Excimer laser crystallization techniques for polysilicon TFTs

The introduction of excimer laser crystallization (ELC) techniques in the fabrication of polysilicon thin-film transistors (TFTs) has produced a tremendous improvement in the device characteristics. When the Super Lateral Growth (SLG) mechanism is triggered, large (> 1 mu m) grains are formed and this crystallization regime appears very attractive from the device fabrication point of view. In fact, using SLG-polysilicon active layers high performance (electron field-effect mobility > 300 cm(2)/V s) TFTs can be obtained and a detailed analysis of the electrical characteristics of such devices is presented. However, the SLG mechanism has a very narrow energy density window and, consequently, highly uniform beam profiles and pulse-to-pulse stability better than 2% are required. This implies that standard ELC-process is technologically quite critical and several approaches have been proposed to improve the process uniformity. Among these we will discuss three main techniques: (1) the use of opportunely (semi-gaussian) profiled beams; (2) the combined use of Solid Phase Crystallization (SPC) and ELC techniques; (3) control of the lateral growth. In particular, we present a novel technique to control the lateral growth, based on a two-pass ELC-process. The proposed technique can be rather attractive for polysilicon TFT fabrication, allowing a precise grain location control (through the mask geometry) and being characterized by a few (3-5) laser shots process and wide energy density windows. (C) 2000 Elsevier Science B.V. All rights reserved.