MEMS deformable mirror based on double-layer electrode collaborative etching and preparation method thereof

Abstract

The invention discloses an MEMS deformable mirror based on double-layer electrode collaborative etching and a preparation method of the MEMS deformable mirror, and belongs to the micro-nano machining technology of a deformable mirror surface structure. The preparation method is realized through the following steps: etching an alignment mark on the back surface of a substrate; depositing a silicon nitride insulating layer on the front surface of the silicon wafer substrate, and opening a back solution etching window; collaborative etching of the double-layer polycrystalline silicon electrode is completed in sequence, and an interdigital electrode, a honeycomb stress hole and a spiral flow guide groove are synchronously formed; lossless structure release is realized through mechanical thinning and back hole directional etching; and finally, sputtering a metal reflecting layer after vacuum bonding and packaging, and implementing an annealing process. According to the invention, by optimizing the electrode etching collaboration and the reflector postposition technology and introducing the back hole release technology, the problems of low etching liquid diffusion efficiency and high structure adhesion rate in the traditional technology are solved, the mirror surface roughness and the release yield are obviously improved, and the method is suitable for manufacturing a high-precision adaptive optical system.

Description

Technical Field

[0001] The present invention relates to micro-nano processing technology of deformable mirror structures, and particularly to a MEMS deformable mirror based on cooperative etching of double-layer electrodes and a preparation method thereof. Background Art

[0002] The manufacturing technology of micro-electro-mechanical system (MEMS) deformable mirrors is a core challenge in the field of adaptive optics, and its performance directly affects the beam wavefront correction accuracy. The traditional process prepares a multi-layer electrode structure through step-by-step deposition and etching methods. The main processes include substrate cleaning, insulating layer deposition, electrode lithography, sacrificial layer release, and reflective mirror coating, etc. However, such a process has the following technical defects: during the wet etching process, the reflective layer is exposed to the etching solution environment for a long time, resulting in an increase in surface residual defects, an increase in mirror surface roughness, and a significant attenuation of optical reflection performance; the difference in the thermal expansion coefficients of the electrode material and the reflective layer causes the accumulation of interface stress, and the mirror deformation error increases after annealing; the single-channel back-hole design limits the diffusion efficiency of the etching solution, resulting in frequent problems of adhesion of movable structures. At the same time, the preparation of multi-layer electrodes requires multiple photolithography mask alignments, and the process complexity is high, seriously restricting the mass production yield.

[0003] Although existing improvement schemes attempt to alleviate the above problems by adjusting the sacrificial layer material or optimizing the etching process, they still face challenges such as insufficient coordination between electrode etching and back-hole release, low control accuracy of the stress release hole morphology, and a decrease in solution transmission efficiency. In addition, although the design of synchronous annealing of the reflective layer and the electrode layer can reduce the thermal stress deformation, the density of mirror defects increases due to the penetration of the etching solution, making it difficult to meet the strict requirements of high-power laser systems for the optical surface quality.

[0004] The above technical bottlenecks indicate that there is an urgent need for a MEMS deformable mirror manufacturing method that can achieve cooperative control of electrode morphology, directional etching of back holes, and low-temperature and high-reliability packaging integration, so as to break through the balance limitation between the existing process's optical performance and structural reliability. Summary of the Invention

[0005] Object of the Invention: Aiming at the above problems, the object of the present invention is to provide a MEMS deformable mirror based on cooperative etching of double-layer electrodes and a preparation method thereof. By optimizing the electrode etching synchronism, directional etching of back holes, and the post-mirror process, the double improvement of optical performance and structural reliability is realized.

[0006] Technical Solution: On the one hand, the present invention provides a MEMS deformable mirror based on cooperative etching of double-layer electrodes, including: A silicon wafer substrate, a silicon nitride insulating layer disposed on the silicon wafer substrate, a first interdigital electrode disposed on the silicon nitride insulating layer, an in-situ doped polysilicon layer disposed on the first interdigital electrode, a second interdigital electrode, stress relief holes and solution diversion trenches etched on the in-situ doped polysilicon layer, a polysilicon support structure disposed on the second interdigital electrode, and a gold-based reflective mirror disposed on the polysilicon support structure.

[0007] Further, the thickness of the silicon nitride insulating layer is 200 - 500 nanometers.

[0008] Further, the thickness of the first interdigital electrode is 0.5 - 1.5 micrometers.

[0009] Further, the stress relief holes are in a honeycomb array with a pore diameter of 10 - 30 micrometers.

[0010] Further, the width of the solution diversion trenches is 2 - 5 micrometers.

[0011] Further, the polysilicon support structure is a columnar array with a diameter of 10 - 20 micrometers and a support density of 50 - 200 per mm².

[0012] On the other hand, the present invention provides a method for fabricating a MEMS deformable mirror based on bilayer electrode collaborative etching, comprising the following steps: Step 1, clean the silicon wafer substrate and etch alignment marks on the back surface of the silicon wafer substrate; Step 2, deposit a silicon nitride insulating layer on the front surface of the silicon wafer substrate and open a back solution etching window; Step 3, deposit a polysilicon layer on the front surface of the silicon nitride insulating layer, and pattern it into a first interdigital electrode after thinning and ion implantation; Step 4, deposit a first phosphosilicate glass sacrificial layer on the front surface of the first interdigital electrode and perform a thinning process; Step 5, deposit an in-situ doped polysilicon layer on the front surface of the first phosphosilicate glass sacrificial layer, and synchronously etch it to form a second interdigital electrode, stress relief holes and solution diversion trenches after thinning; Step 6, deposit a second phosphosilicate glass sacrificial layer on the front surface of the in-situ doped polysilicon layer and selectively remove the lead region; Step 7, deposit and pattern a polysilicon support structure on the front surface of the second phosphosilicate glass sacrificial layer; Step 8, mechanically thin the back surface of the silicon wafer substrate to the target thickness; Step 9, according to the position of the back solution etching window formed in Step 2, etch the silicon wafer substrate to the first phosphosilicate glass sacrificial layer deposited on the front surface of the first interdigital electrode in Step 4; Step 10, remove the first phosphosilicate glass sacrificial layer and the second phosphosilicate glass sacrificial layer to complete the structure release; Step 11: Sputter a metal reflective layer on the front of the polysilicon support structure and pattern it to form a gold-based reflective mirror surface. Perform an annealing process and carry out vacuum bonding and encapsulation.

[0013] Further, the thickness of both the first phosphosilicate glass sacrificial layer and the second phosphosilicate glass sacrificial layer is 1 - 3 microns.

[0014] Further, in Step 2, the cone angle of the solution etching window is 54.7° ± 2°.

[0015] Further, in Step 9, anisotropic wet etching is carried out using a TMAH solution with a mass fraction of 20% - 30%, and the etching temperature is 70 - 90°C.

[0016] Advantageous effects: Compared with the prior art, the significant advantages of the present invention are as follows: 1. Through the double-layer electrode collaborative etching technology, the present invention synchronously forms interdigital electrodes, honeycomb stress holes, and spiral flow guiding grooves, reduces the alignment times of the photolithography mask from 7 times to 4 times, increases the mass production yield rate to over 92%, improves the etching solution diffusion efficiency by 60%, and the structure adhesion rate is lower than 5%; 2. By using the process of sputtering a gold-based reflective layer after vacuum bonding and low-temperature annealing, the surface roughness RMS of the mirror surface is stably up to 0.2 - 0.3 nm, the surface residual particle density is less than 10 particles / cm², and the reflectivity attenuation is less than 3%, breaking through the bottleneck of the surface quality of high-power laser optics; 3. By innovatively combining the mechanical thinning process to 200 - 400 μm and the TMAH directional etching process to a cone angle of 54.7° ± 2°, precise control is achieved such that the distance between the end of the back hole and the sacrificial layer is less than 10 μm. With the cooperation of the polysilicon support column array, the mirror surface curvature deviation after encapsulation is less than 0.8%; 4. By using the phosphosilicate glass sacrificial layer combined with gas-phase hydrogen fluoride etching, the etching uniformity is increased by 35%, the stress accumulation in the electrode layer is reduced by 80%, and the support structure fracture rate is reduced from 15% to 0.5%. Description of the Drawings

[0017] Figure 1 It is a schematic structural diagram of a MEMS deformable mirror; Figure 2 It is a schematic structural diagram of the silicon wafer substrate pretreatment in Step 1; Figure 3 It is a schematic structural diagram of the silicon nitride insulating layer and the back hole forming in Step 2; Figure 4 It is a schematic structural diagram of the first interdigital electrode in Step 3; Figure 5 It is a schematic structural diagram of the phosphosilicate glass sacrificial layer treatment in Step 4; Figure 6 It is a schematic structural diagram of the collaborative etching in Step 5; Figure 7 Schematic diagram of the phosphosilicate glass sacrificial layer structure for Step 6; Figure 8 Schematic diagram of the polysilicon support structure for Step 7; Figure 9 Schematic diagram of the silicon wafer substrate thinning structure for Step 8; Figure 10 Schematic diagram of the back hole etching structure for Step 9; Figure 11 Schematic diagram of the structure release for Step 10.

[0018] In the figure, 1 is the silicon wafer substrate; 2 is the silicon nitride insulating layer; 3 is the first interdigital electrode; 4 is the first phosphosilicate glass sacrificial layer; 5 is the in-situ doped polysilicon layer; 6 is the second phosphosilicate glass sacrificial layer; 7 is the polysilicon support structure; 8 is the gold-based reflecting mirror surface. Detailed implementation manners

[0019] In order to make the objectives, technical solutions and advantages of this application clearer and more understandable, the following further elaborates on this application in combination with the attached drawings and embodiments.

[0020] Embodiment 1

[0021] Combined with Figure 1 As shown, the MEMS deformable mirror based on the cooperative etching of double-layer electrodes described in this embodiment includes: A silicon wafer substrate 1, a silicon nitride insulating layer 2 provided on the silicon wafer substrate 1, a first interdigital electrode 3 provided on the silicon nitride insulating layer 2, an in-situ doped polysilicon layer 5 provided on the first interdigital electrode 3, a second interdigital electrode, stress release holes and solution diversion grooves etched on the in-situ doped polysilicon layer 5, a polysilicon support structure 7 provided on the second interdigital electrode, and a gold-based reflecting mirror surface 8 provided on the polysilicon support structure 7.

[0022] Furthermore, the thickness of the silicon nitride insulating layer is 200 - 500 nanometers.

[0023] Furthermore, the thickness of the first interdigital electrode is 0.5 - 1.5 micrometers.

[0024] Furthermore, the stress release holes are in a honeycomb array, and the aperture is 10 - 30 micrometers.

[0025] Furthermore, the width of the solution diversion groove is 2 - 5 micrometers.

[0026] Furthermore, the polysilicon support structure is a columnar array with a diameter of 10 - 20 micrometers, and the support density is 50 - 200 pieces / mm².

[0027] Embodiment 2

[0028] The preparation method of the MEMS deformable mirror based on double-layer electrode collaborative etching according to this embodiment includes the following steps: Combined with Figure 2 , step 1, clean the silicon wafer substrate 1 and etch alignment marks on the back of the silicon wafer substrate 1.

[0029] A 6-inch double-sided polished n-type silicon wafer can be selected, and the surface particle density is less than 10 particles / cm² after being treated with a cleaning solution with a specific formula. Then, deep reactive ion etching is used to form alignment marks with a depth of 5-15 μm on the back, and the etching gas is a fluorine-based mixed gas, and the sidewall steepness is controlled at 88°±2°.

[0030] Combined with Figure 3 , step 2, deposit a silicon nitride insulating layer 2 on the front of the silicon wafer substrate 1 and open a back solution etching window.

[0031] Deposit a 200-500 nm silicon nitride insulating layer 2 on the front of the silicon wafer substrate 1 by plasma-enhanced chemical vapor deposition, and the dielectric strength is greater than 10 MV / cm. Anisotropic wet etching is used to form a solution channel window with a taper angle of 54.7°±2° on the back of the silicon wafer substrate 1, and the etching end point is monitored in real time by an optical interferometer.

[0032] Combined with Figure 4 , step 3, deposit a polysilicon layer on the front of the silicon nitride insulating layer 2, and pattern it to form the first interdigital electrode 3 after thinning and ion implantation.

[0033] After depositing a 2-3 μm polysilicon layer on the front of the silicon nitride insulating layer 2, chemical mechanical polishing is carried out using a special polishing solution and thinned to 0.5-1.5 μm. Then, boron ion implantation is implemented, and the dose , and an interdigital electrode array with a sheet resistance less than 50 Ω / sq is formed after annealing.

[0034] Combined with Figure 5 , step 4, deposit a first phosphosilicate glass sacrificial layer 4 on the front of the first interdigital electrode 3 and carry out a thinning treatment.

[0035] Grow a 2-4 μm first phosphosilicate glass sacrificial layer 4 on the front of the first interdigital electrode 3 by low-pressure chemical vapor deposition, and the phosphorus doping concentration is optimized to enhance the etching selectivity. Then, chemical mechanical polishing is carried out to thin it to 1-3 μm, and the thickness uniformity is less than ±3%. The thickness of the first phosphosilicate glass sacrificial layer is 1-3 μm.

[0036] Combined with Figure 6 , step 5, deposit an in-situ doped polysilicon layer 5 on the front of the first phosphosilicate glass sacrificial layer 4, and synchronously etch it after thinning to form the second interdigital electrode, stress release holes, and solution diversion grooves.

[0037] Deposit an in-situ doped polysilicon layer 5 with a thickness of 2 - 5 μm on the front side of the first phosphosilicate glass sacrificial layer 4. Use deep reactive ion etching to simultaneously etch the second interdigital electrode, honeycomb stress holes, and spiral flow channels. The etching selectivity is controlled within the range of 20:1 - 40:1 through a specific gas ratio. The depth-to-width ratio of the honeycomb holes is 1:3 - 1:5, the spiral angle of the flow channels is 30° - 60°, and the etching endpoint is monitored in real-time by optical emission spectroscopy. The aperture of the honeycomb stress holes is 10 - 30 μm, and the groove width of the spiral flow channels is 2 - 5 μm.

[0038] Combine Figure 7 , step 6, deposit a second phosphosilicate glass sacrificial layer 6 on the front side of the in-situ doped polysilicon layer 5, and selectively remove the lead area.

[0039] Deposit a 2 - 4 μm thick second phosphosilicate glass sacrificial layer 6 on the front side of the in-situ doped polysilicon layer 5 by low-pressure chemical vapor deposition, and thin it to 1 - 3 μm by reactive ion etching. The etching selectivity is greater than 50:1, and the surface roughness Ra is less than 5 nm. The thickness of the second phosphosilicate glass sacrificial layer is 1 - 3 μm.

[0040] Combine Figure 8 , step 7, deposit and pattern a polysilicon support structure 7 on the front side of the second phosphosilicate glass sacrificial layer 6.

[0041] Photolithographically form an array of polysilicon support pillars with a diameter of 10 - 20 μm on the front side of the second phosphosilicate glass sacrificial layer 6. The support density is 50 - 200 pillars / mm², the height consistency error is less than ±5%, and the Young's modulus is 160 - 180 GPa.

[0042] Combine Figure 9 , step 8, mechanically thin the back side of the silicon wafer substrate 1 to the target thickness.

[0043] Rough grind the back side of the silicon wafer substrate 1 to 200 - 400 μm using a diamond grinding wheel, and then perform chemical mechanical polishing to make the surface roughness Ra less than 10 nm and the surface damage layer less than 50 nm.

[0044] Combine Figure 10 , step 9, etch the silicon wafer substrate to the first phosphosilicate glass sacrificial layer 4 deposited on the front side of the first interdigital electrode in step 4 according to the position of the back-side solution etching window formed in step 2; Etch the back holes of the silicon wafer substrate using a 20 - 30 wt% TMAH solution at a temperature of 70 - 90 °C with a flow rate of 5 - 15 mL / min. Monitor the etching endpoint through an infrared sensor to ensure that the end is less than 10 μm away from the phosphosilicate glass sacrificial layer.

[0045] Combine Figure 11 , step 10, remove the first phosphosilicate glass sacrificial layer 4 and the second phosphosilicate glass sacrificial layer 6 to complete the structure release.

[0046] Using gaseous hydrogen fluoride to etch and completely remove the first phosphosilicate glass sacrificial layer 4 and the second phosphosilicate glass sacrificial layer 6 at 40 - 60 sccm and 80 - 100 °C, the etching uniformity is greater than 95%, and the release time is 30 - 60 minutes.

[0047] Combined with Figure 1 , step 11, sputter a metal reflective layer on the front side of the polysilicon support structure 7 and pattern it to form a gold-based reflective mirror surface 8, implement an annealing process and perform vacuum bonding and encapsulation.

[0048] Magnetron sputter a 150 - 300 nm gold-based reflective layer on the front side of the polysilicon support structure 7, including a 5 - 10 nm transition layer, anneal at 300 - 400 °C to form a mirror surface with an RMS less than 0.3 nm, and complete wafer-level bonding under a vacuum of less than Pa.

[0049] Example 3

[0050] The preparation method of the MEMS deformable mirror based on double-layer electrode collaborative etching described in this example is different from that of Example 2 in that: (1) Substrate specification: Use an 8-inch silicon wafer, and control the back hole cone angle to be 54.7° ± 1.5°; (2) Sacrificial layer material: Use borophosphosilicate glass, and the boron / phosphorus doping concentration is 2 - 5 wt% each; (3) Etching process: In step 5, use time-modulated deep reactive ion etching, and increase the etching rate ratio to 35:1; (4) Encapsulation optimization: Increase the bonding temperature to 220 - 250 °C, and control the pressure at 5 - 10 MPa.

[0051] Example 4

[0052] The preparation method of the MEMS deformable mirror based on double-layer electrode collaborative etching described in this example is different from that of Example 2 in that: (1) Electrode material: The first interdigital electrode uses a polysilicon / tungsten composite structure with a thickness of 0.8 - 1.2 μm; (2) Flow guide groove design: Use a fractal groove to replace the spiral structure, the groove width is 1.5 - 3 μm, and the fractal iteration times are 3 - 5 times; (3) Detection technology: In step 9, introduce electronic speckle interferometry technology, and reconstruct the etching stress distribution in real time through laser speckle displacement field analysis; (4) Reflective layer optimization: Use an Au / ITO composite reflective layer, and the reflectivity in the visible light band is greater than 99%.

Claims

1. A MEMS deformable mirror based on double-layer electrode collaborative etching, characterized in that, Including: A silicon wafer substrate, a silicon nitride insulating layer disposed on the silicon wafer substrate, a first interdigital electrode disposed on the silicon nitride insulating layer, an in-situ doped polysilicon layer disposed on the first interdigital electrode, a second interdigital electrode, stress relief holes and solution diversion grooves etched on the in-situ doped polysilicon layer, a polysilicon support structure disposed on the second interdigital electrode, and a gold-based reflective mirror disposed on the polysilicon support structure.

2. The MEMS deformable mirror based on the collaborative etching of double-layer electrodes according to claim 1, wherein The thickness of the silicon nitride insulating layer is 200 - 500 nanometers.

3. The MEMS deformable mirror based on double-layer electrode collaborative etching according to claim 1, wherein The thickness of the first interdigital electrode is 0.5 - 1.5 microns.

4. The MEMS deformable mirror based on double-layer electrode collaborative etching according to claim 1, characterized in that The stress relief holes are in a honeycomb array with a pore diameter of 10 - 30 microns.

5. The MEMS deformable mirror based on the collaborative etching of double-layer electrodes according to claim 1, wherein The width of the solution diversion grooves is 2 - 5 microns.

6. The MEMS deformable mirror based on the collaborative etching of double-layer electrodes according to claim 1, characterized in that The polysilicon support structure is a columnar array with a diameter of 10 - 20 microns and a support density of 50 - 200 pieces / mm².

7. A method for fabricating a MEMS deformable mirror based on collaborative etching of double-layer electrodes, characterized in that, Including the following steps: Step 1, clean the silicon wafer substrate and etch alignment marks on the back of the silicon wafer substrate; Step 2, deposit a silicon nitride insulating layer on the front of the silicon wafer substrate and open a back solution etching window; Step 3, deposit a polysilicon layer on the front of the silicon nitride insulating layer, and pattern it into the first interdigital electrode after thinning and ion implantation; Step 4, deposit a first phosphosilicate glass sacrificial layer on the front of the first interdigital electrode and perform a thinning process; Step 5, deposit an in-situ doped polysilicon layer on the front of the first phosphosilicate glass sacrificial layer, and synchronously etch to form the second interdigital electrode, stress relief holes and solution diversion grooves after thinning; Step 6, deposit a second phosphosilicate glass sacrificial layer on the front of the in-situ doped polysilicon layer and selectively remove the lead area; Step 7, deposit and pattern a polysilicon support structure on the front of the second phosphosilicate glass sacrificial layer; Step 8, mechanically thin the back of the silicon wafer substrate to the target thickness; Step 9, according to the position of the back solution etching window formed in Step 2, etch the silicon wafer substrate to the first phosphosilicate glass sacrificial layer deposited on the front of the first interdigital electrode in Step 4; Step 10, remove the first phosphosilicate glass sacrificial layer and the second phosphosilicate glass sacrificial layer to complete the structure release; Step 11, sputter a metal reflective layer on the front of the polysilicon support structure and pattern it into a gold-based reflective mirror, perform an annealing process and carry out vacuum bonding and encapsulation.

8. The manufacturing method of the MEMS deformable mirror based on the collaborative etching of double-layer electrodes according to claim 7, characterized in that, The thicknesses of both the first phosphosilicate glass sacrificial layer and the second phosphosilicate glass sacrificial layer are 1 - 3 microns.

9. The preparation method of the MEMS deformable mirror based on the collaborative etching of the double-layer electrodes according to claim 7, characterized in that, The taper angle of the solution etching window in Step 2 is 54.7° ± 2°.

10. The manufacturing method of the MEMS deformable mirror based on collaborative etching of double-layer electrodes according to claim 7, characterized in that In Step 9, an anisotropic wet etching is carried out using a TMAH solution with a mass fraction of 20% - 30%, and the etching temperature is 70 - 90°C.

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