Chemical fabrication of ultrahigh-density arrays of nanopores
from the go-self-assembly dept.
Senior Associate MarkMuhlestein writes "This looks like interesting work, reported in the Dec 15 Science. If you have access, the entire article is at http://www.sciencemag.org/cgi/content/full/290/5499/2126 Here's the abstract: Science, 290, 2126 (15 Dec 2000) 'We show a simple, robust, chemical route to the fabrication of ultrahigh-density arrays of nanopores with high aspect ratios using the equilibrium self-assembled morphology of asymmetric diblock copolymers. The dimensions and lateral density of the array are determined by segmental interactions and the copolymer molecular weight. Through direct current electrodeposition, we fabricated vertical arrays of nanowires with densities in excess of 1.9 x 10^11 wires per square centimeter. We found markedly enhanced coercivities with ferromagnetic cobalt nanowires that point toward a route to ultrahigh-density storage media. The copolymer approach described is practical, parallel, compatible with current lithographic processes, and amenable to multilayered device fabrication.' Thurn-Albrecht et al., U Mass @ Amherst, IBM Watson, LANL"
January 2nd, 2001 at 3:15 PM
comment
They use a mixture of two polymers (71% PS and 29% PMMA by volume) which they spin-coat onto a gold-coated substrate. They evaporate an aluminum layer on top. Then they heat the substrate above the glass transition of both polymers, and the PMMA, being the minority component, separates out into a regular array of cylindrical bubbles (14 nm dia. cylinders in a 24 nm hexagonal lattice). Applying an voltage between the gold and aluminum electrodes, the cylindrical domains orient vertically, parallel to the electric field. After cooling, the aluminum electrode is removed, and the material is exposed to UV light, which weakens the PMMA (bond breaking) while strengthening the PS (cross-linking) so that the PMMA can then be selectively dissolved out. This leaves a nicely ordered array of 14 nm holes in a thin plastic film with a gold electrode at the bottom. They can fill the holes with cobalt or other materials by electrodeposition.
The array of cobalt wires they obtain this way would make a nice magnetic storage medium. The regular array permits higher bit densities than disordered dispersions of magnetic particles, where the disorder creates nucleation sites for errors. The elongated geometry of the wires increases their coercivity, or hardness of magnetization, allowing them to have a small diameter and be densely packed. The authors apparently believe one wire per bit will do.
This is a nice piece of work, which could lead to terabyte (1000 gigabyte) hard drives. I don't see any show-stoppers, but maybe I'm missing something.
Other possible applications include high-efficiency thermoelectric devices that could be used to cool computer chips, for example.
January 2nd, 2001 at 4:34 PM
This is very interesting
I wasn't able to get at the Sciencemag article. So, thanks for posting the process for us. I'll have to do a literature search, but this looks like something that I could do. I work with a variety of PVD (sputter, evaporation, ion-beam) and CVD process. I haven't done spin-coating though.
Hard disk media is an interesting application. So is micro-fluidics. These polymers could also be used whereever zeolites are used in industry (petroleum and chemical processing). One question though. Is it possible to control the hole-size in the resulting material? Say, by altering the PS/PMMA ratio? This could be useful as a paterning tool, in place of photolithography.
January 3rd, 2001 at 9:54 AM
Re:comment
> Other possible applications include high-efficiency thermoelectric devices that could be used to cool computer chips, for example. An interesting notion – FAR higher junction density than conventional TE devices. But (a quibble) – TE devices require their junctions to be series connected, and I'm not sure how you could do that with the described process. The Al side would be "easy", but how would you deal with the Au side? Aqua Regia?
Just curious if you had some notion on this…
January 3rd, 2001 at 1:32 PM
followup
Petroleum cat cracking takes place at rather high temperatures (~1500 C if I recall correctly), too high for the polymers.
Sounds reasonable, but the authors don't say. A clue is that they say you could use an e-beam to position isolated cylindrical domains arbitrarily, which should be very interesting to nanotechers, but indicates that the size of the domains may be intrinsic to the material (i.e. reduce the PMMA fraction, you get more sparse 14 nm domains). However, another knob is the molecular weights of the components. You could use different polymers, too. I would guess that by varying these parameters you get different spatial phases, not always hex lattices: stripe phases, branching meanders, dendrites, etc.
The Al side isn't so easy either, if you're trying to get them all in series. Probably you wire bunches of them in series, to lower the resolution required for the wiring. Getting to the backside would be another problem. Probably you'd need to actually lift the ~1 um thick polymer membrane off the substrate. That might be too delicate, but after patterning the top side, you could grow a support layer of something, then either etch away the original substrate or liftoff by dissolving an underlayer. Another possibility is that after forming the junctions in the cylindrical holes, you could dissolve out the PS and replace it with a different matrix.