Nanometer optical accelerometer and preparation method thereof

Abstract

The invention belongs to the field of accelerometers, and particularly relates to a nanometer optical accelerometer and a preparation method thereof.The preparation method comprises the steps that silicon nitride films are deposited on the two faces of a silicon wafer, and a target pattern is formed on the top silicon nitride film on one face through the photoetching technology; etching the silicon nitride film at the target pattern by adopting an etching method to form a hole of the target pattern, then etching the silicon wafer by adopting the etching method, and releasing the silicon nitride film to obtain a suspended surface with a mass block; a suspended surface with a mass block is used as a top chip of the nano accelerometer; a quartz plate of which the two surfaces are respectively plated with a high-reflective film and an antireflection film and the middle is provided with a groove is used as a bottom chip, and the top chip and the bottom chip are integrated to prepare the nano optical accelerometer; only one photoetching process is needed, the micro-nano machining process is simple and compatible with the CMOS process, the cost is lower, and the yield of device preparation is improved.

Description

Technical Field

[0001] The present invention belongs to the field of accelerometers, and in particular relates to a nano-optical accelerometer and a preparation method thereof. Background Art

[0002] An accelerometer is a sensor that measures changes in velocity. Whether it's the automatic deployment of airbags in cars, the automatic rotation of smartphone screens, or the flight path maintenance of aerospace vehicles, accelerometers are essential.

[0003] Currently, traditional accelerometers use sensor technology to measure acceleration by converting changes in the sensor's velocity into an electrical signal output. However, their measurement range and accuracy are limited by the device's sensitivity and environmental factors, making them prone to errors. Furthermore, due to size constraints, the coupling between devices is insufficient, and the level of integration is low, making it difficult to achieve large-scale, simultaneous acceleration measurements of multiple targets. Micro-opto-electromechanical accelerometers, on the other hand, convert changes in an object's velocity into changes in an optical signal output for measurement. Compared to traditional accelerometers, they offer a simpler measurement process, higher integration, and greater sensitivity, resulting in a garnering significant attention in recent years.

[0004] Generally, the sensing part of current accelerometers requires the design of relatively complex sub-wavelength Si gratings and in-plane structures such as rotating folded beams, and needs to undergo two photolithography-reactive ion etching-wet etching processes. This not only increases the complexity of the process, but also the two photolithography processes also place higher requirements on micro-nano processing technology (such as the alignment accuracy of overlay). Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a nano-optical accelerometer and a preparation method thereof, which only requires one photolithography process, has a simple micro-nano processing process and lowers the cost.

[0006] The present invention provides a nano-optical accelerometer and a preparation method thereof, comprising 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, two silicon nitride films located at the top and bottom of the mass block respectively, and a support structure arranged on the periphery of the mass block, and the top silicon nitride film has a plurality of through holes distributed in a circular shape; The bottom chip includes a quartz plate, a high-reflection film and an anti-reflection film respectively arranged on both sides of the quartz plate. There is a groove in the middle of the quartz plate, and the high-reflection film is located at the bottom of the groove. The silicon nitride film at the bottom of the top chip covers the groove to form an optical resonant cavity, and the mass block is located directly above the groove.

[0007] Optionally, the through holes on the silicon nitride film are circular holes, and the aperture of the through holes is 220-260 μm, and the distance between adjacent through holes is 40-60 μm.

[0008] Optionally, the number of the through holes is 16-24.

[0009] Optionally, the silicon nitride film has a thickness of 200-650 nm.

[0010] Optionally, the typical volume of the mass block is 0.3-1.1 mm in length, width and height × 0.3-1.1 mm × 0.525 mm.

[0011] Compared with the existing technology, the typical sensitivity measurement value of the nano-optical accelerometer provided by the present invention is above 1000mV / μg, where g is the acceleration due to gravity; the sensitivity is improved by about four orders of magnitude compared with the traditional accelerometer composed solely of optical reflectors, breaking through the sensitivity limitations of traditional acceleration sensor structures.

[0012] The present invention provides a method for preparing a nano-optical accelerometer, comprising the following steps: The preparation method of the top chip is: Silicon nitride films are deposited on both sides of a silicon wafer, with the silicon nitride film on one side serving as the top. A target pattern is then formed on the top silicon nitride film using a photolithography process. A first etching method is used to remove the silicon nitride film at the target pattern to form a hole in the target pattern, exposing the silicon wafer underneath. A second etching method is then used to etch the silicon wafer within the hole in the target pattern, resulting in a suspended surface with a mass block. The preparation method of the bottom chip is: A groove is machined on the top surface of the quartz plate, and a high-reflection film is coated on the bottom surface of the groove, and an anti-reflection film is coated on the bottom surface of the quartz plate; The top chip is placed on top of the bottom chip to create a nano-optical accelerometer.

[0013] Optionally, the photolithography process includes the following steps: Spin coating: Spin coating photoresist on the surface of the top silicon nitride film; Pre-baking: After the coating is completed, baking is performed to fully evaporate the solvent in the photoresist; Photolithography: Use a UV lithography machine to expose the photoresist at the target pattern; Development: The silicon wafer after UV exposure is placed in a developer for development to remove the photoresist at the exposed area and obtain the target pattern on the photoresist layer.

[0014] Optionally, the thickness of the coating is 1.6-2.2 μm; the temperature of the pre-bake is 80° C.-100° C., and the time is 50-70 s; and the exposure light source of the photolithography is an LED with a wavelength of 405 nm.

[0015] Optionally, the first etching method is inductively coupled plasma etching, and the conditions of the inductively coupled plasma etching are: the etching gases are SF6 and SHF3, the flow rates of the two 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.

[0016] Optionally, the second etching method is wet etching using potassium hydroxide or sodium hydroxide solution, the concentration of the potassium hydroxide or sodium hydroxide solution is 0.1-0.5 mol / L, the temperature is 70° C.-85° C., and the etching time is 29-40 h.

[0017] The principle of this nanoaccelerometer is as follows: a proof mass is suspended by flexible silicon nitride suspended surfaces at the top and bottom, enabling tiny vertical mechanical displacements. The suspended surfaces are created by defining a microbeam pattern on a silicon nitride film and then wet-etching the single-crystalline silicon underneath to release the film. The bottom silicon nitride suspended surface and a highly reflective film coated on a quartz plate in the bottom chip form an optical resonant cavity. The accelerometer is excited by a diode laser. Within this resonant cavity, light of appropriate wavelengths reflects back and forth between the bottom silicon nitride suspended surface and the highly reflective film, creating a resonance enhancement effect. When the accelerometer's velocity changes, the proof mass undergoes mechanical displacement, which in turn changes the distance between the bottom silicon nitride suspended surface and the highly reflective film, shifting the corresponding resonant wavelength and causing a change in the optical signal detected by the accelerometer's bottom chip. This change in optical signal is then used to obtain the desired acceleration information.

[0018] The beneficial effect of the accelerometer preparation method of the present invention is that by first depositing a certain thickness of silicon nitride film on both sides of the silicon wafer, only one photolithography process and an etching process need to be performed on the silicon nitride film on one side to obtain the required suspended mass block and micro-beam structure. The micro-nano processing technology is simple and compatible with CMOS technology, the preparation difficulty is greatly reduced, and large-scale low-cost batch production is possible, which also improves the yield rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the cross-section of the nano-optical accelerometer structure of the present invention; Figure 2 A flow chart for preparing a suspended surface with a mass block according to the present invention; Figure 3The photolithographic pattern designed for releasing the silicon nitride film in Example 1 of the present invention; Figure 4 is the measurement result of the accelerometer sensitivity in Example 1 of the present invention; Figure 5 This is the photolithographic pattern designed in Example 2 of the present invention for releasing the silicon nitride film.

[0020] In the figure: 101, mass block; 102, silicon nitride film; 103, silicon wafer; 201, quartz wafer; 202, groove; 203, high-reflection film; 204, anti-reflection film. DETAILED DESCRIPTION

[0021] like Figure 1 As shown, the present invention provides a method for preparing a nano-optical accelerometer, comprising the following steps: Silicon nitride films 102 are deposited on both sides of a silicon wafer 103, with the silicon nitride film 102 on one side serving as the top. A target pattern is then formed on the top silicon nitride film 102 using a photolithography process. A first etching method is used to etch the silicon nitride film 102 at the target pattern to form a hole of the target pattern. A second etching method is then used to etch the silicon wafer 103 to release the silicon nitride film 102, thereby obtaining a suspended surface with a mass block 101. The suspended surface with the mass block 101 is used as the top chip of the nano-accelerometer to sense velocity changes; the quartz plate 201 with a high-reflection 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 produce a nano-optical accelerometer.

[0022] Compared with the existing technology, the preparation method of the nano-optical accelerometer provided by the present invention first deposits a silicon nitride film 102 of a certain thickness on both sides of a silicon wafer 103. Only a single photolithography process and an etching process are required on one side of the silicon nitride film 102 to obtain the required suspended mass block 101 and micro-beam structure. The micro-nano processing technology is simple and compatible with CMOS technology, which greatly reduces the difficulty of preparation and enables large-scale, low-cost batch production, also improving the yield rate.

[0023] Example 1 Pretreatment of double-polished single-crystal silicon wafers: Place the double-polished single-crystal silicon wafers in deionized water for ultrasonic cleaning. The size of the double-polished single-crystal silicon wafers is four inches. The ultrasonic cleaner used is the model KQ3200DE. The ultrasonic input power is 150 W and the cleaning time is 15 minutes. After that, take out and blow dry with nitrogen. Depositing a silicon nitride film 102: depositing a silicon nitride film 102 on the top and bottom of the double-polished single-crystal silicon wafer by LPCVD, with the thickness of the silicon nitride film 102 deposited on the top and bottom being 600 nm; Spin coating: AZ5214 positive photoresist was used, with a thickness of 2.0 μm. The spin coating machine operating parameters were: speed 500 r / min, working time 5 s; then speed 2000 r / min, working time 30 s; Pre-baking: After the coating is completed, the silicon wafer 103 is placed on a baking table at 100°C and baked for 60 seconds; Photolithography: The lithography machine used was a TTT-07-UV Litho(L) maskless UV lithography machine. The exposure light source was a 405nm wavelength LED. The lithography precision was 1μm. The exposure parameters were set to 300ms exposure time and 0.3 UV intensity. The lithography patterns included micro-beam structures and high-reflectivity photonic crystal patterns. Development: placing the UV-exposed silicon wafer 103 in a developer for development for 30 seconds; ICP etching of silicon nitride: Coupled plasma etching was used, and the equipment model used was the DISC-ICP-601 nanostructure ICP etching system. The etching gases were SF6 and SHF3, and the flow rate of both gases was set to 35 sccm. The upper and lower RF powers were set to 500 W and 120 W, respectively. The etching time was 103 s, and the etching rate of silicon nitride was about 6 nm / s. The etching rate of photoresist was about 10 nm / s. After the etching was completed, the pattern in the photoresist layer was transferred to the silicon nitride layer. Removing residual photoresist from the top of the sample after ICP etching: After ICP etching, the sample was sequentially treated with acetone, isopropanol, and deionized water to remove residual photoresist from the top of the silicon wafer 103. The silicon wafer 103 was immersed in acetone for 5 minutes, immersed in isopropanol for 2 minutes, and immersed in deionized water for 3 minutes. The silicon wafer 103 (single crystal silicon wafer) is wet-etched with a potassium hydroxide or sodium hydroxide solution. The concentration of the potassium hydroxide or sodium hydroxide solution is 0.1 mol / L, the volume of the solution is 150 ml, the reaction container is made of polytetrafluoroethylene, the water bath temperature is 80°C, and the wet etching time is 29 hours. This results in a suspended surface with a mass block 101. The typical volume (length, width, and height) of the mass block 101 is 0.6 mm × 0.6 mm × 0.525 mm. The suspended surface with the mass 101 is used as the top chip of the nano-accelerometer to sense velocity changes; the quartz plate 201 with a high-reflection 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 and bottom chips are integrated to produce a nano-optical accelerometer. The measurement results are shown in Figure 2. Figure 4 Shown: Sensitivity is 1600 mV / μg.

[0024] Example 2 Pretreatment of double-polished single-crystal silicon wafers: Place the double-polished single-crystal silicon wafers in deionized water for ultrasonic cleaning. The size of the double-polished single-crystal silicon wafers is four inches. The ultrasonic cleaner used is the model KQ3200DE. The ultrasonic input power is 150 W and the cleaning time is 20 minutes. After that, take out and blow dry with nitrogen. Depositing a silicon nitride film 102 on the top and bottom of the double-polished single-crystal silicon wafer by LPCVD, wherein the thickness of the silicon nitride film 102 deposited on the top and bottom is 600 nm; For the spin coat, AZ5214 positive photoresist was selected with a thickness of 1.8 μm. The operating parameters of the spin coater were: speed 500 r / min, working time 5 s; then speed 2500 r / min, working time 30 s; Pre-baking: After the coating is completed, the silicon wafer 103 is placed on a baking table at 100°C and baked for 60 seconds; The photolithography machine used was a TTT-07-UV Litho(L) maskless UV photolithography machine. The exposure light source was a 405 nm wavelength LED. The photolithography precision was 1 μm. The exposure parameters were set to 300 ms exposure time and 0.3 UV intensity. The photolithography patterns included micro-beam structures and high-reflectivity photonic crystal patterns. Development: placing the UV-exposed silicon wafer 103 in a developer for development, with a development time of 30 s; ICP etching of silicon nitride uses coupled plasma etching. The equipment model used is the DISC-ICP-601 nanostructure ICP etching system. The etching gases are SF6 and SHF3. The flow rate of both gases is set to 35 sccm. The upper and lower RF powers are set to 500 W and 120 W, respectively. The etching time is 103 s. The silicon nitride etching rate is about 6 nm / s, and the photoresist etching rate is about 10 nm / s. After the etching is completed, the pattern in the photoresist layer is transferred to the silicon nitride layer. Removing the residual photoresist on the top of the sample after ICP etching. The sample after ICP etching is sequentially treated with acetone, isopropanol, and deionized water to remove the residual photoresist on the top of the silicon wafer 103. The silicon wafer 103 is immersed in acetone for 5 minutes, immersed in isopropanol for 2 minutes, and immersed in deionized water for 3 minutes. Si was wet-etched with 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 container was made of polytetrafluoroethylene, the water bath temperature was 80°C, and the wet etching time was 36 h. A suspended surface with mass block 101 was obtained. The typical volume (length, width, and height) of the mass block 101 was 0.3 mm × 0.3 mm × 0.525 mm. The nano-optical accelerometer is fabricated by integrating a suspended surface with a mass 101 as the top chip, sensing velocity changes. A quartz plate 201, coated on both sides with a high-reflection coating 203 and an anti-reflection coating 204, and with a groove 202 in the middle, serves as the bottom chip. The top and bottom chips are integrated to create a nano-optical accelerometer. The measured sensitivity is 1200 mV / μg.

[0025] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of protection of the present application is limited to these examples. In line with the present application, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of different aspects of one or more embodiments of the present application as described above, which are not provided in detail for the sake of simplicity.

[0026] The one or more embodiments of this application are intended to encompass 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 of this application should be included in the scope of protection of this application.

Claims

1. A nano-optical accelerometer, characterized in that: The acceleration sensing unit includes a top chip and a bottom chip stacked together; The top chip comprises: a mass block (101), two silicon nitride films (102) respectively located on the top and bottom of the mass block (101), and a support structure arranged on the periphery of the mass block, wherein the top silicon nitride film (102) has a plurality of through holes distributed in a circular shape; The bottom chip comprises a quartz plate (201), a high-reflection film (203) and an anti-reflection film (204) respectively arranged on both sides of the quartz plate (201); a groove (202) is provided in the middle of the quartz plate (201); the high-reflection film (203) is located at the bottom of the groove (202); the silicon nitride film (102) at the bottom of the top chip covers the groove (202) to form an optical resonant cavity; and the mass block (101) is located directly above the groove (202).

2. The nano-optical accelerometer according to claim 1, wherein: The through holes on the silicon nitride film (102) are circular holes, the aperture 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 the through holes is 16-24.

4. The nano-optical accelerometer according to claim 3, wherein: The thickness of the silicon nitride film (102) is 200-650 nm.

5. The nano-optical accelerometer according to claim 1, wherein: The typical volume of the mass block (101) is 0.3-1.1 mm in length, width and height × 0.3-1.1 mm × 0.525 mm.

6. A method for preparing a nano-optical accelerometer according to any one of claims 1 to 5, characterized in that: The following steps are involved: The preparation method of the top chip is: A silicon nitride film (102) is deposited on both sides of a silicon wafer (103), with the silicon nitride film (102) on one side being the top, and then a target pattern is formed on the top silicon nitride film (102) using a photolithography process, and the silicon nitride film (102) at the target pattern is removed using a first etching method to form a hole of the target pattern, exposing the silicon wafer (103) below the target pattern, and then a 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 preparation method of the bottom chip is: A groove (202) is machined on the top surface of the quartz plate (201), a high-reflection film (203) is plated on the bottom surface of the groove (202), and an anti-reflection film (204) is plated on the bottom surface of the quartz plate (201); The top chip is placed on top of the bottom chip to create a nano-optical accelerometer.

7. The method for preparing a nano-optical accelerometer according to claim 6, wherein: The photolithography process comprises the following steps: Spinning the photoresist: spin-coating the photoresist on the surface of the top silicon nitride film (102); Pre-baking: After the coating is completed, baking is performed to fully evaporate the solvent in the photoresist; Photolithography: Use a UV lithography machine to expose the photoresist at the target pattern; Development: The silicon wafer (103) after ultraviolet exposure is placed in a developer for development, the photoresist at the exposed portion is removed, and a target pattern is obtained on the photoresist layer.

8. The method for preparing a nano-optical accelerometer according to claim 7, wherein: The thickness of the coating is 1.6-2.2 μm; the temperature of the pre-bake is 80° C.-100° C., and the time is 50-70 s; the exposure light source of the photolithography is an LED with a wavelength of 405 nm.

9. The method for preparing a nano-optical accelerometer according to claim 6, wherein: The first etching method is inductively coupled plasma etching. The conditions of the inductively coupled plasma etching are: the etching gases are SF6 and SHF3, the flow rates of the two gases are both 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 to 9, wherein: The second etching method is wet etching with potassium hydroxide or sodium hydroxide solution, the concentration of the potassium hydroxide or sodium hydroxide solution is 0.1-0.5 mol / L, the temperature is 70° C.-85° C., and the etching time is 29-40 h.

Images