The internal stress can be of importance in the tungsten metallization process used for the fabrication of large−scale integrated circuits. The present work is an extension of an earlier study on the dependence of the stress in low−voltage triode sputtered tungsten films upon deposition conditions and substrate materials. As a function of film thickness, the stress was found to decrease with increasing thickness at various substrate temperatures. The effect of higher substrate temperatures is just to change from large compressive stress to smaller compressive stress and finally into tension. For example, the stress in a 5000−Å film decreases from 1.6×1010 dyn/cm2 in compression to 5×109 dyn/cm2 in tension as substrate temperature increases from 370 to 850 °C. Generally, no gross difference was found for films deposited on SiO2, Al2O3, or Si3N4 at higher substrate temperatures. As a function of deposition rate, the stress can be described in three regions. The stress was found to be small and relatively constant in the low−deposition−rate region. However, as the deposition rate increased to the transition region, a sharp stress increase was found. Finally, above a critical deposition rate, the large internal stress remains constant again. By measuring the stress and characterizing the microstructure of the film, it is concluded that the grain size is the dominant factor determining the stress. These experimental results are discussed in light of the grain boundary relaxation model of Finegan and Hoffman and annealing of disorder proposed by Klokholm and Berry.

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.

Internal stresses and resistivity of lowvoltage sputtered tungsten films

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