The technology to fabricate microelectromechanical systems (MEMS) and micromachines has been around for over two decades. Unfortunately, only the most basic MEMS and micro-devices are utilized commercially due to persistent reliability issues such as “stiction,” the permanent adherence of two contacting micromechanism surfaces. This adhesion is caused by the inherent attractive forces between relatively smooth, microscale features or between a microstructure and the underlaying substrate. Although the development of organic self-assembled monolayers (SAMs) as anti-stiction coatings has nearly eliminated stiction, liquid-phase deposition of these monolayers presents several drawbacks including cost, repeatability, and process sensitivity. This work describes a novel processing method for the deposition of dodecanethiol-capped colloidal gold nanoparticles onto micromechanical devices to reduce stiction by increasing the roughness of micro-structured surfaces. Previous attempts to reduce stiction by reducing contact surface area were abandoned due to only moderate success. Therefore, this method contradicts the conventional wisdom that adhesion reduction is only achieved by chemical alteration.

Colloidal nanoparticles are deposited onto all surfaces of a micromechanical device chip using gas-expanded liquids (GXLs), which experience a reduction in solvent strength with increased gas headspace pressure allowing for particle precipitation. Due to the miniscule size of the nanoparticles, attractive van der Waals forces dominate gravitational forces allowing precipitated particles to attract to all surfaces of the device. Immediately following deposition, the CO₂/solvent mixture is heated to the supercritical state in order to remove the liquid-vapor interface. Doing so effectively eliminates evaporative effects which may disturb nanoparticle films and capillary forces that may destroy microstructures on the device. Following depressurization, the dry, nanoparticle-coated device is removed for analysis.

Colloidal gold nanoparticle coatings on micromechanical cantilever beam arrays are evaluated via contact angle measurements, detachment length and work of adhesion measurements, SEM, and AFM. SEM imaging affirms the uniform, conformal coating of nanoparticles onto microdevice components. Contact angle and work of adhesion measurements are compared to that of octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosilane (FDTS) self-assembled monolayers. The results indicate that the application of colloidal gold nanoparticles onto micromechanical devices reduces the work of adhesion compared to natural silicon oxide layers by more than four orders of magnitude.