Effects of residual phosphorus in the channel region of amorphous silicon thin film transistors(a-Si TFTs) on the TFT characteristics were quantitatively investigated. Concentration and the depth profile of the residual phosphorus were measured by high resolution secondary ion mass spectroscopy(SIMS). The OFF characteristics of a-Si TFTs were also measured.
The SIMS data showed that the phosphorus exists about 100nm deep into intrinsic a-Si(i-a-Si), but the OFF characteristics showed that the activity of the residual phosphorus is 4 order of magnitude lower than that of heavily phosphorus doped a-Si(+-a-Si). The residual phosphorus is found to be inactive and stable, and has little effect on a-Si TFT characteristics.
These results enabled us to fabricate inverted staggered a-Si TFTs by the simplest process using only 2 photo-mask steps and 1 self-aligned exposure.