A nano-optical accelerometer and a method for manufacturing the same
By fabricating a nano-optical accelerometer through a single photolithography process, the problems of measurement accuracy and integration of traditional accelerometers have been solved, realizing a high-sensitivity and large-scale production nano-optical accelerometer and simplifying the micro-nano fabrication process.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- NAT UNIV OF DEFENSE TECH
- Filing Date
- 2025-06-25
- Publication Date
- 2026-04-28
AI Technical Summary
Traditional accelerometers have limited measurement range and accuracy due to device sensitivity and environmental factors. They have low integration levels, making it difficult to achieve large-scale synchronous measurement of the acceleration of multiple targets. Furthermore, existing processes are complex and require two photolithography processes.
A nano-optical accelerometer was fabricated using a single photolithography process, including the integration of a top chip and a bottom chip. This was achieved by depositing silicon nitride thin films on both sides of a silicon wafer and forming vias and suspended mass blocks on the top silicon nitride thin film. The process was simplified by combining wet etching and plasma etching.
It achieves a four-order-of-magnitude improvement in sensitivity, simplifies micro-nano fabrication processes, reduces costs, increases yield, and is suitable for large-scale mass production.
Smart Images
Figure CN120629632B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of accelerometers, specifically relating to a nano-optical accelerometer and its preparation method. Background Technology
[0002] An accelerometer is a sensor that measures changes in velocity. Whether it's the automatic deployment of airbags in cars, the automatic directional control of screen rotation in smartphones, or the maintaining of flight paths in aerospace vehicles, accelerometers are indispensable.
[0003] Currently, traditional accelerometers measure acceleration using sensor technology, converting changes in the sensor's velocity into electrical signals. However, their measurement range and accuracy are limited by the sensitivity of the device itself and environmental factors, making them prone to errors. Furthermore, due to size constraints, the coupling between devices is insufficient, resulting in low integration and making it difficult to meet the need for large-scale simultaneous measurement of the acceleration of multiple targets. In contrast, micro-opto-electro-mechanical accelerometers convert changes in an object's velocity into changes in optical signals for measurement. Compared to traditional accelerometers, they offer a simpler measurement process, higher integration, and higher sensitivity, and have attracted considerable attention in recent years.
[0004] The sensing part of a typical accelerometer requires a relatively complex design of subwavelength Si gratings and in-plane structures such as rotating and folding beams. It also requires two photolithography-reactive ion etching-wet etching processes, which not only increases the complexity of the process, but also places higher demands on micro-nano fabrication processes (such as the alignment accuracy of overlay) during the two photolithography processes. Summary of the Invention
[0005] The technical problem to be solved by the present invention is to provide a nano-optical accelerometer and its preparation method, which only requires one photolithography process, and the micro-nano fabrication process is simple and the cost is lower.
[0006] This invention provides a nano-optical accelerometer and its fabrication method, including an acceleration sensing unit, wherein the acceleration sensing unit includes a top chip and a bottom chip stacked together;
[0007] The top chip includes: a mass block, two silicon nitride films located at the top and bottom of the mass block respectively, and a support structure disposed around the mass block, wherein the top silicon nitride film has multiple through holes distributed in a U-shape.
[0008] The bottom chip includes a quartz plate, a high-reflectivity film and an anti-reflection film respectively disposed on both sides of the quartz plate, a groove in the middle of the quartz plate, the high-reflectivity film being located at the bottom of the groove, a silicon nitride thin film at the bottom of the top chip covering the groove to form an optical resonant cavity, and the mass block being located directly above the groove.
[0009] Optionally, the vias on the silicon nitride film are circular, and the diameter of the vias is 220-260 μm, with a spacing of 40-60 μm between adjacent vias.
[0010] Optionally, the number of through holes is 16-24.
[0011] Optionally, the thickness of the silicon nitride film is 200-650 nm.
[0012] Optionally, the typical volume of the mass block is 0.3-1.1 mm × 0.3-1.1 mm × 0.525 mm.
[0013] Compared with existing technologies, the nano-optical accelerometer provided by this invention has a typical sensitivity measurement value of over 1000mV / μg, where g is the gravitational acceleration; the sensitivity is improved by about four orders of magnitude compared with the traditional accelerometer composed of only optical mirrors, breaking through the limitations of traditional accelerometer structures in terms of sensitivity.
[0014] This invention provides a method for fabricating a nano-optical accelerometer, comprising the following steps:
[0015] The fabrication method of the top chip is as follows:
[0016] Silicon nitride thin films are deposited on both sides of a silicon wafer. One side of the silicon nitride thin film is used as the top. Then, a target pattern is formed on the top silicon nitride thin film using photolithography. The first etching method is used to remove the silicon nitride thin film at the target pattern to form a hole in the target pattern, exposing the silicon wafer below the target pattern. Then, the second etching method is used to etch the silicon wafer in the hole of the target pattern to obtain a suspended surface with a mass block.
[0017] The fabrication method of the bottom chip is as follows:
[0018] A groove is machined on the top surface of the quartz plate, and a high-reflectivity film is coated on the bottom surface of the groove. An anti-reflection film is then coated on the bottom surface of the quartz plate.
[0019] A nano-optical accelerometer is fabricated by placing the top chip on the top surface of the bottom chip.
[0020] Optionally, the photolithography process includes the following steps:
[0021] Spin coating: Photoresist is spin-coated onto the surface of the top silicon nitride film;
[0022] Pre-baking: After homogenization, baking is performed to fully evaporate the solvent in the photoresist;
[0023] Photolithography: using an ultraviolet lithography machine to expose the photoresist at the target pattern;
[0024] Development: The silicon wafer exposed to ultraviolet light is placed in a developer to remove the photoresist in the exposed areas, and the target pattern is obtained on the photoresist layer.
[0025] Optionally, the thickness of the homogenized coating is 1.6-2.2 μm; the pre-baking temperature is 80℃-100℃ and the time is 50-70 s; the exposure light source for the photolithography is an LED with a wavelength of 405 nm.
[0026] Optionally, the first etching method is inductively coupled plasma etching, wherein the conditions for inductively coupled plasma etching are: the etching gases are SF6 and SHF3, the flow rates of both gases are set to 35 sccm, the upper RF power and the lower RF power are set to 500 W and 120 W respectively, and the etching time is 100-106 s.
[0027] Optionally, the second etching method is wet etching with potassium hydroxide or sodium hydroxide solution, wherein the concentration of potassium hydroxide or sodium hydroxide solution is 0.1-0.5 mol / L, the temperature is 70℃-85℃, and the etching time is 29-40 h.
[0028] The principle of this nano-accelerometer is as follows: a mass block is suspended by flexible silicon nitride (SiN) suspended surfaces at the top and bottom, enabling minute vertical mechanical displacement. These suspended SiN surfaces are obtained by defining microbeam patterns on the surface of a SiN film and then wet etching the single-crystal silicon at the bottom of the SiN film to release the SiN film. The suspended SiN surface at the bottom and the high-reflectivity film deposited on a quartz plate in the bottom chip form an optical resonant cavity. The accelerometer is excited by a diode laser. In this resonant cavity, light of an appropriate wavelength can be reflected back and forth between the suspended SiN surface and the high-reflectivity film, creating a resonance enhancement effect. When the velocity of the accelerometer changes, the mass block undergoes mechanical displacement, the distance between the suspended SiN surface and the high-reflectivity film changes, the corresponding resonant wavelength changes, and the light signal detected from the bottom chip of the accelerometer also changes. The desired acceleration information can be obtained by analyzing the change in the light signal.
[0029] The beneficial effects of the accelerometer fabrication method of the present invention are that by first depositing a silicon nitride thin film of a certain thickness on both sides of the silicon wafer, only one photolithography process and etching process are needed on one side of the silicon nitride thin film to obtain the required suspended mass block and microbeam structure. The micro-nano fabrication process is simple and compatible with CMOS process, greatly reducing the difficulty of fabrication, and thus enabling large-scale low-cost batch fabrication, which also improves the yield. Attached Figure Description
[0030] Figure 1 This is a schematic cross-sectional view of the nano-optical accelerometer structure of the present invention;
[0031] Figure 2 This is a flowchart illustrating the preparation process of the suspended surface with a mass block according to the present invention.
[0032] Figure 3 The photolithographic pattern for releasing the silicon nitride thin film designed in Embodiment 1 of the present invention;
[0033] Figure 4 The above are the measurement results of the accelerometer sensitivity in Embodiment 1 of the present invention;
[0034] Figure 5 This is the photolithographic pattern designed in Embodiment 2 of the present invention for releasing the silicon nitride thin film.
[0035] In the diagram: 101, mass block; 102, silicon nitride thin film; 103, silicon wafer; 201, quartz plate; 202, groove; 203, high reflectivity film; 204, antireflection film. Detailed Implementation
[0036] like Figure 1 As shown, the present invention provides a method for fabricating a nano-optical accelerometer, comprising the following steps:
[0037] Silicon nitride thin films 102 are deposited on both sides of silicon wafer 103. One side of the silicon nitride thin film 102 is used as the top. Then, a target pattern is formed on the top silicon nitride thin film 102 using photolithography. The first etching method is used to etch the silicon nitride thin film 102 at the target pattern to form the hole of the target pattern. Then, the silicon wafer 103 is etched using the second etching method to release the silicon nitride thin film 102 and obtain a suspended surface with mass block 101.
[0038] A suspended surface with a mass block 101 is used as the top chip of a nano-accelerometer to sense speed changes; a quartz plate 201 with a high-reflectivity film 203 and an anti-reflection film 204 on both sides and a groove 202 in the middle is used as the bottom chip. The top chip and the bottom chip are integrated to make a nano-optical accelerometer.
[0039] Compared with the prior art, the method for fabricating a nano-optical accelerometer provided by the present invention first deposits a silicon nitride thin film 102 of a certain thickness on both sides of a silicon wafer 103. Only one photolithography process and etching process are required on one side of the silicon nitride thin film 102 to obtain the required suspended mass block 101 and microbeam structure. The micro-nano fabrication process is simple and compatible with CMOS process, which greatly reduces the difficulty of fabrication and enables large-scale, low-cost batch fabrication, thereby improving the yield.
[0040] Example 1
[0041] Pretreatment of double-polished monocrystalline silicon wafers: The double-polished monocrystalline silicon wafers are placed in deionized water for ultrasonic cleaning. The specifications of the double-polished monocrystalline silicon wafers are four inches. The ultrasonic cleaner used is model KQ3200DE, the ultrasonic input power is 150 W, the cleaning time is 15 min, and then they are taken out and dried with nitrogen.
[0042] Silicon nitride thin film 102: Silicon nitride thin film 102 is deposited on the top and bottom of a double-polished single crystal silicon wafer by LPCVD. The thickness of the silicon nitride thin film 102 deposited on the top and bottom is 600 nm.
[0043] Spin coat: AZ5214 positive photoresist was used, with a spin coat thickness of 2.0 μm. The spin coater operating parameters were: 500 r / min rotation speed for 5 s; followed by 2000 r / min rotation speed for 30 s.
[0044] Pre-baking: After the homogenization is completed, the silicon wafer 103 is placed on a baking table at 100 ℃ for baking for 60 seconds;
[0045] Photolithography: The photolithography machine used was a TTT-07-UV Litho(L) maskless ultraviolet photolithography machine. The exposure light source was an LED with a wavelength of 405nm. The photolithography precision was 1μm. The exposure parameters were set as follows: exposure time was 300 ms, ultraviolet light intensity was 0.3, and the photolithography pattern included microbeam structure and high reflectivity photonic crystal pattern.
[0046] Development: The ultraviolet-exposed silicon wafer 103 is placed in the developer for development, and the development time is 30 seconds;
[0047] ICP etching of silicon nitride: Coupled plasma etching was used, with the equipment being the DISC-ICP-601 nanostructure ICP etching system. The etching gases were SF6 and SHF3, with the flow rates of both gases set to 35 sccm. The upper and lower RF power were set to 500 W and 120 W, respectively. The etching time was 103 s, the etching rate of silicon nitride was approximately 6 nm / s, and the etching rate of photoresist was approximately 10 nm / s. After etching, the pattern in the photoresist layer was transferred to the silicon nitride layer.
[0048] Removal of residual photoresist on the top of the sample after ICP etching: The sample after ICP etching was sequentially treated with acetone-isopropanol-deionized water to remove the residual photoresist on the top of silicon wafer 103. The silicon wafer 103 was immersed in acetone for 5 min, in isopropanol for 2 min, and in deionized water for 3 min.
[0049] Silicon wafer 103 (single crystal silicon wafer) was wet-etched using potassium hydroxide or sodium hydroxide solution. The concentration of the potassium hydroxide or sodium hydroxide solution was 0.1 mol / L, the solution volume was 150 ml, the reaction vessel was made of polytetrafluoroethylene, the water bath temperature was 80℃, and the wet etching time was 29 h. A suspended surface with mass block 101 was obtained, and the typical volume (length, width, and height) of mass block 101 was 0.6 mm × 0.6 mm × 0.525 mm.
[0050] A suspended surface with a mass block 101 serves as the top chip of a nano-accelerometer, used to sense velocity changes; a quartz plate 201 with a high-reflectivity film 203 and an anti-reflection film 204 coated on both sides and a groove 202 in the middle serves as the bottom chip. The top and bottom chips are integrated to fabricate a nano-optical accelerometer. Measurement results are as follows: Figure 4 As shown: the sensitivity is 1600 mV / μg.
[0051] Example 2
[0052] Pre-treatment of double-polished monocrystalline silicon wafers: The double-polished monocrystalline silicon wafers are placed in deionized water for ultrasonic cleaning. The specifications of the double-polished monocrystalline silicon wafers are four inches. The ultrasonic cleaner used is model KQ3200DE, the ultrasonic input power is 150 W, the cleaning time is 20 min, and then they are taken out and dried with nitrogen.
[0053] A silicon nitride thin film 102 is deposited on the top and bottom of a double-polished single-crystal silicon wafer by LPCVD. The thickness of the silicon nitride thin film 102 deposited on the top and bottom is 600 nm.
[0054] For the spin coating, AZ5214 positive photoresist was selected, and the coating thickness was 1.8 μm. The spin coating machine operating parameters were: rotation speed 500 r / min, working time 5 s; then rotation speed 2500 r / min, working time 30 s.
[0055] After pre-baking and homogenization, the silicon wafer 103 is placed on a baking table at 100 ℃ for baking for 60 seconds.
[0056] The lithography used was a TTT-07-UV Litho(L) maskless ultraviolet lithography machine, the exposure light source was a 405nm wavelength LED, the lithography precision was 1μm, the exposure parameters were set as follows: exposure time was 300 ms, ultraviolet light intensity was set to 0.3, and the lithographic patterns included microbeam structures and high reflectivity photonic crystal patterns.
[0057] Development: The ultraviolet-exposed silicon wafer 103 is placed in a developer for development, and the development time is 30 seconds.
[0058] ICP etching of silicon nitride was performed using coupled plasma etching. The equipment used was a DISC-ICP-601 nanostructure ICP etching system. The etching gases were SF6 and SHF3, both with a flow rate of 35 sccm. The upper and lower RF power were set to 500 W and 120 W, respectively. The etching time was 103 s. The etching rate of silicon nitride was approximately 6 nm / s, and the etching rate of photoresist was approximately 10 nm / s. After etching, the pattern in the photoresist layer was transferred to the silicon nitride layer.
[0059] After ICP etching, the residual photoresist on the top of the sample was removed. The sample after ICP etching was then treated with acetone-isopropanol-deionized water to remove the residual photoresist on the top of silicon wafer 103. The silicon wafer 103 was immersed in acetone for 5 min, in isopropanol for 2 min, and in deionized water for 3 min.
[0060] Si was wet-etched using potassium hydroxide or sodium hydroxide solution with a concentration of 0.1 mol / L and a solution volume of 150 ml. The reaction vessel was made of polytetrafluoroethylene, the water bath temperature was 80℃, and the wet etching time was 36 h. A suspended surface with mass block 101 was obtained, and the typical volume (length, width, and height) of mass block 101 was 0.3 mm × 0.3 mm × 0.525 mm.
[0061] A suspended surface with a mass block 101 serves as the top chip of a nano-accelerometer, used to sense velocity changes; a quartz plate 201 with a high-reflectivity film 203 and an anti-reflection film 204 coated on both sides and a groove 202 in the middle serves as the bottom chip. The top and bottom chips are integrated to fabricate a nano-optical accelerometer. The sensitivity measurement result is 1200 mV / μg.
[0062] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; within the framework of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of one or more embodiments of this application as described above, which are not provided in detail for the sake of brevity.
[0063] One or more embodiments in this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments in this application should be included within the protection scope of this application.
Claims
1. A nano-optical accelerometer, characterized in that, Includes an acceleration sensing unit, wherein the acceleration sensing unit comprises a top chip and a bottom chip stacked together; The top chip includes: a mass block (101), two silicon nitride films (102) located at the top and bottom of the mass block (101) respectively, and a support structure disposed around the mass block, wherein the top silicon nitride film (102) has multiple through holes distributed in a U-shape; The bottom chip includes a quartz plate (201), a high-reflectivity film (203) and an anti-reflection film (204) respectively disposed on both sides of the quartz plate (201). The quartz plate (201) has a groove (202) in the middle. The high-reflectivity film (203) is located at the bottom of the groove (202). The silicon nitride thin film (102) at the bottom of the top chip covers the groove (202) to form an optical resonant cavity. The mass block (101) is located directly above the groove (202). The sensitivity measurement value of the nano-optical accelerometer is above 1000 mV / μg.
2. The nano-optical accelerometer according to claim 1, characterized in that, The through holes on the silicon nitride thin film (102) are circular holes, and the diameter of the through holes is 220-260 μm, and the spacing between adjacent through holes is 40-60 μm.
3. The nano-optical accelerometer according to claim 2, characterized in that, The number of through holes is 16-24.
4. The nano-optical accelerometer according to claim 3, characterized in that, The thickness of the silicon nitride thin film (102) is 200-650 nm.
5. The nano-optical accelerometer according to claim 1, characterized in that, The mass block (101) has a volume of 0.3-1.1 mm × 0.3-1.1 mm × 0.525 mm.
6. A method for preparing a nano-optical accelerometer according to any one of claims 1-5, characterized in that, Includes the following steps: The fabrication method of the top chip is as follows: Silicon nitride thin films (102) are deposited on both sides of a silicon wafer (103). One side of the silicon nitride thin film (102) is used as the top. Then, a target pattern is formed on the top silicon nitride thin film (102) using photolithography. The first etching method is used to remove the silicon nitride thin film (102) at the target pattern to form a hole in the target pattern, exposing the silicon wafer (103) below the target pattern. Then, the second etching method is used to etch the silicon wafer (103) in the hole of the target pattern to obtain a suspended surface with a mass block (101). The fabrication method of the bottom chip is as follows: A groove (202) is machined on the top surface of the quartz plate (201), and a high-reflection film (203) is deposited on the bottom surface of the groove (202), and an anti-reflection film (204) is deposited on the bottom surface of the quartz plate (201). A nano-optical accelerometer is fabricated by placing the top chip on the top surface of the bottom chip.
7. The method for preparing a nano-optical accelerometer according to claim 6, characterized in that, The photolithography process includes the following steps: Spin-coating: Photoresist is spin-coated onto the surface of the top silicon nitride film (102); Pre-baking: After homogenization, baking is performed to fully evaporate the solvent in the photoresist; Photolithography: using an ultraviolet lithography machine to expose the photoresist at the target pattern; Development: The ultraviolet-exposed silicon wafer (103) is placed in a developer to develop and remove the photoresist at the exposed area, thus obtaining the target pattern on the photoresist layer.
8. The method for preparing a nano-optical accelerometer according to claim 7, characterized in that, The thickness of the homogenized coating is 1.6-2.2 μm; the pre-baking temperature is 80℃-100℃ and the time is 50-70 s; the exposure light source for the photolithography is an LED with a wavelength of 405 nm.
9. The method for preparing a nano-optical accelerometer according to claim 6, characterized in that, The first etching method is inductively coupled plasma etching. The conditions for inductively coupled plasma etching are as follows: the etching gases are SF6 and CHF3, the flow rates of both gases are set to 35 sccm, the upper RF power and the lower RF power are set to 500 W and 120 W respectively, and the etching time is 100-106 s.
10. The method for preparing a nano-optical accelerometer according to any one of claims 6-9, characterized in that, The second etching method is wet etching with potassium hydroxide or sodium hydroxide solution. The concentration of potassium hydroxide or sodium hydroxide solution is 0.1-0.5 mol / L, the temperature is 70℃-85℃, and the etching time is 29-40 h.