Hydrogen-Plasma-Enhanced Crystallization of Hydrogenated Amorphous Silicon Films: Fundamental Mechanisms and Applications

The crystallization of hydrogenated amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition (PECYD) by thermal annealing is of great interest for display and silicon-on-insulator (SOl) technologies. For large area electronics, there has been considerable interest to integrate both a-Si:H TFTs, for low leakage in the OFF state, and poly-Si TFTs, for high drive currents, on the same substrate, for example, to integrate polysilicon drivers in flat panel displays using aSi:H TFTs in pixels. There is interest also to achieve high mobility polysilicon TFTs fabricated at low temperatures (~ 600°C). Low thermal budgets are necessary so that glass substrates can be used in case of display applications, and damage to preexisting devices is minimized in case of 3-D integrated circuit applications. In this thesis we will describe the use of selective crystallization using hydrogen plasma seeding treatment of a-Si:H layers to achieve both these aims. We have found that a room temperature hydrogen plasma exposure in a parallelplate-diode-type Reactive Ion Etcher (RIB) can reduce crystallization time of a-Si:H by a factor of five. Exposure to hydrogen plasma reduces the incubation time, while the rate of crystallization itself is not greatly affected. This plasma-enhanced crystallization can be spatially controlled by masking with patterned oxide, so that both amorphous and polycrystalline areas can be realized simultaneously at desired locations on a single substrate. The enhancement of crystallization rate is probably due to the creation of seed nuclei at the surface and the effect is limited to the top 30-40 nm of the a-Si:H layer. We have used these films to fabricate low-temperature (~600°C) self-aligned nchannel polycrystalline transistors. Well-behaved characteristics were obtained in all 'j