Soft Lithography To Pattern 2d Patterns On Polyion Multilayers

Soft lithography is an alternative, nonphotolithographic set of microfabrication methods [134-136]. All of its members share the common feature of using a patterned elastomer as the stamp, mold, or mask (rather than a rigid photomask) to generate micropatterns and microstructures. Such techniques include: microcontact printing (^CP) [137], replica molding (REM) [138], microtransfer molding (^TM) [139], micromolding in capillaries (MIMIC) [140], solvent-assisted micromolding (SAMIM) [141], phase-shift photolithography [142], cast molding [143, 144], embossing [145, 146], and injection molding [147-149]. These techniques have been explored and developed by The Whitesides Group and several other groups.

The key element of soft lithography is an elastomeric block with a patterned relief structure on its surface and poly(dimethylsiloxane) (PDMS) elastomers (or silicone rubbers) are used in most demonstrations. Poly(dimethy-lsiloxanes) have a unique combination of properties resulting from the presence of an inorganic siloxane backbone and organic methyl groups attached to silicon [150]. These liquid materials can be poured over a master having a relief structure on its surface, then easily converted into solid elastomer by curing and peeling off. Poly(dimethylsiloxane) has been widely used because they can make conformal contact with surfaces over relatively large areas and they can be released easily from rigid masters or from complex structures. The formulation, fabrication, and applications of PDMS elastomers have been extensively studied and are well-documented in the literature [150]. The master is, in turn, fabricated using microlithography techniques such as photolithography, micromachining, e-beam writing, or from available relief structures such as diffraction gratings [151], and relief structures etched in metals or Si [151, 152], or even by directly printing. Figure 6 illustrates the procedure for fabricating PDMS stamps.

Microcontact printing (^CP) is a flexible, nonphotolitho-graphic method that routinely forms patterned SAMs containing regions terminated by different chemical functionalities with submicron lateral dimensions [153-158]. An elastomeric PDMS stamp is used to transfer molecules of the "ink" to the surface of the substrate by contact. After printing, a different SAM can be formed on the relief pattern on the surface of a PDMS stamp to form patterns. However, it differs from other printing methods [159] in the use of self-assembly (especially, the use of SAMS) to form micropatterns and microstructures of various materials.

Self-assembly is the spontaneous aggregation and organization of subunits (molecule or meso-scale objects) into a stable, well-defined structure via nonconvalent interactions. The information that guides the assembly is coded in the properties (e.g., topologies, shapes, and surface functionalities) of the subunits; the individual subunits will reach the final structure simply by equilibrating to the lowest energy form.

Self-assembled monolayers are one of the most intensively studied examples of nonbiological self-assembling systems [161]. Self-assembled monolayers can be easily prepared by

I Fabrication and silanization of master 4

W- T photoresists or wax ^ Pouring of PDMS over master

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