Low temperature polysilicon growth on glass suitable for TFT fabrication

Abstract

Polysilicon films are grown on ordinary glass substrates at temperatures as low as 380°C using a novel ultraviolet assisted metal-induced-crystallization technique. The silicon films grown using this method are suitable for the fabrication of thin film transistors. Samples prepared, consist of 15008, of silicon film deposited on 10008, silicon nitride and 2000A of chromium layers, and Ni is used as the seed for crystallization. Annealing occurred in the presence of an ultra-violet exposure, leads to a high crystallinity of silicon film as examined using XRD and SEM. The lateral growth as the main feature of this technique is presented using optical microscopy analysis. The preliminary results of transistor fabrication on ordinary glass is reported. Transistors fabricated using this technique show a hole mobility of 50 cm2Ns. 1-IntroductionPoly-silicon films are the most suitable candidates for the fabrication of thin film transistors for large area electronics. Amorphous silicon technology, although quite successful in achieving TFT fabrication on glass and even on plastic, does not offer high mobility transistors. Also transistors made using such techniques suffer from long term instability. So low temperature'u-ystallization of aSi is an important objective in thin film technology to fabricate high-performance low cost devices. Conventional methods of crystallization such as laser-crystallization (LC) and solid-phase crystallization (SPC) are being used to reduce the annealing temperature [ 1-31. Although LC method is a fast and low temperature process, it is rather complex and expensive. On the other hand, SPC is an energy-consuming technique, which cannot crystallize a-Si at temperatures lower than 550°C. Processing temperatures above 450°C cannot support the use of ordinary glass, which is a good choice as substrate for low-cost large area applications [4]. Metal induced crystallization (MIC for short) is a newly developed technique to grow polysilicon films on glass using metal as the seed of crystallization [5][6][7][8]. High-performance thin-film transistors have been successfully fabricated using nickel-induced-crystallization[9]. However, the processing temperature reported so far is still beyond the softening temperature of ordinary glass. We have recently developed a novel UV-assisted MIC technique in which part of the energy required for the crystallization is imparted from energetic UV photons[lO-1 11. In this paper a back reflecting UV-assisted MIC method is introduced. The preliminary results of transistor fabrication are reported.In the following sections, we describe the proposed structure followed by the study of crystallinity of the films using XRD, SEM and optical microscopy. The transistor structure is then presented and preliminary I-V results are reported. 2-ExperimentThe starting substrate is ordinary 150pm thick glass which is cleaned using RCA#1 solution. The samples are coated with a layer of chromium with a thickness of 2000A followed by sputter deposition of Si3N4 with a thickness of...