Internal stresses and resistivity of low-voltage sputtered tungsten films

Abstract

The continuing development of microelectronic circuits toward greater complexity has stimulated interest in new materials and processes compatible with the currently known silicon device technology. Tungsten has been considered as the first‐level conductor for a multilevel structure due to its relatively low electrical resistivity, its thermal expansion coefficient which matches fairly well to that of silicon, its demonstrated good adherence to the dielectrics of interest, and its ability to withstand high‐temperature processing. The present work is a part of a study of the dependence of the properties of low‐voltage triode sputtered tungsten films upon deposition parameters. The effects on internal stress and resistivity of tungsten films are reported here. Tungsten films have been deposited with thicknesses from 1000 to 15000 Å and with resistivities as low as 8 μω cm (1.55 of the bulk). These films were deposited at 1 μ argon pressure at rates in the range of 50–400 Å/min. The electrical resistivity was observed to increase with increasing deposition rate, decreasing film thickness, and decreasing substrate temperature. The impurity concentration was found to be small by electron microprobe and ion probe techniques and, hence, did not completely account for the observed changes in resistivity. The internal stress was determined by two x‐ray methods: (i) precision lattice parameter determination and (ii) a two‐exposure technique. In general, depending upon the deposition conditions, tensile or compressive stresses of the order 10⁹−10¹⁰ dyn/cm² were observed. The compressive stress was observed to increase with decreasing film thickness and increasing deposition rate. Increasing the substrate temperature caused the compressive stress to decrease to zero and become tensile. This changeover temperature was observed to be 650°C for a 5000‐Å film deposited at 115 Å/min. The observed results are discussed briefly in terms of microstructure changes.