In high performance polysilicon thin film transistors ?TFTs. the uniformity of electrical characteristics remain a major problem. This situation has stimulated a growing activity aiming to control the lateral growth phenomenon. However, most of the techniques require additional processing steps or a rather high shot density. We present a technique based on a two-pass excimer laser crystallization process: during the first irradiation the sample is irradiated through a patterned mask, while the second irradiation, performed without the mask, results in the homogeneous crystallization of the sample. This technique allows the possibility of forming uniform polysilicon layers, with large ?;2 micron. and aligned grains, with a reduced number of shots and a relatively large process energy window. The results of crystallization performed at different laser energy densities, sample thickness and laser pulse duration are analyzed.

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.