Filtering structure, optical detection system and optical detection method

Abstract

The invention relates to the technical field of optical detection, in particular to a filtering structure, an optical detection system and an optical detection method.The filtering structure comprises a first filtering assembly used for filtering secondary diffraction signals of structural units periodically arranged in an object to be detected; wherein the first filtering assembly comprises a plurality of light blocking elements which are arranged in parallel, the projection of the light blocking elements on the plane where the secondary diffraction light spots are located is not smaller than the diameter of the secondary diffraction light spots, and at least the accuracy of obtaining defect signals of an object to be detected can be improved.

Description

Technical Field

[0001] The present invention belongs to the field of optical detection technology, and in particular relates to a filtering structure, an optical detection system and an optical detection method. Background Art

[0002] Optical imaging detection is the main technology relied upon by the semiconductor industry for process defect detection. With the continuous development of technology, the semiconductor industry's process has shrunk from the micron level to the nanometer level, and more complex patterns require more sophisticated defect detection technology.

[0003] In the imaging detection system, the laser beam is irradiated at a certain angle onto the characteristic area of ​​the object to be detected and / or the rough surface of the object to be detected. The scattered light of the frequency domain information of the characteristic area and / or the scattered light on the rough surface of the object to be detected is collected by the objective lens system, and the parallel coherent light with the same incident angle on the pupil plane forms a light spot, which is then re-imaged on the sensing surface of the detector through the tube lens system.

[0004] However, as the periodic structural units in the object to be detected become smaller, the defect signals in the object to be detected will be buried in the scattered light signals of the periodic structural units and / or the rough surface of the object to be detected, resulting in low accuracy in extracting the defect signals in the object to be detected. Summary of the invention

[0005] In view of this, the present invention aims to provide a filtering structure, an optical detection system and an optical detection method, which are at least helpful in improving the accuracy of obtaining defect signals of an object to be detected.

[0006] To achieve the above object, the technical solution created by the present invention is implemented as follows: On the one hand, the present invention provides a filtering structure, including: a first filtering component, the first filtering component is used to filter the secondary diffraction signal of the periodically arranged structural units in the object to be detected; wherein the first filtering component includes a plurality of light-blocking elements arranged in parallel, and the projection of the light-blocking elements on the plane where the secondary diffraction spot is located is not less than the diameter of the secondary diffraction spot.

[0007] Furthermore, the filtering structure is used to be arranged at the position where the pupil plane is located in the optical detection system, and the number N of light blocking elements satisfies the following formula: ; Where D is the diameter of the pupil plane and d is the width of the light blocking element.

[0008] Furthermore, the plurality of light blocking elements are arranged in at least one plane, and each plane in the at least one plane is perpendicular to a propagation direction of a scattered light beam of the object to be detected.

[0009] Furthermore, the plurality of light blocking elements are arranged on a plurality of planes, and the light blocking elements located in different planes are partially stacked.

[0010] Furthermore, multiple light-blocking elements located in the same plane are arranged at intervals along a first direction, and the filtering structure also includes a first driving component. The multiple light-blocking elements located in the same plane correspond to the same first driving component, and the first driving component is used to drive the multiple light-blocking elements to move along the first direction so that the light-blocking elements block the secondary diffraction signals at corresponding positions.

[0011] Furthermore, for a plurality of light-blocking elements located on the same plane, the first driving assembly can independently drive at least one of the plurality of light-blocking elements to move along the first direction.

[0012] Furthermore, the first driving assembly includes a first driving unit and a second driving unit, and the first driving unit is connected to the second driving unit; wherein the second driving unit rotates to select a light-blocking element to be driven to move, and the first driving unit drives the second driving unit to move along the first direction, so that the second driving unit drives the selected light-blocking element to move along the first direction.

[0013] Furthermore, the first driving component also includes a first guide rail and multiple sliders, the sliders correspond to the light blocking elements one by one, the light blocking elements are connected to the corresponding sliders, and the multiple sliders are arranged on the first guide rail; wherein the second driving unit rotates to select the slider to be pushed.

[0014] Furthermore, the lower surface of the slider has a boss, the first drive component includes a displacement motor, the second drive component includes a rotary motor, the rotary motor has a stop pin on its shaft, the shaft is located on the side of the slider having the boss, and multiple bosses are staggered from each other along the circumference of the shaft; wherein the rotary motor drives the stop pin to rotate via the shaft to select the boss opposite to the stop pin, and the displacement motor drives the rotary motor to move in a first direction so that the stop pin pushes the opposite boss to move in the first direction.

[0015] Furthermore, the edge of the light blocking element is configured as a non-sharp edge structure.

[0016] Furthermore, the light blocking element includes a strip-shaped light blocking element, a cylindrical light blocking element, a sawtooth-shaped light blocking element or a wavy light blocking element.

[0017] Furthermore, the filtering structure also includes a second filtering component, which is used to filter the main diffraction signal of the periodically arranged structural units in the object to be detected. The second filtering component includes at least one filtering unit with a closed outer periphery, and a shading structure connecting any two ends of the filtering unit is arranged in the filtering unit.

[0018] Furthermore, the shading structure includes: a main body, two ends of the main body are connected to any two ends of the filtering unit, and the main body is used to filter the main diffraction signal of the periodically arranged structural units in the object to be detected; a protruding portion, the protruding portion extends from the main body to the filtering unit, the position of the protruding portion corresponds to the position of the scattered light spot on the rough surface of the object to be detected, and the shape of the protruding portion corresponds to the shape of the scattered light spot on the rough surface of the object to be detected.

[0019] Furthermore, the protrusion includes an arched light-blocking structure arranged at one end of the main body, an arched light-blocking structure arranged at both ends of the main body, a polygonal light-blocking structure arranged at any position of the main body, or a circular light-blocking structure arranged at any position of the main body.

[0020] Furthermore, the filtering structure is used to be arranged at the position of the pupil plane in the optical detection system, and the second filtering component includes multiple filtering units, the multiple filtering units are arranged in the same plane, and the distance between the centers of two adjacent filtering units is not less than the diameter of the pupil plane.

[0021] Furthermore, the filtering structure also includes a second driving component, the second filtering component is arranged on the second driving component, and the multiple filtering units are driven by the second driving component. When the object to be detected is switched, different filtering units are driven by the second driving component to overlap with the pupil plane.

[0022] Furthermore, the inner peripheral wall of the filter unit and the edge of the light shielding structure are configured as non-sharp edge structures.

[0023] Furthermore, the surface of the first filter component and / or the second filter component is coated with an anti-reflective film to reduce the reflectivity of the first filter component and / or the second filter component.

[0024] Furthermore, the extending direction of the light shielding structure is perpendicular to the extending direction of the light blocking element.

[0025] Another aspect of the present invention provides an optical detection system, comprising: a light source, used to emit a coherent light beam of a preset band to an object to be detected at a preset angle; an objective lens, used to receive a scattered light beam of the object to be detected, and focus a parallel light beam or a nearly parallel light beam in the scattered light beam to a pupil plane; a filtering structure such as any of the above items, the filtering structure being arranged at the position where the pupil plane is located; a tube lens system, used to image the object to be detected onto a detector; and a detector, used to generate an image of the object to be detected.

[0026] Furthermore, the optical detection system also includes: a cut-through reflector, a pupil imaging tube lens and a pupil imaging detector; wherein the cut-through reflector is used to cut between the tube lens system and the detector to change the propagation direction of the scattered light beam so that the scattered light beam is incident on the pupil imaging tube lens; or, the cut-through reflector is used to cut between the filtering structure and the tube lens system to change the propagation direction of the scattered light beam so that the scattered light beam is incident on the pupil imaging tube lens; the pupil imaging detector receives the light beam emitted by the pupil imaging tube lens for imaging.

[0027] Further, the filtering structure includes a first filtering component and a second filtering component, wherein the first filtering component includes a plurality of light blocking elements arranged in a plurality of planes, one of the plurality of planes is arranged to coincide with the pupil plane, the other planes in the plurality of planes are arranged 0.1 mm to 2 mm in front of the pupil plane along the propagation direction of the scattered light beam, and the second filtering component is arranged 0.1 mm to 2 mm behind the pupil plane along the propagation direction of the scattered light beam; or, the other planes in the plurality of planes are arranged 0.1 mm to 2 mm behind the pupil plane along the propagation direction of the scattered light beam, and the second filtering component is arranged 0.1 mm to 2 mm in front of the pupil plane along the propagation direction of the scattered light beam.

[0028] Furthermore, the optical detection system also includes a beam expansion and shaping module arranged between the light source and the object to be detected, wherein the beam expansion and shaping module includes a beam expander and a shaping element, and the shaping element includes a phase element with a two-dimensional structure.

[0029] The invention also provides an optical detection method, including: using a pupil imaging detector to observe the image of the pupil plane to obtain the distribution state of the scattered light beam in the pupil plane; driving the first filter component to filter the secondary diffraction signal of the periodically arranged structural units in the object to be detected in the scattered light beam; driving the adapted filter unit in the second filter component to filter both or the latter of the scattered signal of the rough surface of the object to be detected in the scattered light beam and the main diffraction signal of the periodically arranged structural units in the object to be detected.

[0030] Compared with the prior art, the invention can achieve the following beneficial effects: The filtering structure provided in an embodiment of the present invention includes a first filtering component. When performing defect detection on an object to be detected, the secondary diffraction signal of the periodically arranged structural units in the object to be detected can be filtered by the first filtering component, thereby improving the accuracy of obtaining the defect signal in the object to be detected. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation on the present invention. In the drawings: Figure 1 A schematic diagram of a first filter component provided by an embodiment of the present invention; Figure 2 A schematic diagram of another first filter component provided by an embodiment of the present invention; Figure 3 A schematic diagram of a stacked light-blocking element provided in an embodiment of the present invention; Figure 4 A schematic diagram of an enlarged structure of a slider and a light-blocking element provided in an embodiment of the present invention; Figure 5 A schematic structural diagram of a first filter assembly and a first drive assembly provided in an embodiment of the present invention; Figure 6 A schematic diagram of a structure including a carrier, a first filter assembly and a first drive assembly provided in an embodiment of the present invention; Figure 7 A schematic diagram of a rotating electrical machine provided by an embodiment of the present invention; Figure 8 A cross-sectional view of a portion of a first drive assembly provided by an embodiment of the present invention on a cross section perpendicular to a first direction; Fig. 9 A schematic diagram of a second filter component provided by an embodiment of the present invention; Fig.10 Schematic diagram of another second filter component provided by an embodiment of the present invention Fig.11 A schematic structural diagram of a servo motor, a ball screw, a second guide rail and a first filter assembly provided in an embodiment of the present invention; Fig.12 A schematic diagram of an optical detection system provided by an embodiment of the present invention; Fig.13 A schematic diagram of the positions of the second filter component and the first filter component provided in an embodiment of the present invention; Fig.14 A schematic diagram of another positional relationship between the second filter component and the first filter component provided in an embodiment of the present invention; Fig.15 A schematic diagram of another optical detection system provided by an embodiment of the present invention; Fig.16 A schematic flow chart of an optical detection method provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0032] After analysis, it was found that the object to be inspected is a wafer. In the process of using optical imaging detection to detect wafer defects, the sources of optical noise are mainly divided into two categories. The first category is the diffraction of the incident light by the wafer background pattern, which specifically includes the main diffraction signal and the secondary diffraction signal. Since the background pattern is regularly distributed, its diffraction light is discretely distributed in the spatial frequency domain, and its distribution pattern is determined by the background pattern. The main diffraction signal is distributed in the main plane, with high intensity and one-dimensional characteristics. The main diffraction signal distribution of different graphics is different, but they are all concentrated in the central strip. The second type of optical noise is the scattered light caused by the rough substrate of the wafer. When the substrate of the wafer is relatively rough, the roughness of the substrate is random, and its scattered light is continuously distributed in the frequency domain.

[0033] In order to reduce the influence of optical noise on defect detection, the embodiment of the present invention adopts a specific filtering structure to filter out the light beam causing the optical noise, so as to improve the accuracy of obtaining the defect signal of the object to be detected.

[0034] In order to make the purpose, technical solution and advantages of the invention more clear, the invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the invention and do not constitute a limitation of the invention.

[0035] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0036] In the description of the invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the invention, unless otherwise specified, the meaning of "multiple" is two or more.

[0037] In the description of the invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the invention can be understood according to specific circumstances.

[0038] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments.

[0039] On the one hand, the present invention provides a filtering structure, including: a first filtering component, the first filtering component is used to filter the secondary diffraction signal of the periodically arranged structural units in the object to be detected; wherein the first filtering component includes a plurality of light-blocking elements 100 arranged in parallel, and the projection of the light-blocking element 100 on the plane where the secondary diffraction spot is located is not less than the diameter of the secondary diffraction spot.

[0040] In some embodiments, the object to be inspected may refer to a semiconductor structure with a pattern such as a wafer or chip. In other embodiments, the object to be inspected may also be glass or a mask. It should be noted that the embodiments of the present invention do not impose specific restrictions on the type of the object to be inspected.

[0041] It can be understood that when inspecting the object to be inspected, if the surface of the object to be inspected has periodically arranged structural units, the incident light beam will form a scattered light beam after being irradiated on the surface of the wafer. The scattered light beam of the periodically arranged structural units will form a light spot at the pupil plane of the objective system (that is, the back focal plane of the objective lens). Since the scattered light beam of the structural unit will affect the identification of defect scattered light, a filtering structure can be set at the pupil plane to filter and block the noise scattered light.

[0042] In some embodiments, the periodically arranged structural units may be structural units periodically arranged along the vertical and horizontal directions. The structural units may be structures such as grooves and protrusions. The embodiments of the present invention do not limit the specific form of the structural units.

[0043] It should be noted that the width of the light-blocking element 100 is related to the diameter of the secondary diffraction spot of the secondary diffraction signal of the periodically arranged structural units at the pupil plane, the length of the light-blocking element 100 is not less than the diameter of the pupil plane, and the number of light-blocking elements 100 is related to the diameter of the pupil plane and the width of each light-blocking element 100.

[0044] Figure 1 A schematic diagram of a first filter assembly composed of a plurality of light blocking elements 100 is given, referring to Figure 1Since the periodically arranged structural units in the object to be detected generally form a plurality of periodically arranged secondary diffraction spots at the pupil plane, it is necessary to set a plurality of light-blocking elements 100 arranged at intervals to block the plurality of periodically arranged secondary diffraction lights. It can be understood that in an actual detection scenario, if the periodically arranged structural units in the object to be detected are structural units periodically arranged along the vertical and horizontal directions, the plurality of light-blocking elements 100 can be arranged in parallel along the horizontal or vertical direction at the pupil plane, and if the periodically arranged structural units in the object to be detected are structural units periodically arranged along the second direction, the plurality of light-blocking elements 100 can be arranged in parallel along the second direction.

[0045] In some embodiments, the light blocking element 100 includes a strip-shaped light blocking element 100, a cylindrical light blocking element 100, a sawtooth-shaped light blocking element 100, or a wavy light blocking element 100. It can be understood that as long as the light blocking element 100 can block the secondary diffracted light, the embodiment of the present invention does not specifically limit the shape of the light blocking element 100. In some examples, the strip-shaped light blocking element 100 can be a strip-shaped light blocking stick.

[0046] In some embodiments, the light blocking element 100 is a strip-shaped light blocking element 100, and the width of the strip-shaped light blocking element 100 is associated with the diameter of the secondary diffraction light spot of the periodically arranged structural unit. For example, the wider the diameter of the secondary diffraction light spot, the wider the width of the strip-shaped light blocking element 100. In practical applications, the width of the strip-shaped light blocking element 100 is generally set to be slightly wider than the diameter of the secondary diffraction light spot. In order to allow the strip-shaped light blocking element 100 to completely block the secondary diffraction light, the length of the strip-shaped light blocking element 100 is generally set to be not less than the diameter of the pupil plane in the optical detection system. In addition, the number of the strip-shaped light blocking elements 100 is associated with the diameter of the pupil plane and the width of each strip-shaped light blocking element 100.

[0047] In some embodiments, the filtering structure is used to be arranged at the position where the pupil plane is located in the optical detection system, and the number N of the light blocking elements 100 satisfies the following formula: Wherein, D is the diameter of the pupil plane, and d is the width of the light blocking element 100. The number of light blocking elements 100 can be designed according to the above formula, thereby improving the convenience and accuracy of setting the light blocking elements 100.

[0048] In some embodiments, the material of the light blocking element 100 may be plastic, wood, or a composite material, etc. The embodiment of the present invention does not impose any specific limitation on the material of the light blocking element 100 .

[0049] In some embodiments, the plurality of light blocking elements 100 are disposed in at least one plane, and each plane in the at least one plane is perpendicular to a propagation direction of a scattered light beam of the object to be detected.

[0050] In some embodiments, multiple light blocking elements 100 are arranged on multiple planes, and parts of the light blocking elements 100 located in different planes are stacked. This is equivalent to widening the width of the light blocking element 100, thereby improving the accuracy of the light blocking element 100 receiving multiple secondary diffraction spots. For ease of understanding, Figure 3 A schematic diagram of a light blocking element 100 in a partially stacked arrangement is given.

[0051] In some embodiments, a plurality of light-blocking elements 100 located on the same plane are arranged at intervals along a first direction X, and the filtering structure further comprises a first driving assembly, and the plurality of light-blocking elements 100 located on the same plane correspond to the same first driving assembly, and the first driving assembly is used to drive the plurality of light-blocking elements 100 to move along the first direction X, so that the light-blocking elements 100 block the secondary diffraction signals at corresponding positions. Using a first driving assembly to adjust the positions of the plurality of light-blocking elements 100 in the same plane is conducive to reducing the number of driving devices in the first driving assembly and reducing the setting cost of the first driving assembly.

[0052] In some embodiments, for multiple light-blocking elements 100 located on the same plane, the first driving component can independently drive at least one of the multiple light-blocking elements 100 to move along the first direction X. That is to say, the first driving component can independently drive any one of the multiple light-blocking elements 100 to move at one time, and can also drive a plurality of continuous light-blocking elements 100 to move at one time. This is conducive to improving the flexibility of the first driving component in driving the light-blocking element 100 to move, so as to adapt to different types of objects to be detected and improve the convenience of detecting different types of objects to be detected.

[0053] For ease of understanding, the following example illustrates that a plurality of light blocking elements 100 are disposed on a plurality of planes: like Figure 2 As shown, for example, there are N*N (N is a positive integer greater than or equal to 2) secondary diffraction spots on the pupil plane. For the convenience of explanation, assuming that N=12, 6 light-blocking elements 100 can be arranged in the first plane, and the other 6 light-blocking elements 100 can be arranged in the second plane, and corresponding first driving components are arranged for the 6 light-blocking elements 100 in the first plane, and corresponding first driving components are arranged for the 6 light-blocking elements 100 in the second plane, and the corresponding first driving components are used to drive the 6 light-blocking elements 100 in the first plane to Figure 2 At the first position 501, the corresponding first driving assembly is used to drive the six light blocking elements 100 in the second plane to Figure 2 At the second position 502, 12*12 secondary diffraction light spots in the pupil plane can fall on the multiple light blocking elements 100, so that the secondary diffraction light is blocked by the light blocking elements 100.

[0054] It should be noted that Figure 2 The first position 501 and the second position 502 are located in two planes at different heights respectively.

[0055] In some embodiments, the first driving assembly includes a first driving unit and a second driving unit, and the first driving unit is connected to the second driving unit; wherein the second driving unit rotates to select a light-blocking element 100 to be moved, and the first driving unit drives the second driving unit to move along the first direction, so that the second driving unit drives the selected light-blocking element 100 to move along the first direction.

[0056] In some embodiments, the first driving component also includes a first guide rail and multiple sliders, the sliders correspond to the light blocking elements 100 one by one, the light blocking elements 100 are connected to the corresponding sliders, and the multiple sliders are arranged on the first guide rail; wherein the second driving unit rotates to select the slider to be pushed.

[0057] In some embodiments, the lower surface of the slider has a boss, the first drive component includes a displacement motor, the second drive component includes a rotary motor, the rotary motor has a stop pin on its shaft, the shaft is located on the side of the slider having the boss, and multiple bosses are staggered from each other along the circumference of the shaft; wherein the rotary motor drives the stop pin to rotate through the shaft to select the boss opposite to the stop pin, and the displacement motor drives the rotary motor to move in a first direction so that the stop pin pushes the opposite boss to move in the first direction.

[0058] Specifically, refer to Figures 4 to 8In some embodiments, the filtering structure further includes a carrier 105, a first driving component is arranged on the carrier 105, and a first filtering component is arranged on the first driving component; the first driving component includes: a first guide rail 102 and a plurality of sliders 101, the sliders 101 correspond to the light blocking elements 100 one by one, the light blocking elements 100 are connected to the corresponding sliders 101, the first guide rail 102 is arranged on the carrier 105, a plurality of sliders 101 are arranged at intervals along the first direction X on the first guide rail 102, and a boss 112 is provided on the lower surface of the slider 101; a displacement motor 104 and a rotation motor 103, the displacement motor 104 is arranged on the carrier 105, the mover of the displacement motor 104 is connected to the stator of the rotary motor 103, the rotating shaft 110 of the rotary motor 103 has a stop pin 111, the rotating shaft 110 is located on the side of the slider 101 having the boss 112, and along the circumference of the rotating shaft 110, a plurality of bosses 112 are staggered with each other; wherein, the rotary motor 103 drives the stop pin 111 to rotate through the rotating shaft 110 to select the boss 112 facing the stop pin 111, and the displacement motor 104 drives the rotary motor 103 to move along the first direction X, so that the stop pin 111 pushes the facing boss 112 to move along the first direction X. In this way, two motors can be used to realize the individual displacement adjustment of any light blocking element 100 in the same plane, and realize the displacement adjustment of multiple light blocking elements 100 arranged continuously in the same plane.

[0059] It can be understood that the stop pin 111 can be moved to the two ends of the boss 112 respectively, and then push the boss 112 in two different relative directions. Specifically, the rotating motor 103 can drive the stop pin 111 to rotate to the gap position between the two bosses 112, and the displacement motor 104 drives the rotating motor 103 to move, and the stop pin 111 moves along the first direction X without pushing the boss 112. When it moves to the end of the boss 112 to be pushed, the displacement motor 104 stops, and the rotating motor 103 drives the stop pin 111 to move so that the stop pin 111 is directly opposite to the end of the boss 112 to be pushed.

[0060] In some embodiments, the number of first guide rails 102 connected to the same slider 101 may be two, which is helpful to improve the guiding stability.

[0061] In some embodiments, an anti-collision piece is disposed on the slider 101 to ensure that there is a gap between adjacent sliders 101 , which helps to avoid collision between adjacent sliders 101 .

[0062] In some embodiments, in order to avoid the sharp edge of the light blocking element 100 causing the scattered light beam to diffract at the edge of the light blocking element 100, the edge of the light blocking element 100 can be set to a non-sharp edge structure, such as a sawtooth state or a wavy shape, etc. There is no specific restriction on the form of the non-sharp edge structure here. Setting the edge of the light blocking element 100 to a non-sharp edge structure is beneficial to reducing the diffraction phenomenon caused by the scattered light beam at the edge of the light blocking element 100, which is beneficial to improving the accuracy of obtaining the defect signal.

[0063] In some embodiments, reference Fig. 9 The filtering structure also includes a second filtering component, which is used to filter the main diffraction signal of the periodically arranged structural units in the object to be detected. The second filtering component includes at least one filtering unit 200 with a closed periphery, and a shading structure 201 connecting any two ends of the filtering unit 200 is provided in the filtering unit 200.

[0064] It should be noted that, in an actual detection scenario, the structural unit on the surface of the object to be detected will generate a bright light at the center of the pupil plane, that is, the main diffraction signal generated by the structural unit. In order to filter out the main diffraction signal, the embodiment of the present invention sets a shading structure 201 in the closed filter unit 200 of the second filter component to filter out the main diffraction signal generated by the structural unit on the surface of the object to be detected at the center of the pupil plane. The width of the main diffraction signal at the pupil plane is directly related to the numerical aperture NA of the illumination light source. Therefore, the width of the shading structure 201 is related to the numerical aperture NA of the illumination light source. When the numerical aperture NA of the illumination light source is larger, the width of the shading structure 201 is wider, and vice versa, the width of the shading structure 201 is narrower.

[0065] In some embodiments, the angle between the extension direction of the light shielding structure 201 and the extension direction of the light shielding element 100 can be in the range of 0° to 90°. Preferably, the extension direction of the light shielding structure 201 is perpendicular to the extension direction of the light shielding element 100, so that the light shielding element 100 can shield more scattered signals that are not blocked by the light shielding structure 201, which is conducive to improving the filtering effect of the noise scattered signal.

[0066] In some embodiments, the closed area of ​​the filter unit 200 may be circular, square, rectangular or polygonal, and the shape of the closed area of ​​the filter unit 200 is not specifically limited herein.

[0067] When the filtering structure includes a first filtering component and a second filtering component, double filtering can be achieved on the same pupil plane, thereby improving the filtering effect. In this way, there is no need to add a relay optical device to achieve two pupil planes, and thus no additional optical errors such as aberrations are introduced, thereby ensuring detection accuracy.

[0068] In some embodiments, reference Fig.10 , the light shielding structure 201 includes: a main body 210, the two ends of the main body 210 are connected to any two ends of the filter unit 200, and the main body 210 is used to filter the main diffraction signal of the periodically arranged structural units in the object to be detected; a protrusion 211, the protrusion 211 extends from the main body 210 to the filter unit 200, the position of the protrusion 211 corresponds to the position of the scattered light spot on the rough surface of the object to be detected, and the shape of the protrusion 211 corresponds to the shape of the scattered light spot on the rough surface of the object to be detected. That is, the second filter component can also be used to filter the scattered light on the rough surface of the object to be detected. In this way, the second filter component can be used to filter out most of the noise scattered signals of the object to be detected, which is conducive to improving the versatility of the second filter component and the signal-to-noise ratio of defect detection.

[0069] In actual detection scenarios, the position of the rough surface in the object to be detected is different, and the size and position of the scattered light spot formed at the pupil plane are also uncertain. Therefore, a protrusion 211 corresponding to the scattered light spot formed by the rough surface at the pupil plane can be set. The protrusion 211 is used to filter the scattered light formed by the rough surface at the pupil plane to reduce the influence of optical noise on defect detection.

[0070] In some embodiments, the protrusion 211 includes an arcuate light-blocking structure disposed at one end of the main body 210, an arcuate light-blocking structure disposed at both ends of the main body 210, a polygonal light-blocking structure disposed at any position of the main body 210, or a circular light-blocking structure disposed at any position of the main body 210. For ease of understanding, Fig.10 A, B and C in the figure respectively give the corresponding schematic diagrams. The present invention does not impose any specific restrictions on the shape of the protrusion 211, as long as the protrusion 211 can filter the scattered light from the rough surface.

[0071] In some embodiments, the protrusions 211 are symmetrically disposed on opposite sides of the main body 210 .

[0072] In some embodiments, the second filter assembly includes a plurality of filter units 200 , the plurality of filter units 200 are arranged on the same plane, and the distance between the centers of two adjacent filter units 200 is not less than the diameter of the pupil plane.

[0073] In some embodiments, the filtering structure further includes a second driving component, the second filtering component is arranged on the second driving component, and the plurality of filtering units 200 are driven by the second driving component, and when the object to be detected is switched, different filtering units 200 are driven by the second driving component to overlap with the pupil plane. The reason for such arrangement is that since the position and shape of the rough surface in different types of objects to be detected are uncertain, in order to filter the scattered light of the rough surface of different types of objects to be detected, the second driving component can be used to switch the filtering unit 200, and the corresponding filtering unit 200 is selected to overlap with the pupil plane according to the type of the rough surface of the object to be detected, thereby filtering the scattered light of the corresponding rough surface.

[0074] In some embodiments, the second driving assembly is also disposed on the carrier 105 .

[0075] It should be noted that the forms of multiple filtering units 200 can be determined according to the type of objects to be detected. When there are three or four objects to be detected, three or four filtering units 200 can be set accordingly, and the three or four filtering structures are arranged in the same plane and driven by the second driving component. In this way, when detecting a certain type of object to be detected, the corresponding filtering unit 200 is used for filtering, thereby improving the adaptability of the second filtering component to different detection scenarios.

[0076] It should be noted that one type of object to be detected can correspond to a filtering unit 200 of one structure, but in actual detection scenarios, one object to be detected can correspond to multiple filtering units 200, or multiple objects to be detected can correspond to the same filtering unit 200. There is no specific restriction on the types of objects to be detected corresponding to different filtering units 200.

[0077] As a specific embodiment of the second driving component, the embodiment of the present invention can use a servo motor to drive a ball screw to push the second filter component to slide along the second guide rail, so as to switch different filter units 200 when detecting different types of objects to be detected. For ease of understanding, Fig.11 A schematic diagram of the servo motor 401 , the ball screw 402 , the second guide rail 403 and the filter unit 200 is given.

[0078] Specifically, in the process of switching the filter unit 200, in order to avoid mutual interference between two adjacent filter units 200, the embodiment of the present invention can also set the distance between the centers of two adjacent filter units 200 to be no less than the diameter of the pupil plane, so as to avoid the two filter units 200 filtering the scattered light in the pupil plane at the same time, thereby facilitating improving the filtering accuracy of the filter unit 200.

[0079] In some embodiments, the inner peripheral wall of the filter unit 200 and the edge of the light shielding structure 201 are set to a non-sharp edge structure, such as a sawtooth state, or a wavy shape, etc., and the shape of the non-sharp edge structure is not specifically limited here. The inner peripheral wall of the filter unit 200 and the edge of the light shielding structure 201 are set to a non-sharp edge structure, thereby reducing the diffraction phenomenon caused by the scattered light beam at the edge of the light shielding structure, which is conducive to improving the signal-to-noise ratio of defect detection, and further conducive to improving the accuracy of obtaining defect signals in the object to be detected.

[0080] In some embodiments, the surface of the first filter assembly and / or the second filter assembly is coated with an anti-reflective film to reduce the reflectivity of the first filter assembly and / or the second filter assembly. Specifically, the surface of the second filter assembly and / or the first filter assembly may be blackened to reduce the reflectivity of the second filter assembly and / or the first filter assembly.

[0081] When the filtering structure includes a second filtering component and a first filtering component, because the embodiment of the present invention requires that both the second filtering component and the first filtering component are arranged at the position where the pupil plane is located, and because the diameter of the secondary diffraction light spot generated by the periodically arranged structural units is relatively small, in order to achieve accurate filtering of the secondary diffraction light generated by the periodically arranged structural units, the embodiment of the present invention sets the first filtering component at the pupil plane, and sets the second filtering component at a position 0.1 mm to 2 mm in front of or behind the pupil plane (such as setting the second filtering component at a distance of ±1 mm from the pupil plane), so that the second filtering component is as close to the position of the pupil plane as possible, so as to achieve accurate filtering of the scattered light formed by the rough surface of the object to be detected and the main diffraction signal generated by the periodic structural units.

[0082] When the first filtering component is arranged in multiple planes, one of the multiple planes can also be arranged at the pupil plane, and the other planes can be arranged at a position of 0.1 mm to 2 mm in front of or behind the pupil plane, so that the other planes are also as close as possible to the position of the pupil plane, so as to achieve accurate filtering of the secondary diffraction signal of the periodic structure unit of the object to be detected.

[0083] The second filter component and the first filter component in the embodiment of the present invention can realize double filtering at the position of the same pupil plane, thereby improving the filtering effect on optical noise. In view of the need for efficient filtering of optical noise in wafer dark field defect detection, filtering of two types of noise is realized on the same pupil plane. Compared with the solution of adding a relay optical device to the optical detection system to realize two pupil planes in the prior art, optical errors such as aberrations are not introduced, thereby ensuring detection accuracy. In addition, in the filtering structure provided by the embodiment of the present invention, the second filter component is used to filter the scattered signal of the rough surface of the object to be detected and the main diffraction signal of the periodically arranged structural unit, or the latter, and the first filter component is used to filter the secondary diffraction signal of the periodically arranged structural unit in the object to be detected, so that when detecting defects of the object to be detected, noise can be better suppressed, the signal-to-noise ratio can be improved, and the defect detection accuracy can be improved.

[0084] Another aspect of the present invention provides an optical detection system, referring to Fig.12 The optical detection system includes: a light source 801, which is used to emit a coherent light beam of a preset wavelength band to the object to be detected 806 at a preset angle. In some examples, the light source 801 can be a red laser, a blue laser or an LED light source 801, etc. The embodiment of the present invention does not specifically limit the type of the light source 801; an objective lens 802, which is used to receive a scattered light beam of the object to be detected 806 and focus a parallel light beam or a nearly parallel light beam in the scattered light beam to a pupil plane, and the pupil plane is the back focal plane of the objective lens 802; a filtering structure 803 as any of the above items, the filtering structure 803 is arranged at the position where the pupil plane is located, and the filtering structure 803 is used to focus the periodic array in the object to be detected 806 The secondary diffraction signal of the structural unit of the cloth is filtered, and / or, both or the latter of the scattering signal of the rough surface of the object to be detected 806 and the main diffraction signal of the periodically arranged structural unit in the object to be detected 806 are filtered. It should be noted that the filtering structure 803 is similar to the description in the above-mentioned embodiment and can refer to each other, and will not be repeated here; the tube lens system 804 is used to image the object to be detected 806 to the detector 805; the detector 805 is used to generate an image of the object to be detected 806. Specifically, the detector 805 can be a photodiode, a photodetector or an avalanche photodetector, etc., and there is no specific limitation on the type of the detector 805.

[0085] In the embodiment of the present invention, the optical inspection system is used to detect defects in the object to be inspected 806. During the inspection process, the first filter component in the filter structure 803 is used to filter the secondary diffraction signal of the periodically arranged structural units in the object to be inspected 806, and the second filter component is used to filter both or the latter of the scattering signal of the rough surface of the object to be inspected 806 and the main diffraction signal of the periodically arranged structural units. Therefore, when detecting defects in the object to be inspected 806, the accuracy of obtaining the defect signal in the object to be inspected 806 can be improved.

[0086] In some embodiments, the edges of the second filter component and / or the first filter component in the filtering structure 803 are set to non-sharp edge structures, thereby avoiding the diffraction phenomenon caused by the second filter component and / or the first filter component to the scattered light beam, thereby improving the Poisson scattering resolution of the optical detection system, so that even if the resolution of the detector 805 is smaller than the Poisson scattering resolution of the optical detection system, defects in the object to be detected 806 can be detected.

[0087] Among them, the Poisson scattering resolution of the optical detection system is also called the PSF (Point Spread Function) resolution accuracy, which is used to evaluate the imaging quality of the optical detection system. Because the edges of the non-sharp structures of the second filter component and / or the first filter component can suppress the diffraction phenomenon caused by the scattered light beam, the light energy in the imaging of the object to be detected 806 is mainly concentrated in the center, so that even if the resolution of the detector 805 is smaller than the Poisson scattering resolution of the optical detection system, defects in the object to be detected 806 can also be detected.

[0088] In some embodiments, reference Fig.15 The optical detection system further includes: a cuttable reflector 807, a pupil imaging tube lens 808 and a pupil imaging detector 809; wherein the cuttable reflector 807 is used to cut between the tube lens system 804 and the detector 805 to change the propagation direction of the scattered light beam so that the scattered light beam is incident on the pupil imaging tube lens 808; or, the cuttable reflector 807 is used to cut between the filter structure 803 and the tube lens system 804 to change the propagation direction of the scattered light beam so that the scattered light beam is incident on the pupil imaging tube lens 808; the pupil imaging detector 809 receives the light beam emitted by the pupil imaging tube lens 808 for imaging. In this way, the observation of the noise spot in the pupil plane can be realized, and then the corresponding filter structure 803 is set according to the noise spot.

[0089] In some embodiments, the filter structure 803 includes a first filter component 8032 and a second filter component 8031, wherein the first filter component 8032 includes a plurality of light blocking elements 100 arranged on a plane, Fig.13 The first filter component 8032 is arranged to coincide with the pupil plane, and the second filter component 8031 ​​is arranged 0.1 mm to 2 mm, for example, 1 mm, behind the pupil plane along the propagation direction of the scattered light beam.

[0090] In some embodiments, the filter structure 803 includes a first filter component 8032 and a second filter component 8031, wherein the first filter component 8032 includes a plurality of light blocking elements 100 disposed on a plurality of planes, Fig.14 , one of the multiple planes is arranged to coincide with the pupil plane, the other planes in the multiple planes are arranged 0.1mm to 2mm in front of the pupil plane along the propagation direction of the scattered light beam, for example, 0.5mm, and the second filter component 8031 ​​is arranged 0.1mm to 2mm behind the pupil plane along the propagation direction of the scattered light beam, for example, 1mm; or, the other planes in the multiple planes are arranged 0.1mm to 2mm behind the pupil plane along the propagation direction of the scattered light beam, and the second filter component 8031 ​​is arranged 0.1mm to 2mm in front of the pupil plane along the propagation direction of the scattered light beam. The reason for this arrangement is that since the diameter of the diffraction spot generated by the periodically arranged structural units of the object to be detected 806 is small, in order to achieve accurate filtering of the diffraction light generated by the periodically arranged structural units, the first filter component 8032 is set at the pupil plane, and the second filter component 8031 ​​is as close to the pupil plane as possible, thereby achieving accurate filtering of the secondary diffraction signal generated by the periodic structural units in the object to be detected 806.

[0091] In some embodiments, reference Fig.15 The optical detection system further includes a beam expansion and shaping module 810 disposed between the light source 801 and the object to be detected 806, wherein the beam expansion and shaping module 810 includes a beam expander and a shaping element, and the shaping element includes a phase element with a two-dimensional structure. In this way, the spot size of the incident light beam irradiated to the surface of the object to be detected 806 can be controlled, wherein the beam expander is used to expand the light source 801 to obtain a spot of a preset size, and the shaping element is used to realize the shaping of the expanded light beam to achieve the purpose of controlling the numerical aperture NA of the incident light.

[0092] The present invention also provides an optical detection method, including: using a pupil imaging detector 805 to observe the image of the pupil plane to obtain the distribution state of the scattered light beam in the pupil plane; driving the first filter component to filter the secondary diffraction signal of the periodically arranged structural units in the object to be detected in the scattered light beam; driving the adapted filter unit 200 in the second filter component to filter both or the latter of the scattered signal of the rough surface of the object to be detected in the scattered light beam and the main diffraction signal of the periodically arranged structural units in the object to be detected.

[0093] The following is based on Fig.15 The optical detection system in the embodiment of the present invention describes a specific implementation of the optical detection method, and refers to Fig.15 and Fig.16 , optical detection methods include: 1201. The image of the pupil surface is observed by using the pupil imaging detector 805 to obtain the distribution state of the scattered signal in the pupil surface. Specifically, the cut-in reflector 807, the pupil imaging tube lens 808 and the pupil imaging detector 809 can cooperate with each other to image the pupil surface to the pupil imaging detector 809, so that the image of the pupil surface can be observed by the pupil imaging detector 809. In this way, the distribution state of the scattered signal in the pupil surface can be obtained. Therefore, according to the position and shape of the scattered signal of the rough surface of the object to be detected, the second filter component is selected to be adapted, that is, according to the position and shape of the scattered signal of the rough surface of the object to be detected, Fig.10 An adaptive filtering unit 200 is selected in the second filtering component.

[0094] 1202. Drive a first filtering component to filter a secondary diffraction signal of a structural unit in the object to be detected in the scattering signal; 1203. Drive the adapted filtering unit 200 to filter both of the scattered signals of the rough surface of the object to be detected and the main diffraction signals of the periodically arranged structural units in the object to be detected, or the latter.

[0095] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0096] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A filtering structure, characterized in that: include: A first filtering component, the first filtering component is used to filter the secondary diffraction signal of the periodically arranged structural units in the object to be detected; Wherein, the first filtering component comprises a plurality of light-blocking elements arranged in parallel, and the projection of the light-blocking elements on the plane where the secondary diffraction spot is located is not less than the diameter of the secondary diffraction spot.

2. The filtering structure according to claim 1, characterized in that: The filtering structure is used to be arranged at the position where the pupil plane is located in the optical detection system, and the number N of the light blocking elements satisfies the following formula: ； Wherein, D is the diameter of the pupil plane, and d is the width of the light blocking element.

3. The filtering structure according to claim 1, characterized in that: The plurality of light blocking elements are arranged in at least one plane, and each of the at least one plane is perpendicular to a propagation direction of the scattered light beam of the object to be detected.

4. The filtering structure according to claim 3, characterized in that: The plurality of light blocking elements are arranged on a plurality of planes, and the light blocking elements located in different planes are partially stacked.

5. The filtering structure according to claim 3 or 4, characterized in that: Multiple light-blocking elements located in the same plane are arranged at intervals along a first direction. The filtering structure also includes a first driving component. The multiple light-blocking elements located in the same plane correspond to the same first driving component. The first driving component is used to drive the multiple light-blocking elements to move along the first direction so that the light-blocking elements block the secondary diffraction signals at corresponding positions.

6. The filtering structure according to claim 5, characterized in that: For a plurality of light-blocking elements located on the same plane, the first driving assembly can independently drive at least one of the plurality of light-blocking elements to move along the first direction.

7. The filtering structure according to claim 6, characterized in that: The first driving assembly includes a first driving unit and a second driving unit, and the first driving unit is connected to the second driving unit; The second driving unit rotates to select a light-blocking element to be driven to move, and the first driving unit drives the second driving unit to move along the first direction, so that the second driving unit drives the selected light-blocking element to move along the first direction.

8. The filtering structure according to claim 7, characterized in that: The first driving assembly further includes a first guide rail and a plurality of sliders, wherein the sliders correspond to the light blocking elements one by one, the light blocking elements are connected to corresponding sliders, and the plurality of sliders are arranged on the first guide rail; The second driving unit rotates to select a sliding block to be pushed.

9. The filtering structure according to claim 8, characterized in that: The lower surface of the slider has a boss, the first drive assembly includes a displacement motor, the second drive assembly includes a rotary motor, a stop pin is provided on the rotary shaft of the rotary motor, the rotary shaft is located on a side of the slider having the boss, and a plurality of the bosses are staggered with each other along the circumference of the rotary shaft; The rotating motor drives the stop pin to rotate via the rotating shaft to select a boss directly opposite to the stop pin, and the displacement motor drives the rotating motor to move along the first direction so that the stop pin pushes the boss directly opposite to the stop pin to move along the first direction.

10. The filtering structure according to claim 1, characterized in that: The edge of the light blocking element is configured as a non-sharp edge structure.

11. The filtering structure according to claim 1, characterized in that: The light blocking element includes a strip-shaped light blocking element, a cylindrical light blocking element, a sawtooth-shaped light blocking element or a wave-shaped light blocking element.

12. The filtering structure according to claim 1, characterized in that: The filtering structure also includes a second filtering component, which is used to filter the main diffraction signal of the periodically arranged structural units in the object to be detected. The second filtering component includes at least one filtering unit with a closed outer periphery, and a shading structure connecting any two ends of the filtering unit is arranged in the filtering unit.

13. The filtering structure according to claim 12, characterized in that: The shading structure comprises: A main body, two ends of which are connected to any two ends of the filtering unit, and the main body is used to filter the main diffraction signal of the periodically arranged structural units in the object to be detected; A protrusion extends from the main body toward the filter unit, the position of the protrusion corresponds to the position of the scattered light spot on the rough surface of the object to be detected, and the shape of the protrusion corresponds to the shape of the scattered light spot on the rough surface of the object to be detected.

14. The filtering structure according to claim 13, characterized in that: The protrusion includes an arched light blocking structure arranged at one end of the main body, an arched light blocking structure arranged at both ends of the main body, a polygonal light blocking structure arranged at any position of the main body, or a circular light blocking structure arranged at any position of the main body.

15. The filtering structure according to claim 12, characterized in that: The filtering structure is used to be arranged at the position of the pupil plane in the optical detection system. The second filtering component includes a plurality of filtering units. The plurality of filtering units are arranged in the same plane, and the distance between the centers of two adjacent filtering units is not less than the diameter of the pupil plane.

16. The filtering structure according to claim 15, characterized in that: The filtering structure also includes a second driving component, the second filtering component is arranged on the second driving component, the multiple filtering units are driven by the second driving component, and when the object to be detected is switched, different filtering units are driven by the second driving component to overlap with the pupil plane.

17. The filtering structure according to claim 12, characterized in that: The inner peripheral wall of the filter unit and the edge of the light shielding structure are configured as non-sharp edge structures.

18. The filtering structure according to claim 12, characterized in that: The surface of the first filter component and / or the second filter component is coated with an anti-reflective film to reduce the reflectivity of the first filter component and / or the second filter component.

19. The filtering structure according to claim 12, characterized in that: An extension direction of the light-shielding structure is perpendicular to an extension direction of the light-blocking element.

20. An optical detection system, characterized in that: include: A light source, used for emitting a coherent light beam of a preset wavelength band at a preset angle toward the object to be detected; An objective lens, used for receiving the scattered light beam of the object to be detected, and focusing a parallel light beam or a nearly parallel light beam in the scattered light beam to a pupil plane; The filter structure according to any one of claims 1 to 19, wherein the filter structure is arranged at the position where the pupil plane is located; A tube lens system, used for imaging the object to be detected onto a detector; The detector is used to generate an image of the object to be detected.

21. The optical detection system according to claim 20, characterized in that: The optical detection system also includes: a cut-in reflector, a pupil imaging tube lens, and a pupil imaging detector; The cuttable reflector is used to cut between the tube lens system and the detector to change the propagation direction of the scattered light beam so that the scattered light beam is incident on the pupil imaging tube lens; or the cuttable reflector is used to cut between the filter structure and the tube lens system to change the propagation direction of the scattered light beam so that the scattered light beam is incident on the pupil imaging tube lens; The pupil imaging detector receives the light beam emitted by the pupil imaging tube lens for imaging.

22. The optical detection system according to claim 20, characterized in that: The filtering structure comprises a first filtering component and a second filtering component, wherein the first filtering component comprises a plurality of light blocking elements arranged on a plurality of planes, one of the plurality of planes is arranged to coincide with the pupil plane, the other planes of the plurality of planes are arranged 0.1 mm to 2 mm in front of the pupil plane along the propagation direction of the scattered light beam, and the second filtering component is arranged 0.1 mm to 2 mm behind the pupil plane along the propagation direction of the scattered light beam; Alternatively, other planes among the multiple planes are arranged 0.1 mm to 2 mm behind the pupil plane along the propagation direction of the scattered light beam, and the second filter component is arranged 0.1 mm to 2 mm in front of the pupil plane along the propagation direction of the scattered light beam.

23. The optical detection system according to any one of claims 20 to 22, characterized in that: The optical detection system further comprises a beam expansion and shaping module disposed between the light source and the object to be detected, wherein the beam expansion and shaping module comprises a beam expander and a shaping element, and the shaping element comprises a phase element having a two-dimensional structure.

24. An optical detection method, characterized in that: include: Observing the image of the pupil plane using a pupil imaging detector to obtain the distribution state of the scattered light beam in the pupil plane; driving the first filter component to filter the secondary diffraction signal of the periodically arranged structural units in the object to be detected in the scattered light beam; The adapted filter unit in the second filter assembly is driven to filter both or the latter of the scattered signal of the rough surface of the object to be detected in the scattered light beam and the main diffraction signal of the periodically arranged structural units in the object to be detected.

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