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Nanoindentation of Microspring Thin Films Mary W. Seto, Brian Dick and Michael J. Brett Department of Electrical and Computer Engineering, University of Alberta Edmonton, Alberta T6G 2G7, Canada ABSTRACT Porous thin films with helical microstructures were fabricated with the Glancing Angle Deposition technique. These films consisted of arrays of “microsprings” whose geometries could be engineered with nanometer scale control. Some of the mechanical properties of these helically structured films were studied with a nanoindentation technique. Several microscopic “springbed” films were tested over a range of forces using a spherical indenter tip. The geometries of the microsprings were varied, and a number of different materials were used to fabricate these films, which were typically a few micrometers thick. Slanted post arrays, resembling micro-cantilevers, were also subjected to nanoindentation tests. Results of initial experiments, theory, and simulations show that these microstructures behave in a manner analogous to macroscopic springs and cantilevers, and may offer some insight into how materials behave at the microscale.

INTRODUCTION The Glancing Angle Deposition (GLAD) technique enables columnar thin films with engineered microstructures to be fabricated [1,2]. Two main mechanisms dominate the growth process for films grown with this technique. Under conditions of highly oblique flux angle (α >80o) and low substrate temperature, the principal processes governing film growth are enhanced shadowing and limited adatom diffusion. The formation of nanometer scale, columnar structures results as the film evolves, and motion of the substrate during film growth causes the developing micro-columns to engineered with a variety of possible morphologies [3,4]. The GLAD apparatus includes two computer controlled stepper motors that vary the motion of the substrate. Rotation can occur about an axis perpendicular to the plane of the substrate, and the incident flux angle can be modified by tilting about an axis parallel to the plane of the substrate. This system can be utilized with thermal and electron beam evaporation, sputtering, and laser ablation techniques, allowing for a wide range of materials to be grown. Further details on the GLAD technique can be found in other literature [5].

MICROSPRING EXPERIMENTAL DETAILS A number of silicon monoxide (SiO) thin films were grown by thermal evaporation, each having a thickness of approximately 2 µm. The helical geometries of these films included 1-, 2and 3-turn microsprings with pitches of 1900 nm, 850 nm, and 600 nm, respectively. Other microspring thin films of titanium (Ti) and chromium (Cr) were grown by electron beam evaporation, and these 3-turn films were approximately 2 µm thick as well. X-ray diffraction studies have shown that the films have a fine-grain polycrystalline nature. EE5.15.1

(a) (b) (c) Figure 1. SEM images of SiO microspring thin films with one turn (a), two turns (b), and three turns (c) grown by thermal evaporation with the GLAD technique.

(a) (b)