Self-Assembled Monolayer-Immobilized Gold Nanoparticles as Durable, Anti-Stiction Coatings for MEMS

Self-assembled monolayer (SAM) films of p-aminophenyl trimethoxysilane (APhTS) and 3-mercaptopropyl trimethoxysilane (MPTS) were used to immobilize gold nanoparticles (AuNPs) on silicon substrates and silicon-based microdevices, which created robust nanoparticle coatings that reduced microstructure adhesion. The terminal groups of APhTS and MPTS have both been previously shown to strongly interact and/or bind with metals and metallic nanoparticles. Scanning electron microscopy (SEM) analysis indicated that APhTS and MPTS monolayers improved the adhesion of gold nanoparticles deposited on silicon substrates and microstructures. SEM analysis also showed that the gold nanoparticle/organic monolayer (AuNP/APhTS or AuNP/MPTS) films were more robust than non-immobilized AuNP coatings toward both cantilever beam mechanical contact and water erosion testing. The combination of the rough, lower-energy surfaces of AuNP/APhTS and AuNP/MPTS films also effectively reduced the adhesion exhibited between microstructured surfaces by nearly two orders of magnitude as measured by the apparent work of adhesion. Smooth native oxide-coated Si(100) in-plane surfaces typically have an adhesion energy in excess of 30 mJ/m² while AuNP/APhTS and AuNP/MPTS coatings reduced the adhesion energy to 0.655 and 1.66 mJ/m², respectively.