Impurity effects in magnetron sputter deposited tungsten films

Abstract

This paper describes the effect of metallic and nonmetallic impurities on the electrical and metallurgical properties of tungsten films sputter deposited onto oxidized silicon wafers. Tungsten films, 200-nm-thick, sputter deposited using a 99.996% pure tungsten target and a deposition rate of 6 nm/s with a substrate temperature of 400 °C had an as-deposited resistivity of 13.5 μΩ cm. This value is within experimental error equal to the lowest values reported for electron-beam evaporated and chemical vapor deposited films. Metallic impurities in the target increase the electrical resistivity of the tungsten films. Use of a 99.98% pure (3N8) tungsten target under identical conditions as those reported above produced films with resistivities of 17.3 μΩ cm or a 28% increase in resistivity due to an increase of less than 160 ppmw in the concentration of metallic impurities. Intentional addition of iron and copper to the tungsten film increased the film resistivity by factors of 5 and 1 nΩ cm/ppmw of impurity, respectively. Air leak rate into the sputtering chamber was also evaluated and found to significantly affect tungsten film resistivity. An air leak rate of 0.001 25 Torr 1/s increased the film resistivity from 16 to 21.8 μΩ cm or an average rate of 4640 μΩ cm/(Torr 1/s).