Research On Vacuum Packaging Of MEMS Devices

Compared with ICs, MEMS devices have specific features such as sensing, actuating, 3D structure and function diversity, those make MEMS packaging technology facing more challenges and a bottleneck of mass production and wide applications for many MEMS devices. Of all the packaging technology, MEMS vacuum packaging is one of the most challenging technologies still under development, many MEMS devices which have potential market applications can only stay in laboratory for demonstration because of lack of adequate, reliable vacuum packaging technology. This dissertation conducted fundamental research into several key issues of MEMS vacuum packaging technology, especially focusing on a lot of common tough technologies by integrating theory, simulation, and experiments. Based on the fundamental work conducted, a vacuum pacakaging equipment by resistance fusion welding has been developed and patented, and MEMS device-level vacuum packaging processes have been developed and implemented in volume production. The major innovation points are summarized below.(1) Systematic research has been conducted on the MEMS vacuum packaging technology for the first time in China. Using permeance and diffuse theory, the disabsorption of surface absorbed gases, the gas penetration and degassing of dissolved gases in packaging materials have been studied and quantatively evaluated the effects on the vacuum packaging processes, especially the dramatic influence on MEMS device vacuum packaging because of MEMS device small volume package, thin package shell. The requirement of vacuum packaging has been theoretical analysed, a relationship between leak rate, vacuum pressure and vacuum maintenance lifetime was derived, which played an important, instructive role on implementing the vacuum packaging. The results demonstrated that the leak rate required for MEMS vacuum packaging is so much low that it is beyond the ability of an instrument to measure and difficult to find out an adequate welding way to achieve the hermeticity. MEMS vacuum packaging could only be implemented by two possible ways, one way is to decrease the effective leak rate by the package structure design, and another way is to use the getter to blance the leak by absorption of leaking gases, the vacuum packaging under 0.1Pa can only be realized by the aid of the getter.(2) The packaged cavity of MEMS devices is too small to find an ordinary vacuum gauge to monitor the vacuum degree, which has been an obstacle in MEMS vacuum research. Using tuning fork cystal in this dissertation gives a way to measure the pressure of the packaged cavity as a monitor chip. The mathematic physical model of tuning fork cystal was created by piezo-electric dynamics, the resonant resistance analytical solution has been derived and gives the relationship between crystal structure, gas damping coefficient, gas pressure and resonant resistance, which plays a great role in crystal optimum design as a sensing chip, and has an instructive significance in crystal new applications. The gas damping effects in rarefied gases have been theoretically studied and verified by the crystal vacuum measurement. Based on the fundamental research work, a crystal vacuum measurement system was designed and carried out, many experiments have been done on measurement repeatability, measuring range, temperature sensibility, and aging characteristic. This system acts as a key vacuum evaluation tool for MEMS vacuum packaging research.(3) Vacuum packaging equipment by resistance fusion welding has been developed for the first time in China. This equipment incorportated the resistance fusion welding process into vacuum packaging with a systematic design of vacuum baking, vacuum pumping, resistance welding,etc. All the subsystems were fitted in the working cabinet with pure gas circulation which supplied a repeatable working environment and guaranteed the vacuum packaging quality. The vacuum packaging process research such as welding parameter optimization, degassing, the fixure influence on the vacuum pressure distribution, etc. has been conducted, and finally established a reliable vacuum metal packaging processes for MEMS devices. The equipment has been patented.(4) The leak mode and leak rate measurement have been studied. The detailed analyse has been conducted on the leak evaluation by helium mass spectrometer leak detector, and theoretically and experimentally authenticated that the helium leak detector as a common leak rate measurement tool can not meet the requirements for leak detection during vacuum packaging research because of its limited sensitivity and measured nolinearity characteristic under vacuum, the measurement results have a big difference compared to the actual leak rate, and can be only taken as a rough reference for comparison with each other. The leak rate was also measured by crystal pressure monitoring. The results were contrasted with that by helium leak detector. The experiments showed that the measured leak rate by helium leak tester had more than three orders difference with the actual leak rate. The leak modes have been thoroughly analyzed based on the micro-tube leak model and degassing model of package materials, and the simulation results greatly satisfied with that from the experiment measurment. The leak rate measurement research showed that it was not a feasible direction to carry out reliable vacuum packaging with 5 more years vacuum maintenance only by increasing the welding hermeticity.(5) An innovated vacuum package design with a small vacuum buffer cavity was proposed, and has been proved in theory that this design can extend the vacuum maintenance at least 20 times more than the common package with the same volume and under the same welding means. The reliability research has been conducted on the vacuum packaging with the vacuum buffer cavity, using 85℃high temperature and 85% relative humidity soak, temperature and humidity cycling to accelerate fatigue. The lifetime of the package cavity vacuum has been evaluated at the same order as that in theory. Another experiment of one more year continued vacuum monitoring has also verified the validity of the vacuum package design. The reliable vacuum packaging has been realized with getter free. The design of the vacuum package has been patented.