The effects of argon pressure on the properties of sputtered tantalum films

Abstract

The properties of tantalum films sputtered onto glass substrates over a wide range of sputtering conditions have been investigated. The films were sputtered at argon pressures between 9 and 80 mtorr and at voltages between 2.0 and 5.8 kV. The sputtering system was basically a d.c. diode system but an electron-emitting filament was also used to increase the range of sputtering conditions and to control the current independently of the pressure. At pressures below 15 mtorr, with voltages between 2.0 and 5.8 kV, the film resistivity was 210 μΩ cm, but it increased with increasing pressure and decreasing voltage; at 50 mtorr, the resistivities of films sputtered at 4.0 and 2.0 kV were 270 and 2100 μΩ cm respectively. Resistivities as high as 8000 μΩ cm were obtained at 2.0 kV and 80 mtorr. As the resistivity increases from 200 to 8000 μΩ cm, there is a decrease in the film density from 14.5 to 8.2 g cm −3, the temperature derivative of resistivity becomes more negative and the β-Ta structure loses its strong (100) orientation. These effects are very similar to those reported for oxygen-doped tantalum films and the structure of the present films is probably similar. In this case, the oxygen is incorporated in the film both during and after sputtering. After deposition, measurements of the film resistance were made as a function of time. The resistivity increased when oxygen was supplied to the system and when the system was vented to atmosphere. The change in resistance increased with the final resistivity; increases between zero and 140% were recorded for films with final resistivities between 210 and 8000 μΩ cm respectively. The surfaces of the films, after reaching their final resistivities, were examined by electron microscopy. As the resistivity increased to 1100 μΩ cm, the film surface became increasingly rough due to the presence of large numbers of almost hemispherical hillocks with diameters of approximately 200 Å. With further increase in resistivity, the surface became smoother again.