Tolis Voutsas, Sharp Laboratories of America, Camas, WA

Sharp Laboratories is the worldwide leader in the development and mass-production of flat panel displays, otherwise known as thin film transistor LCDs, or TFT-LCDs. Recently there has been an explosive growth of low-temperature polycrystalline silicon (poly-Si) TFT technology that promises to deliver novel, high performance, high-content displays. The new concept of a “sheet-computer,” where the display is the heart of the system, offers multiple functions (input/output, data/video imaging, etc.) on a very thin and portable device. For such concepts to materialize, the development of new process technology is needed to understand the complex interactions between individual process parameters. Sharp Labs of America is focusing on the development of such new processes, equipment, and materials to advance the state of LCD technology.

One area of particular interest and complexity is the crystallization of amorphous silicon to form poly-Si films. The quality of the poly-Si microstructure impacts the performance of devices made with these films and profoundly affects the display capabilities. FIDAP has been used to simulate the transformation of amorphous-Si thin films to poly-Si through irradiation of the former by a pulsed laser beam. This is a highly complicated process in which the thin film experiences ultra-rapid heating, melting, and equally rapid cooling. The process is complicated by several factors: phase change occurs far from thermal equilibrium; nucleation occurs in the molten material as it cools, and the crystals grow and coalesce. A number of modifications have been implemented in FIDAP through user-defined subroutines to incorporate these complexities into the existing phase change model.

Equipped with this advanced simulation tool, the temperature history in the film as a function of the relevant problem parameters can be computed, and the final microstructure within the laser-irradiated area can be predicted. The predicted microstructure has been found to compare favorably with images of the actual material obtained experimentally. As a result, FIDAP has been used as a reliable substitute for experimental work to identify promising operating regimes that optimize the material properties (microstructure). In addition, simulation has been used to investigate different irradiation schemes that are either difficult or expensive to implement experimentally, unless sufficient evidence exists to warrant the value of the expenditure. As new features have been added to the model, the value of accurate simulation has proved to be invaluable in the investigation of these highly complex processes. A vast array of operating regimes can now be explored without having to resort to tedious and time consuming traditional methods.

Temperature history at various locations within the film stack	Position and temperature of the solid-liquid interface within the top Si layer as a function of elapsed time

Comparison of simulated (left) and experimental (right) poly-Si microstructure for the case of laser irradiation that results in random nucleation at the center of the irradiated domain, a scenario that is typically undesirable

FluentNEWS Supplement