Effects of Excimer Laser Annealing on InGaZnO4 Thin-Film Transistors Having Different Active-Layer Thicknesses Compared with Those on Polycrystalline Silicon

Abstract

Excimer laser annealing (ELA), which can raise the temperature of InGaZnO4 (IGZO) films for a desired very short time, is effective for obtaining good transfer characteristics in IGZO thin-film transistors (TFTs) on plastic substrates. In this study, we investigate the dependence of the effects of ELA on IGZO-TFTs in comparison with that of its effects on low-temperature polycrystalline silicon (LTPS) for various film thicknesses. We show that the optimum laser energy density with respect to TFT performance decreases with increasing IGZO thickness. Results for IGZO film properties such as carrier density and Hall mobility show the same tendency. This is contrary to the tendency for the ELA of LTPS, in which the threshold energy density for micro crystallization increases with increasing film thickness. In order to gain an insight into the mechanisms at work here, we have numerically estimated the rise in temperature of the IGZO and Si films on the basis of a heat-flow equation. Unlike the case for LTPS, the calculated maximum temperature in IGZO films increases with increasing film thickness. We show that this discrepancy is influenced significantly by the difference in penetration depth between the IGZO film (roughly 70 nm) and Si film (roughly 6 nm) for excimer laser light. To improve the IGZO-TFT characteristics, it is necessary to take into account IGZO penetration depth when determining a suitable IGZO thickness and laser energy density.