Investigation Of Device Level MEMS Packaging Based On Vacuum Resistance Welding

The MEMS devices are widely used in various radio frequency, inertia and mechanical resonators which are often operated under the high vacuum environment to ensure good vibration performance, high quality and long-term stablity and reliability. Therefore, vacuum packaging is necessary for these MEMS devices. In this article, a new method to achieve the vacuum packaging of MEMS devices using vacuum resistance welding was developed. To establish a standard crafts of the MEMS devices in a vacuum resistance packaging process, the key technologies were investigated intensively by theory, numerical simulation and experimental methods. The main research and innovation are as follows:Through a large number of encapsulation testing and analysis about characteristics of hermetic package, the key factors that influence the vacuum of MEMS encapsulation were found. According to the new features of welding in the vacuum environment, the vacuum resistance welding equipment, combined the resistance fusion welding device together with the vacuum encapsulation, with independent intellectual property rights has been developed. The glove box, vacuum drying oven, vacuum-pumping system and welding mechanism of the equipment form as an organic whole which provides repeatable working environment for the whole vacuum encapsulation process and guarantees the quality of vacuum encapsulation.The vacuum of small volume in MEMS devices should be measured before vacuum packaging. This paper makes use of the existing theory that the resonant resistance of quartz crystal changes with the vacuum of the environment to measure the vacuum of these MEMS devices and studies various factors that influence the vacuum measurement accuracy.The thermo-electricity-structure finite element simulation on vacuum resistance welding process under projection welding was performed. The displacement field, stress field, temperature field and the current distribution by contact analysis at the prepressing stage of the projection welding, the process of the convex welding steel conquassation and the forming process of the nugget was further discussed. Results indicated that the melt nuclear form is influenced by the prepressing and the size of the convex welding steel.In order to achieve the vacuum packaging of the MEMS devices below lOPa, a vacuum shell with cushion chamber to ensure the leakage rate and a special shell of the vacuum encapsulation of MEMS devices were developed. To improve the rate of finished products of the vacuum packaging, the effects of the metal coating on the vacuum packaging was further investigated. And then experiments on the weld strength, the vacuum maintenance of the vacuum packaging hull were performed to ensure the technical reliability of the vacuum packaging.The vacuum resistance welding of the MEMS device package models based on the theory of vacuum physics and the primary factors leading to the loss of vacuum were analyzed. According to the theoretical calculation of the leak rate for the special vacuum packaging shell, the lifetime estimation approach of the vacuum packaging MEMS devices was established. At last, a packaging technology for resistance welding MEMS devices was found. Experimental results showed that the self-designed vacuum packaging shell meets the requirements of long time of vacuum maintenance.