In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. Such crystallized thin films can be used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid-crystal display (“AMLCD”) devices. In the latter, a regular array of thin-film transistors (“TFTs”) is fabricated on an appropriate transparent substrate, and each transistor serves as a pixel controller.
Crystalline semiconductor films, such as silicon films, have been processed to provide pixels for liquid crystal displays using various laser processes including excimer laser annealing (“ELA”) and sequential lateral solidification (“SLS”) processes. SLS is well suited to process thin films for use in AMLCD devices, as well as active-matrix organic light emitting diode (AMOLED) devices.
In ELA, a region of the film is irradiated by an excimer laser to partially melt the film, which subsequently crystallizes. The process typically uses a long, narrow beam shape that is continuously advanced over the substrate surface, so that the beam can potentially irradiate the entire semiconductor thin film in a single scan across the surface. ELA produces small-grained polycrystalline films; however, the method often suffers from microstructural non-uniformities, which can be caused by pulse to pulse energy density fluctuations and/or non-uniform beam intensity profiles. FIG. 8A illustrates a random microstructure that can be obtained with ELA. The Si film is irradiated multiple times to create the random polycrystalline film with a uniform grain size. This figure, as well as all other figures, are not drawn to scale, and are intended to be illustrative in nature. FIG. 8E, is a top view SEM image of a film process via ELA and shows the random microstructure of the film, containing randomly located grain boundaries 8002.