Lithography Beyond Light: Microcontact Printing with Monolayer Resists

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. Stamp is first formed by curing poly(dimethyl siloxane) (PDMS) on a master with negative of 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 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 strengths and limitations of microcontact printing with SAMs, necessary to assess its worth to technology. We used lithography based on scanning tunneling microscopy (STM) to demonstrate disruption of SAMs on gold allowed the formation of etched features as small as 20 nm using a CN(2)-/O(2) etch. This result implied 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...