Polysilicon thin-film transistors fabricated from laser-processed sputtered-silicon films

Excimer lasers have been successfully applied to fabrication of low temperature TFTs, since their laser fluence is sufficient to melt silicon, and its pulse-duration is short enough (i.e. 30 ns) to avoid overheating the substrate. With excimer laser processes available for crystallization and doping of TFTs, the major remaining bottleneck in development of a low temperature TFT process is deposition of the silicon film. Although a great deal of work has been put into investigation of PECVD a-Si films, little work has been put into examination of sputtered a-Si films. While both PECVD and sputtering methods are low temperature processes, the H concentration in the silicon films is more easily controlled using the sputtering technique. H-rich films are unattractive for laser processing, since the laser fluence must be gradually increased to avoid ablating the film due to explosive H evolution, and this use of multiple fluences reduces manufacturing throughput. A sputtered silicon film with little H content can be laser processed in similar fashion to an LPCVD film. This work presents TFTs fabricated from laser-crystallized sputtered silicon films of various H concentrations, and compares them to similarly processed LPCVD TFTs.