Nanoporous Gold

This thesis investigates fabrication methods for Nanoporous Gold (NPG), the complex nature of the film stress evolution in thin films during their fabrication and during surface charging. Fabrication of microstructures comprised of NPG requires a precise control over selective corrosion of the precursor alloy. In many designs, the precursor alloy is constrained to a substrate, and complex surface reconstruction during dealloying potentially leads to a high overall stress in the newly forming NPG. Hence, constrained NPG thin films or suspended NPG structures often develop cracks in fabrication. Similarly, nanovoids in thin film precursor alloys or low Au content in the precursor alloys may lead to fractures in the NPG thin films, which compromise integrity and functionality of the resulting architecture. Recently developed scalable electrochemical methods, for which the rate of removal of the less noble elements in the precursor alloy can be precisely controlled, produce crack-free blanket films constrained to a substrate. In this study, conventional as well as newly developed NPG fabrication techniques are assessed from a microscale fabrication perspective, with regard to the NPG product quality, means of tailoring the final porous structure and their compatibility with the standard microdevice fabrication techniques. A galvanostatic dealloying method is introduced, and shown to be effective in fabrication of constrained, crack-free, blanket NPG films on stiff substrates. Surface stress evolution during NPG fabrication is investigated using an optical multi-beam stress sensor (MOSS). The characteristic stress variation, range of the film stress, and effects of fabrication parameters are presented. The findings suggest that the film stress increases fast in the earlier stages of dealloying, proportional to the Ag dissolution rate. Film stress relief dominates during fabrication, with enhanced Au diffusion rate and increase of surface area. Upon surface charging NPG can expand significantly due to the capillary forces created at the double layer interface. A NPG based actuation device is designed to demonstrate its mechanical response with regard to surface charging. The performance of NPG proves to be sufficient for use in MEMS actuators.