Excimer laser annealing of amorphous and solid-phase-crystallized silicon films

Very thin (25–50-nm-thick) amorphous silicon (a-Si) films were crystallized into polycrystalline silicon (polysilicon) films by the combination of low temperature solid phase crystallization (SPC) and subsequent excimer laser annealing (ELA). These films are, then, subjected to a standard low temperature process (<600 °C) of thin film transistor (TFT) fabrication. The performance of resultant TFTs was compared to those fabricated on polysilicon films obtained by simple excimer laser annealing of amorphous silicon films. The electrical characteristics of the TFTs were correlated with the structural characteristics of the polysilicon films, using transmission electron microscopy and x-ray diffraction as analytical tools. The polysilicon films obtained by the SPC process consist of large and heavily defected crystalline grains. These defects, however, could be eliminated by melting and solidifying the polysilicon films during the ELA process. As a result, the electrical properties of the 50-nm-thick polysilicon film subjected to SPC+ELA process improved dramatically when the laser energy density was between 280 and 360 mJ cm−2. The SPC+ELA method did not work successfully for the 25-nm-thick films due to the instability of the laser energy density. Hillock formation at the grain boundaries during the ELA process is also discussed in this article.