Effects of bubbles in coating gel on the performance of MEMS pressure sensor

The process of dispensing gel above the silicon chip aimed at protecting chip surface and bonding wire is one of the most critical steps in the production of highly accurate low-pressure sensor modules. Numerous research efforts focused on the influences of dispensing process parameters, configuration and mechanical behavior of cured gel on the performance of sensors, while few concerns were taken on the air bubbles formed in cured gel induced by inappropriate technology control that adversely influence the zero offset in full temperature range and sensitivity. In this paper, the packaged modules of sensor chip attached on the ceramic substrate were chosen as the subject investigated, a series of hydrostatic fluid-element based 2D models in consideration of different positions and various diameters of air bubbles were conducted to explore this phenomenon. The simulation results showed that the inflation of bubble and elevated air pressure give rise to extra load on the sensing membrane, and this phenomenon becomes more evident as the initial distance between bubble and sensing element becomes shorter and the original diameter becomes larger. Even for the case of smallest diameter and farest distance, the maximum output variation could be 0.5% for a pressure point, this is still unacceptable for the production of high-precision and reliable low pressure sensors. So a vaccumizing process between the dispensing and curing processes could be preferred.