Thermal stability and electrical characteristics of tungsten nitride gates in metal-oxide-semiconductor devices

Abstract

Tungsten nitride (WN(x)) was investigated to be used as a metal gate of metal-oxide-semiconductor field effect transistors (MOSFETs). WN(x) films with various N/W atomic ratios were deposited on SiO(2) and HfO(2) by reactive sputter deposition at different N(2)/Ar ratio flows. Nitrogen concentration in WN(x) films increases rapidly with the N(2)/Ar gas ratio and tends to saturate. WN(x) films with nitrogen atomic ratio higher than 44% have a main phase of WN(x) and the WN phase is stable up to 800 degrees C. The higher-order WN(x) phase does not form even if the nitrogen concentration is as high as 61 %. Many of the excess nitrogen atoms in WN(x) films are desorbed at temperatures below 766 degrees C. The excess nitrogen in WN(x) films can cause effective work function lowering. A weak Fermi-level pinning effect is observed on the HfO(2) film. In this case, WN(x) is not suitable to be metal gate of bulk p-channel MOSFETs. Fully depleted silicon-on-insulator (FD SOI) devices require a work function of 0.2 eV from the midgap of the Si energy band. Therefore, a WN(x)/HfO(2) gate stack can be applied to p-channel FD SOI devices. The good integrity of the WN(x)/HfO(2) gate stack also suggests that WN(x) is a promising gate material.