Self-assembled Monolayer Films: Microcontact Printing

, Microcontact printing (PCP), one of the non-photolithographic techniques that make up "soft lithography" (Xia and Whitesides 1998), uses an elastomeric stamp to generate two-dimensional patterns by print* ing the molecules of an "ink" onto the surface of a solid substrate. In its most common configuration, the stamp is fabricated from an elastomeric polymer, poly(dimethylsiloxane) (PDMS), the "ink" is an alkanethiol, and the substrate on which the printing occurs is a thin, evaporated film of gold or silver. Alkylsiloxanes are also printed on Si/Si02 and glass. Microcontact printing is an experimentally simple methodology; the elastomeric stamp is easily fabricated by molding a liquid prepolymer against a master that has the desired pattern in relief on its surface. Once the master is available, multiple copies of the pattern can be produced reproducibly, using straightforward experimental procedures. The initial products of PCP are usually patterned regions of self-assembled monolayers (SAMs). These printed SAMs can subsequently beused as ultrathin (2-3nm) resists in selective wet etching, or as templates to control the wetting, dewetting, adsorption, nucleation, growth, and deposition of a rich variety of materials. The printing process is inherently parallel-that is, it forms the pattern over the entire area of the substrate in contact with the stamp at the same time-and thus allows for patterning over a relatively large area in a single impression. The minimum size of features that can be directly generated using PCP still has not been completely defined. It is possible to print routinely 300nm features of hexadecanethiolate SAMs over 50cm2 areas (Xia et al. 1997), and it has also been demonstrated that PCP is capable of generating parallel lines of dodecanethiolate SAMs on gold that are 35nm in width and separated by 350nm (Biebuyck et al. 1997).