Techniques and diagnostics on laser recrystallization of thin amorphous silicon films for flat panel display applications

Active matrix liquid crystal displays (AMLCDs) are now commonly used on notebook computers and advance multi-media applications. In order to reduce the cost of materials, low temperature glass substrates were suggested. Presently, AMLCDs are mainly manufactured by using a-Si:H for the pixel switching device. In order to improve display performance and integrate both the driving circuitry and pixel TFTs in a monolithic CMOS technology, high quality poly-Si with low defect density and high carrier mobility must be adopted. Laser recrystallization is found to be a promising technique to obtain high quality poly-Si on low temperature glass substrate. It is the aim of this work to enhance the understanding of excimer laser recrystallization and to develop a new recrystallization technique that can produce large, location-controlled, and direction-controlled poly-Si grains.; First, a specially designed ellipsometry system is used to measure the high temperature spectral optical properties of a-Si and poly-Si. The system can measure the spectral range from wavelength of 250 nm to 1700 nm. The temperature range of interest is from room temperature to about 500°C since higher temperature may cause micro-structural changes in the silicon films. The optical properties found are important parameters for Si film temperature calculations and in-situ optical probing of the recrystallization process.; Conventional excimer laser recrystallization (ELC) is studied next. The average grain size dependence on laser fluence, number of laser pulses, and film thickness are investigated. Although increasing the number of laser pulses enhances grain size, the general problems associated with conventional ELC are narrow process window, nonuniform grain sizes, and randomly oriented grain boundaries. In order to control the grain growth direction, some research efforts have been focused on controlling the temperature distribution in the Si film by using beam shaping techniques. However, the lateral grain growth dependence on fluence gradient has not been demonstrated. By shaping the energy profile by using a beam mask in this study, it is found that higher fluence gradients produce longer and more directional grains.; Finally, a double laser recrystallization technique capable of inducing ultra-large direction- and location-controlled poly-Si grains is developed. This technique has a wide process window and is insensitive to laser energy fluctuations. An elliptical Ar+ laser beam is applied to melt the a-Si film. At a certain time delay, a spatially uniform excimer laser beam or alternatively a more adaptable Nd:YLF laser is irradiated over the molten region. By using high resolution laser flash photography, the liquid/solid interface dynamics is observed during the double laser recrystallization process. The lateral solidification velocity is estimated to be about 10 m/s. The revealed resolidification dynamics suggests two crucial factors for inducing ultra-large lateral grain growth. First, the formation of nuclei triggered by the nanosecond laser pulse is responsible for seeding the subsequent lateral grain growth. Second, the cooling rate into the substrate must be reduced by the Ar + laser so that sufficient amount of time is allowed for lateral grain growth.