A method for detecting the thickness of a terahertz film

Abstract

The application provides a method for detecting the thickness of a terahertz film, and comprises the following steps: step one, thick film detection: terahertz detection is performed on a thick film to obtain a time-domain signal; step two, extraction of a function model of a front pulse signal waveform: a function model of a complete front pulse signal waveform is extracted from the time-domain signal; step three, thin film detection: terahertz detection is performed on a thin film to obtain a time-domain signal of a damaged front pulse signal; step four, fitting of a thin film waveform: step five, calculation of the thickness of the thin film: the thickness of the thin film is calculated based on the time of flight between the complete peak waveform of the restored front pulse signal and the complete peak waveform of a detected rear pulse signal. The application can improve the minimum detection thickness of a terahertz time-domain spectroscopy system from more than 20 microns to several microns by processing the signal. The method is simple in principle, high in detection efficiency, and accurate in result, and is expected to be applied to the nondestructive and high-precision detection of the thickness of ultrathin materials in practical engineering.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of spectrum detection and signal processing, and particularly relates to a method for detecting the thickness of a thin film by using terahertz. BACKGROUND

[0002] Non-destructive testing of the thickness of a thin film, a coating or other ultra-thin objects is an important problem in engineering applications. Terahertz time-domain spectroscopy technology based on terahertz waves has a high time resolution, and is non-ionizing, non-destructive and non-contact, and is considered to be a promising tool for non-destructive evaluation of the thickness of a thin coating. Among them, the terahertz time-domain spectroscopy technology can detect the terahertz time-domain signal of the sample, and the thickness of the sample can be quickly calculated according to the time-domain signal by using the time-of-flight method. However, when detecting ultra-thin objects, the two pulse signals in the time-domain signal are too close, and the terahertz pulse signal itself has a certain pulse width, so the two pulse signals will overlap, resulting in damage to the waveform of the front pulse signal, so that the time of flight between the front and rear pulse signals cannot be determined. This makes the minimum thickness of the object that can be measured by the terahertz time-domain spectroscopy technology combined with the time-of-flight method remain at more than twenty microns, and the thickness of some ultra-thin thin films, coatings and other objects is often only a few microns or even a few microns, which exceeds the detection limit of the terahertz spectrum, limiting the application of the terahertz detection technology in the field of ultra-thin thickness thin films. SUMMARY

[0003] In view of the deficiencies in the prior art, the present application provides a method for detecting the thickness of a thin film by using terahertz. The present application can improve the minimum detection thickness of a terahertz time-domain spectroscopy system from more than twenty microns to a few microns by processing the signal. The method of the present application has a simple principle, high detection efficiency and accurate results, and is expected to be applied to non-destructive high-precision detection of the thickness of ultra-thin materials in practical engineering.

[0004] The present application achieves the above technical object by the following technical means.

[0005] A method for detecting the thickness of a thin film by using terahertz, specifically comprising the following steps:

[0006] Step one, detection of thick and thin films: terahertz detection of thick and thin films to obtain a time-domain signal;

[0007] Step two, extraction of a function model of the waveform of the front pulse signal: extracting a complete function model of the waveform of the front pulse signal from the time-domain signal;

[0008] Step three, detection of thin films: terahertz detection of thin films to obtain a time-domain signal of a damaged front pulse signal;

[0009] Step four, fitting of the thin film waveform: the function model of the extracted pre-pulse signal waveform in step two is used to fit the damaged time domain signal of the pre-pulse signal in step three, and the complete peak waveform of the pre-pulse signal is restored.

[0010] Step five, calculation of the thickness of the thin film: based on the time of flight between the complete peak waveform of the restored pre-pulse signal and the complete peak waveform of the detected post-pulse signal, the thickness of the thin film is calculated.

[0011] In the above scheme, the time domain signal of the thick-thin film is obtained by terahertz time domain spectroscopy detection in step one, and two pulse signals will exist in the time domain signal; the first pulse signal appearing in the time domain is called the pre-pulse signal, and the waveform data thereof is derived.

[0012] In the above scheme, the model fitting program is used to extract the model parameters of the derived pre-pulse signal waveform data in step two.

[0013] In the above scheme, the model fitting program adopts a Gaussian function model, and the selected pre-pulse signal waveform is fitted by the Gaussian function model to extract specific model parameters.

[0014] In the above scheme, the thick-thin film is detected by terahertz time domain spectroscopy in step three, wherein the peak waveform of the pre-pulse signal of the thin film is destroyed by the post-pulse signal, and the peak position of the pre-pulse signal cannot be identified, thereby obtaining a damaged time domain signal of the pre-pulse signal.

[0015] In the above scheme, the time of flight in step five refers to the time interval between the peak of the post-pulse signal existing only in the time domain signal of the thin film and the peak of the pre-pulse signal fitted by the thin film, and the specific formula for calculating the thickness of the thin film by the time of flight method is d, the thickness of the thin film, Δt, the time interval, n, the refractive index of the thin film, and c, the speed of light.

[0016] In the above scheme, the thick-thin film and the thin film are made of the same material.

[0017] In the above scheme, the thickness of the thick-thin film is greater than 30 μm.

[0018] In the above scheme, the thickness of the thin film is 4 μm to 25 μm.

[0019] The beneficial effects of the present application are as follows:

[0020] 1) The present application adopts a model fitting method to extract a complete Gaussian function model of the peak waveform of the pre-pulse signal from the time domain signal of the thick-thin film, and applies the model to the fitting of the incomplete peak waveform of the pre-pulse signal of the thin film, thereby obtaining the complete peak waveform of the pre-pulse signal of the thin film and determining the peak position.

[0021] 2) The method provided by the application can greatly improve the detection level of the terahertz time-domain spectroscopy system, and improve the minimum detection thickness from more than 20 microns to several microns, so that the thickness of a thin film can be detected.

[0022] 3) The signal processing method provided by the application is realized by programming, and a large amount of data involved in the fitting process is calculated by a computer, so that the calculation is simple, efficient and accurate, and the method can be applied to the nondestructive high-precision detection of the thickness of an ultrathin material in practical engineering. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 A schematic diagram of a time-domain signal waveform measured for a thick film;

[0024] Figure 2 A schematic diagram of extracting a specific function model from the time-domain signal of a thick film;

[0025] Figure 3 A schematic diagram of a time-domain signal waveform measured for a thin film;

[0026] Figure 4 A schematic diagram of a complete pre-pulse signal waveform obtained after model fitting of an incomplete pre-pulse signal waveform in the time-domain signal of a thin film. DETAILED DESCRIPTION

[0027] The embodiments of the application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the application, and cannot be understood as a limitation of the application.

[0028] Specifically, as shown in the accompanying drawings, a method for efficiently and accurately detecting the thickness of a film by terahertz time-domain spectroscopy according to an embodiment of the application comprises:

[0029] Step 1, detection of a thick film: as shown in Figure 1 , a thick film is detected by terahertz to obtain its time-domain signal;

[0030] Figure 1 In the thick film, the signal detected by the terahertz time-domain spectroscopy system will have two pulse signals before and after, and the amplitude of the pre-pulse signal will be smaller than that of the post-pulse signal. The amplitude of a single complete pulse signal will first rise, reach a peak, then start to decline, and then rise again after reaching the lowest point below the horizontal line (the initial amplitude line without signal), and finally return to the horizontal line. Figure 1 In the thick film, the pre-pulse signal and the post-pulse signal both conform to the above waveform change, indicating that the waveforms of the two are relatively complete, and the positions of the respective peaks can be identified.

[0031] Step two, extraction of the pre-pulse model: as shown in Figure 2 , the specific parameters of the Gaussian function model of the pre-pulse signal waveform in the time domain signal of the thick film are extracted;

[0032] Figure 2 In the method, the waveform data of the peak part of the pre-pulse signal in the time domain signal is selected, and the specific model parameters of the Gaussian function model that best fit the waveform data are extracted through a fitting program, that is, a specific Gaussian function is used to describe the peak waveform of the pre-pulse signal.

[0033] Step three, detection of the thin film: as shown in Figure 3 , the time domain signal of the thin film is measured, wherein the pre-pulse signal is affected by the post-pulse signal, the peak part is damaged, and the pre-pulse signal is incomplete, and the peak position cannot be identified.

[0034] Figure 3 In the method, it can be seen from the waveform of the measured time domain signal of the thin film that the pre-pulse signal is interrupted by the appearance of the post-pulse signal in the initial amplitude rising stage, resulting in only part of the pre-pulse signal in the time domain signal of the thin film, the pre-pulse signal is incomplete, and especially the peak part disappears, so that the peak position cannot be identified.

[0035] Step four, fitting of the waveform of the thin film: as shown in Figure 4 , the part of the signal belonging to the pre-pulse signal in the time domain signal of the thin film is fitted, the complete peak waveform of the pre-pulse signal is obtained, and the position of the peak and the flight time are determined.

[0036] Figure 4 In the method, it can be seen from the waveform of the time domain signal that the pre-pulse signal is incomplete, and only part of the initial rising signal exists. The waveform data of the signal is selected and input into the fitting program, and the Gaussian model function of the pre-pulse signal peak waveform extracted from the time domain signal of the thick film of the same kind is input into the fitting program. The complete peak waveform data and waveform of the pre-pulse signal in the time domain signal of the thin film are fitted, the peak position of the pre-pulse signal is determined, and the flight time between the pre-pulse signal and the post-pulse signal is calculated.

[0037] Step five, calculation of the thickness of the thin film: on the basis of obtaining the flight time, the thickness of the thin film can be quickly calculated by the classical flight time method, and the thickness detection of the thin film is realized.

[0038] Example 1

[0039] First, the terahertz time domain spectroscopy detection is performed on the PI film with a thickness of 108 microns, and the time domain signal is obtained.

[0040] Then the model fitting program written by Gaussian function model and particle swarm algorithm is used to extract the Gaussian model parameters of the waveform data of the front pulse signal peak part in the time domain signal of the 108-micron PI film, and the specific function of the PI film based on the Gaussian function model is obtained.

[0041] Then the terahertz time-domain spectroscopy is performed on the 23-micron PI film, and the time domain signal thereof is obtained.

[0042] Then the waveform data of the front pulse signal part in the time domain signal waveform graph of the 23-micron PI film is imported into the model fitting program, and the specific function of the front pulse signal peak waveform of the 108-micron PI film based on the Gaussian function model is filled into the model fitting program. Running the model fitting program can obtain the complete peak waveform of the front pulse signal in the time domain signal of the 23-micron PI film and determine the position of the peak, and further obtain the flight time between the front and rear pulse signals as 0.334 ps. The fitting thickness of the 23-micron PI film is calculated as 25.7 microns by using the flight time method.

[0043] Example 2

[0044] First, the terahertz time-domain spectroscopy is performed on the 108-micron PI film, and the time domain signal thereof is obtained.

[0045] Then the model fitting program written by Gaussian function model and particle swarm algorithm is used to extract the Gaussian model parameters of the waveform data of the front pulse signal peak part in the time domain signal of the 108-micron PI film, and the specific function of the PI film based on the Gaussian function model is obtained.

[0046] Then the terahertz time-domain spectroscopy is performed on the 14-micron PI film, and the time domain signal thereof is obtained.

[0047] Then the waveform data of the front pulse signal part in the time domain signal waveform graph of the 14-micron PI film is imported into the model fitting program, and the specific function of the front pulse signal peak waveform of the 108-micron PI film based on the Gaussian function model is filled into the model fitting program. Running the model fitting program can obtain the complete peak waveform of the front pulse signal in the time domain signal of the 14-micron PI film and determine the position of the peak, and further obtain the flight time between the front and rear pulse signals as 0.198 ps. The fitting thickness of the 14-micron PI film is calculated as 15.2 microns by using the flight time method.

[0048] Example 3

[0049] First, the terahertz time-domain spectroscopy is performed on the 108-micron PI film, and the time domain signal thereof is obtained.

[0050] Then, the waveform data of the front pulse signal peak part in the time domain signal of the 108-micron PI film is input into the model fitting program, and the specific function of the front pulse signal peak waveform based on the Gaussian function model of the 108-micron PI film is filled into the model fitting program.

[0051] Then, the time domain signal of the 9-micron PI film is obtained.

[0052] Then, the waveform data of the front pulse signal peak part in the time domain signal of the 108-micron PI film is input into the model fitting program, and the specific function of the front pulse signal peak waveform based on the Gaussian function model of the 108-micron PI film is filled into the model fitting program.

[0053] Example 4

[0054] First, the time domain signal of the 108-micron PI film is obtained.

[0055] Then, the waveform data of the front pulse signal peak part in the time domain signal of the 108-micron PI film is input into the model fitting program, and the specific function of the front pulse signal peak waveform based on the Gaussian function model of the 108-micron PI film is filled into the model fitting program.

[0056] Then, the time domain signal of the 7-micron PI film is obtained.

[0057] Then, the waveform data of the front pulse signal peak part in the time domain signal of the 108-micron PI film is input into the model fitting program, and the specific function of the front pulse signal peak waveform based on the Gaussian function model of the 108-micron PI film is filled into the model fitting program.

[0058] In combination with Examples 1-4, the film thickness obtained by the method of the present application is close to the actual thickness, and according to the above data, the relative error of the fitting result of the film thickness is kept at 7%-12%, which indicates that the actual thickness of the film can be accurately measured by the method of the present application, and the detection lower limit of the terahertz time-of-flight method for measuring thickness is reduced to 7 microns from 25 microns

[0059] Example 5

[0060] TGO film

[0061] First, the 204-micron-thick alumina block with the same material composition as the TGO film was subjected to terahertz time-domain signal detection to obtain its time-domain signal.

[0062] Next, the time-domain signal of the alumina block was processed by using the model fitting program written by the high function model and the particle swarm algorithm, the waveform data of the front pulse signal in the signal were extracted for Gaussian model parameter, and the specific function based on the Gaussian function model was obtained.

[0063] Then, the 4.8-micron-thick TGO film sample was subjected to terahertz time-domain spectroscopy detection to obtain its time-domain signal.

[0064] Next, the part of the signal waveform data in the time-domain signal waveform graph of the 4.8-micron-thick TGO coating that belongs to the front pulse signal was imported into the model fitting program, and the specific function of the front pulse signal peak waveform of the 204-micron-thick alumina block based on the Gaussian function model was filled into the model fitting program. Running the model fitting program can obtain the complete peak waveform of the front pulse signal in the time-domain signal of the 4.8-micron-thick TGO coating and determine the position of the peak, and further obtain the flight time between the front and rear pulse signals as 0.0759 ps. The fitting thickness of the 4.8-micron-thick TGO coating was calculated as 3.93 microns by using the flight time method.

[0065] Example 6

[0066] TGO film

[0067] First, the 204-micron-thick alumina block with the same material composition as the TGO film was subjected to terahertz time-domain signal detection to obtain its time-domain signal.

[0068] Next, the time-domain signal of the alumina block was processed by using the model fitting program written by the high function model and the particle swarm algorithm, the waveform data of the front pulse signal in the signal were extracted for Gaussian model parameter, and the specific function based on the Gaussian function model was obtained.

[0069] Then, the 4-micron-thick TGO film sample was subjected to terahertz time-domain spectroscopy detection to obtain its time-domain signal.

[0070] Then, the part of the signal waveform data of the front pulse signal in the time-domain signal waveform diagram measured by the 4-μm TGO coating is introduced into the model fitting program, and the specific function of the front pulse signal peak waveform of the 204-μm alumina block based on the Gaussian function model is filled into the model fitting program. Running the model fitting program, the complete peak waveform of the front pulse signal in the time-domain signal measured by the 4-μm TGO coating is obtained, and the position of the peak is determined, and then the flight time between the front and rear pulse signals is 0.0635 ps, and the fitting thickness of the 4-μm TGO coating is calculated by the flight time method to be 3.28 μm.

[0071] In combination with Embodiments 5 and 6, the TGO coating thickness is relatively close to the actual thickness, and the actual error is about 0.8 μm, and the minimum detection thickness is 3-4 μm.

[0072] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily mean the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0073] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application without departing from the principles and purposes of the present application.

Claims

1. A method of terahertz detection of film thickness, characterized by, Specifically comprising the following steps: Step one, detection of thick film: terahertz detection is performed on the thick film to obtain a time-domain signal; the thick film has a thickness greater than 30 μm; Step two, extraction of a function model of a pre-pulse signal waveform: a function model of a complete pre-pulse signal waveform is extracted from the time-domain signal; Step three, detection of thin film: terahertz time-domain spectroscopy is performed on the thin film to obtain a time-domain signal of a damaged pre-pulse signal; The thin film has a thickness of 4 μm to 25 μm; terahertz time-domain spectroscopy is performed on the thin film; the peak waveform of the pre-pulse signal of the thin film is destroyed by the post-pulse signal, and the position of the peak of the pre-pulse signal cannot be identified, so that the time-domain signal of the damaged pre-pulse signal is obtained; Step four, fitting of a thin film waveform: the function model of the pre-pulse signal waveform extracted in step two is used to fit the time-domain signal of the damaged pre-pulse signal in step three, so as to restore the complete peak waveform of the pre-pulse signal. Step five, calculation of the thickness of the thin film: based on the complete peak waveform of the pre-pulse signal before reduction and the complete peak waveform of the detected post-pulse signal, the thickness of the thin film is calculated; the time interval Δt between the only existing post-pulse signal peak in the time domain signal of the thin film and the pre-pulse signal peak fitted out of the thin film is the flight time, and the specific formula for calculating the thickness of the thin film by the flight time method is d thickness of the thin film, Δt time interval, n refractive index of the thin film, c speed of light.

2. The method of claim 1, wherein the terahertz detection film thickness is determined by: In step one, terahertz time-domain spectroscopy is performed on the thick film to obtain a time-domain signal; two pulse signals exist in the time-domain signal; the first pulse signal appearing in the time domain is referred to as a pre-pulse signal, and the waveform data thereof is derived.

3. The method of claim 1, wherein the terahertz detection film thickness is determined by: In step two, a model fitting program is used to extract model parameters from the derived pre-pulse signal waveform data.

4. The method of claim 3, wherein the terahertz detection film thickness is determined by: The model fitting program uses a Gaussian function model; the Gaussian function model is used to fit the selected pre-pulse signal waveform, and the specific model parameters thereof are extracted.

5. The method of claim 1, wherein the terahertz detection film thickness is determined by a terahertz time-domain spectroscopy (THz-TDS) method. The thick film and the thin film are made of the same material.

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