Ultra Low Cycle Fatigue of Axisymmetric Freestanding Nanoscale Gold Films

We present ultra low-cycle fatigue experiments of axisymmetric nano-crystalline gold films of nano-thickness deformed by a spherical indenter using a recently developed freestanding membrane test. Freestanding membranes of gold were centrally deflected using a spherical indenter attached to a MEMS load cell. Fabrication of the films for these experiments yielded films 100 nm thick and 500 μm in diameter. We observed that the plastically deformed thin films recover completely in time at room temperature. One particular film was loaded 4 consecutive times, with the fourth loading leading to its fracture after full recovery from the three prior loadings. Observation of this phenomenon directed us to term this behavior “Ultra Low cycle fatigue”. INTRODUCTION Recently, Micro Electro Mechanical Systems (MEMS) and its offshoot Nano Electro Mechanical Systems (NEMS) have generated a lot of interest amongst the engineering and scientific community. This interest has further been bolstered by the presence of the very mature semiconductor technology and its associated industry. As a result, there are now many applications that require MEMS/NEMS devices. These devices include microactuators, microsensors, micro scale strain gauges, RF switches, micro pumps, optical switches and tunable filters etc. [1]. Many of these devices utilize metallic thin films as mechanical structures. The elastic and plastic properties of these thin films are significantly different from those of the bulk material [2-4]. At these scales the volume fraction of material defects such as: grain boundaries, dislocations and interstitials become quite significant and become a chief contributor the physical and mechanical material properties of the thin films. Aluminum (Al), Copper (Cu), Nickel (Ni) and Gold (Au) are popular thin film materials used in MEMS/NEMS. However Gold (Au) is usually preferred due to its high electrical conductivity, chemical inertness and resistance to oxidation. Various studies have been conducted in recent years to determine the mechanical properties of freestanding thin films. Vinci et al [4] developed and described several specialized techniques to determine the mechanical properties and stress strain states of both free standing and films bonded to a substrate. He described nanoindentation as a popular and effective way of determining the elastic and plastic properties as well as hardness of free standing thin films. Landman et al [5] provides the detailed theoretical and experimental research of the atomistic and molecular mechanism of adhesion, contact formation, nanoindentation, and fracture that occurs when a Ni diamond shaped nanoindenter interacts with the Au surface. Kalkman et al [3] made measurements of Young’s modulus on free standing thin films and observed the relaxation of thin films at room temperature with frequency dependence. They