A new tungsten gate process for VLSI applications

In spite of the growing demand for MOS gates and interconnections of higher conductivity, the refractory metal gate process has not received as much attention as those using silicides because it is incompatible with the Si-gate process. The metal gate cannot withstand oxidizing annealing ambients, and source-drain formation by ion implantation is difficult because of the channeling of doping ions through the gate metal during ion implantation. In a new process developed for use in MOS VLSI fabrication, tungsten (W) is used as a gate metal because degradation of SiO
2
by annealing the metal/SiO
2
/Si structure at around 1000°C can be minimized if the metal is W. Metal oxidation is prevented by using a H
2
/H
2
O ambient for this annealing, which also allows Si to be oxidized in the same ambient. The channeling mentioned above is stopped by forming a thin layer of PSG or WO
x
on the W. This gate process is believed to be a step forward toward the desired compatibility.