Method for simultaneously measuring thickness and optical constant of thin film

Abstract

The invention relates to the field of optical precision measurement, and discloses a method for simultaneously measuring film thickness and optical constants based on a surface plasma resonance holographic microscopy system. When surface plasmon resonance occurs in the structure with the film attached to the surface of the metal film, the thickness and the optical constant of the film above the metal film have a differential modulation effect on a reflection phase curve near a resonance angle. And performing parameter inversion on a theoretical resonance curve calculated based on a multilayer film reflection theory and a reflected light wave phase difference under different incident angles measured by using a surface plasma resonance holographic microscopy system to realize simultaneous measurement of the film thickness and the optical constant.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical precision measurement, and particularly relates to a method for simultaneously measuring film thickness and optical constants based on surface plasmon resonance holographic microscopic system. BACKGROUND

[0002] Thin films (such as polymers, graphene, transition metal dichalcogenides, etc.) have been widely studied and applied in various fields due to their unique atomic thickness and excellent physical and chemical properties. The thickness and optical constants of thin films are key parameters for their photoelectric properties, and accurate measurement of these parameters is crucial for understanding the unique properties of thin films. Nahmad-Rohen et al. used heterodyne interferometric reflectometry to measure the thickness and refractive index of thin layers simultaneously. This technique can only obtain the real part of the optical constants and the numerical inversion is complex (Alexander Nahmad-Rohen, et al. “Simultaneous microscopic imaging of thickness and refractive index of thin layers by heterodyne interferometric reflectometry (HiRef),” J. Phys. D: Appl. Phys. 55, 054001-054013 (2022)). Garrity et al. studied monolayer WS2 on a gold substrate using dual scanning near-field optical microscopy. By imaging the local dielectric changes and extracting the dielectric function values, they obtained the optical constants of WS2. Atomic force microscopy (AFM) and Raman spectroscopy were used to characterize the thickness of the material (O. Garrity, et al. “Probing the local dielectric function of WS2 on an au substrate by near field optical microscopy operating in the visible spectral range,” Appl. Surf. Sci. 574, 151672-151679 (2022)). Surface plasmon resonance microscopy (SPRM) is a method that can measure the physical and chemical properties of the near-field region of the interface with high sensitivity. When surface plasmons are excited, the physical and chemical properties of the near-field region of the interface and their subtle changes cause the intensity and phase of the reflected light to change dramatically. By detecting these changes, we can obtain information about the optical constants of the near-field region of the interface. To quantitatively obtain the phase information of light waves, researchers combined SPRM with digital holographic microscopy (DHM) to develop surface plasmon resonance holographic microscopy (SPRHM).Dai et al. measured the optical constants of few-layer graphene using this technique, and characterized the sample thickness using Raman spectroscopy and AFM (S. Dai, et al.“Complex refractive index measurement for atomic-layer materials via surface plasmon resonance holographic microscopy,” Opt. Lett. 44, 2982-2985 (2019)). Most of the existing techniques separately measure the thickness and optical constants of thin films, which is tedious and the existing thickness measurement methods (such as AFM) require contact with the sample, which can damage the sample and result in large measurement errors.

[0003] Therefore, the present application provides a method for simultaneously measuring the thickness and optical constants of a thin film based on a surface plasmon resonance holographic microscopy system to solve the above problems. SUMMARY

[0004] To overcome the shortcomings of existing methods and techniques and achieve simultaneous acquisition of the thickness and optical constants of a thin film, the present application provides a method for simultaneously measuring the thickness and optical constants of a thin film based on a surface plasmon resonance holographic microscopy system.

[0005] The idea of the present application is to use the steep change effect of the reflection phase at the metal-dielectric interface when surface plasmon resonance occurs, and to detect the high-sensitivity response of the reflection phase shift at different incident angles near the resonance angle to achieve simultaneous measurement of the thickness and optical constants of a thin film. Using a surface plasmon resonance holographic microscopy system, a series of holograms of the uppermost layer of the dielectric attached to the metal film surface under different incident angles are collected, and the reflection phase difference of the reflected light wave at each incident angle is obtained by numerical reconstruction. The theoretical curve of the reflection phase shift difference with respect to the incident angle is calculated using the multilayer film reflection theory, and a multi-parameter joint inversion is performed to demodulate the thickness and optical constant parameters of the thin film.

[0006] To achieve the above purpose, the present application adopts the following technical solutions:

[0007] In a first aspect, the present application provides a method for simultaneously measuring the thickness and optical constants of a thin film, characterized by comprising the following steps:

[0008] Step 1: When the thickness, optical constants of the remaining layers of dielectric except the thin film to be measured, and the incident light wavelength are determined, the surface plasmon resonance excitation angle θ SPR and different incident angles θ iReflection phase shift when the top dielectric layer of the structure with thin film attached to the metal film surface (i=1,2…N) is air and water respectively and Plotting the reflection phase shift difference With the incident angle θ of the light wave i The change curve of

[0009] Step 2: Parallel light of a specific polarization state is incident at an angle θ i The incident light is incident on the structural interface of the thin film attached to the metal film surface in step 1, and the reflected light wave carrying the thin film sample information and the reference light wave that is not modulated by the sample interfere with each other off-axis to form a sample hologram H. i (i=1,2…N) and collected by the photoelectric imaging device;

[0010] Step 3: Drop water on the structure with the thin film attached to the metal film surface in step 2 to change the dielectric from air to water. The other conditions remain unchanged and repeat step 2 to collect the corresponding background hologram Η 0i (i=1,2…N);

[0011] Step 4: Reconstruct the phase distribution and Calculate the phase difference distribution of the reflected light wave The phase difference of the reflected light wave is theoretically equal to the reflected phase shift difference in step 1; where (x, y) is the two-dimensional space coordinate;

[0012] Step 5: The reflection phase shift difference ΔΦ calculated in step 1 i With the incident angle θ of the light wave i The theoretical curve of the change and the experimental measurement of different incident angles θ i Downward reflection phase difference distribution ΔΦ′ i Multi-parameter joint inversion is performed on (x,y) discrete data to demodulate the optical constant distribution and thickness distribution of the film.

[0013] Furthermore, in step 1, the thin film is used as a sensitive medium layer, when its optical constant When the thickness d changes, the reflection phase shift difference ΔΦ i With the incident angle θ of the light wave i The change curve changes; wherein n is the refractive index of the film, k is the extinction coefficient of the film, and j is the imaginary unit; the range of variation of the film thickness d is: the penetration depth of the single atomic layer to the evanescent wave.

[0014] Furthermore, the structure of the thin film attached to the surface of the metal film in step 1 includes: a glass substrate, a chromium layer, a gold layer, a thin film and a dielectric layer.

[0015] Furthermore, the incident angle range is θ1~θN The condition θ1< θ should be met SPR <θ N , and the change range of the incident angle is ensured to be ΔΦ i The change curve of the phase difference with θ i contains a surface plasmon resonance interval.

[0016] Further, the sample hologram H i and the background hologram H 0i are reconstructed by using the light wave diffraction theory in step 4.

[0017] In a second aspect, the application provides a system for implementing the method for simultaneously measuring the thickness and optical constants of a thin film.

[0018] Technical effects

[0019] Compared with the prior art, the method for simultaneously measuring the thickness and optical constants of a thin film based on the surface plasmon resonance holographic microscopic system has the following advantages. The sample hologram and the background hologram are recorded at different incident angles near the surface plasmon resonance excitation angle in sequence, the reflection phase difference of the sample hologram and the background hologram is reconstructed based on the light field diffraction theory, and the simultaneous measurement of the thickness and optical constants is realized by combining the multi-parameter joint inversion method. The measurement system involved has a compact structure, is less affected by the environment, and has a simple parameter demodulation method, so that wide-field and high-sensitivity measurement can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 FIG. 1 is a structural schematic diagram of a metal film surface attached film involved in the application;

[0021] Figure 2 FIG. 2 is an optical path diagram of a surface plasmon resonance holographic microscopic system involved in the application;

[0022] Figure 3 FIG. 3 is a theoretical curve of the reflection phase shift difference with the change of the incident angle, in which three curves correspond to different refractive indexes, respectively;

[0023] Figure 4 FIG. 4 is a theoretical curve of the reflection phase shift difference with the change of the incident angle, in which three curves correspond to different extinction coefficients, respectively;

[0024] Figure 5 FIG. 5 is a theoretical curve of the reflection phase shift difference with the change of the incident angle, in which three curves correspond to different thicknesses, respectively;

[0025] Figure 6 FIG. 6 is a surface plasmon resonance phase image of a sample in an embodiment of the application;

[0026] Figure 7The experimental discrete data and the theoretical curve of the sample area reflection phase difference changing with the light wave incident angle in the embodiment of the present application.

[0027] Reference signs:

[0028] 1-He-Ne laser, 2-attenuation sheet, 3-objective lens, 4-needle hole, 5-convex lens one, 6-one-dimensional galvanometer, 7-polarizer, 8-half wave plate, 9-convex lens two, 10-dichroic prism, 11-high numerical aperture oil immersion microscope objective, 12-structure of metal film surface attached film, 13-mirror, 14-convex lens three, 15-Wollaston prism, 16-polarizer, 17-optoelectronic imaging device. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the present application 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 some of the embodiments of the present application, but not all the embodiments of the present application.

[0030] Embodiment 1

[0031] A method for simultaneously measuring the thickness and optical constants of a film, the structure of a metal film surface attached film as shown in the present application comprises a glass substrate, a chromium layer, a gold layer, a film and a dielectric layer. Figure 1

[0032] The method steps are as follows:

[0033] Step 1: when the thickness, optical constants of the remaining layers of medium in the structure of the metal film surface attached film except the film to be measured and the incident light wavelength are determined, the surface plasmon resonance excitation angle θ SPR of the structure of the metal film surface attached film under different incident angles θ i (i = 1, 2…N) is calculated by using the multi-layer film reflection theory, and the reflection phase shift of the uppermost layer of the dielectric in the structure of the metal film surface attached film when the uppermost layer of the dielectric is air and water respectively is calculated. and The difference of the reflection phase shift ΔΦ is drawn with the change curve of the light wave incident angle θ i ; the film as a sensitive medium layer, when the optical constants and the thickness d change, the change curve of ΔΦ i with θ i changes obviously.

[0034] The structure of the metal film surface attached film comprises a glass substrate, a chromium layer, a gold layer, a film and a dielectric layer.

[0035] The incident angle range θ1~θ N should satisfy θ1<θ SPR <θ N ​and can ensure that in this incident angle variation range ΔΦ i with the change of θ i contains the surface plasmon resonance interval;

[0036] The n in said is the refractive index of the film, k is the extinction coefficient of the film, and j is the imaginary unit;

[0037] The film thickness d varies in the range of a single atomic layer to the penetration depth of the evanescent wave;

[0038] Step 2: The parallel light of a specific polarization state is incident on the interface of the structure in step 1 at incident angles θ i The reflected light wave carrying the sample information of the film and the reference light wave not modulated by the sample form an off-axis interference to form a sample hologram H i (i = 1, 2…N) and is collected by an optoelectronic imaging device;

[0039] Step 3: Drop water on the structure in step 2 with the film attached to the surface of the metal film, change the dielectric from air to water, keep the rest unchanged, repeat step 2, and collect the corresponding background hologram H 0i (i = 1, 2…N);

[0040] Step 4: Use the light field diffraction theory to respectively reconstruct the sample hologram H i and the background hologram H 0i to obtain the phase distribution and Calculate the phase difference distribution of the reflected light wave The phase difference of the reflected light wave is theoretically equal to the reflection phase shift difference in step 1; wherein (x, y) is the two-dimensional space coordinate;

[0041] Step 5: The theoretical curve of the reflection phase shift difference ΔΦ i with the change of the incident angle θ i of the light wave is different from the reflection phase difference distribution ΔΦ′ i (x, y) discrete data measured at different incident angles θ i (x, y) can be demodulated to obtain the optical constant distribution and thickness distribution of the film.

[0042] The working principle of the method is as follows:

[0043] When the structure with the film attached to the surface of the metal film occurs surface plasmon resonance and the thickness of the metal layer is small, the surface plasmon wave will penetrate to the upper part of the metal layer and couple with the film at that position, and the characteristics (such as thickness and optical constant) of the film will have a greater impact on the phase of the reflected light wave. For example Figure 3As shown, when the refractive index increases, the theoretical curve shifts right, that is, the refractive index affects the position of the curve; as shown Figure 4 As shown, when the extinction coefficient increases, the slope of the theoretical curve decreases, that is, the extinction coefficient affects the slope of the curve; as shown Figure 5 As shown, if the thickness increases, the theoretical curve shifts right, accompanied by a decrease in the slope of the curve. Therefore, the thickness and optical constant parameters of the sample to be measured can be uniquely determined according to the corresponding relationship between the incident angle and the difference in reflection phase shift.

[0044] Example 2

[0045] The embodiment of the present application provides a system for applying the measurement method of example 1, Figure 2 As shown, the surface plasmon resonance holographic microscopic imaging experimental system, the linearly polarized laser beam emitted by the He-Ne laser 1 (wavelength 632.8 nm) is expanded and collimated into parallel light after passing through the attenuator 2, the objective lens 3, the pinhole 4 and the convex lens one 5. The parallel light converges to the back focal plane of the oil immersion objective lens 11 after passing through the polarizer 7, the half-wave plate 8, the convex lens two 9 and the beam splitter prism 10. The collimated light is incident on the interface of the surface plasmon resonance excitation structure 12 in the form of wide-field illumination. The laser incidence angle is adjusted by the one-dimensional galvanometer 6. The polarizer 7 modulates the polarization state of the incident light beam to 45° linear polarization, including s and p polarization components. After the reflected light passes through the beam splitter prism 10 and the convex lens three 14 (the focal length is the same as that of the convex lens two 9), off-axis interference is realized through the Wollaston prism 15 and the polarizer 16, and the formed hologram is received by the photoelectric imager 17. Among them, the p polarization component of the incident light beam excites the surface plasmon resonance, carries the sample information and serves as the object light wave, and the s polarization component does not excite the surface plasmon resonance and serves as the reference light wave. After the reflected light passes through the Wollaston prism 15, the object light wave and the reference light wave are separated at a small angle, and after passing through the polarizer 16, the two beams interfere in the overlapping area, and the wavefront recording of the off-axis digital hologram is completed.

[0046] A 1-nanometer chromium layer and a 50-nanometer gold layer are deposited on a standard glass substrate by electron beam evaporation, ReS2 is preliminarily peeled off by a mechanical peeling method, and is transferred to the upper part of the gold layer as a sample to be measured by a dry transfer technology. First, the galvanometer is controlled to gradually increase the incidence angle, and the sample area appears a “bright-dark-bright” phenomenon. When θ i = θ SPR , the sample area excites surface plasmon resonance, and the reflected light intensity reaches the minimum, and the sample area is observed to be “dark”; when θ i < θ SPR or θ i > θ SPR , the reflected light intensity increases, and the sample area is observed to be “bright”, so as to determine the incident angle scanning range θ i(i=1,2…N). Next, adjust the incident angle to the minimum incident angle θ1 within the scanning range and gradually increase it by 0.05° to collect the sample hologram H. i (i=1,2…N); drip water on the sample layer of the structure with a thin film attached to the surface of the metal film, that is, change the uppermost dielectric layer from air to water, repeat the experimental steps, and collect the corresponding background hologram Η 0i (i=1,2…N). Using the light field diffraction theory, the sample hologram Η i and background hologram Η 0i Reconstructed phase and Further calculations are performed to obtain the phase difference distribution of the reflected light wave (like Figure 6 As shown), a small area is selected from the sample area of ​​this distribution image, and the mean value of each reflection phase difference distribution image in this area is taken as the reflection phase difference value ΔΦ′ of the sample. i , plot the phase difference ΔΦ′ of the reflected light wave i With the incident angle θ i Finally, a multi-parameter joint inversion is performed with the theoretical curve drawn using multilayer film reflection theory to demodulate the sample thickness and optical constants (such as Figure 7 shown).

[0047] This paper proposes a method for simultaneously measuring film thickness and optical constants using a surface plasmon resonance holographic microscopy system. By sequentially recording sample and background holograms at different incident angles near the surface plasmon resonance excitation angle, the reflection phase difference between the sample and background holograms is reconstructed based on light field diffraction theory. Combined with a multi-parameter joint inversion method, this method enables simultaneous measurement of film thickness and optical constants. The resulting measurement system is compact, minimally affected by environmental factors, and employs a simple parameter demodulation method, enabling wide-field, high-sensitivity measurements.

[0048] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. The replacement may be a replacement of a portion of a structure, device, or method step, or it may be a complete technical solution. Any equivalent replacement or modification based on the technical solution and inventive concept of the present invention shall be covered by the scope of protection of the present invention.

Claims

1. A method for simultaneously measuring film thickness and optical constants, characterized in that: The steps include: Step 1: When the thickness, optical constants and incident light wavelength of all layers of the medium except the film to be measured in the structure of the thin film attached to the metal film surface are determined, the surface plasmon resonance excitation angle θ is calculated using the multilayer film reflection theory. SPR With different incident angles θ i Reflection phase shift when the top dielectric layer of the structure with thin film attached to the metal film surface (i=1,2…N) is air and water respectively and Plotting the reflection phase shift difference With the incident angle θ of the light wave i The change curve of Step 2: Parallel light of a specific polarization state is incident at an angle θ i The incident light is incident on the structural interface of the thin film attached to the metal film surface in step 1, and the reflected light wave carrying the thin film sample information and the reference light wave that is not modulated by the sample interfere with each other off-axis to form a sample hologram H. i (i=1,2…N) and collected by the photoelectric imaging device; Step 3: Drop water on the structure with the thin film attached to the metal film surface in step 2 to change the dielectric from air to water. The other conditions remain unchanged and repeat step 2 to collect the corresponding background hologram Η 0i (i=1,2…N); Step 4: Reconstruct the phase distribution and Calculate the phase difference distribution of the reflected light wave The phase difference of the reflected light wave is theoretically equal to the reflected phase shift difference in step 1; where (x, y) is the two-dimensional space coordinate; Step 5: The reflection phase shift difference ΔΦ calculated in step 1 i With the incident angle θ of the light wave i The theoretical curve of the change and the experimental measurement of different incident angles θ i Lower reflection phase difference distribution ΔΦ′ i Multi-parameter joint inversion is performed on (x,y) discrete data to demodulate the optical constant distribution and thickness distribution of the film.

2. The method for simultaneously measuring film thickness and optical constants according to claim 1, characterized in that: In step 1, the film is used as a sensitive medium layer, when its optical constant When the thickness d changes, the reflection phase shift difference ΔΦ i With the incident angle θ of the light wave i The change curve changes; wherein n is the refractive index of the film, k is the extinction coefficient of the film, and j is the imaginary unit; the range of variation of the film thickness d is: the penetration depth of the single atomic layer to the evanescent wave.

3. The method for simultaneously measuring film thickness and optical constants according to claim 1, wherein: The structure of the thin film attached to the surface of the metal film in step 1 includes: a glass substrate, a chromium layer, a gold layer, a thin film and a dielectric layer.

4. The method for simultaneously measuring film thickness and optical constants according to claim 1, wherein: The incident angle range θ1~θ N Should satisfy θ1<θ SPR <θ N , and can ensure that within this incident angle variation range ΔΦ i With θ i The variation curve of contains the surface plasmon resonance region.

5. The method for simultaneously measuring film thickness and optical constants according to claim 1, characterized in that: In step 4, the light wave diffraction theory is used to respectively analyze the sample hologram H i and background hologram Η 0i reconstruction.

6. A system for implementing the method for simultaneously measuring film thickness and optical constants according to any one of claims 1 to 5.

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