Mitigation of mechanical fracture of polycrystalline silicon structure in MEMS capacitive microphones

Abstract

Micro-machined capacitive microphones, currently well sought-after owing to increasing proliferation of handheld electronic devices, contain two parallel diaphragms that are made of thin-film polycrystalline silicon (polysilicon) - a relatively brittle material. Without sound process and fabrication controls, undesirable repercussions such as elevated production costs and unacceptable device yield levels may occur. This paper describes our experience in the microfabrication of capacitive microphones and proposed solution to minimize the risks of thin polysilicon diaphragms mechanical failure. We found the unintended presence of buried oxide keyholes beneath a polysilicon layer as the leading cause of thin-film rupture when the substrate was processed at elevated temperatures as part of downstream process procedures. We believe the keyholes were formed as a result of the “bread-loafing” effect and the reduction in keyhole size was verified as a contributing factor to minimizing the possibility of material rupture, which leads to better device yield.