a-Si:H TFTs Made on Polyimide Foil by PE-CVD at 150°C

Abstract

We have fabricated high-performance amorphous silicon thin-film transistors (a-Si:H TFTs) on 2 mil. (51 µm) thick polyimide foil substrates. The TFT structure was deposited by r.f.-excited plasma enhanced chemical vapor deposition (PECVD). All TFT layers, including the gate silicon nitride, the undoped, and the n⁺ amorphous silicon were deposited at a substrate temperature of 150°C. The transistors have inverted-staggered back-channel etch structure. The TFT off-current is ∼ 10⁻¹² A, the on-off current ratio is > 10⁷, the threshold voltage is 3.5 V, the sub-threshold slope is ∼ 0.5V/decade, and the linear-regime mobility is ∼ 0.5 cm²V⁻¹s⁻¹. We compare the mechanical behavior of a thin film on a stiff and on a compliant substrate. The thin film stress can be reduced to one half by changing from a stiff to a compliant substrate. A new equation is developed for the radius of curvature of thin films on compliant substrates.