(331d) Nanoparticle-Based Surface Modifications for Microtribology Control

Due to persistent reliability issues such as strong adhesion, high friction and structural wear only the most basic microelectromechanical systems (MEMS) are currently used in commercial applications. Several approaches have been examined to overcome the inherent forces which can lead to stiction and friction related problems and device failure. Previous studies have focused on chemically altering silicon-based microstructure surfaces via self-assembled monolayers (SAMs) in order to change the chemical nature of adhesion and friction. While these SAM coatings have provided reduced adhesion and coefficients of friction between two contacting microstructures, SAM formation processes using organic molecular precursors are limited by high reaction sensitivity and low reproducibility. Another approach to overcome inherent attractive forces which cause adhesion and friction is to reduce the real area of contact between two contacting microstructures by increasing the surface roughness. Our previous work has illustrated that ligand-stabilized gold nanoparticles can be conformally deposited onto all surfaces of a micromachine or microdevice using a CO₂-expanded liquid/supercritical drying process. These fundamentally new nanoparticle coatings were shown to effectively increase the surface roughness of polysilicon microstructures, thereby, reducing the adhesion of contacting structures by four orders of magnitude. However, under further examination these gold nanoparticle coatings were not robust as particles were easily translated along the surfaces when contact was made eventually leading to microstructure adhesion. In this work, various methods of immobilizing or entrapping rough nanoparticle-based surfaces are examined. One approach incorporated organic SAMs such as 3-mercaptopropyltrimethoxysilane and p-aminophenyltrimethoxysilane to immobilize gold nanoparticles to a silica surface. A second approach investigated the entrapment of gold nanoparticles beneath a vapor phase deposited silica layer. The durability of the nanoparticle-based surface modifications were investigated along with the tribological effect of the coatings on microstructure adhesion.