Soft Lithography

Due to the significant cost and technological complexity associated with photolithography, an ensemble of techniques that are collectively termed soft lithography has been invented to construct meso-microscale surface features in a parallel fashion on a variety of substrates.

In the most commonly used variant of soft lithography, microcontact printing (mCP), a silicon master with relief structures is fabricated by photolithography and is used to cast many copies of a soft polymeric stamp (typically an elastomer such as poly(dimethyl siloxane)) that is a negative of the master. The stamp is inked with the molecule of interest, and is transferred to the substrate by direct conformal contact between the soft polymeric stamp and the substrate. Soft lithography was originally developed to pattern alkanethiol self-assembled monolayers (SAMs) on gold [49-51]. However, the practical application of this technology was quite limited. The simultaneous realization by several groups that mCP was not restricted to alkanethiols on gold [52-56] has greatly increased its utility. mCP has now been used to pattern a large number of chemical species including nanoparticles and proteins down to a critical dimension of around 30 nm onto a variety of substrates [52-61]. An advantage of soft lithography over photolithography not only lies in its simplicity, the fact that it does not require access to a clean-room environment (aside from the first step in fabrication of the master, though rapid prototyping methods that obviate this necessity have been developed) [62-64], but also in its ability to transfer a multitude of chemical species in a defined pattern to many different substrates. Soft lithography is appealing for nanofabrication due to its inherent ability to transfer almost any species to a candidate substrate with mesoscale resolution, though we note that achieving submicron resolution is far from trivial [65-68].

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