Mechanisms of Ionizing Radiation Response in Silicon Piezoresistive Micromachined Cantilevers

Abstract

A T-shaped, asymmetric, piezoresistive, micromachined, resonating cantilever is used to investigate the effects of 10-keV X-rays on resonance frequency and resistivity. Total-ionizing-dose-induced resonance frequency and resistivity shifts depend on dopant type and concentration, as well as dose-rate. Lower rate exposures produced up to 4-times more negative frequency shifts and 3-times more positive resistivity shifts than higher rate exposures in devices with lower doping. Devices that were hydrogenated in a steam bath for 1 h showed shifts similar to those of control (not hydrogenated) devices at higher dose rates, and larger shifts than control devices at lower rates. All devices recovered to levels close to pre-irradiation after several hours of post-irradiation annealing. The dose-rate dependence is attributed to differences in carrier concentration and mobility caused by varying efficiencies of the depassivation of dopants by hydrogen at higher and lower dose rates and/or surface charging effects, and the subsequent differences in Young's modulus and shear elastic constants of silicon that occur as a result. Many of these processes are similar to effects that lead to ELDRS in linear bipolar transistors, emphasizing the need to include low-dose-rate testing of MEMS devices when considering them for use in space systems.