Measurement equipment and frequency estimation method and denoising method for equipment noise in measurement equipment

Abstract

The invention relates to the technical field of photoacoustic measurement, and particularly provides measuring equipment and a frequency estimation method and a denoising method of equipment noise therein, the frequency estimation method comprises the following steps: controlling pump light and probe light to irradiate a first sample to be measured, the pump light generating ultrasonic waves on the surface of the first sample to be measured, the detection light is reflected by the surface of the first to-be-detected sample to form first detection signal light; acquiring a first photoacoustic time domain signal formed by the first detection signal light; removing a non-noise signal and a random noise signal in the first photoacoustic time domain signal to obtain a first time domain noise signal; performing time-frequency transformation on the first time-domain noise signal to obtain a first noise spectrum; and determining the frequency of the equipment noise according to the spectrum amplitude of the first noise spectrum. Thus, the device noise in the photoacoustic signal can be removed in a targeted manner according to the frequency of the device noise subsequently, and compared with a traditional overall noise removal manner, the signal-to-noise ratio and the measurement sensitivity can be improved.

Description

Technical Field

[0001] This invention relates to the field of photoacoustic measurement technology, specifically to a measuring device and a method for frequency estimation and noise reduction of the device noise therein. Background Technology

[0002] In the field of semiconductor testing, photoacoustic measurement technology is often used to measure film thickness and other parameters. In photoacoustic measurement, pump light irradiates the sample surface to excite heat generation, leading to thermoelastic deformation and the formation of acoustic pulses. These acoustic pulses propagate longitudinally between the two surfaces of the sample, such as a thin film. When they reach the upper surface, they cause a change in reflectivity and a slight deformation. By irradiating the sample surface with probe light and receiving the reflected probe signal light, a corresponding photoacoustic signal is formed. This signal allows the detection of changes on the upper surface, and the film thickness can be determined based on these changes.

[0003] Noise is inevitably introduced during photoacoustic measurements. Traditional filtering methods are typically used to remove noise from photoacoustic signals, but this can weaken the signal itself, reducing the signal-to-noise ratio and measurement sensitivity. Summary of the Invention

[0004] This invention provides a frequency estimation method, a noise reduction method, and a measurement device for equipment noise, aiming to specifically filter out equipment noise generated by photoacoustic measurement equipment, thereby solving the problem that overall noise removal from photoacoustic signals leads to signal attenuation.

[0005] According to a first aspect, one embodiment provides a method for estimating the frequency of device noise, applied to a photoacoustic measurement device, comprising:

[0006] A pump light and a probe light are controlled to illuminate a first sample to be tested. The pump light is used to generate ultrasonic waves on the surface of the first sample to be tested, and the probe light is reflected by the surface of the first sample to be tested to form a first probe signal light.

[0007] Acquire the first photoacoustic time-domain signal formed by the first detection signal light;

[0008] Remove the non-noise signal and random noise signal from the first photoacoustic time-domain signal to obtain the first time-domain noise signal;

[0009] The first noise spectrum is obtained by performing a time-frequency transformation on the first time-domain noise signal;

[0010] The frequency of the equipment noise is determined based on the spectral amplitude of the first noise spectrum.

[0011] In some embodiments, there are N sets of the first photoacoustic time domain signals. The N sets of the first photoacoustic time domain signals are formed by repeatedly irradiating the first sample to be tested with the probe light N times, and are respectively formed by the first probe signal light during each irradiation.

[0012] The step of removing non-noise signals and random noise signals from the first photoacoustic time-domain signal to obtain the first time-domain noise signal includes: for each group of the first photoacoustic time-domain signals, removing common signals present in N groups of the first photoacoustic time-domain signals to obtain N groups of first time-domain noise signals.

[0013] In some embodiments, for each group of the first photoacoustic time-domain signals, common signals present in the N groups of the first photoacoustic time-domain signals are removed to obtain N groups of first time-domain noise signals, including:

[0014] The first photoacoustic time-domain signal of N groups is averaged to obtain the first photoacoustic average signal.

[0015] The N groups of the first photoacoustic time-domain signals are respectively differentiated from the first photoacoustic average signal to obtain the N groups of first time-domain noise signals.

[0016] In some embodiments, after removing non-noise signals and random noise signals from the first photoacoustic time-domain signal to obtain a first time-domain noise signal, the method further includes: removing signal interference generated by heat accumulation due to repeated irradiation of the first test sample from the first time-domain noise signal.

[0017] In some embodiments, removing signal interference from the first time-domain noise signal caused by heat accumulation due to repeated irradiation of the first test sample includes: obtaining a pre-constructed curve of the change in surface reflectivity of the first test sample caused by heat accumulation, and differentiating each set of the first time-domain noise signal from the curve.

[0018] Alternatively, each group of the first photoacoustic time-domain signals includes multiple first photoacoustic signal data collected at different times, and the first time-domain noise signal includes multiple first noise data corresponding to the first photoacoustic signal data; removing signal interference caused by heat accumulation due to repeated irradiation of the first test sample in the first time-domain noise signal includes: for each group of the first time-domain noise signals, using the first noise data therein to perform curve fitting to obtain a fitting curve, and performing difference between the first time-domain noise signal and the fitting curve.

[0019] In some embodiments, there is a time delay between the pump light and the probe light, and the first photoacoustic time-domain signal is a signal that varies with time delay formed by irradiating the first sample under test with the pump light and the probe light having different time delays;

[0020] Before removing non-noise signals and random noise signals from the first photoacoustic time-domain signal, the method further includes: converting the time delay into the actual measurement time to obtain the first photoacoustic time-domain signal that varies with the actual measurement time.

[0021] In some embodiments, determining the frequency of the device noise based on the spectral amplitude of the first noise spectrum includes: averaging the first noise spectra of N groups of the first time-domain noise signals to obtain a first average spectrum; and determining the frequency corresponding to the peak value of the spectral amplitude of the first average spectrum as the frequency of the device noise.

[0022] According to a second aspect, one embodiment provides a noise reduction method applied to a photoacoustic measurement device, the noise reduction method comprising:

[0023] A pump light and a probe light are controlled to illuminate a second sample to be tested. The pump light is used to generate ultrasonic waves on the surface of the second sample to be tested, and the probe light is reflected by the surface of the second sample to be tested to form a second probe signal light.

[0024] Acquire the second photoacoustic time-domain signal formed by the second detection signal light;

[0025] The frequency of the device noise determined by the frequency estimation method of device noise according to any embodiment of the first aspect is used to remove the signal with the frequency in the second photoacoustic time domain signal to obtain a second photoacoustic time domain signal with the device noise removed, wherein the second test sample and the first test sample are samples of the same type.

[0026] In some embodiments, the noise reduction method further includes:

[0027] The second photoacoustic time-domain signal, after removing the ultrasonic echo signal, is subjected to time-frequency transformation to obtain the second noise spectrum.

[0028] The frequency corresponding to the peak value of the second noise spectrum is determined as the second noise frequency;

[0029] For the second photoacoustic time-domain signal used to remove device noise, remove the signal having the second noise frequency.

[0030] According to a second aspect, one embodiment provides a measuring device, comprising:

[0031] A light source is used to emit pump light and probe light to illuminate the first sample to be tested. The pump light is used to generate ultrasonic waves on the surface of the first sample to be tested, and the probe light is reflected by the surface of the first sample to be tested to form a first probe signal light.

[0032] A detector is used to receive the first detection signal light to form a first photoacoustic time-domain signal;

[0033] The processor, connected to the light source and the detector, is used to execute the frequency estimation method for device noise as described in any embodiment of the first aspect.

[0034] In some embodiments, the pump light and probe light emitted by the light source are also used to illuminate a second sample to be tested, the pump light is also used to generate ultrasonic waves on the surface of the second sample to be tested, and the probe light is reflected by the surface of the second sample to be tested to form a second probe signal light;

[0035] The detector is also used to receive the second detection signal light to form a second photoacoustic time-domain signal;

[0036] The processor is further configured to execute the denoising method described in any embodiment of the second aspect to denoise the second photoacoustic time-domain signal, and to determine the film properties of the second test sample based on the denoised second photoacoustic time-domain signal.

[0037] According to the frequency estimation method, denoising method, and measuring device for device noise described in the above embodiments, after acquiring the first photoacoustic time-domain signal formed by the first detection signal light reflected from the surface of the first sample to be tested, the non-noise signal and random noise signal in the first photoacoustic time-domain signal are first removed to obtain a first time-domain noise signal containing high-purity device noise. Then, the first time-domain noise signal is subjected to time-frequency transformation to obtain a first noise spectrum, thereby enabling the determination of the frequency of the device noise based on the spectral amplitude of the first noise spectrum. In this way, subsequent targeted filtering and other methods can be used to remove device noise from the photoacoustic signal based on the frequency of the device noise, which improves the signal-to-noise ratio and measurement sensitivity compared to traditional overall removal methods. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of a photoacoustic measuring device according to one embodiment;

[0039] Figure 2 This is a schematic diagram illustrating the principle of photoacoustic measurement using pump light and probe light.

[0040] Figure 3 An image of a photoacoustic time-domain signal according to one embodiment;

[0041] Figure 4 A flowchart illustrating a method for estimating the frequency of device noise in a photoacoustic measurement device according to one embodiment;

[0042] Figure 5 An image of a first photoacoustic time-domain signal according to one embodiment;

[0043] Figure 6 According to Figure 5 An image of the first time-domain noise signal obtained from the first photoacoustic time-domain signal;

[0044] Figure 7 To be Figure 5 The image of the first photoacoustic time-domain signal obtained after converting the time delay in the image into the actual measurement time;

[0045] Figure 8 To remove Figure 6 The image of the first time-domain noise signal obtained after the signal interference caused by heat accumulation due to repeated irradiation of the first test sample in the first time-domain noise signal;

[0046] Figure 9 To Figure 8 The first noise spectrum is obtained by performing a time-frequency transformation on the first time-domain noise signal shown.

[0047] Figure 10 This is a flowchart of a noise reduction method according to one embodiment. Detailed Implementation

[0048] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of this application. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, certain operations related to this application are not shown or described in the specification. This is to avoid obscuring the core parts of this application with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0049] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments. At the same time, the steps or actions in the method description can be rearranged or adjusted in a manner obvious to those skilled in the art. Therefore, the various orders in the specification and drawings are only for the clear description of a particular embodiment and do not imply a necessary order, unless otherwise stated that a particular order must be followed.

[0050] The serial numbers assigned to components or physical quantities in this document, such as "first," "second," etc., are used only to distinguish the described objects and have no sequential or technical meaning. They should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. "Multiple" means two or more. Unless otherwise specified, "connection" or "linkage" in this application includes both direct and indirect connections (linkages).

[0051] In photoacoustic measurements, in addition to common noises such as white noise, there is also equipment noise caused by the photoacoustic measurement equipment itself. The applicant argues that simply removing noise from the photoacoustic signal as a whole using filtering methods, without specifically filtering for the characteristics of the equipment noise, weakens the photoacoustic signal itself during the filtering process, thereby reducing the signal-to-noise ratio and measurement sensitivity. Therefore, it is necessary to perform targeted noise removal based on the specific characteristics of the equipment noise.

[0052] Therefore, the applicant proposes a technical solution to estimate the frequency of equipment noise and then perform denoising at that frequency. In this application's technical solution, based on the principle of photoacoustic measurement, the time-domain signal is preprocessed to remove useful non-noise signals and eliminate interference from random noise such as white noise and thermal noise. Then, noise frequency characteristics are extracted through spectral analysis, facilitating targeted denoising. This solution solves the problem of the lack of specificity in photoacoustic signal denoising; by targeting noise at specific frequencies, it can improve the signal-to-noise ratio and measurement sensitivity of the photoacoustic signal.

[0053] To facilitate understanding of the technical solution of this application, the photoacoustic measurement equipment provided in this application will be described below. Please refer to... Figure 1 The photoacoustic measurement device includes a light source 10, a detector 11, and a processor 12.

[0054] Light source 10 is used to emit pump light and probe light to illuminate the sample 13 under test. Please refer to... Figure 1 In some embodiments, the light source 10 includes a laser 101 and a beam splitter 102, wherein the laser 101 is used to generate a pulsed beam, and the beam splitter 102 is used to split the pulsed beam into two beams, one for pump light and the other for probe light. In some examples, the pulsed beam generated by the laser 101 can be an ultrashort pulse beam, and the beam splitter 102 can include a polarizing beam splitter prism. In other embodiments, the light source 10 can include a first laser and a second laser, wherein the first laser is used to generate pump light and the second laser is used to generate probe light.

[0055] In photoacoustic measurements, the pump light and probe light are incident on the same location on the sample 13. Please refer to... Figure 2The excitation light generates ultrasonic waves at the light spot on the surface of the sample 13. The ultrasonic waves propagate into the interior of the sample 13 and are reflected when they encounter the interface of the film layers of different materials below, forming an ultrasonic echo signal that returns to the surface of the sample 13, causing changes in the surface morphology and reflectivity of the sample 13.

[0056] When the probe light shines on the same location, it is reflected by the surface of the sample 13 to form a probe signal light. The detector 11 receives the probe signal light and detects its intensity. When the ultrasonic wave reflects back to the surface of the sample 13, the intensity of the reflected light changes due to the change in reflectivity. By performing a time-delay scan on the probe light, the signal of the reflected light intensity changing with the propagation time of the ultrasonic wave is obtained. Because it is a time-varying signal, it is referred to as a photoacoustic time-domain signal in this application. Figure 3 As shown.

[0057] Detector 11 is used to receive the detection signal light to form a photoacoustic time-domain signal. Detector 11 can be a photodetector used to convert the optical signal into an electrical signal, such as converting light intensity into a voltage value.

[0058] The processor 12 is connected to the light source 10 and the detector 11 to control the pump light and the probe light to illuminate the sample 13 under test, acquire the photoacoustic time-domain signal formed by the probe signal light, and process the photoacoustic time-domain signal to estimate the frequency of the device noise and / or determine the film properties of the sample 13 under test.

[0059] It should be noted that the film layer of the sample 13 to be tested can be at least one of a metal film or a dielectric film, and this application does not specifically limit the type of the sample 13 to be tested. The film layer properties include the film layer thickness.

[0060] In this application, the frequency of equipment noise in the photoacoustic measurement device is estimated using a first test sample. Subsequently, when detecting the film properties of a second test sample of the same type as the first test sample, the equipment noise in the photoacoustic time domain signal can be specifically removed based on this frequency. The first test sample can be any sample in the photoacoustic measurement scenario.

[0061] The pump light emitted by the light source 11 can be used to generate ultrasonic waves on the surface of the first sample to be tested, and the probe light is reflected by the surface of the first sample to be tested to form a first probe signal light. The detector 11 is used to receive the first probe signal light to form a first photoacoustic time-domain signal. The processor 12 can execute the frequency estimation method for device noise in the photoacoustic measurement device of any embodiment of this application. The pump light emitted by the light source 11 can also be used to generate ultrasonic waves on the surface of the second sample to be tested, and the probe light is reflected by the surface of the second sample to be tested to form a second probe signal light. The detector 11 is also used to receive the second probe signal light to form a second photoacoustic time-domain signal. The processor 12 can also execute the denoising method of any embodiment of this application to denoise the second photoacoustic time-domain signal, and determine the film properties of the second sample to be tested based on the denoised second photoacoustic time-domain signal. The first sample to be tested and the second sample to be tested can be the same sample or different samples.

[0062] The frequency estimation method for equipment noise provided in this application is described below. Please refer to it. Figure 4 In some embodiments, the method includes steps 100 to 500, which are described in detail below.

[0063] Step 100: Control the pump light and probe light to illuminate the first sample to be tested.

[0064] Step 200: Acquire the first photoacoustic time-domain signal formed by the first detection signal light.

[0065] In some embodiments, to improve the accuracy of frequency estimation, the probe light can be controlled to repeatedly irradiate the first test sample multiple times, performing multiple measurements to obtain N sets of first photoacoustic time-domain signals (N is an integer not less than 2). These N sets of first photoacoustic time-domain signals are formed by repeatedly irradiating the first test sample with the probe light N times, each time by the first probe signal light. These N sets of first photoacoustic time-domain signals are used to estimate the noise frequency. Preferably, N ≥ 5. During the acquisition of different first photoacoustic time-domain signals, the probe light can irradiate the same location on the first test sample. In one embodiment, the five sets of first photoacoustic time-domain signals acquired are as follows: Figure 5 As shown, different colored curves represent different first photoacoustic time-domain signals. A photodetector is used to receive the first detection signal light and convert the light intensity into a voltage value.

[0066] Step 300: Remove the non-noise signal and random noise signal from the first photoacoustic time domain signal to obtain the first time domain noise signal.

[0067] Among them, non-noise signals are the useful signals used to perform photoacoustic measurements on samples, including ultrasonic echo signals, and random noise signals include white noise signals, thermal noise signals, etc.

[0068] In some embodiments, removing non-noise signals and random noise signals from the first photoacoustic time-domain signal to obtain a first time-domain noise signal includes: for each group of first photoacoustic time-domain signals, removing common signals present in N groups of first photoacoustic time-domain signals to obtain N groups of first time-domain noise signals.

[0069] It is understandable that the common signals appearing in multiple measurements include ultrasonic echo signals and noise signals with the same phase and frequency. The noise signals with the same phase and frequency include white noise signals, thermal noise signals, etc. Therefore, by acquiring N sets of first photoacoustic time-domain signals and removing the common signals present in the N sets of first photoacoustic time-domain signals, the resulting first time-domain noise signal basically contains only equipment noise, providing a basis for subsequent estimation of equipment noise frequency.

[0070] In some embodiments, for each group of first photoacoustic time-domain signals, common signals present in N groups of first photoacoustic time-domain signals are removed to obtain N groups of first time-domain noise signals. This includes: averaging the N groups of first photoacoustic time-domain signals to obtain a first photoacoustic average signal; and differentiating each of the N groups of first photoacoustic time-domain signals from the first photoacoustic average signal to obtain N groups of first time-domain noise signals. Figure 5 Based on this, the five sets of first time-domain noise signals obtained are as follows: Figure 6 As shown.

[0071] In this embodiment, by subtracting the N groups of first photoacoustic time-domain signals from the averaged first photoacoustic signal, the common signal present in the N groups of first photoacoustic time-domain signals is simply and effectively removed.

[0072] Step 400: Perform time-frequency transformation on the first time-domain noise signal to obtain the first noise spectrum.

[0073] Time-frequency transformation can employ techniques such as Fourier transform.

[0074] Step 500: Determine the frequency of the equipment noise based on the spectral amplitude of the first noise spectrum.

[0075] Specifically, the peak frequency of the first noise spectrum can be searched, and the frequency corresponding to the peak frequency can be determined as the frequency of the device noise. Existing peak-finding algorithms can be used to obtain the frequency corresponding to the peak frequency. The obtained device noise frequency is then used for targeted removal of device noise through filtering and other methods.

[0076] If N sets of first photoacoustic time-domain signals are obtained and the noise frequency is estimated, and non-noise signals and random noise signals are removed from the first photoacoustic time-domain signals to obtain N sets of first time-domain noise signals, then N first noise spectra will be obtained. At this time, the first noise spectra of the N sets of first time-domain noise signals can be averaged to obtain the first average spectrum. The frequency corresponding to the peak value of the spectrum amplitude of the first average spectrum is determined as the frequency of the equipment noise.

[0077] In some embodiments, to avoid interference from possible low-frequency signals, after obtaining the peak value of the spectral amplitude, a filtering process can be performed, and the frequency corresponding to the peak value of the spectral amplitude that is greater than a preset frequency threshold and / or the spectral amplitude that is greater than a preset amplitude threshold can be used as the frequency of the device noise.

[0078] In this embodiment, after obtaining the first photoacoustic time-domain signal formed by the first detection signal light reflected from the surface of the first sample to be tested, the non-noise signal and random noise signal in the first photoacoustic time-domain signal are first removed to obtain a first time-domain noise signal containing high-purity equipment noise. Then, the first time-domain noise signal is subjected to time-frequency transformation to obtain a first noise spectrum, so that the frequency of the equipment noise can be determined according to the spectral amplitude of the first noise spectrum. In this way, the equipment noise in the photoacoustic signal can be selectively removed according to the frequency of the equipment noise.

[0079] In photoacoustic measurements, there is usually a time delay between the pump light and the probe light. For example, the probe light illuminates the sample after the pump light, after a certain time delay. The time delay is typically adjusted, using pump and probe lights with different time delays to illuminate the first sample, thus obtaining photoacoustic signals under different time delays. Therefore, the resulting first photoacoustic time-domain signal is a signal that varies with time delay when the first sample is illuminated by pump and probe lights with different time delays. Figure 3 and Figure 5 In the diagram, the horizontal axis represents the delay time.

[0080] To facilitate time-frequency transformation, before removing non-noise and random noise signals from the first photoacoustic time-domain signal, the time delay can be converted into the actual measurement time to obtain the first photoacoustic time-domain signal that varies with the actual measurement time. Figure 5 The first photoacoustic time-domain signal in the image is converted into a signal that varies with the actual measurement time, and the result is as follows: Figure 7 As shown, the horizontal axis represents the actual measurement time.

[0081] The applicant discovered that if the noise frequency is estimated by repeatedly irradiating the first test sample with the probe light to obtain N sets of first photoacoustic time-domain signals, low-frequency interference will occur due to heat accumulation caused by repeated measurements at the same location. Therefore, in some embodiments, after removing non-noise and random noise signals from the first photoacoustic time-domain signals to obtain the first time-domain noise signal, the signal interference caused by heat accumulation due to repeated irradiation of the first test sample is also removed from the first time-domain noise signal. This removes the heat difference between different first time-domain noise signals, eliminates low-frequency interference in the first time-domain noise signal, and obtains a high-purity device noise signal.

[0082] Specifically, in one embodiment, a curve showing the change in surface reflectance of the first test sample due to heat accumulation can be pre-constructed. This curve can be simulated based on the material properties of the first test sample (including at least one of density, thermal conductivity, Poisson's ratio, and thermoelastic coefficient). Removing signal interference from the first time-domain noise signal caused by heat accumulation due to repeated irradiation of the first test sample specifically includes: acquiring the pre-constructed curve showing the change in surface reflectance of the first test sample due to heat accumulation, and performing a difference analysis between each set of first time-domain noise signals and this curve.

[0083] Typically, detector 11 samples the first detection signal light at different times to obtain multiple first photoacoustic signal data (such as light intensity, voltage, etc.). A set of first photoacoustic time-domain signals includes multiple first photoacoustic signal data collected at different times, which are obtained by connecting the points to form a line. Figure 5 , Figure 7 The signal image shown; correspondingly, the first time-domain noise signal includes multiple first noise data corresponding to the first photoacoustic signal data. In another embodiment, removing signal interference caused by heat accumulation due to repeated irradiation of the first test sample from the first time-domain noise signal specifically includes: for each group of first time-domain noise signals, using the first noise data therein to perform curve fitting to obtain a fitted curve, and differentiating the first time-domain noise signal from the fitted curve.

[0084] Figure 8 The removal is given Figure 6 The first time-domain noise signal shown is an image of the first time-domain noise signal obtained after signal interference caused by heat accumulation due to repeated irradiation of the first test sample. Figure 9 To Figure 8 The first noise spectrum obtained by performing time-frequency transformation on the first time-domain noise signal shown can be seen to have two peaks. X represents the horizontal axis of the peak, i.e., frequency, and Y represents the vertical axis, i.e., amplitude. It can be found that the frequency of the device noise is approximately 7Hz and 50Hz.

[0085] Based on the noise estimation method described above, this application also provides a denoising method, please refer to... Figure 10 In some embodiments, the method includes steps 600 to 800, which are described in detail below.

[0086] Step 600: Control the pump light and probe light to illuminate the second sample to be tested;

[0087] Step 700: Acquire the second photoacoustic time-domain signal formed by the second detection signal light;

[0088] Step 800: Based on the frequency estimation method for device noise in the photoacoustic measuring device according to any embodiment of this application, remove the signal with that frequency from the second photoacoustic time domain signal to obtain the second photoacoustic time domain signal with the device noise removed.

[0089] For example, bandpass filtering can be applied to the frequency of equipment noise to reduce interference. Figure 9 In this system, bandpass filtering can be applied to signals in the 7Hz-50Hz frequency band, or bandpass filtering can be applied to the frequency bands around 7Hz and around 50Hz respectively.

[0090] It should be noted that the second test sample and the first test sample are of the same type, and their films exhibit the same or similar behavior under excitation light. For example, both the first and second test samples are metallic films.

[0091] By selectively filtering and other methods based on the frequency of equipment noise to remove equipment noise from photoacoustic signals, the signal-to-noise ratio and measurement sensitivity are improved compared to traditional overall removal methods.

[0092] The denoising method in some embodiments further includes: removing the ultrasonic echo signal from the second photoacoustic time-domain signal for removing equipment noise, and then performing time-frequency transformation to obtain a second noise spectrum; determining the frequency corresponding to the peak value of the spectral amplitude of the second noise spectrum as the second noise frequency; and removing the signal with the second noise frequency from the second photoacoustic time-domain signal for removing equipment noise.

[0093] After denoising in step 800, some other noise may still exist, such as white noise and thermal noise. Therefore, in this embodiment, for the second photoacoustic time-domain signal with removed equipment noise, useful ultrasonic echo signals are first excluded, leaving mainly noise signals. Then, a time-frequency transformation is performed to obtain the second noise spectrum. The second noise frequency is determined based on the second noise spectrum. For the second photoacoustic time-domain signal with removed equipment noise, signals with the second noise frequency are removed, thereby removing other noise and further improving signal quality.

[0094] Those skilled in the art will understand that all or part of the functions of the various methods in the above embodiments can be implemented by hardware or by computer programs. When all or part of the functions in the above embodiments are implemented by computer programs, the program can be stored in a computer-readable storage medium, which may include: read-only memory, random access memory, disk, optical disk, hard disk, etc., and the program is executed by a computer to achieve the above functions. For example, the program can be stored in the memory of a device, and when the program in the memory is executed by the processor, all or part of the above functions can be achieved. In addition, when all or part of the functions in the above embodiments are implemented by computer programs, the program can also be stored in a server, another computer, disk, optical disk, flash drive, or external hard drive, etc., and can be downloaded or copied to the memory of a local device, or the system of the local device can be updated. When the program in the memory is executed by the processor, all or part of the functions in the above embodiments can be achieved.

[0095] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the principles of this invention.

Claims

1. A method for estimating the frequency of equipment noise, characterized in that, Applications in photoacoustic measurement equipment, including: A pump light and a probe light are controlled to illuminate a first sample to be tested. The pump light is used to generate ultrasonic waves on the surface of the first sample to be tested, and the probe light is reflected by the surface of the first sample to be tested to form a first probe signal light. Acquire the first photoacoustic time-domain signal formed by the first detection signal light; Remove the non-noise signal and random noise signal from the first photoacoustic time-domain signal to obtain the first time-domain noise signal; The first noise spectrum is obtained by performing a time-frequency transformation on the first time-domain noise signal; The frequency of the equipment noise is determined based on the spectral amplitude of the first noise spectrum.

2. The frequency estimation method as described in claim 1, characterized in that, There are N sets of the first photoacoustic time domain signal. The N sets of the first photoacoustic time domain signal are formed by repeatedly irradiating the first sample under test with the probe light N times, and are respectively formed by the first probe signal light during each irradiation. The step of removing non-noise signals and random noise signals from the first photoacoustic time-domain signal to obtain the first time-domain noise signal includes: for each group of the first photoacoustic time-domain signals, removing common signals present in N groups of the first photoacoustic time-domain signals to obtain N groups of first time-domain noise signals.

3. The frequency estimation method as described in claim 2, characterized in that, For each group of the first photoacoustic time-domain signals, remove the common signals present in the N groups of the first photoacoustic time-domain signals to obtain N groups of first time-domain noise signals, including: The first photoacoustic time-domain signal of N groups is averaged to obtain the first photoacoustic average signal. The N groups of the first photoacoustic time-domain signals are respectively differentiated from the first photoacoustic average signal to obtain the N groups of first time-domain noise signals.

4. The frequency estimation method as described in claim 2, characterized in that, After removing the non-noise signal and random noise signal from the first photoacoustic time-domain signal to obtain the first time-domain noise signal, the method further includes: removing the signal interference caused by heat accumulation due to repeated irradiation of the first test sample from the first time-domain noise signal.

5. The frequency estimation method as described in claim 4, characterized in that, Removing signal interference from the first time-domain noise signal caused by heat accumulation due to repeated irradiation of the first test sample includes: obtaining a pre-constructed curve of the change in surface reflectivity of the first test sample caused by heat accumulation, and differentiating each set of the first time-domain noise signal from the curve. Alternatively, each group of the first photoacoustic time-domain signals includes multiple first photoacoustic signal data collected at different times, and the first time-domain noise signal includes multiple first noise data corresponding to the first photoacoustic signal data; removing signal interference caused by heat accumulation due to repeated irradiation of the first test sample in the first time-domain noise signal includes: for each group of the first time-domain noise signals, using the first noise data therein to perform curve fitting to obtain a fitting curve, and performing difference between the first time-domain noise signal and the fitting curve.

6. The frequency estimation method according to any one of claims 1 to 5, characterized in that, There is a time delay between the pump light and the probe light, and the first photoacoustic time-domain signal is a signal that changes with time delay formed by irradiating the first sample under test with the pump light and the probe light having different time delays; Before removing non-noise signals and random noise signals from the first photoacoustic time-domain signal, the method further includes: converting the time delay into the actual measurement time to obtain the first photoacoustic time-domain signal that varies with the actual measurement time.

7. The frequency estimation method as described in claim 2, characterized in that, Determining the frequency of the device noise based on the spectral amplitude of the first noise spectrum includes: averaging the first noise spectra of N groups of the first time-domain noise signals to obtain a first average spectrum; and determining the frequency corresponding to the peak value of the spectral amplitude of the first average spectrum as the frequency of the device noise.

8. A noise reduction method, characterized in that, The noise reduction method, applied to photoacoustic measurement equipment, includes: A pump light and a probe light are controlled to illuminate a second sample to be tested. The pump light is used to generate ultrasonic waves on the surface of the second sample to be tested, and the probe light is reflected by the surface of the second sample to be tested to form a second probe signal light. Acquire the second photoacoustic time-domain signal formed by the second detection signal light; The frequency of the equipment noise is determined by the frequency estimation method for equipment noise as described in any one of claims 1 to 7. The signal with the frequency in the second photoacoustic time domain signal is removed to obtain a second photoacoustic time domain signal with the equipment noise removed, wherein the second test sample and the first test sample are samples of the same type.

9. The noise reduction method as described in claim 8, characterized in that, Also includes: The second photoacoustic time-domain signal, after removing the ultrasonic echo signal, is subjected to time-frequency transformation to obtain the second noise spectrum. The frequency corresponding to the peak value of the second noise spectrum is determined as the second noise frequency; For the second photoacoustic time-domain signal used to remove device noise, remove the signal having the second noise frequency.

10. A measuring device, characterized in that, include: A light source is used to emit pump light and probe light to illuminate the first sample to be tested. The pump light is used to generate ultrasonic waves on the surface of the first sample to be tested, and the probe light is reflected by the surface of the first sample to be tested to form a first probe signal light. A detector is used to receive the first detection signal light to form a first photoacoustic time-domain signal; A processor, connected to the light source and the detector, is used to execute the frequency estimation method for device noise as described in any one of claims 1 to 7.

11. The measuring device as described in claim 10, characterized in that, The pump light and probe light emitted by the light source are also used to illuminate the second sample to be tested. The pump light is also used to generate ultrasonic waves on the surface of the second sample to be tested. The probe light is reflected by the surface of the second sample to be tested to form a second probe signal light. The detector is also used to receive the second detection signal light to form a second photoacoustic time-domain signal; The processor is further configured to perform the denoising method as described in claim 8 or 9 to denoise the second photoacoustic time-domain signal, and to determine the film properties of the second test sample based on the denoised second photoacoustic time-domain signal.

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