"Nanofabrication" is the process of making functional structures with arbitrary patterns having sizes of 100 nm or less in at least one dimension. Scanning-beam techniques, such as EBL and focused-ion-beam (FIB) writing, are the principal methods of generating arbitrary nanoscale patterns (mastering); photolithography is the principal method of transferring these patterns from one substrate to another (replication). These techniques, while the workhorses of nanofabrication, come with high capital and operating costs, limited accessibility to general users (and users of materials considered incompatible with electronics fabrication). They are generally only applicable to the two-dimensional patterning of resist materials on planar substrates.
The limitations of these conventional approaches when applied to innovative problems - high capital and operating costs, the difficulty in accessing the facilities necessary to use them, and their restricted applicability to many important classes of problems - motivate our exploration and development of new, or "unconventional" nanofabrication techniques.
Unconventional techniques have the potential to be the ultimate, low-cost method for certain types of nanomanufacturing; approaches based on reel-to-reel processing are particularly attractive for low-cost processes. Unconventional approaches are also operationally much simpler to use than are conventional techniques, and thus help to open nanoscience and nanotechnology to exploration by a wide range of disciplines, especially those historically only weakly connected to electrical engineering and applied physics.
Nanoskiving is a process that combines thin deposition of metal on a topographically contoured substrate with thin sectioning using an ultramicrotome. This technique introduces “cutting” as a method of replicating patterns that is complementary to techniques like printing and molding.
Nanoskiving converts the perimeters of the molded relief features into the geometries of the nanostructures. The procedure used for two-dimensional patterning requires us to create a three-dimensional master that determines the geometry of the features. The thicknesses of the thin films determine the line widths of the features; and the ultramicrotome determines the height of the features.
This technique uses a nanoindenter, equipped with a diamond tip, to form patterns of indentations on planar substrates. Nanoindentation Lithography makes it possible to indent hard materials, to produce patterns with multiple levels of relief by changing the loading force, and to control the profiles of the indentations by using indenters with different shapes. The indentations can also be used as molds to create PDMS stamps.
The Whitesides group has developed four unique methods for fabricating nanostructures by molding (Figures 3, 4): (1) Replica Molding (RM) consists of three steps: i) creating a topographically patterned master (usually by conventional techniques; see, for example, ii) transferring the pattern of this master into PDMS by replica molding; and iii) fabricating a replica of the original master by solidifying a liquid precursor against the PDMS mold. (2) Solvent-Assisted Micromolding (SAMIM) uses an elastomeric mold and an appropriate solvent to emboss polymer films.
Using a stamping technique we have developed two methods for patterning molecules on surfaces with high resolution (Figure 3). In microcontact printing (µCP), molecules are transfered from a patterned PDMS stamp to a substrate by the formation of covalent bonds. In electrical microcontact printing (e-µCP), a flexible electrode is used to pattern a thin film of electret-based material (i.e., that accepts and maintains an electrostatic potential), probably by injecting and trapping charges.
We are exploring several methods for creating nanostructures from using the topographical changes in the edges of patterns. Exposing the edge of a thin film can lead to the formation of nanostructure. This method of edge lithography takes advantage of the numerous methods that can grow thin films over large areas with a thickness between 1 and 50 nm. Converting these films - which are thin in the vertical direction - into structures that are thin in the lateral direction is an approach to fabricating nanostructures.
We have demonstrated the use of spherical silica colloids on substrate as template on which metallic half-shells are formed. Dissolution of the template releases hollow metallic (Au, Pt, Pd) hemispheres with nanometric-scale dimensions.
1. Qin, D.; Xia, Y. N.; Whitesides, G. M. Nature Protocols 2010, 5, (3), 491-502.
2. Elhadj, S.; Rioux, R. M.; Dickey, M. D.; DeYoreo, J. J.; Whitesides, G. M. Nano Letters 2010, 10, (10), 4140-4145.
3. Gong, J. L.; Lipomi, D. J.; Deng, J. D.; Nie, Z. H.; Chen, X.; Randall, N. X.; Nair, R.; Whitesides, G. M. Nano Letters 2010, 10, (7), 2702-2708.
4. Dickey, M. D.; Russell, K. J.; Lipomi, D. J.; Narayanamurti, V.; Whitesides, G. M. Small 2010, 6, (18), 2050-2057.
5. Dickey, M. D.; Lipomi, D. J.; Bracher, P. J.; Whitesides, G. M. Nano Letters 2008, 8, (12), 4568-4573.
6. Dickey, M. D.; Weiss, E. A.; Smythe, E. J.; Chiechi, R. C.; Capasso, F.; Whitesides, G. M. Acs Nano 2008, 2, (4), 800-808.
7. Lipomi, D. J.; Ilievski, F.; Wiley, B. J.; Deotare, P. B.; Loncar, M.; Whitesides, G. M. ACS Nano 2009, 3, (10), 3315-25.
8. Lipomi, D. J.; Kats, M. A.; Kim, P.; Kang, S. H.; Aizenberg, J.; Capasso, F.; Whitesides, G. M. Acs Nano 2010, 4, (7), 4017-4026.
9. McGuigan, A. P.; Bruzewicz, D. A.; Glavan, A.; Butte, M.; Whitesides, G. M. PLoS One 2008, 3, (5), Article No.: e2258.
10. Whitesides, G. M.; Lipomi, D. J. Faraday Discussions 2009, 143, 373-384.