Structural properties of reactively sputtered W–Si–N thin films

Tungsten-silicon-nitrogen, W–Si–N, ternary thin films have been reactively sputter deposited from W5Si3 and WSi2 targets using several nitrogen partial pressures. The films have been thermal annealed in the 600–1000°C temperature range and a wide region of the W–Si–N ternary phase diagram has been explored by changing the N2∕Ar ratio during the deposition. Multitechnique approach was adopted for the analysis of the samples. Composition has been determined via ion beam analysis; chemical states were investigated using x-ray photoelectron spectroscopy (XPS); crystalline structure was studied using transmission electron microscopy (TEM) and x-ray diffraction (XRD) and surface morphology by scanning electron microscope. The films deposited in pure argon atmosphere are tungsten rich and approach the target contents as N2∕Ar ratio is varied during deposition. Tungsten enrichment in the films is caused by resputtering of silicon which can be inhibited by the formation of silicon nitride, allowing films with Si∕W ratio closer to the target compositions. The higher capability to form nitrides with silicon than with tungsten favors enhancement of nitrogen content in samples deposited from the silicon rich target (WSi2). The samples with excess nitrogen content have shown losses of this element after thermal treatment. XPS measurements show a break of W–N bonds caused by thermal instability of tungsten nitrides. TEM and XRD revealed the segregation of tungsten in form of metallic or silicide nanoclusters in samples with low nitrogen content (W58Si21N21 and W24Si42N34). High amounts of nitrogen were revealed to be highly effective in inhibiting metallic cluster coalescence. Measurements of electrical resistivity of as deposited films were performed using four point probe technique. They were found to lie in the range between 0.4 and 79mΩcm depending on sample composition.