Method for nondestructive measurement of thickness of non-transparent film based on acoustic surface wave resonance

Abstract

The invention belongs to the field of surface acoustic wave nondestructive testing equipment, and particularly relates to a method for nondestructive measurement of the thickness of a non-transparent film in a film / substrate layered structure based on a surface acoustic wave resonator, and the method comprises the steps: building an experimental platform for nondestructive testing of the thickness of the non-transparent film through the surface acoustic wave resonator; obtaining an S11 parameter frequency domain curve of the surface acoustic wave resonator under different film thicknesses by using a vector network analyzer, obtaining a corresponding resonant frequency, and fitting a relation curve of the film thickness and the resonant frequency; and the film thickness of the to-be-measured sample is experimentally measured, and the accuracy of measuring the thickness of the non-transparent film by the surface acoustic wave resonator is verified. According to the method, the limitation of an SEM method and an ellipsometry method in the field of lossless characterization of the thickness of the non-transparent film is effectively made up.

Description

Technical Field

[0001] The invention belongs to the field of surface acoustic wave nondestructive testing equipment, and in particular relates to a method for nondestructively measuring the thickness of a non-transparent film in a film / substrate layered structure based on a surface acoustic wave resonator. Background Art

[0002] With the increasing application of ultra-thin film materials in modern integrated circuit research and industry, the technology for non-destructive testing of non-transparent film properties is becoming more and more important. At present, common film thickness measurement technologies include scanning electron microscopy (SEM) and ellipsometry. Among them, the SEM method is a destructive test and requires slicing of the sample. The film measured by the ellipsometry method should be transparent, and there are limitations in the thickness measurement of non-transparent films. Surface acoustic wave technology is a new detection method that can be used to detect the mechanical properties of thin films. The surface acoustic wave resonator non-destructive measurement system uses interdigitated electrodes to excite surface acoustic waves. It has the advantages of low cost and easy portability, which is conducive to the instrumentation of surface acoustic wave non-destructive testing technology. It has become an important means for non-destructive testing of thin film material properties. Based on the surface acoustic wave resonator non-destructive testing method, the present invention constructs a surface acoustic wave resonator non-destructive testing system, and obtains the S of the surface acoustic wave resonator under different film thicknesses through a vector network analyzer (VNA). 11 The parameter curve is obtained to obtain the resonant frequency of the resonator, and the thickness of the film to be measured and the resonant frequency curve are fitted to realize the characterization of the thickness of the non-transparent Pt metal film. Summary of the invention

[0003] The purpose of the present invention is to provide a method for non-destructively measuring the thickness of a non-metallic film in a film / substrate layered structure using a surface acoustic wave resonator. An experimental platform for non-destructive testing of film thickness using a surface acoustic wave resonator is built. The S of the surface acoustic wave resonator under different metal film thicknesses is measured experimentally. 11 The parameter frequency domain curve is used to obtain the resonant frequency of the resonator, and the relationship curve between the film thickness and the resonant frequency is fitted to achieve the characterization of the thickness of the non-transparent Pt metal film. The technical solution of the present invention is as follows: (1) Build an experimental platform for nondestructive testing of film thickness of surface acoustic wave resonators to study the effect of the film thickness on the resonant frequency of the surface acoustic wave resonator; (2) By replacing samples with different film thicknesses, the S of the surface acoustic wave resonator under different film thicknesses was obtained on a vector network analyzer. 11 Parameter frequency domain curve, and obtain the corresponding resonant frequency; (3) The relationship curve between film thickness and resonant frequency is fitted according to the obtained resonant frequency. A sample with a specific film thickness is placed on the experimental platform. The S of the surface acoustic wave resonator is experimentally measured. 11 The curve and resonant frequency are substituted into the obtained fitting curve to verify the accuracy of the surface acoustic wave resonator in measuring the thickness of non-transparent films. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figure 1 Schematic diagram of the surface acoustic wave resonator nondestructive testing equipment.

[0005] Figure 2 Non-destructive characterization of film thickness experiment using surface acoustic wave resonator.

[0006] Figure 3 The S of the surface acoustic wave resonator is measured under different metal film thicknesses. 11 Parameter frequency domain curve.

[0007] Figure 4 Fitting curve of metal film thickness and resonant frequency of surface acoustic wave resonator. DETAILED DESCRIPTION

[0008] In order to better understand the present invention, the content of the present invention is further described below in conjunction with the accompanying drawings and examples.

[0009] (1) Resonant surface acoustic wave sensors detect the resonant frequency f The offset of 0 establishes the corresponding relationship with the external environment parameters, and the characteristic frequency of the interdigital electrode is:

[0010] In the formula, v s The propagation velocity of the surface acoustic wave on the piezoelectric substrate. The film / substrate structure sample to be tested is placed above the SAW resonator, completely covering the interdigital electrodes and reflective grating structure of the surface acoustic wave resonator. At this time, the propagation velocity of the surface acoustic wave v s The structure of the film / substrate covering the resonator changes. Changing the film thickness will increase the propagation speed of the surface acoustic wave. v s As the thickness of the film changes, the resonant frequency of the surface acoustic wave resonator also changes. By establishing a fitting curve of the film thickness and the resonant frequency of the surface acoustic wave resonator based on the corresponding relationship between the film thickness and the resonant frequency of the resonator, the thickness of the film to be measured can be characterized.

[0011] (2) Figure 1 This is a schematic diagram of the nondestructive testing equipment for surface acoustic wave resonators. The piezoelectric substrate is made of 128°YX lithium niobate. The interdigital electrodes and the reflective grating are placed on the lithium niobate substrate. The electrode material is all made of metal aluminum. The electrode thickness is h The width of the reflective grating and the interdigital electrode is 600 nm. a The electrode spacing is 3.5 mm b The gap between the reflector and the interdigital electrode is 3.5 mm. LThe interdigital electrode aperture is 5.25 mm. W The thickness of the reflector is 5 mm, the number of interdigital electrode pairs is 40, the number of short-circuit grids is 50, and the silicon dioxide guide layer covers and wraps the metal electrode to reduce the temperature effect of the surface acoustic wave resonator. h 1 is 1.5 mm, and the film thickness measurement range of the surface acoustic wave resonator is 100 nm~1000 nm;

[0012] (3) Build a surface acoustic wave resonator non-destructive characterization of film thickness experimental platform such as Figure 2 As shown, the surface acoustic wave resonator is pasted on the PCB board. The surface acoustic wave resonator is connected to the SMA connector through conductive silver paste, and the surface acoustic wave resonator is connected to the vector network analyzer through a wire with an SMA connector. The sample tested is a Pt film prepared by magnetron sputtering. The film is sputtered on a low-resistance heavily doped P-type Si

[100] substrate with a film density of 21.45 g / cm 3 , Young's modulus is 168 GPa. The film thicknesses are 98.2 nm, 203.5 nm, 298.7 nm, 502.7 nm, 704.5 nm, and 995.9 nm, respectively. The 203.5 nm film thickness sample is used as the verification group, and the other thickness samples are used as the experimental group. (4) Based on the SAW resonator non-destructive characterization film thickness experimental platform in step (3), the Pt film thickness in the sample with a film / substrate structure of about 1 cm × 2 cm was non-destructively characterized. All tests were performed at room temperature (20°C) and in an environment without mechanical vibration or electromagnetic interference that would interfere with the test accuracy. During the test, each time the sample to be tested was changed to a different thickness, the same sample was repeated three times. 11 After the curve is stable, extract the curve data and filter the curve to remove the influence of noise on the curve to obtain a smooth curve. Take the average of the three curves to get the S value of the sample. 11 The parameter curve is Figure 3 As shown; (5) For any film thickness S 11 Curve, when S 11 When the value of is the smallest, the surface acoustic wave resonator is in a resonant state, and the corresponding frequency value at this time is the resonant frequency of the surface acoustic wave resonator. Figure 3 It can be seen that the change in the thickness of the material to be tested will cause S 11The curve shifts, that is, the resonant frequency changes. When the thickness of the material to be tested is 98.2 nm, the resonant frequency is 332.31 MHz. As the thickness of the material to be tested increases, the resonant frequency decreases monotonically. When the thickness of the material to be tested is 995.9 nm, the resonant frequency is 266.69 MHz. Record the thickness of the material to be tested and the corresponding resonant frequency value, perform curve fitting on the data, and obtain the theoretical fitting curve of the resonant frequency of the surface acoustic wave resonator and the thickness of the Pt film as shown below: Figure 4 As shown, the fitting polynomial is:

[0013] in, th is the thickness of the film to be measured, f 0 is the resonant frequency of the surface acoustic wave resonator. Three consecutive experiments were carried out on the verification group samples with a film thickness of 203.5nm, and three groups of S 11 The parameter frequency domain curve is obtained, and the corresponding resonator resonant frequency parameters are extracted according to the curve. The resonant frequency obtained by the test is substituted into the Pt film thickness-frequency fitting polynomial to calculate the predicted value of the film thickness of the measured sample, and then the relative error and relative standard deviation are calculated. The measurement results are shown in Table 1. It can be concluded from the table that the maximum relative error of the surface acoustic wave resonator non-destructive thickness measurement system for the non-transparent Pt film thickness measurement is 1.84%, and the measurement accuracy is high. The relative standard deviation is 0.46%, indicating that this method has good precision and repeatability for the non-transparent Pt film thickness measurement.

[0014] Table 1 Pt metal film thickness measurement results of the verification group

[0015] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

[0016] The above are only embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included within the scope of the claims of the present application.

Claims

1. A method for non-destructive measurement of the thickness of a non-transparent film in a film / substrate layered structure based on a surface acoustic wave resonator, characterized in that The following steps are involved: (1) Build an experimental platform for nondestructive testing of film thickness of surface acoustic wave resonators to study the effect of the film thickness on the resonant frequency of the surface acoustic wave resonator; (2) By replacing samples with different film thicknesses, the S of the surface acoustic wave resonator under different film thicknesses was obtained on a vector network analyzer. 11 Parameter frequency domain curve, and obtain the corresponding resonant frequency; (3) Fitting the relationship curve between film thickness and resonant frequency, placing a sample with a specific film thickness on the experimental platform, and experimentally measuring the S of the surface acoustic wave resonator 11 The curve and resonant frequency are substituted into the obtained fitting curve to verify the accuracy of the surface acoustic wave resonator in measuring the thickness of non-transparent films.

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