Effect of humidity and temperature on the fatigue behavior of polysilicon thin film

The fatigue behavior of polysilicon thin films was investigated under three different environmental conditions, i.e., air with relative humidity 80% at 22 °C, inert nitrogen gas at 22 °C and 180 °C. A new evaluation technique was utilized, where the experiment applying a stress amplitude gradually increased with the number of fatigue cycles was combined with a statistical analysis using an equivalent fatigue crack extension model on the basis of Paris’ well-known law. The parameter values in Paris’ law for relative humidity 80% to control fatigue behavior were found to lie close to those obtained previously with conventional fatigue tests with constant stress amplitudes, when a sufficiently slow increasing rate of stress amplitude was applied. The fracture stress level appeared to be smaller than the static strength even in nitrogen gas at 22 °C, which suggested that fatigue damage was introduced by repeated loading even under an inert environment. Fatigue damage was observed to be more significant at a higher temperature (180 °C) in the same nitrogen gas atmosphere, which is an evidence of the contribution of intrinsic defects on the fatigue behavior.