Soft lithography is a group of non-photolithographic techniques that uses organic (soft) materials to transfer patterns to the substrate material.
Soft lithography is a field that covers a number of different techniques. Soft lithography can be broken into 3 different major categories (each of which can be further divided up); printing, replica molding and embossing. It can be noted that these definitions may vary slightly depending on the reference source.
Printing can be defined as a process that involves material transfer from the mold onto the substrate. Printing can further be broken down into subfields of microcontact printing and nanotransfer printing.
Microcontact printing (µCP) is considered one of the most useful techniques for large area (>cm2) patterning of functional organic surfaces. The process is very similar to using a common stamp to transfer ink from an ink pad to a piece of paper. The mold is "inked" with the material to be transfered to the substrate and contact is made between the substrate and the protruding features of the mold. This allows for the material to be transfered from the mold to the substrate, thus patterning the substrate as desired.
Nanotransfer printing (nTP) is a technique that allows for printing of micro and nanoscale features onto a substrate, without using any etching steps. Nanotransfer printing is commonly used to transfer metals onto the surface on the substrate. Similarly to microcontact printing, a hard or elastomeric mold is coated with the metal, the mold is put in contact with the substrate, which is coated with an adhesion layer to promote bonding, and the metal pattern is transfered to the substrate.
Replica Molding Edit
Replica molding can be defined a process that transfers a pattern from a rigid or elastomeric mold into another material by way of a liquid solidifying when in contact with the mold. Replica molding can further be broken down into subfields microtransfer molding, micromolding in capillaries and UV-molding.
Microtransfer molding uses a elastomeric poly(dimethylsiloxance) (PDMS) mold as a master, and a liquid prepolymer is used to fill the recessed regions of the the master mold. An example of the liquid prepolymer a polyurethane, or any thermally curable epoxy. The excess polymer is removed from the master from the master and the prepolymer is then pressed against the substrate and the entire assembly is cured. After the curing process is complete, the PDMS master is removed. There will be some excess polymer that lies between the features on the substrate that needs to be removed. This polymer film is often referred to as "scum", and it is removed using reactive ion etching (RIE), so that the substrate is exposed in between the protruding features of the polymer.
Micromolding in Capillaries is similar to microtransfer molding, however in this case the elastomeric poly(dimethysiloxance) (PDMS) master has a microchannel structure. The PDMS master is first placed face down on the substrate, and liquid prepolymer is place at the open ends of the channels. The capillary forces pull the liquid into the channels. The assembly is then cured, and after the curing process is complete the PDMS master is removed, leaving protruding polymer channels on the substrate.
Ultraviolet-molding (UV-molding) commonly uses a rigid master mold, as opposed to a soft PDMS master as in microtransfer molding and micromolding in capillaries. The major difference in the UV-molding process in comparison to the previous two replica molding techniques is the curing process uses UV radiation as opposed to heat. Apart from the curing step, the UV-molding process is extremely similar to microtransfer molding. Liquid prepolymer is first used to fill the recessed regions of the master mold. The excess polymer is removed from the master from the master and a transparent substrate is placed on top of the prepolymer. The entire assembly is then cured using UV radiation through the transparent substrate. Alternatively, the UV ratdiation can be exposed to the prepolymer through a transparent master, such as quartz. The master is then removed and the excess polymer film, or scum, is removed using reactive ion etching (RIE), so that the substrate is exposed in between the protruding features of the polymer.
Embossing involves imprinting a pattern on an initially flat surface by way of pressing a mold into the surface. Embossing can further be broken down into subfields nanoimprinting (using rigid or soft molds) and solvent assisted micromolding (SAMIM)
- Gates B.D., Xu Q., Love J. C., Wolfe D. B., Whitesides G. M., “Unconventional Nanofabrication”, Annu. Rev. Mater. Res., Vol. 34, pp.339-72, 2004.
- Y. Xia and G. M. Whitesides, “Soft lithography”, Annu. Rev. Mater. Sci., Vol. 37, pp. 550-575, 1998.