Surface modification with self-assembled monolayers for nanoscale replication of photoplastic MEMS
Journal of Microelectromechanical Systmes, Vol., No. 3, June 2002
A release technique that enables to lift microfabricated structures mechanically off the surface without using wet chemistry is presented. A self-assembled monolayer of dodecyltrichlorosilane forms a very uniform ~1.5-nm-thick anti-adhesion coating on the silicon dioxide surface, on full wafer scale. The structural layers are formed directly onto the organic layer. They consist here of a 100-nm-thick aluminum film and a high-aspect ratio photoplastic SU-8 structure. After the microfabrication the structure can be lifted off the surface together with the aluminum layer. This generic technique was used to make a variety of novel structures. First, aluminum electrodes that are embedded in plastic are made using lithography, etching and surface transfer techniques.. Second, using a patterned monolayer as defined by microcontact printing, resulted in a spatial variation of the surface adhesion forces. This was used to directly transfer the stamped pattern into a metal structure without using additional transfer etching steps. Third, the monolayer's ability to cover surface features down to nanometer scale was exploited to replicate sharp surface molds into metal coated photoplastic tips with ~30-nm radii for use in scanning probe instruments such as near-field optical techniques. The advantage compared to standard sacrificial layer techniques is the ability of replication at the nanoscale and the absence of etchants or solvents in the final process steps.
Figure showing an overview and process flow chart of (a) SAMs meet MEMS2 growing; (b) SAM coating; (c) metal deposition and patterning; (d) SU8 process; (e) lift-off. CASE 1: photoplastic MEMS with flat, embedded electrodes fabricated on a planar surface with homogenous SAM coating. CASE 2: for transferring a SAM pattern made by μCP directly into a thin Al film by mechanical layer disruption. CASE 3: replication of a photoplastic near-field optical probe with Al coating from an oxidation-sharpened mold. In principle, other variations of these cases are possible but are not presented here