Uniform Square Polycrystalline Silicon Fabricated by Employing Periodic Metallic Pads and SiON Absorption Layer for Thin Film Transistors

The polycrystalline silicon with regular square grains is fabricated by employing metallic (Cr/Al) periodic pads as the heat sinks and with underlying silicon oxynitride (SiON) as the heat absorption layer. The maximum lateral growth length of the poly-Si is 1.78 $\mu\hbox{m}$ by this method. If the metal pads are periodically arranged, the poly-Si can grow to regular square grains following the high power excimer laser annealing. After removing the metallic pads, the low power laser shot transfers the a-Si:H under the original metallic pads to poly-Si without destroying the square grains. The TFTs fabricated by this method achieve a field effect mobility of 450 $\hbox{cm}^{2}/\hbox{V}\cdot \hbox{s}$ and an on/off current ratio exceeding $\hbox{10}^{7}$ . It is found that the TFT with smaller channel width and length results in a better subthreshold swing because it contains fewer grain boundaries and defects.