Photoacoustic film thickness measuring system and measuring method

Abstract

According to the photoacoustic film thickness measuring system and method provided by the invention, the laser polarization adjusting device or the wafer rotating assembly is introduced into the measuring system, and the polarization state of femtosecond pulse laser or the wafer angle is adjusted according to the signal-to-noise ratio of the measured data; the influence of polarization direction of femtosecond pulse laser and wafer lattice orientation mismatch on measurement stability is eliminated, and the signal-to-noise ratio of a measurement signal is improved.

Description

Technical Field

[0001] The present application relates to the field of semiconductor technology, and in particular to a photoacoustic film thickness measurement system and method. Background Art

[0002] In the field of semiconductor chips, the thickness of the film layer is a key technical indicator that affects chip performance. Picosecond ultrasonic metal film thickness measurement equipment is used to measure key parameters such as film thickness and Young's modulus of metal film and dielectric film chips. Its main technical principle is: when a beam of femtosecond to picosecond pulsed laser as pump light excites the film to be tested to produce transient thermal expansion and emit ultrashort sound waves. The sound wave is transmitted along the film layer in the film and reflected at the interface of the film layer to form an echo. At this time, another beam of ultrashort pulsed laser as detection light will irradiate the film excitation position to detect the reflectivity change and surface morphology change of the film caused by the echo. By obtaining the time when the echo reaches the sample surface and the sound velocity in the film layer, the film thickness value of the film to be tested can be obtained.

[0003] Picosecond ultrasonic metal film thickness measurement equipment is a non-destructive measurement equipment based on optical detection. The laser used is a femtosecond pulse laser with a fixed polarization direction. The metal film layer of the chip is generally prepared by thin film epitaxial growth technology, and the film layer has a specific lattice orientation. Since the interaction between laser and matter is affected by the lattice orientation of the material and the polarization direction of the laser, when the polarization direction of the femtosecond pulse laser is mismatched with the lattice orientation of the wafer, the signal-to-noise ratio of the picosecond ultrasonic metal film thickness measurement will be poor, which will seriously affect the accuracy and stability of the metal film thickness measurement results. Summary of the invention

[0004] In view of this, the present invention provides a photoacoustic film thickness measurement system and method to eliminate the influence of the mismatch between the polarization direction of femtosecond pulse laser and the wafer lattice orientation on the measurement stability.

[0005] To solve the above problems, this application adopts the following technical solutions:

[0006] One of the purposes of the present application is to provide a photoacoustic film thickness measurement system, comprising:

[0007] A light source module, wherein the light source module is used to emit a detection beam and a pump beam;

[0008] A detection optical path module, the detection optical path module comprising a first polarization state adjustment component, the first polarization state adjustment component is used to adjust the polarization state of the detection light beam;

[0009] A pump optical path module, the pump optical path module comprising a second polarization state adjustment component, the second polarization state adjustment component is used to adjust the polarization state of the pump light beam;

[0010] and processing modules;

[0011] Among them: the detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal, the processing module receives the optical signal and obtains a signal-to-noise ratio according to the optical signal, and then adjusts the first polarization state adjustment component and / or the second polarization state adjustment component according to the signal-to-noise ratio.

[0012] In some embodiments, the state of one of the first polarization state adjustment component and the second polarization state adjustment component is fixed, the state of the other of the first polarization state adjustment component and the second polarization state adjustment component is adjusted, and then the signal-to-noise ratio in each state is obtained, and the optimal state of the first polarization state adjustment component or the second polarization state adjustment component is determined based on the obtained signal-to-noise ratio.

[0013] In some embodiments, the first polarization state adjusting component and the second polarization state adjusting component include half glass plates.

[0014] In some embodiments, the light source module includes a pulse laser and a beam splitter, and the femtosecond pulse laser emitted by the pulse laser is split into a reflected light beam and a transmitted light beam by the beam splitter.

[0015] In some embodiments, the detection optical path module or the pump optical path module also includes a delay line, and the delay line includes an electric linear displacement platform, a broadband hollow retroreflector and a first reflector. The electric linear displacement platform is movable and used to fix the broadband hollow retroreflector, and the detection light beam is reflected by the broadband hollow retroreflector and the first reflector in sequence and then enters the first polarization state adjustment component.

[0016] In some embodiments, the pump optical path module further includes a modulator, and the modulator is used to dynamically modulate the intensity of the pump light beam, and the pump light beam modulated by the modulator is incident on the second polarization state adjustment component.

[0017] In some embodiments, the processing module includes a photodetector, a lock-and-release amplifier connected to the photodetector signal, and a signal processor connected to the lock-and-release amplifier signal. The photodetector converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier. The lock-and-release amplifier obtains a signal-to-noise ratio based on the electrical signal and transmits the output ultrafast signal to the signal processor.

[0018] In some of the embodiments, the modulator is also electrically connected to a signal generator, and the signal generator is also signal-connected to a reference signal input terminal of the lock-in amplifier.

[0019] In some of the embodiments, a wafer rotation assembly is further included, wherein the wafer rotation assembly is used to fix the wafer to be tested and the wafer rotation assembly is rotatable. By adjusting the angle of the wafer rotation assembly and obtaining the signal-to-noise ratio at each angle, the optimal angle of the wafer rotation assembly is determined based on the quality of the obtained signal-to-noise ratio.

[0020] The second object of the present application is to provide a photoacoustic film thickness measurement system, comprising:

[0021] A light source module, the light source module is used to emit a detection beam and a pump beam;

[0022] A wafer rotating assembly, wherein the wafer rotating assembly is used to fix the wafer to be tested and the wafer rotating assembly is rotatable;

[0023] and processing modules;

[0024] Among them: the detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal, the processing module receives the optical signal and obtains a signal-to-noise ratio according to the optical signal, and determines the rotation angle of the wafer rotation component according to the signal-to-noise ratio.

[0025] In some of the embodiments, the angle of the wafer rotation assembly is adjusted, and the signal-to-noise ratio of the measurement signal at each angle is obtained, and the optimal angle of the wafer rotation assembly is determined based on the obtained signal-to-noise ratio.

[0026] In some embodiments, the light source module includes a pulse laser and a beam splitter, and the femtosecond pulse laser emitted by the pulse laser is split into a reflected light beam and a transmitted light beam by the beam splitter.

[0027] In some embodiments, the detection optical path module or the pump optical path module also includes a delay line, and the delay line includes an electric linear displacement platform, a broadband hollow retroreflector and a first reflector. The electric linear displacement platform is movable and used to fix the broadband hollow retroreflector, and the detection light beam enters the first reflector through the broadband hollow retroreflector in turn.

[0028] In some embodiments, the pump optical path module further includes a modulator, and the modulator is used to dynamically modulate the intensity of the pump optical beam.

[0029] In some embodiments, the processing module includes a photodetector, a lock-and-release amplifier connected to the photodetector signal, and a signal processor connected to the lock-and-release amplifier signal. The photodetector converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier. The lock-and-release amplifier obtains a signal-to-noise ratio based on the electrical signal and transmits the output ultrafast signal to the signal processor.

[0030] In some of the embodiments, the modulator is also electrically connected to a signal generator, and the signal generator is also signal-connected to a reference signal input terminal of the lock-in amplifier.

[0031] The third object of the present application is to provide a photoacoustic film thickness measurement method, comprising the following steps:

[0032] The light source module emits a detection beam and a pump beam;

[0033] The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and stimulate the wafer to be tested to generate an optical signal;

[0034] Acquire a signal-to-noise ratio according to the optical signal;

[0035] The polarization state of the detection beam and / or the pump beam is adjusted according to the signal-to-noise ratio.

[0036] The fourth object of the present application is to provide a photoacoustic film thickness measurement method, comprising the following steps:

[0037] The light source module emits a detection beam and a pump beam;

[0038] The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal;

[0039] Acquire a signal-to-noise ratio according to the optical signal;

[0040] The angle of the wafer to be tested is adjusted according to the signal-to-noise ratio.

[0041] This application adopts the above technical solution, and its beneficial effects are as follows:

[0042] The photoacoustic film thickness measurement system and method provided in the present application introduce a laser polarization adjustment device or a wafer rotation component into the measurement system, and adjust the polarization state of the femtosecond pulse laser or the wafer angle according to the signal-to-noise ratio of the measurement data to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability, thereby improving the signal-to-noise ratio of the measurement signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments of the present application or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0044] Figure 1This is a schematic diagram of the structure of the photoacoustic film thickness measurement system provided in Example 1 of the present application.

[0045] Figure 2 It is a structural schematic diagram of the photoacoustic film thickness measurement system provided in Example 2 of the present application.

[0046] Figure 3 This is a step flow chart of the photoacoustic film thickness measurement method provided in Example 3 of the present application.

[0047] Figure 4 This is a step flow chart of the photoacoustic film thickness measurement method provided in Example 4 of the present application. DETAILED DESCRIPTION

[0048] Embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0049] In the description of the present application, it should be understood that the terms "upper", "lower", "horizontal", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0050] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0051] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0052] Example 1

[0053] See also Figure 1 , is a schematic diagram of the structure of the photoacoustic film thickness measurement system provided in Example 1 of the present application, including a light source module, a detection optical path module, a pump optical path module and a processing module. The technical solution implemented is described in detail below.

[0054] The light source module is used to emit a detection beam and a pump beam.

[0055] In this embodiment, the light source module includes a pulse laser 10 and a beam splitter 11. The femtosecond pulse laser emitted by the pulse laser 10 is split into a reflected beam and a transmitted beam by the beam splitter 11. The reflected beam is used as a pump beam, and the transmitted beam is used as a detection beam. The pulse width of the femtosecond pulse laser output by the pulse laser 10 is less than 1ps.

[0056] It can be understood that the light source module is not limited to the above structure, and can also be a single light source or multiple light sources. The light beam emitted by the light source is split to meet the light path requirements to ensure the flexibility of the entire system.

[0057] The detection optical path module comprises a first polarization state adjusting component 16, and the first polarization state adjusting component 16 is used to adjust the polarization state of the detection light beam.

[0058] In this embodiment, the first polarization state adjustment component 16 can continuously change the polarization state of the detection light beam. The first polarization state adjustment component 16 can usually use half a glass slide. For example, the motor can be rotated to drive the deflection of half a glass slide to achieve polarization state adjustment of the detection light beam. The first polarization state adjustment component 16 is not limited to the above structure, and can also be other schemes and devices that can achieve laser polarization state adjustment.

[0059] It can be understood that since the lattice orientation has various anisotropies, the ultrasound generated by the detection beam may also have various anisotropies. By adjusting the polarization angle of the detection beam, the sensitivity of the detection light in detecting ultrasound can be maximized, thereby improving the overall detection signal-to-noise ratio.

[0060] The pump optical path module comprises a second polarization state adjusting component 25, and the second polarization state adjusting component 25 is used to adjust the polarization state of the pump light beam.

[0061] In this embodiment, the second polarization state adjustment component 25 can continuously change the polarization state of the pump light beam. The second polarization state adjustment component 25 can usually use half a glass slide. For example, the motor can be rotated to drive the deflection of the half glass slide to achieve polarization state adjustment of the pump light beam. The second polarization state adjustment component 25 is not limited to the above structure, and can also be other solutions and devices that can achieve laser polarization state adjustment.

[0062] It can be understood that this embodiment changes the polarization state of the pump beam to maximize the absorption rate of the wafer to the pump beam, thereby maximizing the ultrasound generation efficiency and ultrasound amplitude, while reducing the stray light intensity of the pump.

[0063] The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer 200 to be tested overlap and excite the wafer 200 to be tested to generate an optical signal. The processing module receives the optical signal and obtains a signal-to-noise ratio based on the optical signal, and then adjusts the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25 based on the signal-to-noise ratio, thereby realizing the adjustment of the polarization state of the detection light beam and / or the polarization state of the pump light beam.

[0064] It can be understood that the polarization state of the femtosecond pulse laser is adjusted according to the signal-to-noise ratio of the measurement data to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability.

[0065] It should be noted that the polarization states of the detection light path and the pump light path are different to avoid the pump light affecting the detection signal of the detection light. When the polarization state of one of the detection light path and the pump light path is 90°, it is the best state, but when the detection light path and the pump light path are both 90°, it may not be the case with the highest signal-to-noise ratio. At this time, the wavelengths of the detection light and the pump light can be adjusted to make them different.

[0066] Furthermore, two light sources are provided to emit detection light and pump light respectively or a frequency doubling crystal is introduced into the detection light path / pump light path, for example, the frequency doubling crystal is provided after the beam splitter, so as to meet the condition that the wavelengths of the detection light and the pump light are different.

[0067] It should be noted that, when determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, the state of the second polarization state adjustment component 25 may be fixed first, the state of the first polarization state adjustment component 16 may be adjusted continuously, and the signal-to-noise ratio of the measurement signal in each state may be obtained, and the optimal state of the first polarization state adjustment component 16 may be determined based on the quality of the signal-to-noise ratio of the measurement signal. Subsequently, the state of the first polarization state adjustment component 16 may be fixed, and the state of the second polarization state adjustment component 25 may be adjusted continuously, and the signal-to-noise ratio of the measurement signal in each state may be obtained, and the optimal state of the polarization state adjustment component 25 may be determined based on the quality of the signal-to-noise ratio of the measurement signal. When determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, no requirement is imposed on the order of fixing and adjusting the polarization state adjustment components. The state of one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be fixed, the state of the other one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be adjusted, and then the signal-to-noise ratio in each state can be obtained, and the optimal state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 can be determined based on the quality of the obtained signal-to-noise ratio.

[0068] Furthermore, the photoacoustic film thickness measurement system provided in this embodiment may also include a wafer rotation assembly (not shown), which is used to fix the wafer to be measured and is rotatable. The wafer is driven to rotate by adjusting the angle of the wafer rotation assembly, and the signal-to-noise ratio at each angle is obtained, and the optimal signal-to-noise ratio is determined.

[0069] It should be noted that: in this embodiment, the state of the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25 can be adjusted to obtain the signal-to-noise ratio in each state, and the optimal state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 can be determined according to the quality of the obtained signal-to-noise ratio; at the same time, when adjusting the polarization state adjustment component, it can also be combined with the adjustment of the angle of the wafer rotation component to obtain the optimal signal-to-noise ratio, thereby obtaining the state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 under the signal-to-noise ratio.

[0070] In this embodiment, the detection optical path module or the pump optical path module also includes a delay line. The delay line includes an electric linear displacement platform 13, a broadband hollow retroreflector 14 and a first reflector 15. The electric linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The detection light beam enters the first reflector 15 through the broadband hollow retroreflector 14 in turn, and then reflects and enters the first polarization state adjustment component 16 for polarization state adjustment. Under the action of the delay line, the time delay scan required for measurement can be provided. The delay line is not limited to the above structure. Other mechanical delay lines that can achieve delay, or methods of achieving delay by adjusting the pulse frequency can meet actual application requirements.

[0071] It should be noted that, in the above-mentioned embodiment, a delay line is set in one of the detection optical path module and the pump optical path module to realize time delay scanning and compensation of optical path, but in practical applications, it is not limited to the above-mentioned scheme. It is also possible to set a technical scheme of a variable delay line in one of the detection optical path module and the pump optical path module, and set an immutable delay line or no delay line in the other corresponding optical path module, or set a variable delay line in both the detection optical path module and the pump optical path module, both of which can meet practical application requirements.

[0072] In this embodiment, the pump optical path module further includes a modulator 101. The pump light emitted by the modulator 101 is incident on the second polarization state adjustment component 25 for polarization state adjustment. Under the action of the modulator 101, the intensity of the pump light beam reflected by the third reflector 24 can be dynamically modulated.

[0073] In this embodiment, the detection optical path module further includes a fourth reflector 17 . The light beam subjected to polarization state adjustment by the first polarization state adjustment component 16 enters the fourth reflector 17 , is focused on the surface of the wafer 200 to be tested, and forms a detection light spot.

[0074] In this embodiment, the pump optical path module further includes a fifth reflector 26 . The pump light beam after polarization state adjustment by the second polarization state adjustment component 25 enters the fifth reflector 26 , is focused on the surface of the wafer 200 to form a pump light spot.

[0075] It can be understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable, and the pump light spot and the detection light spot can be overlapped on the wafer 200 to be tested by adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26 .

[0076] It can be understood that this embodiment achieves the change of light path direction by setting the above-mentioned reflector, but in practice, it is not limited to the above-mentioned optical structure. Other methods that can achieve light path direction change and focusing are within the protection scope of this application.

[0077] In this embodiment, the processing module includes a photodetector 104, a lock-and-release amplifier 103 connected to the photodetector 104 signal, and a signal processor 105 connected to the lock-and-release amplifier 103 signal; the photoacoustic film thickness measurement system also includes a sixth reflector 20, and the detection light beam and the pump light beam reflected by the surface of the wafer 200 to be measured are then incident on the photodetector 104 through the sixth reflector 20, the photodetector 140 converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier 103, and the lock-and-release amplifier 103 obtains the signal-to-noise ratio according to the electrical signal and transmits the output ultrafast signal to the signal processor 105.

[0078] Furthermore, the modulator 101 is also electrically connected to the signal generator 102 , and the signal generator 102 is also signal-connected to a reference signal input terminal of the lock-in amplifier 103 .

[0079] It can be understood that the modulation trigger signal of the modulator 101 is provided by the signal generator 102, and the modulation trigger signal is connected to the reference signal input terminal of the phase-locked amplifier 103 through a BNC line to provide the reference frequency required for the operation of the phase-locked amplifier 103. Other wired and wireless methods that can achieve the connection between the phase-locked amplifier 103 and the signal generator 102 are within the protection scope of this application.

[0080] The photoacoustic film thickness measurement system provided in the present application introduces a laser polarization adjustment device into the measurement system to adjust the polarization state of the femtosecond pulse laser according to the signal-to-noise ratio of the measurement data, so as to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability, thereby improving the signal-to-noise ratio of the measurement signal.

[0081] Example 2

[0082] See also Figure 2, is a schematic diagram of the structure of the photoacoustic film thickness measurement system provided in the embodiment of the present application, including a light source module, a wafer rotation component 201 and a processing module. The technical solution implemented is described in detail below.

[0083] The light source module is used to emit a detection beam and a pump beam.

[0084] In this embodiment, the light source module includes a pulse laser 10 and a beam splitter 11. The femtosecond pulse laser emitted by the pulse laser 10 is split into a reflected beam and a transmitted beam by the beam splitter 11. The reflected beam is used as a pump beam, and the transmitted beam is used as a detection beam. It can be understood that the light source module is not limited to the above structure, and can also be a single light source or multiple light sources. The light beam emitted by the light source is split to meet the optical path requirements to ensure the flexibility of the entire system.

[0085] The wafer rotating assembly 201 is used to fix the wafer to be tested and is rotatable. The rotation of the wafer rotating assembly 201 can drive the wafer to be tested to rotate.

[0086] The detection light spot and pump light spot formed by the detection light beam and the pump light beam on the wafer 200 to be tested overlap and stimulate the wafer 200 to be tested to generate an optical signal. The processing module receives the optical signal and obtains a signal-to-noise ratio based on the optical signal, and determines the rotation angle of the wafer rotation component 201 based on the signal-to-noise ratio.

[0087] It can be understood that the photoacoustic film thickness measurement system provided in this embodiment, by introducing a wafer rotation component 201 into the measurement system, adjusts the wafer angle of the femtosecond pulse laser according to the signal-to-noise ratio of the measurement data, so as to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability, thereby improving the measurement signal-to-noise ratio.

[0088] It should be noted that the photoacoustic film thickness measurement system provided in Example 2 of the present application may also include some of the solutions in Example 1, the detection optical path module includes a first polarization state adjustment component and the pump optical path module includes a second polarization state adjustment component. The specific structure and implementation method can refer to Example 1 and will not be repeated here.

[0089] Furthermore, a half wave plate is provided in one of the detection optical path and the pump optical path to make the two optical paths have different polarizations. At this time, the polarization state of one of the detection optical path and the pump optical path is 90°, which is the optimal state.

[0090] In this embodiment, the detection optical path module or the pump optical path module also includes a delay line, which includes an electric linear displacement platform 13, a broadband hollow retroreflector 14 and a first reflector 15. The electric linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The detection beam sequentially enters the first reflector 15 through the broadband hollow retroreflector 14. Under the action of the delay line, the time delay scan required for measurement can be provided. The delay line is not limited to the above structure. Other mechanical delay lines that can achieve delay, or methods of achieving delay by adjusting the pulse frequency can meet actual application requirements.

[0091] It should be noted that, in the above-mentioned embodiment, a delay line is set in one of the detection optical path module and the pump optical path module to realize time delay scanning and compensation of optical path, but in practical applications, it is not limited to the above-mentioned scheme. It is also possible to set a technical scheme of a variable delay line in one of the detection optical path module and the pump optical path module, and set an immutable delay line or no delay line in the other corresponding optical path module, or set a variable delay line in both the detection optical path module and the pump optical path module, both of which can meet practical application requirements.

[0092] In this embodiment, the pump optical path module further includes a modulator 101 , and the modulator 101 is used to dynamically modulate the light intensity of the pump light beam reflected by the third reflector 23 .

[0093] In this embodiment, the detection light path module further includes a fourth reflector 17 . The light beam passing through the first reflector 15 enters the fourth reflector 17 , is focused on the surface of the wafer 200 to be tested, and forms a detection light spot.

[0094] In this embodiment, the pump light path module further includes a fifth reflector 26 . The pump light beam dynamically modulated by the modulator 101 is incident on the fifth reflector 26 again, focused on the surface of the wafer 200 to be tested and forms a pump light spot.

[0095] It can be understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable, and the pump light spot and the detection light spot can be overlapped on the wafer 200 to be tested by adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26 .

[0096] It can be understood that this embodiment achieves the change of light path direction and focusing by setting the above-mentioned reflector, but in practice, it is not limited to the above-mentioned optical structure. Other methods that can achieve the change of light path direction and focusing are within the protection scope of this application.

[0097] In this embodiment, the processing module includes a photodetector 104, a lock-and-release amplifier 103 connected to the photodetector 104 signal, and a signal processor 105 connected to the lock-and-release amplifier 103 signal; the photoacoustic film thickness measurement system also includes a sixth reflector 20, and the detection light beam and the pump light beam reflected by the surface of the wafer 200 to be measured are then incident on the photodetector 104 through the sixth reflector 20, the photodetector 140 converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier 103, and the lock-and-release amplifier 103 obtains the signal-to-noise ratio according to the electrical signal and transmits the output ultrafast signal to the signal processor 105.

[0098] Furthermore, the modulator 101 is also electrically connected to the signal generator 102 , and the signal generator 102 is also signal-connected to a reference signal input terminal of the lock-in amplifier 103 .

[0099] It can be understood that the modulation trigger signal of the modulator 101 is provided by the signal generator 102, and the modulation trigger signal is connected to the reference signal input terminal of the phase-locked amplifier 103 through a BNC line to provide the reference frequency required for the operation of the phase-locked amplifier 103. Other wired and wireless methods that can achieve the connection between the phase-locked amplifier 103 and the signal generator 102 are within the protection scope of this application.

[0100] The photoacoustic film thickness measurement system provided in the present application introduces a wafer rotation component 201 into the measurement system and adjusts the wafer angle according to the signal-to-noise ratio of the measurement data to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability, thereby improving the signal-to-noise ratio of the measurement signal.

[0101] Example 3

[0102] See also Figure 3 , is a flowchart of the photoacoustic film thickness measurement method of the embodiment of the present application, which is applied to the photoacoustic film thickness measurement system of embodiment 1, including the following steps:

[0103] Step S310: The light source module emits a detection beam and a pump beam.

[0104] In this embodiment, the light source module includes a pulse laser 10 and a beam splitter 11. The femtosecond pulse laser emitted by the pulse laser 10 is split into a reflected beam and a transmitted beam by the beam splitter 11. The reflected beam is used as a pump beam, and the transmitted beam is used as a detection beam. The pulse width of the femtosecond pulse laser output by the pulse laser 10 is less than 1ps.

[0105] It can be understood that the light source module is not limited to the above structure, and can also be a single light source or multiple light sources. The light beam emitted by the light source is split to meet the light path requirements to ensure the flexibility of the entire system.

[0106] Step S320: The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and stimulate the wafer to be tested to generate an optical signal.

[0107] Please combine Figure 1 The optical path involved in this embodiment includes a detection optical path module and a pump optical path module, which are as follows:

[0108] The detection optical path module comprises a first polarization state adjusting component 16, and the first polarization state adjusting component 16 is used to adjust the polarization state of the detection light beam.

[0109] In this embodiment, the first polarization state adjustment component 16 can continuously change the polarization state of the detection light beam. The first polarization state adjustment component 16 can usually use half a glass slide. For example, the motor can be rotated to drive the deflection of half a glass slide to achieve polarization state adjustment of the detection light beam. The first polarization state adjustment component 16 is not limited to the above structure, and can also be other schemes and devices that can achieve laser polarization state adjustment.

[0110] It can be understood that since the lattice orientation has various anisotropies, the ultrasound generated by the detection beam may also have various anisotropies. By adjusting the polarization angle of the detection beam, the sensitivity of the detection light in detecting ultrasound can be maximized, thereby improving the overall detection signal-to-noise ratio.

[0111] The pump optical path module comprises a second polarization state adjusting component 25, and the second polarization state adjusting component 25 is used to adjust the polarization state of the pump light beam.

[0112] In this embodiment, the second polarization state adjustment component 25 can continuously change the polarization state of the pump light beam. The second polarization state adjustment component 25 can usually use half a glass slide. For example, the motor can be rotated to drive the deflection of the half glass slide to achieve polarization state adjustment of the pump light beam. The second polarization state adjustment component 25 is not limited to the above structure, and can also be other solutions and devices that can achieve laser polarization state adjustment.

[0113] It can be understood that this embodiment changes the polarization state of the pump beam to maximize the absorption rate of the wafer to the pump beam, thereby maximizing the ultrasound generation efficiency and ultrasound amplitude, while reducing the stray light intensity of the pump.

[0114] The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer 200 to be tested overlap and excite the wafer 200 to be tested to generate an optical signal. The processing module receives the optical signal and obtains a signal-to-noise ratio based on the optical signal, and then adjusts the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25 based on the signal-to-noise ratio, thereby realizing the adjustment of the polarization state of the detection light beam and / or the polarization state of the pump light beam.

[0115] It can be understood that the polarization state of the femtosecond pulse laser is adjusted according to the signal-to-noise ratio of the measurement data to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability.

[0116] It should be noted that the polarization states of the detection light path and the pump light path are different to avoid the pump light affecting the detection signal of the detection light. When the polarization state of one of the detection light path and the pump light path is 90°, it is the best state, but when the detection light path and the pump light path are both 90°, it may not be the case with the highest signal-to-noise ratio. At this time, the wavelengths of the detection light and the pump light can be adjusted to make them different.

[0117] Furthermore, two light sources are provided to emit detection light and pump light respectively or a frequency doubling crystal is introduced into the detection light path / pump light path, for example, the frequency doubling crystal is provided after the beam splitter, so as to meet the condition that the wavelengths of the detection light and the pump light are different.

[0118] It should be noted that, when determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, the state of the second polarization state adjustment component 25 may be fixed first, the state of the first polarization state adjustment component 16 may be adjusted continuously, and the signal-to-noise ratio of the measurement signal in each state may be obtained, and the optimal state of the first polarization state adjustment component 16 may be determined based on the quality of the signal-to-noise ratio of the measurement signal. Subsequently, the state of the first polarization state adjustment component 16 may be fixed, and the state of the second polarization state adjustment component 25 may be adjusted continuously, and the signal-to-noise ratio of the measurement signal in each state may be obtained, and the optimal state of the polarization state adjustment component 25 may be determined based on the quality of the signal-to-noise ratio of the measurement signal. When determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, no requirement is imposed on the order of fixing and adjusting the polarization state adjustment components. The state of one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be fixed, the state of the other one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be adjusted, and then the signal-to-noise ratio in each state can be obtained, and the optimal state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 can be determined based on the quality of the obtained signal-to-noise ratio.

[0119] Furthermore, the photoacoustic film thickness measurement system provided in this embodiment may also include a wafer rotation assembly (not shown), which is used to fix the wafer to be measured and is rotatable. The wafer is driven to rotate by adjusting the angle of the wafer rotation assembly, and the signal-to-noise ratio at each angle is obtained, and the optimal signal-to-noise ratio is determined.

[0120] It should be noted that: in this embodiment, the state of the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25 can be adjusted to obtain the signal-to-noise ratio in each state, and the optimal state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 can be determined according to the quality of the obtained signal-to-noise ratio; at the same time, when adjusting the polarization state adjustment component, it can also be combined with the adjustment of the angle of the wafer rotation component to obtain the optimal signal-to-noise ratio, thereby obtaining the state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 under the signal-to-noise ratio.

[0121] In this embodiment, the detection optical path module or the pump optical path module also includes a delay line, which includes an electric linear displacement platform 13, a broadband hollow retroreflector 14 and a first reflector 15. The electric linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The detection beam enters the first reflector 15 through the broadband hollow retroreflector 14 in turn, and then reflects and enters the first polarization state adjustment component 16 for polarization state adjustment. Under the action of the delay line, the time delay scan required for measurement can be provided. The delay line is not limited to the above structure. Other mechanical delay lines that can achieve delay, or methods of achieving delay by adjusting the pulse frequency can meet actual application requirements.

[0122] It should be noted that, in the above-mentioned embodiment, a delay line is set in one of the detection optical path module and the pump optical path module to realize time delay scanning and compensation of optical path, but in practical applications, it is not limited to the above-mentioned scheme. It is also possible to set a technical scheme of a variable delay line in one of the detection optical path module and the pump optical path module, and set an immutable delay line or no delay line in the other corresponding optical path module, or set a variable delay line in both the detection optical path module and the pump optical path module, both of which can meet practical application requirements.

[0123] In this embodiment, the pump optical path module further includes a modulator 101. The pump light emitted by the modulator 101 is incident on the second polarization state adjustment component 25 for polarization state adjustment. Under the action of the modulator 101, the intensity of the pump light beam reflected by the third reflector 24 can be dynamically modulated.

[0124] In this embodiment, the detection optical path module further includes a fourth reflector 17 . The light beam subjected to polarization state adjustment by the first polarization state adjustment component 16 enters the fourth reflector 17 , is focused on the surface of the wafer 200 to be tested, and forms a detection light spot.

[0125] In this embodiment, the pump optical path module further includes a fifth reflector 26 . The pump light beam after polarization state adjustment by the second polarization state adjustment component 25 enters the fifth reflector 26 , is focused on the surface of the wafer 200 to form a pump light spot.

[0126] It can be understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable, and the pump light spot and the detection light spot can be overlapped on the wafer 200 to be tested by adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26 .

[0127] It can be understood that this embodiment achieves the change of light path direction by setting the above-mentioned reflector, but in practice, it is not limited to the above-mentioned optical structure. Other methods that can achieve light path direction change and focusing are within the protection scope of this application.

[0128] Step S330: Acquire a signal-to-noise ratio according to the optical signal.

[0129] Specifically, the detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal, and the processing module receives the optical signal and obtains a signal-to-noise ratio according to the optical signal.

[0130] Please combine Figure 1 The processing module includes a photodetector 104, a lock-and-release amplifier 103 connected to the photodetector 104 signal, and a signal processor 105 connected to the lock-and-release amplifier 103 signal; the photoacoustic film thickness measurement system also includes a sixth reflector 20, and the detection light beam and the pump light beam reflected by the surface of the wafer 200 to be measured are incident on the photodetector 104 through the sixth reflector 20, and the photodetector 140 converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier 103, and the lock-and-release amplifier 103 obtains the signal-to-noise ratio according to the electrical signal and transmits the output ultrafast signal to the signal processor 105.

[0131] Step S340: adjusting the polarization state of the detection beam and / or the pump beam according to the signal-to-noise ratio.

[0132] Please combine Figure 1 The detection optical path module includes a first polarization state adjustment component 16, and the first polarization state adjustment component 16 is used to adjust the polarization state of the detection light beam.

[0133] It can be understood that the first polarization state adjusting component 16 can continuously change the polarization state of the detection light beam. The first polarization state adjusting component 16 can usually use half a glass slide, but is not limited to other solutions and devices that can achieve laser polarization state adjustment.

[0134] The pump optical path module comprises a second polarization state adjusting component 25, and the second polarization state adjusting component 25 is used to adjust the polarization state of the pump light beam.

[0135] It can be understood that the second polarization state adjusting component 25 can continuously change the polarization state of the pump light beam. The second polarization state adjusting component 25 can usually use half a glass plate, but is not limited to other solutions and devices that can achieve laser polarization state adjustment.

[0136] It should be noted that, when determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, the state of the second polarization state adjustment component 25 may be fixed first, the state of the first polarization state adjustment component 16 may be adjusted continuously, and the signal-to-noise ratio of the measurement signal in each state may be obtained, and the optimal state of the first polarization state adjustment component 16 may be determined based on the quality of the signal-to-noise ratio of the measurement signal. Subsequently, the state of the first polarization state adjustment component 16 may be fixed, and the state of the second polarization state adjustment component 25 may be adjusted continuously, and the signal-to-noise ratio of the measurement signal in each state may be obtained, and the optimal state of the polarization state adjustment component 25 may be determined based on the quality of the signal-to-noise ratio of the measurement signal. When determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, no requirement is imposed on the order of fixing and adjusting the polarization state adjustment components. The state of one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be fixed, the state of the other one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be adjusted, and then the signal-to-noise ratio in each state can be obtained, and the optimal state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 can be determined based on the quality of the obtained signal-to-noise ratio.

[0137] The photoacoustic film thickness measurement method provided in the present application adjusts the polarization state of the femtosecond pulse laser according to the signal-to-noise ratio of the measurement data to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability, thereby improving the signal-to-noise ratio of the measurement signal.

[0138] Example 4

[0139] See also Figure 4 , is a flowchart of the photoacoustic film thickness measurement method of the embodiment of the present application, which is applied to the photoacoustic film thickness measurement system of embodiment 2, including the following steps:

[0140] Step S410: The light source module emits a detection beam and a pump beam.

[0141] In this embodiment, the light source module includes a pulse laser 10 and a beam splitter 11. The femtosecond pulse laser emitted by the pulse laser 10 is split into a reflected beam and a transmitted beam by the beam splitter 11. The reflected beam is used as a pump beam, and the transmitted beam is used as a detection beam. The pulse width of the femtosecond pulse laser output by the pulse laser 10 is less than 1ps.

[0142] It can be understood that the light source module is not limited to the above structure, and can also be a single light source or multiple light sources. The light beam emitted by the light source is split to meet the light path requirements to ensure the flexibility of the entire system.

[0143] Step S420: The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal.

[0144] Please combine Figure 2 The optical path involved in this embodiment includes a detection optical path module and a pump optical path module.

[0145] It should be noted that the photoacoustic film thickness measurement method provided in Example 4 of the present application may also include some of the solutions in Example 1, the detection optical path module includes a first polarization state adjustment component and the pump optical path module includes a second polarization state adjustment component, and the specific structure and implementation method can refer to Example 1 and will not be repeated here.

[0146] Furthermore, a half wave plate is provided in one of the detection optical path and the pump optical path to make the two optical paths have different polarizations. At this time, the polarization state of one of the detection optical path and the pump optical path is 90°, which is the optimal state.

[0147] In this embodiment, the detection optical path module or the pump optical path module also includes a delay line. The delay line includes an electric linear displacement platform 13, a broadband hollow retroreflector 14 and a first reflector 15. The electric linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The detection beam enters the first reflector 15 through the broadband hollow retroreflector 14 in sequence. Under the action of the delay line, the time delay scan required for measurement can be provided. The delay line is not limited to the above structure. Other mechanical delay lines that can achieve delay, or methods of achieving delay by adjusting the pulse frequency can meet actual application requirements.

[0148] It should be noted that, in the above-mentioned embodiment, a delay line is set in one of the detection optical path module and the pump optical path module to realize time delay scanning and compensation of optical path, but in practical applications, it is not limited to the above-mentioned scheme. It is also possible to set a technical scheme of a variable delay line in one of the detection optical path module and the pump optical path module, and set an immutable delay line or no delay line in the other corresponding optical path module, or set a variable delay line in both the detection optical path module and the pump optical path module, both of which can meet practical application requirements.

[0149] In this embodiment, the pump optical path module further includes a modulator 101 , and the modulator 101 is used to dynamically modulate the light intensity of the pump light beam reflected by the third reflector 23 .

[0150] In this embodiment, the detection light path module further includes a fourth reflector 17 . The light beam passing through the first reflector 15 enters the fourth reflector 17 , is focused on the surface of the wafer 200 to be tested, and forms a detection light spot.

[0151] In this embodiment, the pump light path module further includes a fifth reflector 26 . The pump light beam dynamically modulated by the modulator 101 is incident on the fifth reflector 26 again, focused on the surface of the wafer 200 to be tested and forms a pump light spot.

[0152] It can be understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable, and the pump light spot and the detection light spot can be overlapped on the wafer 200 to be tested by adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26 .

[0153] It can be understood that this embodiment achieves the change of light path direction by setting the above-mentioned reflector, but in practice, it is not limited to the above-mentioned optical structure. Other methods that can achieve light path direction change and focusing are within the protection scope of this application.

[0154] Step S430: obtaining a signal-to-noise ratio according to the optical signal.

[0155] Specifically, the detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal, and the processing module receives the optical signal and obtains a signal-to-noise ratio according to the optical signal.

[0156] Please combine Figure 2 The processing module includes a photodetector 104, a lock-and-release amplifier 103 connected to the photodetector 104 signal, and a signal processor 105 connected to the lock-and-release amplifier 103 signal; the photoacoustic film thickness measurement system also includes a sixth reflector 20, and the detection light beam and the pump light beam reflected by the surface of the wafer 200 to be measured are incident on the photodetector 104 through the sixth reflector 20, and the photodetector 140 converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier 103, and the lock-and-release amplifier 103 obtains the signal-to-noise ratio according to the electrical signal and transmits the output ultrafast signal to the signal processor 105.

[0157] Step S440: adjusting the angle of the wafer to be tested according to the signal-to-noise ratio.

[0158] Please combine Figure 2 In this embodiment, a wafer rotating assembly 201 is used to fix the wafer to be tested, and the wafer rotating assembly 201 is rotatable. The rotation of the wafer rotating assembly 201 drives the wafer to be tested to rotate.

[0159] The photoacoustic film thickness measurement method provided in the present application adjusts the rotation angle of the wafer to be measured according to the signal-to-noise ratio of the measurement data to eliminate the influence of the polarization direction of the femtosecond pulse laser and the mismatch of the wafer lattice orientation on the measurement stability, thereby improving the signal-to-noise ratio of the measurement signal.

[0160] The above are only preferred embodiments of the present application, and only specifically describe the technical principles of the present application. These descriptions are only for explaining the principles of the present application and cannot be interpreted as limiting the scope of protection of the present application in any way. Based on the explanation here, any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application, and other specific implementation methods of the present application that can be associated with the technicians in this field without creative work, should be included in the scope of protection of the present application.

Claims

1. A photoacoustic film thickness measurement system, characterized in that: include: A light source module, wherein the light source module is used to emit a detection beam and a pump beam; A detection optical path module, the detection optical path module comprising a first polarization state adjustment component, the first polarization state adjustment component is used to adjust the polarization state of the detection light beam; A pump optical path module, the pump optical path module comprising a second polarization state adjustment component, the second polarization state adjustment component is used to adjust the polarization state of the pump light beam; and processing modules; Among them: the detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal, the processing module receives the optical signal and obtains a signal-to-noise ratio according to the optical signal, and then adjusts the first polarization state adjustment component and / or the second polarization state adjustment component according to the signal-to-noise ratio.

2. The photoacoustic film thickness measurement system according to claim 1, characterized in that: Fix the state of one of the first polarization state adjustment component and the second polarization state adjustment component, adjust the state of the other one of the first polarization state adjustment component and the second polarization state adjustment component, obtain the signal-to-noise ratio in each state, and determine the optimal state of the first polarization state adjustment component or the second polarization state adjustment component based on the obtained signal-to-noise ratio.

3. The photoacoustic film thickness measurement system according to claim 1 or 2, characterized in that: The first polarization state adjusting component and the second polarization state adjusting component include half glass sheets.

4. The photoacoustic film thickness measurement system according to claim 1, characterized in that: The light source module comprises a pulse laser and a beam splitter. The femtosecond pulse laser emitted by the pulse laser is split into a reflected light beam and a transmitted light beam by the beam splitter.

5. The photoacoustic film thickness measurement system according to claim 4, characterized in that: The detection optical path module or the pump optical path module also includes a delay line, which includes an electric linear displacement platform, a broadband hollow retroreflector and a first reflector. The electric linear displacement platform is movable and used to fix the broadband hollow retroreflector. The detection light beam is reflected by the broadband hollow retroreflector and the first reflector in turn and then enters the first polarization state adjustment component.

6. The photoacoustic film thickness measurement system according to claim 4, characterized in that: The pump optical path module further includes a modulator, which is used to dynamically modulate the light intensity of the pump light beam. The pump light beam modulated by the modulator is incident on the second polarization state adjustment component.

7. The photoacoustic film thickness measurement system according to claim 4, characterized in that: The processing module includes a photodetector, a lock-and-release amplifier connected to the photodetector signal, and a signal processor connected to the lock-and-release amplifier signal. The photodetector converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier. The lock-and-release amplifier obtains the signal-to-noise ratio according to the electrical signal and transmits the output ultrafast signal to the signal processor.

8. The photoacoustic film thickness measurement system according to claim 7, characterized in that: The modulator is also electrically connected to a signal generator, and the signal generator is also signal-connected to a reference signal input terminal of the lock-in amplifier.

9. The photoacoustic film thickness measurement system according to claim 1, characterized in that: It also includes a wafer rotation assembly, which is used to fix the wafer to be tested and is rotatable. By adjusting the angle of the wafer rotation assembly and obtaining the signal-to-noise ratio at each angle, the optimal angle of the wafer rotation assembly is determined based on the quality of the obtained signal-to-noise ratio.

10. A photoacoustic film thickness measurement system, characterized in that: include: A light source module, the light source module is used to emit a detection beam and a pump beam; A wafer rotating assembly, wherein the wafer rotating assembly is used to fix the wafer to be tested and the wafer rotating assembly is rotatable; and processing modules; Among them: the detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal, the processing module receives the optical signal and obtains a signal-to-noise ratio according to the optical signal, and determines the rotation angle of the wafer rotation component according to the signal-to-noise ratio.

11. The photoacoustic film thickness measurement system according to claim 10, characterized in that: By adjusting the angle of the wafer rotation assembly and obtaining the signal-to-noise ratio of the measurement signal at each angle, the optimal angle of the wafer rotation assembly is determined based on the obtained signal-to-noise ratio.

12. The photoacoustic film thickness measurement system according to claim 10, characterized in that: The light source module comprises a pulse laser and a beam splitter. The femtosecond pulse laser emitted by the pulse laser is split into a reflected light beam and a transmitted light beam by the beam splitter.

13. The photoacoustic film thickness measurement system according to claim 12, characterized in that: The detection optical path module or the pump optical path module also includes a delay line, which includes an electric linear displacement platform, a broadband hollow retroreflector and a first reflector. The electric linear displacement platform is movable and used to fix the broadband hollow retroreflector, and the detection light beam enters the first reflector through the broadband hollow retroreflector in sequence.

14. The photoacoustic film thickness measurement system according to claim 12, wherein: The pump optical path module further includes a modulator, which is used to dynamically modulate the light intensity of the pump optical beam.

15. The photoacoustic film thickness measurement system according to claim 12, wherein: The processing module includes a photodetector, a lock-and-release amplifier connected to the photodetector signal, and a signal processor connected to the lock-and-release amplifier signal. The photodetector converts the acquired optical signal into an electrical signal and transmits it to the lock-and-release amplifier. The lock-and-release amplifier obtains the signal-to-noise ratio according to the electrical signal and transmits the output ultrafast signal to the signal processor.

16. The photoacoustic film thickness measurement system according to claim 12, wherein: The modulator is also electrically connected to a signal generator, and the signal generator is also signal-connected to a reference signal input terminal of the lock-in amplifier.

17. A photoacoustic film thickness measurement method, characterized in that: The steps include: The light source module emits a detection beam and a pump beam; The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and stimulate the wafer to be tested to generate an optical signal; Acquire a signal-to-noise ratio according to the optical signal; The polarization state of the detection beam and / or the pump beam is adjusted according to the signal-to-noise ratio.

18. A photoacoustic film thickness measurement method, characterized in that: The steps include: The light source module emits a detection beam and a pump beam; The detection light spot and the pump light spot formed by the detection light beam and the pump light beam on the wafer to be tested overlap and excite the wafer to be tested to generate an optical signal; Acquire a signal-to-noise ratio according to the optical signal; The angle of the wafer to be tested is adjusted according to the signal-to-noise ratio.

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