Template-induced formation of α-W and size-dependent properties of tungsten thin films

Tungsten thin films with different thicknesses were deposited on Si substrate and Mo seed-layer by magnetron sputtering. X-ray diffraction (XRD) analyses and field emission scanning electronic microscopy (FESEM) observations show that stable α-W in equiaxial-grain shape is preferred on Mo layer driven by template effect while the metastable β-W with non-equiaxed grain structure appears to form on silicon substrate. Additionally, residual stress and electronic resistivity depend upon the film thickness considerably, but with different mechanisms. For the case of β-W, electronic resistivity and residual tensile stress increase with decreasing film thickness indeed because of reduced grain size. Whereas, for α-W case, at film thicknesses equal to or smaller than tens of nanometers, the constraint of coherent interface between α-W and Mo will dominate and enhance the resistivity and residual compressive stress.