A dual-sided reflective piezoelectric MEMS biaxial micromirror unit, array and method of manufacture

Abstract

The application relates to a double-sided reflection piezoelectric MEMS biaxial micromirror unit, array and preparation method, belonging to the technical field of micro-electro-mechanical systems (MEMS), and comprising a mirror platform, a peripheral fixed frame, a support beam and a piezoelectric driving stack, the support beam connects the mirror platform and the peripheral fixed frame, the piezoelectric driving stack is arranged on the support beam and is used for driving the support beam to deform so as to drive the mirror platform to realize biaxial deflection; the application can solve the core technical problems of high main light path loss, complex system light path and difficulty in large-scale array integration inherent in the existing single-sided reflection micromirror scheme.

Description

Technical Field

[0001] This invention belongs to the field of microelectromechanical systems (MEMS) technology, and specifically relates to a dual-sided reflective piezoelectric MEMS biaxial micromirror unit, array, and fabrication method. Background Technology

[0002] With the rapid evolution of applications such as large-scale data center interconnection and integrated space-ground laser communication, all-optical switching technology, with its core advantages of low latency, low power consumption, high bandwidth, and transparent transmission, has become a core direction for the reconstruction of optical network architectures. Optical cross-connection schemes based on MEMS dual-axis micromirror arrays, possessing the characteristics of non-blocking interconnection and easy port scalability, are the core execution devices of all-optical switching systems. As all-optical switching systems continue to develop towards high port density and low insertion loss, core requirements are placed on MEMS micromirrors that balance large-angle dual-axis deflection, low-loss transmission in the main optical path, high-precision closed-loop attitude control, and large-scale array manufacturability.

[0003] In existing technologies, piezoelectric actuation has become the mainstream technology for biaxial MEMS micromirrors due to its advantages of low driving power, high control precision, and good process compatibility. An existing invention patent with application number 202311067233.9 discloses a two-dimensional biaxial piezoelectric MEMS micromirror. This solution involves etching symmetrically arranged cantilever beams and reflective mirrors onto a silicon substrate, placing piezoelectric driving blocks on the cantilever beams, and using these blocks to drive the cantilever beams to bend, achieving a two-dimensional biaxial large-angle flipping of the reflective mirrors. This provides a feasible technical solution for large-angle deflection of piezoelectric MEMS micromirrors.

[0004] However, the existing technology has the following core defects: The above-mentioned solutions and similar mainstream piezoelectric MEMS micromirrors all adopt a single-sided reflective mirror design, which can only realize the beam reflection function on one side of the mirror surface. It is impossible to achieve physical decoupling between the main optical path signal transmission and the micromirror angle detection / attitude calibration function at the device level. In order to achieve closed-loop control of micromirror deflection, a beam splitter and a matching detection optical path must be added to the main optical path. This will not only directly cause energy diversion in the main optical path, resulting in a significant increase in system insertion loss, but also greatly increase the complexity and assembly difficulty of the optical path system. At the same time, the coupling interference between the detection optical path and the main optical path will also affect the micromirror control accuracy, which cannot meet the engineering application requirements of high port density, low loss and high integration of all-optical switching systems. Summary of the Invention

[0005] This invention provides a dual-sided reflective piezoelectric MEMS biaxial micromirror unit, array, and fabrication method, which can solve the core technical problems inherent in existing single-sided reflective micromirror solutions, such as high main optical path loss, complex system optical path, and difficulty in large-scale array integration.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0007] This application provides a dual-sided reflective piezoelectric MEMS dual-axis micromirror unit, including a mirror platform, an outer fixing frame, a support beam, and a piezoelectric driving stack. The support beam connects the mirror platform and the outer fixing frame. The piezoelectric driving stack is disposed on the support beam and is used to drive the support beam to deform so as to drive the mirror platform to achieve dual-axis deflection.

[0008] The aforementioned micromirror unit uses an SOI substrate as its base. The SOI substrate includes a top silicon device layer, a buried oxide layer, and a bottom silicon support layer stacked sequentially from top to bottom. The mirror platform, the support beam, and the peripheral fixing frame are all integrally formed on the top silicon device layer. The bottom silicon support layer has a back cavity that penetrates the bottom silicon support layer in the area corresponding to the mirror platform, which is used to release the movable structure formed by the mirror platform and the support beam.

[0009] There are four support beams, which are symmetrically arranged around the mirror platform and are paired up to correspond to the deflection drive of two mutually orthogonal rotation axes.

[0010] The piezoelectric drive stack is disposed in the deformable flexible section of the support beam. The piezoelectric drive stack includes, from bottom to top, a Pt lower electrode layer, a PZT piezoelectric thin film layer and an Au upper electrode layer that are in direct contact with the top silicon device layer.

[0011] The aforementioned mirror platform has a front mirror on its front side and a back mirror on its back side. Both the front and back mirrors have a Cr / Au layered structure, wherein the Cr layer is an adhesive layer and the Au layer is a reflective layer.

[0012] Through the above technical solution, dual reflective surfaces with independent functions are integrated on the same movable mirror platform. The front mirror can be dedicated to the reflection and exchange of signals in the main optical path, while the back mirror can be dedicated to the reflection of beams for back-side angle detection and attitude calibration. This eliminates the energy loss and optical path interference caused by beam splitting detection in traditional single-sided mirror solutions from the source. It achieves physical isolation and decoupling of transmission and detection functions at the device level, eliminating the beam splitter and complex detection optical path that are indispensable in traditional solutions. This minimizes the energy shunting of the main optical path, significantly reduces system insertion loss, and greatly simplifies the optical system architecture. At the same time, the mirror platform, support beam, and peripheral fixing frame, which are integrally formed on the SOI substrate, together with the symmetrically arranged four-beam drive structure, ensure the stability and consistency of the dual-axis deflection of the mirror platform. This meets the integration requirements of large-scale micromirror arrays and provides a key device solution for a new generation of all-optical switching systems with low insertion loss and high integration.

[0013] In this invention, the aforementioned mirror platform is a rectangular mirror platform; the four support beams are cantilever beams or folding beams, with one end of the support beam connected to the outer fixed frame and the other end connected to the vertex region or the midpoint region of the four sides of the rectangular mirror platform; when the support beam is connected to the vertex region of the rectangular mirror platform, a two-degree-of-freedom flexible connection structure is provided at the connection between the support beam and the mirror platform, and the two-degree-of-freedom flexible connection structure is one of a narrow-neck flexible hinge, a symmetrical compliant beam, or a slotted flexible mechanism, or a combination of two or more of them; the piezoelectric drive stack corresponding to the four support beams adopts a four-way independent drive mode or a two-to-two group drive mode.

[0014] Through the above technical solutions, the support form of cantilever beams or folding beams, as well as the connection method between the support beam and the vertex or midpoint region of the rectangular mirror platform, can be flexibly selected according to the deflection requirements, driving voltage threshold, and layout constraints of the actual application, adapting to different engineering scenarios. At the same time, the two-degree-of-freedom flexible connection structure set when the support beam is connected to the vertex region of the mirror can effectively suppress the mirror tilt during the dual-axis deflection process, alleviate the stress concentration at the connection, and more efficiently convert the bending displacement generated by piezoelectric drive into the desired rotational degree of freedom, reduce the torsional coupling between orthogonal axes, and improve the attitude consistency, deflection efficiency, and fatigue life of the mirror deflection. The four independent drive or two-in-two group drive methods further improve the flexibility and pointing accuracy of dual-axis deflection control.

[0015] This application also provides a dual-reflective piezoelectric MEMS micromirror array, comprising multiple dual-axis micromirror units, wherein the multiple dual-axis micromirror units are arranged in rows and columns to form a two-dimensional array, wherein the dual-axis micromirror units are the dual-reflective piezoelectric MEMS dual-axis micromirror units as described in claim 1 or 2, and an isolation groove or gap is provided between adjacent dual-axis micromirror units to reduce mechanical coupling and residual stress transmission between units.

[0016] The above technical solution enables large-scale two-dimensional array integration of double-sided reflective micromirror units, which can meet the core application requirements of high port density and non-blocking interconnection in all-optical switching systems. The isolation slots or intervals set between adjacent micromirror units effectively reduce mechanical coupling and residual stress transmission between units, improving the overall stability of the array and the consistency of the performance of each unit. At the same time, relying on the double-sided reflective structure of the micromirror units, the angle detection module can be calibrated in parallel on the back side using a unified light source and detector layout, reducing optical stacking on the main optical path side and significantly improving the compactness and assembly tolerance of the array package.

[0017] This application also provides a method for fabricating a biaxial micromirror unit of a double-sided reflective piezoelectric MEMS, which is used to fabricate the biaxial micromirror unit of the double-sided reflective piezoelectric MEMS as described in claim 1 or 2. The fabrication method includes the following steps:

[0018] Step S1: Provide an SOI wafer and clean it. The SOI wafer includes a top silicon device layer, a buried oxide layer and a bottom silicon support layer stacked sequentially from top to bottom. A Pt lower electrode layer, a PZT piezoelectric thin film layer and an Au upper electrode layer are sequentially formed on the surface of the top silicon device layer to obtain a Pt-PZT-Au thin film stack.

[0019] Step S2: A mask is formed by photolithography to pattern the upper electrode layer of Au, thereby obtaining the upper electrode pattern and lead connection area of ​​the support beam driving area.

[0020] Step S3: Using photoresist or the upper electrode pattern as a mask, perform patterned etching or etching on the PZT piezoelectric thin film layer to retain PZT in the driving area and the necessary transition area, and expose the area required for subsequent lower electrode etching.

[0021] Step S4: Perform patterned etching on the Pt lower electrode layer to complete the definition of the Pt-PZT-Au piezoelectric drive stack structure;

[0022] Step S5: In the region of the top silicon device layer corresponding to the mirror platform, first deposit a Cr adhesion layer, then deposit an Au reflective layer to form a front mirror.

[0023] Step S6: Deposit a SiN protective layer on the front side of the SOI wafer and etch openings at locations where pads need to be exposed or release windows need to be formed;

[0024] Step S7: Perform deep silicon etching from the back side of the SOI wafer to form a back cavity and etch the bottom silicon support layer to a predetermined depth, using the buried oxide layer as an etching stop layer to control the etching endpoint.

[0025] Step S8: Continue deep silicon etching from the back of the SOI wafer to the buried oxide layer and form a through-cavity structure. Then, locally remove or open the buried oxide layer exposed in the cavity to enable the mirror platform and support beam to release.

[0026] Step S9: First, deposit a Cr adhesion layer on the back of the mirror platform, and then deposit an Au reflective layer to form the back mirror surface;

[0027] Step S10: Perform final release, cleaning and drying on the wafer to make the mirror platform and support beam a movable structure, thus obtaining the double-sided reflective piezoelectric MEMS biaxial micromirror unit.

[0028] The above technical solution provides a wafer-level complete fabrication process that is perfectly matched to the double-sided reflective micromirror structure. It can complete the batch manufacturing of all micromirror units at the wafer level at one time, solving the process compatibility problems of front and back mirror consistency, piezoelectric drive stack patterning, and deep silicon etching and release of the structure when manufacturing double-sided reflective micromirror arrays. By first completing the fabrication of the front piezoelectric drive stack, front mirror and structure contour, then performing deep silicon etching on the back to form the back cavity and release the structure, and finally completing the metallization of the back mirror, the process design ensures the consistency of the double-sided optical performance of each micromirror unit in the large-scale array, greatly improving the array manufacturing yield and batch stability, and providing a replicable process foundation for the engineering implementation of all-optical switching systems.

[0029] In this invention, in steps S7 and S8, the deep silicon etching adopts the DRIE process; in steps S5 and S9, the front mirror and the back mirror adopt the same layered structure and deposition process.

[0030] The above technical solution employs DRIE (Deep Silicon Etching) technology for back-side deep silicon etching, enabling high-precision, high aspect ratio etching of the cavity structure. Combined with a buried oxide layer as the etching stop layer, the etching endpoint can be precisely controlled, avoiding damage to the movable structure on the front side and improving batch consistency of the cavity structure. Simultaneously, the front and back mirrors utilize the same layered structure and deposition process, further ensuring the uniformity of reflection performance on both sides, enhancing the adhesion between the metal mirror and the underlying structure, reducing the long-term failure risk of mirror peeling and cracking, and further improving the performance consistency between array units and the long-term reliability of the device. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 A schematic diagram of the structure of a dual-axis micromirror unit of a double-sided reflective piezoelectric MEMS (four cantilever beams are arranged around the mirror platform and connected to the mirror platform through a flexible connection area) provided for an embodiment of the present invention;

[0033] Figure 2 A schematic diagram of the structure of a biaxial micromirror unit of a double-sided reflective piezoelectric MEMS (four folded beams are arranged around the mirror platform and connected to the mirror platform through a flexible connection area) provided for an embodiment of the present invention;

[0034] Figure 3Another structural schematic diagram of a dual-axis micromirror unit of a double-sided reflective piezoelectric MEMS (four folded beams are arranged around the mirror platform and connected to the mirror platform through a flexible connection area) provided for an embodiment of the present invention;

[0035] Figure 4 Another structural schematic diagram of a dual-axis micromirror unit of a double-sided reflective piezoelectric MEMS (four cantilever beams are arranged around the mirror platform and connected to the mirror platform through a flexible connection area) provided for an embodiment of the present invention;

[0036] Figure 5 This is a schematic diagram of the fabrication process of the micromirror unit and array provided in an embodiment of the present invention.

[0037] icon:

[0038] 1, 5, 9, 13 – External fixed frame;

[0039] 2, 6, 10, 14 – Mirrored platforms;

[0040] 3, 7, 11, 15 — Flexible connection structure;

[0041] 4.16 – Cantilever beam;

[0042] 8, 12 – Folded beams. Detailed Implementation

[0043] The embodiments of this application will now be described in detail with reference to the accompanying drawings.

[0044] In the description of this application, it should be understood that the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0045] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0046] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to welding, bolting, or riveting; they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0047] Example 1:

[0048] like Figure 1 As shown, the four supporting beams are cantilever beams 4, extending inward from the outer fixed frame 1 and connecting to the four vertex regions of the rectangular mirror platform 2. A two-degree-of-freedom flexible connection structure 3 is provided at the connection point between each cantilever beam 4 and the mirror platform 2, enabling controllable compliance at the connection point in two orthogonal directions. Purpose: To obtain a larger moment arm at the vertex connection to improve deflection efficiency, while absorbing torsional coupling caused by inconsistencies between processing and drive through the flexible connection. Result: Higher deflection efficiency can be obtained under the same drive conditions, and mirror tilting is suppressed, improving attitude consistency and repeatability. Optionally, the two-degree-of-freedom flexible connection structure 3 can be implemented by a narrow-neck flexible hinge, a symmetrical compliant beam, a slotted flexible mechanism, or a combination thereof.

[0049] Example 2

[0050] like Figure 2 As shown, the four supporting beams are folded beams 8 (e.g., U-shaped or multi-segment folding structures) extending inward from the outer fixed frame 5 to the four vertex regions of the rectangular mirror platform 6. Two-degree-of-freedom flexible connection structures 7 are installed at the vertices. Objective: To increase the equivalent beam length within a limited area to enhance flexibility and reduce driving voltage requirements, while simultaneously dispersing stress and improving reliability; and to suppress skew coupling under vertex driving through flexible connections. Results: A large deflection capability is achieved under lower driving voltage conditions, reducing local stress concentration and improving fatigue life, while maintaining good attitude consistency.

[0051] Optionally, the folding beam 8 can be U-shaped, serpentine, or multi-segment folding form; the flexible connection structure 7 is the same as in Embodiment 1.

[0052] Example 3

[0053] like Figure 3 As shown, the four supporting beams are folded beams 12, which extend inward from the outer fixed frame 9 to the midpoint area of ​​the four sides of the rectangular mirror platform 10, and are also connected by a flexible connection structure 11.

[0054] Objective: To reduce torsional coupling in certain directions by altering the torque distribution path through the midpoint of application, while simultaneously utilizing the folded beam 12 to achieve greater flexibility and reduce the driving voltage. Results: Mirror deflection more closely resembles pure rotation, improving attitude stability; the target deflection angle can still be achieved at lower voltages. Optionally, the device stiffness, resonant frequency, and stress level can be comprehensively optimized by adjusting the length, linewidth, and spacing of the folded-back section.

[0055] Example 4

[0056] like Figure 4 As shown, the four supporting beams are cantilever beams 16, extending inward from the outer fixed frame 13 and connecting to the midpoint regions of the four sides of the rectangular mirror platform 14. Flexible connection structures 15 are used at the connection points. Purpose: To improve layout implementation and array replicability by using simple cantilever beams, and to reduce torsional coupling by utilizing midpoint connections.

[0057] Results: Easy routing and pad layout, suitable for array scaling; improved attitude stability and scan linearity of mirror deflection.

[0058] Preparation method examples

[0059] The following is an embodiment of a wafer-level fabrication method matching the above structure, used to fabricate a dual-axis micromirror unit and its array device for double-sided reflective piezoelectric MEMS. This method is based on an SOI wafer and sequentially completes the fabrication and patterning of the piezoelectric drive stack, front mirror formation, protective layer deposition, back cavity formation and release via deep silicon etching, back mirror formation, and final release.

[0060] Step S1: Wafer Preparation

[0061] Objective: To provide a substrate with a device layer, a sacrificial layer, and a support layer, and to establish a basis for piezoelectric-driven stack deposition. Process and Results: An SOI wafer was provided and cleaned; a Pt lower electrode layer, a PZT piezoelectric thin film layer, and an Au upper electrode layer were sequentially formed on the top silicon surface to obtain a Pt-PZT-Au thin film stack. Thin film deposition can be performed using processes such as magnetron sputtering; PZT can undergo crystallization-annealing after deposition to obtain piezoelectric properties.

[0062] Step S2: Etching of the upper electrode ( Figure 5 Step A)

[0063] Objective: To define the upper electrode and lead pattern of the driving region, providing an alignment reference for subsequent piezoelectric thin film patterning. Process and Results: A photolithography mask was formed, and the Au upper electrode was patterned and etched to obtain the upper electrode pattern and lead connection area of ​​the support beam driving region. The etching method can be IBE, RIE, or Au-compatible wet etching.

[0064] Step S3: PZT etching ( Figure 5(Step B)

[0065] Objective: To confine PZT within the drive region of the support beam, reducing film residue and parasitic coupling in non-drive areas. Process and Results: Using photoresist or the upper electrode pattern as a mask, the PZT piezoelectric film layer was patterned and etched to retain PZT within the drive region and necessary transition areas, while exposing the areas required for subsequent lower electrode etching.

[0066] Step S4: Lower electrode etching ( Figure 5 (Step C)

[0067] Objective: To define the driving capacitor area and form electrode leads that connect to the pads.

[0068] Process and Results: The Pt lower electrode layer was patterned and etched to complete the definition of the Pt-PZT-Au driven stack structure.

[0069] Step S5: Cr / Au deposition on the front side to form a front mirror surface ( Figure 5 (Step D)

[0070] Objective: To form a highly reflective mirror surface on the front of a mirror platform while ensuring reliable metal adhesion. Process and Results: A Cr adhesion layer is first deposited in the mirror platform area, followed by an Au reflective layer to form the front mirror surface. The typical thickness of the Au reflective layer is approximately 200 nm, and the Cr thickness is 5 nm–50 nm. Deposition can be performed by sputtering or evaporation, and patterning can be achieved through lift-off or etching methods to ensure clear mirror boundaries and complete coverage.

[0071] Step S6: Deposit a silicon nitride protective layer ( Figure 5 (Step E)

[0072] Objective: To improve the structural reliability and electrical insulation during subsequent deep silicon etching and release processes.

[0073] Process and Results: A SiN protective layer was deposited, and openings were made at locations where pads needed to be exposed or release windows needed to be formed.

[0074] Step S7: Etching the bottom silicon portion to form the back cavity ( Figure 5 (Step F)

[0075] Objective: To create a back cavity opening to provide space for the movement of the mirror and support beam. Process and Results: Deep silicon etching (DRIE) is performed from the back of the wafer to form the back cavity and the bottom silicon is etched to a predetermined depth. The buried oxide layer is used as an etch stop layer to control the endpoint. The etched area on the back side can be defined by back alignment lithography and hard mask.

[0076] Step S8: Bottom silicon release ( Figure 5 (Middle step G)

[0077] Objective: To create a through-cavity and establish conditions for the release of the movable structure. Process and Results: Continue DRIE etching on the back side down to the buried oxide layer to create a through-cavity structure; subsequently, locally remove or window the buried oxide layer to enable the mirror platform and support beam to be released. The buried oxide layer can be removed using HF wet etching or HF vapor phase etching.

[0078] Step S9: Deposit Cr-Au on the back side to form a back mirror ( Figure 5 (Middle step H)

[0079] Objective: To form a highly reflective interface on the back side of a mirror platform that is consistent with the front mirror surface. Process and Results: A Cr adhesion layer is first deposited on the back side of the mirror platform, followed by an Au reflective layer to form the back mirror surface; the typical thickness of the Au reflective layer is approximately 200 nm. Backside metallization is preferably performed in a single wafer-level operation to improve array uniformity.

[0080] Step S10: Frontal release and cleaning / drying ( Figure 5 (Step I)

[0081] Objective: To remove the sacrificial layer and obtain a final movable structure while reducing the risk of adhesion failure after release. Process and Results: Final release, cleaning, and drying were performed to make the mirror platform and support beam a movable structure; optional anti-adhesion treatment or supercritical drying were used to reduce the risk of adhesion failure. This resulted in a biaxial micromirror unit and its array device for a double-sided reflective piezoelectric MEMS.

[0082] Array-based extended implementation

[0083] In this embodiment, the micromirror units can be repeatedly arranged in rows and columns to form a two-dimensional array (e.g., 2×2, 3×3, and larger-scale arrays). Preferably, isolation trenches or gaps are provided between the units to reduce mechanical coupling and residual stress transmission. The Cr-Au back mirror is preferably deposited in a single wafer-level deposition to achieve higher back-side reflection uniformity and improve the assembly tolerance of array-level calibration or inspection.

[0084] Typical values ​​and selectable ranges of key parameters

[0085] Without altering the basic technical solution of "rectangular mirror platform + four-beam support + Pt-PZT-Au driving stack + double-sided Cr-Au mirrors (Au≈200nm) + back cavity release," the following typical values ​​and optional ranges can be adopted: top silicon thickness 5μm–30μm (typically 20μm); PZT thickness 1μm–5μm (typically 2μm); minimum linewidth of support beams 40μm–300μm; Au thickness of both mirrors 100nm–500nm (typically 200nm); Cr adhesion layer thickness 5nm–50nm; driving voltage 0V–60V (typically 0V–45V). These ranges cover existing device design and process capabilities while also reserving space for subsequent iterations.

[0086] Packaging Method Examples

[0087] The packaging structure includes a micromirror array, a packaging substrate, a top cover, an upper glass protection plate, and a lower glass protection plate.

[0088] A micromirror array protects N*N micromirror units, with each unit protecting both a front mirror and a back mirror. Both the front and back mirrors of the bifacial reflective MEMS micromirror unit can reflect light. The front mirror is used to control the optical signal link of the switch. The back mirror is used to monitor the micromirror's rotation angle. The principle is as follows: a laser is emitted onto the back mirror, which reflects the laser onto a position-sensitive detector (PSD). The position of the laser spot center on the PSD depends on the micromirror's rotation angle. By monitoring the position of the laser spot center, the micromirror angle is monitored.

[0089] The substrate of the packaging structure has multiple back-side light-transmitting holes, the number of which is the same as the number of micromirror units, ensuring that the back mirror surface of each micromirror unit can receive laser signals. Similarly, the top cover of the packaging structure has multiple front-side light-transmitting holes, the number of which is the same as the number of micromirror units, ensuring that the front mirror surface of each micromirror unit can receive light signals.

[0090] The top cover is fabricated using multilayer PCB or ceramic PCB technology, with the internal package pins located close to the internal micromirror units. Interconnect leads connect the internal micromirror unit pins to the internal package pins, and the internal package pins are interconnected with the external package pins via package wires on the top cover. This interconnection scheme enables the micromirror units to be interconnected in close proximity, overcoming the challenges of wire arrangement in micromirror arrays, reducing the complexity of wire fabrication processes, and lowering manufacturing costs.

[0091] To achieve both dust protection and light transmission requirements, the top and bottom of the encapsulation structure are covered with an upper glass protective plate and a lower glass protective plate, respectively.

[0092] The main steps in the packaging process include:

[0093] (1) The micromirror array is attached to the packaging substrate using epoxy resin or silicone and then cured by heating;

[0094] (2) The top cover plate is attached to the substrate using epoxy resin or silicone and then cured by heating;

[0095] (3) Wire bonding calculations are used to achieve interconnection between the micromirror pins and the internal pins of the package;

[0096] (4) Use epoxy resin or silicone to attach the glass protective plate to the top cover plate and the substrate, and heat to cure it to achieve dust protection for the micromirror array.

[0097] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope described in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A dual-reflective piezoelectric MEMS biaxial micromirror unit, comprising a mirror platform, an outer fixing frame, a support beam, and a piezoelectric driving stack, wherein the support beam connects the mirror platform and the outer fixing frame, and the piezoelectric driving stack is disposed on the support beam for driving the support beam to deform in order to drive the mirror platform to achieve biaxial deflection, characterized in that: The micromirror unit uses an SOI substrate as its base. The SOI substrate includes a top silicon device layer, a buried oxide layer, and a bottom silicon support layer stacked sequentially from top to bottom. The mirror platform, the support beam, and the peripheral fixing frame are all integrally formed on the top silicon device layer. The bottom silicon support layer has a back cavity that penetrates the bottom silicon support layer in the area corresponding to the mirror platform, which is used to release the movable structure formed by the mirror platform and the support beam. There are four support beams, which are symmetrically arranged around the mirror platform and are paired up to correspond to the deflection drive of two mutually orthogonal rotation axes. The piezoelectric drive stack is disposed in the deformable flexible section of the support beam, and the piezoelectric drive stack includes, from bottom to top, a Pt lower electrode layer, a PZT piezoelectric thin film layer and an Au upper electrode layer that are in direct contact with the top silicon device layer. The mirror platform has a front mirror on its front side and a back mirror on its back side. Both the front and back mirrors have a Cr / Au layered structure, where the Cr layer is an adhesive layer and the Au layer is a reflective layer.

2. The biaxial micromirror unit of double-sided reflective piezoelectric MEMS according to claim 1, characterized in that, The mirror platform is a rectangular mirror platform; the four support beams are cantilever beams or folding beams, with one end of the support beam connected to the outer fixed frame and the other end connected to the vertex region or the midpoint region of the four sides of the rectangular mirror platform; when the support beam is connected to the vertex region of the rectangular mirror platform, a two-degree-of-freedom flexible connection structure is provided at the connection between the support beam and the mirror platform, which is one of a narrow-neck flexible hinge, a symmetrical compliant beam, or a slotted flexible mechanism, or a combination of two or more of them; the piezoelectric drive stack corresponding to the four support beams adopts a four-way independent drive mode or a two-to-two group drive mode.

3. A double-sided reflective piezoelectric MEMS micromirror array, comprising multiple biaxial micromirror units, wherein the multiple biaxial micromirror units are arranged in a row-column configuration to form a two-dimensional array, characterized in that, The biaxial micromirror unit is the biaxial micromirror unit of double-sided reflective piezoelectric MEMS as described in claim 1 or 2. An isolation groove or spacer area is provided between adjacent biaxial micromirror units to reduce mechanical coupling and residual stress transmission between units.

4. A method for fabricating a biaxial micromirror unit of a double-sided reflective piezoelectric MEMS, characterized in that, The fabrication method for the biaxial micromirror unit of double-sided reflective piezoelectric MEMS as described in claim 1 or 2 includes the following steps: Step S1: Provide an SOI wafer and clean it. The SOI wafer includes a top silicon device layer, a buried oxide layer and a bottom silicon support layer stacked sequentially from top to bottom. A Pt lower electrode layer, a PZT piezoelectric thin film layer and an Au upper electrode layer are sequentially formed on the surface of the top silicon device layer to obtain a Pt-PZT-Au thin film stack. Step S2: A mask is formed by photolithography, and the Au upper electrode layer is patterned and etched to obtain the upper electrode pattern and lead connection area of ​​the support beam driving area. Step S3: Using photoresist or the upper electrode pattern as a mask, perform patterned etching or etching on the PZT piezoelectric thin film layer to retain PZT in the driving area and the necessary transition area, and expose the area required for subsequent lower electrode etching. Step S4: Perform patterned etching on the Pt lower electrode layer to complete the definition of the Pt-PZT-Au piezoelectric drive stack structure; Step S5: In the region of the top silicon device layer corresponding to the mirror platform, first deposit a Cr adhesion layer, then deposit an Au reflective layer to form a front mirror. Step S6: Deposit a SiN protective layer on the front side of the SOI wafer and etch openings at locations where pads need to be exposed or release windows need to be formed; Step S7: Perform deep silicon etching from the back side of the SOI wafer to form a back cavity and etch the bottom silicon support layer to a predetermined depth, using the buried oxide layer as an etching stop layer to control the etching endpoint. Step S8: Continue deep silicon etching from the back of the SOI wafer to the buried oxide layer and form a through-cavity structure. Then, locally remove or open the buried oxide layer exposed in the cavity to enable the mirror platform and support beam to release. Step S9: First, deposit a Cr adhesion layer on the back of the mirror platform, and then deposit an Au reflective layer to form the back mirror surface; Step S10: Perform final release, cleaning and drying on the wafer to make the mirror platform and support beam a movable structure, thus obtaining the double-sided reflective piezoelectric MEMS biaxial micromirror unit.

5. The method for fabricating a biaxial micromirror unit of a double-sided reflective piezoelectric MEMS according to claim 4, characterized in that, In steps S7 and S8, the deep silicon etching adopts the DRIE process; in steps S5 and S9, the front mirror and the back mirror adopt the same layered structure and deposition process.

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