TFT threshold voltage adjustment with in-situ doped PVD silicon films

For laser crystallization of amorphous silicon, plasma enhanced chemical vapor deposition (PECVD) is the method of choice for a-Si precursor deposition. This situation is likely to change, however, with the transition to higher performance polysilicon material produced via advanced laser annealing techniques. Two factors make the use of sputtered a-Si precursors particularly attractive for laser annealing technologies. First, owing to their low hydrogen content, sputtered a-Si films are uniquely suited as precursors for laser crystallization techniques. Second, the ability to dope the target material (and thus produce doped silicon films) allows for control of the threshold voltage of the resulting TFTs. To that end we evaluated sputter deposited doped silicon as an a-Si precursor for excimer laser annealing. We established process conditions necessary to shift the Vth of both N and P transistors such that they were centered near zero. In addition we determined levels of target doping, DC power, and chamber pressure that produced TFT's with balanced N and P Vth values and satisfactory mobility. We also found that the off-state leakage and subthreshold slope of the PVD films were better than PECVD deposited films.