Optical height measurement system for electron beam scanning imaging

Abstract

The invention belongs to the field of electron beam scanning imaging, and particularly relates to an optical height measurement system for electron beam scanning imaging, which comprises a transmitting optical module and a receiving optical module which are positioned on two opposite sides above a workpiece, the emission optical module comprises a light source assembly; the grating is used for projecting a light beam emitted by the light source assembly to a workpiece and reflecting the light beam to the receiving optical module to form a grating image; the polarization device is located at the downstream of the grating, the emitted light beam is separated into a first polarization component and a second polarization component, the first polarization component is transmitted out of the polarization device and projected to the surface of the workpiece to form a grating image, imaging interference in the view of the camera is improved, and the reliability of height measurement is improved; the receiving optical module comprises a double-telecentric lens group and an optical imaging sensor, the double-telecentric lens group projects reflected light beams on the surface of a workpiece to the optical imaging sensor in a double-telecentric mode, the height measurement system has the advantages of being large in depth of field, constant in amplification factor and small in distortion, and the precision and stability of the height measurement system are improved.

Description

Technical Field

[0001] The invention belongs to the field of electron beam scanning imaging, and in particular relates to an optical height measurement system for electron beam scanning imaging. Background Art

[0002] Electron beam scanning imaging equipment is the main equipment used for measuring key dimensions and detecting defects in integrated circuits during the production of semiconductor chips. One of its core technical points is to maintain stability during the electron beam scanning imaging process. However, during the entire working process, as the position of the workpiece table carrying the wafer moves, due to the influence of factors such as the mechanical structure, the stability of the drive components, and the flatness of the wafer, the surface height of the wafer area receiving the electron beam scanning will change to a certain extent, and the focal plane of the electron beam will also change accordingly. In order to ensure the clarity of the scanned image, it is necessary to accurately measure the height of the current scanning area and feed it back to the control system to match the electron beam focus.

[0003] At present, electron beam scanning imaging equipment used for wafer inspection generally uses triangulation to directly measure or estimate the height. The traditional triangulation height measurement system structure is as follows: Figure 1 As shown, single-point measurement is usually used, which includes: a laser 100, a transmitter lens 101, a receiver lens 102, and a photosensitive element 103. The laser emits laser light, and after reflection, the photosensitive element receives the laser light. When the height of the wafer changes, the light sensing position of the photosensitive element is different, so as to calculate the height change of the wafer.

[0004] However, in actual working process, the traditional single-point measurement method is difficult to ensure the stability and consistency of the optical signal after reflection from the wafer due to changes in conditions such as wafer material, integrated circuit graphic design, and process nodes. This will lead to increased deviation or even loss of height measurement or estimation.

[0005] Chinese invention patent CN 108548490 B discloses a method for determining the height of an object, comprising: projecting light from a light source onto the surface of the object through a grating, and then reflecting the light onto the imaging surface of the optical imaging sensor to form the grating image; using the aforementioned method to determine the displacement of the grating image on the imaging plane, and determining the height based on a linear relationship between the displacement of the grating image on the imaging plane and the height of the object.

[0006] The height control stability requirement of the workpiece stage of the electron beam scanning imaging wafer inspection equipment usually needs to reach within tens to hundreds of nanometers. Under this stability requirement, the clarity of the optical system imaging has an important impact on the accuracy of the height measurement system. However, the optical height calculation method of the electron beam scanning imaging disclosed in the above patent depends on the displacement value of the grating image on the imaging plane. According to the disclosure in its specification, the calculation method for obtaining the displacement value of the grating image on the imaging plane is relatively complicated, which is not conducive to promotion and application; if the focus ring is adjusted in the optical receiving module, the height of the camera relative to the focus ring changes, which may cause image blur or distortion on the image side, reducing the height measurement accuracy and stability. Summary of the invention

[0007] Aiming at the problems of camera image blur or distortion, low imaging accuracy, low height measurement reliability and stability when the height of a workpiece changes in the existing electron beam scanning imaging height measurement system, the present invention provides an optical height measurement system for electron beam scanning imaging.

[0008] The optical height measurement system for electron beam scanning imaging comprises: a transmitting optical module and a receiving optical module located on opposite sides above a workpiece to be scanned; The transmitting optical module comprises: A light source assembly, for providing an emission light beam; A grating, located in the optical path of the emission light beam, for projecting the emission light beam onto the surface of the workpiece and reflecting it to the receiving optical module to form a grating image; A polarizing device, located downstream of the grating, for separating the emission light beam passing through the grating into a first polarization component and a second polarization component, wherein the first polarization component is transmitted through the polarizing device and projected onto the surface of the workpiece to form a grating image; The receiving optical module includes a double telecentric lens group and an optical imaging sensor; The double telecentric lens group double telecentrically projects the reflected light beam on the workpiece surface to the optical imaging sensor to form a grating image.

[0009] Preferably, the emission optical module further comprises an emission lens group located between the grating and the polarization device, and the emission lens group satisfies: f1 <d0<2×f1(1) d1 = N × d0 (2) Among them, d0 is the parallel center axis distance between the emitting lens group and the grating, f1 is the focal length of the emitting lens group, and d1 is the distance from the axial light of the emitting lens group to the wafer surface.

[0010] Furthermore, the double telecentric lens group includes a first receiving lens group, a biconcave lens group, and a second receiving lens group arranged in sequence along the reflected light path, and the arrangement of the double telecentric lens group satisfies: d3 = f2(3) d4 = f3(4) Wherein, d3 is the axial center distance between the first receiving lens group and the biconcave lens group, f2 is the focal length of the first receiving lens group, d4 is the axial center distance between the biconcave lens group and the second receiving lens group, and f3 is the focal length of the second receiving lens group.

[0011] Preferably, the polarization device comprises a polarization beam splitter, and the first polarization component and the second polarization component separated by the polarization beam splitter are respectively a p component and an s component; the light beam of the first polarization component is approximately totally reflected on the workpiece surface to the receiving optical module.

[0012] Preferably, the emitting optical module includes a coaxially mounted focusing ring, a centering ring, a mounting flange and an objective lens barrel, the light source assembly and the grating are installed in the focusing ring, and the emitting lens group and the polarization device are installed in the objective lens barrel; the focusing ring is used to adjust the object distance and image distance of the optical system, and the centering ring is used to adjust the measuring position of the emitting light beam on the surface of the workpiece to be consistent with the scanning position of the electron beam.

[0013] Preferably, the receiving optical module also includes a coaxially mounted lens, a second mounting flange, a second focusing ring, a second centering ring and a camera, the first receiving lens group, the second receiving lens group and the biconcave lens group are locked to the axis of the lens, and the optical imaging sensor is fixed to the axis of the camera.

[0014] Wherein, the light source assembly comprises a light guide column and a light source, a concave and smooth light source groove is arranged on the light incident side of the light guide column, and a light emitting surface of the light source is tightly embedded in the light source groove; The outer wall of the light guide column is covered with a reflective film for reflecting the light emitted by the light source inside the light guide column; The grating is located at the light-emitting side of the light-guiding column, and the grating includes at least two groups of periodic grids, and the periods of the grids of each group are different.

[0015] Preferably, the light guide column comprises a light guide side wall located on the light incident side and a reflective end face; A plurality of tooth-shaped grooves are formed on the light-guiding side wall, and the tooth-shaped grooves include a first inclined surface for reflecting the light on both sides of the emission angle of the light source toward the reflecting end surface, and a second inclined surface for reflecting the light on both sides of the emission angle of the light source toward the light exit side of the light guide column; The side of the reflective end surface close to the light source is inclined toward the light source slot, and the reflective end surface reflects the light from the first inclined surface toward the light emitting side.

[0016] Preferably, a gap is provided between two adjacent tooth-shaped grooves, and the light from the second inclined surface is reflected into the gap and continues to be reflected toward the light emitting side.

[0017] Preferably, the depth of the tooth-shaped groove on the side close to the reflective end surface is greater than the depth of the tooth-shaped groove on the side away from the reflective end surface.

[0018] The beneficial effects of the present invention are: The present invention utilizes a polarization device to separate an emitted light beam into a first polarization component and a second polarization component, and filters the second polarization component that interferes with the quality of light reflected from the wafer surface, retains the first polarization component and projects it onto the wafer surface, thereby eliminating the influence of multiple reflections of refracted light caused by wafer material and process, obtaining a stable and clear grating illumination image, ensuring normal optical path imaging, avoiding interference of imaging in the camera field of view, and improving the reliability of height measurement.

[0019] A double telecentric lens group is arranged in the receiving optical module of the present invention. When the height of the workbench changes slightly, the object distance side telecentricity of the double telecentric lens group is utilized to realize that the imaging ratio within the depth of field remains unchanged, and the image distance side telecentricity ensures that the imaging ratio is stable when the focus ring is adjusted, thereby eliminating the perspective error and image blur problems caused by the change of wafer height. The system has the characteristics of large depth of field and small distortion, and ensures the accuracy and stability of the height measurement system.

[0020] The light source assembly of the present invention adopts a special light guide column structure to distribute the light emitted by the light source to the grating more evenly, thereby improving the clarity of the grating image. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings: Figure 1 It is a schematic diagram of an optical height measurement system of electron beam scanning imaging in the prior art; Figure 2 is a schematic diagram of the structural layout of optical elements of the optical altimeter system of the present invention; Figure 3 is a schematic diagram of the optical principle of the optical altimeter system of the present invention; Figure 4 It is a schematic diagram of a double telecentric lens group and its parameters of the present invention; Figure 5 is a schematic diagram of another double telecentric lens assembly of the present invention; Figure 6 is a schematic diagram of the polarization device of the present invention separating linearly polarized light; Figure 7 is a schematic diagram of partial light propagation on the light incident side of a light guide column according to an embodiment of the present invention; Figure 8 is a schematic diagram of partial light propagation on the light incident side of a light guide column according to another embodiment of the present invention; Fig. 9 yes Figure 8 A partial enlarged view of the .

[0022] The markings in the figure are: 100, light source base; 101, focus ring 1; 102, centering ring 1; 103, assembly flange 1; 104, objective lens barrel; 105, lens; 106, assembly flange 2; 107, focus ring 2; 108, centering ring 2; 109, camera; 200, light source; 201, light guide column; 2011, light source groove; 2012, reflective film; 2013, light guide side wall; 2014, reflective end face; 2015, serrated groove; 2016, first inclined plane; 2017, second inclined plane; 202, grating; 203, emitting lens group; 204, polarizer; 205, first receiving lens group; 206, second receiving lens group; 207, optical imaging sensor; 208, double concave lens group; 209, incident light beam; 210, first polarization component; 211, second polarization component. DETAILED DESCRIPTION

[0023] Example 1 like Figures 2 to 5 As shown, an optical height measurement system for electron beam scanning imaging includes a workbench, on which a sample to be scanned is placed. The sample in this application is described using a wafer as an example, but it can also be used to scan other types of samples with similar material properties to the wafer, not limited to semiconductor chips, semiconductor workpieces or other workpieces, such as storage disks, etc. An emitting optical module and a receiving optical module are arranged oppositely on the left and right sides of the sample. The emitting optical module is used to project a light beam onto the sample surface through a grating to construct an irradiated image of a grating, and further reflected to the receiving optical module to form a grating image on the optical imaging sensor.

[0024] The present application utilizes the characteristics of partial or entire grating image signal enhancement to significantly reduce the risk of failure caused by distortion or loss of reflection measurement points due to changes in conditions such as wafer material, circuit pattern design, and process nodes when relying on traditional single-point measurement or single-characteristic measurement.

[0025] like Figure 3 As shown, the emission optical module of this embodiment includes: a light source assembly, a grating 202, an emission lens group 203 and a polarizer 204, which are arranged in sequence on the main optical axis of the emission light beam.

[0026] The light source assembly includes a light source 200 and a light guide column 201. The light source 200 is fixed at the center of the light source base 100 and is used to provide an emission light beam. The light source 200 preferably uses monochromatic light with a shorter wavelength, because under the same lens parameter conditions, the shorter the wavelength of the light wave of the illumination light source, the higher the resolution of the obtained image. Therefore, choosing monochromatic light with a short wavelength, high energy and strong penetration is helpful to improve the accuracy of the system.

[0027] The entrance of the light guide column 201 is provided with a concave and smooth light source groove 2011, the light emitting side of the light source 200 is embedded in the light source groove 2011 of the light guide column and is close to the light source groove 2011, the light radiation angle of the light source matches the light receiving angle of the light guide column 201, and the coupling efficiency is improved. The smooth structure of the light source groove 2011 more effectively improves the capture rate of the light flux.

[0028] The grating 202 is close to the end of the light source 200, that is, located on the light-emitting side of the light guide column 201, and is used to project the emitted light beam onto the surface of the workpiece and reflect it to the receiving optical module to form a grating image. The grating 202 is composed of two or more groups of periodic grids, and the period of each group of gratings needs to be greatly different; within the full range of the height variation of the wafer surface, the grating image is unique, that is, a certain height value of the wafer surface corresponds to a unique grating image in the imaging device. Figure 3 An optional raster shape diagram is illustrated, but is not limited to the shape shown in the figure.

[0029] Ordinary wafers are mostly made of silicon, and their surface is reflective. If they are reflected by ordinary light sources, they can be imaged normally, and single-point measurement methods can also be used for stable measurement. This application needs to solve two problems: Problem 1. After the wafer is processed by various processes, a multi-layer structure will be formed on the surface of the wafer due to reasons such as photoresist stacking and multi-layer printed circuits. When the surface structure is translucent, the reflection imaging of ordinary light sources will be interfered by refracted light. Problem 2. The problem of interference from refracted light of transparent wafers needs to be solved. Wafer materials include quartz glass, sapphire, silicon carbide and other translucent materials. Because of the influence of refracted light, height measurement will be unreliable or even unable to work.

[0030] This embodiment uses a polarization device 204 to solve the above problem. Figure 3 and Figure 6As shown, the polarizer 204 is located downstream of the grating 202, and is used to separate the incident light beam 209 passing through the grating 202 into a first polarization component 210 and a second polarization component 211. The second polarization component 211 will be refracted into the wafer after reaching the surface of the wafer. The polarizer 204 can be a polarization beam splitter, and the first polarization component 210 and the second polarization component 211 separated by the polarizer 204 are p component and s component respectively; wherein the first polarization component is transmitted through the polarizer 204 and projected onto the surface of the workpiece to form an illumination image of the grating, and the first polarization component will not be refracted into the wafer after reaching the surface of the wafer. This is because the polarizer 204 is designed according to Brewster's law. When the incident angle of its light is the Brewster's angle, the majority of the reflected light is the second polarization component vibrating perpendicular to the incident plane, and the majority of the refracted light is the first polarization component vibrating parallel to the incident plane. The separated second polarization component is equivalent to being filtered by the polarizer 204, and only the first polarization component reaches the wafer surface. Therefore, the second polarization component is avoided from being refracted into the interior of the wafer after reaching the sample surface, and then being reflected out of the wafer inside the wafer to affect the normal optical path imaging, thereby avoiding interference of the imaging in the field of view of the camera 109, and improving the reliability of height measurement.

[0031] The light beam of the first polarization component 210 of this embodiment is projected to the receiving optical module in a manner of nearly total reflection on the workpiece surface to obtain a stable and clear grating image. According to the Jones matrix algorithm, the brightness of the grating projection light will decrease after passing through the polarizer 204, so the brightness of the light source needs to be appropriately increased.

[0032] The assembly structure of the transmitting optical module is introduced below.

[0033] like Figure 2 As shown, the emission optical module also includes a coaxially mounted focus ring 101, a centering ring 102, an assembly flange 103 and an objective lens barrel 104. The light source assembly and the grating 202 are installed in the focus ring 101, and the emission lens group 203 and the polarizer 204 are installed in the objective lens barrel 104. The clarity of the grating projection image depends on the focusing member of the optical system. The focus ring 101 is threadedly connected with the centering ring 102 and fixed by a fastening screw. The object distance and image distance of the optical system can be easily adjusted by rotating the focus ring 101. The centering ring 102 is used to adjust the measurement position of the emission light beam on the surface of the workpiece to be consistent with the scanning position of the electron beam. The centering ring 102 has a radial three-claw adjustment screw structure. The top of the assembly flange 103 has an inverted cone surface. The centering ring 102 is tightly matched with the cone surface of the assembly flange to form a centering fastening structure. An internal thread groove is provided at the center of the bottom of the mounting flange, and the objective lens barrel 104 is installed in the internal thread groove of the mounting flange and is locked and fixed to the mounting flange.

[0034] The optical system structure of the receiving optical module is introduced below.

[0035] Please refer to Figure 3 and Figure 4 The receiving optical module includes a double telecentric lens group and an optical imaging sensor 207 located on the optical axis of the reflected light beam. The double telecentric lens group projects the reflected light beam on the workpiece surface to the optical imaging sensor 207 with double telecentricity. The telecentricity on the object distance side is used to achieve a constant imaging ratio within the depth of field range. The telecentricity on the image distance side ensures that the imaging ratio is stable when the focus ring is adjusted. When the object plane is within the working distance range, the magnification of the system remains unchanged, eliminating the perspective error and image blurring problems caused by the change in wafer height, ensuring the accuracy of the height measurement system, having the characteristics of large depth of field and small distortion within the working range, and improving the stability of the height measurement system.

[0036] The assembly structure of the receiving optical module is introduced below.

[0037] like Figure 2 As shown, the receiving optical module also includes a coaxially mounted lens 105, a second mounting flange 106, a second focus ring 107, a second centering ring 108 and a camera 109. The lens 105 is threadedly connected to the second mounting flange 106 and faces the scanning position of the wafer. The second mounting flange 106 is threadedly connected to the second focus ring 107 and is fixed by radial fastening screws. An inverted cone is provided on the top of the second focus ring 107, and the cone cooperates with the three-claw adjustment screw structure of the second centering ring 108 to form a centering fastening structure. The second centering ring 108 is threadedly connected to the camera 109.

[0038] Please refer to Figure 4 The double telecentric lens group is locked to the axis of the lens 105 , and the double telecentric lens group includes a first receiving lens group 205 , a double concave lens group 206 and a second receiving lens group 208 arranged in sequence. The optical imaging sensor 207 is fixed at the center of the camera 109 .

[0039] In a specific implementation, the first receiving lens group 205 and the second receiving lens group 208 are both biconvex lenses. Figure 4 A schematic diagram of the optical path of the double telecentric lens group is shown.

[0040] like Figure 4 As shown, the configuration of the emitting lens group 203 of this embodiment satisfies: f1 <d0<2×f1(1) d1 = N × d0 (2) Wherein, d0 is the parallel center axis distance between the emitting lens group 203 and the grating, f1 is the focal length of the emitting lens group 203, and d1 is the distance from the axial light of the emitting lens group 203 to the wafer surface. Formula (1) ensures that the grating projection image on the wafer is a real image, and formula (2) makes the grating projection image magnified N times and focused on the wafer surface.

[0041] The setting of the double telecentric lens group satisfies: d3 = f2(3) d4 = f3(4) Wherein, d3 is the axial center distance between the first receiving lens group 205 and the biconcave lens group 206, f2 is the focal length of the first receiving lens group 205, d4 is the axial center distance between the biconcave lens group 206 and the second receiving lens group 208, and f3 is the focal length of the second receiving lens group 208. Formula (3) and formula (4) form a double telecentric lens, which ensures the working accuracy of the system.

[0042] In another specific embodiment, please refer to Figure 5 The first receiving lens group 205 includes a plano-convex lens with a convex surface on the light-incoming side, a concave-convex lens with a convex surface on the light-incoming side and a concave surface on the light-emitting side, and the second receiving lens group 208 includes a biconcave lens and a biconvex lens that are closely attached, and another biconvex lens. Figure 5 A schematic diagram of the optical path of the double telecentric lens group is shown.

[0043] The specific construction method of the double telecentric lens group of this system can adopt the control variable method to first simulate the basic double telecentric imaging, and then keep the image distance side lens group unchanged according to the needs, optimize the object distance side lens group to achieve a specific object distance and depth of field, and then use a similar method to fix the object distance side and optimize the image distance side. After several repeated adjustments, the appropriate double telecentric lens group arrangement is obtained.

[0044] Example 2 After the polarizing device 204 filters the incident light into the first polarization component 210, the brightness of the light projected onto the surface of the wafer will be reduced. In order to compensate for the brightness loss of the light source and improve the utilization rate of light, this embodiment is based on Embodiment 1, and a layer of reflective film 2012 is attached to the outer side of the light guide column 201 of the light source assembly. The reflective film 2012 allows the light emitted by the light source to reach the grating after being reflected once or multiple times inside the light guide column 201, thereby preventing the light from being refracted outside the light guide column 201.

[0045] like Figures 8 to 9As shown, the light guide column 201 includes a light guide side wall 2013 located on the light incident side and a reflective end face 2014; the reflective end face 2014 of the light guide column 201 is located on the left end face of the light guide column 201, and is similar to a V-shaped structure as a whole, and the side close to the light source is inclined in the direction of the light source groove 2011, that is, inclined to the right. A plurality of concave tooth-shaped grooves 2015 are formed on the light guide side wall 2013, and the tooth-shaped grooves 2015 include a first inclined surface 2016 and a second inclined surface 2017. The first inclined surface 2016 reflects part of the light located on both sides of the light source emission angle to the reflective end face of the light guide column, and the reflective end face 2014 reflects the light from the first inclined surface 2016 to the light emitting side, that is, to the right, thereby reducing the amount of light loss that cannot be utilized due to diffuse reflection near the light source and improving the efficiency of light utilization. Furthermore, the inclination angles of the first bevel 2016, the second bevel 2017 and the reflective end surface 2014 are adjusted so that the reflected light of the reflective end surface 2014 propagates approximately in the horizontal direction, thereby reducing the number of reflections of this part of the light on the light guide column 201 and the resulting light energy loss, thereby improving the utilization rate of light.

[0046] The second inclined surface 2017 of the tooth-shaped groove 2015 reflects part of the light on both sides of the emission angle of the light source to the right, and then enters the grating after one or more reflections by the reflective film 2012 on the light-guiding side wall 2013 of the light-guiding column 201. Figure 7 As shown, if there is no toothed groove 2015 near the light source of the light guide column 201, the reflection angle of the light on both sides of the emission angle of the light source on the light guide column 201 is small, as shown in FIG. Figure 7 As shown, this part of the reflected light needs to be reflected more times near the light source before reaching the grating, resulting in too high brightness near the light source of the light guide column 201 and too low brightness at the tail end of the light guide column 201, affecting the clarity of the grating imaging. The second inclined surface 2017 in this embodiment increases the reflection angle of the light on the light guide column 201 and reduces the light loss caused by multiple reflections on the light guide column 201.

[0047] In other specific embodiments, a gap is set between two adjacent tooth-shaped grooves 2015, such as Fig. 9 As shown, the light from the second inclined surface 2017 is reflected into the interval and continues to be reflected toward the light-emitting side. The tooth-shaped groove on the side close to the reflective end surface 2014 is defined as the first tooth-shaped groove, and the tooth-shaped groove on the side away from the reflective end surface 2014 is defined as the second tooth-shaped groove. The depth of the first tooth-shaped groove is greater than the depth of the second tooth-shaped groove, so that most of the reflected light on the second inclined surface 2017 of the first tooth-shaped groove can be smoothly transmitted to the right without being blocked by the second tooth-shaped groove.

[0048] The toothed groove 2015 of the present application can be formed into an annular groove around the circumference of the light guide column 201, and the toothed groove is covered with a reflective film 2012. The present application does not limit the number and inclination angle of the toothed grooves, which can be flexibly adjusted. The other structures of this embodiment are the same as those of embodiment 1.

[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An optical height measurement system for electron beam scanning imaging, characterized in that: It includes a transmitting optical module and a receiving optical module located on opposite sides above the workpiece to be scanned; The transmitting optical module comprises: A light source assembly, for providing an emission light beam; A grating, located in the optical path of the emission light beam, for projecting the emission light beam onto the surface of the workpiece and reflecting it to the receiving optical module to form a grating image; a polarizing device, located downstream of the grating, for separating the emission light beam passing through the grating into a first polarization component and a second polarization component, wherein the first polarization component is transmitted through the polarizing device and projected onto the surface of the workpiece to form an illumination image of the grating; The receiving optical module includes a double telecentric lens group and an optical imaging sensor; The double telecentric lens group double telecentrically projects the reflected light beam on the workpiece surface to the optical imaging sensor to form a grating image.

2. The optical altimeter system according to claim 1, characterized in that: The emission optical module further comprises an emission lens group located between the grating and the polarization device, and the emission lens group satisfies: f1 < d0 < 2×f1(1) d1 = N × d0 (2) Among them, d0 is the parallel center axis distance between the emitting lens group and the grating, f1 is the focal length of the emitting lens group, and d1 is the distance from the axial light of the emitting lens group to the wafer surface.

3. The optical altimeter system according to claim 2, characterized in that: The double telecentric lens group includes a first receiving lens group, a biconcave lens group and a second receiving lens group arranged in sequence along the reflection light path, and the arrangement of the double telecentric lens group satisfies: d3 = f2(3) d4 = f3(4) Wherein, d3 is the axial center distance between the first receiving lens group and the biconcave lens group, f2 is the focal length of the first receiving lens group, d4 is the axial center distance between the biconcave lens group and the second receiving lens group, and f3 is the focal length of the second receiving lens group.

4. The optical altimeter system according to claim 1, characterized in that: The polarization device comprises a polarization beam splitter, wherein the first polarization component and the second polarization component separated by the polarization beam splitter are respectively a p component and an s component; the light beam of the first polarization component is approximately totally reflected on the surface of the workpiece to the receiving optical module.

5. The optical altimeter system according to claim 4, characterized in that: The emission optical module comprises a coaxially mounted focus ring 1, a centering ring 1, a mounting flange 1 and an objective lens barrel, the light source assembly and the grating are mounted in the focus ring 1, and the emission lens group and the polarization device are mounted in the objective lens barrel; The focusing ring 1 is used to adjust the object distance and image distance of the optical system, and the centering ring 1 is used to adjust the measuring position of the emission light beam on the workpiece surface to be consistent with the scanning position of the electron beam.

6. The optical altimeter system according to claim 5, characterized in that: The receiving optical module also includes a coaxially mounted lens, a second mounting flange, a second focusing ring, a second centering ring and a camera. The first receiving lens group, the second receiving lens group and the biconcave lens group are locked to the axis of the lens, and the optical imaging sensor is fixed to the axis of the camera.

7. The optical altimeter system according to any one of claims 1 to 6, characterized in that: The light source assembly comprises a light guide column and a light source, the light incident side of the light guide column is provided with a concave and smooth light source groove, and the light emitting surface of the light source is tightly embedded in the light source groove; The outer wall of the light guide column is covered with a reflective film for reflecting the light emitted by the light source inside the light guide column; The grating is located at the light-emitting side of the light-guiding column, and the grating includes at least two groups of periodic grids, and the periods of the grids of each group are different.

8. The optical altimeter system according to claim 7, characterized in that: The light guide column comprises a light guide side wall located on the light incident side and a reflective end face; The light guide side wall is formed with a tooth-shaped groove, and the tooth-shaped groove includes a first inclined surface for reflecting the light on both sides of the emission angle of the light source toward the reflective end surface, and a second inclined surface for reflecting the light on both sides of the emission angle of the light source toward the light exit side of the light guide column; The side of the reflective end surface close to the light source is inclined toward the light source slot, and the reflective end surface reflects the light from the first inclined surface toward the light emitting side.

9. The optical altimeter system according to claim 8, characterized in that: A gap is arranged between two adjacent tooth-shaped grooves, and the light from the second inclined surface is reflected into the gap and continues to be reflected toward the light emitting side.

10. The optical altimeter system according to claim 8, characterized in that: The depth of the tooth-shaped groove on the side close to the reflective end surface is greater than the depth of the tooth-shaped groove on the side far from the reflective end surface.

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