Grain Engineering Approaches for High-Performance Polysilicon Thin-Film Transistor Fabrication

Using an approach we call “grain engineering,” we discuss several techniques to control grain growth during excimer laser annealing, to create low-defect density polysilicon films. By adjusting of laser parameters, for example, we obtain polysilicon films with grain sizes of more than 9 µm, without substrate heating. These high-quality films are used in the fabrication of low-temperature unhydrogenated polysilicon thin-film transistors (TFT's) yielding mobilities of > 260 cm2/Vs and on/off current ratios > 107. We investigate the laser recrystallization of “prepatterned” films as another technique of grain engineering. We find the performance of TFT's fabricated in active areas that are prepatterned before laser recrystallization is dramatically improved compared to those TFT's fabricated from the laser recrystallization of blanket polysilicon films. A novel “recessed” structure is also examined as a new grain engineering tool. By depositing a blanket silicon film on a patterned oxide layer on a heat sink, the heat flow through the continuous silicon film may be controlled during laser recrystallization to simultaneously produce adjacent regions of remarkably different grain microstructure.