Stamps for Nanoprint Lithography & Microcontact Printing

Nanoimprint Lithography

Imprinting or embossing, is a well-known technique to generate microstructures in hard polymers by pressing a rigid master containing surface-relief feature into a thin thermoplastic polymer film that is then heated close to or, more generally, above Tg.1 Nanoimprint lithography (NIL) has the potential of high-throughput due to parallel processing, does not require sophisticated tools, and allows nanoscale replication for data storage.2,3 NIL is also compatible with conventional device processing techniques. The quality of the nanoimprinting process depends on a number of experimental parameters like Tg, viscosity in the melt, adhesion of the polymer to the mold, etc.4 PMMA has been most widely used as the imprintable material, but a range of thermoplastic and thermosetting polymers is under investigation to optimize the imprinting and subsequent etching steps.5,6

The rigid master is usually prepared via e-beam lithography and has feature sizes in the 10-100 nm range. After imprinting the polymer film, further etching can transfer the pattern into the underlying substrate. Alternatively, metal evaporation and lift-off of the polymer mask produces nanopattern metal features.
References:
1. Herzig, E.P. Micro-optics, London: Taylor & Francis (1997).
2. Terris, B.D. et al. Appl. Phys. Lett., 69, 4262 (1996).
3. Li, M. et al. Appl Phys. Lett., 76, 673 (2000).
4. Faircloth, B. et al. J. Vac. Sci. Technol. B, 18, 1866 (2000).
5. Schulz, H. et al. J. Vac. Sci. Technol. B, 18, 1861 (2000).
6. Schulz, H. et al. J. Vac. Sci. Technol. B, 18, 3582 (2000).
Microcontact Printing

Nanoimprint lithography (NIL) has primarily been used to emboss hard thermoplastic polymers. The micro-molding and embossing of elastomers have attracted considerable interest as these materials have found important applications is soft-lithographic techniques such as microcontact printing (μCP).1,2 In this technique, a monolayer of a material is printed off an elastomeric stamp [made of poly(dimethylsiloxane) (PDMS)] after forming conformal contact between stamp and substrate. Sub-micron surface relief structures can easily be introduced in PDMS by curing the polymers against a lithographically prepared master. The advantage of μCP is the ability to pattern surfaces chemically at the sub-micron level.

An elastomeric stamp inked with small molecules (thiols or silanes) and pressed against a clean substrate (gold or silicon wafer). Where the stamp is in contact with the surface, a monolayer of material is transferred to the substrate. A second thiol or silane is used to fill in the background to provide a chemically patterned surface.
References:
1. Xia, Y. Angew. Chem. Int. Ed., 37, 550 (1998).
2. Michel, B. et al. IBM J. Res. Dev., 45, 697 (2001).