Titanium (Ti) film has been used as a hydrogen storage material. The effect of the thickness of a molybdenum (Mo) nano-interlayer on the cohesive strength between a Mo/Ti multilayer film and a single crystal silicon (Si) substrate was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and nano-indenter. Four groups of Si/Mo/Ti multilayer films with different thicknesses of Mo and Ti films were fabricated. The XRD results showed that the introduction of the Mo layer suppressed the chemical reaction between the Ti film and Si substrate. The nano-indenter scratch results demonstrated that the cohesion between the Mo/Ti film and Si substrate decreased significantly with increasing Mo interlayer thickness. The XRD stress analysis indicated that the residual stress in the Si/Mo/Ti film was in-plane tensile stress which might be due to the lattice expansion at a high film growth temperature of 700 °C and the discrepancy of the thermal expansion coefficient between the Ti film and Si substrate. The tensile stress in the Si/Mo/Ti film decreased with increasing Mo interlayer thickness. During the cooling of the Si substrate, a greater decrease in tensile stress occurred for the thicker Mo interlayer sample, which became the driving force for reducing the cohesion between the Mo/Ti film and Si substrate. The results confirmed that the design of the Mo interlayer played an important role in the quality of the Ti film grown on Si substrate.
Keywords: cohesion; interlayer; nano-indenter; residual stress; titanium film.