Abstract
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.
Highlights
Thin film materials maintain the fantastic properties of bulk materials [1,2,3,4,5,6] and have the significant advantages of being more economic [7], having small device size [4,5] and having the new physical performance of two-dimensional material [8,9,10]
Pure Ti films were successfully grown on single crystal Si substrates by the introduction of Mo interlayers
It was found that the thickness of the Mo interlayer played an important role in the cohesion between the Mo/Ti multilayer film and the Si substrate, which significantly decreased with increasing Mo interlayer thickness
Summary
Thin film materials maintain the fantastic properties of bulk materials [1,2,3,4,5,6] and have the significant advantages of being more economic [7], having small device size [4,5] and having the new physical performance of two-dimensional material [8,9,10]. Postolnyi et al [23] has proposed that a lower individual layer thickness with smaller grain size and more interlayer interfaces can significantly improve the mechanical properties of CrN/MoN multilayers. The cohesive strength between multilayer and substrate is another crucial problem in multilayer film design and is affected by substrate roughness [25], substrate temperature [25,26], different thermal expansion coefficient in the film-substrate couple [27,28], and film thickness [29,30,31], etc. Screening and designing membrane material systems and film thicknesses are of great importance for the fabrication of high quality multilayer materials [18,23]. The cohesion between the Ti film and the Si substrate has been discussed in relation to the residual stress [6,44,45] of the Ti film and the different thermal expansion coefficients of the Ti and Si materials [27,28]
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