Size Effects and Passivation Effects on the Plasticity of Freestanding Submicron Gold Films

The Membrane Deflection Experiment was used to examine size effects on freestanding thin film gold membranes. It is the first micro-scale testing scheme where the loading procedure is straightforward and accomplished in a highly sensitive manner while preserving the independent measurement of stress and strain. Stress-strain curves were obtained on films 0.3, 0.5 and 1.0 μm thick including membrane widths of 2.5, 5.0, 10.0 and 20.0 μm for each thickness. Both membrane thickness and width were shown to cause size effects on the mechanical properties. By far, thickness played a major role in deformation behavior exhibiting a major transition in the material inelastic response occuring when thickness was changed from 1.0 to 0.5 μm. In this transition, the yield stress more than doubled when film thickness was decreased, with the 0.5 μm thick specimen exhibiting a more brittle-like failure and the 1.0 μm thick specimen exhibiting a strain softening behavior. Results of the effect of surface passivation, with 30 nm SiO2 layers, showed a decrease in yield stress with passivation opposite to that reported in other studies. INTRODUCTION Most knowledge of material properties exists at the bulkscale regime where known constitutive laws material describe behavior. When specimen size becomes small, in the micron regime, these laws fail to describe material response. Thin films, which are commonly employed in microelectronic components and MEMS devices, often display mechanical behavior that cannot be described by traditional means. Their mechanical properties are often essential to the device function and therefore accurate identification is key in determining the device reliability. The affect of specimen size on material mechanical behavior has been experimentally studied by several researchers. Results on nanoindentation [1-5], torsion of microscale rods [6], and bending of thin films [7] have all shown that as specimen size decreased to the micron regime the strength of the film increased. However, in these studies each method subjects the specimens to large strain gradients. Modified plasticity theories have incorporated these strain gradients into a continuum description of microscale deformation behavior [6,8-11]. In this work the Membrane Deflection Experiment (MDE) is used to test the mechanical response of sub-micron gold films [12-15]. The MDE test has certain advantages for the microscale mechanical testing of thin films. The simplicity of sample microfabrication and ease of handling lend confidence in repeatability. The loading procedure is straightforward and accomplished in a highly sensitive manner while preserving the independent measurement of stress and strain. The measurements are also performed under macroscopic homogeneous axial deformation, i.e., in the absence of deformation gradients, in contrast to nanoindentation, torsion, and bending of thin films where deformation gradients naturally occur. We will also evaluate the effect of 30 nm thick SiO2 passivation layers on plasticity and fracture of thin gold film. EXPERIMENTAL PROCEDURE Specially designed thin film Au specimens were microfabricated on (100) Si wafers. Specimen shape was defined on the topside by photolithography and lift off with selective etching of bottom side windows with the purpose of creating suspend membranes, see Espinosa et al. [13] for further details. Passivation layers were grown on both sides of the gold membrane through plasma assisted CVD. The geometry of the suspended thin-film membranes can be described best as a double dog-bone tensile specimen. Fig. 1(a) shows an optical image of the Au membranes. Membrane width was varied in each die, to examine size effects, while preserving the ratio of gauge-length/width. Dimensions of four differently sized membranes can be described by their widths, W, of 2.5, 5, 10 and 20 um. Fig. 1. (a) An optical image showing the topside of the Au membranes and (b) a side view of the MDE test. Parameters are defined in the text. PV Wafer