<title>Anodic oxidation and reliability of MEMS polysilicon electrodes at high relative humidity and high voltages</title>

Abstract

We present a full factorial study of the effect of relative humidity and voltage on the oxidation of surface-micromachined poly-silicon wiring and electrodes. Our system consists of 500 nm thick poly-Si wires and electrodes insulated from the substrate wafer by 600 nm of Si-rch Si<SUB>x</SUB>N<SUB>y</SUB>, fabricated using a surface-micromachinging process. In dry ambients, oxidation or damage to the bottom poly-Si layer (the Poly0 layer) in MicroElectroMechanical Systems (MEMS) devices occurs so slowly that little can be learned in a timely manner, even when stressing the electrodes at electric fields close to dielectric breakdown. We observe however that in ambient with elevated relative humidity the Poly0 wires and electrodes anodically oxidize within a short period of time when operated at moderately large voltages. Only the most positively biased poly-Si structures oxidize, and we describe the anodic oxidation and association volume expansion as a function of a number of accelerating factors including relative humidity and voltage. A threshold is observed in relative humidity bot not in voltage.