Resistivity, grain size, and impurity effects in chemically vapor-deposited tungsten films

Abstract

The room-temperature electrical resistivity, grain size, and impurity content of tungsten films deposited at low pressure on silicon wafers from tungsten hexafluoride and hydrogen reactants were determined. These properties were examined as functions of deposition temperature and film thickness. The resistivity is independent of thickness at a value of approximately 13.5 μΩ cm for films deposited at 300 °C. For films deposited at 400 °C, the resistivity decreases from 24 to 8.5 μΩ cm as thickness increases from 0.075 to 1 μm. The resistivity behavior is interpreted in terms of grain-boundary scattering with a zero reflection coefficient for the 300 °C films. For the 400 °C films, a reflection coefficient that decreases from 0.67 to 0.38 over the above thickness range and a linear dependence of grain size on thickness are utilized. For both deposition temperatures, the grain size exhibits a rapid initial growth to 30 nm followed by growth at a slope of 0.32 with respect to thickness. The rapid initial growth is associated with the rapid, self-limiting tungsten hexafluoride/silicon reaction. The oxygen content of the films tracks the trend of the reflection coefficient in that it increases with increased deposition temperature or decreased film thickness. Evidence suggests that the oxygen is located predominantly at grain boundaries and may be the primary determinant of boundary scattering. Fluorine is found in films at concentrations (0.03–0.05 at. %) similar to those of oxygen but does not vary with thickness and decreases with increased deposition temperature. No evidence is found for surface scattering of carriers suggesting that all are specularly scattered. Grain-boundary scattering, with a constant reflection coefficient, adequately interprets previously published data. Utilizing such results, together with the present ones, leads to the conclusions that the grain size is relatively independent of deposition temperature while the intragrain resistivity decreases with temperature and approaches the bulk value for 600 °C deposition.