Photoacoustic film thickness measurement system and measurement method

Abstract

The photoacoustic film thickness measurement system and method provided in the application introduce a laser polarization adjusting device or a wafer rotating assembly in the measurement system, adjust the polarization state of the femtosecond pulse laser or the wafer angle according to the signal-to-noise ratio of the measurement data, eliminate the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability, and improve the signal-to-noise ratio of the measurement signal.

Description

TECHNICAL FIELD

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

[0002] In the field of semiconductor chips, the thickness of the film layer is a key technical index affecting the performance of the chip. The picosecond ultrasonic metal film thickness measurement device is applied to the measurement of key parameters such as film thickness and Young's modulus of metal film and dielectric film chips. The main technical principle is that when a femtosecond-picosecond pulsed laser as a pump light excites the transient thermal expansion of the measured film and emits ultrashort sound waves. The sound wave transmits in the film layer direction and reflects to form a return wave at the interface of the film layer. At this time, another ultrashort pulsed laser as a probe light will irradiate the film excitation position, and the reflectivity change and surface topography change caused by the return wave are detected. By obtaining the time of the return wave reaching the sample surface and the sound velocity in the film layer, the film thickness value of the measured film layer can be obtained.

[0003] The picosecond ultrasonic metal film thickness measurement device is a non-destructive measurement device based on optical detection, and the laser used is a femtosecond pulsed 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 crystal lattice orientation. Since the interaction between laser and matter is jointly affected by the crystal lattice orientation of the matter and the polarization direction of the laser, when the polarization direction of the femtosecond pulsed laser is mismatched with the wafer crystal lattice orientation, it will cause the signal-to-noise ratio of the picosecond ultrasonic metal film thickness measurement to be poor, which seriously affects the accuracy and stability of the metal film thickness measurement result. SUMMARY

[0004] Therefore, the present application provides a photoacoustic film thickness measurement system and method to eliminate the influence of the mismatch between the polarization direction of the femtosecond pulsed laser and the wafer crystal lattice orientation on the measurement stability.

[0005] To solve the above problems, the technical scheme adopted by the present application is as follows:

[0006] One of the objects of the present application is to provide a photoacoustic film thickness measurement system, which comprises:

[0007] A light source module for emitting a probe light beam and a pump light beam;

[0008] A probe light path module comprising a first polarization state adjusting component for adjusting the polarization state of the probe light beam;

[0009] A pump light path module comprising a second polarization state adjusting component for adjusting the polarization state of the pump light beam;

[0010] and a processing module.

[0011] Wherein: the probe beam and the pump beam form a probe spot and a pump spot on the wafer under test, which overlap and excite the wafer under test to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal. Then, it adjusts the first polarization state adjustment component and / or the second polarization state adjustment component based on 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 the signal-to-noise ratio of each state is obtained. Based on the quality of the obtained signal-to-noise ratio, the optimal state of the first polarization state adjustment component or the second polarization state adjustment component is determined.

[0013] In some embodiments, the first polarization state adjustment component and the second polarization state adjustment component comprise a half glass slide.

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

[0015] In some embodiments, the probe optical path module or the pump optical path module further 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 probe beam is reflected sequentially by the broadband hollow retroreflector and the first reflector before entering the first polarization state adjustment component.

[0016] In some embodiments, the pump optical path module further includes a modulator for dynamically modulating the intensity of the pump beam, and the pump 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 latch amplifier connected to the photodetector, and a signal processor connected to the latch amplifier. The photodetector converts the acquired optical signal into an electrical signal and transmits it to the latch amplifier. The latch amplifier obtains the signal-to-noise ratio based on the electrical signal and transmits the output ultrafast signal to the signal processor.

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

[0019] In some embodiments, a wafer rotation component is also included, which is used to fix the wafer under test and is rotatable. The angle of the wafer rotation component is adjusted and the signal-to-noise ratio at each angle is obtained. The optimal angle of the wafer rotation component is determined based on the quality of the obtained signal-to-noise ratio.

[0020] A second objective of this application is to provide a photoacoustic diaphragm thickness measurement system, comprising:

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

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

[0023] and processing modules;

[0024] Wherein: the detection beam and the pump beam form a detection spot and a pump spot on the wafer under test, which overlap and excite the wafer under test to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal. Based on the signal-to-noise ratio, it determines the rotation angle of the wafer rotation component.

[0025] In some embodiments, the optimal angle of the wafer rotation component is determined based on the quality of the obtained signal-to-noise ratio (SNR) of the measurement signal at each angle, by adjusting the angle of the wafer rotation component and obtaining the SNR of the measurement signal at each angle.

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

[0027] In some embodiments, the probe optical path module or the pump optical path module further includes a delay line, which includes an electrically driven linear displacement platform, a broadband hollow retroreflector, and a first reflector. The electrically driven linear displacement platform is movable and used to fix the broadband hollow retroreflector. The probe beam passes through the broadband hollow retroreflector and enters the first reflector in sequence.

[0028] In some embodiments, the pump optical path module further includes a modulator for dynamically modulating the intensity of the pump beam.

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

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

[0031] A third objective of this application is to provide a method for measuring the thickness of a photoacoustic diaphragm, comprising the following steps:

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

[0033] The detection beam and the pump beam form a detection spot and a pump spot on the wafer under test, which overlap and excite the wafer under test to generate an optical signal.

[0034] The signal-to-noise ratio is obtained based on the optical signal;

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

[0036] Fourthly, this application also provides a method for measuring the thickness of a photoacoustic diaphragm, comprising the following steps:

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

[0038] The detection beam and the pump beam form a detection spot and a pump spot on the wafer under test, which overlap and excite the wafer under test to generate an optical signal.

[0039] The signal-to-noise ratio is obtained based on the optical signal;

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

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

[0042] The photoacoustic film thickness measurement system and method provided in this application introduce a laser polarization adjustment device or a wafer rotation component into the measurement system. By adjusting the polarization state of the femtosecond pulse laser or the wafer angle according to the signal-to-noise ratio of the measurement data, the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability is eliminated, thereby improving the signal-to-noise ratio of the measurement signal. Attached Figure Description

[0043] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

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

[0045] Figure 2 This is a schematic diagram of the photoacoustic film thickness measurement system provided in Embodiment 2 of this application.

[0046] Figure 3 This is a flowchart of the photoacoustic film thickness measurement method provided in Embodiment 3 of this application.

[0047] Figure 4 This is a flowchart of the photoacoustic film thickness measurement method provided in Embodiment 4 of this application. Detailed Implementation

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

[0049] In the description of this application, it should be understood that the terms "upper", "lower", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0050] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0051] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments.

[0052] Example 1

[0053] Please see Figure 1 This is a schematic diagram of the photoacoustic film thickness measurement system provided in Embodiment 1 of this application, including a light source module, a probe optical path module, a pump optical path module, and a processing module. The technical solution for its implementation is described in detail below.

[0054] The light source module is used to emit the probe beam and the pump beam.

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

[0056] It is understandable that the light source module is not limited to the above structure. It can also be a single light source or multiple light sources. By splitting the beam emitted from the light source, the optical path requirements can be met, thus ensuring the flexibility of the entire system.

[0057] The detection optical path module includes a first polarization state adjustment component 16, which is used to adjust the polarization state of the detection beam.

[0058] In this embodiment, the first polarization state adjustment component 16 can continuously change the polarization state of the probe beam. The first polarization state adjustment component 16 can typically use a half-glass slide. For example, the polarization state of the probe beam can be adjusted by rotating a motor to drive the deflection of the half-glass slide. The first polarization state adjustment component 16 is not limited to the above structure, and can also be other schemes and devices that can realize laser polarization state adjustment.

[0059] It is understandable that since the crystal lattice orientation is anisotropic, the ultrasound generated by the probe beam may also be anisotropic. By adjusting the polarization angle of the probe beam, the sensitivity of the probe beam to detect ultrasound can be maximized, thereby improving the overall detection signal-to-noise ratio.

[0060] The pump optical path module includes a second polarization state adjustment component 25, which is used to adjust the polarization state of the pump beam.

[0061] In this embodiment, the second polarization state adjustment component 25 can continuously change the polarization state of the pump beam. The second polarization state adjustment component 25 can typically use a half-glass slide. For example, the polarization state of the pump beam can be adjusted by rotating a motor to drive the deflection of the half-glass slide. The second polarization state adjustment component 25 is not limited to the above structure and can also be other schemes and devices that can realize laser polarization state adjustment.

[0062] It is understood that this embodiment maximizes the absorption rate of the pump beam by the wafer by changing the polarization state of the pump beam, thereby maximizing the ultrasonic generation efficiency and ultrasonic amplitude, while reducing the intensity of stray light from the pump.

[0063] The probe beam and the pump beam form probe and pump spots on the wafer under test 200 and then merge to excite the wafer under test 200 to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal. Then, it adjusts the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25 according to the signal-to-noise ratio, thereby realizing the adjustment of the polarization state of the probe beam and / or the polarization state of the pump beam.

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

[0065] It should be noted that the probe and pump optical paths have different polarization states to avoid the pump light affecting the probe signal. The optimal state is when one of the probe or pump optical paths has a polarization state of 90°. However, if both the probe and pump optical paths have a polarization state of 90°, it may not be the case with the highest signal-to-noise ratio. In this case, the wavelengths of the probe and pump light can be adjusted to make them different.

[0066] Furthermore, by setting up two light sources to emit probe light and pump light respectively, or by introducing a frequency doubling crystal in the probe light path / pump light path, for example, by placing the frequency doubling crystal after the beam splitter, the condition that the wavelengths of the probe light and pump light are different can be met.

[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 can be fixed first, and the state of the first polarization state adjustment component 16 can be continuously adjusted. The signal-to-noise ratio (SNR) of the measured signal in each state can be obtained, and the optimal state of the first polarization state adjustment component 16 can be determined based on the quality of the SNR. Subsequently, the state of the first polarization state adjustment component 16 can be fixed, and the state of the second polarization state adjustment component 25 can be continuously adjusted again. The SNR of the measured signal in each state can be obtained, and the optimal state of the polarization state adjustment component 25 can be determined based on the quality of the SNR. When determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, the order of fixing and adjusting the aforementioned polarization state adjustment components is not required. The state of one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be fixed, and the state of the other of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be adjusted. Then, the signal-to-noise ratio of 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 component (not shown in the figure). The wafer rotation component is used to fix the wafer to be measured and the wafer rotation component can rotate. By adjusting the angle of the wafer rotation component, the wafer can be rotated, and the signal-to-noise ratio at each angle can be obtained, and the optimal signal-to-noise ratio can be determined.

[0069] It should be noted that: In this embodiment, the signal-to-noise ratio (SNR) of each state can be obtained by adjusting the state of the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25. The optimal state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 is determined based on the quality of the obtained SNR. At the same time, when adjusting the polarization state adjustment component, the adjustment of the wafer rotation component angle can also be combined to obtain the optimal SNR, thereby obtaining the state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 under this SNR.

[0070] In this embodiment, the probe optical path module or pump optical path module further includes a delay line. The delay line includes an electrically driven linear displacement platform 13, a broadband hollow retroreflector 14, and a first reflecting mirror 15. The electrically driven linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The probe beam sequentially passes through the broadband hollow retroreflector 14 into the first reflecting mirror 15, and is then reflected and incident on 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 capable of achieving delay, or methods that achieve delay by adjusting the pulse frequency, can also meet practical application requirements.

[0071] It should be noted that the above embodiments set a delay line in one of the probe optical path module and the pump optical path module to achieve time delay scanning and optical path compensation. However, in practical applications, the above scheme is not limited. A technical scheme can also be used to set a variable delay line in one of the optical path modules of the probe optical path module and the pump optical path module, while the other optical path module is set with an invariable delay line or no delay line is set, or both the probe optical path module and the pump optical path module are set with variable delay lines. All of these can meet the needs of practical applications.

[0072] In this embodiment, the pump optical path module further includes a modulator 101. The pump light emitted from 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 light intensity of the pump beam reflected by the third reflector 24 can be dynamically modulated.

[0073] In this embodiment, the detection optical path module also includes a fourth reflector 17. The beam whose polarization state is adjusted 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 also includes a fifth reflector 26. The pump beam, after being polarized by the second polarization state adjustment component 25, enters the fifth reflector 26, is focused on the surface of the wafer 200 under test, and forms a pump light spot.

[0075] It is understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable. By adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26, the pump light spot and the probe light spot can be made to coincide on the wafer 200 under test.

[0076] It is understood that this embodiment achieves the change of light path direction by setting the above-mentioned reflector. However, 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 all within the protection scope of this application.

[0077] In this embodiment, the processing module includes a photodetector 104, a latch amplifier 103 connected to the photodetector 104, and a signal processor 105 connected to the latch amplifier 103. The photoacoustic diaphragm thickness measurement system also includes a sixth reflector 20. The probe beam and pump beam reflected from the surface of the wafer 200 under test 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 latch amplifier 103. The latch amplifier 103 obtains the signal-to-noise ratio based on 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 the reference signal input terminal of the lock-in amplifier 103.

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

[0080] The photoacoustic film thickness measurement system provided in this 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, thereby eliminating the influence of the mismatch between the polarization direction of the femtosecond pulse laser and the wafer lattice orientation on the measurement stability and improving the signal-to-noise ratio of the measurement signal.

[0081] Example 2

[0082] Please see Figure 2This is a schematic diagram of the photoacoustic diaphragm thickness measurement system provided in an embodiment of this application, including a light source module, a wafer rotation assembly 201, and a processing module. The technical solution for its implementation is described in detail below.

[0083] The light source module is used to emit the probe beam and the pump beam.

[0084] In this embodiment, the light source module includes a pulsed laser 10 and a beam splitter 11. The femtosecond pulsed laser emitted from the pulsed laser 10 is split into a reflected beam and a transmitted beam by the beam splitter 11. The reflected beam serves as the pump beam, and the transmitted beam serves as the probe beam. It is 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. By splitting the beam emitted from the light source, the optical path requirements are met, thereby ensuring the flexibility of the entire system.

[0085] The wafer rotation component 201 is used to fix the wafer under test and the wafer rotation component can rotate. By rotating the wafer rotation component 201, the wafer under test can be driven to rotate.

[0086] The probe beam and pump beam form a probe spot and a pump spot on the wafer under test 200, which are then combined to excite the wafer under test 200 to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal. Based on the signal-to-noise ratio, it determines the rotation angle of the wafer rotation component 201.

[0087] It is understood that the photoacoustic film thickness measurement system provided in this embodiment introduces a wafer rotation component 201 into the measurement system and adjusts the wafer angle 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 and improve the measurement signal-to-noise ratio.

[0088] It should be noted that the photoacoustic film thickness measurement system provided in Embodiment 2 of this application may also include some of the schemes in Embodiment 1. The probe 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 be referred to Embodiment 1, and will not be repeated here.

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

[0090] In this embodiment, the probe optical path module or pump optical path module further includes a delay line, which comprises an electrically driven linear displacement platform 13, a broadband hollow retroreflector 14, and a first reflecting mirror 15. The electrically driven linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The probe beam sequentially passes through the broadband hollow retroreflector 14 and enters the first reflecting mirror 15. The delay line provides the time delay scan required for measurement. The delay line is not limited to the above structure; other mechanical delay lines capable of achieving delay, or methods that achieve delay by adjusting the pulse frequency, can also meet practical application requirements.

[0091] It should be noted that the above embodiments set a delay line in one of the probe optical path module and the pump optical path module to achieve time delay scanning and optical path compensation. However, in practical applications, the above scheme is not limited. A technical scheme can also be used to set a variable delay line in one of the optical path modules of the probe optical path module and the pump optical path module, while the other optical path module is set with an invariable delay line or no delay line is set, or both the probe optical path module and the pump optical path module are set with variable delay lines. All of these can meet the needs of practical applications.

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

[0093] In this embodiment, the detection optical path module also 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 under test, and forms a detection light spot.

[0094] In this embodiment, the pump optical path module also includes a fifth reflector 26. The pump beam dynamically modulated by the modulator 101 is re-injected into the fifth reflector 26, focused on the surface of the wafer 200 under test, and forms a pump light spot.

[0095] It is understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable. By adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26, the pump light spot and the probe light spot can be made to coincide on the wafer 200 under test.

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

[0097] In this embodiment, the processing module includes a photodetector 104, a latch amplifier 103 connected to the photodetector 104, and a signal processor 105 connected to the latch amplifier 103. The photoacoustic diaphragm thickness measurement system also includes a sixth reflector 20. The probe beam and pump beam reflected from the surface of the wafer 200 under test 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 latch amplifier 103. The latch amplifier 103 obtains the signal-to-noise ratio based on 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 the reference signal input terminal of the lock-in amplifier 103.

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

[0100] The photoacoustic film thickness measurement system provided in this 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] Please see Figure 3 The following is a flowchart of the photoacoustic diaphragm thickness measurement method according to an embodiment of this application. Applied to the photoacoustic diaphragm thickness measurement system of Embodiment 1, it includes the following steps:

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

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

[0105] It is understandable that the light source module is not limited to the above structure. It can also be a single light source or multiple light sources. By splitting the beam emitted from the light source, the optical path requirements can be met, thus ensuring the flexibility of the entire system.

[0106] Step S320: The probe beam and the pump beam form a probe spot and a pump spot on the wafer under test, respectively, and then excite the wafer under test 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, as detailed below:

[0108] The detection optical path module includes a first polarization state adjustment component 16, which is used to adjust the polarization state of the detection beam.

[0109] In this embodiment, the first polarization state adjustment component 16 can continuously change the polarization state of the probe beam. The first polarization state adjustment component 16 can typically use a half-glass slide. For example, the polarization state of the probe beam can be adjusted by rotating a motor to drive the deflection of the half-glass slide. The first polarization state adjustment component 16 is not limited to the above structure, and can also be other schemes and devices that can realize laser polarization state adjustment.

[0110] It is understandable that since the crystal lattice orientation is anisotropic, the ultrasound generated by the probe beam may also be anisotropic. By adjusting the polarization angle of the probe beam, the sensitivity of the probe beam to detect ultrasound can be maximized, thereby improving the overall detection signal-to-noise ratio.

[0111] The pump optical path module includes a second polarization state adjustment component 25, which is used to adjust the polarization state of the pump beam.

[0112] In this embodiment, the second polarization state adjustment component 25 can continuously change the polarization state of the pump beam. The second polarization state adjustment component 25 can typically use a half-glass slide. For example, the polarization state of the pump beam can be adjusted by rotating a motor to drive the deflection of the half-glass slide. The second polarization state adjustment component 25 is not limited to the above structure and can also be other schemes and devices that can realize laser polarization state adjustment.

[0113] It is understood that this embodiment maximizes the absorption rate of the pump beam by the wafer by changing the polarization state of the pump beam, thereby maximizing the ultrasonic generation efficiency and ultrasonic amplitude, while reducing the intensity of stray light from the pump.

[0114] The probe beam and the pump beam form probe and pump spots on the wafer under test 200 and then merge to excite the wafer under test 200 to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal. Then, it adjusts the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25 according to the signal-to-noise ratio, thereby realizing the adjustment of the polarization state of the probe beam and / or the polarization state of the pump beam.

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

[0116] It should be noted that the probe and pump optical paths have different polarization states to avoid the pump light affecting the probe signal. The optimal state is when one of the probe or pump optical paths has a polarization state of 90°. However, if both the probe and pump optical paths have a polarization state of 90°, it may not be the case with the highest signal-to-noise ratio. In this case, the wavelengths of the probe and pump light can be adjusted to make them different.

[0117] Furthermore, by setting up two light sources to emit probe light and pump light respectively, or by introducing a frequency doubling crystal in the probe light path / pump light path, for example, by placing the frequency doubling crystal after the beam splitter, the condition that the wavelengths of the probe light and pump light are different can be met.

[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 can be fixed first, and the state of the first polarization state adjustment component 16 can be continuously adjusted. The signal-to-noise ratio (SNR) of the measured signal in each state can be obtained, and the optimal state of the first polarization state adjustment component 16 can be determined based on the quality of the SNR. Subsequently, the state of the first polarization state adjustment component 16 can be fixed, and the state of the second polarization state adjustment component 25 can be continuously adjusted again. The SNR of the measured signal in each state can be obtained, and the optimal state of the polarization state adjustment component 25 can be determined based on the quality of the SNR. When determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, the order of fixing and adjusting the aforementioned polarization state adjustment components is not required. The state of one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be fixed, and the state of the other of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be adjusted. Then, the signal-to-noise ratio of 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 component (not shown in the figure). The wafer rotation component is used to fix the wafer to be measured and the wafer rotation component can rotate. By adjusting the angle of the wafer rotation component, the wafer can be rotated, and the signal-to-noise ratio at each angle can be obtained, and the optimal signal-to-noise ratio can be determined.

[0120] It should be noted that: In this embodiment, the signal-to-noise ratio (SNR) of each state can be obtained by adjusting the state of the first polarization state adjustment component 16 and / or the second polarization state adjustment component 25. The optimal state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 is determined based on the quality of the obtained SNR. At the same time, when adjusting the polarization state adjustment component, the adjustment of the wafer rotation component angle can also be combined to obtain the optimal SNR, thereby obtaining the state of the first polarization state adjustment component 16 or the second polarization state adjustment component 25 under this SNR.

[0121] In this embodiment, the probe optical path module or pump optical path module further includes a delay line. The delay line includes an electrically driven linear displacement platform 13, a broadband hollow retroreflector 14, and a first reflecting mirror 15. The electrically driven linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The probe beam sequentially passes through the broadband hollow retroreflector 14 and enters the first reflecting mirror 15, and is then reflected and incident on 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 that achieve delay by adjusting the pulse frequency, can also meet the practical application requirements.

[0122] It should be noted that the above embodiments set a delay line in one of the probe optical path module and the pump optical path module to achieve time delay scanning and optical path compensation. However, in practical applications, the above scheme is not limited. A technical scheme can also be used to set a variable delay line in one of the optical path modules of the probe optical path module and the pump optical path module, while the other optical path module is set with an invariable delay line or no delay line is set, or both the probe optical path module and the pump optical path module are set with variable delay lines. All of these can meet the needs of practical applications.

[0123] In this embodiment, the pump optical path module further includes a modulator 101. The pump light emitted from 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 light intensity of the pump beam reflected by the third reflector 24 can be dynamically modulated.

[0124] In this embodiment, the detection optical path module also includes a fourth reflector 17. The beam whose polarization state is adjusted 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 also includes a fifth reflector 26. The pump beam, after being polarized by the second polarization state adjustment component 25, enters the fifth reflector 26, is focused on the surface of the wafer 200 under test, and forms a pump light spot.

[0126] It is understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable. By adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26, the pump light spot and the probe light spot can be made to coincide on the wafer 200 under test.

[0127] It is understood that this embodiment achieves the change of light path direction by setting the above-mentioned reflector. However, 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 all within the protection scope of this application.

[0128] Step S330: Obtain the signal-to-noise ratio based on the optical signal.

[0129] Specifically, the probe beam and the pump beam form a probe spot and a pump spot on the wafer under test, which overlap and excite the wafer under test to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal.

[0130] Please combine Figure 1 The processing module includes a photodetector 104, a latch amplifier 103 connected to the photodetector 104, and a signal processor 105 connected to the latch amplifier 103. The photoacoustic diaphragm thickness measurement system also includes a sixth reflector 20. The probe beam and pump beam reflected from the surface of the wafer 200 under test 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 latch amplifier 103. The latch amplifier 103 obtains the signal-to-noise ratio based on the electrical signal and transmits the output ultrafast signal to the signal processor 105.

[0131] Step S340: Adjust the polarization state of the probe 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, which is used to adjust the polarization state of the detection beam.

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

[0134] The pump optical path module includes a second polarization state adjustment component 25, which is used to adjust the polarization state of the pump beam.

[0135] It is understood that the second polarization state adjustment component 25 can continuously change the polarization state of the pump beam. The second polarization state adjustment component 25 can typically use a half glass plate, but is not limited to other schemes 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 can be fixed first, and the state of the first polarization state adjustment component 16 can be continuously adjusted. The signal-to-noise ratio (SNR) of the measured signal in each state can be obtained, and the optimal state of the first polarization state adjustment component 16 can be determined based on the quality of the SNR. Subsequently, the state of the first polarization state adjustment component 16 can be fixed, and the state of the second polarization state adjustment component 25 can be continuously adjusted again. The SNR of the measured signal in each state can be obtained, and the optimal state of the polarization state adjustment component 25 can be determined based on the quality of the SNR. When determining the optimal configuration of the first polarization state adjustment component 16 and the second polarization state adjustment component 25, the order of fixing and adjusting the aforementioned polarization state adjustment components is not required. The state of one of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be fixed, and the state of the other of the first polarization state adjustment component 16 and the second polarization state adjustment component 25 can be adjusted. Then, the signal-to-noise ratio of 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 this 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] Please see Figure 4 The above is a flowchart of the photoacoustic diaphragm thickness measurement method according to an embodiment of this application. Applied to the photoacoustic diaphragm thickness measurement system of Embodiment 2, it includes the following steps:

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

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

[0142] It is understandable that the light source module is not limited to the above structure. It can also be a single light source or multiple light sources. By splitting the beam emitted from the light source, the optical path requirements can be met, thus ensuring the flexibility of the entire system.

[0143] Step S420: The probe beam and the pump beam form a probe spot and a pump spot on the wafer under test, which are then combined and excited to generate an optical signal on the wafer under test.

[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 Embodiment 4 of this application may also include some of the schemes in Embodiment 1. The probe 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 can be referred to Embodiment 1, and will not be repeated here.

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

[0147] In this embodiment, the probe optical path module or pump optical path module further includes a delay line. The delay line includes an electrically driven linear displacement platform 13, a broadband hollow retroreflector 14, and a first reflecting mirror 15. The electrically driven linear displacement platform 13 is movable and used to fix the broadband hollow retroreflector 14. The probe beam sequentially passes through the broadband hollow retroreflector 14 and enters the first reflecting mirror 15. 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 capable of achieving delay, or methods that achieve delay by adjusting the pulse frequency, can also meet practical application requirements.

[0148] It should be noted that the above embodiments set a delay line in one of the probe optical path module and the pump optical path module to achieve time delay scanning and optical path compensation. However, in practical applications, the above scheme is not limited. A technical scheme can also be used to set a variable delay line in one of the optical path modules of the probe optical path module and the pump optical path module, while the other optical path module is set with an invariable delay line or no delay line is set, or both the probe optical path module and the pump optical path module are set with variable delay lines. All of these can meet the needs of practical applications.

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

[0150] In this embodiment, the detection optical path module also 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 under test, and forms a detection light spot.

[0151] In this embodiment, the pump optical path module also includes a fifth reflector 26. The pump beam dynamically modulated by the modulator 101 is re-injected into the fifth reflector 26, focused on the surface of the wafer 200 under test, and forms a pump light spot.

[0152] It is understood that the angles of the fourth reflector 17 and the fifth reflector 26 are adjustable. By adjusting the angles of the fourth reflector 17 and / or the fifth reflector 26, the pump light spot and the probe light spot can be made to coincide on the wafer 200 under test.

[0153] It is understood that this embodiment achieves the change of light path direction by setting the above-mentioned reflector. However, 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 all within the protection scope of this application.

[0154] Step S430: Obtain the signal-to-noise ratio based on the optical signal.

[0155] Specifically, the probe beam and the pump beam form a probe spot and a pump spot on the wafer under test, which overlap and excite the wafer under test to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal.

[0156] Please combine Figure 2 The processing module includes a photodetector 104, a latch amplifier 103 connected to the photodetector 104, and a signal processor 105 connected to the latch amplifier 103. The photoacoustic diaphragm thickness measurement system also includes a sixth reflector 20. The probe beam and pump beam reflected from the surface of the wafer 200 under test 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 latch amplifier 103. The latch amplifier 103 obtains the signal-to-noise ratio based on the electrical signal and transmits the output ultrafast signal to the signal processor 105.

[0157] Step S440: Adjust the angle of the wafer under test according to the signal-to-noise ratio.

[0158] Please combine Figure 2 In this embodiment, a wafer rotation component 201 is used to fix the wafer under test, and the wafer rotation component is rotatable. The wafer under test is rotated by the rotation of the wafer rotation component 201.

[0159] The photoacoustic film thickness measurement method provided in this application adjusts the rotation angle of the wafer under test 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.

[0160] The above are merely preferred embodiments of this application, and only specifically describe the technical principles of this application. These descriptions are only for explaining the principles of this application and should not be construed as limiting the scope of protection of this application in any way. Based on this explanation, any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application, as well as other specific embodiments of this application that can be conceived by those skilled in the art without creative effort, should be included within the scope of protection of this application.

Claims

1. A photoacoustic diaphragm thickness measurement system, characterized in that, include: The light source module is used to emit a probe beam and a pump beam; A detection optical path module, the detection optical path module including a first polarization state adjustment component, the first polarization state adjustment component being used to adjust the polarization state of the detection beam; A pump optical path module, the pump optical path module including a second polarization state adjustment component, the second polarization state adjustment component being used to adjust the polarization state of the pump beam; The detection beam and the pump beam form a detection spot and a pump spot on the wafer under test, which are then combined to excite the wafer under test to generate an optical signal. The processing module receives the optical signal and obtains the signal-to-noise ratio based on the optical signal. Wherein: The state of the second polarization state adjustment component is fixed, the state of the first polarization state adjustment component is continuously adjusted, and the signal-to-noise ratio of the measured signal is obtained in each state. The optimal state of the first polarization state adjustment component is determined based on the quality of the signal-to-noise ratio of the measured signal. The state of the first polarization state adjustment component is fixed, the state of the second polarization state adjustment component is continuously adjusted, and the signal-to-noise ratio of the measured signal is obtained in each state. The optimal state of the second polarization state adjustment component is determined based on the quality of the signal-to-noise ratio of the measured signal. The processing module adjusts the first polarization state adjustment component and the second polarization state adjustment component according to the optimal state of the first polarization state adjustment component and the optimal state of the second polarization state adjustment component.

2. The photoacoustic diaphragm thickness measurement system as described in claim 1, characterized in that, The first polarization state adjustment component and the second polarization state adjustment component each include a half glass slide.

3. The photoacoustic diaphragm thickness measurement system as described in claim 1, characterized in that, The light source module includes a pulsed laser and a beam splitter. The femtosecond pulsed laser emitted by the pulsed laser is split into a reflected beam and a transmitted beam by the beam splitter.

4. The photoacoustic diaphragm thickness measurement system as described in claim 3, characterized in that, The probe optical path module or the pump optical path module further 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 probe beam is reflected sequentially by the broadband hollow retroreflector and the first reflector before entering the first polarization state adjustment component.

5. The photoacoustic diaphragm thickness measurement system as described in claim 3, characterized in that, The pump optical path module also includes a modulator, which is used to dynamically modulate the intensity of the pump beam. The pump beam modulated by the modulator is then incident on the second polarization state adjustment component.

6. The photoacoustic diaphragm thickness measurement system as described in claim 5, characterized in that, The processing module includes a photodetector, a latch amplifier connected to the photodetector, and a signal processor connected to the latch amplifier. The photodetector converts the acquired optical signal into an electrical signal and transmits it to the latch amplifier. The latch amplifier obtains the signal-to-noise ratio based on the electrical signal and transmits the output ultrafast signal to the signal processor.

7. The photoacoustic diaphragm thickness measurement system as described in claim 6, characterized in that, The modulator is also electrically connected to a signal generator, which is further connected to the reference signal input of the latch amplifier.

8. The photoacoustic diaphragm thickness measurement system as described in claim 1, characterized in that, It also includes a wafer rotation component, which is used to fix the wafer under test and is rotatable. By adjusting the angle of the wafer rotation component and obtaining the signal-to-noise ratio at each angle, the optimal angle of the wafer rotation component is determined based on the quality of the obtained signal-to-noise ratio.

9. A method for measuring the thickness of a photoacoustic diaphragm in a photoacoustic diaphragm thickness measurement system as described in claim 1, characterized in that, Includes the following steps: The light source module emits a detection beam and a pump beam; The detection beam and the pump beam form a detection spot and a pump spot on the wafer under test, which overlap and excite the wafer under test to generate an optical signal. The signal-to-noise ratio is obtained based on the optical signal; The polarization state of the probe beam and / or the pump beam is adjusted according to the signal-to-noise ratio.

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