Nanoimprint – Page 2

Status of Patterned Media Development

Ricardo Ruiz of Hitachi GST gave a short lecture on the status of patterned media development, to the local section of the IEEE Magnetics Society yesterday. I summarize by pointing to four publications listed below. R Ruiz, et al. (Hitachi GST, U Wisc, Hitachi) gives a compact, readable overview of patterned media development circa 2008. Figure 2 is particularly noteworthy; in order to become commercially interesting, one needs a bit density above 1Tb/in^2. Since the read/write head design constrains one to a 2:1 or (better) 4:1 aspect ratio, the dot pitch has to be below 20 nm. [SST 51(9)(2008)]

For reliable operation, every bit on a hard disk should switch in the same way. Happily, electron-beam directed assembly yields tighter switching field distributions than either e-beam lithography or block copolymer self-assembly alone. [Appl. Phys. Lett. 96, 052511 (2010), doi:10.1063/1.3293301]

Recently, Hitachi GST has also reported on an integrated thermally-assisted recording head and its operation on patterned media. Going under the moniker of “BP-TAR”, the authors have achieved 1Tb/in^2 on a 24 nm track pitch. This represents a solid advance over writing on continuous media, even with thermal assist. [Nature Photonics 4, 484 – 488 (2010), doi:10.1038/nphoton.2010.90]

By means of orthogonal double patterning, Hitachi is able to show 27 nm full pitch dots of arbitrary aspect ratio. This is achieved by using e-beam directed self-assembly in the radial direction of the media, and then cutting the resulting long (radial) lines into tracks with EBL. Directed self-assembly can accomodate small (~5%) variations from the block-copolymer’s preferred lattice period, thereby making equi-angular spacing feasible. [ACS Nano, 2011, 5 (1), pp 79–84, doi:10.1021/nn101561p]

In summary, patterned media development appears to be on track. Like anything complex, a lot of engineering and learning remains to be done.

Anodic Aluminum Oxide

Anodic aluminum oxide (AAO) has long been known to display self-organized structure. Here is a nice SEM image. Since alumina is a durable material, AAO can be used as masks and molds for patterning materials. For example, polycyanurate nanorods were recently polymerised in AAO molds for sensing applications.

While self-organization yields largely periodic structures, long-range order must be induced by other means. This turns out to be quite simple: one embosses the aluminum starting layer before starting the anodization. See this article. Click through to the downloadable Supplementary info for a process pictorial.

The same Berkeley group now reports a variant of the technique to yield dual-diameter pores. They used these to grow germanium arrays which absorb 99% of all incoming radiation in the wavelength range of 300 nm – 900 nm. This is almost as absorptive as the SWNT carpet reported some time ago, (SEM image) but a good deal more useful for photonics.

Center for Nano-CEMMS

The NSF Center for Nanoscale Chemical-Electrical-Mechanical Manufacturing Systems has produced many interesting advances during its existence. It is well worth visiting the web site.

Superionic solid state stamping (or S4) is one such advance. S4 electrochemically transfers a pattern written in a superionic solid state conductor into a compatible substrate. For published details on the S4 process, refer to JVST B 25(6) Nov/Dec 2007, 2419-24 or Nano Lett., 2007, 7, 446-51 . For a pictorial overview, see the link at the bottom of the project page.

The Center recently combined S4 with metal-assisted chemical etching [Appl. Phys. Lett. 77, 2572 (2000)] to create silicon nanowire arrays as a demonstration project. At least for silicon substrates, the technique is reported [Nano Lett. 2010, 10, 1582-1588] to be capable of lateral resolutions approaching 10 nm along with extremely high aspect ratio. (The latter will likely be limited by mechanical integrity of the free-standing features.)

Along with S4, five other printing-related processes are under development at Nano-CEMMS. I encourage you to take a look at the site.

ASML Step-and-Flash Control

The question “are there any bubbles trapped” has been asked many times of step-and-flash nanoimprinting. ASML was granted a U.S. patent on Tuesday, 6 April, with the title “Imprint lithography” [US7692771] The invention envisions a visible wavelength illumination and a scattering detector focussed on the template/resist interface. By watching the amount of light scattered from the interface, one can determine whether bubbles are present. The feedback loop permits real-time control of the delay between the template positioning and resist hardening steps, for example.

This patent has a sibling [US2007-0018360] which is ready to issue. Its allowed claims are more explicit, stating

the output providing an indication of whether or not the imprintable material has substantially fully flowed into a recess of the imprint template.