Biaxial laminated scanning mirror with centering property and preparation method thereof

Abstract

The invention belongs to the related technical field of optical devices, and discloses a biaxial laminated scanning mirror with centering performance and a preparation method thereof. The scanning mirror comprises an upper-layer structure and a lower-layer structure, wherein the lower-layer structure serves as a carrier of the upper-layer structure and provides swinging driving force for the upper-layer structure; the upper layer structure comprises a reflecting mirror and centering spring beams symmetrically arranged on the side face of the mirror body. The invention also discloses a preparation method of the scanning mirror. According to the invention, the problems that the stability of the mirror body is low and the mirror body interferes with the driver during large-rotation-angle movement are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field related to optical devices, and more specifically, relates to a dual-axis stacked scanning mirror with centering capability and a preparation method thereof. Background Art

[0002] Scanning mirrors are an important class of optical devices, primarily used to deflect, focus, and scan light, enabling observation, imaging, or measurement of target objects. With technological advancements, MEMS scanning mirrors based on semiconductor processes have garnered widespread attention. Compared to traditional mechanical scanning mirrors, MNEMS scanning mirrors offer advantages such as miniaturization and integration, low power consumption, high precision, and high reliability. They are widely used in a wide range of fields, including consumer electronics, optical communications, the automotive industry, and biomedicine.

[0003] Based on the actuation method, MEMS scanning mirrors can be categorized into architectures such as electromagnetic, electrostatic, piezoelectric, and thermoelectric. Structurally, they can be divided into planar and stacked architectures. A planar architecture involves the scanning mirror's actuator and mirror surface being arranged on the same plane. While the process flow is relatively simple, achieving a high fill factor is difficult. Large mirror surfaces (typically ≥10mm) can generate significant stress distribution, impacting device reliability. Against this backdrop, a stacked architecture has been proposed: the actuator and mirror surface are fabricated on separate structural layers and connected by a linkage. This design significantly increases chip fill factor (typically >90%), effectively utilizing wafer area. This approach is particularly advantageous in scanning mirror arrays, enabling highly dense layouts. Furthermore, for large-mirror scanning mirrors, the stacked architecture offers relatively low overall stress, making it easier to meet reliability design requirements. Therefore, the stacked architecture is a preferred solution for large-scale scanning mirror arrays and large-mirror scanning mirrors.

[0004] Existing laminated mirror designs suffer from the following common issues: Because the driver and mirror are not coplanar and are connected via a connecting rod, the mirror's stability is reduced, particularly when subjected to external shock and vibration. Furthermore, large-angle mirror motion can interfere with the driver, leading to device failure. To address these shortcomings, a novel structural solution is proposed. Summary of the Invention

[0005] In response to the above defects or improvement needs of the prior art, the present invention provides a dual-axis laminated scanning mirror with centering properties and a preparation method thereof, which solves the problems of low mirror stability and interference with the driver during large-angle movement.

[0006] To achieve the above-mentioned objectives, according to one aspect of the present invention, a dual-axis stacked MEMS scanning mirror with centering properties is provided, which includes an upper structure and a lower structure, wherein the lower structure serves as a carrier of the upper structure and provides a driving force for the upper structure to swing; the upper structure includes a reflector and a centering spring beam symmetrically arranged on the side of the mirror body.

[0007] Further preferably, the upper structure further includes a centering plate, the centering plate serves as a carrier of the mirror body, and the centering spring beam is connected to the centering plate.

[0008] Further preferably, the center of the upper structure is connected to the center of the lower structure through a connecting rod, and the connecting rod is connected to the mirror body through the centering plate.

[0009] Further preferably, the lower structure includes a base, a driver and a connecting spring beam, the connecting spring beam is arranged around the connecting rod to connect the connecting rod and the base, and the driver provides a driving force for the upper structure to swing.

[0010] Further preferably, one end of the centering spring is anchored to the base of the lower structure.

[0011] Further preferably, the parallel equivalent stiffness k1 of the connecting spring beam and the parallel equivalent stiffness k2 of the centering spring beam should satisfy the relationship: k1>k2.

[0012] Further preferably, the centering spring beam and the connecting spring beam are serpentine folding beams, straight beams, curved beams, L-shaped spring beams or U-shaped spring beams.

[0013] According to another aspect of the present invention, a method for preparing the aforementioned dual-axis stacked scanning mirror with centering properties is provided, the method comprising the following steps:

[0014] Select the wafer as the base of the lower structure, grow a buffer layer on the front of the wafer, prepare the driver on the buffer layer and

[0015] A protective film is covered on the formed driver; a connecting rod is etched on the back of the wafer to release the buried oxide layer in the wafer, thereby releasing the constraints of the driver and spring beam attached to the buried oxide layer and obtaining a lower structure preform;

[0016] A wafer is selected as the base of the upper structure, and a centering plate and a centering spring beam are etched on the front surface of the wafer to obtain a prefabricated upper structure;

[0017] The upper structure preform and the lower structure preform are bonded together, and a reflective film is plated on the mirror surface of the upper structure preform; the buried oxide layer in the wafer is released to release the constraint of the centering spring beam attached to the buried oxide layer; and the protective film of the lower structure preform is removed to obtain the required scanning mirror.

[0018] Further preferably, the scanned wafers are all SOI wafers.

[0019] Further preferably, the reflective film is plated on the surface of the mirror body of the upper structure by a magnetron sputtering process or an electron beam evaporation coating process.

[0020] In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:

[0021] 1. The present invention has centering spring beams installed on both sides of the mirror body of the superstructure. These centering spring beams limit the mirror surface when it deflects at large angles, preventing interference with the actuator during deflection. Furthermore, the installation of the centering spring beams significantly reduces the effects of external shocks and vibrations on the mirror body, facilitating long-term operational reliability and increasing the stability of the mirror surface.

[0022] 2. The scanning mirror of the present invention is dually constrained by the centering spring beam and the connecting spring beam. Both the swing end (connected to the centering spring beam) and the supporting end (connected to the connecting rod) of the mirror are constrained by the corresponding spring beams, thus forming a double-end constraint. Compared with the single-end constraint of existing designs on the market, the centering performance is greatly improved, thereby enhancing the accuracy and controllability of the scanning mirror.

[0023] 3. In the preparation method provided in the present invention, SOI silicon wafers are used in both the upper structure and the lower structure and the front and back surfaces are processed respectively, and finally assembled through a bonding process. Based on this process, independent processing of the four-layer structure can be achieved, and finally an integrated integrated chip of the driver, connecting rod, centering spring and mirror is realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of the overall structure of a scanning mirror constructed according to a preferred embodiment of the present invention;

[0025] Figure 2 is a side view of a scanning mirror constructed in accordance with a preferred embodiment of the present invention;

[0026] Figure 3 is a top view of a superstructure constructed according to a preferred embodiment of the present invention;

[0027] Figure 4 is a top view of a driver constructed according to a preferred embodiment of the present invention;

[0028] Figure 5is a schematic cross-sectional view of a piezoelectric actuator constructed according to a preferred embodiment of the present invention;

[0029] Figure 6 is a schematic plan projection diagram of a cantilever beam of a driver constructed according to a preferred embodiment of the present invention;

[0030] Figure 7 This is a schematic structural diagram of a connecting spring beam or a centering spring beam constructed according to a preferred embodiment of the present invention:

[0031] Figure 8 is a schematic cross-sectional view of a film structure on the surface of a mirror body constructed according to a preferred embodiment of the present invention;

[0032] Figure 9 is a schematic diagram of the surface configuration of a mirror body constructed according to a preferred embodiment of the present invention;

[0033] Figure 10 This is a process flow for preparing a superstructure constructed according to a preferred embodiment of the present invention;

[0034] Figure 11 This is a process flow for preparing a lower structure constructed according to a preferred embodiment of the present invention;

[0035] Figure 12 The present invention relates to the bonding of the upper and lower structures and the subsequent process flow constructed according to the preferred embodiment of the present invention.

[0036] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0037] 1-base, 2-driver, 3-connecting spring beam, 4-centering plate, 5-centering spring beam, 6-mirror body, 7-connecting rod. DETAILED DESCRIPTION

[0038] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0039] like Figure 1 Figure 1 shows a schematic diagram of a dual-axis stacked scanning mirror with centering capability. The scanning mirror comprises a base 1, at least two drivers 2, at least two connecting spring beams 3, a connecting rod 7, a centering plate 4, at least two centering spring beams 5, and a mirror body 6.

[0040] One end of the actuator 2 is connected to the base 1 and fixed, and the other end is connected to the connecting rod 7 through the connecting spring beam 3. The connecting rod 7 is further connected to the centering plate 4, which is directly connected to the mirror body 6; at the same time, one end of the centering spring beam 5 is connected to the centering plate 4 and the other end is anchored.

[0041] like Figures 2-4 As shown, the cross-sectional structure along the xOz plane of an embodiment, in this embodiment, the driver 2 adopts a piezoelectric drive, that is, the inverse piezoelectric effect is used to apply voltage to the piezoelectric material to cause displacement. The piezoelectric drive comprises at least a sandwich membrane structure consisting of a top electrode-piezoelectric material-bottom electrode, and a passive layer (such as a silicon film) bonded thereto, together forming a cantilever drive structure. The cross-sectional structure diagram of the piezoelectric drive is shown in FIG. Figure 5 , the passive layer and the electrode-piezoelectric material-electrode composite film layer are bonded to each other, and the projection of the electrode-piezoelectric material-electrode composite film on the xOy plane is included in the projection of the passive layer on the xOy plane; specifically, as for the plane shape of the driver 2, any reasonable polygon such as triangle, rectangle, trapezoid, fan, leaf, etc. can be used to design the cantilever beam, such as Figure 6 As shown in Figure 1 , in actual operation, by applying a driving voltage to the top and bottom electrodes, the piezoelectric material tends to displace longitudinally due to the inverse piezoelectric effect, thereby accumulating stress in the mutually constrained passive layer. Because one end of the passive layer is fixedly constrained by the base 1, warping occurs at its free end, causing displacement. This, in turn, drives the connecting rod 7 and the centering plate 4 through the connecting spring, ultimately driving the mirror body 6.

[0042] It should be pointed out that when the piezoelectric drive method is used to implement this solution, the piezoelectric material can be freely selected according to the specific application scenario and parameter requirements, including but not limited to PZT (lead zirconate titanate), AlN (aluminum nitride), AlScN (scandium-doped aluminum nitride), ZnO (zinc oxide), KNN (sodium potassium niobate), PVDF (polyvinylidene fluoride) and other piezoelectric materials.

[0043] In an embodiment of the present invention, the driver 2 can be any one of piezoelectric drive, electromagnetic drive, electrostatic drive or electrothermal drive. Under a given driving electrical signal, the driver 2 can generate out-of-plane (outside the xOy plane) motion, thereby driving the connecting rod 7 to move; when the movement direction of the driver 2 on the opposite side is opposite or the displacement is different, the connecting rod 7 will be deflected, thereby driving the top centering plate 4 and the mirror body 6 to rotate. It should be noted that the movement direction of the driver 2 is bidirectional, that is, under different driving signals, positive and negative out-of-plane deflections can be achieved respectively. These deflection motions are ultimately transmitted to the mirror body 6, so the rotational motion of the mirror body 6 is highly free.

[0044] In the embodiment of the present invention, the stiffness matching of the connecting spring beam 3 and the centering spring beam 5 can be reasonably adjusted as needed. Preferably, the parallel equivalent stiffness k1 of the connecting spring beam 3 and the parallel equivalent stiffness k2 of the centering spring beam 5 should satisfy the relationship: k1>k2, so as to avoid excessive stiffness of the centering spring beam 5, which would over-constrain the mirror body 6 and affect the scanning range of the mirror body 6.

[0045] In the embodiment of the present invention, the connecting spring beam 3 and the centering spring beam 5 are both designed as serpentine folding beams. It should be noted that the present invention does not limit the structural types of the connecting spring beam 3 and the centering spring beam 5, but can arbitrarily select different structures and combinations according to actual design requirements, including but not limited to: straight beams, curved beams, L-shaped spring beams, and U-shaped spring beams. Figure 7 Some spring design examples are given.

[0046] The mirror body 6 in the present invention can be realized by a single-layer structure, a double-layer structure or a multi-layer structure. When a single-layer structure is adopted, a material with high reflectivity such as metal can be selected. When a double-layer or multi-layer structure is adopted, a substrate material with a highly flat surface and various coating materials can be selected. The surface coating can be used to improve reflectivity, suppress light loss, adjust the reflection wavelength, protect the mirror body 6, etc. Figure 8 Several embodiments of the mirror body 6 film layer structure are shown.

[0047] In addition, the surface of the mirror body 6 can be a flat structure, or a convex or concave structure, which is used to expand or reduce the angle of the light path; the mirror body 6 can also be a grating structure, which can select the wavelength. Figure 9 Several embodiments of the surface configuration of the mirror body 6 are shown.

[0048] The scanning mirror operates as follows: Driver 2, under a given drive signal, drives connecting rod 7 to deflect, thereby rotating mirror body 6. By properly controlling the input signal, mirror body 6 can be made to rotate along a predetermined trajectory, thereby performing functions such as beam scanning and reflection signal capture.

[0049] The centering principle of the above-mentioned scanning mirror is as follows: the centering spring beam 5 and the centering plate 4 are used to directly constrain the mirror body 6, and the centering property of the mirror body 6 is enhanced through the symmetrical design and the stiffness of the spring.

[0050] In order to realize the centering structure and function of the stacked MEMS scanning mirror, it is necessary to use SOI wafers to separate the mirror body and the centering layer (including the centering spring beam 5 and the centering plate 4). Therefore, it is necessary to first complete the corresponding process steps on two SOI (Silicon on Insulator) wafers to form the upper structure and the lower structure respectively, and then realize the wafer-level bonding of the two SOI wafers through the silicon-silicon bonding process, and finally complete the remaining processing flow on the composite structure wafer combined into one. After completing all the process flows, the complete wafer is divided into independent MEMS chips by mechanical scribing or laser scribing. Among them, the SOI wafer is composed of top silicon (i.e., device layer), bottom silicon (i.e., substrate layer) and a buried oxide layer separated therein. The surface corresponding to the top silicon of the SOI wafer is the front side of the wafer, and the surface corresponding to the bottom silicon is the back side of the wafer.

[0051] The lower structure, including the base 1, actuator 2, connecting spring beam 3, and connecting rod 7, can be fabricated from a single SOI wafer. The actuator 2 and connecting spring beam 3 are fabricated on the SOI device layer, while the base 1 and connecting rod 7 are fabricated on the SOI substrate layer. The upper structure, including the centering plate 4, centering spring beam 5, and mirror body 6, is fabricated from a separate SOI wafer. The centering plate 4 and centering spring beam 5 are fabricated on the SOI device layer, while the mirror body 6 is fabricated on the SOI substrate layer. After fabrication, the two SOI wafers are assembled through bonding or adhesive bonding, completing the complete scanning mirror assembly.

[0052] The details are as follows:

[0053] (1) Figure 10 As shown, the processing flow of the superstructure is:

[0054] S11 prepares the SOI wafer and cleans it. In order to increase the adhesion of the subsequent film on the wafer surface and ensure the film quality, a buffer layer is grown on the surface of the SOI wafer. Thermal silicon oxide is usually grown using a thermal oxidation method.

[0055] S12: Prepare driver 2 on the buffer layer

[0056] The deposited piezoelectric film stack usually includes a bottom electrode film layer, a piezoelectric film, and a top electrode film to form a sandwich structure. The bottom and top electrodes can be made of metal materials commonly used in magnetron sputtering or electron beam evaporation processes, such as gold, platinum, aluminum, molybdenum, etc. The piezoelectric film can be made of common piezoelectric materials such as PZT (lead zirconate titanate), AlN (aluminum nitride), AlScN (scandium-doped aluminum nitride), KNN (sodium potassium niobate), etc., and the preparation process can be magnetron sputtering or sol-gel method;

[0057] The top electrode is patterned by a photolithography-etching process;

[0058] Patterning of the piezoelectric layer is achieved through a photolithography-etching process;

[0059] The bottom electrode is patterned by a photolithography-etching process;

[0060] The buffer layer and device layer silicon are patterned by photolithography-etching process;

[0061] The front-side processing of the first SOI wafer has been completed, resulting in the main structure of driver 2. It is now time to switch to back-side processing. To protect the front-side pattern from damage, a flexible protective film is applied to the front of the wafer.

[0062] S13 flips the SOI wafer over and performs backside processing: back cavity etching is completed through a photolithography-etching process. This step also completes the processing of the connecting rod 7;

[0063] S14 releases the buried oxide layer. After completion, the driver 2 can flip over based on its clamping end and drive the corresponding electrode to realize its driving function.

[0064] (2) Figure 11 As shown, the preparation process of the lower structure is as follows:

[0065] S21 prepares and cleans the SOI wafer. Unlike the SOI wafer in the upper structure, the SOI wafer in the lower structure serves as the mirror body (substrate layer) and the centering layer. The centering layer includes the centering spring beam 5 and the centering plate 4. The substrate layer is relatively thin to reduce the mass of the mirror body and lower the overall system load.

[0066] S22 uses a photolithography-etching process to pattern the centering spring beam 5 and the centering plate 4. A key process for achieving the centering architecture function of the present invention is to separate the process of the centering layer and the mirror body layer, which requires the introduction of a physical separation layer. Therefore, the wafer for the lower structure also needs to be an SOI wafer. The centering spring beam 5 and the centering plate 4 are processed on the device layer, and the buried oxide layer is used to achieve physical isolation between the centering layer and the mirror body layer.

[0067] (3) Bonding of the superstructure and the substructure.

[0068] After the two SOI wafers are processed separately, they need to be bonded together to achieve integration and then proceed to the subsequent processes. Figure 12 The process flow shown:

[0069] S31 aligns the back surface of the first wafer with the front surface of the second wafer, and then directly connects them through a silicon-silicon bonding process. After completing this step, the two wafers are combined into a composite structure wafer. The two surfaces of the composite structure wafer are the front surface of the original first wafer and the back surface of the original second wafer, which are defined as the driving surface and mirror surface of the composite wafer respectively.

[0070] S32: coating a reflective film on the surface of the mirror body 6 of the composite wafer by a magnetron sputtering process or an electron beam evaporation coating process. The coating material may be a metal material with high reflectivity such as gold, silver, aluminum, or other optical reflective materials;

[0071] S33: Patterning of the reflective film layer of the mirror body 6 is achieved through a photolithography-etching process. The pattern of the reflective film layer of the mirror body 6 is usually consistent with the pattern of the mirror body. Patterning of the mirror body is achieved through a photolithography-etching process.

[0072] S34 releases the original buried oxide layer of the second wafer through a dry or wet etching process. This step is also a key step in achieving system centering. After this step, the centering spring beam 5 can deform freely to achieve a restraining effect on the mirror body, thereby achieving important functions such as improving system stability, improving system centering, and avoiding interference between the mirror body and the driver 2;

[0073] S35 removes the protective film previously attached to the front side of the SOI wafer on the upper structure.

[0074] Subsequently, the discrete stacked MEMS scanning mirror chip can be completed by wafer dicing.

[0075] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A dual-axis stacked MEMS scanning mirror with centering properties, characterized in that: The scanning mirror comprises an upper structure and a lower structure, wherein the lower structure serves as a carrier of the upper structure and provides a driving force for the upper structure to swing; the upper structure comprises a mirror body (6) and a centering spring beam (5) symmetrically arranged on the side of the mirror body (6).

2. The dual-axis stacked scanning mirror with centering capability according to claim 1, wherein: The upper structure also includes a centering plate (4), which serves as the carrier, and the centering spring beam (5) is connected to the centering plate (4).

3. The dual-axis stacked scanning mirror with centering capability according to claim 1, wherein: The center of the upper structure is connected to the center of the lower structure via a connecting rod (7), and the connecting rod (7) is connected to the mirror body (6) via the centering plate (4).

4. The dual-axis stacked scanning mirror with centering capability according to claim 3, wherein: The lower structure comprises a base (1), a driver (2) and a connecting spring beam (3); the connecting spring beam (3) is arranged around the connecting rod (7) and is used to connect the connecting rod (7) and the base (1); the driver (2) provides a driving force for the upper structure to swing.

5. The dual-axis stacked scanning mirror with centering capability according to claim 4, wherein: One end of the centering spring beam (5) is anchored to the base of the lower structure.

6. The dual-axis stacked scanning mirror with centering capability according to claim 1, wherein: The parallel equivalent stiffness k1 of the connecting spring beam (3) and the parallel equivalent stiffness k2 of the centering spring beam (5) should satisfy the relationship: k1>k2.

7. The dual-axis stacked scanning mirror with centering capability according to claim 1, wherein: The centering spring beam (5) and the connecting spring beam (3) are serpentine folding beams, straight beams, curved beams, L-shaped spring beams or round-shaped spring beams.

8. A method for preparing a dual-axis stacked scanning mirror with centering properties according to any one of claims 1 to 7, characterized in that: The preparation method comprises the following steps: Select the wafer as the base of the lower structure, grow a buffer layer on the front of the wafer, prepare the driver on the buffer layer and A protective film is covered on the formed driver; a connecting rod is etched on the back of the wafer to release the buried oxide layer in the wafer, thereby releasing the constraints of the driver and spring beam attached to the buried oxide layer and obtaining a lower structure preform; A wafer is selected as the base of the upper structure, and a centering plate and a centering spring beam are etched on the front surface of the wafer to obtain a prefabricated upper structure; The upper structure preform and the lower structure preform are bonded together, and a reflective film is plated on the mirror surface of the upper structure preform; the buried oxide layer in the wafer is released to release the constraint of the centering spring beam attached to the buried oxide layer; and the protective film of the lower structure preform is removed to obtain the required scanning mirror.

9. The preparation method according to claim 8, wherein The scanned wafers are all SOI wafers.

10. The preparation method according to claim 9, characterized in that The reflective film is plated on the surface of the mirror body of the upper structure by a magnetron sputtering process or an electron beam evaporation coating process.

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