1 opposite to the first surface 12. Figure 1 A flowchart of a roughness measurement method according to an embodiment of the present invention is shown.

[0041] Figure 2 A flowchart of a roughness measurement method according to an embodiment of the present invention is shown.

[0042] like Figures 1-2 As shown, the measurement method includes operations S1 to S5.

[0043] In operation S1, the probe light is incident perpendicularly from the second surface of the structure to be detected and reflected light is obtained. A portion of the probe light beam can be incident on the bottom surface of the groove structure.

[0044] In operation S2, the spectrum of the reflected light is divided into a first sub-spectrum and a second sub-spectrum. The first sub-spectrum is the spectrum corresponding to the first band of the probe light that is not inside the structure to be detected and is directly reflected by the second surface. The second sub-spectrum is the spectrum corresponding to the second band of the probe light that is able to enter the structure to be detected and is reflected by the interior of the structure to be detected.

[0045] In operation S3, the roughness of the second surface is obtained based on the first sub-spectrum;

[0046] In operation S4, the bottom thickness of the structure to be detected is obtained based on the second sub-spectrum. The bottom thickness is the distance between the bottom surface of the groove structure and the second surface.

[0047] In operation S5, the bottom surface roughness of the groove structure is obtained based on the bottom thickness of the structure to be detected, the roughness of the second surface, and the spectrum of the reflected light.

[0048] According to the detection method provided in the embodiments of the present invention, the first wavelength band does not enter the interior of the receiving structure but is reflected directly from the second surface, carrying information about the roughness of the second surface. The second wavelength band can enter the interior of the structure to be detected and return from the bottom surface of the groove structure, thus carrying information about the bottom surface roughness and bottom thickness of the microgroove structure. Therefore, the roughness of the second surface can be obtained using the first sub-spectrum, the bottom thickness can be obtained using the second sub-spectrum, and the bottom surface roughness of the groove structure can be obtained using the roughness of the second surface, the bottom thickness, and the spectrum of the reflected light, thereby realizing the measurement of the bottom surface roughness.

[0049] According to the detection method provided in this embodiment of the invention, the probe light is incident perpendicularly from the second surface of the structure to be tested. By utilizing the principle of perpendicular incident reflection, it replaces contact-based measurements such as AFM / profilometers, achieving non-contact measurement of the bottom surface roughness of deep groove structures. The detection method provided in this embodiment of the invention employs the principle of optical reflection, completing the measurement without damaging the sample, avoiding the problems of traditional SEM / TEM requiring slicing and maintaining the integrity of the structure to be tested. According to the embodiments of the invention, in conventional methods, the measurement method using a traditional confocal / white light interferometer is limited by the Abbe diffraction limit. When the aspect ratio is >10:1, multiple reflections of the incident light within the deep groove lead to energy attenuation of over 90%. The method of this embodiment of the invention avoids the signal attenuation problem of confocal / white light interferometers when the aspect ratio is >10:1, making it suitable for measuring the bottom surface roughness of groove structures with large aspect ratios (>10:1).

[0050] According to embodiments of the present invention, the material of the detection structure can be, for example, a conductor, a semiconductor, or an insulator. Further, the material of the structure to be detected is silicon, the structure to be detected is a silicon wafer, and the wavelength range of the probe light is 350 nm to 1700 nm.

[0051] According to an embodiment of the present invention, when the material of the structure to be detected is silicon and the structure to be detected is a silicon wafer, selecting a wavelength range of 350nm to 1700nm for the probe light can make a portion of the probe light (the first band) reflected by the second surface (i.e., the lower surface) of the silicon wafer, while the other portion (the second band) can enter the interior of the silicon wafer.

[0052] In operation S2, the spectrum of the reflected light is divided into a first sub-spectrum and a second sub-spectrum, including operations S21 to S22.

[0053] In operation S21, the cutoff wavelength that does not penetrate the structure to be detected is determined based on the oscillation characteristics of the spectral curve of the reflected light.

[0054] In operation S22, the spectrum of the reflected light is divided into a first sub-spectrum and a second sub-spectrum according to the cutoff wavelength.

[0055] According to the detection method provided in the embodiments of the present invention, the first waveband, since it does not enter the interior of the structure to be detected, is directly reflected by the second surface, resulting in a first sub-reflected light. A portion of the beam in the second waveband enters the interior of the structure to be detected and is reflected by the bottom surface of the groove structure inside the structure, resulting in a second sub-reflected light. Another portion of the beam is directly reflected by the second surface, resulting in a third sub-reflected light. The second and third sub-reflected lights have the same wavelength and an optical path difference, causing interference outside the structure to be detected. The spectrum of the interfering light is the second sub-spectrum corresponding to the second waveband. Therefore, the cutoff wavelength that does not penetrate the structure to be detected can be determined by the oscillation characteristics of the reflected light's spectral curve (i.e., whether an interference curve is generated).

[0056] Figure 3 The spectrum of reflected light according to an embodiment of the present invention is shown.

[0057] like Figure 3 As shown, when the structure to be detected is a silicon wafer, the spectrum of reflected light can be divided into two segments. The first segment has no interference signal, while the second segment does. The second surface roughness of the silicon wafer is 5 nm, and the bottom thickness of the silicon wafer is 50 μm. The boundary between the first and second segments is 900 nm. It should be noted that the boundary between the first and second segments is not fixed and varies depending on the bottom thickness. When the bottom thickness is thinner, the spectral interference will shift to shorter wavelengths. For example, when the bottom thickness is 5 μm, the boundary may be at a wavelength of 500 nm or below.

[0058] Figure 4 It shows Figure 3 The spectrum of the 350~750nm band.

[0059] like Figure 4 As shown, in the 350~750nm wavelength band, there is no interference phenomenon in the reflected light, and the incident light in the 350~750nm wavelength band is directly reflected by the second surface.

[0060] Figure 5 It shows Figure 3 The spectrum of the 1150~1700nm band.

[0061] like Figure 5 As shown, interference occurs in the reflected light in the 1150~1700nm band, and the incident light in the 1150~1700nm band enters the structure to be tested.

[0062] According to an embodiment of the present invention, in operation S3, obtaining the bottom thickness of the structure to be detected based on the second sub-spectrum includes operations S31 to S32.

[0063] In operation S31, a fast Fourier transform is performed on the second sub-spectrum, and the spectral oscillation frequency is extracted. In operation S32, the bottom thickness of the structure under test is calculated based on the spectral oscillation frequency and the refractive index dispersion relationship of the material under test.

[0064] According to embodiments of the present invention, when the structure to be detected is a silicon wafer, the bottom thickness of the groove structure on the silicon wafer can be obtained by analyzing the near-infrared wavelength range of the reflected light spectral data. For example, by using fast Fourier analysis and spectral curve regression of near-infrared spectroscopy, an initial value of 50 μm for the bottom thickness can be obtained.

[0065] According to an embodiment of the present invention, in operation S5, the bottom surface roughness of the groove structure is obtained based on the bottom thickness of the structure to be detected, the roughness of the second surface, and the spectrum of the reflected light, including operations S51 to S53.

[0066] In operation S51, the intensity of the reflected light is obtained based on the spectrum of the reflected light.

[0067] During operation S52, based on the intensity of the reflected light... and the intensity of incident light The actual reflectivity of the structure under test to the probe light is obtained. .

[0068] In operation S53, the bottom surface roughness of the groove structure is obtained based on the actual reflectivity, the bottom thickness of the structure to be tested, and the roughness of the second surface.

[0069] According to an embodiment of the present invention, the actual reflectivity The bottom thickness of the structure to be tested Roughness of the second surface Obtain the bottom surface roughness of the groove structure Actual reflectivity It can be expressed as equation (1).

[0070] (1)

[0071] Among them, I λo I represents the intensity of the returning light wave. λi Let f represent the intensity of the incident light wave, and f represent the functional relationship.

[0072] According to equation (1), the actual reflectivity and the bottom thickness of the structure to be tested Roughness of the second surface and the surface roughness of the groove structure. Therefore, the bottom surface roughness of the groove structure is related. It can be obtained from the actual reflectivity, the bottom thickness of the structure under test, and the roughness of the second surface.

[0073] According to an embodiment of the present invention, the determination process of equation (1) is described below. Taking a silicon wafer as the structure to be tested as an example, firstly, based on the equivalent medium approximation theory, the second surface of the structure to be tested is equivalently regarded as a uniform dielectric layer, that is, a rough layer on the second surface or a rough layer on the lower surface. The bottom surface of the groove structure is equivalently regarded as a uniform dielectric layer, that is, a rough layer on the bottom surface of the groove structure. Then, a multilayer dielectric model is established: air dielectric / rough layer on the lower surface of the structure to be tested / silicon wafer / rough layer on the bottom of the groove structure. A mathematical model is established using the optical propagation matrix method of multilayer dielectric. This mathematical model is equation (1).

[0074] Specifically, when the structure to be tested is a silicon wafer, the spectral data of the reflected light, the roughness of the lower surface of the wafer, the bottom thickness of the groove structure, and the refractive index of each medium can be substituted into equation (1) and the bottom surface roughness of the groove structure can be determined by least squares, iterative fitting, and other methods.

[0075] According to an embodiment of the present invention, the measurement method further includes evaluating the surface roughness of the bottom surface of the groove structure. Evaluating the surface roughness of the bottom surface of the groove structure includes the following steps.

[0076] Step A: Establish optical propagation models with different bottom surface roughness based on the structure to be tested, and determine the simulated reflectivity of each of the multiple optical propagation models under the probe light.

[0077] Step B: Select the simulated reflectance with the smallest deviation from the actual reflectance as the target simulated reflectance.

[0078] Step C: Evaluate the bottom surface roughness of the groove structure based on the bottom surface roughness of the optical propagation model corresponding to the target simulated reflectivity.

[0079] According to an embodiment of the present invention, an optical propagation model analysis method is used to establish an optical propagation model based on the bottom surface of the groove with different bottom surface roughness. Based on a specific incident angle and the geometric parameters of the groove, a diffraction analysis is performed to establish the optical propagation model.

[0080] A fixed-angle incident light beam is perpendicularly incident on the lower surface (second surface) of the structure under test. The visible light component of the beam returns from the lower surface, carrying roughness information. The near-infrared component penetrates the lower surface and reaches the bottom of the groove. The light wave returning from the bottom of the groove carries information about the bottom roughness and thickness of the groove structure. The actual reflectivity curve of the structure under test is measured with the incident light wavelength as the independent variable. By analyzing the changes in the actual reflectivity curve and adapting it to the established optical propagation model of the groove bottom surface with different roughnesses, the roughness of the groove structure's bottom surface can be evaluated.

[0081] According to an embodiment of the present invention, the light spot of the second band is located on the bottom surface of the groove structure, and the diameter of the light spot is smaller than the opening width of the groove structure.

[0082] According to embodiments of the present invention, the method further includes downsampling the spectrum of the reflected light. This downsampling can be achieved, for example, by reducing the pixel resolution of the spectral curve of the reflected light. For instance, a moving average method can be used to achieve a spectral pixel resolution of 10 nm / pixel, thereby improving the signal-to-noise ratio and computational efficiency.

[0083] This invention provides an optical measurement method for the bottom roughness of deep silicon etched structures. Combining the advantages of mature optical measurement technology, it establishes an optical propagation model of the microgroove bottom and calculates the bottom roughness by measuring the reflectivity of the returned light wave. This forms an optical measurement method for microgroove bottom roughness based on reflectivity curve analysis, which has high measurement accuracy and enables non-destructive and rapid measurement. This invention can be used for bottom roughness measurement of microgrooves made of materials such as conductors, semiconductors, and insulators.

[0084] The above specific embodiments further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for measuring a bottom surface roughness, the method being adapted to measure a bottom surface roughness of a groove structure formed on a structure to be detected, the groove structure extending from a first surface of the structure to be detected to a second surface of the structure to be detected, the second surface being a surface of the structure to be detected opposite to the first surface, the method comprising: vertically incident a probe light from the second surface of the structure to be detected, and obtaining a reflected light, a part of a probe light beam being capable of being incident to a bottom surface of the groove structure; splitting a spectrum of the reflected light into a first sub-spectrum and a second sub-spectrum, the first sub-spectrum being a spectrum corresponding to a first waveband of the probe light which is not entered into an interior of the structure to be detected and directly reflected by the second surface, the second sub-spectrum being a spectrum corresponding to a second waveband of the probe light which is entered into the interior of the structure to be detected and reflected by the interior of the structure to be detected; obtaining a roughness of the second surface according to the first sub-spectrum; obtaining a bottom thickness of the groove structure according to the second sub-spectrum, the bottom thickness being a distance between the bottom surface of the groove structure and the second surface; and obtaining the bottom surface roughness of the groove structure according to the bottom thickness of the structure to be detected, the roughness of the second surface, and the spectrum of the reflected light. 2.The method of claim 1, wherein: obtaining the bottom surface roughness of the groove structure according to the bottom thickness of the structure to be detected, the roughness of the second surface, and the spectrum of the reflected light comprises: obtaining an optical intensity of the reflected light according to the spectrum of the reflected light; obtaining an actual reflectivity of the structure to be detected to the probe light according to the optical intensity of the reflected light and an optical intensity of the incident light; and obtaining the bottom surface roughness of the groove structure according to the actual reflectivity, the bottom thickness of the structure to be detected, and the roughness of the second surface. The method further comprises: evaluating the bottom surface roughness of the groove structure. Evaluating the bottom surface roughness of the groove structure comprises: establishing a plurality of optical propagation models with different bottom surface roughnesses according to the structure to be detected, and determining a plurality of simulated reflectivities of the plurality of optical propagation models under the probe light respectively; selecting a simulated reflectivity with a minimum deviation from the actual reflectivity as a target simulated reflectivity; and evaluating the bottom surface roughness of the groove structure according to a bottom surface roughness of an optical propagation model corresponding to the target simulated reflectivity. The material of the structure to be detected is silicon, and the wavelength range of the probe light is 350 nm-1700 nm. The second waveband spot is located on the bottom surface of the groove structure, and a diameter of the spot is less than an opening width of the groove structure. The method further comprises performing a down-sampling processing on the spectrum of the reflected light. The obtaining of the bottom thickness of the structure to be detected according to the second sub-spectrum comprises: performing a fast Fourier transform on the second sub-spectrum, and extracting a spectral oscillation frequency; and calculating the bottom thickness of the structure to be detected according to the spectral oscillation frequency and a refractive index dispersion relationship of a material of the structure to be detected. ​ ​ ​ 3. The measurement method of claim 1, wherein, ​ ​ 4. The measurement method of claim 1, wherein, ​ ​ ​ ​ 5. The measurement method of claim 1, wherein, ​ 6. The measurement method of claim 1, wherein, ​ 7. The measurement method of claim 1, wherein, ​ 8. The measurement method of claim 1, wherein, ​ ​ 9. The measurement method of claim 1, wherein, The step of splitting the spectrum of the reflected light into a first sub-spectrum and a second sub-spectrum comprises: determining a cut-off wavelength of the structure to be detected based on the oscillation feature of the spectrum curve of the reflected light; splitting the spectrum of the reflected light into a first sub-spectrum and a second sub-spectrum based on the cut-off wavelength.

10. The measurement method of claim 1, wherein, The material of the structure to be detected is a conductor, a semiconductor, or an insulator.

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