High-performance thin-film transistors fabricated using excimer laser processing

Future cost-effective manufacturing of large-area active-matrix liquid-crystal displays will require high-performance polysilicon thin-film transistors (TFT's) for driving and switching pixels. A further requirement is that these devices be fabricated on inexpensive glass substrates that limit processing temperatures to below 600{dollar}spcirc{dollar}C. The two major barriers to fabricating these TFT's are, low temperature processes for (i) producing low-defect-density polysilicon films for the active areas, and (ii) incorporating and activating dopants for source-drain junctions. The goal of this work is to develop excimer-laser-based processes that overcome these barriers and push the performance limits for polysilicon TFT's.; The first barrier is overcome by "grain engineering" high-quality polysilicon films. One grain engineering approach is to optimize the laser processing parameters, such as laser fluence, pulse frequency, and number of pulses. A detailed study of the laser recrystallized grain microstructure as a function of these laser parameters is presented. By understanding the laser process, an average grain size of {dollar}sim{dollar}4{dollar}mu{dollar}m is achieved in 90 nm thick polysilicon films, in which the largest grains are more than 9 {dollar}mu{dollar}m long. A second approach is to manipulate the sample structure before laser recrystallization. For example, prepatterning the polysilicon film into active areas before recrystallization results in large lateral grain growth along the edge of the patterned island after recrystallization. A novel "recessed" structure is also introduced, and provides similar results. The second barrier mentioned above is overcome by applying gas-immersion laser doping (GILD) to heavily dope polysilicon films. Using GILD, only twenty laser pulses are needed to dope a 90 nm thick polysilicon film below 1000 {dollar}Omega{dollar}/sq.; By combining the grain-engineered channel polysilicon regions with the laser-doped source-drain regions, high-performance TFT's are fabricated with electron mobilities up to 260 cm{dollar}sp2{dollar}/Vs and on/off current ratios greater than 10{dollar}sp7{dollar}. These devices represent the best performing laser-annealed TFT's reported without using substrate heating or hydrogenation.; In developing the materials used in these TFT's, we discovered a new ripple-formation mechanism observed after laser irradiation of polysilicon films sandwiched between oxide layers. The ripples appear chaotic, with a periodicity of 1 {dollar}mu{dollar}m. The characteristics of these ripples are investigated in the context of trying to identify the underlying physical phenomena.