Light spot adjustment method and film thickness measurement method

Abstract

The present invention provides a method for adjusting a light spot and a method for measuring film thickness. The method for adjusting the light spot acquires images of an excitation light spot and a detection light spot through an imaging system, calculates the relative position between the centroid of the excitation light spot and the centroid of the detection light spot based on the images, and performs a first adjustment on the attitude of the deflecting mirror of the detection optical path and / or the excitation optical path according to the relative position acquired by the imaging system, and through the theoretical maximum LIA max Perform a first adjustment on the attitude of the deflecting mirror of the detection optical path and / or the excitation optical path, so that the centroid of the excitation light spot and the centroid of the detection light spot coincide, and a signal with the maximum intensity can be obtained. Applying this to a photoacoustic measurement system, by precisely adjusting the relative position between the detection light spot and the excitation light spot, the reflectivity of the detection light can be improved, and the time interval when the sound wave returns to the sample surface twice continuously can be accurately obtained, improving the detection accuracy and detection stability, and enhancing the accuracy of film thickness calculation.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a light spot adjustment method and a film thickness measurement method. Background Art

[0002] As semiconductor technology continues to increase in complexity, there are methods for depositing coatings in the back-end process. One of the most widely used methods is to deposit a seed layer of copper on top of a barrier metal (such as tantalum), and then electroplate a thicker layer of copper on top of the seed layer. Once the deposition is complete, it is important to verify that the deposited film meets the specifications. Therefore, a technology that can accurately measure film thickness is needed.

[0003] The traditional method for measuring copper film thickness is the four-point probe method. This technique was originally developed in the 1950s, where an array of four sharp probes arranged in a straight line are pressed into the conductive copper film. Current is applied to one outer probe and returned through the opposing outer probe. The voltage between the two middle probes, combined with the amount of current and knowledge of the film's bulk resistivity, is used to determine the film's thickness. However, the four-point probe method requires the probes to penetrate the surface of the conductive layer. This can cause scratches and may also lead to the formation of small particles, which can cause defects elsewhere on the wafer. Therefore, non-contact techniques for measuring film thickness are also necessary.

[0004] Measurement methods using one existing measurement system generate a signal that originates from the induced change in reflectivity caused by the stress field propagating into the sample. When the stress field encounters an acoustic impedance mismatch, a portion of the stress field is scattered back to the surface. A probe beam that monitors the time-dependent change in reflectivity detects the change in reflectivity as the stress field propagates. However, this signal is weak, and the presence of noise complicates the analysis method. A particularly problematic type of noise is low-frequency (within the data acquisition bandwidth) "multiplicative noise," which arises from power fluctuations in the laser, vibrations in the sample or system optics, and temperature fluctuations or air currents in the system that manipulates the beam. These 1 / f noise sources can be reduced by proper laser selection, vibration isolation of the measurement system, and shielding against air currents around the system. However, the noise levels generated can still be so high that repeatable measurements cannot be made on certain film samples with very thin layers or thin buried layers.

[0005] Another example of a photoacoustic system is provided in the article "Picosecond Ultrasonic Study Of Mo / Si Multilayer Structures Using An Alternating-Pump Technique". This article was published by Nen-Wen Pu et al. in Applied Physics Letters, January 11, 1999, Vol. 74, No. 2, pp. 320-322. The system disclosed in this article uses a pump and probe transient reflectivity technique in which acoustic waves are pulsed by optical absorption of an ultrashort "pump" laser pulse and detected as reflectivity changes of a time-delayed "probe" laser beam. The article discloses the use of an acousto-optic modulator (AOM) and other components to provide enhanced signal-to-noise ratio and higher sensitivity. However, because this system relies on acousto-optic diffraction of the pump beam, the ability to make measurements at different wavelengths is limited. More specifically, the limitations of this system include the need to vary the diffraction angle of the pump beam with wavelength and the limited useful wavelength range of commonly available acousto-optic materials (e.g., TeO2, SiO2). Therefore, the versatility of this system is also limited.

[0006] Therefore, an improved technology is needed to improve the signal-to-noise ratio of the reflected signal during photoacoustic measurement, improve the detection accuracy and detection stability, and accurately calculate the thickness of the film from the data. Summary of the invention

[0007] The object of the present invention is to provide a light spot adjustment method and a film thickness measurement method, so as to improve the problems of low signal-to-noise ratio and low detection accuracy in the prior art.

[0008] In a first aspect, the present invention provides a method for adjusting a light spot, comprising: generating excitation light and detection light based on a light source module, wherein the excitation light is used to generate an acoustic wave in a sample to be tested, wherein the acoustic wave is transmitted back to the surface of the sample to be tested through an interface in the sample to be tested to form a deformation area, and the detection light is irradiated to the deformation area to form a reflected signal light with varying intensity; acquiring images of an excitation light spot and a detection light spot respectively formed by the excitation light and the detection light on the surface of the sample to be tested through an imaging system, and acquiring the position of the centroid of the excitation light spot and the distance between the centroid of the detection light spot based on the image; performing a first adjustment on the posture of a deflection mirror located in a detection light path and / or an excitation light path according to the acquired distance, so that the distance between the position of the centroid of the excitation light spot and the centroid of the detection light spot is less than a preset threshold; and acquiring a theoretical maximum LIA of the signal light intensity. max , and according to the theoretical maximum LIA maxA second adjustment is performed on the posture of the deflection mirror of the detection light path and / or the excitation light path so that the centroid of the excitation light spot coincides with the centroid of the detection light spot.

[0009] The beneficial effect of the light spot adjustment method of the present invention is that the acoustic wave and the deformation area are generated by irradiating the excitation light onto the sample to be tested, and the signal light is obtained by irradiating the detection light onto the deformation area, and the images of the excitation light spot and the detection light spot are acquired by the imaging system, and the distance between the centroid of the excitation light spot and the centroid of the detection light spot is calculated based on the image, and the relative positions of the centroid of the excitation light spot and the centroid of the detection light spot are adjusted by the first adjustment and the second adjustment according to the distance, so that the centroid of the excitation light spot and the centroid of the detection light spot coincide with each other, thereby improving the reflectivity of the signal light and the signal-to-noise ratio of the reflected signal, facilitating the subsequent detection and collection of the signal light, and improving the detection accuracy and detection stability.

[0010] In a feasible solution, the theoretical maximum LIA of the signal light intensity is obtained. max , including: establishing a mathematical relationship between the signal light intensity and the distance between the centroid of the excitation light spot and the centroid of the detection light spot; obtaining the value of the signal light intensity at the peak of the mathematical relationship, the value at the peak is the theoretical maximum LIA max The beneficial effect is that the relative position of the centroid of the excitation light spot and the centroid of the detection light spot can be adjusted by adjusting the deflection mirror of the detection light path and / or the excitation light path, and the mathematical relationship can be established by obtaining the light intensity values ​​of the signal light at multiple relative positions, and the light intensity value at the peak can be calculated according to the mathematical relationship, and the light intensity value at the peak is the theoretical maximum LIA max .

[0011] In a feasible solution, the second adjustment includes: adjusting the posture of the deflection mirror of the detection light path and / or the excitation light path and detecting the intensity of the signal light in real time, when the intensity of the signal light reaches the theoretical maximum value LIA max The beneficial effect is that: since the smaller the distance between the centroid of the excitation light spot and the centroid of the detection light spot, the higher the reflectivity, it can be known that the light intensity of the signal light is the largest when the centroid of the excitation light spot and the centroid of the detection light spot coincide with each other, and the theoretical maximum LIA can be calculated accordingly. max As the basis for adjusting the deflection mirror of the detection optical path and / or the excitation optical path, when the intensity of the signal light detected in real time reaches the theoretical maximum value LIA maxWhen , it means that the centroid of the detection light spot coincides with the centroid of the excitation light spot. At this time, the purpose of making the centroid of the detection light spot coincide with the centroid of the excitation light spot is achieved.

[0012] In a feasible solution, the excitation light and the detection light are both Gaussian beams. The beneficial effect is that when the excitation light and the detection light are Gaussian beams, the light intensity distribution of the excitation light and the detection light can be calculated by a Gaussian function, which is convenient for confirming and adjusting the centroid position.

[0013] In a feasible solution, the imaging system includes a focusing lens, an imaging lens, a magnifying glass group and an imaging unit which are set up in sequence, and the focusing lens and the imaging lens constitute an optical 4F system; the excitation light spot and the detection light spot generate an excitation light intermediate image and a detection light intermediate image respectively after passing through the focusing lens and the imaging lens, and the excitation light intermediate image and the detection light intermediate image are magnified by the magnifying glass group and converged to the imaging unit to form the images of the excitation light spot and the detection light spot respectively. Its beneficial effect is that the excitation light spot and the detection light spot are processed by the focusing lens and the imaging lens to obtain the intermediate image of the two, which can minimize the system aberration and improve the imaging resolution; then the intermediate image is processed by the magnifying glass group, and the intermediate image can be magnified by the magnifying glass group with a magnification, and finally the spot imaging is performed by the imaging unit, and the magnifying glass group and the imaging unit are set to effectively control the working distance of the entire optical path, which is conducive to engineering design and has high flexibility.

[0014] In a feasible scheme, the method of obtaining the images of the excitation light spot and the detection light spot respectively formed by the excitation light and the detection light on the surface of the sample to be tested through an imaging system, and obtaining the distance between the centroid of the excitation light spot and the centroid of the detection light spot based on the image, includes: preprocessing the image, and the preprocessing includes image median filtering, brightness adjustment and threshold segmentation; based on the preprocessed image, obtaining the positions of the centroid of the excitation light spot and the centroid of the detection light spot on the image through a graphic processing algorithm, so as to calculate the distance between the centroid of the excitation light spot and the centroid of the detection light spot. Its beneficial effects are: through preprocessing, noise can be filtered out, image edges can be maintained, images can be made clear, target areas in images can be determined, etc., and through image algorithm processing, spot images and position information can be extracted, so as to facilitate the acquisition of the position of the centroid of the excitation light spot on the excitation light spot image and the position of the centroid of the detection light spot on the detection light spot image.

[0015] In a feasible solution, the image processing algorithm includes centroid extraction, and the centroid extraction obtains the positions of the centroid of the excitation light spot and the centroid of the detection light spot on the image respectively through the following formulas:

[0016]

[0017] Wherein, p is the pixel size of the imaging unit, β is the total magnification of the imaging system, n is the total number of pixels corresponding to the spot image, and I i is the gray value of the i-th pixel of the spot image, X i and Y i The X coordinate and Y coordinate of the ith pixel i are obtained by obtaining the position of the centroid of the excitation light spot and the centroid of the detection light spot, respectively, so as to facilitate the adjustment of the deflection mirror of the detection light path and / or the excitation light path so that the centroid of the detection light spot is located at the theoretical position.

[0018] In a feasible solution, the graphics processing algorithm includes: extracting the spot edge position information in the spot image through an edge detection operator; fitting the spot edge position information to obtain the position of the spot centroid. Its beneficial effect is that the edge position information of the image spot is obtained through edge information detection, which facilitates the extraction of the coordinate information of the spot centroid.

[0019] In the second aspect, the present invention provides a film thickness measurement method, which uses the spot adjustment method described in any feasible solution of the first aspect to adjust the relative position of the excitation light spot and the detection light spot; adjust the optical path of the detection light by an optical delay device, obtain the intensity change of the signal light as the optical path of the detection light changes, and calculate the film thickness information. Its beneficial effect is that by adjusting the relative position of the excitation light spot and the detection light spot, the centroid of the excitation light spot and the centroid of the detection light spot are overlapped, and the signal light with the highest reflectivity can be obtained at this time, the optical path of the detection light is adjusted by the optical delay device, and the intensity change of the signal light as the optical path of the detection light changes can be obtained, and the time difference between two adjacent peaks in the intensity change of the signal light can be obtained to know the movement of the acoustic wave in the sample, and the film thickness can be calculated based on this, and by coinciding the centroid of the excitation light spot with the centroid of the detection light spot, the intensity peak of the obtained signal light will be more obvious and clear, and the thickness of the film can be accurately calculated. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a flowchart of a light spot adjustment method in a first embodiment of the present invention;

[0021] Figure 2Schematic diagram of the incident detection light and the excitation light in the second embodiment of the present invention;

[0022] Figure 3 Schematic diagram of the relationship between the intensity of the signal light and the distance between the centroid of the excitation light spot and the centroid of the detection light spot in the third embodiment of the present invention;

[0023] Figure 4 is a schematic structural diagram of an imaging system in a fourth embodiment of the present invention;

[0024] Figure 5 This is a flowchart of obtaining the centroid position of the imaging system in the fifth embodiment of the present invention. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be understood by people with general skills in the field to which the present invention belongs. "Including" and similar words used in this article mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0026] In view of the problems existing in the prior art, embodiments of the present invention provide a light spot adjustment method and a film thickness measurement method.

[0027] Figure 1 is a flowchart of a light spot adjustment method in a first embodiment of the present invention, Figure 2 Schematic diagram of the incident detection light and excitation light in the second embodiment of the present invention.

[0028] In some embodiments of the present invention, referring to Figure 1 and Figure 2 , the light spot adjustment method comprises:

[0029] S1. Generate excitation light and detection light based on the light source module, wherein the excitation light is used to generate sound waves in the sample to be tested, the sound waves are transmitted back to the surface of the sample to be tested through the interface in the sample to be tested to form a deformation area, and the detection light is irradiated to the deformation area to form a reflected signal light with varying intensity;

[0030] S2, acquiring images of the excitation light spot and the detection light spot respectively formed by the excitation light and the detection light on the surface of the sample to be tested through an imaging system, and acquiring the position of the centroid of the excitation light spot and the distance between the centroid of the detection light spot based on the images;

[0031] S3, performing a first adjustment on the posture of the deflection mirror located in the detection light path and / or the excitation light path according to the acquired distance, so that the distance between the position of the centroid of the excitation light spot and the centroid of the detection light spot is less than a preset threshold;

[0032] S4. Obtaining the theoretical maximum value LIA of the signal light intensity max , and according to the theoretical maximum LIA max A second adjustment is performed on the posture of the deflection mirror of the detection light path and / or the excitation light path so that the centroid of the excitation light spot coincides with the centroid of the detection light spot.

[0033] In some specific embodiments of the present invention, the light source module is a pulse laser, and the pulse laser generated by the pulse laser is divided into the detection light and the excitation light by a beam splitter. The excitation light is irradiated onto the surface of the sample to be tested to generate the sound wave, and the sound wave moves into the sample to be tested to the bottom surface and then bounces back to the surface of the sample to be tested to form the deformation area. The detection light is irradiated onto the light spot of the excitation light and the deformation area, and the reflected light of the detection light is the signal light. Since the movement of the sound wave in the sample will affect the light intensity of the signal light, the intensity of the signal light will change.

[0034] In some embodiments, the imaging system is capable of acquiring the excitation light spot image and the detection light spot image, and is capable of acquiring the position of the excitation light spot centroid and the position of the detection light spot centroid based on the excitation light spot image and the detection light spot image. By making the excitation light spot centroid and the detection light spot centroid coincide with each other through the first adjustment and the second adjustment, the signal-to-noise ratio of the signal light can be enhanced, which facilitates the subsequent acquisition of the intensity change of the signal light through the detector.

[0035] In some embodiments, the first adjustment is a coarse adjustment, and the second adjustment is a fine adjustment. The adjustment is performed by combining coarse and fine adjustments to ensure adjustment accuracy and efficiency.

[0036] In some embodiments, the posture adjustment of the yaw mirror includes adjustment of the yaw angle and adjustment of the pitch angle.

[0037] In some embodiments, a polarizer and an electro-optic modulator are provided on the optical path of the excitation light.

[0038] In some embodiments, the optical path of the detection light is provided with a reflector group and an optical delay device for adjusting the optical path of the detection light.

[0039] In some embodiments, a beam expander is provided between the laser and the beam splitter, and the beam expander is used to expand and collimate the laser.

[0040] In some embodiments, the split detection light and the excitation light are irradiated onto the surface of the sample to be tested via a beam combiner and a deflection mirror to form a detection light spot and an excitation light spot. The excitation light spot and the detection light spot can be acquired by the imaging system to obtain the excitation light spot image and the detection light spot image.

[0041] In some embodiments, the sample to be tested is usually a semiconductor product such as a coated wafer or chip, the detection light and the excitation light are irradiated on the sample surface through the coating side, and the sound wave formed by the excitation light moves to the bottom of the film layer and then rebounds. The interface is the film layer.

[0042] In some embodiments, the excitation light is incident perpendicularly to the sample surface, and the detection light is incident on the sample surface in an inclined manner.

[0043] In some embodiments, when the excitation light is incident perpendicularly to the sample surface, the deformation region coincides with the excitation light spot.

[0044] In some embodiments, the light intensity of the signal light is collected by a photodetector.

[0045] In some embodiments, the preset threshold is 10 pixel size values.

[0046] In some embodiments, the deflection mirror of the detection light path is adjusted so that the distance between the centroid of the excitation light spot and the centroid of the detection light spot is smaller than the preset threshold.

[0047] In some embodiments, the deflection mirror of the excitation light path is adjusted so that the distance between the centroid of the excitation light spot and the centroid of the detection light spot is smaller than the preset threshold.

[0048] In some embodiments, the deflection mirror of the excitation light path and the deflection mirror of the detection light path are adjusted simultaneously so that the distance between the centroid of the excitation light spot and the centroid of the detection light spot is less than the preset threshold.

[0049] In some embodiments, the deflection mirror of the detection light path is adjusted so that the centroid of the excitation light spot coincides with the centroid of the detection light spot.

[0050] In some embodiments, the deflection mirror of the excitation light path is adjusted so that the centroid of the excitation light spot coincides with the centroid of the detection light spot.

[0051] In some embodiments, the deflection mirror of the excitation light path and the deflection mirror of the detection light path are adjusted simultaneously so that the centroid of the excitation light spot coincides with the centroid of the detection light spot.

[0052] In some embodiments, the optical path of the detection light is adjusted by a reflector group and an optical delay device so that the optical path of the detection light and the excitation light are equal, that is, the detection light and the excitation light reach the surface of the sample to be measured at the same time.

[0053] Figure 3 Schematic diagram of the relationship between the intensity of signal light and the distance between the centroid of the excitation light spot and the centroid of the detection light spot in the third embodiment of the present invention.

[0054] In some embodiments of the present invention, referring to Figures 1 to 3 , the theoretical maximum LIA of the signal light intensity is obtained max , including: establishing a mathematical relationship between the signal light intensity and the distance between the centroid of the excitation light spot and the centroid of the detection light spot; obtaining the value of the signal light intensity at the peak of the mathematical relationship, the value at the peak is the theoretical maximum LIA max .

[0055] In some specific embodiments of the present invention, a plurality of signal light intensities and data on the distance between the detection light spot and the centroid of the excitation light spot are obtained by multi-point measurement, the mathematical relationship between the signal light intensity and the centroid distance is established, and the maximum light intensity value is calculated according to the mathematical relationship. The maximum light intensity value is the value at the peak value, that is, the theoretical maximum LIA. max .

[0056] In some embodiments, when the detection light is at the maximum intensity of the excitation light, that is, when the detection light spot and the excitation light spot coincide, the reflectivity of the detection light is the highest, and the intensity of the signal light is the highest at this time. When the detection light deviates from the center of the excitation light, the intensity of the signal light decreases with the change in the intensity of the excitation light and decays in the form of a Gaussian distribution.

[0057] In some embodiments, before establishing the mathematical relationship, the optical paths of the detection light and the excitation light are adjusted by an optical delay device so that the optical paths of the two are equal.

[0058] In some embodiments of the present invention, referring to Figures 1 to 3The second adjustment includes: adjusting the posture of the deflection mirror of the detection light path and / or the excitation light path and detecting the intensity of the signal light in real time, when the intensity of the signal light reaches the theoretical maximum value LIA max When the detection light spot mass center coincides with the excitation light spot mass center.

[0059] In some specific embodiments of the present invention, after the first adjustment is completed, the intensity of the signal light is detected in real time by adjusting the posture of the deflection mirror of the detection light path and / or the excitation light path, so that the intensity of the signal light reaches the theoretical maximum value LIA max At this time, the centroid of the detection light spot coincides with the centroid of the excitation light spot.

[0060] In some embodiments of the present invention, referring to Figures 1 to 3 , the excitation light and the detection light are both Gaussian beams.

[0061] In some specific embodiments of the present invention, the excitation light is a Gaussian beam.

[0062] In some embodiments, the Gaussian beam intensity distribution formula is:

[0063]

[0064] A is the maximum light intensity of the light spot, x0 is the center position of the light spot, and c is related to the full width at half maximum.

[0065] Figure 4 is a schematic structural diagram of an imaging system in a fourth embodiment of the present invention, Figure 5 This is a flowchart of obtaining the centroid position of the imaging system in the fifth embodiment of the present invention.

[0066] In some embodiments of the present invention, referring to Figures 1 to 5 The imaging system includes a focusing lens, an imaging lens, a magnifying glass group and an imaging unit which are sequentially arranged, and the focusing lens and the imaging lens constitute an optical 4F system; the excitation light spot and the detection light spot are respectively generated as an excitation light intermediate image and a detection light intermediate image after passing through the focusing lens and the imaging lens, and the excitation light intermediate image and the detection light intermediate image are magnified by the magnifying glass group and converged to the imaging unit to form images of the excitation light spot and the detection light spot respectively.

[0067] In some specific embodiments of the present invention, the focusing lens and the imaging lens form an optical 4F system, which can minimize system aberrations and improve imaging resolution. The focal length of the focusing lens is f B , the focal length of the imaging lens is f C , under the 4F imaging system, the imaging magnification of the two is β = f C / fB , when the focal length of the imaging lens is equal to the focal length of the focusing lens, the imaging magnification of the two is β = 1. The magnifying glass group is further set, and a magnifying glass group is added to the above imaging system, and the magnification of the magnifying glass group is β', so that the final imaging magnification is β×β', and the final imaging magnification can be adjusted by the subsequent magnifying glass group, so the working distance of the entire optical path can be effectively controlled.

[0068] In some embodiments of the present invention, referring to Figures 1 to 5 , the imaging system is used to obtain images of the excitation light spot and the detection light spot respectively formed by the excitation light and the detection light on the surface of the sample to be tested, and the distance between the centroid of the excitation light spot and the centroid of the detection light spot is obtained based on the image, including: preprocessing the image, the preprocessing including image median filtering, brightness adjustment and threshold segmentation; based on the preprocessed image, the positions of the centroid of the excitation light spot and the centroid of the detection light spot on the image are obtained through a graphics processing algorithm to calculate the distance between the centroid of the excitation light spot and the centroid of the detection light spot.

[0069] In some specific embodiments of the present invention, after the excitation light spot image and the detection light spot image are acquired based on the imaging system, the spot image needs to be further processed to obtain the positions of the excitation light spot and the excitation light spot. Specifically, after the excitation light spot image and the detection light spot image are acquired by the imaging system, they are pre-processed by median filtering, brightness processing and threshold segmentation. The image processing algorithm performs edge detection or centroid extraction on the pre-processed image to extract the spot morphology and position information. Based on the spot position information of the excitation light and the detection light, the deflection mirror of the detection light path and / or the excitation light path is adjusted so that the difference in the centroid positions of the two is less than the preset threshold, and the coarse adjustment (first adjustment) is completed.

[0070] In some embodiments, the median filter can filter out noise while maintaining the image edge well, overcoming the blurring of image details caused by the average filter. Specifically, the grayscale values ​​of all pixels in a small window centered on a certain pixel are sorted in ascending order, and the middle value of the sorting result is output as the grayscale value of the pixel.

[0071] In some embodiments, the brightness processing is performed by adjusting the exposure time of the imaging system, with the maximum grayscale value of the light spot being less than 255 and greater than a certain grayscale value (such as 200) as the standard, and taking this exposure time as the optimal brightness, so as to meet the requirements of not overexposure and maximizing the contrast of the light spot image.

[0072] In some embodiments, the threshold segmentation utilizes the difference in grayscale value between the desired spot image and the background to select a suitable grayscale threshold, and uses the grayscale threshold to determine whether the pixel belongs to the target area or the background area.

[0073] In some embodiments of the present invention, referring to Figures 1 to 5 The image processing algorithm includes centroid extraction, and the centroid extraction obtains the positions of the centroid of the excitation light spot and the centroid of the detection light spot on the image respectively through the following formulas:

[0074]

[0075] Wherein, p is the pixel size of the imaging unit, β is the total magnification of the imaging system, n is the total number of pixels corresponding to the spot image, and I i is the gray value of the i-th pixel in the spot image, X i and Y i are the X coordinate and Y coordinate of the i-th pixel i.

[0076] In some embodiments, the centroid information of the spot image is extracted to calculate the spot position, and the centroid extraction is a weighted centroid method using grayscale as a weight.

[0077] In some embodiments of the present invention, referring to Figures 1 to 5 The graphics processing algorithm includes: extracting the spot edge position information in the spot image through an edge detection operator; fitting the spot edge position information to obtain the position of the spot centroid.

[0078] In some specific embodiments of the present invention, edge information of the light spot in the image is extracted through an edge detection operator; the edge result is used as the light spot morphology, and then the information such as the size of the light spot in the XY direction and the ellipticity is further obtained through methods such as ellipse fitting.

[0079] In some embodiments, the ellipse fitting extracts coordinate information from edge detection and fits the returned contour points to obtain ellipse information.

[0080] In some embodiments of the present invention, referring to Figures 1 to 5 , using the spot adjustment method described in any of the above embodiments to adjust the relative positions of the excitation light spot and the detection light spot; adjusting the optical path of the detection light through an optical delay device, obtaining the change in the light intensity of the signal light as the optical path of the detection light changes, and calculating the film thickness information.

[0081] In some specific embodiments of the present invention, the relative positions of the excitation light spot and the detection light spot are adjusted by a deflection mirror in the detection light path and / or the excitation light path, so that the centroid of the detection light spot coincides with the centroid of the excitation light spot. At this time, the amplitude of the intensity change of the collected signal light can be enhanced, and the signal-to-noise ratio of the measurement can be improved. At the same time, the optical path of the detection light is adjusted by an optical delay device, and the intensity of the signal light is collected by a photodetector to obtain a waveform diagram of the intensity change over time. The time when the sound wave is first transmitted back to the surface of the sample to be tested and the time when it is secondly transmitted back to the surface of the sample to be tested are collected through the waveform diagram. The time difference between the two time points, that is, the time length of the sound wave's back and forth movement in the interface of the sample to be tested, can be used to calculate the film thickness based on the movement speed of the sound wave in the sample.

[0082] Although the embodiments of the present invention are described in detail above, it is obvious to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations are within the scope and spirit of the present invention as described in the claims. Moreover, the present invention described herein may have other embodiments and may be implemented or realized in a variety of ways.

Claims

1. A light spot adjustment method, characterized in that: include: Generate excitation light and detection light based on the light source module, the excitation light is used to generate sound waves in the sample to be tested, the sound waves are transmitted back to the surface of the sample to be tested through the interface in the sample to be tested, and a deformation area is formed on the surface of the sample to be tested, and the detection light is irradiated to the deformation area to form a reflected signal light with varying intensity; Acquire images of the excitation light spot and the detection light spot respectively formed by the excitation light and the detection light on the surface of the sample to be tested through an imaging system, and acquire the position of the centroid of the excitation light spot and the distance between the centroid of the detection light spot based on the images; Performing a first adjustment on the posture of the deflection mirror located in the detection light path and / or the excitation light path according to the acquired distance, so that the distance between the position of the centroid of the excitation light spot and the centroid of the detection light spot is less than a preset threshold; Obtaining the theoretical maximum value of the signal light intensity , and according to the theoretical maximum A second adjustment is performed on the posture of the deflection mirror of the detection light path and / or the excitation light path so that the centroid of the excitation light spot coincides with the centroid of the detection light spot.

2. The light spot adjustment method according to claim 1, characterized in that: The theoretical maximum value of the intensity of the signal light is obtained ,include: Establishing a mathematical relationship between the intensity of the signal light and the distance between the centroid of the excitation light spot and the centroid of the detection light spot; Obtain the value of the signal light intensity at the peak value of the mathematical relationship, the value at the peak value is the theoretical maximum value .

3. The light spot adjustment method according to claim 1, characterized in that: The second adjustment includes: Adjust the posture of the deflection mirror of the detection light path and / or the excitation light path and detect the intensity of the signal light in real time. When the intensity of the signal light reaches the theoretical maximum value, When the detection light spot mass center coincides with the excitation light spot mass center.

4. The light spot adjustment method according to claim 1, characterized in that: The excitation light and the detection light are both Gaussian beams.

5. The light spot adjustment method according to any one of claims 1 to 4, characterized in that: The imaging system comprises a focusing lens, an imaging lens, a magnifying lens group and an imaging unit which are sequentially arranged, and the focusing lens and the imaging lens constitute an optical 4F system; The excitation light spot and the detection light spot generate an excitation light intermediate image and a detection light intermediate image respectively after passing through the focusing lens and the imaging lens. The excitation light intermediate image and the detection light intermediate image are magnified by the magnifying glass group and converged to the imaging unit to form images of the excitation light spot and the detection light spot respectively.

6. The light spot adjustment method according to claim 5, characterized in that: The step of acquiring images of the excitation light spot and the detection light spot respectively formed by the excitation light and the detection light on the surface of the sample to be tested through an imaging system, and acquiring the distance between the centroid of the excitation light spot and the centroid of the detection light spot based on the images comprises: Preprocessing the image, wherein the preprocessing includes image median filtering, brightness adjustment and threshold segmentation; Based on the preprocessed image, the positions of the centroid of the excitation light spot and the centroid of the detection light spot on the image are obtained by a graphics processing algorithm to calculate the distance between the centroid of the excitation light spot and the centroid of the detection light spot.

7. The light spot adjustment method according to claim 6, characterized in that: The image processing algorithm includes centroid extraction, and the centroid extraction obtains the positions of the centroid of the excitation light spot and the centroid of the detection light spot on the image respectively through the following formulas: in, is the pixel size of the imaging unit, is the total magnification of the imaging system, is the total number of pixels corresponding to the spot image, The spot image The gray value of a pixel, and For the Pixels The X and Y coordinates of the 8. The light spot adjustment method according to claim 6, characterized in that: The graphics processing algorithm includes: The edge position information of the light spot in the light spot image is extracted by the edge detection operator; The light spot edge position information is fitted to obtain the position of the light spot centroid.

9. A film thickness measurement method, comprising: Adopting the light spot adjustment method described in any one of claims 1 to 8 to adjust the relative positions of the excitation light spot and the detection light spot; The optical path length of the detection light is adjusted by an optical delay device, the intensity change of the signal light as the optical path length of the detection light changes is obtained, and the film thickness information is calculated.

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