Light spot adjustment method and film thickness measurement method

Abstract

The present invention provides a light spot adjustment method and a film thickness measurement method. The light spot adjustment method obtains 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 image, obtains the theoretical position of the maximum value of the bulge deformation gradient according to theoretical data, and adjusts the posture of the deflection mirror of the detection light path and / or the excitation light path so that the detection light spot is located at the theoretical position. At this time, the signal light is collected by a position-sensitive detector to obtain a signal with the maximum intensity. When this is applied to a photoacoustic measurement system, the relative position of the detection light spot and the excitation light spot can be accurately adjusted so that the detection light spot is at an optimized position, that is, the maximum position of the bulge gradient, and then the maximum light spot deflection amount can be detected, and the time interval when the sound wave is transmitted back to the sample surface twice in a row can be accurately obtained, thereby improving the accuracy of the 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] A measurement method using an existing measurement system generates a signal, which is a signal propagated into the sample. The film thickness is calculated by measuring the time difference between the signal being transmitted back to the sample surface twice. However, when the signal is actually detected, a bulge will form on the sample surface, and the surface deformation will cause the position of the detection spot to shift. In addition, during the actual measurement process, due to the stress release of the mechanical components of the measurement system, the relative position of the detection spot and the bulge will change over time, so the deflection signal of the detection spot will also change, causing the long-term stability and repeatability of the system to deteriorate.

[0005] Since the position change of the signal propagated into the sample and the position offset of the detection spot in photoacoustic measurement have a direct impact on the detected signal intensity, it is necessary to design a high-precision relative position of the detection spot and the pump light spot to improve the detection accuracy and detection stability and accurately calculate the film thickness from the data. Summary of the invention

[0006] 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 position accuracy of the detection light spot during measurement and improve the accuracy of film thickness measurement.

[0007] 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, 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 bulge deformation area, and the detection light is irradiated to the deformation area to form a signal light with a changing reflection angle; 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; acquiring the theoretical position of the maximum value of the deformation gradient in the bulge deformation area relative to the centroid of the excitation light spot; and adjusting the posture of a deflection mirror located in 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.

[0008] The beneficial effect of the light spot adjustment method of the present invention is: the images of the excitation light spot and the detection light spot are obtained through the imaging system, the relative position between the centroid of the excitation light spot and the centroid of the detection light spot is calculated based on the image, and the theoretical position of the maximum value of the bulge deformation gradient is obtained according to theoretical data, and the posture of the deflection mirror of the detection light path and / or the excitation light path is adjusted so that the detection light spot is located at the theoretical position. At this time, the signal light is collected by the position-sensitive detector to obtain the signal with the maximum intensity. Applying this to the photoacoustic measurement system can accurately obtain the time interval when the sound wave is transmitted back to the sample surface twice in a row, which can improve the accuracy of the acquisition time point when calculating the film thickness and improve the accuracy of the film thickness calculation. The spot adjustment method of the present application can obtain the optimal position of the detection light relative to the excitation light, and can accurately adjust the relative positions of the detection light spot and the excitation light spot, so that the detection light spot is in the optimal position, that is, the position with the maximum bulge gradient, and then the maximum spot deflection amount can be detected, that is, the intensity of the collected signal light position offset change is greatly enhanced, the signal-to-noise ratio of the measurement is improved, and the accuracy of the film thickness calculation results is improved.

[0009] In a feasible scheme, obtaining the theoretical position of the maximum value of the deformation gradient in the bulge deformation area relative to the center of mass of the excitation light spot includes: establishing a light intensity distribution function of the excitation light; calculating the maximum slope of the light intensity distribution function, and the position corresponding to the maximum slope is the theoretical position of the maximum value of the deformation gradient in the bulge deformation area relative to the center of mass of the excitation light spot, and obtaining the distance d0 between the theoretical position and the center of mass position of the excitation light spot. Its beneficial effects are: the sound wave forms a bulge on the surface of the sample to be tested, and the height of the bulge on the surface of the sample to be tested gradually decreases as it moves away from the center of the sound wave, and the gradient change of the bulge changes approximately synchronously with the intensity gradient of the excitation light. The maximum slope of the intensity distribution of the excitation light is calculated through the intensity distribution of the excitation light, and the position of the maximum slope in the light spot is calculated, and the percentage of the position at the maximum slope offset from the center of the excitation light spot is calculated. The theoretical position of the maximum value of the bulge deformation gradient can be obtained according to the intensity distribution of the excitation light, the maximum slope of the intensity of the excitation light spot, and the position of the maximum slope in the excitation light spot or the percentage of the position at the maximum slope, and the distance d0 between the theoretical position and the center of mass of the excitation light spot is obtained. Then, the detection light path and / or the deflection mirror of the excitation light path can be adjusted according to the theoretical position and the distance d0, so that the center of mass of the detection light spot is located at the theoretical position.

[0010] In a feasible solution, the posture of the deflection mirror located in the detection light path and / or the excitation light path is adjusted so that the centroid of the detection light spot is located at the theoretical position, including: obtaining the theoretical light intensity value LIA of the signal light when the detection light is at the theoretical position best Based on the image, the posture of the deflection mirror located in the detection light path and / or the excitation light path is first adjusted to make the distance between the centroid of the excitation light spot and the centroid of the detection light spot less than a preset threshold; the posture of the deflection mirror located in the detection light path and / or the excitation light path is secondly adjusted and the signal light intensity is detected in real time. When the signal light intensity reaches the theoretical light intensity value LIA best The centroid of the detection light spot is located at the theoretical position. The beneficial effect is that the theoretical light intensity value LIA is obtained by calculation best Then, the posture of the deflection mirror of the detection light path and / or the excitation light path is adjusted for the first time, i.e., a rough adjustment is performed. When the distance between the centroid of the excitation light spot and the centroid of the detection light spot is less than the length threshold, the rough adjustment is completed. Then, the second adjustment is performed, i.e., a fine adjustment is performed, so that the light intensity of the collected signal light is the theoretical light intensity value LIA best, that is, at this time, the centroid of the detection light spot is located at the theoretical position, and the adjustment efficiency can be improved by adjusting by combining coarse and fine adjustment methods.

[0011] In a feasible solution, the acquisition of the theoretical light intensity value LIA of the signal light at the theoretical position of the detection light best , including: establishing a mathematical relationship between the signal light intensity distribution and the distance between the centroid of the excitation light spot and the centroid of the detection light spot, wherein the signal light intensity distribution is the signal light intensity value and the signal light intensity maximum LIA when the detection light spot is at different positions max Based on the mathematical relationship and the distance d0 between the theoretical position and the centroid position of the excitation light spot, the light intensity distribution coefficient Q of the signal light at the theoretical position is obtained and the theoretical light intensity value LIA of the signal light is calculated. best , LIA best =Q*LIA max The beneficial effect is that since the size of the excitation light spot and the size of the detection light spot are usually fixed, by continuously adjusting the overlap of the excitation light spot and the detection light spot and adjusting the distance between the centroid of the excitation light spot and the centroid of the detection light spot, the signal light intensity and the maximum value of the signal light intensity LIA at multiple different distances can be obtained. max , and obtain the signal light intensity and light intensity maximum LIA at multiple different spacings max The ratio of the signal light intensity distribution to the distance between the centroid of the excitation light spot and the centroid of the detection light spot can be established based on this, and then the theoretical light intensity value LIA of the signal light when the detection light is located at the theoretical position can be obtained based on the mathematical relationship. best .

[0012] In a feasible solution, the second adjustment includes: adjusting the posture of the deflection mirror located in the detection light path and / or the excitation light path and detecting the intensity of the signal light in real time; obtaining the maximum value LIA of the signal light intensity max The relative position between the centroid of the excitation light spot and the centroid of the detection light spot is fixed, and the posture of the deflection mirror located in the detection light path is adjusted until the signal light intensity reaches the theoretical light intensity value LIA. best The beneficial effect is that the intensity of the signal light is adjusted by adjusting the deflection mirror of the detection light path and / or the excitation light path, and then when the intensity of the signal light is at the maximum value LIA maxThe relative position between the centroid of the excitation light spot and the centroid of the detection light spot is recorded at the same time, and the relative position is used as the position when the centroid of the excitation light spot and the centroid of the detection light spot coincide with each other, and the state of the excitation light is maintained on the basis of this position, the excitation light spot is kept stationary, and the posture of the deflection mirror of the detection light path is adjusted so that the light intensity of the signal light reaches the theoretical light intensity value LIA best , that is, the centroid of the detection light spot is located at the theoretical position.

[0013] 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 facilitates the calculation and acquisition of the theoretical position.

[0014] 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.

[0015] 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.

[0016] 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:

[0017]

[0018] 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.

[0019] 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.

[0020] 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; collects the signal light through a position-sensitive detector, and detects the position offset change of the signal light on the receiving surface of the position-sensitive detector and its corresponding time, so as to obtain the time interval between the echoes of the sound wave being transmitted to the surface of the sample to be measured twice in succession; and obtains the film thickness of the sample to be measured based on the time interval. Its beneficial effect is that by adjusting the relative position of the excitation light spot and the detection light spot, the detection light spot is located at the theoretical position, and the signal light is collected by the position-sensitive detector, so that the time interval between the two consecutive transmissions of the sound wave to the sample surface can be accurately obtained, which can improve the accuracy of the acquisition time point when calculating the film thickness, and improve the accuracy of the film thickness calculation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a flowchart of a light spot adjustment method in a first embodiment of the present invention;

[0022] Figure 2 Schematic diagram of the incident detection light and the excitation light in the second embodiment of the present invention;

[0023] Figure 3 Schematic diagram of the relationship between the intensity distribution 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;

[0024] Figure 4 is a schematic structural diagram of an imaging system in a fourth embodiment of the present invention;

[0025] Figure 5 This is a flowchart of the imaging system obtaining the centroid position in the fifth embodiment of the present invention. DETAILED DESCRIPTION

[0026] 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.

[0027] In view of the problems existing in the prior art, the embodiments of the present invention provide a light spot adjustment method and a film thickness measurement method.

[0028] 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.

[0029] In some embodiments of the present invention, referring to Figure 1 and Figure 2 , the light spot adjustment method comprises:

[0030] 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, and 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 bulge deformation area, and the detection light is irradiated to the deformation area to form a signal light with a changing reflection angle;

[0031] 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;

[0032] S3, obtaining a theoretical position of the maximum value of the deformation gradient in the bulge deformation area relative to the centroid of the excitation light spot;

[0033] S4. Adjust the posture of the deflection mirror located in 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.

[0034] In some specific embodiments of the present invention, the light source module is a pulsed laser, and the pulsed laser generated by the pulsed laser is divided into the detection light and the excitation light by a beam splitter. The excitation light is irradiated on 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 bulge deformation area. The detection light is irradiated on 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 deformation area is formed on the surface of the sample to be tested, the detection light irradiated at different positions will result in different reflection angles, that is, the direction of the signal light will change with the irradiation position of the detection light.

[0035] In some embodiments, the imaging system can acquire the excitation light spot image and the detection light spot image, and can acquire 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. Through the theoretical position of the maximum value of the deformation gradient in the bulge deformation area, the posture of the deflection mirror of the detection light path and / or the excitation light path can be adjusted so that the detection light spot centroid is located at the theoretical position, which is convenient for subsequent calculation of the film thickness by acquiring signals through a position-sensitive detector.

[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, the light intensity of the signal light is collected by a photodetector, and the position offset change on its receiving surface is collected by a position sensitive detector (PSD). In some specific embodiments, the position offset change refers to the offset between the real-time position of the signal light and a fixed point in the collection surface. In some other embodiments, the fixed point is the position where the signal light is collected on the position sensitive detector when the detection light spot coincides with the excitation light spot.

[0044] In some embodiments, when adjusting the deflection mirror in the detection light path to move the detection light spot on the sample surface, the detector position needs to be adjusted accordingly to ensure that the detection light spot is completely received in the detector.

[0045] In some embodiments of the present invention, referring to Figure 1 and Figure 2 , the obtaining of the theoretical position of the maximum value of the deformation gradient in the bulge deformation area relative to the centroid of the excitation light spot includes: establishing a light intensity distribution function of the excitation light; calculating the maximum slope of the light intensity distribution function, the position corresponding to the maximum slope is the theoretical position of the maximum value of the deformation gradient in the bulge deformation area relative to the centroid of the excitation light spot, and obtaining the distance d0 between the theoretical position and the centroid position of the excitation light spot.

[0046] In some specific embodiments of the present invention, an intensity distribution function of the excitation light is established based on empirical data or based on an excitation light spot image, and the maximum slope of the intensity distribution function is calculated. The theoretical position of the maximum slope of the bulge gradient function is obtained based on the position of the maximum slope of the intensity distribution function within the excitation light spot, and the distance d0 between the theoretical position and the center of mass position of the excitation light spot is obtained.

[0047] In some embodiments, the intensity distribution of the excitation light and the gradient change of the bulge deformation are both Gaussian distributions.

[0048] In some embodiments, the center of the excitation light spot coincides with the center of the bulge deformation.

[0049] Figure 3 Schematic diagram of the relationship between the intensity distribution 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.

[0050] In some embodiments of the present invention, referring to Figures 1 to 3 , the theoretical light intensity value LIA of the signal light at the theoretical position of the detection light is obtained best , including: establishing a mathematical relationship between the signal light intensity distribution and the distance between the centroid of the excitation light spot and the centroid of the detection light spot, wherein the signal light intensity distribution is the signal light intensity value and the signal light intensity maximum LIA when the detection light spot is at different positions max Based on the mathematical relationship and the distance d0 between the theoretical position and the centroid position of the excitation light spot, the light intensity distribution coefficient Q of the signal light at the theoretical position is obtained and the theoretical light intensity value LIA of the signal light is calculated. best , LIA best =Q*LIA max .

[0051] In some specific embodiments of the present invention, the maximum value LIA of the signal light intensity is obtained by overlapping the detection light spot with the excitation light spot. max And obtain several signal light intensity and light intensity maximum LIA through multi-point measurement max The ratio of the detection light spot and the excitation light spot centroid distance data is used to establish a mathematical relationship between the ratio and the centroid distance, and the light intensity value when the centroid distance is d0 is calculated and obtained, that is, the theoretical light intensity value LIA best , LIA best =Q*LIA max , then according to the theoretical light intensity value LIA best A first adjustment and a second adjustment are performed on the deflection mirror of the detection light path and / or the excitation light path.

[0052] 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 value collected by phase locking is LIA max When the detection light deviates from the center of the excitation light, the phase-locked reading will decrease as the intensity of the excitation light changes and decay in the form of a Gaussian distribution.

[0053] In some embodiments of the present invention, referring to Figures 1 to 3 The posture of the deflection mirror located in the detection light path and / or the excitation light path is adjusted so that the centroid of the detection light spot is located at the theoretical position, including: obtaining the theoretical light intensity value LIA of the signal light when the detection light is at the theoretical position best Based on the image, the posture of the deflection mirror located in the detection light path and / or the excitation light path is first adjusted to make the distance between the centroid of the excitation light spot and the centroid of the detection light spot less than a preset threshold; the posture of the deflection mirror located in the detection light path and / or the excitation light path is secondly adjusted and the signal light intensity is detected in real time. When the signal light intensity reaches the theoretical light intensity value LIA best When the centroid of the detection light spot is located at the theoretical position.

[0054] In some specific embodiments of the present invention, the theoretical light intensity value LIA is obtained by the mathematical relationship between the ratio and the centroid distance. best , under the observation of the imaging system, adjust the distance between the centroid of the excitation light spot and the centroid of the detection light spot so that the distance between the centroids is less than the preset threshold. Then, 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 light intensity reaches the theoretical light intensity value LIA best , it means that the centroid of the detection light spot is located at the theoretical position.

[0055] In some embodiments, the detection light spot may be first adjusted based on the distance d0 between the theoretical position and the centroid position of the excitation light spot.

[0056] In some embodiments, the preset threshold is 10 pixel size values.

[0057] In some embodiments, the centroid of the detection light spot is located at the theoretical position by adjusting the deflection mirror of the detection light path.

[0058] In some embodiments, the centroid of the detection light spot is located at the theoretical position by adjusting the deflection mirror of the excitation light path.

[0059] 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 detection light spot is located at the theoretical position.

[0060] In some embodiments of the present invention, referring to Figures 1 to 3 The second adjustment includes: adjusting the posture of the deflection mirror located in the detection light path and / or the excitation light path and detecting the intensity of the signal light in real time; obtaining the maximum value LIA of the signal light intensity max The relative position between the centroid of the excitation light spot and the centroid of the detection light spot is fixed, and the posture of the deflection mirror located in the detection light path is adjusted until the signal light intensity reaches the theoretical light intensity value LIA. best .

[0061] 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 detection light spot and the excitation light spot coincide with each other, and at this time, the intensity of the signal light reaches the maximum value LIA max , obtain the maximum value LIA max Then, according to the maximum LIA max The theoretical light intensity value LIA is calculated best , and the spot position of the detection light can be adjusted according to the position at this time to reach the theoretical position. When the theoretical position is reached, the measured signal light intensity is the theoretical light intensity value LIA best .

[0062] In some embodiments of the present invention, referring to Figures 1 to 3 , the excitation light and the detection light are both Gaussian beams.

[0063] In some specific embodiments of the present invention, the excitation light is a Gaussian beam.

[0064] In some embodiments, the Gaussian beam intensity distribution formula is:

[0065]

[0066] 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.

[0067] The second-order derivative of the Gaussian beam intensity distribution formula is taken, and the second-order derivative formula is 0 to obtain the extreme position after the derivative of the Gaussian beam intensity distribution formula, and the slope is the maximum at the extreme position x=x0+c. This extreme position is the theoretical position of the maximum value of the stress bulge gradient on the sample surface. Based on formula (1), the light intensity at this extreme position is obtained:

[0068] f(x0+c)≈0.6A

[0069] When calculating the theoretical position, the signal light and the maximum signal light intensity LIA max The optimized light intensity ratio coefficient Q, that is, the ratio coefficient of the excitation light under Gaussian distribution Q = 0.6

[0070] Therefore, the point where the slope of the intensity distribution function of the excitation light is the largest is also the point where the deflection of the bulge is the largest, that is, the Gaussian-distributed excitation light, the light intensity energy at the point where the slope of the intensity distribution function is the largest is about 0.6 times the maximum light intensity.

[0071] At this time, the detection light is at the position with the maximum bulge gradient, that is, the optimal position of the detection light relative to the excitation light is obtained, so that the maximum light spot deflection can be detected.

[0072] 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 the imaging system obtaining the centroid position in the fifth embodiment of the present invention.

[0073] 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.

[0074] 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 / f B , 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.

[0075] 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.

[0076] 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 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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 position of the centroid of the excitation light spot and the position of the centroid of the detection light spot on the image respectively through the following formula:

[0081]

[0082] 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.

[0083] 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.

[0084] 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.

[0085] In some specific embodiments of the present invention, edge information of the light spot in the image is extracted by an edge detection operator; the edge result is used as the light spot morphology, and then the XY direction size, ellipticity and other information of the light spot are further obtained by ellipse fitting and other methods.

[0086] In some embodiments, the ellipse fitting extracts coordinate information from edge detection and fits the returned contour points to obtain ellipse information.

[0087] In some embodiments of the present invention, the film thickness measurement method adopts 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; collects the signal light through a position-sensitive detector, and detects the position offset change of the signal light on the receiving surface of the position-sensitive detector and its corresponding time, so as to obtain the time interval between two consecutive echoes of the sound wave transmitted to the surface of the sample to be measured; and obtains the film thickness of the sample to be measured based on the time interval.

[0088] In some specific embodiments of the present invention, the relative positions of the excitation light spot and the detection light spot are adjusted by 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 of the maximum deformation gradient in the bulge deformation area. At this time, the intensity of the collected signal light position offset change can be greatly enhanced, and the measurement signal-to-noise ratio can be improved. At the same time, the position sensitive detector can be used to record the time information of the position offset change, so as to accurately obtain the time interval when the sound wave is transmitted back to the sample surface twice in a row, and then the film thickness of the sample to be measured can be accurately calculated.

[0089] 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 to form a bulge deformation area, and the detection light is irradiated to the deformation area to form a signal light with a changing reflection angle; 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; Obtaining a theoretical position of a location where the deformation gradient in the bulge deformation region is at its maximum relative to the centroid of the excitation light spot; The posture of the deflection mirror located in the detection light path is adjusted so that the centroid of the detection light spot is located at the theoretical position.

2. The light spot adjustment method according to claim 1, characterized in that: The obtaining of the theoretical position of the maximum value of the deformation gradient in the bulge deformation region relative to the centroid of the excitation light spot comprises: Establishing a light intensity distribution function of the excitation light; Calculate the maximum slope of the light intensity distribution function, the position corresponding to the maximum slope is the theoretical position of the maximum deformation gradient in the bulge deformation area relative to the centroid of the excitation light spot, and obtain the distance between the theoretical position and the centroid position of the excitation light spot .

3. The light spot adjustment method according to claim 2, characterized in that: The posture of the deflection mirror located in the detection light path is adjusted so that the centroid of the detection light spot is located at the theoretical position, including: Obtaining the theoretical light intensity value of the signal light when the detection light is at the theoretical position ; Based on the image, a first adjustment is performed on the posture of the deflection mirror located in the detection light path, so that the distance between the centroid of the excitation light spot and the centroid of the detection light spot is less than a preset threshold; The posture of the deflection mirror located in the detection light path is adjusted for the second time and the intensity of the signal light is detected in real time. When the intensity of the signal light reaches the theoretical intensity value, When the centroid of the detection light spot is located at the theoretical position.

4. The light spot adjustment method according to claim 3, characterized in that: The step of obtaining the theoretical light intensity value of the signal light at the theoretical position of the detection light ,include: Establish a mathematical relationship between the signal light intensity distribution and the distance between the centroid of the excitation light spot and the centroid of the detection light spot, wherein the signal light intensity distribution is the signal light intensity value when the detection light spot is at different positions and the maximum value of the signal light intensity proportional relationship; Based on the mathematical relationship and the distance between the theoretical position and the centroid position of the excitation light spot , obtain the light intensity distribution coefficient Q of the signal light at the theoretical position and calculate the theoretical light intensity value of the signal light , Q* .

5. The light spot adjustment method according to claim 4, characterized in that: The second adjustment includes: Adjusting the posture of the deflection mirror located in the detection light path and detecting the intensity of the signal light in real time; Obtaining the maximum value of the signal light intensity The relative position between the centroid of the excitation light spot and the centroid of the detection light spot; Fix the position of the excitation light spot and adjust the posture of the deflection mirror located in the detection light path until the signal light intensity reaches the theoretical light intensity value. .

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

7. The light spot adjustment method according to any one of claims 1 to 6, 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.

8. The light spot adjustment method according to claim 7, 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.

9. The light spot adjustment method according to claim 8, 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 The X and Y coordinates of the pixel.

10. The light spot adjustment method according to claim 8, 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.

11. A film thickness measurement method, comprising: Adopting the light spot adjustment method described in any one of claims 1 to 10 to adjust the relative positions of the excitation light spot and the detection light spot; The signal light is collected by a position-sensitive detector, and the position offset change of the signal light on the receiving surface of the position-sensitive detector and the corresponding time are detected to obtain the time interval between two consecutive echoes of the sound wave transmitted to the surface of the sample to be tested; The film thickness of the sample to be tested is obtained based on the time interval.

Images