Mask defect detection system and method based on fiber laser array

Abstract

According to the multi-beam confocal mask defect detection system and method based on the fiber laser array, a multi-point scanning light field is obtained on the basis of the fiber laser array to serve as an illumination light source, the scanning light field can be regulated and controlled on the basis of arrangement of the laser array, and through the combined action of a motion table and a multi-face rotating mirror, the defect of the multi-beam confocal mask is detected. Rapid and accurate mask two-dimensional scanning imaging is achieved, and high-resolution detection of mask defects is achieved on the basis of a confocal microscopic imaging method in cooperation with a high-power microscope objective and a photomultiplier. Compared with a traditional point scanning type mask defect detection system, the system has the advantages that the detection resolution is high, the scanning speed is high, a multi-point scanning light field can be freely regulated and controlled, and no extra diffraction optical element needs to be introduced.

Description

Technical Field

[0001] The present invention relates to the field of photolithography technology, and in particular to a multi-beam confocal mask defect detection system and method based on an optical fiber laser array. Background Art

[0002] The mask is an indispensable part of the lithography system. Its quality has a nearly decisive influence on the yield of the final lithography element. Even a tiny defect at the nanoscale will cause the light field amplitude or phase to change, and eventually the defect will be copied to the wafer surface, significantly affecting the yield of the lithography process. Therefore, the defect detection technology for the mask is indispensable, and its detection accuracy and speed have become the main difficulties restricting the development of the lithography process.

[0003] Although the existing line scanning mask defect detection system has a fast detection speed, it is limited by its imaging method and only has a high detection accuracy in the scanning direction, and cannot accurately locate the defects in two dimensions. Although the existing point scanning mask defect detection system can accurately locate the defects, it generally has problems such as slow detection speed and complex system structure.

[0004] Therefore, improving the detection speed while ensuring high detection accuracy is a technical problem that needs to be solved urgently. Summary of the invention

[0005] The purpose of this application is to provide a multi-beam confocal mask defect detection system and method based on a fiber laser array, which can improve the detection accuracy and speed as a whole.

[0006] In order to solve the above technical problems, the present application provides a multi-beam confocal mask defect detection system based on a fiber laser array, comprising: A fiber laser array including a multi-point scanning light field as the system light source; A collimating lens, a reflector and a first beam splitter, wherein the fiber laser array is connected to the collimating lens, and the collimated incident light passes through the reflector and is split into two independent light beams by the first beam splitter; A polygonal rotating mirror, a second beam splitter and a third beam splitter, wherein the polygonal rotating mirror is used to deflect one of the incident light beams independently separated from the first beam splitter, so that the light beam passes through the imaging lens and is expanded to the second beam splitter and the third beam splitter; A quarter wave plate, used for modulating the linearly polarized light after being split by the second beam splitter and the third beam splitter into circularly polarized light; A microscope objective lens, used to focus circularly polarized light onto the surface of the mask to be tested; A motion stage is used to fix the mask to be tested and realize fast two-dimensional scanning of the mask; Differential confocal module, used to help the microscope objective lens to accurately focus; The photomultiplier tube detector analyzes the reflected light from the surface of the mask to be tested after it is returned to the first beam splitter along the original optical path and split, thereby obtaining the intensity information of the corresponding reflected light; The clock synchronization module is used to analyze another beam of linear polarization after the incident light is split by the second beam splitter to obtain the time series of the impulse signal.

[0007] Optionally, the fiber laser array is arranged in an X-shape, and triangular grooves are etched on both sides of the fiber laser array, and the depth and spacing of the etching are determined by the arrangement of the array and the size of the outer diameter of the optical fiber.

[0008] Optionally, the arrangement of the fiber laser array is consistent with the arrangement of the photomultiplier tube detector array.

[0009] Optionally, the multi-faceted rotating mirror and the motion stage can realize fast two-dimensional scanning of the mask, thereby improving the detection speed of the system, which specifically includes: using the multi-faceted rotating mirror and the imaging lens, the multi-faceted rotating mirror has an angular velocity of β, and the imaging axial coordinate z has an angle with the incident light. Satisfy the corresponding relationship , is the focal length of the imaging lens, and the scanning speed at this point , matched with a polygonal mirror rotating at a constant angular velocity.

[0010] Optionally, the differential confocal module has a built-in detection light source. The emitted light beam is collimated by a collimating lens, reflected by a third beam splitter, and focused by a microscope objective onto the surface of a mask to be tested after passing through a quarter-wave plate. Similarly, the reflected light beam passes through the microscope objective, a quarter-wave plate and the third beam splitter again and returns along the original optical path to be incident on the differential confocal module. The incident light is respectively incident on the back focus optical path and the front focus optical path by a beam splitter, and is collected by corresponding detectors to obtain corresponding back focus signals and front focus signals. The corresponding axial differential confocal response curve is obtained after scanning by an actuator, and the accurate focusing information of the microscope objective is obtained after fitting the data, so that the microscope objective is always in an ideal fixed focus position.

[0011] Optionally, the built-in detection light source point, the focal plane of the microscope objective lens and the detector pinhole of the differential confocal module are in a conjugate relationship with each other. The scattered light generated at the focal plane of the microscope objective lens is normally collected by the detector through the detector pinhole, while for the scattered light at the non-focal plane, its imaging position deviates from the pinhole position, and only a small part of the scattered light can be collected by the detector through the pinhole, thereby eliminating the interference of out-of-focus light outside the focal plane.

[0012] Optionally, the linearly polarized light in the reflection direction split by the second beam splitter is incident on the clock synchronization module to obtain the time series of the impulse signal, and combined with the reflected light intensity information sequence, the corresponding two-dimensional mask detection icon is obtained to achieve rapid positioning of defects and type identification.

[0013] The embodiment of the present invention further provides a multi-beam confocal mask defect detection method based on a fiber laser array, comprising: A fiber laser array is used to obtain a multi-point light beam, which is continuously deflected by a collimating lens, a reflector, a first beam splitter, and a multifaceted mirror. After deflection, the light beam is expanded by an imaging lens and incident on a second beam splitter. At this time, the light beam is divided into linear polarized light in a transmission direction and a reflection direction. The linear polarized light in the transmission direction is modulated into circular polarized light after continuously passing through a third beam splitter and a quarter-wave plate. The light beam is focused onto a mask sample to be tested fixed on a moving stage through a microscope objective lens placed on an actuator. The reflected light on the surface of the mask sample to be tested is returned along the original optical path to the first beam splitter through which the incident light passes. The split reflected light is focused by the first lens onto a photomultiplier light detection array to obtain a sequence of reflected light intensity information at each position. The linearly polarized light in the reflection direction split by the second beam splitter is incident on the clock synchronization module to obtain the time sequence of the impulse signal. Combined with the reflected light intensity information sequence, the corresponding two-dimensional mask detection icon is obtained to achieve rapid positioning and type identification of defects. The reflected light from the surface of the mask sample to be tested is split along the original optical path through the third beam splitter through which the incident light passes, and is then split into the differential confocal module. The beam splitter in the differential confocal module splits the light beam into the back focus optical path and the front focus optical path respectively, and the light beams are collected by the corresponding detectors to obtain the corresponding back focus signals and front focus signals. After scanning by the actuator, the corresponding axial differential confocal response curve is obtained, and the accurate focusing information of the microscope objective is obtained after fitting the data, so that the microscope objective is always in the ideal fixed focus position.

[0014] Optionally, the differential confocal module is used to accurately place the mask sample to be tested at the focal plane of the microscope objective, the fiber laser array and the photomultiplier tube detector are both located at the conjugate plane of the sample plane, and a pinhole is placed close to the detector. The pinhole and the fiber laser array are in an imaging relationship with each other, thereby removing out-of-focus light outside the focal plane and improving detection accuracy.

[0015] Optionally, the differential confocal module further includes the following steps for improving the detection accuracy: The built-in detection light source point, microscope objective focal plane and detector pinhole of the differential confocal module are in a conjugate relationship with each other. The scattered light generated at the focal plane of the microscope objective is normally collected by the detector through the detector pinhole. As for the scattered light at the non-focal plane, its imaging position deviates from the pinhole position, and only a small part of the scattered light can be collected by the detector through the pinhole, thereby eliminating the interference of out-of-focus light outside the focal plane.

[0016] This application has the following beneficial effects: By using a fiber laser array to obtain a multi-point scanning light field, the scanning light field can be freely controlled based on the arrangement of the laser array, getting rid of the dependence of the traditional system on diffractive optical elements, being simple to manufacture and having a high degree of freedom in adjustment; through the joint action of the motion table and the multi-faceted rotating mirror, combined with the signal obtained by the clock synchronization module, fast and accurate mask two-dimensional scanning imaging can be achieved. The present invention uses a differential confocal system to obtain the corresponding differential confocal axial response, realizes real-time and accurate focusing of the microscope objective lens, and improves the defect detection accuracy. The present invention combines the above advantages to realize a high-precision and fast mask defect detection system with greater freedom of adjustment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of a multi-beam confocal mask defect detection system based on a fiber laser array provided in an embodiment of the present application; Figure 2 A schematic diagram of the structure of a fiber laser array provided in an embodiment of the present application; Figure 3 A schematic diagram of a differential confocal response simulation result provided in an embodiment of the present application; Figure 4 A schematic diagram of the signal detected by the time synchronization module provided in an embodiment of the present application. DETAILED DESCRIPTION

[0018] In order to make the core, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0019] At present, although the existing line scanning mask defect detection system has a fast detection speed, it cannot accurately locate the defects in two dimensions, and although the existing point scanning mask defect detection system can accurately locate the defects, it generally has the problem of slow detection speed. The present application provides a multi-beam confocal mask defect detection system and method based on a fiber laser array, which can solve the above problems as a whole. Figure 1As shown, the multi-beam confocal mask defect detection system based on the fiber laser array of this embodiment includes: The optical fiber laser array 1 is used as a scanning light field, including a multi-point scanning light field that can be freely arranged; after the emitted light beam is collimated by the collimating lens 2, it is continuously transmitted to the multi-faceted mirror 5 through the first reflector 3 and the first beam splitter 4, the light beam is deflected, and continuously expands through the imaging lens (including the second lens 6 and the third lens 7), and enters the second beam splitter 8. At this time, the light beam is divided into linear polarized light in the transmission direction and the reflection direction; the linear polarized light in the transmission direction is modulated into circular polarized light after passing through the third beam splitter 9 and the quarter wave plate 10, and is focused to the mask sample to be tested fixed on the moving stage 13 through the microscope objective lens 11 placed on the actuator 12; the reflected light on the surface of the mask sample to be tested carries the mask related information and returns along the original light path, passing through the microscope objective lens 11, the quarter wave plate 10, the third lens 7, the second lens 6, and after being deflected by the multi-faceted mirror 5, it passes through the first beam splitter 4, and is focused by the first lens 14 to the photomultiplier light detection array 15 to obtain the intensity information sequence of the mask surface; The linearly polarized light in the reflection direction of the second beam splitter 8 enters the clock synchronization module 16, is focused by the lens in the module, and is transmitted to the grating, and is collected by the detector therein to obtain the time series of the impulse signal; The differential confocal module 17 has a built-in 632nm laser as a detection light source. The emitted light beam is collimated by a collimating lens, reflected by the third beam splitter 9, and focused by the microscope objective lens 11 to the mask surface to be measured after passing through the quarter wave plate 10. Similarly, the reflected light beam passes through the microscope objective lens 11, the quarter wave plate 10 and the third beam splitter 9 again and returns to the differential confocal module 17 according to the original optical path. The incident light is divided into linearly polarized light in the transmission direction and the reflection direction by a beam splitter, and is incident on the back focus optical path and the front focus optical path respectively, and is collected by the corresponding detector to obtain the corresponding back focus signal and front focus signal. After scanning by the actuator 12, the corresponding axial differential confocal response curve is obtained, and the accurate focusing information of the microscope objective lens is obtained after fitting the data, so that the microscope objective lens is always in the ideal fixed focus position.

[0020] Furthermore, the sample to be detected is accurately placed at the focal plane of the microscope objective through a differential confocal module, and the fiber laser array and the photomultiplier tube detector are both located at the conjugate plane of the sample plane, wherein a pinhole is placed close to the detector, and the scattered light generated at the focal plane can be normally collected by the detector through the detector pinhole, while for the scattered light at the non-focal plane, its imaging position deviates from the pinhole position, so only a small part of the scattered light can be collected by the detector through the pinhole, thereby eliminating interference outside the focal plane and improving the accuracy of system detection; The differential confocal module can be used with an actuator to achieve accurate focusing of the microscope objective. The beam splitter in the differential confocal module divides the reflected light of the mask sample to be measured into two measuring beams, which are received by the two detectors PMT1 and PMT2 after passing through the focusing lens and the pinhole. The offsets of the intersection of the pinhole and the measuring beam are and , then the light intensity signal obtained by the corresponding detector can be expressed by the formula:

[0021]

[0022] in and Represents the normalized optical coordinates of the radial and axial directions in the object space, respectively, and the actual radial coordinates And the axial coordinate There is a conversion relationship between them. represents the amount of defocus Optical coordinates under normalized conditions. and are the three-dimensional point spread functions of the illumination light path and the collection light path in the differential confocal module, represents the three-dimensional amplitude distribution of the illumination light field, and the operator It represents the convolution operation in three-dimensional space.

[0023] Since the focal length of the system is much larger than the offset , the normalized radius of the available system pupil , System Normalized Pupil Function and the zero-order Bessel function Represents the point spread function of the system and , the formula used is:

[0024] Among them, the normalized optical coordinates With actual coordinates The relationship between is determined by formula (4):

[0025]

[0026] in To detect the wavelength of the light source, is the effective numerical aperture of the microscope objective, when The value of defocus is When the normalized value is calculated, the optical coordinate , represents the additional noise caused by the difference between the two detectors, , which has good linearity in the center of the response curve; based on the above formula, the corresponding differential confocal curve is calculated, and the simulation results are as follows Figure 4 As shown, the central part of the response curve has good linearity. Based on the differential response curve, the focus offset can be calculated by formula (6) To perform an exact calculation: Formula (6) in Represents the change value of the differential response signal, represents the slope of the linear region; the differential confocal module periodically acquires the differential response signal and calculates the corresponding offset in real time , corresponding to the actuator moving a distance , actuator With the calculated offset Satisfaction relationship , It is a proportional parameter, whose value is determined by the system numerical aperture and the focus rate requirement. It is used to limit the distance that the fixed focus moves and achieve real-time and accurate focus tracking of the system.

[0027] Furthermore, when the scanning light field is incident on the surface of the mask area to be tested, if there are defects such as protrusions and depressions on the mask surface, the intensity of the reflected light collected by the photomultiplier tube detector will change accordingly due to the scattering effect of the defects. By analyzing the intensity information on the detector obtained from different areas, the location of the defect can be located. At the same time, according to the transformation law of the intensity information obtained by detection, the type of defect can also be distinguished. The system uses a multi-faceted rotating mirror and a motion stage to realize two-dimensional detection of the mask, and uses a time synchronization module to correspond to the deflection angle of the light beam and the corresponding scanning area to achieve accurate positioning of the scanning area.

[0028] Furthermore, in order to make the fiber laser array arrangement more compact and firm, according to the following Figure 2 As shown, the clamping structure fixes the optical fiber, and triangular grooves are etched on both sides of the structure. The etching depth and spacing are determined by the arrangement of the array and the size of the outer diameter of the optical fiber. During preparation, the optical fiber is installed in the corresponding groove to ensure the fixing effect. In particular, the optical fiber used in the device uses a corrosive agent to corrode the outer cladding at the front end of the optical fiber before use, thereby further reducing the spacing between the optical fiber arrays and improving the scanning speed of the system.

[0029] In the process of arranging the fiber laser array, the actual use requirements of the system are first considered, the corresponding microscope objective is determined, and the arrangement of the fiber laser array is determined according to the field of view size of the microscope objective used and the required detection accuracy requirements to optimize the scanning rate of the system; and the arrangement of the photomultiplier tube array should be consistent with the arrangement of the fiber laser array to meet the multi-point confocal relationship to improve the detection resolution.

[0030] Furthermore, for a multi-faceted rotating mirror, if the rotating mirror rotation angular velocity is , the current rotation angle of the mirror is , the imaging distance is , then on the imaging plane, there is an image point scanning speed , the image point scanning speed changes with the current rotation angle changes, and cannot satisfy the linear relationship; an imaging lens is introduced after the multi-faceted rotating mirror, and its imaging axial coordinate Angle with incident light Satisfy the corresponding relationship , is the focal length of the imaging lens, and the scanning speed is , which matches the rotating mirror rotating at a constant angular velocity, thus eliminating the need to introduce complex nonlinear compensation algorithms.

[0031] Furthermore, the clock synchronization module is introduced to match the signal collected by the photomultiplier tube detector with the corresponding detection position. When the linearly polarized light in the reflection direction is incident on the clock synchronization module, it is deflected by the converging lens. A grating is placed behind the converging lens. Only part of the light at a specific angle can pass through the grating and be collected by the detector. The signal collected by the detector is as follows Figure 3 As shown, represents the mirror rotation frequency of the multi-faceted mirror, represents the number of faces of the multi-faceted mirror in a single deflection cycle In the method, the position of the impact signal obtained is the lateral scanning calibration position, and there is a corresponding relationship between the scanning calibration position in the grating and the lateral coordinate of the detection area in the mask to be tested. Therefore, according to the lateral coordinate-time relationship obtained by the time synchronization module and the longitudinal coordinate-time relationship obtained by the motion stage recording, the defect position is accurately located, and the light intensity-time signal obtained by the detector array can be combined to complete the accurate mapping of the scanning position and the light intensity information obtained by detection.

[0032] This embodiment also provides a multi-beam confocal mask defect detection method based on fiber laser, comprising: The actuator is adjusted to scan in the axial direction, and the front focus optical path and the back focus optical path in the differential confocal module are used to obtain the light intensity signals at the corresponding positions respectively, and the differential confocal response curve in the axial direction is calculated and obtained, and the microscope objective lens is positioned to the desired focus in real time based on the signal. A multi-faceted rotating mirror and a motion stage are used to realize two-dimensional scanning of the mask to be tested, and a photomultiplier tube detector is used to obtain the corresponding light intensity-time series. The horizontal coordinate-time series obtained by the time synchronization module and the vertical coordinate-time series obtained by the motion stage are combined to establish an accurate mapping relationship between the corresponding scanning position on the mask and the obtained light intensity information, and the corresponding mask detection pattern is obtained based on the corresponding data processing model. By comparing and analyzing the obtained patterns and combining the corresponding relationship of the mask detection patterns under different defect types, the accurate positioning and type distinction of the mask defects are realized. Furthermore, the trained neural network model can be used to analyze the obtained mask detection results to realize accurate positioning of the defect position and rapid discrimination of the defect type.

[0033] The above is a detailed introduction to a multi-beam confocal mask defect detection system and method based on fiber laser provided by the present application; for ordinary technicians in this technical field, several improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A multi-beam confocal mask defect detection system based on a fiber laser array, characterized in that: include: A fiber laser array including a multi-point scanning light field as the system light source; A collimating lens, a reflector and a first beam splitter, wherein the fiber laser array is connected to the collimating lens, and the collimated incident light passes through the reflector and is split into two independent light beams by the first beam splitter; A polygonal rotating mirror, a second beam splitter and a third beam splitter, wherein the polygonal rotating mirror is used to deflect one of the incident light beams independently separated from the first beam splitter, so that the light beam passes through the imaging lens and is expanded to the second beam splitter and the third beam splitter; A quarter wave plate, used for modulating the linearly polarized light after being split by the second beam splitter and the third beam splitter into circularly polarized light; A microscope objective lens, used to focus circularly polarized light onto the surface of the mask to be tested; The motion stage is used to fix the mask to be tested and realize fast two-dimensional scanning of the mask; Differential confocal module, used to help the microscope objective lens to accurately focus; The photomultiplier tube detector analyzes the reflected light from the surface of the mask to be tested after it is returned to the first beam splitter along the original optical path and split, thereby obtaining the intensity information of the corresponding reflected light; The clock synchronization module is used to analyze another beam of linear polarization after the incident light is split by the second beam splitter to obtain the time series of the impulse signal.

2. The multi-beam confocal mask defect detection system based on a fiber laser array according to claim 1, characterized in that: The fiber laser array is arranged in an X-shaped cross pattern, and triangular grooves are etched on both sides of the fiber laser array. The depth and spacing of the etching are determined by the arrangement of the array and the size of the outer diameter of the optical fiber.

3. The multi-beam confocal mask defect detection system based on a fiber laser array according to claim 2, characterized in that: The arrangement of the fiber laser array is consistent with the arrangement of the photomultiplier tube detector array.

4. The multi-beam confocal mask defect detection system based on a fiber laser array according to claim 1, characterized in that: Also includes: The multi-faceted rotating mirror realizes fast two-dimensional scanning of the mask through the cooperation with the motion stage, thereby improving the detection speed of the system. Specifically, the multi-faceted rotating mirror and the imaging lens are used, the rotation angular velocity of the multi-faceted rotating mirror is β, and the angle between the imaging axial coordinate z and the incident light is Satisfy the corresponding relationship , is the focal length of the imaging lens, and the scanning speed at this point , matched with a polygonal mirror rotating at a constant angular velocity.

5. The multi-beam confocal mask defect detection system based on a fiber laser array according to claim 1, characterized in that: The differential confocal module has a built-in detection light source. The emitted light beam is collimated by a collimating lens, reflected by the third beam splitter, and focused by the microscope objective onto the surface of the mask to be tested after passing through a quarter-wave plate. Similarly, the reflected light beam passes through the microscope objective, the quarter-wave plate and the third beam splitter again and returns along the original optical path to be incident on the differential confocal module. The incident light is incident on the back focus optical path and the front focus optical path respectively by a beam splitter, and is collected by the corresponding detector to obtain the corresponding back focus signal and front focus signal. The corresponding axial differential confocal response curve is obtained after scanning by the actuator, and the accurate focusing information of the microscope objective is obtained after fitting the data, so that the microscope objective is always in the ideal fixed focus position.

6. The multi-beam confocal mask defect detection system based on a fiber laser array according to claim 5, characterized in that: The built-in detection light source point, the focal plane of the microscope objective lens and the detector pinhole of the differential confocal module are in a conjugate relationship with each other. The scattered light generated at the focal plane of the microscope objective lens is normally collected by the detector through the detector pinhole. For the scattered light at the non-focal plane, its imaging position deviates from the pinhole position, and only a small part of the scattered light can be collected by the detector through the pinhole, thereby eliminating the interference of out-of-focus light outside the focal plane.

7. The multi-beam confocal mask defect detection system based on a fiber laser array according to claim 1, characterized in that: The linearly polarized light in the reflection direction split by the second beam splitter is incident on the clock synchronization module to obtain the time series of the impulse signal. Combined with the reflected light intensity information sequence, the corresponding two-dimensional mask detection icon is obtained to achieve rapid positioning and type identification of defects.

8. A multi-beam confocal mask defect detection method based on a fiber laser array, characterized in that: include: A fiber laser array is used to obtain a multi-point light beam, which is continuously deflected by a collimating lens, a reflector, a first beam splitter, and a multifaceted mirror. After deflection, the light beam is expanded by an imaging lens and incident on a second beam splitter. At this time, the light beam is divided into linear polarized light in a transmission direction and a reflection direction. The linear polarized light in the transmission direction is modulated into circular polarized light after continuously passing through a third beam splitter and a quarter-wave plate. The light beam is focused onto a mask sample to be tested fixed on a moving stage through a microscope objective lens placed on an actuator. The reflected light on the surface of the mask sample to be tested is returned along the original optical path to the first beam splitter through which the incident light passes. The split reflected light is focused by the first lens onto a photomultiplier light detection array to obtain a sequence of reflected light intensity information at each position. The linearly polarized light in the reflection direction split by the second beam splitter is incident on the clock synchronization module to obtain the time sequence of the impulse signal. Combined with the reflected light intensity information sequence, the corresponding two-dimensional mask detection icon is obtained to achieve rapid positioning and type identification of defects. The reflected light from the surface of the mask sample to be tested is split along the original optical path through the third beam splitter through which the incident light passes, and is then split into the differential confocal module. The beam splitter in the differential confocal module splits the light beam into the back focus optical path and the front focus optical path respectively, and the light beams are collected by the corresponding detectors to obtain the corresponding back focus signals and front focus signals. After scanning by the actuator, the corresponding axial differential confocal response curve is obtained, and the accurate focusing information of the microscope objective is obtained after fitting the data, so that the microscope objective is always in the ideal fixed focus position.

9. The multi-beam confocal mask defect detection method based on fiber laser array according to claim 8, characterized in that: The differential confocal module is used to accurately place the mask sample to be tested on the focal plane of the microscope objective lens. The fiber laser array and the photomultiplier tube detector are both located at the conjugate plane of the sample plane. A pinhole is placed close to the detector. The pinhole and the fiber laser array are in an imaging relationship with each other, thereby removing out-of-focus light outside the focal plane and improving detection accuracy.

10. The multi-beam confocal mask defect detection method based on fiber laser array according to claim 8, characterized in that: The differential confocal module also includes the following steps to improve the detection accuracy: The built-in detection light source point, microscope objective focal plane and detector pinhole of the differential confocal module are in a conjugate relationship with each other. The scattered light generated at the focal plane of the microscope objective is normally collected by the detector through the detector pinhole. As for the scattered light at the non-focal plane, its imaging position deviates from the pinhole position, and only a small part of the scattered light can be collected by the detector through the pinhole, thereby eliminating the interference of out-of-focus light outside the focal plane.

Images