Focusing method and device of maskless photoetching machine

Abstract

The invention relates to the technical field of optical technology, in particular to a focusing method and device of a maskless photoetching machine, and the method comprises the following steps: S1, determining the initial position of a displacement platform, and collecting the light intensity of the initial position; S2, controlling the displacement platform to move on a z axis, and collecting the light intensity to determine the position when the light intensity is maximum; s3, driving the displacement table to move to a position which is spaced with the maximum value of the light intensity by a coarse step length; s4, performing coarse positioning by using a hill climbing method to determine an approximate position of a focal plane; s5, fine positioning of the focal plane: setting a fine step length, plus or minus a coarse step length near the step S4, driving the displacement table to move in the interval according to the length of the fine step length, and obtaining the maximum light intensity P1; and S6, determining an optimal focal position: continuously searching the optimal focal position near the focal point by adopting the hill climbing method, calculating an evaluation function value of each time, comparing the calculated value of this time with the database, determining a searching direction and a step length according to the change of the function value, and efficiently searching the optimal focal position.

Description

Technical Field

[0001] The present invention relates to the field of optical technology, and in particular to a focusing method and device for a maskless lithography machine. Background Art

[0002] As a key micro-nano processing technology in the semiconductor manufacturing process, photolithography is widely used in integrated circuit (IC) manufacturing, micro-electromechanical systems (MEMS), optoelectronic devices and other fields. Traditional photolithography machines (including mask photolithography machines) usually rely on masks to project optical patterns onto photosensitive materials to form the required microstructures. However, as the integrated circuit manufacturing process develops towards smaller nodes, photolithography technology faces a series of technical challenges, especially in terms of focusing accuracy, exposure accuracy and processing speed.

[0003] During the mask lithography process, the focusing error between the mask and the lithography substrate will directly affect the exposure effect, thereby affecting the accuracy and performance of the chip. Therefore, modern lithography machines are usually equipped with high-precision focusing systems to ensure accurate alignment between the mask and the substrate. However, mask lithography machines have certain limitations, especially in high-resolution lithography, where the manufacturing cost of the mask is high and the lithography process has more stringent requirements on the mask, which requires more precise focusing technology to improve the lithography quality.

[0004] In recent years, in maskless lithography, traditional masks are no longer used. Instead, lithography patterns are written directly on photosensitive substrates through digital light sources. Maskless lithography technology has advantages such as high flexibility, low cost, and suitability for small batch production. However, maskless lithography technology still faces the problem of how to focus quickly and accurately and reduce focusing errors.

[0005] Therefore, the present application has developed a focusing method and device for a maskless lithography machine to solve the problems existing in the prior art. Summary of the invention

[0006] The purpose of the present invention is to provide a focusing method and device for a maskless lithography machine to solve the problems in the prior art of slow focusing speed and inability to determine which layer the light spot is focused on when focusing on a multi-layer structure sample.

[0007] The technical solution of the present invention is: a focusing method for a maskless lithography machine, comprising the following steps: S1: Determine the initial position of the translation stage and collect the light intensity at the initial position: determine the total length L of the translation stage in the z-axis direction, and when the translation stage is not at the initial position, drive the translation stage to move to the initial position; S2: Control the displacement platform to move on the z-axis and collect the light intensity to determine the position at which the light intensity is maximum; S3: driving the translation stage to move to a position that is one coarse step away from the maximum light intensity; S4: Use the hill climbing method to perform rough positioning to determine the approximate position of the focal plane; S5: Fine positioning of the focal plane: set a fine step length, ± a coarse step length near step S4, drive the translation stage to move in this interval according to the length of the fine step length, and obtain the maximum light intensity P1; S6: Determine the best focus position: Continue to use the hill climbing method to search for the best focus position near the focus, calculate the evaluation function value each time, compare the calculated value with the database, determine the search direction and step size based on the change in the function value, and find the focus position.

[0008] Preferably, step S2 comprises the following steps: S2.1: Set a coarse step size; S2.2: The translation stage moves along the z-axis direction according to the length of the coarse step, and collects the light intensity of each step until the moving distance l=L, and collects the maximum light intensity P during the movement.

[0009] Preferably, step S4 includes the following steps: S4.1: Capture and denoise images: Capture the original photolithography image and use an adaptive function to adjust the brightness and contrast of the image, and use Gaussian filtering and median filtering to process the grayscale image to remove image noise; S4.2: Calculate the gradient values ​​of the image in four directions and take the extreme value; S4.3: Perform edge suppression processing and morphological processing on the image, determine whether the current position of the image is a positive focus position, and determine the approximate position of the focal plane.

[0010] Preferably, in step S4.2, the gradient values ​​of the image in four directions, namely horizontal, vertical, 45 degrees and 135 degrees, are calculated by an edge gradient operator, and the difference between the maximum and minimum values ​​of the gradient in the four directions is obtained as the gradient value.

[0011] A maskless lithography device comprises: a DMD chip, an imaging device, a focusing device and a displacement stage, a substrate to be processed is placed on the displacement stage, an image on the DMD chip is imaged on the substrate to be processed through multiple reflections by the imaging device, the focusing device comprises a laser light source, a spectrometer and a photomultiplier tube, the spectrometer is located in the emission direction of the laser light source, and the laser is reflected to the substrate to be processed through the spectrometer, and the laser is received by the photomultiplier tube after being reflected by the substrate to be processed.

[0012] Preferably, the imaging device includes an LED light source, a reflecting prism, a first reflecting mirror, a second reflecting mirror and an eyepiece, the reflecting prism is located in the irradiation direction of the LED light source, the DMD chip is located in the reflection direction of the reflecting prism, the reflecting prism reflects the image of the DMD chip through the first reflecting mirror and the second reflecting mirror, and forms an image on the substrate to be processed through the eyepiece, and the first reflecting mirror and the second reflecting mirror both make the reflected light present a 90-degree reflection.

[0013] Preferably, the spectroscopic part includes a first spectroscope, a second spectroscope, a third reflector and a spectroscopic prism, the spectroscopic prism is located between the first reflector and the second reflector, the laser light source emits laser light, which passes through the first spectroscope, the second spectroscope, the third reflector and the spectroscopic prism in sequence, and reflects the laser light to the second reflector, and the photomultiplier tube is located and installed in the reflection direction of the second spectroscope.

[0014] Preferably, the beam splitter prism is also provided with a dust cover.

[0015] Compared with the prior art, the advantages of the present invention are: (1) The photomultiplier tube is used to quickly collect the light intensity at different times and compare them. The maximum light intensity is selected to preliminarily determine the optimal focal plane. The hill climbing method is then used to perform image processing at the optimal focal plane position to preliminarily determine the focal position. The stage moves in fine steps near the focal point. The photomultiplier tube captures the maximum light intensity again. The two light intensities are compared to determine the focal position at this time. The hill climbing method is used to calculate the data near the focal point at this time to obtain the focal position. The initial focal plane position is quickly determined by the light intensity. The focal position is further determined by the hill climbing method. This reduces the complexity of the image processing process and overcomes the problem that the optimal focal position cannot be determined by the light intensity when determining the multi-layer structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described below in conjunction with the accompanying drawings and embodiments: Figure 1 This is a flow chart of a focusing method for a maskless lithography machine according to the present invention; Figure 2 It is a structural schematic diagram of a maskless lithography device according to the present invention; Figure 3 A top view of a maskless lithography device according to the present invention; Figure 4 It is the Gamma curve diagram of the adaptive function of the present invention; Figure 5 A schematic diagram of a hill climbing search method according to the present invention; Figure 6 This is a schematic diagram of the Gaussian filter processing image principle of the present invention; Figure 7 This is a diagram of the median filtering calculation process of the present invention.

[0017] Among them: 1. DMD chip; 2. Imaging device; 21. LED light source; 22. Reflecting prism; 23. First reflector; 24. Second reflector; 25. Eyepiece; 3. Focusing device; 31. Laser light source; 32. Spectroscopic part; 321. First spectroscopic mirror; 322. Second spectroscopic mirror; 323. Third reflector; 324. Spectroscopic prism; 325. Dust cover; 33. Photomultiplier tube; 4. Translation stage. DETAILED DESCRIPTION

[0018] The present invention is further described in detail below in conjunction with specific embodiments: like Figure 1-Figure 3 As shown, a focusing method for a maskless lithography machine comprises the following steps: S1: Determine the initial position of the displacement stage 4 and collect the light intensity at the initial position: determine the total length L of the displacement stage 4 in the z-axis direction. When the displacement stage 4 is not in the initial position, drive the displacement stage 4 to move to the initial position. The return of the displacement stage 4 to the initial position can ensure that the photolithography pattern is accurately aligned with the specific position on the substrate, thereby improving the position accuracy of the photolithography pattern and ensuring the uniformity and stability of the light intensity during exposure at this position. By returning to the initial position, it can be ensured that a consistent light intensity distribution can be obtained during each exposure, thereby improving the quality and consistency of the photolithography pattern.

[0019] S2: Control the displacement platform to move on the z-axis and collect the light intensity to determine the position at which the light intensity is maximum; S2.1: Set a coarse step size. Specifically, the coarse step size is between 0.1μm and 1μm. The purpose of setting the coarse step size is to quickly approach the optimal focus position during the focusing process. Since the focus range of the lithography machine may be relatively large, the coarse step size can quickly narrow the gap with the optimal focus position at the beginning of focusing, thus laying the foundation for the subsequent fine focusing steps and significantly reducing the time required to reach the initial focus state, thereby improving the production efficiency of the entire lithography process; S2.2: the translation stage 4 moves along the z-axis direction according to the length of the coarse step, and collects the light intensity of each step until the moving distance l=L, and collects the maximum light intensity P1 during the movement; After the displacement stage 4 moves to the initial position, the laser light source 31 is turned on to irradiate the first beam splitter 321. After the laser is reflected by the first beam splitter 321, the second beam splitter 322, the third reflector 323 and the beam splitter prism 324, the laser is reflected to the second reflector 24 and then reflected again through the eyepiece 25 to hit the substrate to be processed. After the laser is reflected by the substrate to be processed again, it passes through the eyepiece 25, the second reflector 24, the beam splitter prism 324, the third reflector 323 and the second beam splitter 322 in sequence. The reflected laser passes through the second beam splitter 322 and then reflects to the photomultiplier tube 33. The photomultiplier tube 33 collects the light intensity at this time. The photomultiplier tube 33 collects the light intensity at the current position every coarse step. When the displacement stage 4 moves a distance l=L in the z-axis direction, the light intensity collection on the z-axis path is completed, and the maximum light intensity value P is recorded.

[0020] Furthermore, the beam splitter prism 324 is provided with a dust cover 325, which can effectively prevent dust and dirt from adhering to the surface of the beam splitter prism 324, thereby maintaining the stability and accuracy of its optical performance and improving the precision and accuracy of measurement.

[0021] S3: Drive the displacement stage 4 to move to a position that is one coarse step away from the maximum light intensity. After determining the maximum light intensity position, the approximate position of the focal plane is initially determined. By finding the maximum light intensity position and moving a coarse step on this basis, the relative position between the lithography machine lens and the silicon wafer can be preliminarily determined, providing a rough reference point for subsequent finer focus adjustments. If the displacement stage 4 is directly moved to the maximum light intensity position, overfocusing may occur, that is, the distance between the lens and the substrate to be processed is too close, resulting in image distortion or damage during exposure. Moving a coarse step can effectively avoid this risk and quickly narrow the focus range, so that subsequent finer focus adjustments can be performed faster, thereby improving the overall focus efficiency.

[0022] In this embodiment, the focal plane is preliminarily determined by contrasting the light intensity and the degree of focus. Since there is a clear correspondence between the change in light intensity and the focus state, the best focus point can be found more accurately, and the response speed of light intensity focus is usually fast, and does not require complex image processing algorithms, which helps to shorten the focus time and improve production efficiency. However, when imaging the DMD chip 1, the light spot focused by the laser covers a large area. For imaging without an image, light intensity can be used for fast focusing. For imaging with an image, the light spot will cover a multi-layer structure, and it is impossible to determine which layer is focused. Therefore, focusing only by light intensity cannot meet the focusing requirements, so image processing needs to be added to determine the focus position.

[0023] S4: Use the hill climbing method for coarse positioning to determine the approximate position of the focal plane. The hill climbing method gradually approaches the optimal focus position by comparing the light intensity or evaluation function values ​​at different positions. It can quickly narrow the search range during the coarse positioning stage.

[0024] S4.1: Capture images and denoise them: Capture the original lithography images and use adaptive functions to adjust the brightness and contrast of the images. Use Gaussian filtering and median filtering to remove the noise from the grayscaled images. The adaptive function can dynamically adjust the brightness and contrast according to the local features of the image, making the details in the image clearer and the contrast more vivid, improving the visual effect of the image, making the lithography structure easier to observe and identify, and at the same time improving the accuracy and efficiency of subsequent image processing, because the enhanced image can provide more useful information.

[0025] Specifically, in the image preprocessing stage, the Gamma adaptive function is selected and the Gamma correction is used to adjust the brightness and contrast of the image. The Gamma correction is derived from the response curve of the CRT display, that is, the nonlinear relationship between its brightness and input voltage. Its working principle is to edit the Gamma curve of the image, find the dark and bright parts in the image signal by performing nonlinear tone adjustment on the image, and increase the ratio between them, thereby expanding the dynamic range of the image and improving the image contrast.

[0026] Gamma correction can be achieved through simple operations, that is, by performing functional transformation on the grayscale value of the image. Its standard form in image processing is as follows:

[0027] Among them, S represents the transformed pixel value, and R represents the original image pixel value mapped to [0, 1]. As a control parameter in the transformation process, it plays a key role in the image enhancement algorithm based on Gamma correction. It determines the final adjustment effect of brightness and contrast.

[0028] For a frame of image in a reasonable state, the average value of all pixels after normalization is roughly around 0.5. In the process of adaptive gamma correction, there is the following corresponding relationship between the γ value and the average brightness X of the image:

[0029] From the above formula, when the mapping range of the brightness average value X is [0,1], the corresponding γ∈ [0.130288,2.847366], according to the size of γ, the corresponding Gamma curve is obtained as follows Figure 4 As shown, according to Figure 4The relationship diagram dynamically and adaptively adjusts brightness and contrast.

[0030] Among them, Gaussian filtering is a linear filtering technology that smoothes the image through a Gaussian function. Figure 6 As shown in the figure, the core of the method is to create a Gaussian kernel whose central value is the largest and the surrounding values ​​gradually decrease. During filtering, this Gaussian kernel is slid on the image, the weighted average of the pixels in the kernel is calculated, and then the value of the pixel in the center of the kernel is replaced by this value. This can effectively remove high-frequency noise while retaining low-frequency image details.

[0031] The median filter is an effective image processing technique that updates the grayscale value of each pixel by calculating the median of all the pixel grayscale values ​​in its specific neighborhood. This process makes the surrounding pixel values ​​closer to the actual situation, thereby effectively eliminating isolated noise points and having a significant filtering effect on impulse noise. More importantly, while removing noise, the median filter can well protect the edge and detail information of the image, preventing the image from becoming blurred due to filtering. Its calculation process is as follows: Figure 7 As shown: a window containing five points is selected, and then the pixels in the window are scanned one by one, the grayscale values ​​corresponding to each pixel are arranged in ascending or descending order, and finally the middle value is selected to replace the grayscale value of the pixel.

[0032] In actual application, there are many different shapes of windows to choose from in the median filtering process, such as square, cross, ring and circle. These windows of different shapes will bring different filtering effects in the application. For example, square and circular windows are more suitable for processing images with long outer edges due to their shape characteristics; while cross-shaped windows perform well in processing images with sharp corners due to their structural characteristics. Figure 7 As shown, the present application selects a rectangular window because the rectangular window processes pixel values ​​in all directions evenly, can effectively smooth the image, and better preserve edge information.

[0033] S4.2: Calculate the gradient values ​​of the image in four directions and take the extreme value. Calculate the gradient values ​​of the image in four directions: horizontal, vertical, 45 degrees and 135 degrees, and obtain the difference between the maximum and minimum values ​​of the gradient in the four directions as the gradient value. Calculating the gradient value can highlight the edge features in the image, because the edge is the area where the grayscale value changes most dramatically. By comparing the gradient values ​​in the four directions, the strength and direction of the edge can be determined. By calculating the gradient values ​​in the four directions and taking the extreme value, richer image feature information can be obtained, which helps to adjust the focus position more accurately, so that the lithography image achieves the best focus effect, thereby reducing the error rate.

[0034] S4.3: Perform edge suppression and morphological processing on the image to determine whether the current position of the image is in focus and determine the approximate position of the focal plane. Edge suppression can reduce or eliminate edge effects in the image, thereby reducing noise, lens distortion and other factors caused during the image acquisition process. Edge suppression can make the edges in the image smoother and reduce the focus error caused by edge effects. Morphological processing can be used to further smooth the image, highlight the key features in the image, and reduce noise interference, which helps to more accurately determine whether the current position of the image is in focus, thereby improving the focus accuracy.

[0035] S5: Fine positioning of the focal plane: Set a fine step size, specifically, the fine step size is between 0nm-10nm, and further refine the coarse step size after focusing to ensure that the lithography machine can meet the final focusing accuracy requirements. Since the photolithography process has extremely high precision requirements, the fine step length can be fine-tuned in the final stage of focusing to maximize the focusing accuracy. This can ensure that the photolithography machine continues to make small adjustments after reaching the initial focusing position to achieve the final focusing accuracy requirements. At around step S4, ± a coarse step length is used to drive the displacement stage 4 to move in this interval according to the length of the fine step length, and the maximum light intensity P1 is obtained. When fine positioning is performed, the light intensity is collected every other fine step length, and the maximum light intensity P1 is determined. When P1>P, the displacement stage 4 moves to the position of P1 to achieve fast focusing of fine positioning. The movement of the displacement stage 4 can quickly complete the collection of light intensity in the interval, further shorten the focus time, and improve work efficiency. After completing the focus of the product to be inspected, the product to be inspected needs to be replaced, and the product to be inspected can be replaced frequently and quickly at the same time.

[0036] S6: Determine the best focus position: Continue to use the hill climbing method to search for the best focus position near the focus, calculate the evaluation function value each time, compare the calculated value with the database, determine the search direction and step size according to the change of the function value, and find the focus position. Move the displacement stage 4 according to the result of the hill climbing method. If the function value of the hill climbing method increases, drive the displacement stage 4 to continue moving forward. If it decreases, turn the direction and reduce the step size. If a peak value appears, turn the direction at the corresponding position and reduce the step size to continue searching until the maximum value of the comprehensive evaluation function is found. By collecting the maximum light intensity twice, the image processing data is greatly reduced, and the focus position can be determined quickly and accurately.

[0037] In this embodiment, the evaluation function in step S4 and step S6 uses the PMBG focusing evaluation function, and its calculation formula is as follows: ; ; : selected direction angle; M, N: rows and columns of pixels respectively; : The value of the corresponding pixel; : The gradient of the corresponding pixel; : General expression of the convolution matrix of the Brenner operator.

[0038] in, .

[0039] Through the PMBG focus evaluation function, the accuracy of the automatic focusing system is significantly improved. The focus is clear or not by calculating the gradient value. The PMBG focus evaluation function is selected based on the following advantages: First, the visual characteristics of the human eye are fully considered. Since human vision is more sensitive to green, the green channel data is used as input, which can not only ensure the effective acquisition of image edge information, but also take into account the human eye's evaluation of the image focus degree, while greatly reducing the computational complexity. Secondly, the operator level is improved, and the Brenner operator in the PMBG focus evaluation function is expanded from the original two directions of a level to 8 directions, and the gradient operators of the four directions of 0°, 45°, 90°, and 135° are selected from them. The focus evaluation function value is determined by multiplying the gradient values ​​of these four directions and then summing them up, which greatly widens the function value gap between the focused image and the slightly defocused image, significantly enhancing the ability to distinguish images with different focus degrees, and thus obtaining a better clarity ratio and sensitivity factor.

[0040] Specifically, the hill climbing method simulates the process of a climber searching for a peak, continuously selecting better solutions, and thus gradually approaching the optimal solution to the problem. Its basic principle is to gradually approach the local optimal solution by continuously selecting the solution that maximizes (or minimizes) the objective function value in the neighborhood, until a local highest point (or lowest point) is reached or no higher point (or lower point) can be found. In this process, the function value serves as a criterion for measuring the quality of the solution and guides the direction of the search. In the hill climbing method, the function value is usually calculated by a predefined evaluation function, that is, the hill climbing function value is the evaluation function value.

[0041] like Figure 5As shown, first start from the starting position with a larger step length, compare the values ​​of the focus evaluation function of the previous and next frames and determine the direction of the next step. If the value of the previous frame is less than the current frame, continue to move in that direction. If the value of the previous frame is greater than the current frame, rotate the motor in the opposite direction. After determining the rotation direction of the zoom motor, continue to search with a large step length, starting from point A and rotating toward point B until the curve trend is downward, that is, the focus evaluation function value of the current frame is less than the value of the previous frame. The large step length causes the focus position to be missed. If the value of point B is less than point F, continue to rotate in the same direction. The value of point F is greater than point H, indicating that the focus evaluation function curve trend is downward. At this time, change the rotation direction of the zoom motor and reduce the search step length. When the value of the previous frame is less than the value of the current frame again, that is, the value at point C is less than point G, change the rotation direction of the zoom motor again and reduce the search step length. Repeat the above steps until the search step length is the minimum step length, and the peak point E of the focus evaluation function curve is found with the minimum step length, indicating that the automatic focus is successful.

[0042] A maskless lithography device is controlled by a focusing method of a maskless lithography device, comprising a DMD chip 1, an imaging device 2, a focusing device 3 and a displacement stage 4, a substrate to be processed is placed on the displacement stage 4, the imaging device 2 comprises an LED light source 21, a reflecting prism 22, a first reflector 23, a second reflector 24 and an eyepiece 25, the focusing device 3 comprises a laser light source 31, a spectrometer 32 and a photomultiplier tube 33, the LED light irradiates to reflect the image on the DMD chip 1 to the substrate to be processed through the imaging device 2, the laser light source 31 emits laser, and the laser is irradiated on the photomultiplier tube 33 through the spectrometer 32 to obtain light intensity, thereby preliminarily judging the focusing position.

[0043] Furthermore, the first reflector 23 and the second reflector 24 both reflect the reflected light at 90 degrees. The 90-degree reflection makes the light more stable when propagating inside the lithography machine, reduces the error caused by light deflection or scattering, thereby improving the accuracy of lithography, and also makes it more flexible to arrange the optical elements and mechanical structures inside the lithography machine, making the spatial layout of the entire system more compact and reasonable.

[0044] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the scope of protection of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is limited by the attached claims rather than the above description, and it is intended to include all changes within the meaning and scope of the equivalent elements of the claims in the present invention.

Claims

1. A focusing method for a maskless lithography machine, characterized in that: The following steps are involved: S1: determining the initial position of the translation stage (4) and collecting the light intensity at the initial position: determining the total length L of the translation stage (4) in the z-axis direction, and when the translation stage (4) is not at the initial position, driving the translation stage (4) to move to the initial position; S2: Control the displacement platform to move on the z-axis and collect the light intensity to determine the position at which the light intensity is maximum; S3: driving the translation stage (4) to move to a position that is a coarse step away from the maximum light intensity; S4: Use the hill climbing method to perform rough positioning to determine the approximate position of the focal plane; S5: Fine positioning of the focal plane: set a fine step length, ± a coarse step length near step S4, drive the translation stage (4) to move in this interval according to the length of the fine step length, and obtain the maximum light intensity P1; S6: Determine the best focus position: Continue to use the hill climbing method to search for the best focus position near the focus, calculate the evaluation function value each time, compare the calculated value with the database, determine the search direction and step size based on the change in the function value, and find the focus position.

2. The focusing method of a maskless lithography machine according to claim 1, characterized in that: Step S2 includes the following steps: S2.1: Set a coarse step size; S2.2: The translation stage (4) moves along the z-axis direction according to the length of the coarse step, and collects the light intensity of each step until the moving distance l=L, and collects the maximum light intensity P during the movement.

3. The focusing method of a maskless lithography machine according to claim 1, characterized in that: Step S4 includes the following steps: S4.1: Capture and denoise images: Capture the original photolithography image and use an adaptive function to adjust the brightness and contrast of the image, and use Gaussian filtering and median filtering to process the grayscale image to remove image noise; S4.2: Calculate the gradient values ​​of the image in four directions and take the extreme value; S4.3: Perform edge suppression processing and morphological processing on the image, determine whether the current position of the image is a positive focus position, and determine the approximate position of the focal plane.

4. The focusing method of a maskless lithography machine according to claim 3, characterized in that: In step S4.2, the gradient values ​​of the image in four directions, namely horizontal, vertical, 45 degrees and 135 degrees, are calculated by using an edge gradient operator, and the difference between the maximum and minimum values ​​of the gradient in the four directions is obtained as the gradient value.

5. A maskless lithography apparatus, controlled by a focusing method for a maskless lithography apparatus according to any one of claims 1 to 4, characterized in that: include: A DMD chip (1), an imaging device (2), a focusing device (3) and a displacement stage (4); a substrate to be processed is placed on the displacement stage (4); an image on the DMD chip (1) is imaged on the substrate to be processed through multiple reflections by the imaging device (2); the focusing device (3) comprises a laser light source (31), a spectroscopic unit (32) and a photomultiplier tube (33); the spectroscopic unit (32) is located in the emission direction of the laser light source (31), and reflects laser light onto the substrate to be processed through the spectroscopic unit (32); the laser light is reflected by the substrate to be processed and received by the photomultiplier tube (33).

6. The maskless lithography apparatus according to claim 5, characterized in that: The imaging device (2) comprises an LED light source (21), a reflecting prism (22), a first reflecting mirror (23), a second reflecting mirror (24) and an eyepiece (25); the reflecting prism is located in the irradiation direction of the LED light source (21); the DMD chip (1) is located in the reflection direction of the reflecting prism (22); the reflecting prism (22) reflects the image of the DMD chip (1) through the first reflecting mirror (23) and the second reflecting mirror (24), and forms an image on the substrate to be processed through the eyepiece (25); the first reflecting mirror (23) and the second reflecting mirror (24) both reflect the reflected light at 90 degrees.

7. The maskless lithography apparatus according to claim 6, characterized in that: The light splitting section (32) comprises a first light splitter (321), a second light splitter (322), a third reflector (323) and a light splitting prism (324); the light splitting prism (324) is located between the first reflector (23) and the second reflector (24); the laser light source (31) emits laser light, which passes through the first light splitter (321), the second light splitter (322), the third reflector (323) and the light splitting prism (324) in sequence, and is reflected to the second reflector (24); the photomultiplier tube (33) is located in the reflection direction of the second light splitter (322).

8. The maskless lithography apparatus according to claim 7, characterized in that: The beam splitter prism (324) is also provided with a dust cover (325).

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