A method of preventing stiction of a MEMS device
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2014-08-28
- Publication Date
- 2016-10-12
AI Technical Summary
[0006]本发明的目的是：解决现有的MEMS器件——硅微加速度计的粘连问题
[0019] This invention modifies the surface of silicon dioxide by depositing silicon nitride using LPCVD, and then etches a silicon dioxide barrier block using a silicon dioxide etching solution. This results in a barrier block surface morphology that is low in the center and high at the edges, effectively reducing the contact area. The principle is that during silicon nitride deposition on the silicon dioxide surface, some nitrogen diffuses to the surface. Because the sides of the barrier block have a relatively large roughness during wet etching, more nitrogen diffuses in; however, because the upper surface of the barrier block is smooth, less nitrogen diffuses in. At the edges of the upper surface of the barrier block, nitrogen diffuses within a 270° range, resulting in a higher nitrogen content. This leads to less nitrogen diffusion in the silicon dioxide in the central region of the barrier block. In the silicon dioxide etching solution, the etching rate in the nitrogen-diffused areas is lower, so the central region of the barrier block is etched quickly, while the edges are etched slowly, resulting in a morphology that is low in the center and high at the edges. This barrier block surface morphology effectively reduces the contact area between the barrier block and the lower electrode plate during accelerometer use, thus reducing the adhesion ratio.
Smart Images
Figure CN122556227B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the development of key technologies for micro sensors and is of great significance in improving the performance of MEMS sensors. Background Technology
[0002] MEMS devices are micro / nano devices, mainly including sensors and actuators, playing a vital role in aerospace, defense technology, industrial production, and daily life. Surface adhesion is a significant form of failure in micro / nano devices. Many surface forces can cause adhesion, the most common being capillary adhesion, van der Waals forces, electrostatic forces, and mechanical forces. Among these, capillary adhesion and van der Waals forces are the two main causes of micro / nano device failure, particularly common during the operation of RF-MEMS devices and MEMS accelerometers, and during the release process of MEMS gyroscope structures.
[0003] Capillary adhesion is related to the area and roughness of the adhered surfaces, as well as wettability. To reduce capillary adhesion, it is necessary to increase the distance between the interfaces, reduce the contact area, and increase the contact angle. Since the change in capillary adhesion with the effective distance is slow, controlling capillary adhesion requires reducing the area of action of the capillary adhesion, changing the surface morphology and surface properties, and reducing the contact area between the two interfaces is the most effective way to prevent surface adhesion.
[0004] During the use of MEMS accelerometers, adhesion often occurs, severely affecting the survival rate of the accelerometers. For example... Figure 1 The diagram shows a typical structure of a "sandwich" pendulum accelerometer. This structure consists of an outer frame 1, two electrode plates 2 on either side, and a central mass block 3. Eight blocking blocks 4 are symmetrically distributed on the surface of the mass block to prevent direct contact between the central mass block and the electrodes, thus preventing short circuits. The blocking blocks are made of SiO2 and have dimensions of 50µm × 50µm × 2µm (length × width × height). The capacitance signal is led out through electrode holes (not shown in the diagram). This accelerometer has an open structure.
[0005] During testing and use, this type of accelerometer exhibits a phenomenon where the central mass block adheres to the electrodes, meaning the blocking block sticks to the electrodes. Because the restoring force generated by the cantilever beam is less than the viscous force between the blocking block and the electrodes, the central mass block of the accelerometer cannot return to its equilibrium position, severely affecting the normal operation of the accelerometer. Summary of the Invention
[0006] The purpose of this invention is to solve the adhesion problem in existing MEMS devices—silicon micro accelerometers. This method can be applied to other MEMS devices, such as RF sensors, and can achieve a good anti-adhesion effect.
[0007] The technical solution of this invention is:
[0008] This invention provides a method for preventing MEMS devices from sticking together, characterized by comprising the following steps:
[0009] 1. A silicon dioxide film layer with a thickness of 2-5 μm is grown on the surface of a silicon wafer;
[0010] 2. The silicon dioxide film is patterned using photolithography, and then the silicon dioxide film is etched using a slow-release hydrofluoric acid solution to obtain silicon dioxide blocking blocks. Then the silicon wafer surface is cleaned to remove the adhesive.
[0011] 3. Using an LPCVD device, silicon nitride is deposited on the surface of a silicon dioxide barrier block, and then the silicon nitride on the surface of the silicon dioxide barrier block is etched away using a concentrated phosphoric acid solution;
[0012] 4. Using a low-concentration hydrofluoric acid solution to etch the silicon dioxide barrier block, the edge of the upper surface of the silicon dioxide barrier block is higher than the middle, forming a protrusion, thereby reducing the adhesion ratio.
[0013] Furthermore, in step 1, the silicon dioxide film layer is 2 μm thick and is deposited using LPCVD deposition. LPCVD can conveniently prepare silicon dioxide films with a thickness ≥2 μm, meeting the requirements for barrier block fabrication.
[0014] Furthermore, in step 3, the silicon nitride film thickness is 160 nm, deposited using LPCVD deposition. LPCVD deposition of silicon nitride serves two purposes: firstly, it acts as a masking layer for wet etching during structure fabrication; secondly, it alters the surface properties of silicon dioxide, ensuring differences in the subsequent etching of the barrier blocks and enabling the fabrication of anti-adhesion barrier blocks.
[0015] Furthermore, in step 2, a slow-release hydrofluoric acid solution is used to etch the silicon dioxide film. The solution concentration is 49% HF: 40% NH3F: H2O = 3:12:1, the temperature is room temperature, and the etching time is 3 minutes. Using this solution ensures that the barrier block is etched into shape in a shorter time, reducing the depth of side corrosion.
[0016] Furthermore, in step 3, the silicon nitride is etched using a concentrated phosphoric acid solution with a concentration of 90%, a temperature of 140°C, and a time of 3 hours to ensure that the silicon nitride film is completely removed.
[0017] Furthermore, in step 4, a low-concentration HF acid solution is used, with a solution concentration of 49% HF:H2O = 1:20. The etching temperature for silicon dioxide is room temperature, and the etching time is 7 minutes. This step is a key process in the preparation of the anti-adhesion blocking block. Different concentrations will result in different surface morphologies, and at this concentration, the etching morphology characteristics can be easily controlled.
[0018] The silicon dioxide blocking block prepared according to the above method effectively prevents MEMS devices from sticking together.
[0019] This invention modifies the surface of silicon dioxide by depositing silicon nitride using LPCVD, and then etches a silicon dioxide barrier block using a silicon dioxide etching solution. This results in a barrier block surface morphology that is low in the center and high at the edges, effectively reducing the contact area. The principle is that during silicon nitride deposition on the silicon dioxide surface, some nitrogen diffuses to the surface. Because the sides of the barrier block have a relatively large roughness during wet etching, more nitrogen diffuses in; however, because the upper surface of the barrier block is smooth, less nitrogen diffuses in. At the edges of the upper surface of the barrier block, nitrogen diffuses within a 270° range, resulting in a higher nitrogen content. This leads to less nitrogen diffusion in the silicon dioxide in the central region of the barrier block. In the silicon dioxide etching solution, the etching rate in the nitrogen-diffused areas is lower, so the central region of the barrier block is etched quickly, while the edges are etched slowly, resulting in a morphology that is low in the center and high at the edges. This barrier block surface morphology effectively reduces the contact area between the barrier block and the lower electrode plate during accelerometer use, thus reducing the adhesion ratio.
[0020] The advantage of this invention is that, without changing the fabrication process of the silicon micro accelerometer, a low-concentration silicon dioxide etchant is used to etch the blocking block, thereby altering the surface morphology of the blocking block's contact surface. This method can be applied not only to all blocking blocks in sandwich accelerometers but also has significant reference value for preventing adhesion in other MEMS devices.
[0021] This method can reduce the accelerometer adhesion failure rate from 30% to below 5%, significantly improving the accelerometer survival rate. This method not only solves the adhesion problem of cantilever accelerometers, but also plays an important role in other torsional accelerometers and RF MEMS. Attached Figure Description
[0022] Figure 1 This is a schematic diagram of a silicon micro accelerometer;
[0023] Figure 2 It is the shape of the block before etching;
[0024] Figure 3 It is the shape of the block after etching. Detailed Implementation
[0025] Step 1: Clean the silicon wafer and grow a 2µm thick silicon dioxide film on the surface of the silicon wafer using an LPCVD device.
[0026] Step 2: Using AZ6124 photoresist as a mask layer, the silicon dioxide film is patterned using photolithography, with a window size of 50um × 50um (i.e., the surface size of the barrier block). Then, the silicon dioxide is etched using a slow-release hydrofluoric acid solution (solution concentration: 49% HF: 40% NH3F: H2O = 3:12:1) for 3 minutes to obtain a single barrier block (size: length × width × height 50um × 50um × 2um). Then, the silicon wafer surface is cleaned to remove the photoresist.
[0027] Step 3: Using an LPCVD device, deposit a silicon nitride layer with a thickness of 160 nm on the surface of the silicon dioxide barrier (optimal deposition process control temperature 840℃, time 1h). Then, use a hot concentrated phosphoric acid solution (concentration 90%) to remove the silicon nitride from the silicon dioxide surface (preferably control the temperature 140℃, etching time 3h).
[0028] Step 4: Etch the barrier block using a hydrofluoric acid solution with a concentration of 1:20 (HF (49%):H2O = 1:20) for approximately 7 minutes. This will create a protrusion of about 200 nm at the edge. (See [link to documentation]). Figure 2 (morphology measured by step-by-step instrumentation before etching of the barrier block) and Figure 3 (The morphology of the block after etching is measured by a step gauge). The height of the protrusion changes depending on the etching time (the longer the etching time, the more obvious the protrusion). This morphology can effectively reduce the contact area and thus reduce the adhesion ratio.
Claims
1. A method for preventing MEMS devices from sticking together, characterized in that, Includes the following steps: (1) A silicon dioxide film layer with a thickness of 2-5 μm is grown on the surface of a silicon wafer; (2) The silicon dioxide film is patterned using photolithography, and then the silicon dioxide film is etched using slow-release hydrofluoric acid solution to obtain silicon dioxide blocking blocks. Then the silicon wafer surface is cleaned by removing the adhesive. (3) Using an LPCVD device, silicon nitride is deposited on the surface of the silicon dioxide barrier block, and then the silicon nitride on the surface of the silicon dioxide barrier block is etched away using a concentrated phosphoric acid solution. (4) Use a low-concentration hydrofluoric acid solution to etch the silicon dioxide block, causing the edge of the upper surface of the silicon dioxide block to be higher than the middle, forming a protrusion, thereby reducing the adhesion ratio.
2. The method for preventing MEMS device adhesion according to claim 1, characterized in that, In step (1), the silicon dioxide film is 2 μm thick and is deposited using the LPCVD deposition process.
3. The method for preventing MEMS device adhesion according to claim 1, characterized in that, The silicon nitride film in step (3) has a thickness of 160 nm and is deposited using LPCVD deposition process.
4. The method for preventing MEMS device adhesion according to claim 1, characterized in that, In step (2), the silicon dioxide film is etched using a slow-release hydrofluoric acid solution with a concentration of 49% HF: 40% NH3F: H2O = 3:12:
1. The temperature is room temperature and the etching time is 3 minutes.
5. The method for preventing MEMS device adhesion according to claim 1, characterized in that, In step (3), silicon nitride is etched using a concentrated phosphoric acid solution with a concentration of 90%, a temperature of 140°C, and a time of 3 hours.
6. The method for preventing MEMS device adhesion according to claim 1, characterized in that, In step (4), a low-concentration HF acid solution is used, with a solution concentration of 49% HF:H2O = 1:
20. The etching temperature of silicon dioxide is room temperature, and the etching time is 7 minutes.