MEMS Hermetic Packaging for Extremely Sensitive Accelerometers with In-Package Pressure Sensing Solution

In Micro-Electro-Mechanical Systems packages, sealing pressure is one of the most important indicators of packaging quality. Traditional hermeticity testing methods are expensive and inconvenient. Solution for in-package pressure monitoring has long been appealing. Pirani sensor is a commonly used pressure sensor that can be integrated into Micro-Electro-Mechanical Systems packages. Our team is developing a hermetic packaging process for extremely sensitive and low noise accelerometers. A eutectic package sealing process is developed and evaluated. To verify the stability of environment inside the package, a lowcost, process flow compatible, space saving bondwire Pirani sensor has been explored. The sensing principle is based on resistance change of the filament is a function of pressure under constant electrical power. The feasibility of using the bondwire Pirani sensor has been thoroughly discussed. A novel four point measurement set up is implemented to achieve the accurate low resistance measurement. The bondwire Pirani sensor has a dynamic range from 0.1 Torr to about 50 Torr and is compatible with any micro-system such as resonators, gyroscopes, micro-mirrors, and micro-display systems among others. The sensor is applied to our own packaging process as pressure sensing element to detect pressure change. Long-term pressure stability for sealed packages is also measured by the bondwire Pirani sensor.