The Effect of TaSi2/n+ Poly Gate on Hot-Electron Instability of Small Channel MOSFETs

This paper reports on a comparative study on the properties of small channel TaSi2/n+ poly gate MOSFETs. Measurements of threshold voltage as a function of effective channel length show that these TaSi2 gate transistors have less short-channel effects for channel lengths ¿1.5 ¿m. In order to determine the threshold stability under voltage stress, poly gate and TaSi2/poly gate MOSFETs were subjected to various drain and gate voltage stresses for a long duration. It is observed that, after the stress, poly gate transistors with channel lengths ¿1.5 ¿m show significant change in the threshold voltage and ß of MOSFETs. This instability seems to be due to hot-electron trapping in the oxide. but under the same stress conditions, these TaSi2/poly gate transistors do not show significant instability. SEM studies show that the poly layer of the TaSi2/poly gate was undercut near the drain-gate and source-gate overlap regions. This undercutting apparently helps in making lightly doped n-junctions near the drain-gate and source-gate overlap region. We postulate that the presence of this lightly doped junction near the drain-gate overlap region moves the high-electric field near the drain junction and reduces the hot-electron instability in the small channel-length TaSi2/poly gate MOSFETs.