A Fast Focusing Method for White Light Interferometer

Abstract

The present invention discloses a method for rapid focusing of a white light interferometer, including: coarse focusing, the stepper motor moves in the positive direction close to the sample at a first precision step size, and one picture is collected each time it moves. The clarity of each picture is calculated according to the energy gradient function algorithm, and the maximum clarity value is recorded; fine focusing, the stepper motor moves in the positive direction at a second precision step size, and the clarity of each picture and the position corresponding to its maximum value are recorded according to the energy gradient function algorithm and the improved EAV point sharpness algorithm; precise focusing, the piezoelectric ceramic driver moves in the reverse direction at a third precision step size, and the clarity of each picture and its maximum value are recorded according to the improved EAV point sharpness algorithm. After judgment, the focusing position is determined. The present invention uses a stepper motor and a piezoelectric ceramic driver to achieve hierarchical focusing and compensate for the error of the stepper motor's backward movement; the energy gradient function and the improved EAV point sharpness algorithm are used to evaluate the clarity of the image, realizing automatic focusing with fast speed, high precision and large search range.

Description

Technical Field

[0001] The present invention relates to the field of white light interference, and in particular to a rapid focusing method for a white light interferometer. Background Art

[0002] White light interference measurement has the characteristics of high precision, high resolution and non-contact, and is mainly used for measuring surface topography, steps and grooves, etc., and has wide applications in the field of high-precision measurement. The coherence length of a white light interferometer is extremely short, and interference fringes will only appear near the zero optical path difference position. At this time, the light intensity has a maximum value. By longitudinally scanning to determine the height of the maximum value of each point of the sample, the topography of the sample can be measured. Therefore, the white light interferometer must find the interference fringes before measurement, and this process is called "focusing". Generally, the spectral width of a white light LED light source is about 0.1 μm, and its coherence length is about several micrometers, which makes it difficult to quickly locate the zero optical path difference position where interference can occur.

[0003] In view of the characteristics of the white light interferometer, there are currently many methods to calculate the clarity or contrast of pictures by collecting pictures at different longitudinal positions, and then select a suitable search algorithm to find the longitudinal position with the maximum fringe clarity, so as to achieve automatic focusing (Jingtao Dong etc, Automated determination of best focus and minimization of optical path difference in Linnik white light interferometry, Applied Optics, 2011; Zhishun Wu etc, Research on the automatic scanning technology of white light interferometer based on threshold judgment method, China Mechanical Engineering, 2012; Tong Guo etc, Measurement of large-scale step structures by variable-speed white light scanning interferometry, Journal of Optoelectronics·Laser, 2012; Qian Liu etc, An automatic focusing device and method for a white light interferometer, 2020). Due to the short coherence length of white light interference, if the axial moving distance is too large, it is easy to miss the focusing position and the accuracy is low; if it is too small, it will take too long and reduce the efficiency. The above methods have achieved automatic focusing to a certain extent, but there is still a large room for improvement in the search range and time consumption. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention proposes a rapid focusing method for a white light interferometer, which solves the problems of slow speed and small search range in the existing white light interference automatic focusing technology, realizes rapid and large-range automatic focusing, and finally enables the objective lens to directly locate to the starting scanning position.

[0005] The specific technical solutions are as follows:

[0006] A rapid focusing method for a white light interferometer, which is realized based on a white light interference optical system; the white light interference optical system is installed on a stepping motor and includes a white light source, a light homogenizer, an illumination objective lens, a beam splitter, a tube lens, a CCD camera, and a Mirau objective lens; the Mirau objective lens is also fixedly connected to a piezoelectric ceramic driver, and the Mirau objective lens is driven by the piezoelectric ceramic driver to move independently along its optical axis; the rapid focusing method for the white light interferometer includes the following steps:

[0007] S1: Turn on the white light source and place the sample directly in the field of view of the Mirau objective lens;

[0008] S2: Coarse focusing; the stepping motor moves in the positive direction close to the sample at a first precision step size, and the CCD camera captures an image each time it moves; calculate the sharpness S1(i) of each image according to the energy gradient function algorithm, i = 1, 2, 3,..., and record the maximum value of the current sharpness as S1max; if S1(i) - S1(i - 1) > 0.1, then proceed to the next step; otherwise, repeat step S2;

[0009] S3: Fine focusing; the stepping motor moves in the positive direction close to the sample at a second precision step size, and the CCD camera captures an image each time it moves; calculate and record the sharpness S1(i) of each image, the maximum value S1max of the current sharpness, and record the position h of S1max max ; According to the improved EAV point sharpness algorithm, calculate the sharpness S2(i) of each image and record the current maximum sharpness S2max; if 0.9S1max ≤ S1(i) ≤ S1max and the movement from the h max position exceeds 10 steps, or S1(i) < 0.9S1max, the stepping motor retreats to the h max position and starts the next step; otherwise, repeat step S3;

[0010] The expression of the improved EAV point sharpness algorithm is as follows:

[0011]

[0012] In the formula, x is the number of the horizontal pixel points of the image, y is the number of the vertical pixel points of the image, the f function represents the light intensity of this point, M is the number of pixel rows of the image, and N is the number of pixel columns of the image;

[0013] S4: Precision focusing; the stepper motor remains stationary, and the piezoelectric ceramic actuator moves in the opposite direction away from the sample by a third precision step size. Each time it moves, the CCD camera captures an image; calculate and record the sharpness S2(i) and S2max of each image according to the improved EAV point sharpness algorithm. If the current S2(i) is greater than S2max, the piezoelectric ceramic actuator continues to move in the opposite direction until the sharpness S2(i - 1) and S2(i) of two consecutive images are both less than S2max. At this time, the piezoelectric ceramic actuator returns to the position where S2max is recorded. If the current S2(i) is less than or equal to S2max, determine whether S2(i) is less than 0.98S2max. If so, repeat step S4; otherwise, the piezoelectric ceramic actuator remains stationary to complete rapid focusing.

[0014] Further, the expression of the energy gradient function algorithm is as follows:

[0015]

[0016] In the formula, x is the number of the horizontal pixel points of the image, y is the number of the vertical pixel points of the image, and the f function represents the light intensity of this point.

[0017] Further, the first precision step size is 20 - 30 μm, the second precision step size is 1 μm, and the third precision step size is 0.1 μm.

[0018] Further, in the optical system, the white light source, the light homogenizer, the illumination objective lens, and the beam splitter are arranged coaxially in sequence, and the CCD camera, the tube lens, the beam splitter, and the Mirau objective lens are arranged coaxially in sequence. The optical axes where the white light source and the CCD camera are located are perpendicular to each other. The sample is placed directly below the Mirau objective lens; the white light source, the light homogenizer, the illumination objective lens, the beam splitter, the tube lens, the CCD camera, the piezoelectric ceramic actuator, and the Mirau objective lens are installed on the platform of the stepper motor and move along the optical axis of the Mirau objective lens driven by the stepper motor; the Mirau objective lens is fixedly connected to the piezoelectric ceramic actuator, and the Mirau objective lens is driven by the piezoelectric ceramic actuator to move independently along its optical axis.

[0019] The beneficial effects of the present invention are:

[0020] (1) The present invention combines the stepper motor and the piezoelectric ceramic actuator to achieve hierarchical focusing, taking into account both efficiency and accuracy; and there is an error when the stepper motor retracts. The piezoelectric ceramic actuator is used to achieve a smaller precision displacement to compensate for this error.

[0021] (2) The present invention utilizes the characteristics that the energy gradient function is sensitive to the focusing position and the improved EAV point sharpness function is sensitive to fringes, and finally obtains an automatic focusing effect that is fast, has a large range, and is highly accurate. Brief Description of the Drawings

[0022] Figure 1 is a schematic diagram of the optical system used in the present invention.

[0023] Figure 2 is a flowchart of the method of the present invention.

[0024] Figure 3 is a schematic diagram showing the change of clarity with the focusing process in an embodiment of the present invention.

[0025] Figure 4 is a field of view diagram of the Mirau objective lens at each stage in an embodiment of the present invention, where (a) is the starting position, (b) is the focused position, and (c) is the starting position of scanning.

[0026] In the figure, white light source 1, light homogenizing sheet 2, illumination objective lens 3, beam splitter 4, tube lens 5, CCD camera 6, stepper motor 7, piezoelectric ceramic actuator 8, Mirau objective lens 9, sample 10. Detailed Description of the Preferred Embodiments

[0027] The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. The objectives and effects of the present invention will become more apparent. The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0028] As Figure 1 shown, the optical system used in the present invention includes: white light source 1, light homogenizing sheet 2, illumination objective lens 3, beam splitter 4, tube lens 5, CCD camera 6, stepper motor 7, piezoelectric ceramic actuator 8, Mirau objective lens 9.

[0029] The white light source 1, light homogenizing sheet 2, illumination objective lens 3, and beam splitter 4 are arranged coaxially in sequence. The CCD camera 6, tube lens 5, beam splitter 4, and Mirau objective lens 9 are arranged coaxially in sequence. The optical axes where the white light source 1 and the CCD camera 6 are located are perpendicular to each other. The sample 10 is placed directly below the Mirau objective lens 9. The white light source 1, light homogenizing sheet 2, illumination objective lens 3, beam splitter 4, tube lens 5, CCD camera 6, piezoelectric ceramic actuator 8, and Mirau objective lens 9 are installed on the platform of the stepper motor 7 and can move along the optical axis of the Mirau objective lens 9 driven by the stepper motor 7; the piezoelectric ceramic actuator 8 is fixedly connected to the Mirau objective lens 9, and the Mirau objective lens 9 can move independently along its optical axis driven by the piezoelectric ceramic actuator 8.

[0030] After the white light emitted by the white light source 1 is evenly illuminated by the light homogenizer 2, it enters the illumination objective lens 3 to form a collimated light beam. The collimated light beam is reflected by the beam splitter 4 and then irradiated on the sample 10 through the Mirau objective lens 9. The light beam reflected by the sample 10 passes through the Mirau objective lens 9, the beam splitter 4, and the tube lens 5 in sequence, and is irradiated on the CCD camera 6.

[0031] As Figure 2 shown, the rapid focusing method of the white light interferometer includes the following steps:

[0032] S1: Turn on the white light source 1 and place the sample 10.

[0033] S2: Coarse focusing. Taking the direction approaching the sample 10 as the positive direction and the direction away from the sample 10 as the negative direction; the stepping motor 7 moves along the positive direction at the first precision step size of 20 - 30 μm, and the CCD camera 6 captures one picture each time it moves. In this embodiment, the coarse precision step size is taken as 30 μm.

[0034] Calculate the clarity S1(i) of each picture according to the energy gradient function algorithm, i = 1, 2, 3,..., and record the maximum value of the current clarity as S1max. The expression of the energy gradient function algorithm is as follows:

[0035]

[0036] In the formula, x is the number of the horizontal pixel points of the image, y is the number of the vertical pixel points of the image, and the f function represents the light intensity at this point.

[0037] If S1(i) - S1(i - 1) > 0.1, then continue to the next step; otherwise, repeat step S2. Here, 0.1 is used as the threshold to avoid the situation of directly crossing the focal plane during the coarse focusing process.

[0038] S3: Fine focusing. The stepping motor 7 moves along the positive direction at the second precision step size of 1 μm, and the CCD camera 6 captures one picture each time it moves. Calculate the clarity S1(i) of each picture according to the energy gradient function algorithm, and record the position h of this picture i ; record the maximum value of the current clarity as S1max, and record the position h at this time max ; according to the improved EAV point sharpness algorithm, calculate the clarity S2(i) of each picture, and record the current maximum clarity S2max. The expression of the improved EAV point sharpness algorithm is as follows:

[0039]

[0040] In the formula, M is the number of pixel rows of the image, and N is the number of pixel columns of the image. Distance weighting calculations are performed on the four neighborhoods at 45 degrees and 135 degrees of the pixels.

[0041] In the prior art, the EAV point sharpness algorithm calculates based on eight points surrounding the pixel point to be calculated. In contrast, the improved EAV point sharpness algorithm of the present invention calculates based on four points surrounding the pixel point to be calculated. These four points are respectively located in the 45°, 135°, 225°, and 275° directions of the pixel point. Compared with the prior art, the calculation amount of the algorithm adopted by the present invention is greatly reduced, and the final obtained effect is not much different from that of the algorithm used in the prior art. Therefore, the improved EAV point sharpness algorithm adopted by the present invention is superior.

[0042] If 0.9S1max ≤ S1(i) ≤ S1max and the distance h max moves more than 10 steps, or S1(i) < 0.9S1max, it indicates that the position of the maximum sharpness has been passed, and the stepping motor 7 is retracted to the h max position when the maximum sharpness was previously recorded, and the next step is started; otherwise, step S3 is repeated.

[0043] S4: Precise focusing. The stepping motor 7 remains stationary, and the piezoelectric ceramic actuator 8 moves in the reverse direction by a third precision step size of 0.1 μm. Each time it moves, the CCD camera 6 acquires a picture. According to the improved EAV point sharpness algorithm, the sharpness S2(i) of each picture is calculated.

[0044] If the current S2(i) is greater than S2max, then update the current S2max with S2(i); the piezoelectric ceramic actuator 8 continues to move in the reverse direction until the sharpnesses S2(i - 1) and S2(i) of two consecutive pictures are both less than S2max. At this time, the piezoelectric ceramic actuator 8 returns to the position where the latest S2max was recorded, which is the quasi-focus position with dense fringes.

[0045] If the current S2(i) is less than or equal to the previously recorded S2max, then determine whether S2(i) is less than 0.98S2max. If so, repeat step S4; otherwise, when S2(i) >= 0.98S2max, the piezoelectric ceramic actuator 8 remains stationary, and the position where the current S2(i) is recorded is considered as the quasi-focus position with dense fringes, and the rapid focusing is completed.

[0046] Moreover, since the third precision step size adopted in step S4 is small enough and a threshold of 0.98 is set, and there are errors in the previous retraction of the stepping motor, the situation where the quasi-focus plane is crossed and S2(i) is always less than 0.98S2max will not occur.

[0047] When it is necessary to collect n pictures by the phase-shifting method, the piezoelectric ceramic actuator 8 moves in the reverse direction to prepare for starting the white light interferometer scan, where In this embodiment, considering that the scanning step is 72nm, 120 pictures are collected, and the total length is 8.64μm, the dense stripes should be located in the middle of 8.64μm as much as possible, so the piezoelectric ceramic driver 8 needs to move in the opposite direction The distance is half of the total length, that is, 4.32μm.

[0048] like Figure 3 As shown, it is a line graph of the change of clarity during the focusing process of this embodiment, wherein segment A is the coarse focusing process, segment B is the fine focusing process, segment C is the precise focusing process, and the search range is 426.2μm. It can be seen from the figure that the focusing method adopted by the present invention can achieve the effect of accurate focusing.

[0049] In this embodiment, when the focusing process starts, the field of view in the Mirau objective lens 9 is as follows: Figure 4 (a); when step S4 is completed to find the quasi-focus position with dense fringes, the field of view in the Mirau objective lens 9 is as follows Figure 4 (b) As shown; when step S5 is completed, the field of view in the Mirau objective lens 9 is as follows; Figure 4 As shown in (c), it can be seen from the figure that the focusing effect of the present invention is better.

[0050] The present invention combines a stepper motor and a piezoelectric ceramic driver to achieve graded focusing. When the defocus position is large, the stepper motor is used for large-range displacement in consideration of efficiency. When the defocus position is small, the stepper motor is used for small-range displacement and the piezoelectric ceramic driver is used for smaller displacement in consideration of accuracy. This takes both efficiency and accuracy into consideration. In addition, the stepper motor has an error when retreating, and the piezoelectric ceramic driver can be used to achieve a smaller displacement to compensate for the error. The present invention also uses the characteristics of the energy gradient function being sensitive to the focusing position and the improved EAV point sharpness function being sensitive to the stripes, and finally obtains a fast, large-range, high-precision automatic focusing effect.

[0051] Those skilled in the art can understand that the above are only preferred examples of the invention and are not intended to limit the invention. Although the invention is described in detail with reference to the above examples, those skilled in the art can still modify the technical solutions recorded in the above examples or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, etc. made within the spirit and principle of the invention shall be included in the protection scope of the invention.

Claims

1. A fast focusing method for a white light interferometer, characterized in that This method is implemented based on a white-light interference optical system; the white-light interference optical system is installed on a stepping motor and includes a white-light source, a light homogenizer, an illumination objective lens, a beam splitter, a tube lens, a CCD camera, and a Mirau objective lens; the Mirau objective lens is also fixedly connected to a piezoelectric ceramic actuator, and the Mirau objective lens is driven by the piezoelectric ceramic actuator to move independently along its optical axis; The rapid focusing method of the white-light interferometer includes the following steps: S1: Turn on the white-light source and place the sample directly in the field of view of the Mirau objective lens; S2: Coarse focusing; the stepping motor moves in the positive direction close to the sample at a first precision step size, and the CCD camera captures one picture each time it moves; calculate the sharpness S1(i) of each picture according to the energy gradient function algorithm, i = 1, 2, 3, …, and record the maximum value of the current sharpness as S1max; if S1(i) - S1(i - 1) > 0.1, then proceed to the next step; otherwise, repeat step S2; S3: Fine focusing; the stepper motor moves in the positive direction close to the sample by a second-precision step length, and the CCD camera captures one picture each time it moves; calculate and record the sharpness S1(i) of each picture, the maximum value S1max of the current sharpness, and record the position h of S1max according to the energy gradient function algorithm max ; calculate the sharpness S2(i) of each picture according to the improved EAV point sharpness algorithm, and record the current maximum sharpness S2max; if 0.9S1max ≤ S1(i) ≤ S1max and the movement from the h max position exceeds 10 steps, or S1(i) < 0.9S1max, the stepper motor retreats to the h max position and starts the next step; otherwise, repeat step S3; The expression of the improved EAV point sharpness algorithm is as follows: In the formula, x is the number of the horizontal pixel points of the image, y is the number of the vertical pixel points of the image, the f function represents the light intensity at this point, M is the number of pixel rows of the image, and N is the number of pixel columns of the image; S4: Fine focusing; the stepping motor remains stationary, and the piezoelectric ceramic actuator moves in the reverse direction away from the sample at a third precision step size, and the CCD camera captures one picture each time it moves; calculate and record the sharpness S2(i) and S2max of each picture according to the improved EAV point sharpness algorithm; if the current S2(i) is greater than S2max, the piezoelectric ceramic actuator continues to move in the reverse direction until the sharpness of two consecutive pictures S2(i - 1) and S2(i) are both less than S2max, at this time the piezoelectric ceramic actuator returns to the position where S2max is recorded; if the current S2(i) is less than or equal to S2max, then determine whether S2(i) is less than 0.98S2max, if so, repeat step S4; otherwise, the piezoelectric ceramic actuator remains stationary and the rapid focusing is completed.

2. The rapid focusing method of the white light interferometer according to claim 1, characterized in that, The expression of the energy gradient function algorithm is as follows: In the formula, x is the number of the horizontal pixel points of the image, y is the number of the vertical pixel points of the image, and the f function represents the light intensity at this point.

3. The rapid focusing method of the white light interferometer according to claim 1, wherein The first precision step size is 20 - 30 μm, the second precision step size is 1 μm, and the third precision step size is 0.1 μm.

4. The rapid focusing method of the white light interferometer according to claim 1, characterized in that In the optical system, the white light source, the light homogenizer, the illumination objective lens, and the beam splitter are arranged coaxially in sequence; the CCD camera, the tube lens, the beam splitter, and the Mirau objective lens are arranged coaxially in sequence. The optical axes where the white light source and the CCD camera are located are perpendicular to each other. The sample is placed directly below the Mirau objective lens. The white light source, the light homogenizer, the illumination objective lens, the beam splitter, the tube lens, the CCD camera, the piezoelectric ceramic actuator, and the Mirau objective lens are installed on the platform of the stepping motor and move along the optical axis of the Mirau objective lens driven by the stepping motor. The Mirau objective lens is fixedly connected to the piezoelectric ceramic actuator, and the Mirau objective lens is driven by the piezoelectric ceramic actuator to move independently along its optical axis.

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