Lithography beyond light: Microcontact printing with monolayer resists

Abstract

We describe high-resolution lithography based on transfer of a pattern from an elastomeric "stamp" to a solid substrate by conformal contact: a nanoscale interaction between substrate and stamp on macroscopic scales that allows transport of material from stamp to substrate. The stamp is first formed by curing poly(dimethyl siloxane) (PDMS) on a master with the negative of the desired surface, resulting in an elastomeric solid with a pattern of reliefs, typically a few microns deep, on its surface. The stamp provides an "ink" that forms a self-assembled monolayer (SAM) on a solid surface by a covalent, chemical reaction. Because SAMs act as highly localized and efficient barriers to some wet etches, microcontact printing forms part of a convenient lithographic system not subject to diffraction or depth of focus limitations while still providing simultaneous transfer of patterned features. Our study helps to define the strengths and limitations of microcontact printing with SAMs, a process that is necessary to assess its worth to technology. We used lithography based on scanning tunneling microscopy (STM) to demonstrate that disruption of SAMs on gold allowed the formation of etched features as small as 20 nm using a CN-/O2 etch. This result implied that etching occurred where damage of a few molecules in the ordered SAM allowed passage of cyanide, whereas adjacent molecules in the SAM remained unperturbed at this scale. Features as small as 30 nm etched in gold over areas greater than 1 cm2 resulted from microcontact printing with replicas of electron-beam-formed masters, with the transfer of these printed SAMs requiring only ≈ 1 s. STM studies of these transferred SAMs revealed an achievable order indistinguishable from that found for SAMs prepared from solution. Facile alignment of printing steps at submicron scales may result from new designs of stamps that exploit their limited deformability and lock-and-key-type approaches to mate stamp and substrate.