Mechanical property characterization of metal nano-particle films for MEMS devices

Abstract

Printing-based fabrication methods have emerged as a promising alternative to conventional lithographic processes in a number of applications. These methods are being exploited in display manufacturing, flexible electronics, and more recently MEMS. Unlike applications in printed electronics, MEMS devices require comparatively thick layers, typically on the order of microns. In the micron-thickness regime, nanoparticle-based inks are the preferred means for material delivery, in large part due to the ability to deliver high solids content. However, sintered nanoparticle films possess unique morphology and hence different properties when compared to bulk material or materials deposited through other methods and are dependent on the processing conditions. As such, careful characterization of the film's properties is critical to successful adoption of this technology. A detailed methodology to identify the process-mechanical property of metal nanoparticle-based films was developed using silver nanoparticles as the case study. Silver nanoparticle-based cantilevers and films were fabricated through inkjet printing and conventional microfabrication techniques. These structures were mechanically characterized by beam deflection analysis and nanoindentation to map Young's moduli versus the processing conditions. The results were coupled with sintering and powder metallurgy models to explain the data. For silver-based nanoparticle films, it was determined that the process and mechanical property have a power law relationship with the ratio of the sintering temperature and the melting point of bulk silver. This relationship enables prediction of mechanical properties and provides guidance for optimization of sintering conditions towards a desired mechanical property. The specific results reported include: i) process flows to fabricate nanoparticle-based microstructures, ii) detailed methodology to map film features and properties versus processing conditions, and iii) an empirical model explaining the data and enabling prediction of the resultant properties. While this methodology was shown to determine the process-mechanical property relationship for silver nanoparticle-based films, it should be generally applicable to other metal nanoparticle-based films and lays the groundwork for characterizing this class of materials.