NMOS Logic Circuits in Laser-Crystallized Silicon on Quartz

Abstract

Depletion mode as well as enhancement mode n-channel thin-film transistors (TFT’s) have been fabricated in CO₂ laser-crystallized silicon on fused quartz. Nearly defect-free islands were obtained by using an offset circular beam to form a tilted melt interface. The optimization of subsequent processing steps to achieve simultaneously low leakage currents and voltage thresholds appropriate for depletion-load NMOS circuits involved adjustments of ion implantation and high temperature cycles with the aid of simulation. The resultant high performance silicon-gate TFT’s have led to NMOS ring oscillators with 2.5 ns delay/stage and dynamic shift registers with MHz clock rates. These are the first logic circuits fabricated in beam-crystallized silicon on bulk amorphous substrates.