SOFT LITHOGRAPHY

Scientific Drivers

Fabrication of new types of electronic, magnetic, and optical devices currently consists of redesigning existing structures in down-sized versions. We are exploring an alternate strategy that uses "soft lithography" for the fabrication and manufacture of nanostructures. With these techniques we are able to generate patterns with critical dimensions as small as 30 nm. These techniques use transparent, elastomeric polydimethylsiloxane (PDMS) "stamps" with patterned relief on the surface to generate features. The stamps can be prepared by casting prepolymers against masters patterned by conventional lithographic techniques, as well as against other masters of interest.

Several different techniques are known collectively as soft lithography. They are as described below:

• Near-Field Phase Shift Lithography. A transparent PDMS phase mask with relief on its surface is placed in conformal contact with a layer of photoresist. Light passing through the stamp is modulated in the near-field. If the relief on the surface of the stamp shifts the phase of light by an odd multiple of (, a node in the intensity is produced. Features with dimensions between 40 and 100 nm are produced in photoresist at each phase edge (Rogers et al. n.d.).

• Replica Molding. A PDMS stamp is cast against a conventionally patterned master. Polyurethane is then molded against the secondary PDMS master. In this way, multiple copies can be made without damaging the original master. The technique can replicate features as small as 30 nm (Xia et al. 1997).

• Micromolding in Capillaries (MIMIC). Continuous channels are formed when a PDMS stamp is brought into conformal contact with a solid substrate. Capillary action fills the channels with a polymer precursor. The polymer is cured and the stamp is removed. MIMIC is able to generate features down to 1 µm in size (Kim, Xia and Whitesides 1995; Xia, Kim, and Whitesides 1996).

• Microtransfer Molding ((TM). A PDMS stamp is filled with a prepolymer or ceramic precursor and placed on a substrate. The material is cured and the stamp is removed. The technique generates features as small as 250 nm and is able to generate multilayer systems (Zhao, Xia, and Whitesides 1996).

• Solvent-assisted Microcontact Molding (SAMIM). A small amount of solvent is spread on a patterned PDMS stamp and the stamp is placed on a polymer, such as photoresist. The solvent swells the polymer and causes it to expand to fill the surface relief of the stamp. Features as small as 60 nm have been produced (Kim et al. n.d.).

• Microcontact Printing ((CP). An "ink" of alkanethiols is spread on a patterned PDMS stamp. The stamp is then brought into contact with the substrate, which can range from coinage metals to oxide layers. The thiol ink is transferred to the substrate where it forms a self-assembled monolayer that can act as a resist against etching. Features as small as 300 nm have been made in this way (Kumar and Whitesides 1993).

• Techniques used in other groups include micromachining of silicon for microelectricalmechanical systems (MEMS) (Mehregany et al. 1992), and embossing of thermoplastic with patterned quartz (Chou, Krauss, and Renstrom 1996).

Our techniques require little in capital investment and most can be carried out in ambient laboratory conditions at low cost. Unlike conventional lithography, these techniques are able to generate features on both curved (Jackman, Wilbur, and Whitesides 1995) and reflective substrates and rapidly pattern large areas. A variety of materials have been patterned using the above techniques, including metals and polymers (Joon et al. 1995; St. John and Craighead 1996; Wang et al. 1997). The methods complement and extend existing nanolithographic techniques and provide new routes to high-quality patterns and structures with feature sizes ( 30 nm. There are, however, important unresolved problems such as distortions in the mask and issues such as runout and registration in multilevel fabrication.

Applications of soft lithography in the near future could include simple optical devices, such as polarizers, filters, wire grids, and surface acoustic wave (SAW) devices (Zhao et al. 1996). Longer term goals include working toward optical data storage systems, flat panel displays, and quantum devices. Soft lithographic techniques are currently not competitive with conventional photolithography for multilayer fabrication where there are critical requirements for pattern regularity.

Critical Parameters

Soft lithography appears to be a promising route to nanostructures and nanosystems. Many issues remain to be solved, however, before the techniques can be brought to the market. Some critical parameters are applicable to all the techniques, while others apply only to specific techniques.

Multilevel structures and devices necessitate the ability to accurately place one layer on top of another. High-resolution registration is problematic with elastomeric, distortion-prone materials. Distortion currently limits soft lithographic techniques to the fabrication of single-layer structures and devices. Registration is possible on micron-sized features using a mask aligner. Studies are also underway in our group to quantify the distortion in elastomeric stamps and to limit this distortion by using thick stamps and rigid backings.

The formation and distribution of defects also must be controlled if these techniques are to be used in device fabrication. Defects arise from dust particles, poor adhesion to the substrate and poor release from the stamp, bubbles in the precursor, and contamination from low molecular weight monomers in the stamp. Another problem is the presence of a thin film of polymer under the nanometer-sized features in certain of the soft lithographic methods; microtransfer molding and solvent-assisted embossing often have problems with underlayer control. The presence of the underlayer makes etching and lift-off difficult. Although reactive ion etching can be used to remove thin layers, the time necessary to remove thick layers would damage small features.

The resist used has been a limiting factor in some cases. Phase shift lithography is able to generate features in photoresist more than an order of magnitude smaller than those for which the resist and mask aligner are designed. The resist is, however, only filtered to particles of 40 nm, suggesting that it will be difficult to achieve smaller features consistently with current materials. In microcontact printing where the SAM functions as a resist for etching, the resolution of the grain size of gold in the substrate produced by evaporation (30 nm) limits the edge resolution of the etched area.

Phase-shift lithography has its own set of problems. Features are formed at a phase edge: a mask with a 4 µm period generates two 90 nm lines with a distance of 2 µm between them. Increasing the density of the features is challenging. The fabrication of features such as T-junctions is complex because of the necessity of the phase edge. Integration of large and small features in phase shift lithography is also difficult. The compatibility of these patterning techniques with the production of microelectronic circuitry is starting to be addressed.

The above difficulties are tractable; creativity, however, will be necessary to overcome them.

Bringing Soft Lithography to the Market

Nanostructures are fabricated ordinarily using advanced nanolithographic techniques such as e-beam writing, X-ray lithography, and proximal-probe lithography. These techniques are capable of providing very small features, but their development into methods for generating large areas of nanostructures rapidly and at low cost will require some ingenuity. Conventional techniques are also limited to the formation of two-dimensional structures in a limited number of materials on planar surfaces. The lack of techniques capable of generating and manufacturing nanostructures rapidly and economically represents a limiting step in the area of nanoscience and nanotechnology.

Soft lithography suggests a new conceptual approach to nanomanufacturing: advanced nanolithographic techniques would be used to make masters, and these structures would then be transferred into organic polymers or other materials using procedures such as printing, molding, embossing, or a combination thereof. The techniques could be automated (Xia, Qin, and Whitesides 1996). The ability to make both positive and negative replicas and to modify the dimensions and shapes of features present on elastomeric masters by mechanical deformation adds further flexibility to this methodology (Xia and Whitesides 1997). Currently, our ability to manufacture is limited to devices where small feature size is important but lateral distortions are not: single-level structures such as polarizers and waveguides. The limitations are due to the yet unresolved issues of registration and distortion. Further practical technological uses of these techniques will require further development.

References

Chou, S.Y., P.R. Krauss, and P.J. Renstrom. 1996. Imprint lithography with 25-nanometer resolution. Science 272: 85-87.

Delamarche, E., H. Schmid, B. Michel, and H. Biebuck. N.d. Stability of molded PDMS microstructures. Adv. Mater. In press.

Haisma, J., M. Verhaijen, K. van den Heuvel, and J. van den Berg. 1996. Mold-assisted nanolithography: A process for reliable pattern replication. J. Vac. Sci. Tech. B 14: 4124-28.

Jackman, R.J., J. Wilbur, and G.M. Whitesides. 1995. Fabrication of submicron features on curved substrates by microcontact printing. Science 269: 664-666.

Joon, N.L., P.G. Clem, R.G. Nuzzo, and D.A. Payne. 1995. Patterning of dielectric oxide thin layers by microcontact printing of self-assembled monolayers. J. Mat. Res. 10: 2996-99.

Kim, E., Y. Xia, and G.M. Whitesides. 1995. Polymer microstructures formed by moulding in capillaries. Nature 376: 581-584.

Kim, E., Y. Xia, X.-M. Zhao, and G.M. Whitesides. N.d. Solvent-assisted microcontact molding: A convenient method for fabricating three-dimensional structures on surfaces of polymers. Adv. Mater. In press.

Kumar, A., and G.M. Whitesides. 1993. Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric stamp and an alkanethiol ink followed by chemical etching. App. Phys. Lett. 63: 2002-4.

Mehregany, M., S.D. Senturia, J.H. Lang, and P. Nagarkar. 1992. Micromotor fabrication. In IEEE Trans. Elec. Dev. 39: 2060-69.

Rogers, J.A., K.E. Paul, R.J. Jackman, and G.M. Whitesides. N.d. Using an elastomeric phase mask for sub-100 nm photolithography in the optical near field. App. Phys. Lett. In press.

St. John, P.M., and H.G. Craighead. 1996. Microcontact printing and pattern transfer using trichlorosilianes on oxide substrates. Appl. Phys. Lett. 68: 1022-24.

Wang, D., S.G. Thomas, K.L. Wang, Y. Xia, and G.M. Whitesides. 1997. Nanometer scale patterning and pattern transfer on amorphous Si, crystalline Si, and SiO₂ surfaces using self-assembled monolayers. Appl. Phys. Lett. 70: 1593-95.

Xia, Y., and G.M. Whitesides. 1997. Extending microcontact printing as a microlithographic technique. Langmuir 13: 2059-67.

Xia, Y., D. Qin, and G.M. Whitesides. 1996. Microcontact printing with a cylindrical rolling stamp: A practical step toward automatic manufacturing of patterns with submicrometer-sized features. Adv. Mater. 8: 1015-17.

Xia, Y., E. Kim, and G.M. Whitesides. 1996. Micromolding in capillaries: Applications in material science. J. Am. Chem. Soc. 118: 5722-5731.

Xia, Y., et al. 1997. Replica molding using polymeric materials: A practical step toward nanomanufacturing. Adv. Mater. 9: 147-149.

Yang, X.M., D.A. Tryk, K. Hashimoto, and A. Fujishima. 1996. Surface enhanced Raman imaging of a patterned self-assembled monolayer formed by microcontact printing on a silver film. Appl. Phys. Lett. 69: 4020-22.

Zhao, X.-M., et al. 1996. Fabrication of single-mode polymeric waveguides using micromolding in capillaries. Adv. Mater. 8: 420-24.

Zhao, X.-M., Y. Xia, and G.M. Whitesides. 1996. Fabrication of three-dimensional micro-structures: Microtransfer molding. Adv. Mater. 8: 420-424.