Non-transparent film thickness measuring method and system based on ultrafast photoacoustic spectrometry

Abstract

The invention discloses a non-transparent film thickness measurement method and system based on ultrafast photoacoustic spectroscopy. The method comprises the following steps: (1) superposing a two-dimensional semiconductor film on a non-transparent film; based on a femtosecond laser pumping detection technology, pump light and detection light with different time delays are generated, the pump light and the detection light with different time delays are focused on the surface of the two-dimensional semiconductor film, and a time-resolved ultrafast photoacoustic spectrum reflected by the surface of the two-dimensional semiconductor film is detected; performing background removal processing on the time-resolved ultrafast photoacoustic spectrum to obtain picosecond sound wave pulse echo signals under different time delays; and extracting a time delay t corresponding to an echo moment in the picosecond sound wave pulse echo signal, and calculating the thickness d of the non-transparent film according to a formula d = vt / 2 in combination with a sound wave transmission speed v in the non-transparent film. The method has the technical advantages of high spatial resolution, rapidness and no damage.

Description

Technical Field

[0001] The invention relates to a non-transparent film thickness measurement technology, and in particular to a non-transparent film thickness measurement method and system based on ultrafast photoacoustic spectroscopy. Background Art

[0002] Non-transparent films such as metal materials are widely used in optoelectronics, microelectronic devices and chips, and are important functional structures of microsystems. For example, in electrically pumped micro-nano lasers (such as vertical cavity surface emitting lasers, VCSELs), non-transparent films of a specific thickness are used as high-reflectivity mirrors or electrode layers, and their performance directly affects the output power and beam quality of the laser; or in microelectronic chips, metal films are usually used as electrode layers, and their thickness has an important influence on the chip's conductivity, heat dissipation and mechanical stability. Accurately measuring the thickness of non-transparent films is essential in the controllable manufacturing and integration process of microsystems.

[0003] However, it is difficult for light waves to penetrate this type of non-transparent film, and conventional optical interference methods for measuring film thickness are no longer applicable. At present, the main methods for accurately measuring the thickness of non-transparent films include scanning electron microscopy (SEM), atomic force microscopy (AFM), etc. However, these methods have some limitations in practical applications: (1) The measurement speed is extremely slow and cannot meet the needs of large-scale rapid detection; (2) The measurement process usually requires a cross-sectional structure, and it is impossible to perform non-destructive measurement of uniform films; (3) The measurement requires a vacuum and vibration-free laboratory environment, which cannot meet the requirements of working condition measurement and is not universal. Therefore, it is important to develop a non-destructive testing method for measuring the thickness of non-transparent metal films to achieve rapid and accurate measurement of non-transparent films to meet the needs of efficient and reliable detection in industrial production, which has important industrial application value. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention aims to provide a non-transparent thin film thickness measurement method and system based on ultrafast photoacoustic spectroscopy, which is fast, highly universal and non-destructive.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] A non-transparent film thickness measurement method based on ultrafast photoacoustic spectroscopy comprises the following steps:

[0007] (1) stacking a two-dimensional semiconductor thin film on a non-transparent film, wherein the two-dimensional semiconductor thin film includes a plurality of two-dimensional semiconductor layers;

[0008] (2) Based on the femtosecond laser pump-probe technology, pump light and probe light with different time delays are generated, and the pump light and probe light with different time delays are focused on the surface of the two-dimensional semiconductor film to detect the picosecond acoustic wave pulse echo signals reflected from the surface of the two-dimensional semiconductor film under different time delays, i.e., time-resolved ultrafast photoacoustic spectroscopy;

[0009] (3) performing background removal processing on the time-resolved ultrafast photoacoustic spectrum to obtain picosecond acoustic wave pulse echo signals under different time delays;

[0010] (4) The time delay t corresponding to the echo moment in the picosecond acoustic pulse echo signal is extracted, and combined with the acoustic wave transmission speed v in the non-transparent film, the thickness d of the non-transparent film is calculated according to the formula d=vt / 2.

[0011] Furthermore, step (2) specifically includes:

[0012] (2.1) generating pump light and probe light with time delay;

[0013] (2.2) focusing the pump light on the surface of the two-dimensional semiconductor film, so that coherent acoustic phonon oscillations are generated between the layers of the two-dimensional semiconductor film, thereby emitting picosecond acoustic wave pulses into the non-transparent film. When the picosecond acoustic wave pulses propagate to the bottom interface of the non-transparent film, they are reflected back to the two-dimensional semiconductor film-opaque film interface;

[0014] (2.3) When the time-delayed probe light reaches the surface of the two-dimensional semiconductor film, the picosecond acoustic wave pulse modulates the probe light reflected from the surface of the two-dimensional semiconductor film, forming a picosecond acoustic wave pulse echo signal;

[0015] (2.4) By setting the time delay of the detection light and the pump light to different values, the time-resolved ultrafast photoacoustic spectrum reflected from the surface of the two-dimensional semiconductor film is detected.

[0016] Furthermore, step (3) specifically includes:

[0017] (3.1) fitting the time-resolved photoacoustic spectrum using a double exponential decay function to obtain a fitting curve;

[0018] (3.2) Subtract the fitting curve from the time-resolved photoacoustic light to extract the picosecond acoustic wave pulse echo signal.

[0019] A non-transparent film thickness measurement system based on ultrafast photoacoustic spectroscopy, comprising:

[0020] A femtosecond laser pump-probe device is used to generate pump light and probe light with different time delays, and focus the pump light and probe light with different time delays on the surface of a two-dimensional semiconductor film. The surface of the two-dimensional semiconductor film reflects picosecond acoustic wave pulse echo signals under different time delays, i.e., time-resolved ultrafast photoacoustic spectroscopy, wherein the two-dimensional semiconductor film includes several two-dimensional semiconductor layers, which are superimposed on a non-transparent film;

[0021] A spectrum processing module, used for performing background removal processing on the time-resolved ultrafast photoacoustic spectrum to obtain picosecond acoustic wave pulse echo signals under different time delays;

[0022] The thickness calculation module is used to extract the time delay t corresponding to the echo moment of the picosecond acoustic wave pulse, and combine the acoustic wave transmission speed v in the non-transparent film to calculate the thickness d of the non-transparent film according to the formula d=vt / 2.

[0023] Furthermore, the femtosecond laser pumping detection device specifically includes:

[0024] A laser for generating a pump light and a probe light with a time delay;

[0025] A time delay device, used to delay the detection light from reaching the surface of the two-dimensional semiconductor film compared to the pump light, and to adjust the time delay to different values;

[0026] A microscope objective lens, used to focus the pump light and the probe light onto the surface of the two-dimensional semiconductor film;

[0027] The optical detection module is used to detect the picosecond acoustic wave pulse echo signal reflected from the surface of the two-dimensional semiconductor film under different time delays when the time delay device adjusts the time delay to different values, that is, time-resolved ultrafast photoacoustic spectroscopy; wherein the picosecond acoustic wave pulse echo signal is formed in the following process: when the pump light is focused on the surface of the two-dimensional semiconductor film, the two-dimensional semiconductor film is stimulated to produce longitudinal coherent acoustic phonon oscillations, thereby emitting picosecond acoustic wave pulses into the non-transparent film; when the picosecond acoustic wave pulse propagates to the bottom interface of the non-transparent film, it is reflected back to the two-dimensional semiconductor film-opaque film interface; when the detection light after time delay reaches the surface of the two-dimensional semiconductor film, the picosecond acoustic wave pulse echo modulates the detection light reflected from the surface of the two-dimensional semiconductor film to form a picosecond acoustic wave pulse echo signal.

[0028] Furthermore, the spectrum processing module specifically includes:

[0029] A fitting unit, used for fitting the time-resolved ultrafast photoacoustic spectrum using a double exponential decay function to obtain a fitting curve;

[0030] The background removal unit is used to subtract the fitting curve from the time-resolved ultrafast photoacoustic signal, thereby extracting a picosecond acoustic wave pulse echo signal.

[0031] Compared with the prior art, the present invention has the following beneficial effects: the present invention uses two-dimensional semiconductor materials as the emission source and detector of picosecond acoustic wave pulses, can realize in-situ low-threshold light emission and high-sensitivity detection of high-frequency ultrasound, its frequency can reach the GHz level, and its wavelength is at the nanometer scale, so it has a spatial resolution detection capability higher than that of optical methods; through femtosecond laser pumping detection technology, it can realize all-optical, rapid, and non-destructive detection of the thickness of non-transparent medium films, without the need for specific conditions such as vacuum, which is extremely convenient. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A schematic flow chart of a non-transparent film thickness measurement method based on ultrafast photoacoustic spectroscopy provided in an embodiment of the present invention;

[0033] Figure 2 Schematic diagram of the femtosecond laser pumping and detection process;

[0034] Figure 3 The time-resolved ultrafast photoacoustic spectrum obtained by the embodiment of the present invention;

[0035] Figure 4 It is the picosecond acoustic wave pulse echo signal separated in the embodiment of the present invention. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0037] Embodiment 1

[0038] The embodiment of the present invention provides a non-transparent film thickness measurement method based on ultrafast photoacoustic spectroscopy, such as Figure 1 As shown, the following steps are included:

[0039] (1) Superimposing a two-dimensional semiconductor film on a non-transparent film.

[0040] In this embodiment, MoS2 is used as a two-dimensional semiconductor film and an aluminum (Al) film is used as a metal non-transparent film to be detected. The MoS2 film specifically includes multiple MoS2 layers. During preparation, the multiple MoS2 layers are first obtained by mechanical stripping, and then the stripped multiple MoS2 layers are superimposed on the aluminum film by dry transfer.

[0041] (2) Based on the femtosecond laser pump-probe technology, pump light and probe light with different time delays are generated, and the pump light and probe light with different time delays are focused on the surface of the two-dimensional semiconductor film. The picosecond acoustic wave pulse echo signals reflected from the surface of the two-dimensional semiconductor film under different time delays are detected, that is, time-resolved ultrafast photoacoustic spectroscopy.

[0042] This step specifically includes:

[0043] (2.1) Generate pump light and probe light with time delay; In this embodiment, a titanium sapphire femtosecond laser with a repetition frequency of 80MHz and a pulse width of about 100fs is selected as the light source, and a femtosecond laser with a wavelength of 800nm ​​is output; it is divided into two beams by a spectroscope, one beam is efficiently converted to 400nm by a BBO frequency doubling crystal, and used as pump light to achieve the excitation of longitudinal coherent acoustic phonon oscillations in MoS2, and the other 800nm ​​is used as the probe light. The reason why a 400nm wavelength is selected as the pump light in this embodiment is that MoS2 has a strong absorption of 400nm laser, which ensures that MoS2 has sufficient light absorption to generate coherent acoustic phonon oscillations and realize the emission of picosecond acoustic wave pulses. The power of the pump light and the probe light is adjusted to 0.5mw and 0.1mw respectively, and the two light spots are overlapped in space, and are focused on the same position on the surface of the two-dimensional semiconductor film through a microscope objective.

[0044] (2.2) The pump light is focused on the surface of the two-dimensional semiconductor film, so that the two-dimensional semiconductor film produces longitudinal coherent acoustic phonon oscillations, thereby emitting picosecond acoustic pulses into the aluminum film. When the picosecond acoustic pulses propagate to the interface formed by the aluminum film and the air, a picosecond acoustic pulse echo is generated; the echo will propagate in the reverse direction until it reaches the interface between the two-dimensional semiconductor film and the aluminum film, such as Figure 2 shown.

[0045] (2.3) When the time-delayed probe light reaches the surface of the two-dimensional semiconductor film, the picosecond acoustic wave pulse echo modulates the probe light reflected from the surface of the two-dimensional semiconductor film, forming a picosecond acoustic wave pulse echo signal. In this embodiment, the probe light is irradiated on the surface of the MoS2 film, and the picosecond acoustic wave pulse echo in step (2.2) modulates the reflected light from the surface of the MoS2 film, thereby forming a time-resolved ultrafast photoacoustic spectroscopy signal.

[0046] (2.4) By setting the time delay of the detection light and the pump light to different values, the time-resolved photoacoustic spectrum of the picosecond acoustic wave pulse echo signal is detected.

[0047] The time delay between the pump light and the probe light can be controlled by a delay device, such as a displacement platform and an optical signal delay line. The moment when the pump light and the probe light reach the surface of the MoS2 film at the same time is time 0. On the contrary, the pump light is irradiated onto the surface of the MoS2 film, and after a period of time, the probe light is irradiated onto the sample with a delay. The detected signal is different due to the different delay times, so that the time-resolved observation of the differential reflection signal of the probe light can be realized, that is, the time-resolved ultrafast photoacoustic spectroscopy. The time-resolved ultrafast photoacoustic spectroscopy detected in this embodiment is as follows Figure 3 shown.

[0048] The time-resolved ultrafast photoacoustic spectrum is obtained by balanced detection using a lock-in amplifier, thereby improving the signal-to-noise ratio. Specifically, it is achieved by using a mechanical chopper-modulated lock-in amplifier and a dual-channel balanced photodetector.

[0049] (3) Background removal is performed on the time-resolved ultrafast photoacoustic spectrum to obtain picosecond acoustic wave pulse echo signals under different time delays.

[0050] This step specifically includes:

[0051] (3.1) fitting the time-resolved photoacoustic spectrum using a double exponential decay function to obtain a fitting curve;

[0052] (3.2) Subtract the fitting curve from the time-resolved ultrafast photoacoustic light to extract the picosecond acoustic wave pulse echo signal. The picosecond acoustic wave pulse echo signal extracted in this embodiment is as follows: Figure 4 shown.

[0053] (4) The time delay t corresponding to the echo moment of the picosecond acoustic wave pulse is extracted, and combined with the acoustic wave transmission speed v in the non-transparent film, the thickness d of the non-transparent film is calculated according to the formula d=vt / 2.

[0054] exist Figure 3 In the figure, an oscillating wave packet of a picosecond acoustic pulse echo appears at a time delay of 413±5ps, and t=413ps can be used as the time delay t corresponding to the echo moment. Combined with the known speed of sound of aluminum of about 6400m / s, the thickness of the aluminum film can be calculated by the formula to be about 1321±16nm.

[0055] Embodiment 2

[0056] The embodiment of the present invention provides a non-transparent film thickness measurement system based on ultrafast photoacoustic spectroscopy, comprising:

[0057] A femtosecond laser pump-probe device is used to generate pump light and probe light with different time delays, and focus the pump light and probe light with different time delays on the surface of a two-dimensional semiconductor film, and detect the picosecond acoustic wave pulse echo signal reflected from the surface of the two-dimensional semiconductor film under different time delays, i.e., time-resolved ultrafast photoacoustic spectroscopy, wherein the two-dimensional semiconductor film includes several two-dimensional semiconductor layers, and is superimposed on a non-transparent film;

[0058] A spectrum processing module, used for performing background removal processing on the time-resolved ultrafast photoacoustic spectrum to obtain picosecond acoustic wave pulse echo signals under different time delays;

[0059] The thickness calculation module is used to extract the time delay t corresponding to the echo moment in the picosecond acoustic pulse echo signal, and combine the acoustic wave transmission speed v in the non-transparent film to calculate the thickness d of the non-transparent film according to the formula d=vt / 2.

[0060] Wherein, the femtosecond laser pumping detection device specifically comprises:

[0061] A laser for generating a pump light and a probe light with a time delay;

[0062] A time delay device, used to delay the detection light from reaching the surface of the two-dimensional semiconductor film compared to the pump light, and to adjust the time delay to different values;

[0063] A microscope objective lens, used to focus the pump light and the probe light onto the surface of the two-dimensional semiconductor film;

[0064] The optical detection module is used to detect the picosecond acoustic wave pulse echo signal reflected from the surface of the two-dimensional semiconductor film under different time delays when the time delay device adjusts the time delay to different values, that is, time-resolved ultrafast photoacoustic spectroscopy; wherein the picosecond acoustic wave pulse echo signal is formed in the following process: when the pump light is focused on the surface of the two-dimensional semiconductor film, it will excite the two-dimensional semiconductor film to produce longitudinal coherent acoustic phonon oscillations, thereby emitting picosecond acoustic wave pulses into the non-transparent film; when the picosecond acoustic wave pulse propagates to the bottom interface of the non-transparent film, it is reflected back to the two-dimensional semiconductor film / non-transparent film interface; when the detection light after time delay reaches the surface of the two-dimensional semiconductor film, the picosecond acoustic wave pulse echo modulates the detection light reflected from the surface of the two-dimensional semiconductor film to form a picosecond acoustic wave pulse echo signal.

[0065] The optical detection module performs balanced detection through a phase-locked amplifier to obtain a time-resolved ultrafast photoacoustic spectrum.

[0066] Wherein, the pump light and the detection light both come from the same femtosecond laser.

[0067] Wherein, the spectrum processing module specifically includes:

[0068] A fitting unit, used for fitting the time-resolved ultrafast photoacoustic spectrum using a double exponential decay function to obtain a fitting curve;

[0069] The background removal unit is used to subtract the fitting curve from the time-resolved ultrafast photoacoustic signal, thereby extracting a picosecond acoustic wave pulse echo signal.

[0070] It is worth noting that in the embodiment of the above system, the various units and modules included are only divided according to functional logic, but are not limited to the above division, as long as the corresponding functions can be achieved; in addition, the specific names of the functional units are only for the convenience of distinguishing each other, and are not used to limit the scope of protection of the present invention.

[0071] The embodiments described above are only illustrative, wherein the modules described as separate components may or may not be physically separated, and the components displayed as modules may or may not be physical modules, i.e., they may be located in one place, or they may be distributed on multiple network modules. Some or all of the modules may be selected according to actual needs to achieve the purpose of the scheme of this embodiment. Those skilled in the art can clearly understand that each implementation method can be implemented by means of software plus a necessary general hardware platform, or of course, it can be implemented only by hardware, as long as the function or effect can be achieved.

[0072] It should be understood that the above embodiments and descriptions only describe the principles, main features and advantages of the present invention. Without departing from the spirit and scope of the present invention, the present invention may be subject to various changes and improvements, and these changes and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for measuring the thickness of a non-transparent film based on ultrafast photoacoustic spectroscopy, characterized in that: The steps include: (1) stacking a two-dimensional semiconductor thin film on a non-transparent film, wherein the two-dimensional semiconductor thin film includes a plurality of two-dimensional semiconductor layers; (2) Based on the femtosecond laser pump-probe technology, pump light and probe light with different time delays are generated, and the pump light and probe light with different time delays are focused on the surface of the two-dimensional semiconductor film to detect the picosecond acoustic wave pulse echo signals reflected from the surface of the two-dimensional semiconductor film under different time delays, i.e., time-resolved ultrafast photoacoustic spectroscopy; (3) performing background removal processing on the time-resolved ultrafast photoacoustic spectrum to obtain picosecond acoustic wave pulse echo signals under different time delays; (4) The time delay t corresponding to the echo moment in the picosecond acoustic pulse echo signal is extracted, and combined with the acoustic wave transmission speed v in the non-transparent film, the thickness d of the non-transparent film is calculated according to the formula d=vt / 2.

2. The non-transparent film thickness measurement method based on ultrafast photoacoustic spectroscopy according to claim 1, characterized in that: Step (2) specifically includes: (2.1) generating pump light and probe light with time delay; (2.2) focusing the pump light on the surface of the two-dimensional semiconductor film, so that coherent acoustic phonon oscillations are generated between the layers of the two-dimensional semiconductor film, thereby emitting picosecond acoustic wave pulses into the non-transparent film. When the picosecond acoustic wave pulses propagate to the bottom interface of the non-transparent film, they are reflected back to the interface between the two-dimensional semiconductor film and the opaque film; (2.3) When the time-delayed probe light reaches the surface of the two-dimensional semiconductor film, the picosecond acoustic wave pulse modulates the probe light reflected from the surface of the two-dimensional semiconductor film, forming a picosecond acoustic wave pulse echo signal; (2.4) By setting the time delay of the detection light and the pump light to different values, the time-resolved ultrafast photoacoustic spectrum of the picosecond acoustic wave pulse echo wave signal is detected.

3. The method for measuring thickness of non-transparent thin films based on ultrafast photoacoustic spectroscopy according to claim 2, characterized in that: The time-resolved ultrafast photoacoustic spectrum described in step (2.4) is obtained by balanced detection using a phase-locked amplifier.

4. The method for measuring thickness of non-transparent thin films based on ultrafast photoacoustic spectroscopy according to claim 2, characterized in that: The pump light and the detection light both originate from the same femtosecond laser.

5. The method for measuring thickness of non-transparent thin films based on ultrafast photoacoustic spectroscopy according to claim 1, characterized in that: Step (3) specifically includes: (3.1) fitting the time-resolved ultrafast photoacoustic spectrum using a double exponential decay function to obtain a fitting curve; (3.2) Subtract the fitting curve from the time-resolved ultrafast photoacoustic spectroscopy signal to extract the picosecond acoustic wave pulse echo signal.

6. A non-transparent film thickness measurement system based on ultrafast photoacoustic spectroscopy, characterized in that: include: A femtosecond laser pump-probe device is used to generate pump light and probe light with different time delays, and focus the pump light and probe light with different time delays on the surface of a two-dimensional semiconductor film, and detect the picosecond acoustic wave pulse echo signal reflected from the surface of the two-dimensional semiconductor film under different time delays, i.e., time-resolved ultrafast photoacoustic spectroscopy, wherein the two-dimensional semiconductor film includes several two-dimensional semiconductor layers, and is superimposed on a non-transparent film; A spectrum processing module, used for performing background removal processing on the time-resolved ultrafast photoacoustic spectrum to obtain picosecond acoustic wave pulse echo signals under different time delays; The thickness calculation module is used to extract the time delay t corresponding to the echo moment in the picosecond acoustic wave pulse signal, and combine the acoustic wave transmission speed v in the non-transparent film to calculate the thickness d of the non-transparent film according to the formula d=vt / 2.

7. The non-transparent film thickness measurement system based on ultrafast photoacoustic spectroscopy according to claim 6, characterized in that: The femtosecond laser pumping detection device specifically comprises: A laser for generating a pump light and a probe light with a time delay; A time delay device, used to delay the detection light from reaching the surface of the two-dimensional semiconductor film compared to the pump light, and to adjust the time delay to different values; A microscope objective lens, used to focus the pump light and the probe light onto the surface of the two-dimensional semiconductor film; The optical detection module is used to detect the picosecond acoustic wave pulse echo signal reflected from the surface of the two-dimensional semiconductor film under different time delays when the time delay device adjusts the time delay to different values, that is, time-resolved ultrafast photoacoustic spectroscopy; wherein the picosecond acoustic wave pulse echo signal is formed in the following process: when the pump light is focused on the surface of the two-dimensional semiconductor film, coherent acoustic phonon oscillations are generated between the layers of the two-dimensional semiconductor film, thereby emitting picosecond acoustic wave pulses into the non-transparent film, and when the picosecond acoustic wave pulses propagate to the bottom interface of the non-transparent film, they are reflected back to the two-dimensional semiconductor film-opaque film interface; when the detection light after time delay reaches the surface of the two-dimensional semiconductor film, the picosecond acoustic wave pulse modulates the detection light reflected from the surface of the two-dimensional semiconductor film to form a picosecond acoustic wave pulse echo signal.

8. The non-transparent film thickness measurement system based on ultrafast photoacoustic spectroscopy according to claim 7, characterized in that: The light detection module performs balanced detection through a phase-locked amplifier to obtain a time-resolved ultrafast photoacoustic spectrum.

9. The non-transparent film thickness measurement system based on ultrafast photoacoustic spectroscopy according to claim 7, characterized in that: The pump light and the detection light both originate from the same femtosecond laser.

10. The non-transparent film thickness measurement system based on ultrafast photoacoustic spectroscopy according to claim 6, characterized in that: The spectrum processing module specifically includes: A fitting unit, used for fitting the time-resolved ultrafast photoacoustic spectrum using a double exponential decay function to obtain a fitting curve; The background removal unit is used to subtract the fitting curve from the time-resolved photoacoustic light, so as to extract the picosecond acoustic wave pulse echo signal.

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