Surface charge extraction methods in offset shift-related failures of MEMS pressure sensors

Abstract

An offset shift failure mode is well known in characterizing MEMS piezoresistive pressure sensors. However, the root cause could be either mechanical or electrical. Electrical charge can create a leakage path between implanted traces, whereas mechanical stress will influence the piezoresistors on the surface. Here we look closely at the charge-induced cause and calculate the contribution of such input. We describe two methods for extracting the surface charge density present in our samples. The first approach employs 3D device simulation for the resistor layout to compare numerically computed parasitic FET-induced resistance shifts, with experimental findings from intentionally charged pressure sensors. The second method takes advantage of the FET nature of the leakage contribution by analyzing the impact of the n− substrate bias as the back-gate voltage of the parasitic FET. In this paper, it is shown that the first analytical method and the second experimental method both predicted a similar charge level on the surface of the pressure sensor. Therefore both methods can be interchangeably used and can verify the results of each other. These methods provide a useful tool to find the root cause of an offset shift failure of a pressure sensor, and further lead the way for a better process and system design.