Identification and Elimination of Hygro-Thermo- Mechanical Stress-Effects in a High-Precision MEMS Pressure Sensor

A micromechanical pressure sensor has been modeled and characterized with a focus on hygro-thermo-mechanical stress originating from a thermoset-polymer die-attach material, and its influence on the sensor output signal. The sensor output signal shows a first-cycle effect during the first thermal cycle after storage, as well as increased hygro-thermal hysteresis. Whereas such effects are normally difficult to explain quantitatively, our model of hygroscopic diffusion and swelling provides an excellent match with experimental values. By implementing suitable stress-isolating structures in the sensor design, the effect of thermoset polymer stress on the sensor output signal can be reduced to non-detectable levels. The resulting sensor designs shows improved performance with a reduced signal drift and hygro-thermal hysteresis.