Effects of mechanical strain on laser crystallized polysilicon thin film transistors and ring oscillators fabricated on stainless steel foil

Abstract

In this paper the influence of mechanical tensile strain on the performance of thin film transistors (TFTs), with various channel geometries, and of ring oscillators, with 3, 7, 11, 21, and 51 number of stages and device channel lengths of 1, 4, and 8 μm, fabricated on stainless steel foil substrate is investigated. TFT parameters such as field effect mobility, threshold voltage, subthreshold slope, leakage and gate current for both n-channel, and p-channel TFTs are studied at various longitudinal tensile strain levels. For strain levels from 0.0% to 0.5%, the field effect mobility of n-channel TFTs increases while that of p-channel ones decreases as the longitudinal tensile strain increases. The field effect mobility, of both n-channel and p-channel TFTs, becomes independent of longitudinal tensile strain at strain levels greater than 0.5%. Threshold voltage and subthreshold slope of p-channel TFTs increases while that of n-channel ones does not follow a specific trend. The leakage current of both type devices tends to decrease by increasing the longitudinal tensile strain. The propagation delay, per inverter stage of a ring oscillator, is investigated at different supply voltages and tensile strain levels. The propagation delay of inverters with longer device channel length (⩾4 μm) tends to decrease while that of shorter length tends to increase as the longitudinal tensile strain increases.