Elasto-Plastic Deformation of Gold Thin Film Investigated by Hole-Nanoindentation

Abstract

Gold is used as an electrode in nano-devices such as MEMS and NEMS because of its superior properties of high electrical conductivity, biocompatibility and chemical stability, among others. It is generally used as a nanometers-thick film, and the mechanical properties of each layer in a device must be measured to confirm reliability. Mechanical properties can be measured by tensile test, compression test, indentation, etc. Above all, the uni-axial tensile test is known as to obtain the most quantitative mechanical properties such as yield strength, ultimate tensile strength, failure elongation and strain-hardening exponent. However, for nanometer-thick films, specimen fabrication and testing technique are complex. In contrast, indentation is nondestructive, has relatively simple sample preparation and testing method, and can measure elastic modulus. Although nanoindentation makes possible the precise measurement of the properties of micro-scale materials, it is not suitable for sub-micron films due to substrate effects. Recently hole-nanoindentation is well used to measure the mechanical properties of 2-D materials. It also has simple sample fabrication method and test method, but it can only measure several mechanical properties such as elastic modulus and fracture strength. In the present study, we introduce method to evaluate yield strength of Au thin film by hole-nanoindentation. Since yield strength is the stress when elasto-plastic behavior begins, measuring yield strength for designing device is significant. For sample preparation, 100 nm-thick gold film is deposited by sputtering on 1 μm-thick wet oxide silicon substrate and by etching silicon dioxide and drying-off the film, freestanding gold thin film is obtained. We perform tensile test and hole-nanoindentation, obtain yield strength from hole-nanoindentation, and compare its results with those from tensile testing.