Characteristics of tungsten gate devices for mos VLSIs

Recently, refractory metals, such as W and Mo, with low resistivity and high melting points have received keen interest as materials for gate electrodes and interconnects for MOS VLSI devices. It is commonly believed that MOS devices with refractory metal gates deposited from sputter sources have low reliability due to mobile ion contamination. In this work, it was confirmed 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 have developed a high-purity W sputtering target fabrication technique through the use of electron melting. A W gate annealing technique was developed to reduce internal stress in W films. Through these techniques, the above-mentioned problems can be eliminated and W gate devices with characteristics comparable to those of poly Si gates can be produced. The minimum feature size of very large scale integrated (VLSI) circuits has been reduced to one micrometer or less, while at the same time the number of elements per chip continues to increase. Under the present D-RAM developmental situation, a 256kb D-RAM has reached the stage of practical use, and 1Mb D-RAM prototypes have been examined and tested by a number of LSI manufacturers. These developments have impose severe demands on the materials and interconnects in MOS VLSIs