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(New page: Entry to be completed by Kevin Tian ''Written by Kevin Tian, AP 225, Fall 2011'' Soft-lithography can be defined to be the category of techniques which, through the use of a stamp/mo...)
 
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Entry to be completed by Kevin Tian

Written by Kevin Tian, AP 225, Fall 2011

Soft-lithography can be defined to be the category of techniques which, through the use of a stamp/mold made from a "soft" material (referring to elastomers, typically PDMS), transfer a pattern to a substrate. It has been used specifically to construct feature sizes ranging from micron to nanometer length scales, making the set of techniques particularly invaluable when it comes to fabrication. Prior to their popularization in ~1995, conventional fabrication techniques typically involved Photolithography. However the introduction of soft-lithography techniques was quite popular as a low-cost alternative for micro- and nano-fabrication.

General

Taken from [6]. Image depicts the SIL fabrication process, a combination of soft- and photo-lithographic techniques.

Although photolithography and e-beam-lithography have been quite popular in high-tech industries, soft-lithography offers several advantages:

  • Significantly lower cost for mass production compared to photolithography
    • Part of this comes from the fact soft-lithography does not require the use of a clean-room facility (to achieve reasonable resolution), though specialized equipment is still needed.
  • Can be applied to both planar and non-planar surfaces
    • Photolithography requires the use of a very clean, flat surface, as otherwise photoresist will not spin on in an even fashion.
    • Soft-lithography is not limited by this, though designs for the stamps/molds must take non-planar surfaces into consideration.
  • Photo-sensitive surfaces not necessary (removing need to work under special lights!)
    • This allows for a greater variety of materials that can be directly patterned onto
  • In terms of resolution, soft-lithography is very competitive (and at lower cost) and easily exceeds the resolution limits of standard photolithography. This is because the physical contact circumvents the diffraction limit that e-beam and photo-lithographic techniques run into.
    • E-beam ~15nm resolution [1]
    • Photolithography ~50-100nm resolution [2]
    • Soft-lithography ~35nm resolution (depending on the mask, can reach up to 6nm even?)[3] [4]

As one notes, these various advantages make soft-lithography very attractive, and not only to researchers who would like to access to cheaper lithography techniques. We reiterate that soft-lithography is classified as techniques that utilize an elastomeric stamp/mold to transfer a pattern to a substrate. In the broadest sense, the hallmark of soft-lithography is, as the name implies, the 'softness' and flexibility of this mold compared to the more rigid, inorganic masks photolithography.

Some fabrication techniques have also looked into combining the two forms of lithography (soft-lithography and photolithography) to develop hybrid methods that utilize a PDMS photomask in addition to a Chromium photomask to create novel structures. One such technique is Soft interference lithography (SIL), which has been used to pattern in new and interesting ways [5].

Key Concept(s)

Figure 1: Taken from [6]. Illustrates the process of fabricating a PDMS stamp from a master mold. This allows for the creation of many stamps from just a single relief structure in a very straightforward fashion.

The most essential idea behind soft-lithography is the use of an elastomeric block with a patterned relief on its surface (no different from a conventional stamp!) and using this structure to transfer patterns onto the surface of a substrate. The details behind each various technique employs slightly different principles, but all soft-lithography techniques will employ this key idea in some form or another.

Materials that have been used for the elastomer have included: polydimethylesiloxane (PDMS), polyurethanes, polyimides, and cross-linked Novolac™ resins (a phenol formaldehyde polymer) [6].

Typically techniques will begin with a 'master', from which elastomeric stamps and molds are subsequently created. This master is typically fabricated by using other lithographic techniques including, but not limited to, photolithography, micromachining, e-beam writing/lithography etc. Once one has the master, the process depicted in Figure 1 can then be used to produce a stamp/mold.

The typical process for PDMS is as follows (assuming a master is at hand):

1. Silanization of the master.

2. PDMS elastomer is poured over the master, cured and released.

Though technical issues do exist with PDMS in particular (such as deformations that are also shown in Figure 1 that can cause defects in the pattern), the overcoming of these problems is an area of active research. In the meantime, designing one's pattern such that the known issues do not occur is the only workaround.

Technique List

This is a non-exhaustive list of techniques that have been developed that one can classify as "soft-lithography". Each one has its own associated advantages, disadvantages, and subtleties that differentiates the process from other soft-lithographic processes.

References

http://en.wikipedia.org/wiki/Soft_lithography

[1] http://en.wikipedia.org/wiki/Electron_beam_lithography#Resolution_capability

[2] http://en.wikipedia.org/wiki/Photolithography#Resolution_in_projection_systems

[3] Waldner, Jean-Baptiste (2008). Nanocomputers and Swarm Intelligence. London: ISTE John Wiley & Sons. p. 93.

[4] Y.Xia and G.M.Whitesides, "Softlithography", Angew. Chem. Int. Ed. (1998), 37, 550-575.

[5] Joel Henzie, Min Hyung Lee and Teri W. Odom, "Multiscale patterning of plasmonic metamaterials," Nature Nanotechnology 2, 549 - 554 (2007)

[6] Xia, Y.; Whitesides, G. M. (1998). "Soft Lithography. In". Annu. Rev. Mater. Sci. 28: 153–184.

Keyword in references:

Propulsion of Flexible Polymer Structures in a Rotating Magnetic Field