Effects of gas bubbles in protection gel on 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 protection gel that adversely influence the sensitivity and zero-drift in full temperature range. In this research, a series of hydrostatic fluid-element based 3D models in consideration of different positions and various diameters of gas bubble were conducted to explore this phenomenon. Results show that the existence of gas bubble brings in an extra load and changes the pressure distribution on the sensing membrane of the chip, which will result in zero-drift and sensitivity variation of packaged sensors. The influence degree is closely correlated with the position and size of gas bubble in protection gel. An experiment was conducted to verify the simulation, the results fit well. Due to the random distribution of bubble it is almost impossible to design signal modulation circuits to compensate for voltage drift in mass production. Therefore, in practical production, the gas bubble should be avoided especially in the region close to the sensing membrane of sensor chip.