A concave-patterned TiN/PECVD-Si3N4/TiN diaphragm MEMS acoustic sensor based on a polyimide sacrificial layer

Abstract

In this paper, we present a concave-patterned TiN/PECVD-Si3N4/TiN diaphragm microelectro-mechanical system (MEMS) acoustic sensor based on a polyimide sacrificial layer. The use of the spin-coated polyimide eliminates the additional Al pad process of conventional device fabrication due to simple O-2 ashing to release the sacrificial layer, simplifying the photolithography process. Also, to adjust the acoustic sensor for a bottom-ported package, its diaphragm was implemented to be placed over the back-plate. The TiN/PECVD-Si3N4/TiN multi-layer diaphragm was formed with the stress controllability of PECVD-Si3N4 from -162 MPa to +109 MPa. Furthermore, a parallel-plate capacitance model on the basis of an approximately linearized electric field method (ALEM) is proposed to evaluate the capacitance of two plates. The modelled capacitance showed less than 3.7% error in FEM simulation, demonstrating the validity of the proposed model. At a zero-bias voltage, the effective intrinsic and parasitic capacitances in the active area were 1.656 pF and 0.388 pF, respectively. Moreover, with a pull-in analytical model by using ALEM, the effective tensile stress for the diaphragm was extracted to +31.5 MPa, where the pull-in voltage was 10.7 V. In succession, the dynamic response for the open-circuit sensitivity was modelled with an equivalent circuit model based on lumped parameters. The measured open-circuit sensitivity of -45.1 dBV Pa-1 at 1 kHz with a bias of 9.6 V was only slightly different from the modelled sensitivity of -45.0 dBV Pa-1. Thus, these results demonstrate that the proposed sensor is suitable for a front-end voice capture module.