Effects of thermal annealing on structural and electrical properties of sputtered W–Ti thin films

In this work we have studied the influence of thermal annealing on the structural and electrical properties of W–Ti thin films, deposited on n-type (100) silicon wafers. The films were deposited by d.c. sputtering from a 90:10 wt.% W–Ti target, using Ar ions, to a thickness of ~ 170 nm. After deposition the samples were annealed at 400 to 700 °C for 60 min, in a nitrogen ambient. Structural characterizations were performed by X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). A four-point probe was used for electrical characterization. It was found that the as-deposited films exhibit a polycrystalline structure in the form of columnar grains (20–50 nm in diameter). Only b.c.c. W phase could be registered in the films, while any presence of crystalline Ti was hindered in XRD and electron diffraction patterns. Annealing at up to 600 °C resulted in an increase of the mean grain size in the films, and a decrease of their sheet resistance. For these annealing temperatures we did not register any interaction at the W–Ti/Si interface. However, upon annealing at 700 °C, a progressed interaction between the film and the substrate occurred. Structural analyses suggest the formation of both W and Ti silicides. The estimated thickness of the formed metal-silicide layer is approximately 125 nm. Sheet resistance increased rapidly after annealing at 700 °C, because a large portion of the original metallic film was consumed in the reaction with silicon.