Simple fabrication of offset-gated poly-Si TFTs by in situ fluorine passivation and excimer-laser doping

We have proposed a simple fabrication method of offset-gated polycrystalline silicon thin film transistors (poly-Si TFTs) by employing excimer-laser-induced in situ fluorine passivation and excimer-laser doping technique. In the proposed process, the crystallization and in situ fluorine passivation of active layer and the doping of source/drain region are performed simultaneously with only one excimer-laser annealing (ELA). Using phosphosilicate glass (PSG) films as a diffusion source, excimer-laser doping was accomplished successfully and the minimum sheet resistance was 500 Omega/square for 5 pulses at 320 mJ/cm(2). The subthreshold and the on-state properties of devices with in situ fluorine passivation were improved compared to those without in situ fluorine passivation. The threshold voltage and the maximum transconductance for the device with in situ fluorine passivation were improved from 5.7 +/- 0.4 V and 8.7 +/- 1.1 mu A/V to 4.3 +/- 0.3 V and 12.8 +/- 0.8 mu A/V, respectively. Such an improvement is due to the fluorine passivation effects of trap states in poly-Si channel, offset region and SiO2/poly-Si interface. Our experimental data show that the proposed method is effective to simply obtain the high performance poly-Si TFTs. (C) 2000 Elsevier Science B.V. All rights reserved.