Thin-film transistors in polycrystalline silicon by blanket and local source/drain hydrogen plasma-seeded crystallization

Thin film n-channel transistors have been fabricated in polycrystalline silicon films crystallized using hydrogen plasma seeding, by using several processing techniques with 600 to 625 °C or 1000 °C as the maximum process temperature. The TFT's from hydrogen plasma-treated films with a maximum process temperature of 600 °C, have a linear field-effect mobility of approximately 35 cm²/Vs and an ON/OFF current ratio of approximately 10⁶, and TFT's with a maximum process temperature of 1000 °C, have a linear field-effect mobility of approximately 100 cm²/Vs and an ON/OFF current ratio of approximately 10⁷. A hydrogen plasma has also then been applied selectively in the source and drain regions to seed large crystal grains in the channel. Transistors made with this method with maximum temperature of 600 °C showed a nearly twofold improvement in mobility (72 versus 37 cm²/Vs) over the unseeded devices at short channel lengths. The dominant factor in determining the field-effect mobility in all cases was the grain size of the polycrystalline silicon, and not the gate oxide growth/deposition conditions. Significant increases in mobility are observed when the grain size is in order of the channel length. However, the gate oxide plays an important role in determining the subthreshold slope and the leakage current.