Nanotechnology

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¯/O 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.

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Lithography beyond light: Microcontact printing with monolayer resists

Microcontact Printing of Alkanethiols on Silver and Its Application in Microfabrication

Abstract

This paper describes the use of microcontact printing (μCP) to generate patterned self‐assembled monolayers (SAMs) of alkanethiolates on the surfaces of evaporated films of silver. Using patterned SAMs of alkanethiolates as nanometer‐thick resists, patterned microfeatures of silver with sizes down to ∼200 nm were fabricated on Formula by selective etching in aqueous solutions containing Formula , and Formula . Complete etching of silver can be achieved more rapidly than that of gold: ∼20 s vs. ∼20 min for 50 nm thick metal films when similar ferricyanide etchants were used. Microstructures of silver produced by the present method have higher edge resolution (typically, ∼20 nm vs. ∼100 nm) and far fewer defects (∼10 pits/mm2 vs. ∼103 pits/mm2) than those of gold fabricated by a similar procedure. Silver lines (0.2 μm in thickness, ∼50 μm in width, and ∼5 mm in length) had the expected metallic conductivity (≈5.56 × 105 S/cm); parallel lines of silver (0.2 μm in thickness, ∼10 μm in width, ∼1 mm in length, and separated by ∼10 μm) were electrically isolated from each other. Aqueous solutions containing other coordinating ligands and oxidants, Formula , and Formula , were also selective etchants for use with patterned SAMs of alkanethiolates on silver. Patterned structures of silver (50 nm thick) on Formula could be used as secondary masks for etching of Formula in aqueous solutions of Formula , and of Si(100) in aqueous solutions of KOH and i‐propanol. Patterned films of silver (0.2 μm thick) on silicon wafers could be used as masters to cast elastomeric stamps with surface relief to be used for μCP. By choosing appropriate etching conditions, microparticles of MX (M = Ag; X = Cl, Br, I, OH, and SCN) could be formed in situ on the underivatized regions of the SAM‐patterned surface during etching of silver.

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Microcontact Printing of Alkanethiols on Silver and Its Application in Microfabrication

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