Semiconductor structure and forming method thereof
By employing a multi-ridge grating structure in a silicon-based photonic integrated chip and using etching technology to form grating strips arranged in an arithmetic sequence, the problems of low coupling efficiency and small bandwidth of optical couplers are solved, achieving more efficient light coupling and wavelength adaptability.
Patent Information
- Application Number
- CN202410940533.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-07-12
- Publication Date
- 2026-01-13
AI Technical Summary
Existing optical couplers have low coupling efficiency, small bandwidth, and significant differences in coupling efficiency across different wavelengths. In particular, in silicon-based photonic integrated chips, the traditional single-ridge grating structure design lacks flexibility and high performance.
A multi-ridge grating structure is adopted. By forming grating strips with different heights on the SOI substrate, and using an etching process to form a second grating structure arranged in an arithmetic sequence, the flexibility and coupling efficiency of the grating structure are improved.
It increases the selectable wavelengths and bandwidth of incident light, improves optical coupling efficiency, and adapts to the coupling needs of light of different wavelengths.
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Figure CN121325317A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of semiconductor technology, and in particular to a semiconductor structure and a method for forming the same. Background Technology
[0002] In silicon-based photonic integrated circuits, optical couplers, serving as input and output devices for optical signals, are highly valued, especially in the fabrication process where they demonstrate significant technological advantages. Currently, there are two main coupling methods: vertical coupling and end-face coupling. Vertical coupling's main advantage lies in its flexible placement; it can be located anywhere within the chip, making it suitable for wafer-level in-circuit testing. Furthermore, its larger alignment tolerance facilitates packaging. However, its coupling efficiency is relatively low, and its bandwidth is limited. The coupling efficiency varies considerably across different wavelengths of light during diffraction within the same wavelength band.
[0003] Currently, most traditional grating couplers are typically designed based on a single-ridge grating structure. Compared to single-ridge gratings, multi-ridge structures offer greater design freedom, and therefore, multi-ridge gratings often exhibit higher performance.
[0004] Therefore, from a process feasibility perspective, research on double-ridge gratings with two or more grating ridge structures and their design for implementation in various polarizers and beam splitters is particularly important. Summary of the Invention
[0005] This application provides a semiconductor structure and a method for forming the same, which can form a grating structure with grating strips of various heights arranged flexibly, thereby increasing the selectable wavelengths of incident light and improving bandwidth.
[0006] One aspect of this application provides a method for forming a semiconductor structure, comprising: providing an SOI substrate, the SOI substrate including a bottom silicon layer, an insulating layer and a top silicon layer, wherein a first grating structure is formed in the top silicon layer, the first grating structure including a plurality of first grating ridges and a first grating groove, and a cover layer covering the SOI substrate and the first grating structure is further formed on the SOI substrate; forming a second grating structure on the surface of the cover layer, the second grating structure including a plurality of second grating strips and a plurality of second grating grooves, wherein the plurality of second grating strips correspond to the positions of the plurality of first grating ridges, and the plurality of second grating grooves correspond to the positions of the plurality of first grating grooves; etching a portion of the second grating strips, such that the plurality of second grating strips include first grating strips, second grating strips... to Nth grating strips of different heights, where N is a positive integer greater than or equal to 2, wherein the first grating strips, second grating strips... to Nth grating strips are arranged cyclically, and the number of second grating strips of each height is greater than or equal to 2.
[0007] In some embodiments of this application, the method of etching portions of the second grating protrusions to make the plurality of second grating protrusions respectively become first grating strips, second grating strips, ... to the Nth grating strip with different heights includes: performing a first etching process to etch portions of the second grating protrusions, wherein the unetched second grating protrusions become first grating strips, and the etched second grating protrusions become second grating strips; performing a second etching process to etch portions of the second grating strips, the etched second grating strips becoming third grating strips; ... performing an (N-1)th etching process to etch portions of the (N-1)th grating strips, the etched (N-1)th grating strip becoming the Nth grating strip.
[0008] In some embodiments of this application, the method of etching portions of the second grating protrusions to make the plurality of second grating protrusions respectively become first grating strips, second grating strips, ... to the Nth grating strip with different heights includes: performing a first etching process to etch portions of the second grating protrusions, wherein the unetched second grating protrusions become first grating strips, and the etched second grating protrusions become second grating strips; performing a second etching process to continue etching portions of the second grating protrusions, and the etched second grating protrusions become third grating strips; ... performing an (N-1)th etching process to continue etching portions of the second grating protrusions, and the etched second grating protrusions become the Nth grating strip.
[0009] In some embodiments of this application, the heights of the first grating strip, the second grating strip, ... up to the Nth grating strip are arranged in an arithmetic sequence.
[0010] In some embodiments of this application, the difference in the arithmetic sequence is 0.2-0.5 micrometers.
[0011] In some embodiments of this application, the material of the second grating structure includes polycrystalline silicon.
[0012] Another aspect of this application provides a semiconductor structure, comprising: an SOI substrate, the SOI substrate including a bottom silicon layer, an insulating layer and a top silicon layer, wherein a first grating structure is formed in the top silicon layer, the first grating structure including a plurality of first grating ridges and a first grating groove, and a cover layer covering the SOI substrate and the first grating structure is further formed on the SOI substrate; and a second grating structure located on the surface of the cover layer, the second grating structure including a plurality of second grating ridges and a plurality of second grating grooves, the plurality of second grating ridges including first grating ridges, second grating ridges, ... to Nth grating ridges of different heights, wherein the plurality of second grating ridges correspond to the plurality of first grating grooves, the plurality of second grating grooves correspond to the plurality of first grating grooves, N is a positive integer greater than or equal to 2, the first grating ridges, second grating ridges, ... to Nth grating ridges are arranged cyclically, and the number of second grating ridges of each height is greater than or equal to 2.
[0013] In some embodiments of this application, the heights of the first grating strip, the second grating strip, ... up to the Nth grating strip are arranged in an arithmetic sequence.
[0014] In some embodiments of this application, the difference in the arithmetic sequence is 0.2-0.5 micrometers.
[0015] In some embodiments of this application, the material of the second grating structure includes polycrystalline silicon.
[0016] This application provides a semiconductor structure and a method for forming the same, which can form a grating structure with grating strips of various heights arranged flexibly, thereby increasing the selectable wavelengths of incident light and improving bandwidth. Attached Figure Description
[0017] The following accompanying drawings describe in detail the exemplary embodiments disclosed in this application. The same reference numerals denote similar structures in several views of the drawings. Those skilled in the art will understand that these embodiments are non-limiting and exemplary, and the drawings are for illustrative purposes only and are not intended to limit the scope of this application. Other embodiments may similarly fulfill the inventive intent of this application. It should be understood that the drawings are not drawn to scale.
[0018] in:
[0019] Figures 1 to 3 This is a schematic diagram of the steps in the method for forming a semiconductor structure according to some embodiments of this application;
[0020] Figure 4 This is a schematic diagram of the semiconductor structure described in other embodiments of this application;
[0021] Figures 5 to 6 This is a schematic diagram of the steps in the method for forming a semiconductor structure according to other embodiments of this application;
[0022] Figures 7 to 8 This is a schematic diagram of the steps in the method for forming a semiconductor structure according to other embodiments of this application. Detailed Implementation
[0023] The following description provides specific application scenarios and requirements for this application, intended to enable those skilled in the art to make and use the content of this application. Various partial modifications to the disclosed embodiments will be apparent to those skilled in the art, and the general principles defined herein can be applied to other embodiments and applications without departing from the spirit and scope of this application. Therefore, this application is not limited to the embodiments shown, but rather to the widest scope consistent with the claims.
[0024] The technical solution of the present invention will be described in detail below with reference to the embodiments and accompanying drawings.
[0025] Figures 1 to 3 These are schematic diagrams illustrating the steps in the method for forming a semiconductor structure according to some embodiments of this application. The method for forming a semiconductor structure according to embodiments of this application will be described in detail below with reference to the accompanying drawings.
[0026] refer to Figure 1 As shown, an SOI substrate 100 is provided. The SOI substrate 100 includes a bottom silicon layer 101, an insulating layer 102, and a top silicon layer 103. A first grating structure 110 is formed in the top silicon layer 103. The first grating structure 110 includes a plurality of first grating ridges 111 and first grating grooves 112. A cover layer 120 covering the SOI substrate 100 and the first grating structure 110 is also formed on the SOI substrate 100.
[0027] In some embodiments of this application, the bottom silicon layer 101 is made of silicon, the insulating layer 102 is made of silicon oxide, and the top silicon layer 103 is made of silicon. SOI substrates (silicon-on-insulator substrates) are a common structure in the semiconductor field, and their specific formation processes and detailed structures are not described in detail here.
[0028] In some embodiments of this application, the first grating structure 110 is formed by etching the top silicon layer 103, so the material and total thickness of the first grating structure 110 are the same as those of the top silicon layer 103.
[0029] In some embodiments of this application, the first grating structure 110 for vertical coupling is used to couple incident light rays in the vertical direction. The first grating structure 110 is a uniform, repeatable structure. Specifically, the width and height of the plurality of first grating ridges 111 are the same. The width and depth of the plurality of first grating grooves 112 are also the same. In this way, every two first grating ridges 111 and their adjacent first grating grooves 112 form a repeatable structure to perform the light coupling function. Here, the height and depth refer to the longitudinal dimensions in the figures, that is, the dimensions in the direction perpendicular to the substrate.
[0030] In some embodiments of this application, the material of the cover layer 120 includes a dielectric material such as silicon oxide. The top surface of the cover layer 120 is higher than the top surface of the first grating structure 110. The cover layer 120 is used to protect the first grating structure 110.
[0031] refer to Figure 2As shown, a second grating structure 130 is formed on the surface of the cover layer 120. The second grating structure 130 includes a plurality of second grating protrusions 131 and a plurality of grating grooves 132. The plurality of second grating protrusions 131 correspond to the positions of the plurality of first grating ridges 111, and the plurality of second grating grooves 132 correspond to the positions of the plurality of first grating grooves 112.
[0032] In some embodiments of this application, the material of the second grating structure 130 is polysilicon. A method for forming the second grating structure 130 includes: depositing a polysilicon material layer on the surface of the capping layer 120; and etching the polysilicon material layer to the surface of the capping layer 120 to form the second grating structure 130.
[0033] In some embodiments of this application, the width of the plurality of second grating protrusions 131 is the same as the width of the plurality of first grating ridges 111. The width of the plurality of second grating grooves 132 is the same as the width of the plurality of first grating grooves 112.
[0034] In some embodiments of this application, the projections of the plurality of second grating ridges 131 in the vertical direction coincide with the protruding portions of the plurality of first grating ridges 111. The projections of the plurality of second grating grooves 132 in the vertical direction coincide with the projections of the plurality of first grating grooves 112 in the vertical direction.
[0035] refer to Figure 3 As shown, the second grating protrusions 131 are etched to make the plurality of second grating protrusions 131 become first grating strips 141, second grating strips 142... to Nth grating strips with different heights, where N is a positive integer greater than or equal to 2. The first grating strips 141, second grating strips 142... to Nth grating strips are arranged in a cycle, and the number of grating strips of each height is greater than or equal to 2.
[0036] Continue to refer to Figure 3 As shown, some embodiments of this application are illustrated here with N being 2, wherein the first grating strip 141 and the second grating strip 142 are alternately and uniformly distributed. Specifically, a first etching process is performed to etch a portion of the second grating protrusion 131, wherein the unetched second grating protrusion 131 becomes the first grating strip 141, and the etched second grating protrusion 131 becomes the second grating strip 142.
[0037] In some embodiments of this application, the height of the second grating strip 142 is half that of the first grating strip 141.
[0038] Continue to refer to Figure 3As shown, the pitch between adjacent first grating strips 141 is T1. The pitch between adjacent first grating strips 141 and second grating strips 142 is T2. T1 is twice T2.
[0039] In some embodiments of this application, each pitch of the grating structure includes at least two second grating ridges and a second grating groove, and the number of second grating ridges and second grating grooves is the same in each pitch of the grating structure.
[0040] In the technical solution of this application, the second grating structure 130 is etched to include second grating ridges 131 of different heights (e.g., first grating ridges 141 and second grating ridges 142), forming combinations of second grating ridges 131 with different spacings (e.g., T1 and T2). For example, all the first grating ridges 141 combined form a grating structure with a spacing of T1; all the first grating ridges 141 and second grating ridges 142 combined form a grating structure with a spacing of T2. Grating structures with different spacings correspond to the coupling of light rays in different wavelength ranges. Therefore, the second grating structure 130 of this application can simultaneously couple light rays in different wavelength ranges, increasing the selectable wavelength types of incident light, improving bandwidth, and enhancing coupling efficiency.
[0041] Figure 4 This is a schematic diagram of the semiconductor structure described in some other embodiments of this application.
[0042] refer to Figure 4 As shown, with Figure 3 The difference is that each first grating strip 141 and two second grating strips 142 are evenly distributed alternately. The pitch between adjacent first grating strips 141 is T1. The pitch between adjacent first grating strips 141 and second grating strips 142 or adjacent second grating ridge strips 142 is T2. T1 is three times T2.
[0043] exist Figure 4 In the structure shown, the spacing T1 can be adjusted by changing the arrangement of the first grating strip 141 and the second grating strip 142. The structure of the second grating structure 130 can be adjusted according to the wavelength of the light to be coupled. The process is simple and easy to implement. Similarly, the arrangement of the first grating strip 141 and the second grating strip 142 can be arbitrarily changed to achieve a suitable spacing T1 to meet the coupling requirements of the required wavelength.
[0044] Figures 5 to 6 This is a schematic diagram of the steps in the method for forming a semiconductor structure according to other embodiments of this application. Figure 5 and Figure 6 In some other embodiments shown, N is 3 as an example.
[0045] refer to Figure 5 As shown, a first etching process is performed to etch a portion of the second grating protrusion 131, wherein the unetched second grating protrusion 131 becomes the first grating strip 141, and the etched second grating protrusion 131 becomes the second grating strip 142.
[0046] refer to Figure 6 As shown, a second etching process is performed to etch a portion of the second grating strip 142, and the etched second grating strip 142 becomes the third grating strip 143. In other embodiments of this application, the second grating structure 130 has a grating combination with three pitches: T1, T2, and T3.
[0047] In some embodiments of this application, the heights of the first grating strip 141, the second grating strip 142, the third grating strip 143, ... up to the Nth grating strip are arranged in an arithmetic sequence.
[0048] In some embodiments of this application, the difference in the arithmetic sequence is 0.2-0.5 micrometers.
[0049] In some embodiments of this application, the maximum height of the first grating strip 141, the second grating strip 142, the third grating strip 143... up to the Nth grating strip is 0.2 micrometers and the minimum height is 0.05 micrometers.
[0050] In some embodiments of this application, when N is greater than 3, the method of etching a portion of the second grating protrusions to make the plurality of second grating protrusions respectively become first grating strips, second grating strips, ... to the Nth grating strip with different heights includes: performing a first etching process to etch a portion of the second grating protrusions, wherein the unetched second grating protrusions become first grating strips, and the etched second grating protrusions become second grating strips; performing a second etching process to etch a portion of the second grating strips, and the etched second grating strip becomes a third grating strip; ... performing an (N-1)th etching process to etch a portion of the (N-1)th grating strip, and the etched (N-1)th grating strip becomes the Nth grating strip.
[0051] Figures 7 to 8 This is a schematic diagram of the steps in the method for forming a semiconductor structure according to other embodiments of this application. Figure 7 and Figure 8 In some other embodiments shown, N is 3 as an example.
[0052] refer to Figure 7 As shown, a first etching process is performed to etch a portion of the second grating protrusion 131, wherein the unetched second grating protrusion 131 becomes the first grating strip 141, and the etched second grating protrusion 131 becomes the second grating strip 142.
[0053] refer to Figure 8 As shown, the second etching process is performed to continue etching a portion of the second grating protrusion 131, and the etched second grating protrusion 131 becomes the third grating strip 143.
[0054] In some embodiments of this application, when N is greater than 3, the method of etching a portion of the second grating protrusions to make the plurality of second grating protrusions respectively become first grating strips, second grating strips, ... to the Nth grating strip with different heights includes: performing a first etching process to etch a portion of the second grating protrusions, wherein the unetched second grating protrusions become first grating strips, and the etched second grating protrusions become second grating strips; performing a second etching process to continue etching a portion of the second grating protrusions, and the etched second grating protrusions become third grating strips; ... performing an N-1th etching process to continue etching a portion of the second grating protrusions, and the etched second grating protrusions become the Nth grating strip.
[0055] This application provides a method for forming a semiconductor structure, which can form a grating structure with grating strips of various heights and flexible arrangement, thereby increasing the selectable wavelengths of incident light and improving bandwidth.
[0056] This application also provides a semiconductor structure, referenced... Figure 3 or Figure 4 or Figure 6 As shown, the system includes: an SOI substrate 100, which includes a bottom silicon layer 101, an insulating layer 102, and a top silicon layer 103. A first grating structure 110 is formed in the top silicon layer 103. The first grating structure 110 includes a plurality of first grating ridges 111 and first grating grooves 112. A cover layer 120 covering the SOI substrate 100 and the first grating structure 110 is also formed on the SOI substrate 100. A second grating structure 130 is located on the surface of the cover layer 120. The second grating structure 130 includes a plurality of second grating ribs 13. 1. The second grating groove 132, wherein the plurality of second grating protrusions 131 include first grating strips 141, second grating strips 142... to the Nth grating strip with different heights, wherein the plurality of second grating protrusions 131 correspond to the positions of the plurality of first grating ridges 111, and the plurality of second grating grooves 132 correspond to the positions of the plurality of first grating grooves 112, N is a positive integer greater than or equal to 2, the first grating strips 141, second grating strips 142... to the Nth grating strip are arranged cyclically, and the number of second grating protrusions 131 of each height is greater than or equal to 2.
[0057] In some embodiments of this application, the bottom silicon layer 101 is made of silicon, the insulating layer 102 is made of silicon oxide, and the top silicon layer 103 is made of silicon. SOI substrates (silicon-on-insulator substrates) are a common structure in the semiconductor field, and their specific formation processes and detailed structures are not described in detail here.
[0058] In some embodiments of this application, the first grating structure 110 is formed by etching the top silicon layer 103, so the material and total thickness of the first grating structure 110 are the same as those of the top silicon layer 103.
[0059] In some embodiments of this application, the first grating structure 110 for vertical coupling is used to couple incident light rays in the vertical direction. The first grating structure 110 is a uniform, repeatable structure. Specifically, the width and height of the plurality of first grating ridges 111 are the same. The width and depth of the plurality of first grating grooves 112 are also the same. In this way, every two first grating ridges 111 and their adjacent first grating grooves 112 form a repeating structure to perform the function of light coupling.
[0060] In some embodiments of this application, the material of the cover layer 120 includes a dielectric material such as silicon oxide. The top surface of the cover layer 120 is higher than the top surface of the first grating structure 110. The cover layer 120 is used to protect the first grating structure 110.
[0061] In some embodiments of this application, the material of the second grating structure 130 is polycrystalline silicon.
[0062] In some embodiments of this application, the width of the plurality of second grating protrusions 131 is the same as the width of the plurality of first grating ridges 111. The width of the plurality of second grating grooves 132 is the same as the width of the plurality of first grating grooves 112.
[0063] In some embodiments of this application, the projections of the plurality of second grating ridges 131 in the vertical direction coincide with the protruding portions of the plurality of first grating ridges 111. The projections of the plurality of second grating grooves 132 in the vertical direction coincide with the projections of the plurality of first grating grooves 112 in the vertical direction.
[0064] refer to Figure 3 As shown, some embodiments of this application are illustrated here with N being 2, wherein the first grating strip 141 and the second grating strip 142 are alternately and uniformly distributed.
[0065] In some embodiments of this application, the height of the second grating strip 142 is half that of the first grating strip 141.
[0066] Continue to refer to Figure 3 As shown, the pitch between adjacent first grating strips 141 is T1. The pitch between adjacent first grating strips 141 and second grating strips 142 is T2. T1 is twice T2.
[0067] In some embodiments of this application, each pitch of the grating structure includes at least two second grating ridges and a second grating groove, and the number of second grating ridges and second grating grooves is the same in each pitch of the grating structure.
[0068] In the technical solution of this application, the second grating structure 130 includes second grating ridges 131 of different heights (e.g., first grating ridges 141 and second grating ridges 142), which form combinations of second grating ridges 131 with different spacings (e.g., T1 and T2). For example, all the first grating ridges 141 combined together constitute a grating structure with a spacing of T1; all the first grating ridges 141 and second grating ridges 142 combined together constitute a grating structure with a spacing of T2. Grating structures with different spacings correspond to the coupling of light rays in different wavelength ranges. Therefore, the second grating structure 130 of this application can simultaneously couple light rays in different wavelength ranges, increasing the selectable wavelength types of incident light, improving bandwidth, and enhancing coupling efficiency.
[0069] refer to Figure 4 As shown, with Figure 3 The difference is that each first grating strip 141 and two second grating strips 142 are evenly distributed alternately. The pitch between adjacent first grating strips 141 is T1. The pitch between adjacent first grating strips 141 and second grating strips 142 or adjacent second grating ridge strips 142 is T2. T1 is three times T2.
[0070] exist Figure 4 In the structure shown, the spacing T1 can be adjusted by changing the arrangement of the first grating strip 141 and the second grating strip 142. The structure of the second grating structure 130 can be adjusted according to the wavelength of the light to be coupled. The process is simple and easy to implement. Similarly, the arrangement of the first grating strip 141 and the second grating strip 142 can be arbitrarily changed to achieve a suitable spacing T1 to meet the coupling requirements of the required wavelength.
[0071] refer to Figure 6 As shown, in some embodiments of this application, N is 3 as an example. The second grating ridge 131 includes a first grating ridge 141, a second grating ridge 142, and a third grating ridge 143. In some embodiments of this application, the second grating structure 130 has grating combinations with three spacings: T1, T2, and T3.
[0072] In some embodiments of this application, the heights of the first grating strip 141, the second grating strip 142, the third grating strip 143, ... up to the Nth grating strip are arranged in an arithmetic sequence.
[0073] In some embodiments of this application, the difference in the arithmetic sequence is 0.2-0.5 micrometers.
[0074] In some embodiments of this application, the maximum height of the first grating strip 141, the second grating strip 142, the third grating strip 143... up to the Nth grating strip is 0.2 micrometers and the minimum height is 0.05 micrometers.
[0075] This application provides a semiconductor structure and a method for forming the same, which can form a grating structure with grating strips of various heights arranged flexibly, thereby increasing the selectable wavelengths of incident light and improving bandwidth.
[0076] In summary, after reading this application, those skilled in the art will understand that the foregoing application content is presented by way of example only and is not restrictive. Although not explicitly stated herein, those skilled in the art will understand that this application is intended to encompass various reasonable changes, improvements, and modifications to the embodiments. These changes, improvements, and modifications are all within the spirit and scope of the exemplary embodiments of this application.
[0077] It should be understood that the term "and / or" as used in this embodiment includes any or all combinations of one or more of the associated listed items. Conversely, the term "directly" means without intermediate elements. It should also be understood that the terms "comprising," "containing," "including," or "comprises," as used in this application, indicate the presence of the described features, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or groups thereof.
[0078] It should also be understood that although the terms first, second, third, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Therefore, without departing from the teachings of this application, a first element in some embodiments may be referred to as a second element in other embodiments. The same reference numerals or the same reference signs denote the same elements throughout the specification.
[0079] Furthermore, this application specification describes exemplary embodiments by referring to idealized exemplary cross-sectional views and / or plan views and / or perspective views. Therefore, differences from the illustrated shapes are foreseeable due to factors such as manufacturing techniques and / or tolerances. Therefore, exemplary embodiments should not be construed as limited to the shapes of the regions shown herein, but should include deviations in shape caused, for example, by manufacturing processes. Thus, the regions shown in the figures are substantially schematic, and their shapes are not intended to illustrate the actual shapes of the regions of the device, nor are they intended to limit the scope of the exemplary embodiments.
Claims
1. A method for forming a semiconductor structure, characterized in that, include: An SOI substrate is provided, the SOI substrate including a bottom silicon layer, an insulating layer and a top silicon layer, a first grating structure is formed in the top silicon layer, the first grating structure includes a plurality of first grating ridges and first grating grooves, and a cover layer covering the SOI substrate and the first grating structure is also formed on the SOI substrate. A second grating structure is formed on the surface of the cover layer. The second grating structure includes a plurality of second grating ridges and a plurality of second grating grooves, wherein the plurality of second grating ridges correspond to the positions of the plurality of first grating ridges, and the plurality of second grating grooves correspond to the positions of the plurality of first grating grooves. The second grating protrusions are etched in such a way that the plurality of second grating protrusions include first gratings, second gratings, ... to Nth gratings with different heights, where N is a positive integer greater than or equal to 2. The first gratings, second gratings, ... to Nth gratings are arranged in a cyclic manner, and the number of second grating protrusions of each height is greater than or equal to 2.
2. The method for forming a semiconductor structure as described in claim 1, characterized in that, The method for etching the second grating ridges to make the plurality of second grating ridges respectively become first grating ridges, second grating ridges... to the Nth grating ridges with different heights includes: A first etching process is performed to etch a portion of the second grating protrusions, wherein the unetched second grating protrusions become the first grating strip, and the etched second grating protrusions become the second grating strip; A second etching process is performed to etch a portion of the second grating strip, and the etched second grating strip becomes the third grating strip; ...The N-1th grating strip is etched by the N-1th etching process, and the etched N-1th grating strip becomes the Nth grating strip.
3. The method for forming a semiconductor structure as described in claim 1, characterized in that, The method for etching the second grating ridges to make the plurality of second grating ridges respectively become first grating ridges, second grating ridges... to the Nth grating ridges with different heights includes: A first etching process is performed to etch a portion of the second grating protrusions, wherein the unetched second grating protrusions become the first grating strip, and the etched second grating protrusions become the second grating strip; The second etching process is performed to further etch a portion of the second grating protrusion, and the etched second grating protrusion becomes the third grating strip; ...The N-1 etching process is performed to continue etching a portion of the second grating ridge, and the etched second grating ridge becomes the Nth grating ridge.
4. The method for forming a semiconductor structure as described in claim 1, characterized in that, The heights of the first grating strip, the second grating strip, ... up to the Nth grating strip are arranged in an arithmetic sequence.
5. The method for forming a semiconductor structure as described in claim 4, characterized in that, The difference in the arithmetic sequence is 0.2-0.5 micrometers.
6. The method for forming a semiconductor structure as described in claim 1, characterized in that, The material of the second grating structure includes polycrystalline silicon.
7. A semiconductor structure, characterized in that, include: The SOI substrate includes a bottom silicon layer, an insulating layer and a top silicon layer. A first grating structure is formed in the top silicon layer. The first grating structure includes a plurality of first grating ridges and first grating grooves. A cover layer covering the SOI substrate and the first grating structure is also formed on the SOI substrate. A second grating structure is located on the surface of the cover layer. The second grating structure includes a plurality of second grating ridges and second grating grooves. The plurality of second grating ridges include first grating strips, second grating strips, ... to Nth grating strips with different heights. The positions of the plurality of second grating ridges correspond to the positions of the plurality of first grating ridges, and the positions of the plurality of second grating grooves correspond to the positions of the plurality of first grating grooves. N is a positive integer greater than or equal to 2. The first grating strips, second grating strips, ... to Nth grating strips are arranged cyclically, and the number of second grating ridges of each height is greater than or equal to 2.
8. The semiconductor structure as described in claim 7, characterized in that, The heights of the first grating strip, the second grating strip, ... up to the Nth grating strip are arranged in an arithmetic sequence.
9. The semiconductor structure as described in claim 8, characterized in that, The difference in the arithmetic sequence is 0.2-0.5 micrometers.
10. The semiconductor structure as claimed in claim 7, characterized in that, The material of the second grating structure includes polycrystalline silicon.