Helically Perforated Thin Films -- Dependence of Mechanical Properties on Microstructure
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Helically Perforated Thin Films -- Dependence of Mechanical Properties on Microstructure Sumudu P. Fernando, Anastasia L. Elias, and Michael J. Brett Electrical and Computer Engineering, University of Alberta, ECERF, 9107 - 116 St., Edmonton, T6G 2V4, Canada ABSTRACT The effects of several microstructural parameters on the mechanical behaviour of a helically perforated thin film structure, or inverse microspring, were investigated using a finite element model [1]. The parameters investigated were the helical pitch angle, the cross-section radius, and the coil spacing. The elastic modulus was found to depend most strongly on the helical pitch angle (changing by a factor of 1.3 as the pitch angle went from 35o to 70o). Variations in the coil radius and the film thickness had a minor effect on the modulus. It was also found that using a finite size model (as opposed to an infinite model using periodic boundary conditions) produced better conditioned results. A preliminary confirmation of the model’s validity was performed by comparison to nanoindentation results of a nickel helically perforated thin film. INTRODUCTION Significant efforts are being made to develop techniques allowing for the prediction, modelling, and measurement of fundamental mechanical properties such as hardness and elastic modulus at the nano-scale, since these properties change dramatically as dimensions are reduced [2][3][4]. One fabrication method that offers control over the mechanical properties of a material is the creation of a nanostructured thin film by glancing angle deposition (GLAD) [5][6]. This process allows for the creation of columns in a variety of shapes such as helices, square spirals, slanted and vertical posts, and zig-zag structures. The fabrication of helical SiO2 features that deform in a spring-like manner has also been demonstrated; the specific stiffness was determined by the pitch angle of the helices and could be changed by adjusting the deposition parameters [7]. As an extension to the GLAD technique, a templating procedure has been developed allowing for the creation of solid thin films with structured pores [8]. In this process a porous GLAD film master is filled with another material, and the original film is etched away. This structure’s geometry is tuneable due to the nature of the GLAD process, and it also offers added mechanical stability over the discrete column template. In this work, the mechanical properties of the inverse helical structure (a thin film perforated by helically shaped pores) were investigated. More specifically, the dependence of the structure’s stiffness on its geometry was examined in order to determine the relationships between the structural parameters and the mechanical properties of the film. THEORY While the problem of the deformation of regular, spring-like helices has been successfully reduced to an equation [9], the mechanical behaviour of the inverted structure is not so readily expressed. Due to the complexity of the theoretical problem, a finite element model create
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