Influence of sensor-package hermeticity-level on long-term drift for a piezoresistive MEMS pressure-sensor

Established industrial hermeticity requirements for electronic devices with internal cavities, such as given by MIL-STD-883 Test Method 1014, are typically motivated by the need to avoid moisture in sealed devices. Avoiding moisture is important to prevent corrosion during the lifetime of the device, especially in the presence of contaminants. The output signal of a sealed pressure-sensor structure will also be influenced by the sensor cap pressure, putting additional requirements on the sensor package hermeticity in order to guarantee a stable operation under variable conditions. A sealed sensor structure was analysed using the finite element method with focus on establishing the sensitivity of the internal sensor structure to a varying cap pressure. The simulation results were confirmed by measurements and a typical cap pressure sensitivity of -0.25 %FS/bar was established. Based on the cap pressure sensitivity, we calculated the effects on the signal for specific actions like storage at altitude and storage at elevated temperature for typical hermeticity levels. We found that the hermeticity levels typically used to avoid moisture in electronic packages can be insufficient when targeting control of long-term drift related mechanisms.