Fracture force analysis at the interface of Pd and SrTiO3
Introduction
Over the past number of years, advances in technology have made it possible to produce continuous force–indentation depth curves, with little effort, even at low loads [1]. Working at low loads enables very small specimens to be considered. This opens up the indentation technique to be used in the area of thin films. To date, efforts have been made to determine fracture mechanics properties and residual stresses from low load indentation tests [2]. Residual stresses generally appear during the deposition of a thin film on the substrates. Such stresses can lead to significant troubles such as film detachment, bending and modification of the interfacial properties of the system. The evaluation of such stresses at the interface is of fundamental characteristics for metallization of the microelectronic devices. There are mainly three kinds of stresses such as intrinsic stresses, thermal stresses and external stresses which a thin film undergoes entirely. However, external stresses which are applied by the outside world depend on the application and are relatively negligible. Intrinsic stress is generated during the deposition of the film while thermal stresses appeared due to thermal expansion coefficient mismatch between the film and the substrate [3]. High mobility materials such as pure FCC metals tend to have low intrinsic stresses when deposited at or near room temperature, but can support high extrinsic stresses [4]. Basically, residual stresses at the interface could be modulated by the substrate relaxation process [5]. In fact, the substrate relaxation process occurs due to lattice mismatch between the film and the substrate which their interfacial layer encompasses the variable lattice parameter from the lattice constant of the film to lattice constant of the substrate. This relaxation process would introduce the crystallographic changes in the order of few nanometer of the thin film thickness which could modify the interface electron scattering. In fact, defects and lattice elongation increase the electrical resistivity of the film due to higher degree of electron scattering.
Previously, it was found by Guisbiers et al. [6] that the residual stresses were enhanced in higher thickness of Pd thin metallic film. Indeed, the effect of parameters such as porosity and hall–petch relationship on the elastic module of Pd thin films have been also previously studied [7], [8], [9]. Certainly, the mechanical properties of the film could straightforwardly introduce the global stresses and interfacial properties of the thin film. Therefore, hardness (H) and elastic modulus (E) were measured with force–indentation depth curves obtained by Nanoindenter. Some of the problems surrounding the use of low load indentation to determine mechanical properties include the difficulty in measuring the correct contact area for the indentation; at low loads there can be considerable elastic recovery and so the final contact area under load can be significantly different to the final contact area upon load removal. In addition, the displacement of material under an indenter is far from uniform [10]. In developing the method, oscillating indentation was performed in order to quantify the fracture force at the interface. The specific objective is to develop a method that would be reasonably straightforward to implement and be suitable for use with very low load. This method indents generally at low indentation applied load to prevent the plastic deformation while repetitive contact of the indenter with the surface using oscillating indentation modifies the final contact area. Scanning electron microscopy (SEM) was utilized to study the indentation vicinity before and after the indentation. Finally, atomic force microscopy (AFM) was carried out over the indentation area to distinguish the piling up and sinking in districts and the ratio between total indentation depth and piling up height. The aim of this work is to introduce a new method to illustrate the relation between the critical fracture forces at the interface with thickness variation for soft thin films over the hard substrates.
Section snippets
Experimental
The materials used in this study were 101B grade both sides polished (1 0 0) oriented SrTiO3 (STO) substrate and 99.99% pure palladium. Electron beam evaporator was utilized to evaporate the palladium. Graphite crucibles were employed for resistive evaporation. The evaporation chamber was pumped with mechanical and turbo molecular vacuum pump while the base pressure of the chamber was 4.7 × 10−6 Torr. Substrate temperature was maintained during deposition at 400 °C and cooling was carried out in the
Results and discussion
XRD θ–2θ measurement revealed highly textured Pd thin film toward the (1 1 1) crystallographic direction. Quaeyhaegens et al. [12] proved that all metals with FCC structures such as Pd tend to have a natural preference in (1 1 1) orientation during the growth whereas the BCC metals prefer to have a texture according to (1 1 0) crystallographic orientation. This behaviour is strongly caused by the low number of the bonds with neighboring atoms in these low dense plans. However, the presence of (2 0 0)
Conclusions
Basically, the hardness, elastic module, grain size, and the ratio between the high intense crystallographic direction of the substrate and film were measured in order to clarify the growth mode of the Pd thin film. Stranski-Krastanov growth mode has been seen for Pd thin film whereas the variation of grain size and elastic module is inconsistent with the hall–petch equation.
The specific objective of the work was to develop an experimental methodology to determine fracture force at the
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