Experimental Investigation of Moisture Induced Offset Drifts on Plastic Packaged MEMS Sensor Devices

Encapsulated microelectromechanical measurement units are known to be sensitive to ambient humidity conditions, due to various effects caused by moisture diffusing into and out of the plastic packaging components. This paper focuses on investigating the effect that a change of moisture content has on the transient offset behavior of a packaged MEMS sensor by investigating the output signals of several test devices that only differ in their response to deformation of the MEMS core area. This comprises a novel method of experimentally analyzing the transient drift behavior of a MEMS caused by a deformation of the MEMS core area. This approach enables a back calculation of possible deformation shapes of the core area that explain the observed offset drifts in a given package. It was shown that the humidity response of a MEMS sensor is a strongly transient function that is not proportional to the uptake of water and can therefore significantly differ from their humidity performance at or close to a saturated state. Humidity performance tests that take only this state as point of reference for a performance evaluation cannot correctly estimate the humidity induced offset drifts. It was also shown that the moisture induced offset drifts can be traced back to a superposition of several deformation fields leading to several stages of the humidity response of MEMS sensors. Based on the example of the test devices it was shown that MEMS cores can be optimized to minimize the response of moisture effects. [2021-0212]