Improving the Characteristics of Ultra-Thin-Film Fully-Depleted Metal-Oxide-Semiconductor Field Effect Transistors on SIMOX (Separation by IMplanted OXygen) by Selective Tungsten Deposition on Source and Drain Region

Abstract

Selective tungsten (W) chemical-vapor-deposition (CVD) with hydrogenation and hydrogen-termination (HHT) is applied to ultra-thin-film fully-depleted (FD) metal-oxide-semiconductor field effect transistors (MOSFETs) on SIMOX (Separation by IMplanted OXygen) for reducing the sheet resistance of source and drain regions. 0.25-µm-gate MOSFETs on SIMOX (MOSFETs/SIMOX) with a top Si layer with a thickness of 50 nm are fabricated using selective W-CVD, and their characteristics, including hot-carrier effects and latch-onset voltage, are systematically investigated. It is found that selective W-CVD with HHT can reduce the source/drain (S/D) sheet resistance in 50-nm-thick ultra-thin-film SIMOX to 10 Ω/sq. or less. This ensures that W deposition increases the drain saturation current. It is also found that W deposition largely suppresses the parasitic bipolar effects. Consequently, the anomalous subthreshold slope diminishes, hot carrier reliability improves, and the latch-onset voltage rises to over 2.5 V. Moreover, it is clarified that the parasitic bipolar effects are largely suppressed when the W layer is within 0.3 µm from the source/body junction. This is because W effectively extracts the holes generated by impact ionization, and thus suppresses the accumulation of holes which would otherwise induce an increase in the body potential.