A Simple Ohmic-Contact Formation Technology using Phosphine Plasma Treatment for Top-Gate Amorphous-Silicon Thin-Film Transistors

The effects of exposing patterned indium-tin-oxide (ITO) on amorphous silicon oxide (a-SiO x ) to a phosphine (PH3) plasma has been investigated. Phosphorous atoms react with ITO to form InPO x and InP, with a much smaller number being deposited on a-SiO x . If amorphous silicon (a-Si) is then deposited onto these layers by plasma enhanced chemical vapor deposition, phosphorous atoms from the ITO are incorporated into the a-Si in the region of the ITO. This effectively dopes the a-Si to form good ohmic contacts between the ITO and a-Si layers. In contrast, the number of phosphorus atoms incorporated into a-Si directly above a-SiO x is too small to have an effect on the electrical properties of a-Si. The doping mechanism for the ITO and a-Si is discussed, and an explanation given as to how these features can be used to mass produce high performance top-gate a-Si thin film transistors using only three photomask steps.