Fabrication of polycrystalline silicon thin‐film transistors by ion shower doping technique

Abstract

In the fabrications of polycrystalline silicon thin‐film transistors (TFTs), doping of impurities is often carried out by ion implantation. In this study, dopings of phosphorus and boron were carried out by an ion shower doping technique, where the rf plasma of H2‐diluted PH3 or H2‐diluted B2H6 gas with a magnetic field was used for the ion source. To form the source‐drain regions using this technique, the doping characteristics such as sheet resistivity and impurity profiles in polycrystalline Si were investigated. Using the conditions obtained experimentally, n and p channel Si gate polycrystalline Si TFTs were fabricated on a fused silica substrate by using a CMOS process. It was also confirmed that the conventional photoresist can be used for this process. For the n‐channel transistor, field‐effect mobility μ = 3 cm2/V·s, threshold voltage Vth = 5 V, and on‐off current ratio ION (VD = 6 V, VG = 10 V)/IOFF (VD = 6 V, VG = 0 V) ≌ 106 were obtained for W/L = 12 μm/6 μm. In the p‐channel transistor, the characteristics were similar to those obtained by ion implantation. The ion shower doping technique made it possible to fabricate polycrystalline Si TFTs with a CMOS structure, using a simple apparatus. By using this technique, a large area process will be possible.