Self-Assembled Monolayers as High-Resolution Etch Masks and Templates for Organic Molecular Assembly
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Self-Assembled Monolayers as High-Resolution Etch Masks and Templates for Organic Molecular Assembly C. K. Harnett, A. G. Lopez, K. M. Satyalakshmi, Y.-F. Chen, and H. G. Craighead School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853 ABSTRACT We have used a variety of self-assembled monolayers as resists for low energy electron beam patterning. These compounds can be used as high-resolution patternable linker molecules for selected area binding of proteins and other organic compounds, as well as nanoparticles with organic chemical coatings. Because these systems can be aligned in registry to existing patterns, the organic systems may be positioned with the accuracy of electron-beam lithography. We have also explored the use of self-assembled monolayers for the creation of sub-wavelength artificial dielectric systems. The ultra-thin patterned monolayer is combined with a contrast-enhancing etch process to create high aspect ratio structures. This technique can be used to fabricate diffractive optical devices in a single-step process.
INTRODUCTION Self-assembled monolayers (SAMs) have been widely explored as coatings for nanofabrication techniques because the SAM molecules can be designed to present closelypacked endgroups with specific chemical functionality. SAMs can bind to technologically relevant surfaces such as semiconductors, oxides, and metals. Applications of patterned SAMs include chemical templates for the patterning of materials that react with the chosen monolayer, and chemical-resistant patterns that act as ultrathin, high-resolution etch masks. In this study, monolayers were patterned in three ways: removal of chemically active monolayers by direct electron beam writing (Figure 1a), “backfilling” by immersing an electron beam patterned inert monolayer surface into a solution of a chemically active monolayer (Figure 1b), and by microcontact printing of inert monolayers (Figure 1c). The electron beam patterned active monolayers were used as chemical templates for the adhesion of particles and antibodies over small areas (1 mm2), while the microcontact printed monolayers were used as imaging layers for high aspect ratio etching over areas of several square millimeters. Optical structures fabricated with this deep etching process include a blazed diffraction grating and a quarter-wave plate.
F10.1.1
(a) Direct electron beam patterning of reactive SAMs
(b) Backfilling patterned inert monolayers
Non-reactive SAM on Si or Au Reactive SAM on Si or Au E-beam exposure removes monolayer Electron beam exposure destroys reactivity Immersion in reactive SAM solution
Materials bind to exposed areas
Materials bind to unexposed areas
(c) SAM patterning by microcontact printing.
Ink elastomer stamp with SAM solution
Bring stamp into contact with surface for 1 s to 2 minutes depending on SAM
Figure 1. Monolayer patterning process diagrams.(a) Direct electron beam removal of chemically active monolayers. (b) Backfilling of patterned inert monolayer with chemically active monolayer. (
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