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 temper ature process (< 600 degrees C) of thin film transistor (TFT) fabrication. The performance of resultant TFTs was compared to those fabricated on polys ilicon films obtained by simple excimer laser annealing of amorphous silico n films. The electrical characteristics of the TFTs were correlated with th e structural characteristics of the polysilicon films, using transmission e lectron microscopy and x-ray diffraction as analytical tools. The polysilic on 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 bet ween 280 and 360 mJ cm(-2). The SPC+ELA method did not work successfully fo r 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 d iscussed in this article. (C) 1999 American Institute of Physics. [S0021-89 79(99)05122-1].