A method for measuring the two-dimensional thickness distribution of double-layer nanometal films

Abstract

This invention discloses a method for measuring the two-dimensional thickness distribution of a double-layer nanometal film. By establishing two surface plasmon resonance models, using double-exposure digital holographic interferometry to detect the phase difference of the reflected light waves corresponding to the two resonance models, and combining the Fresnel formula theory to calculate the corresponding reflection phase shift difference surface, two monotonic curves with the thickness of the two metal films as variables are obtained, and the coordinates of the curve intersection are extracted. This method can achieve highly sensitive and accurate measurement of the two-dimensional thickness distribution of a double-layer nanometal film.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement, in particular to a method for measuring the two-dimensional thickness distribution of a double-layer nano-metal film by using surface plasmon resonance holographic microscopy. Background Art

[0002] Nanofilm refers to a single or multilayer film with a thickness of nanometers. In recent decades, great progress has been made in the preparation methods, analytical techniques and application research of nanofilms. Accurate measurement and characterization of nanofilm thickness are of great significance in basic research and engineering applications. Atomic force microscopy (AFM) can be used to measure the thickness of nanofilms in situ (Lobo RFM, et al. "Insitu thickness measurements of ultra-thin multilayerpolymer films by atomic force microscopy," Nanotechnology 10(4), 389-393(2000)). However, AFM is a contact measurement method and the time cost of point-by-point scanning makes it impossible to achieve wide-field dynamic measurement. Surface plasmon resonance technology has been widely studied and applied in the field of thin film parameter sensing. Among them, surface plasmon resonance measurement technology based on phase detection has attracted widespread attention due to its high sensitivity. Conventional techniques utilize single-layer metal films to excite plasmon resonance. Bi-layer metal film structures are widely used in optical sensing and measurement due to their advantages, such as improved detection sensitivity and protection of the outer metal layer. However, the film thickness parameter significantly influences measurement accuracy, making accurate calibration of the thickness parameter of bi-layer metal films of great significance. By extracting the interference fringes formed by polarizer-modulated TM polarized light (parallel to the incident plane) and TE polarized light (perpendicular to the incident plane) at different incident angles, the phase difference between the two reflected beams can be obtained to calculate the thickness of the bi-layer metal film (Liu C, Liu Q, et al. “Determination of the Bimetallic Layers' Film Thicknesses by Phase Detection of SPR Prism Coupler,” Plasmonics, 12, 1199–1204 (2017)). However, this approach cannot achieve wide-field measurement, the data processing involved is cumbersome, and the Mach-Zehnder interferometer structure, in which the object parameter beam is non-co-path in the measurement optical path, is susceptible to environmental influences, resulting in large measurement errors. Therefore, it is particularly important to realize high-sensitivity measurement of the thickness of double-layer metal films based on a high-stability optical path system. Summary of the Invention

[0003] Technical problems to be solved

[0004] To overcome the shortcomings of existing methods and technologies and achieve wide-field, high-precision measurement of the two-dimensional thickness distribution of double-layer nano-metal films, the present invention proposes a method for measuring the thickness of double-layer nano-metal films based on surface plasmon resonance holographic microscopy. The concept of the present invention is that when a physical parameter such as the refractive index or thickness of the medium in the surface plasmon resonance excitation structure undergoes a slight change, the phase of the reflected light wave will undergo a dramatic change. Using a surface plasmon resonance holographic microscopy imaging system, the phase difference of the reflected light wave under the two resonance conditions is measured. A theoretical model is then used to calculate a surface plot of the phase shift difference after the light wave is reflected by the multilayer film system in the excitation structure. Based on the numerical equality between the experimentally measured reflected light wave phase difference and the theoretically calculated reflected phase shift difference, the corresponding double-layer metal film thickness value is found, achieving accurate demodulation of the thickness distribution of the double-layer nano-metal film.

[0005] Technical Solution

[0006] The technical solution adopted by the present invention is characterized by the following steps:

[0007] Step 1: Construct a four-layer Kretschmann surface plasmon resonance (SPR) excitation structure consisting of dielectric layer 1, metal layer 1 (thickness d1), metal layer 2 (thickness d2), and dielectric layer 2. Using the wave vector matching condition for SPR, calculate the incident angles θ1 and θ2 of the excitation light wave required to excite SPR when dielectric layer 2 is liquid 1 (refractive index n1) and liquid 2 (refractive index n2), respectively. When the incident angle is θ1, use the Fresnel formula to calculate the reflection phase shift of the incident light wave after reflection from the multilayer film system of the excitation structure when dielectric layer 2 is liquid 1 or air, respectively. The difference between the two is the reflection phase shift difference φ1(n1, d1, d2). Given the range of d1 and d2, the resonance surface Φ1(φ1, d1, d2) is obtained, where φ1 varies with d1 and d2. Similarly, calculate the reflection phase shift difference surface Φ2(φ2, d1, d2) corresponding to liquid 2 when the incident angle is θ2.

[0008] Step 2: Build a surface plasmon resonance holographic microscopy measurement system based on the Kretschmann structure. Set the incident angle of the excitation light wave to θ1. When the dielectric layer 2 is liquid 1, record the reflected light wave as the digital hologram H1 formed by the interference between the object light wave and the reference light wave. Replace the dielectric layer 2 with air and record the digital hologram H2.

[0009] Step 3: Adjust the incident angle of the excitation light wave in the measurement system to θ2, set the dielectric layer 2 to liquid 2, and record digital holograms H3 and H4 using the same method as step 2;

[0010] Step 4: Use the principle of light diffraction to calculate the phase distributions 1-4 of the four digital holograms. Subtract phase distribution 1 from phase distribution 2 to get the phase difference distribution. Phase distribution 4 minus phase distribution 3 to get the phase difference distribution

[0011]

[0012] The (x, y) represents the two-dimensional space coordinates;

[0013] Step 5: The reflected phase shift difference φ1(x,y) is numerically compared with the experimentally measured reflected light wave phase difference Equal, φ2(x,y) and Equal, according to experimental measurements and Corresponding to the theoretically calculated resonance surfaces Φ1(φ1, d1, d2) and Φ2(φ2, d1, d2), two monotonically changing resonance curves are projected in the coordinate system with d1 and d2 as variables. The two thickness values ​​corresponding to the unique intersection of the curves are d1(x, y) and d2(x, y) to be measured, that is, the two-dimensional thickness distribution of metal layer 1 and metal layer 2.

[0014] Beneficial effects

[0015] This paper proposes a method for measuring the two-dimensional thickness distribution of a double-layer nanometal film using surface plasmon resonance holographic microscopy. By recording digital holograms under different resonance conditions, the phase distribution is obtained by numerically reconstructing the complex amplitude of the reflected light wave from the digital hologram. The resonance surface is then calculated using the Fresnel formula to demodulate the thickness distribution of the double-layer metal film. The experimental system involved features simple components and a compact structure, effectively avoiding environmental disturbances. Furthermore, the system is simple to operate and minimizes experimental measurement errors. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 : is a schematic diagram of the optical path of the surface plasmon resonance holographic microscopy imaging system under two resonance conditions involved in the present invention;

[0017] In the figure: 1-He-Ne laser, 2-fiber coupling device, 3-collimating lens, 4-half-wave plate, 5-right-angle prism, 6-surface plasmon resonance excitation structure, 7-microscope objective lens, 8-imaging lens, 9-beam splitter prism, 10-image acquisition device.

[0018] Figure 2 : This is a surface calculated theoretically to show how the reflection phase shift difference when surface plasmon resonance occurs when the dielectric layer 2 is composed of two dielectric liquids varies with the thickness of the two metal films. The figure takes water and ethanol as dielectric liquids and chromium and gold as two metal films as examples.

[0019] Figure 3:is based on Figure 2 The theoretical surface is drawn by selecting the reflection phase shift difference corresponding to 3nm chromium and 45nm gold, and the theoretical curve is drawn with the thickness of the chromium layer and the gold layer as the variable coordinate system. The coordinates of the intersection are the corresponding metal film thickness.

[0020] Figure 4 : is the reflection phase difference distribution recorded in the experiment. DETAILED DESCRIPTION

[0021] The present invention will now be further described with reference to the embodiments and accompanying drawings:

[0022] The present invention relates to a surface plasmon resonance holographic microscopy system for measuring the two-dimensional thickness distribution of a double-layer nano-metal film. Figure 1 、 2 As shown, it includes a He-Ne laser 1, a fiber coupling device 2, a collimating lens 3, a half-wave plate 4, a right-angle prism 5, a surface plasmon resonance excitation structure 6, a microscope objective lens 7, an imaging lens 8, a beam splitter prism 9, and an image acquisition device 10.

[0023] The working principle of the method for measuring the two-dimensional thickness distribution of a double-layer nano-metal film is as follows:

[0024] When p-polarized light is incident on a single-layer metal-dielectric interface at a surface plasmon resonance angle, surface plasmon resonance is excited. If all parameters in the excited structure except the metal film thickness are known, the metal film thickness can be determined by combining the experimentally measured phase difference with the theoretically calculated reflection phase shift curve. If the excited structure includes a double metal film, the phase difference of the reflected light wave under another resonance condition must be measured and two reflection phase shift curves calculated. Based on the two experimentally measured phase differences, two resonance curves are derived with the double metal thickness as a variable. The thickness of the double metal film can be determined by extracting the coordinates of the curve intersection.

[0025] The method for measuring the two-dimensional thickness distribution of a double-layer nano-metal film has the following workflow:

[0026] like Figure 1 、 Figure 2As shown, linearly polarized light emitted by a He-Ne laser 1 (wavelength 632.8 nm) is coupled into an optical fiber via a fiber coupling device 2. After passing through a collimating lens 3 and a half-wave plate 4, it is converted into p-polarized parallel light. It then enters a right-angle prism 5, refracted, and incident at a specific angle on the metal-dielectric interface in the excitation structure 6 to stimulate surface plasmon resonance. The reflected light wave, after exiting the right-angle prism 5, passes through a microscope objective 7 and an imaging lens 8 to image the dielectric sample above the metal layer. The light wave passes through a beam splitter prism 9 positioned at a 45-degree angle and is split into two parts, one above and one below. After transmission and reflection within the prism, the light waves exit from both sides of the prism, each containing both parts. Therefore, the dielectric sample must be placed halfway above the prism, halfway between the light spot and the sample spot, so that the exiting light wave contains both parts, one carrying sample information and the other not. The former serves as the object wave, while the latter serves as the reference wave. A digital hologram is recorded using an image acquisition device 10 placed on one side of the prism. First, the rotation angle of right-angle prism 5 is adjusted so that when dielectric layer 2 is water, the light wave is incident on the metal-dielectric interface at the surface plasmon resonance angle. When dielectric layer 2 is air, a background hologram is recorded. Water is dripped onto the metal layer, and a sample hologram is recorded. The phase distribution of the two holograms is obtained through numerical reconstruction. The phase difference distribution of the sample hologram and the background hologram is subtracted to obtain the phase difference distribution of the reflected light wave. Next, dielectric layer 2 is replaced with ethanol, and the optical path is adjusted using the above method. Two digital holograms are recorded again and the phase difference distribution is demodulated. Finally, the Fresnel formula is used to calculate the theoretical surface of the reflection phase shift difference as a function of the thickness of the two metal layers when dielectric layer 2 is water and ethanol, respectively. Based on the characteristic that the experimentally measured phase difference of the reflected light wave is equal to the numerical value of the reflected phase shift difference in the theoretical curve, the above-mentioned measured phase difference is projected onto the theoretical surface to obtain two monotonic curves with the thickness of the double metal layer as the variable coordinate system. The horizontal and vertical coordinate values ​​of the intersection of the curves are found, which are the thickness values ​​of the two layers of metal films, thereby realizing the measurement of the two-dimensional distribution of the thickness of the double-layer nano-metal film.

Claims

1. A method for measuring the two-dimensional thickness distribution of a double-layer nanometal film, characterized in that Here are the steps: Step 1: Construct a four-layer Kretschmann surface plasmon resonance excitation structure consisting of a dielectric layer 1, a metal layer 1 with a thickness of d1, a metal layer 2 with a thickness of d2, and a dielectric layer 2. Using the wave vector matching condition of the surface plasmon resonance, calculate the incident angles θ1 and θ2 of the excitation light wave required to excite the surface plasmon resonance when the dielectric layer 2 is liquid 1 with a refractive index of n1 and liquid 2 with a refractive index of n2, respectively. When the light wave incident angle is θ1, use the Fresnel formula to calculate the reflection phase shift of the incident light wave after reflection from the multilayer film system of the excitation structure when the dielectric layer 2 is liquid 1 and air, respectively. The difference between the two is the reflection phase shift difference φ1(n1, d1, d2). Given the variation range of d1 and d2, the resonance surface Φ1(φ1, d1, d2) where φ1 varies with d1 and d2 is obtained. Similarly, calculate the reflection phase shift difference surface Φ2(φ2, d1, d2) corresponding to liquid 2 when the light wave incident angle is θ2. Step 2: Build a surface plasmon resonance holographic microscopy measurement system based on the Kretschmann structure. Set the incident angle of the excitation light wave to θ1. When the dielectric layer 2 is liquid 1, record the reflected light wave as the digital hologram H1 formed by the interference between the object light wave and the reference light wave. Replace the dielectric layer 2 with air and record the digital hologram H2. Step 3: Adjust the incident angle of the excitation light wave in the measurement system to θ2, set the dielectric layer 2 to liquid 2, and use the same method as step 2 to record digital holograms H3 and H4; Step 4: Use the principle of light diffraction to calculate the phase distributions 1-4 of the four digital holograms. Subtract phase distribution 1 from phase distribution 2 to get the phase difference distribution. Phase distribution 4 minus phase distribution 3 to get the phase difference distribution (x,y) represents the coordinates in two-dimensional space; Step 5: The reflected phase shift difference φ1(x,y) is numerically compared with the experimentally measured reflected light wave phase difference Equal, φ2(x,y) and Equal, according to experimental measurements and Corresponding to the theoretically calculated resonance surfaces Φ1(φ1, d1, d2) and Φ2(φ2, d1, d2), two monotonically changing resonance curves are projected in the coordinate system with d1 and d2 as variables. The two thickness values ​​corresponding to the unique intersection of the curves are the measured d1(x, y) and the measured d2(x, y), that is, the two-dimensional thickness distribution of metal layer 1 and metal layer 2.

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