The traditional inertial sensor has limited its application in many aspects because of its large volume and high cost. The micromachining method can be used to manufacture micro-machined vibrating gyros, which can realize mass production and make the products have low cost, small volume and good repeatability and consistency. These characteristics make the MEMS vibrating gyro have distinct dual-use for military and civilian purposes, which has attracted extensive attention of scholars at home and abroad. Since the mid-1980s, the United States, Britain, Japan, Germany and other industrialized countries have invested a lot of manpower and financial resources in the research of micromechanical vibration inertial devices.
Because most semiconductor integrated circuits are made of silicon, people have a deeper understanding of silicon. Many micromechanical inertial sensors reported in domestic and foreign literatures are fabricated on silicon. However, micromachining materials are not limited to silicon, and many other materials are also suitable for making special micro devices. Piezoelectric crystal is also a common crystal.
From the point of view of single crystal structure and high elastic modulus, timely crystal and silicon material are ideal vibrator materials for MEMS inertial sensors. They have different crystal symmetries. It belongs to triangular system, while silicon belongs to cubic system, and silicon has higher symmetry. Therefore, some of their physical characteristics are completely different. Time is an insulator, while silicon is a semiconductor. There is no current leakage between the conductors in the device made of Synchrotron. Synchrotron is the first widely used piezoelectric crystal, which is very suitable for making the microstructure of resonator. Silicon is not a piezoelectric body, so using it as a resonant device requires more complicated technology. We selected the timely crystal material and developed a micro-mechanical vibrating gyro prototype by micro-machining technology. The following mainly introduces the structure, principle and test results we adopted.
Second, the basic structure and working principle
The basic structure of the timely MEMS vibrating gyro is shown in figure 1.
The driving tuning fork is excited by a closed-loop constant gain oscillation circuit to make it resonate around 10kHz. When the tuning fork rotates along the input shaft at an angular velocity w, the reading tuning fork vibrates back and forth in the direction perpendicular to the vibration plane of the driving tuning fork because the tuning fork swings at a certain speed (see figure 1 for the vibration directions of the driving tuning fork and the reading tuning fork). Due to the piezoelectric effect, charge is generated on the reading tuning fork, and it is detected that the reading tuning fork is generated.
Thirdly, the principle of anisotropic etching.
There are two kinds of anisotropic etching of crystal: dry etching (reactive ion etching) and wet etching (chemical etching).
Reactive ion etching technology
Plasma etching technology uses plasma instead of chemical etching solution. The shape obtained by micromachining is not controlled by the crystal orientation of the substrate, and plasma will not bring great stress to the microstructure, but the equipment is complex and many parameters must be controlled. Typical parameters are gas properties and flow rate, substrate properties and area, electrode structure, excited electromagnetic parameters and the shape of vacuum chamber. Different combinations will produce different corrosion processes. The ideal plasma micromachining process with high speed, anisotropy and selectivity is still to be developed. However, with the help of the completed research, it can be predicted that this process will become a reality in recent years. From the point of view of micromachining, the etching depth of only tens of microns is too limited, so it is necessary to further study the anisotropic etching of trenches with hundreds of microns by reactive ion etching process.
2. Chemical etching technology
The etching speed of the crystal depends on the crystal orientation of the etched surface, and the principle of this anisotropic etching is still unclear. The atomic density of the etched crystal surface seems to be the decisive factor, and the slowest etching speed corresponds to the densest surface. A very useful feature of timely crystal is that its corrosion rate is almost zero for all planes parallel to the Z axis. Using this characteristic, steep walls with different shapes perpendicular to the Z axis can be formed on the Z-cut substrate. The corrosion equation in HF and NH4F buffer is:
SiO2+4HF→SiF4+2H2O
SiF4 is a gas under normal circumstances, and it will form a complex with HF if it is too late to volatilize in the solution containing HF.
SiF4+2HF→H2SiF6
SiF4+2NH4F→(NH4)2SiF6
The corrosion rate depends on the composition and temperature of the etching solution, and the properties of the etched surface depend on the positive ions and temperature in the etching solution. The etching solution containing H+ has steps and no pits, the etching solution containing NH4+ has steps and pits (there are fewer pits at 25℃ than at 55℃), and the etching solution containing K+ has no steps and no pits at 25℃ ... Finally, it is necessary to accurately monitor the temperature of the etching solution according to the etching time. When the temperature changes by 65438±0°C, the etching rate will change by 0.65438 0 mm/min.
Four, the key processing technology of MEMS inertial sensor.
From the discussion of chemical etching and reactive ion etching technology, it can be seen that chemical etching has fast etching speed, good anisotropy and low cost, while reactive ion etching has slow etching speed and high cost. The thickness of the vibrator to be machined is several hundred microns, which requires anisotropic corrosion rate. Therefore, chemical etching technology is used to process the vibrator. Comprises the steps of substrate processing, protective film deposition, photoetching, chemical anisotropic etching and the like. Every step is important. If there are many surface defects and scratches on the substrate, these scratches will become wider and deeper after corrosion, which will affect the surface flatness of the vibrator. If the substrate is not cleaned, the dust on the surface of the substrate will reduce the adhesion between the substrate and the protective film. The poor adhesion of the protective film will lead to the failure of oscillator etching. However, the most critical of all processes is the chemical anisotropic etching process.
The corrosion rate depends on the composition and temperature of the corrosion solution. The higher the temperature, the faster the corrosion rate. We used HF, NH4F, NH4HF and KF as the main components of the corrosive solution, and conducted a lot of tests on corrosive solutions with different components and concentrations at different temperatures. The main problem encountered in the experiment is that the shape of the X side wall of the tuning fork oscillator is inconsistent after corrosion. As shown in Figure 2, only one direction of the X sidewall is steep, and the other side has a prominent edge, which makes the subsequent process impossible to complete. Based on the analysis of the corrosion mechanism of silicon dioxide in HF, a variety of combination tests were carried out on a variety of etching solutions with different concentrations and temperatures, and finally a more reasonable composition and working temperature conditions of the etching solution were obtained. The corrosion pattern of post-treatment after improving the process conditions is shown in Figure 3.
Verb (abbreviation of verb) test results
We use the anisotropic etching method of chemical etching to process the tuning fork oscillator. At present, we have developed a prototype of the timely MEMS gyroscope. The zero stability of MEMS gyro prototype is 0.3/s, which is the leading level in China at present. Figure 4-5 shows the impedance-phase characteristic curves of the driving surface and the reading surface of the MEMS vibrating gyro in air. The frequencies of the driving surface and the reading surface of the micromachined vibrating gyroscope in air are10.5 ~1khz and10.8 ~11.2khz, respectively, and the q values of the driving surface and the reading surface are 2000~5000.
Conclusion of intransitive verbs
A MEMS gyroscope prototype was developed by using chemical anisotropic etching technology. The processing and testing results of the timely MEMS vibration gyroscope are briefly described. The next step is to focus on the research of high-precision MEMS vibrating gyro and the engineering research of MEMS vibrating gyro. With the development of MEMS technology, it can be said for sure that in the near future, the domestic timely micromechanical vibrating gyro will fully meet the needs of military and civil use.