GOALI/IUCRP: Uniform Grain Size Polysilicon for Thin Film Transistors

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9424399 Collins The goal is to define low temperature processes to form uniform grain silicon films for use in fabricating thin film transistors to be benchmarked in static Random Access Memory System (RAMS), in collaboration with industrial partners. At present, transistors in amorphous silicon achieve low leakage, while in polysilicon, they achieve carrier high mobility. But one material is not able to provide both desirable properties at the same time unless crystalline silicon is used. The research approach is to use anodic aluminum oxide as a seed bed, where the seeds are uniformly spaced so that the subsequent polysilicon grains are forced to have an extremely narrow distribution of grain sizes thereby providing a tighter distribution of threshold voltages, mobilites and leakage currents in the thin film transistors. The first step is to use aluminum substrates that are anodized to form alumina and then move on to Al metalized Si wafers as substrates for final Si wafers processing through device fabrication . Since the processed wafer must never exceed 350 degrees celcius during the polysilicon formation on alumina due to the temperature limits of the underlying devices, non-thermal electron irradiation will be incorporated to cause weakly-bound species to desorb or re-order more efficiently, thereby allowing new microstructural pathways for uniform grain silicon. First, a silicon seed layer that has a single strong preferred orientation will be grown on the anodized Al, with as grown grain size as large as possible. Second, energetic particle bombardment-assisted Chemical Vapor Deposition will be used for the thick Si-on-Si film growth to achieve uniform size grains. Finally, grain boundary passivation is achieved via either low temperature plasma-based anneals or plasma immersion ion implantation. The electrical performance of transistors built with these silicon wafers will be measured with a target to exceed on/off current ratios above 10 to the power of 10 and mobilitie s exceeding 500 square cm./V-sec. Silicon remains the workhorse semiconductor material in the manufacture of computers, telecommunication equipment and myriad's of electronics devices. Successful completion of this research project has the potential to drive down the cost of silicon wafers for many electronic applications and improve the efficiency of many electronic devices. Further, industrial partners are collaborating closely by hosting university faculty and one student for three months and sending their own researcher to the university for one year. Cross fertilization of ideas between academia and industry impacts the Nation's ability to select relevant areas for research and then transform the results into the commercial sector to maintain global competitiveness.