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 mm. 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 cm2rV 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.