Silicon-based MEMS micromirror structure and forming method thereof

Abstract

This invention relates to the field of MEMS optical scanning device technology, and discloses a silicon-based MEMS micromirror structure and its molding method. The structure includes a fixed frame, a movable mirror, and an elastic support structure. A support skeleton is provided on the back of the movable mirror. The support skeleton includes a main support rib extending perpendicular to the torsion axis, two side ribs located on either side of the main support rib, a load-bearing plate integrally extended from the end of the main support rib for fixing a central magnet, and perforated grooves formed between the main support rib and side ribs one and two, respectively, extending through the thickness direction. The inner contours of side ribs one and two are arc-shaped or bow-shaped, concave towards the main support rib. The molding method uses the non-optical area on the back as the design domain. Under the constraint of a volume retention rate not exceeding 50%, the main load-bearing path is extracted using the SIMP variable density method, the moving asymptote algorithm, and Helmholtz density filtering. After converting the etching pattern according to the minimum feature size requirement, the support skeleton is integrally molded.

Description

Technical Field

[0001] This invention relates to the field of microelectromechanical systems and optical scanning devices, specifically to a silicon-based MEMS micromirror structure and its molding method, which is particularly suitable for lightweight load-bearing design and silicon-based integrated molding of the movable mirror back structure in electromagnetically driven MEMS micromirrors. Background Technology

[0002] MEMS micromirrors are a type of movable optical reflective device fabricated using microelectromechanical processes. They typically consist of a fixed frame, a movable mirror, and an elastic support structure connecting the two. The movable mirror deflects under electromagnetic, electrostatic, piezoelectric, or electrothermal driving forces, thereby enabling the scanning, modulation, or redirection of an incident light beam. Due to their small size, fast response speed, low power consumption, ease of mass production, and convenient system integration, MEMS micromirrors have been widely used in fields such as lidar, projection displays, optical communication, spectral detection, and biomedical imaging. As the requirements for scanning field of view, scanning frequency, and long-term stability in related applications continue to increase, MEMS micromirrors not only need to have a large optical scanning angle but also need to maintain good structural strength and minimal mirror deformation under high-speed motion.

[0003] Existing electromagnetically driven MEMS micromirrors typically generate driving torque through the electromagnetic interaction between a magnet and a coil. Patent document CN106249402A discloses an electromagnetically driven one-dimensional micromirror that uses one side of the mirror as a reflective surface and an electromagnetic coil for generating output torque on the other side. A magnet-encapsulated structure is used to enhance the magnetic induction intensity, thereby improving the micromirror's rotation angle. This approach can improve the micromirror's deflection capability from the perspective of driving torque, but it primarily focuses on magnetic field enhancement and coil torque output, with relatively limited consideration for back support, mass distribution, and local stress control during large-angle, high-frequency movements of the movable mirror. When the micromirror needs to further increase its operating frequency or incorporate magnetic components, simply increasing the driving torque may simultaneously introduce greater inertial loads and structural stresses, thus affecting device reliability.

[0004] Patent document CN103399402A discloses an electromagnetically driven micro two-dimensional scanning mirror device, which achieves two-dimensional scanning through a reflective mirror, an inner substrate, an outer substrate, an inner torsion beam, an outer torsion beam, and two sets of orthogonal magnetic fields. This type of solution is advantageous for achieving dual-axis deflection and structural compactness, but its main focus is on the two-dimensional scanning support relationship and the arrangement of orthogonal magnetic fields. For silicon-based MEMS micromirrors that need to balance large-angle scanning and high operating frequencies, achieving two-dimensional deflection solely through inner and outer torsion beams and orthogonal magnetic fields cannot fully address the potential issues of localized stress concentration, out-of-plane deformation, and rotational inertia control that may arise during high-speed torsion of the movable mirror itself.

[0005] In addition, some MEMS scanning devices incorporate reinforcing ribs or support structures on the back of the scanning plate or mirror body. Patent document US9946062B1 discloses a MEMS scanner with multiple ribs beneath the scanning plate to provide additional stiffness to the scanning surface without excessively increasing weight. This technology demonstrates that back-side rib structures can improve the stiffness of the scanning plate, but the disclosed rib structures are more of a general stiffness enhancement design, primarily addressing the support problem of the scanning surface. For electromagnetically driven silicon-based MEMS micromirrors, the movable parts often need to withstand the additional mass introduced by magnetic components and the inertial load under high-speed deflection. Ordinary straight ribs, regular ribs, or empirical weight-reduction structures may still struggle to simultaneously achieve lightweight, load-bearing stability, and fabrication feasibility.

[0006] Therefore, there is still room for improvement in existing silicon-based MEMS micromirror structures. Especially in application scenarios where electromagnetic drive, large optical rotation angles, and high operating frequencies coexist, how to improve the structural stability of the movable mirror, reduce the risk of local stress concentration and mirror deformation, and take into account the feasibility of silicon-based micro-nano fabrication, without significantly increasing the mass of the movable part, remains a technical problem that needs to be solved in this field. Summary of the Invention

[0007] The technical problem to be solved by this invention is: how to improve the load-bearing stability of the back of the movable mirror in an electromagnetically driven silicon-based MEMS micromirror while maintaining the lightweight of the movable mirror, reducing the risk of local stress concentration, and taking into account the usage requirements of higher operating frequency and larger optical rotation angle.

[0008] To address the aforementioned technical problems, this invention provides a silicon-based MEMS micromirror structure, comprising a fixed frame, a movable mirror, and an elastic support structure connecting the two. The front of the movable mirror is an optical reflection plane, and the back of the movable mirror is provided with a support frame. The support frame includes a main support rib extending along a direction perpendicular to the torsion axis of the movable mirror, side rib one and side rib two located on both sides of the main support rib, a load-bearing plate integrally extended from the end of the main support rib, and hollow slots formed between the main support rib and side rib one, and between the main support rib and side rib two, respectively. The inner contours of side rib one and side rib two are both arc-shaped or bow-shaped with concave inward toward the main support rib. The hollow slots penetrate through the thickness direction of the support frame. The load-bearing plate is used to fix the central magnet.

[0009] Furthermore, side rib one and side rib two are symmetrically arranged about the main support rib.

[0010] Furthermore, the outer sides of side rib one and side rib two extend to the connection anchor point between the elastic support structure and the movable mirror, respectively.

[0011] Furthermore, the load-bearing plate is a circular load-bearing area formed by the local widening of the end of the main support rib, and the load-bearing plate and the main support rib are an integral continuous structure.

[0012] Furthermore, the inner contour radii of curvature of side rib one and side rib two are 0.2 to 0.8 times the width of the movable mirror along the direction perpendicular to the torsion axis, respectively.

[0013] Furthermore, the diameter of the support plate accounts for 5% to 20% of the width of the movable mirror along the direction perpendicular to the torsion axis; the total opening area of ​​the hollowed-out groove accounts for 30% to 70% of the total area of ​​the back of the movable mirror.

[0014] Furthermore, both side rib one and side rib two have a gradually changing structure that is wider in the middle and narrower at both ends along their extension direction.

[0015] Furthermore, the supporting frame and the movable mirror are integrally formed, and the volume of the solid material of the supporting frame accounts for no more than 50% of the overall enveloping volume of the back of the movable mirror.

[0016] Furthermore, the elastic support structure includes a first folded serpentine beam, a second folded serpentine beam, a third folded serpentine beam, and a fourth folded serpentine beam; the minimum beam width of the first folded serpentine beam, the second folded serpentine beam, the third folded serpentine beam, and the fourth folded serpentine beam is not less than 80μm.

[0017] The present invention also provides a method for forming a silicon-based MEMS micromirror structure, which is used to prepare the above-mentioned silicon-based MEMS micromirror structure, including the following steps: S1. Taking the non-optical area on the back of the movable mirror as the initial design domain, the installation area of ​​the central magnet as the boundary load application area, the connection area between the elastic support structure and the movable mirror as the fixed constraint area, and setting the constraint condition that the volume retention rate does not exceed 50%. S2. With the objective function of maximizing the global stiffness of the structure and minimizing the dynamic strain energy, the SIMP interpolation model of the variable density method is adopted, and the moving asymptote algorithm is combined for iteration to obtain the continuous material density distribution that matches the principal stress transmission path. S3. The Helmholtz density filtering technique is used to filter the density distribution of continuous materials to suppress the numerical instability of checkerboard and mesh-dependent phenomena, and the main load-bearing path is extracted after the convergence criterion is reached. S4. In accordance with the requirement that the minimum feature size of the deep reactive ion etching process is not less than 10μm, the topology corresponding to the main load-bearing path is smoothed and corrected to generate a mask pattern containing the geometric features of the main support rib, side rib one, side rib two, load-bearing plate and hollow groove that are integrally continuous with the main support rib. S5. Perform deep reactive ion etching on the back of the silicon substrate according to the mask pattern to form an integral support framework.

[0018] This invention features a support frame on the back of the movable mirror, consisting of a main support rib, side rib one, side rib two, a load-bearing plate, and a hollowed-out groove. This structurally integrates the mounting area of ​​the central magnet with the load-bearing area on the back of the movable mirror. Compared to solutions that rely solely on enhancing the electromagnetic driving force or adjusting the torsional beam stiffness, this invention improves the load-bearing stability of the back of the movable mirror without occupying the front optical reflection plane, and reduces the risk of localized load concentration after the central magnet is installed.

[0019] Meanwhile, the inner contours of side rib one and side rib two are arc-shaped or bow-shaped, concave towards the main support rib, and can extend to the connection anchor point between the elastic support structure and the movable mirror, so that the support frame forms a smoother and more continuous support relationship between the central magnet mounting area and the elastic support area. Compared with ordinary straight ribs or regular reinforcing ribs, this structure helps to reduce stress concentration at abrupt joints and improve the structural reliability under high-speed torsion.

[0020] In addition, the hollowed-out groove runs through the thickness of the support frame, which can reduce the solid material in the non-primary load-bearing areas of the back of the movable mirror; combined with the setting that the volume of the solid material of the support frame does not exceed 50% of the overall envelope volume of the back of the movable mirror, it can reduce the mass and rotational inertia of the movable part while maintaining the necessary back support strength, thus taking into account both lightweight and load-bearing stability.

[0021] Furthermore, the minimum beam width of the folded serpentine beam in the elastic support structure is not less than 80 μm, which is beneficial for balancing the feasibility of silicon-based micro / nano fabrication, beam structural strength, and torsional compliance. Testing showed that under stress less than 500 MPa, the fast-axis resonant frequency of the sample of this invention can reach 1447.2 Hz, and the maximum fast-axis optical angle can reach 66°, indicating that this structure can meet the application requirements of higher operating frequencies and larger optical rotation angles. Attached Figure Description

[0022] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in this specification are briefly described below.

[0023] Figure 1 This is a schematic planar view of a silicon-based MEMS micromirror structure according to the present invention; Figure 2 This is a schematic diagram of the molding method for the silicon-based MEMS micromirror structure of the present invention; Figure 3 This is a three-dimensional schematic diagram of a silicon-based MEMS micromirror structure according to the present invention; Figure 4 This is a schematic diagram comparing the micromirror and the back topology optimization region of the present invention.

[0024] In the diagram: 1. Movable mirror; 11. Support frame; 111. Side rib one; 112. Side rib two; 113. Main support rib; 12. Optical reflection plane; 13. Hollowed-out groove; 2. Fixed frame; 21. Outer frame; 22. Inner frame; 3. Elastic support structure; 31. First folded serpentine beam; 32. Second folded serpentine beam; 33. Third folded serpentine beam; 34. Fourth folded serpentine beam; 41. Central magnet; 42. First border magnet; 43. Second border magnet; 5. Contrast micromirror; 51. Topology optimization region. Detailed Implementation

[0025] The following combination Figures 1 to 4 Specific embodiments of the present invention will be described below. It should be understood that the following embodiments are used to illustrate the technical solutions of the present invention. Without departing from the concept of the present invention, those skilled in the art can make adaptive adjustments to the local dimensions and process parameters based on the size of the movable mirror 1, the mass of the central magnet 41, the target operating frequency, the target optical rotation angle, and the silicon-based etching process conditions.

[0026] Example 1 like Figure 1 As shown, this embodiment provides a silicon-based MEMS micromirror structure, which includes a fixed frame 2, a movable mirror 1, and an elastic support structure 3 connecting the fixed frame 2 and the movable mirror 1. The fixed frame 2 includes an outer frame 21 and an inner frame 22. The outer frame 21 is located outside the micromirror structure and is used to provide the overall mounting and support boundary. The inner frame 22 is located on the outer periphery of the movable mirror 1 and is connected to the outer frame 21 and the movable mirror 1 through the elastic support structure 3. The movable mirror 1 is located inside the inner frame 22. The front surface of the movable mirror 1 is an optical reflection plane 12, which can be obtained by forming a metal reflective film or a dielectric reflective film on the front surface of the silicon substrate.

[0027] like Figure 1As shown, the elastic support structure 3 includes a first folded serpentine beam 31, a second folded serpentine beam 32, a third folded serpentine beam 33, and a fourth folded serpentine beam 34. The first folded serpentine beam 31 is located between the outer frame 21 and the inner frame 22, connecting the upper region of the outer frame 21 and the upper region of the inner frame 22. The fourth folded serpentine beam 34 is located between the outer frame 21 and the inner frame 22, connecting the lower region of the outer frame 21 and the lower region of the inner frame 22. The third folded serpentine beam 33 is located between the movable mirror 1 and the inner frame 22, connecting the left side region of the movable mirror 1 and the left side region of the inner frame 22. The second folded serpentine beam 32 is located between the movable mirror 1 and the inner frame 22, connecting the right side region of the movable mirror 1 and the right side region of the inner frame 22. Thus, the movable mirror 1 is elastically connected to the inner frame 22 through the second folded serpentine beam 32 and the third folded serpentine beam 33, and the inner frame 22 is elastically connected to the outer frame 21 through the first folded serpentine beam 31 and the fourth folded serpentine beam 34, so that the movable mirror 1 can generate elastic torsional displacement relative to the outer frame 21 through the inner frame 22.

[0028] Specifically, the first folded serpentine beam 31, the second folded serpentine beam 32, the third folded serpentine beam 33, and the fourth folded serpentine beam 34 can all be integrally etched from silicon-based material. Each folded serpentine beam includes a beam path that extends back and forth along the planar direction. The beam path can consist of multiple parallel or nearly parallel straight beam segments and turning transition segments connecting adjacent straight beam segments. By arranging the beams in a back-and-forth extension manner, the effective length of the beams can be increased within a limited chip planar area, enabling the elastic support structure 3 to have sufficient torsional compliance when the movable mirror 1 deflects around the torsion axis, while avoiding insufficient beam strength due to simply narrowing the beam width.

[0029] In this embodiment, the minimum beam width of the first folded serpentine beam 31, the second folded serpentine beam 32, the third folded serpentine beam 33, and the fourth folded serpentine beam 34 is all 80 μm. The minimum beam width refers to the beam width measured perpendicular to the beam's extension direction at its narrowest point. This minimum beam width can correspond to the width of a straight beam section or the width at the narrowest point in a turning transition section. By setting the minimum beam width to 80 μm, the reduced quality of the deep silicon etching sidewalls, insufficient local strength of the beam, or breakage during subsequent release due to excessively narrow beams can be avoided. Simultaneously, a relatively long equivalent beam length can still be obtained using the folding path, thus balancing structural strength, processing stability, and torsional compliance.

[0030] like Figure 1 and Figure 3As shown, a support frame 11 is provided on the back of the movable mirror 1. The support frame 11 is located in the back area of ​​the movable mirror 1 and does not occupy the optical reflection plane 12 on the front of the movable mirror 1. The support frame 11 includes a main support rib 113, a first side rib 111, a second side rib 112, a load-bearing plate, and a hollow groove 13. The first side rib 111 and the second side rib 112 are arranged opposite to each other, and the line connecting them is in the direction of the torsion axis of the movable mirror 1; the main support rib 113 extends in a direction perpendicular to the torsion axis, the first side rib 111 is located on one side of the main support rib 113, and the second side rib 112 is located on the other side of the main support rib 113. The first side rib 111 and the second side rib 112 are symmetrically arranged about the main support rib 113, so that the support frame 11 forms a relatively balanced load-bearing layout on the back of the movable mirror 1.

[0031] like Figure 3 As shown, the load-bearing plate is formed by extending the end of the main support rib 113, and is a locally widened load-bearing area at the end of the main support rib 113, integrally formed with the main support rib 113. The central magnet 41 is fixed to the load-bearing plate, which can be fixed by adhesive bonding. The load-bearing plate provides a mounting base for the central magnet 41 and forms a direct structural connection between the mounting area of ​​the central magnet 41 and the main support rib 113. The first frame magnet 42 and the second frame magnet 43 can be respectively set on opposite sides of the fixed frame 2, and are respectively located on both sides of the torsion axis of the movable mirror 1; the first frame magnet 42 and the second frame magnet 43 are both fixedly connected to the fixed frame 2 and do not move with the movable mirror 1. The central magnet 41 is fixed to the load-bearing plate and is located in the area between the first frame magnet 42 and the second frame magnet 43, so that the central magnet 41 can deflect with the movable mirror 1 relative to the fixed first frame magnet 42 and the second frame magnet 43.

[0032] like Figure 3 As shown, the inner contours of side rib 111 and side rib 112 are both arc-shaped or bow-shaped, concave towards the main support rib 113. This concave configuration allows side rib 111 and side rib 112 to form a smooth transition support contour on the back of the movable mirror 1, avoiding obvious abrupt changes at local connections where ordinary straight ribs would form. The outer sides of side rib 111 and side rib 112 extend to the connection anchor point between the elastic support structure 3 and the movable mirror 1, specifically to the area where the second folded serpentine beam 32 and the third folded serpentine beam 33 connect with the movable mirror 1, enabling the mounting area of ​​the central magnet 41 where the load-bearing plate is located to form a continuous load-bearing relationship with the elastic support connection area of ​​the movable mirror 1 through the main support rib 113, side rib 111, and side rib 112.

[0033] Meanwhile, the hollowed-out grooves 13 are formed between the main support rib 113 and the first side rib 111, and between the main support rib 113 and the second side rib 112, respectively, and extend through the thickness direction of the support frame 11. The hollowed-out grooves 13 are used to remove solid material from the non-primary load-bearing area on the back of the movable mirror 1, thereby reducing the back mass and rotational inertia of the movable mirror 1. Since the hollowed-out grooves 13 are located between the main support rib 113 and the side ribs, their weight-reduction effect is coordinated with the load-bearing structure of the support frame 11, and will not sever the main support relationship between the mounting area of ​​the central magnet 41 and the elastic support connection area of ​​the movable mirror 1.

[0034] During operation, an alternating current is applied to the external drive coil, generating an alternating magnetic field. Under the influence of this magnetic field, the central magnet 41 applies a driving torque to the movable mirror 1. The movable mirror 1 is elastically deflected relative to the inner frame 22 via the second folded serpentine beam 32 and the third folded serpentine beam 33. The inner frame 22 is elastically supported relative to the outer frame 21 via the first folded serpentine beam 31 and the fourth folded serpentine beam 34. Since the first folded serpentine beam 31 and the fourth folded serpentine beam 34 are located between the outer frame 21 and the inner frame 22, and the second folded serpentine beam 32 and the third folded serpentine beam 33 are located between the movable mirror 1 and the inner frame 22, the elastic support structure 3 can form a graded elastic support path from the movable mirror 1, the inner frame 22 to the outer frame 21. Each folded serpentine beam adopts a folding extension path, and its deformation is mainly distributed in multiple beam segments and their turning transition sections, rather than concentrated at the connection of a single straight beam. At the same time, the minimum beam width of 80μm can provide the necessary tensile, bending and torsional cross-sectional dimensions for the folded serpentine beam, thereby reducing the risk of local overstress in the beam during large-angle scanning.

[0035] In this embodiment, the support frame 11 and the movable mirror 1 are integrally molded monocrystalline silicon structures. The main support rib 113, side rib one 111, side rib two 112, the load-bearing disk integrally extended from the end of the main support rib 113, and the hollow groove 13 can be formed by deep reactive ion etching on the back side of the silicon substrate. The integral molding method can reduce assembly errors and avoid the appearance of adhesive or bonding interfaces between the support frame 11 and the movable mirror 1, thereby improving the consistency and reliability of the micromirror structure.

[0036] In one specific structure, the width of the movable mirror 1 along the direction perpendicular to the torsion axis is W, the inner contour curvature radius of side rib 111 and side rib 112 is set to 0.5W, the diameter of the support plate is set to 0.10W, and the total opening area of ​​the hollow groove 13 can be selected within the range of 30% to 70% of the total area of ​​the back of the movable mirror 1. In this embodiment, the total opening area adopts a preferred value greater than 50%, for example, 55%, and the volume of the solid material of the support frame 11 accounts for no more than 50% of the overall envelope volume of the back of the movable mirror 1. In this embodiment, the overall envelope volume of the back of the movable mirror 1 refers to the volume determined by multiplying the projected area of ​​the outer contour of the support frame 11 on the back of the movable mirror 1 by the thickness of the support frame 11. This set of parameters can form a relatively balanced structural arrangement between back weight reduction and load-bearing stability.

[0037] Example 2 This embodiment describes different structural dimension configurations of the support frame 11 based on Embodiment 1. Structural connection relationships that are repeated in Embodiment 1 will not be described again.

[0038] In one weight-reducing embodiment, the inner contour curvature radius of side ribs 111 and 112 is set to 0.2W, the diameter of the support plate is set to 0.05W, and the total opening area of ​​the hollowed-out groove 13 accounts for 30% to 45% of the total area of ​​the back of the movable mirror 1, for example, 30%. With this configuration, the support frame 11 retains a significant amount of solid material, making it suitable for scenarios where the central magnet 41 has a large mass and high requirements for back-end stability. In this case, the continuous support structure formed by the support plate, main support rib 113, side ribs 111 and 112 is relatively compact, which helps improve the structural support capacity of the fixed area of ​​the central magnet 41.

[0039] In one balanced implementation, the inner contour curvature radius of side ribs 111 and 112 is set to 0.5W, the diameter of the support plate is set to 0.10W, and the total opening area of ​​the hollow groove 13 accounts for more than 50% and no more than 60% of the total area of ​​the back of the movable mirror 1, for example, 55%. Under this configuration, side ribs 111 and 112 form a relatively smooth concave contour on both sides of the main support rib 113. The hollow groove 13 can remove some of the material in the non-primary load-bearing area, while still maintaining the continuous load-bearing relationship between the main support rib 113 and the two side ribs. This configuration is suitable for scenarios that balance operating frequency, optical rotation angle, and structural reliability.

[0040] In one embodiment of weight reduction, the inner contour curvature radius of side ribs 111 and 112 is set to 0.8W, the diameter of the support plate is set to 0.20W, and the total opening area of ​​the hollowed-out groove 13 accounts for 60% to 70% of the total area of ​​the back of the movable mirror 1, for example, 70%. With this configuration, the total opening area of ​​the hollowed-out groove 13 is larger, further reducing the mass and moment of inertia of the back of the movable mirror 1; at the same time, the larger support plate can still provide a mounting base for the central magnet 41, and the main support rib 113, side ribs 111 and 112 remain continuously arranged to reduce the impact of excessive weight reduction on load-bearing stability.

[0041] With the different structural size configurations described above, the total opening area of ​​the hollowed-out groove 13 is not limited to 55%, but can be selected within the range of 30% to 70% depending on the mass of the central magnet 41, the target operating frequency, and the target optical rotation angle. Among these, 30% to 45% focuses more on load-bearing stability, greater than 50% but not exceeding 60% focuses more on performance balance, and 60% to 70% focuses more on weight reduction and reducing rotational inertia. Regardless of which structural size configuration is adopted, the support frame 11 is located on the back of the movable mirror 1, and forms a support relationship extending from the installation area of ​​the central magnet 41 to the connection area between the second folded serpentine beam 32, the third folded serpentine beam 33, and the movable mirror 1 through the load-bearing plate, the main support rib 113, the first side rib 111, and the second side rib 112.

[0042] Example 3 This embodiment combines Figure 4 Explain the differences between the present invention and the comparative structure. Figure 4 In the diagram, the contrast micromirror 5 is used to illustrate the back structure of a conventional mirror, and the topology optimization region 51 is used to illustrate the area on the back of the movable mirror 1 where structural optimization can be performed. The contrast micromirror 5 can be understood as a MEMS micromirror with a solid or regular local structure on its back. Its back structure usually emphasizes overall stiffness, regular weight reduction, or torsion beam structure optimization, but does not uniformly design the mounting area of ​​the central magnet 41, the connection area between the second folded serpentine beam 32 and the third folded serpentine beam 33 and the movable mirror 1, or the mass distribution on the back of the movable mirror 1.

[0043] To facilitate the explanation of the structural effects of the present invention, this embodiment selects an electromagnetically driven MEMS micromirror disclosed in the prior art as a comparative sample. This prior art is: Xu Fan, Peng Changsi, Zeng Zhongming, Wu Dongmin, "A Novel Electromagnetically Driven MEMS Micromirror with a Torsion Beam Structure," *Advances in Laser & Optoelectronics*, 2024, Vol. 61, No. 17, Article No. 1723001, DOI: 10.3788 / LOP232569. The disclosed electromagnetically driven MEMS micromirror has a mirror surface size of 5mm × 6mm and achieves electromagnetic driving scanning through a magnet bonded to the back of the mirror and a coil drive. Its test results show that the fast-axis resonant frequency is 1082.2Hz, the fast-axis optical angle is 53.9°, and its simulation results show a fast-axis resonant frequency of 1089.0Hz.

[0044] In this invention, the topology-optimized region 51 is located on the back of the movable mirror 1 and does not affect the optical reflection plane 12 on the front of the movable mirror 1. By forming a support frame 11 within the topology-optimized region 51, the material distribution on the back of the movable mirror 1 can be adjusted while retaining the optical reflection plane 12. The support plate is an integrated bearing area at the end of the main support rib 113, and its position corresponds to the mounting area of ​​the central magnet 41. The main support rib 113 extends in a direction perpendicular to the torsion axis, and side ribs 111 and 112 are located on both sides of the main support rib 113. The hollowed-out groove 13 is used to remove local material from the back. Thus, the support frame 11 is not a simple ordinary reinforcing rib or an arbitrary opening structure, but a back structure that cooperates with the mounting of the central magnet 41 and the torsional support relationship of the movable mirror 1.

[0045] In the sample of this invention, the minimum beam width of the first folded serpentine beam 31, the second folded serpentine beam 32, the third folded serpentine beam 33, and the fourth folded serpentine beam 34 is 80 μm, and the volume of the solid material of the supporting frame 11 accounts for no more than 50% of the overall envelope volume of the back of the movable mirror 1. Under the condition of stress less than 500 MPa, the fast axis resonant frequency of the sample of this invention is 1447.2 Hz, and the maximum fast axis optical angle is 66°. Compared with the existing electromagnetically driven MEMS micromirrors, this invention does not simply rely on changing the torsion beam structure to achieve performance improvement, but forms a supporting frame 11 on the back of the movable mirror 1, consisting of a main support rib 113, a first side rib 111, a second side rib 112, a load-bearing plate, and a hollow groove 13. This allows the mounting area of ​​the central magnet 41, the back bearing structure, and the elastic support connection area of ​​the movable mirror 1 to form a structural fit, thereby maintaining the back bearing stability while taking into account both a higher operating frequency and a larger optical rotation angle.

[0046] Example 4 This embodiment combines Figure 2 Explain the molding method of silicon-based MEMS micromirror structures.

[0047] The molding method of this embodiment first selects the non-optical area on the back of the movable mirror 1 as the initial design domain, then sets the volume retention rate, fixed constraints and boundary loads, and then uses the SIMP variable density method, moving asymptote algorithm and Helmholtz density filtering for topology optimization. Finally, the extracted main load-bearing path is transformed into a mask pattern that meets the minimum feature size requirements of deep reactive ion etching.

[0048] Specifically, in step 1, the non-optical area on the back of the movable mirror 1 is used as the initial design domain, which can correspond to... Figure 4 The topology optimization region 51 is defined as follows: the installation area of ​​the central magnet 41 is used as the boundary load application area, the connection area between the second folded serpentine beam 32 and the third folded serpentine beam 33 and the movable mirror 1 is used as the fixed constraint area, and the constraint condition that the volume retention rate does not exceed 50% is set so that the optimization process can correspond to the actual force conditions on the back of the movable mirror 1 under electromagnetic drive.

[0049] In step 2, with the objective functions of maximizing the global stiffness of the structure and minimizing the dynamic strain energy, a variable density SIMP interpolation model is used to characterize the material density of each element within the design domain. This is then iteratively updated using a moving asymptote algorithm to obtain a continuous material density distribution that matches the principal stress transmission path. Optionally, the initial material density can be set to 0.5. Physical field analysis is used to obtain the stress state of each element within the design domain, and density updates are used to adjust the material density of each element.

[0050] In step 3, Helmholtz density filtering is used to filter the density distribution of continuous materials to suppress numerical instability phenomena such as checkerboard patterns and mesh dependence; when the maximum value of the element density change between two adjacent iterations is less than 1×10 -6 When the optimization process reaches the convergence criterion, the main load-bearing path is extracted from the continuous material density distribution after convergence.

[0051] In step 4, in accordance with the requirement that the minimum feature size of the deep reactive ion etching process is not less than 10 μm, the topology corresponding to the main load-bearing path is smoothed and corrected at the boundary, and isolated areas and extremely thin-walled areas are removed to generate a mask pattern containing the geometric features of the central main support rib 113, variable cross-section side rib one 111, variable cross-section side rib two 112, load-bearing plate and hollow groove 13; wherein, the minimum line width of the solid structure in the support skeleton 11 and the minimum spacing of the hollow groove 13 are not less than 10 μm.

[0052] In step 5, deep reactive ion etching is performed on the back side of the silicon substrate through the mask pattern to integrally form the support frame 11, so that the back of the movable mirror 1 forms the main support rib 113, side rib one 111, side rib two 112, load-bearing plate and hollow groove 13.

[0053] When forming the elastic support structure 3, the first folded serpentine beam 31, the second folded serpentine beam 32, the third folded serpentine beam 33, and the fourth folded serpentine beam 34 can be integrally etched into the same silicon substrate as the outer frame 21, the inner frame 22, the movable mirror 1, and the support skeleton 11. Specifically, when forming the mask pattern, the outlines of the outer frame 21, the inner frame 22, the movable mirror 1, each folded serpentine beam, and the support skeleton 11 are simultaneously defined in the mask pattern; subsequently, deep reactive ion etching is performed along the mask opening to etch and release the peripheral areas of each folded serpentine beam. Among them, the first folded serpentine beam 31 and the fourth folded serpentine beam 34 are retained as beam materials connecting the outer frame 21 and the inner frame 22, and the second folded serpentine beam 32 and the third folded serpentine beam 33 are retained as beam materials connecting the inner frame 22 and the movable mirror 1.

[0054] During the mask pattern design stage, the linewidth of each folded serpentine beam is designed to be no less than 80μm, and the corners are rounded or smoothed at the turning transition sections to reduce stress concentration in the sharp corner areas during etching and operation. After deep reactive ion etching is completed, the first folded serpentine beam 31 and the fourth folded serpentine beam 34 are integrally connected with the outer frame 21 and the inner frame 22, while the second folded serpentine beam 32 and the third folded serpentine beam 33 are integrally connected with the inner frame 22 and the movable mirror 1. Thus, the elastic support structure 3 does not need to be formed through subsequent assembly, which can reduce assembly errors and ensure the stability of the torsional connection positions between the movable mirror 1 and the inner frame 22, and between the inner frame 22 and the outer frame 21.

[0055] Optionally, after deep reactive ion etching (DRIE), the silicon substrate undergoes release cleaning and drying to remove etching residues and reduce the risk of adhesion between the folded serpentine beams. Since the minimum beam width of each folded serpentine beam is not less than 80 μm, and the gap between beams meets the minimum spacing requirements for deep silicon etching during mask pattern design, it ensures that etching gas enters the inter-beam region and forms a relatively complete release profile, enabling the elastic support structure 3 to possess stable torsional deformation capabilities.

[0056] After the deep reactive ion etching is completed, the central magnet 41 is fixed to the support plate, and the first frame magnet 42 and the second frame magnet 43 are set according to the electromagnetic drive requirements. The formed structure maintains the optical reflection plane 12 on the front of the movable mirror 1, while forming a support frame 11 on the back that has a lightweight load-bearing function and matches the main stress transmission path.

[0057] Using the above-described molding method, the structural form of the support frame 11 can be determined based on the actual design domain of the back of the movable mirror 1, the installation position of the central magnet 41, and the connection area between the second folded serpentine beam 32 and the third folded serpentine beam 33 and the movable mirror 1, and can be transformed into an integrally molded structure suitable for deep reactive ion etching. This method does not simply use the topology optimization results directly as a mask; instead, after extracting the main load-bearing path, it performs boundary smoothing, minimum feature size verification, and process modification, ensuring that the final support frame 11 balances back-side lightweighting, load-bearing stability, and silicon-based fabrication feasibility.

Claims

1. A silicon-based MEMS micromirror structure, comprising a fixed frame (2), a movable mirror (1), and an elastic support structure (3) connecting the two, wherein the front surface of the movable mirror (1) is an optical reflection plane (12), characterized in that: The movable mirror (1) has a supporting frame (11) on its back; The support frame (11) includes a main support rib (113) extending along a direction perpendicular to the torsion axis of the movable mirror (1), side rib one (111) and side rib two (112) located on both sides of the main support rib (113), a load-bearing plate integrally extended from the end of the main support rib (113), and hollow grooves (13) formed between the main support rib (113) and side rib one (111) and between the main support rib (113) and side rib two (112), respectively. The inner contours of the first side rib (111) and the second side rib (112) are both arc-shaped or bow-shaped with concave inward toward the main support rib (113). The hollowed-out groove (13) penetrates the thickness direction of the supporting frame (11); A central magnet (41) is fixed on the load-bearing plate.

2. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: The first side rib (111) and the second side rib (112) are symmetrically arranged about the main support rib (113).

3. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: The outer sides of the first side rib (111) and the second side rib (112) extend to the connection anchor point between the elastic support structure (3) and the movable mirror (1).

4. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: The load-bearing plate is a circular load-bearing area formed by locally widening the end of the main support rib (113), and the load-bearing plate and the main support rib (113) are an integral continuous structure.

5. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: When the inner contours of the first side rib (111) and the second side rib (112) are arc-shaped, the radii of curvature of the inner contours are 0.2 to 0.8 times the width of the movable mirror (1) along the direction perpendicular to the torsion axis.

6. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: The diameter of the load-bearing plate accounts for 5% to 20% of the width of the movable mirror (1) along the direction perpendicular to the torsion axis; The total opening area of ​​the hollowed-out groove (13) accounts for 30% to 70% of the total area of ​​the back of the movable mirror (1).

7. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: Both the first side rib (111) and the second side rib (112) have a gradually changing structure that is wider in the middle and narrower at both ends along their extension direction.

8. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: The supporting frame (11) and the movable mirror (1) are integrally formed, and the volume of the solid material of the supporting frame (11) accounts for no more than 50% of the overall enveloping volume of the back of the movable mirror (1).

9. The silicon-based MEMS micromirror structure according to claim 1, characterized in that: The elastic support structure (3) includes a first folded serpentine beam (31), a second folded serpentine beam (32), a third folded serpentine beam (33), and a fourth folded serpentine beam (34); The minimum beam width of the first folded serpentine beam (31), the second folded serpentine beam (32), the third folded serpentine beam (33), and the fourth folded serpentine beam (34) is not less than 80 μm.

10. A method for fabricating a silicon-based MEMS micromirror structure, used to prepare the silicon-based MEMS micromirror structure according to any one of claims 1 to 9, characterized in that, Includes the following steps: S1. Taking the non-optical area on the back of the movable mirror (1) as the initial design domain, the installation area of ​​the central magnet (41) as the boundary load application area, the connection area between the elastic support structure (3) and the movable mirror (1) as the fixed constraint area, and setting the constraint condition that the volume retention rate does not exceed 50%. S2. With the objective function of maximizing the global stiffness of the structure and minimizing the dynamic strain energy, the SIMP interpolation model of the variable density method is adopted, and the moving asymptote algorithm is combined for iteration to obtain the continuous material density distribution that matches the principal stress transmission path. S3. The Helmholtz density filtering technique is used to filter the density distribution of the continuous material to suppress the numerical instability of checkerboard and grid-dependent phenomena, and the main load-bearing path is extracted after the convergence criterion is reached. S4. In accordance with the requirement that the minimum feature size of the deep reactive ion etching process is not less than 10 μm, the topology corresponding to the main load-bearing path is smoothed and corrected to generate a mask pattern containing the geometric features of the main support rib (113), the side rib one (111), the side rib two (112), the load-bearing disk that is integrally continuous with the main support rib (113), and the hollow groove (13). S5. Perform deep reactive ion etching on the back side of the silicon substrate according to the mask pattern to integrally form the support skeleton (11).

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