Fabrication of Highly Reliable Tungsten Gate MOS VLSI's

Abstract

Recently, refractory metals, such as W and Mo, with low resistivity and a high melting point have received keen interest as materials for gate electrodes and interconnects for MOS VLSI's. However, it is commonly believed that MOS devices with refractory metal gates have low reliability due to mobile ion contamination. In this work, it was clarified that W gates with commercially available W sputtering targets introduced high level mobile charges into the gate oxide. It was also found that the rates of degradation of W gate MOS transistors due to hot carriers were higher than those of poly‐Si gates, and the internal stresses in W gates increased the degradation rates. Therefore, the authors developed a high purity W sputtering target fabrication technique through the use of electron melting. A W gate annealing technique was developed to reduce the internal stress in W films. Through these techniques, the above‐mentioned problems can be eliminated, and W gate devices that have characteristics comparable to those of poly‐Si gates can be produced.