Zero-Balance Method for Evaluation of Sealed Cavity Pressure Down to Single Digit Pa Using Thin Silicon Diaphragm

Abstract

The sealed cavity pressure of a micro-package often becomes the determining factor of the performance of an encapsulated micro-electro mechanical system (MEMS). In this study, a sealed cavity pressure evaluation method for a micro-package using a thin diaphragm is studied comprehensively. The sealed cavity pressure is identical to the surrounding pressure when the diaphragm separating both volumes becomes flat, i.e. zero-balance method. The zero-balance method has advantages to other techniques in terms of fabrication process simplicity and pressure evaluation range. Two approaches are introduced to reduce the impact of the residual stress, which typically exists in the conventionally used silicon-on-insulator (SOI) diaphragm. The introduction of a stress compensation layer was able to reduce the impact of the residual stress of SOI-based diaphragms. However, a significant hysteresis is developed on the diaphragm, which limits the precise measurement of a sealed cavity. On the other hand the residual stress can be avoided by the usage of intentionally tensile stressed boron doped Si diaphragm, which is produced by alkaline wet etching. Using such a diaphragm, a sealed cavity pressure of several Pa can be measured with a single digit Pa accuracy. [2020-0014]