A method for measuring anisotropic thin films based on zero-reflection differential signal

Abstract

A method for measuring anisotropic film based on zero reflection difference differential signal relates to the field of film thickness detection, and comprises the following steps: measuring wide-band reflection difference spectrum of a sample by using a reflection difference spectrometer and a photoelastic modulation reflection difference spectrometer; determining an incident wavelength corresponding to a zero reflection difference signal: traversing all wavelength channels in the wide-band reflection difference spectrum, and determining a wavelength corresponding to a zero-crossing of the reflection difference signal; determining the thickness of the anisotropic film: determining the thickness of the anisotropic film according to a mapping relationship between the wavelength corresponding to the zero reflection difference signal and the film thickness. The application proposes using the position of the zero-crossing point of the reflection difference signal ΔR / R=0 to determine the thickness of the anisotropic film, so that the error introduced by the difference between the polarization angle of incident light and the crystal axis angle of the anisotropic film can be removed, and further, the time for determining the crystal axis can be saved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of thin film thickness detection, in particular to a method for measuring the thickness of anisotropic thin film based on the zero-crossing point of reflectance difference spectrum. BACKGROUND

[0002] With the size of various components and devices becoming smaller and lighter, the requirement for system integration is becoming higher and higher. Anisotropic thin films (such as low-symmetry two-dimensional materials) have attracted extensive attention of researchers due to their unique direction-dependent properties, which broaden the freedom of device design. The measurement of the thickness of anisotropic thin films is one of the basic physical properties that need to be concerned in the process of device development and property research. Existing thin film thickness measurement methods mainly include contact type and non-contact type. The contact type thin film measurement method, represented by AFM and scanning tunneling microscope (SEM), can accurately measure the thickness of thin films, and has the advantages of high resolution and stereoscopic imaging, but the operation is complex, time-consuming and laborious. The non-contact thin film measurement method includes Raman spectroscopy, photoluminescence spectroscopy, polarization reflection\absorption spectroscopy, optical microscope interference method, etc. The Raman spectroscopy and photoluminescence spectroscopy are relatively simple to measure, but they are mainly limited to measuring materials with a thickness of less than 10 layers, and cannot accurately characterize the thickness of materials with more layers. The polarization reflection\absorption spectroscopy and optical microscope interference method need to determine the intrinsic crystal axis angle of the anisotropic thin film first. However, the error of the angle between the incident polarized light and the crystal axis angle seriously affects the measurement results. On the other hand, in order to determine the crystal axis angle of the anisotropic thin film, the azimuth angle of the in-plane polarized light needs to be rotated, which wastes measurement time. SUMMARY

[0003] In view of the problem of measuring the thickness of anisotropic thin film in the prior art, the present application provides a method for measuring the thickness of anisotropic thin film based on the zero-crossing point of reflectance difference spectrum, which directly obtains the thickness of the sample to be measured without determining the crystal axis angle of the sample.

[0004] The technical solution adopted by the present application to solve the technical problem is as follows:

[0005] A method for measuring anisotropic thin film based on zero reflectance difference signal, comprising the following steps:

[0006] S1: measuring the wide-band reflectance difference spectrum of the sample: using a reflectance difference spectrometer and a photoelastic modulation reflectance difference spectrometer to measure the wide-band reflectance difference spectrum of the sample; wherein the reflectance difference spectrometer can use the device disclosed in patent No. ZL201820817059.3, and the photoelastic modulation reflectance difference spectrometer can use the device disclosed in patent No. ZL200710059903.7;

[0007] S2: determining the incident wavelength corresponding to the zero reflection difference signal: traversing all wavelength channels in the wide-band reflection difference spectrum, determining the wavelength corresponding to the zero reflection difference signal;

[0008] S3: determining the thickness of the anisotropic film: according to the mapping relationship between the wavelength corresponding to the zero reflection difference signal and the film thickness, determining the thickness of the anisotropic film.

[0009] Further, in the step S3, when the optical refractive index of the sample to be measured is known, the mapping relationship between the wavelength (λ) of the zero reflection difference signal and the thickness (d) of the corresponding anisotropic film is calculated according to the Fresnel law theory and the beam propagation matrix method.

[0010] Further, in the step S3, when the optical refractive index of the sample to be measured is unknown, first, the mapping relationship between the wavelength of the zero reflection difference signal and the thickness of the sample is calibrated, including the following steps:

[0011] S31: obtaining a series of anisotropic films with different thicknesses;

[0012] S32: using a film thickness measuring device to test the thickness of the series of anisotropic films with different thicknesses;

[0013] S33: using the reflection difference spectrum to test the series of samples with known thicknesses;

[0014] S34: according to the test results of S32 and S33, establishing the mapping relationship between the wavelength of the zero reflection difference signal and the thickness, and then using the established mapping relationship to determine the thickness of other anisotropic films.

[0015] Further, the spectrum type of the wide-band reflection difference spectrum is used as a fingerprint feature to judge the consistency of the measured sample and the theoretically calculated or calibrated sample in composition and optical properties.

[0016] Further, the thickness interval of the film material is determined according to the number of wavelength channels of the zero reflection difference signal, and the thickness of the film is accurately determined according to the corresponding linear curve of wavelength (λ) and thickness (d).

[0017] Further, the angle between the polarization incident plane of the reflection difference spectrometer and the crystal axis of the anisotropic film is not equal to any one of 45°, 135°, 225°, and 315°.

[0018] Further, the range of the wavelength is 400 nanometers to 1 micrometer.

[0019] Further, in the step S3, the film thickness measuring device is an atomic force microscope or an ellipsometer.

[0020] The beneficial effects of the present application are:

[0021] 1、The present application uses the position of the zero-crossing point of the reflected differential signal ΔR / R=0 to determine the thickness of the anisotropic film. Since the position of the zero-crossing point is zero at all polarization angles of the incident light, this method can eliminate the error introduced by the difference between the polarization angle of the incident light and the crystal axis angle of the anisotropic film, and further, can eliminate the time for determining the crystal axis.

[0022] 2、For existing optical methods for measuring the thickness of anisotropic films, such as ellipsometers, the accuracy of the measurement results is related to the arrangement of the crystal axis direction of the anisotropic film. Generally, the crystal axis direction of the anisotropic film needs to be determined first, and then the film thickness is measured based on this, which leads to the accuracy of the film thickness measurement and the crystal axis direction measurement being intertwined and influencing each other. The method described in the present technical solution makes the measurement of the two key parameters of film thickness and crystal axis direction independent of each other and has no influence on each other, because the angle between the linearly polarized incident light of the reflected differential spectrum measurement and the crystal axis of the anisotropic film to be measured only affects the amplitude of the reflected differential signal, and has no effect on the wavelength position of the zero-crossing signal.

[0023] 3、The azimuth angle of the linearly polarized incident light of the reflected differential spectrum measurement can be quickly determined by rotating the azimuth angle, so the present technical solution can realize a measurement method for simultaneously measuring the crystal axis direction and the film thickness, and there is no interference in the measurement accuracy between them.

[0024] 4、Benefiting from the easy realization of high spatial resolution measurement by micro-reflected differential microscopy, the present method can conveniently realize the measurement of anisotropic films with small lateral size, such as anisotropic two-dimensional materials prepared by mechanical exfoliation, such as black phosphorus, rhenium disulfide, rhenium diselenide, etc. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Figure is a multi-layer reflection model diagram for a homogeneous medium;

[0026] Figure 2 Figure is a reflection coefficient curve diagram in two mutually perpendicular directions for anisotropic two-dimensional material black phosphorus (BP) placed on a 268-nanometer-thick silicon dioxide layer on a silicon substrate;

[0027] Figure 3 Figure is a wavelength (λ)-thickness (d) corresponding curve diagram for the zero reflected differential signal when the two-dimensional material black phosphorus is placed on a silicon substrate with a 268-nanometer-thick silicon dioxide layer;

[0028] Figure 4A four-phase model diagram for a silicon / silicon dioxide substrate. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0030] In the description of the present application, it should be understood that the terms “center”, “longitudinal”, “transverse”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inner”, “outer” and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the protection scope of the present application.

[0031] First embodiment

[0032] The sample to be measured 1: for an anisotropic two-dimensional material black phosphorus sample with known optical refractive index, the sample to be measured is placed on a silicon / silicon dioxide substrate. As shown in Figure 4 , the specific measurement steps are as follows:

[0033] (a) Measure the wide-band reflection difference spectrum of the sample, and traverse all wavelength channels in the wide-band reflection difference spectrum to find the wavelength (λ) corresponding to the zero-crossing of the reflection difference signal;

[0034] (b) Based on the Fresnel law theory, the optical refractive index and thickness of air, the sample to be measured, and the substrate are brought into formulas 1-6 to calculate the reflection coefficients R X and R Y of the two mutually perpendicular crystal axis directions (x direction and y direction) on the surface of the sample to be measured with different thicknesses (typically when the thickness of black phosphorus is 6 nm, the reflection coefficients along the two crystal axis directions are as shown in Figure 2 ), and are brought into the formula ΔR / R=2*(R X -R Y ) / (R X +R Y ) to calculate the corresponding mapping relationship between the wavelength and the thickness when the zero reflection difference signal in the wide-band, as shown in Figure 3 ;

[0035] The calculation method of the optical reflection coefficient is:

[0036] wherein:

[0037]

[0038]

[0039]

[0040]

[0041]

[0042]

[0043] wherein m represents the layer number, p represents two crystal axis directions of the anisotropic material, which are a axis or b axis, is the refractive index coefficient of the mth layer material in the p axis direction, and β is the phase delay value of light propagating in different layer materials.

[0044] c) using the wavelength (λ) and sample thickness (d) obtained in step (b), a wavelength (λ)-thickness (d) linear curve of the sample is drawn, and due to the periodic principle of interference, multiple sets of wavelength (λ)-thickness (d) linear curves will appear in the test wavelength range as the sample thickness increases, and the curve with the highest linearity and the widest application range is selected;

[0045] d) according to the wavelength corresponding to the zero reflection difference signal in step (a), the thickness of the sample to be measured is determined by using the mapping relationship between the wavelength (λ) corresponding to the zero reflection difference signal and the film thickness (d). Figure 3

[0046] Second embodiment

[0047] The sample to be measured 2: the optical refractive index of the sample to be measured is unknown, and the sample to be measured is placed on a silicon substrate. As shown in the figure, the specific measurement steps are as follows: Figure 4

[0048] a) using other film thickness measurement methods (such as AFM, etc.) to characterize the film thickness (d);

[0049] b) measure the wide-band reflection difference signal spectrum of the sample to be measured with a known thickness (d), traverse all wavelength channels, and find the wavelength (λ) corresponding to the zero reflection difference signal;

[0050] ​​c) according to the film thickness (d) and wavelength (λ) obtained in step (a) and step (b), a wavelength (λ)-thickness (d) linear curve of the film to be measured is drawn, and due to the periodicity principle of interference, as the sample thickness increases, there will be multiple sets of wavelength (λ)-thickness (d) linear curves in the test wavelength range, and the curve with the highest linearity and the widest range is selected;

[0051] d) for the same kind of film to be measured, the reflection difference signal of the film is measured, all wavelength channels in the wide band reflection difference spectrum are traversed, the wavelength (λ) corresponding to the reflection difference signal zero is found, and is brought into the curve drawn in step (c), and the thickness of the sample to be measured is determined according to the mapping relationship between the wavelength (λ) corresponding to the reflection difference zero signal and the film thickness (d).

[0052] In addition to the technical features described in the specification, they are known to those skilled in the art.

Claims

1. A method for measuring anisotropic thin films based on zero-reflection differential signals, characterized in that, Includes the following steps: S1: Measurement of the wideband reflectance differential spectrum of the sample: The wideband reflectance differential spectrum of the sample is measured using a reflectance differential spectrometer or a photoelastic modulation reflectance differential spectrometer. S2: Determine the incident wavelength corresponding to the zero-reflection differential signal: Traverse all wavelength channels in the wideband reflection differential spectrum to determine the wavelength corresponding to the zero-crossing of the reflection differential signal; S3: Determination of the thickness of anisotropic thin films: The thickness of anisotropic thin films is determined based on the mapping relationship between the wavelength corresponding to the obtained differential zero signal and the thickness of the thin films. In step S3, when the optical refractive index of the sample to be tested is unknown, the mapping relationship between the wavelength that generates a zero-reflection differential signal and the sample thickness is first determined, including the following steps: S31: Obtain a series of anisotropic films of different thicknesses; S32: Use a thin film thickness measuring device to test the thickness of this series of anisotropic thin films with different thicknesses. S33: This series of samples with known thicknesses are tested using differential reflectance spectroscopy; S34: Based on the test results of S32 and S33, establish the mapping relationship between the wavelength and thickness of the zero-reflection differential signal, and then use the established mapping relationship to determine the thickness of other anisotropic films.

2. The method for measuring anisotropic thin films based on zero-reflection differential signals according to claim 1, characterized in that, In step S3, when the optical refractive index of the sample to be tested is known, the mapping relationship between the wavelength of the zero-reflection differential signal and the thickness of the corresponding anisotropic thin film is calculated according to Fresnel's law theory and the beam propagation matrix method.

3. The method for measuring anisotropic thin films based on zero-reflection differential signals as described in claim 1, characterized in that, By using the spectral pattern of broadband reflectance differential spectroscopy as a fingerprint feature, the consistency of the tested sample with the theoretically calculated or calibrated sample in terms of composition and optical properties can be determined.

4. The method for measuring anisotropic thin films based on zero-reflection differential signals as described in claim 1 or 2, characterized in that, The thickness range of the thin film material is determined by the number of wavelength channels of the zero-reflection differential signal, and the thickness of the thin film is accurately determined by the linear curve of wavelength versus thickness.

5. The method for measuring anisotropic thin films based on zero-reflection differential signals as described in claim 1, characterized in that, The angle between the polarization incident plane of the reflection differential spectrometer and the crystal axis of the anisotropic thin film is not equal to any one of 45°, 135°, 225°, or 315°.

6. The method for measuring anisotropic thin films based on zero-reflection differential signals as described in claim 1, characterized in that, The wavelength range is from 400 nanometers to 1 micrometer.

7. The method for measuring anisotropic thin films based on zero-reflection differential signals as described in claim 1, characterized in that, In step S3, the thin film thickness measuring device is an atomic force microscope or an ellipsometer.

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