Soft lithography

Figure 1 - "Inking" a stamp. PDMS stamp with pattern is placed in Ethanol and ODT solution

Figure 2 - ODT from the solution settles down onto the PDMS stamp. Stamp now has ODT attached to it which acts as the ink.

Figure 3 - The PDMS stamp with the ODT is placed on the gold substrate. When the stamp is removed, the ODT in contact with the gold stays stuck to the gold. Thus the pattern from the stamp is transferred to the gold via the ODT "ink."

In technology, soft lithography refers to a set of methods for fabricating or replicating structures using "elastomeric stamps, molds, and conformable photomasks" (in the words of Rogers and Nuzzo, p. 50, as cited in "References"). It is called "soft" because it uses elastomeric materials. Soft lithography is generally used to construct features measured on the nanometer scale. According to Rogers and Nuzzo (2005), development of soft lithography expanded rapidly during the period 1995 to 2005.

Procedure

Soft lithography includes the technologies of microcontact printing (mCP), replica molding (REM), microtransfer molding (mTM), micromolding in capillaries (MIMIC) and solvent-assisted micromolding (SAMIM) (From Xia et al.) One of the soft lithography procedures, which was discussed by Xia and Whitesides, is as follows:

1. The steps of any of your favorite micro- or nano- scale lithography procedures (photolithography, EBL, etc.) are followed to etch a desired pattern onto a substrate (usually silicon)
2. Next, the stamp is created by pouring a degassed resin overtop of the etched wafer. Common resins include PDMS and Fluorosilicone.
3. Removing the cured resin from the substrate, a stamp contoured to your pattern is acquired.
4. The stamp is then "inked" by placing it, pattern-up, in a bath of inking solution (for example, ODT in ethanol) for a short period of time(Figure 1). The ink molecules will fall and adhere to the surface of the stamp (Figure 2) creating a single-molecule layer of the ink on the stamp.
5. The inked stamp is then pressed on the substrate and removed, leaving the desired single-molecule thick pattern on the substrate (Figure 3)
6. Steps 4 and 5 are repeated for each substrate on which the pattern is desired

Advantages

Soft lithography has some unique advantages over other forms of lithography (such as photolithography and electron beam lithography). They include the following:

• Lower cost than traditional photolithography
• Well-suited for applications in biotechnology
• Well-suited for applications in plastic electronics
• Well-suited for applications involving large or nonplanar (nonflat) surfaces
• More pattern-transferring methods than traditional lithography techniques (more "ink" options)
• Does not need a photo-reactive surface to create a nanostructure
• Smaller details than photolithography in laboratory settings (~30nm vs ~100nm)
• ~2500 prints from one master (stamp) compared to 1 with traditional lithography

See also

• Nanolithography

References

• Xia, Y. and Whitesides, G. M., (1998) Soft Lithography. In Angew. Chem. Int. Ed. Engl. 37, 551-575.[1]
• Xia, Y. and Whitesides, G. M., (1998) Soft Lithography. In Annu. Rev. Mater. Sci. 28, 153-184.
• Quake, S. R. & Scherer, A. (2000, November 24). From micro- to nanofabrication with soft materials. In Issues in nanotechnology. In Science, 290, 1536 – 1540.
• Rogers, J. A. & Nuzzo, R. G. (2005, February). Recent progress in soft lithography. In Materials today, 8, 50 – 56.

External links

• Whitesides research group at Harvard University [2]
• Xia research group at the University of Washington [3]
• Rogers research group at the University of Illinois at Urbana-Champaign [4]
• Soft Lithography and Atomic Force Microscopy: A Research Experience [5]
• Center for Advanced Microstructures and Devices at Louisiana State University [6]