 |
| Fig
1. A cross-sectional scanning-electron micrograph of
a 50-nm-wide line in photoresist with a 500-nm-wide
undercut region on either side. The widening of the
line towards the top of the image is an artifact of
the imaging technique. |
Liftoff
and shadow evaporation are fabrication methods used to define
metal features on surfaces. Liftoff uses a metal deposition
on a patterned surface (usually a photoresist), followed by
the chemical removal of the underlying layer, to leave a metal
pattern. Shadow evaporation uses a suspended resist structure
(such as is shown in figure 1, where the suspended structure
is 50-nm wide and the undercut is 500-nm wide) with subsequent
angled depositions of metal to create bilayer metal nanojunctions
for applications in single-electronics and quantum computing.
Typically
wet-chemical etching is used to define the undercut feature.
This approach results in poor control of the undercut's extent,
as well as some widening of the narrow suspended feature.
The result is great difficulty in obtaining sub-50-nm features
and robust undercut resist, wide enough to perform shadow
evaporation, but not wide enough to collapse.
 |
| Fig
2. A 16-nm-wide gold line on a silicon surface defined
using liftoff and processing methods developed in our
group. |
We
have developed a new technique for liftoff and shadow-evaporation
using a PMMA/PMGI bilayer that demonstrates superior resolution
and undercut control relative to existing methods. This method
uses novel development methods and defines the undercut using
electron-beam lithography, rather than wet-chemical etching.
The result is reproducible fabrication of nanowire features
as narrow as 16 nm (see figure 2) using resist structures
with undercuts that vary in dimension by less than 50 nm from
run to run.
We
have recently developed new resist processes capable of feature
dimensions down to 11 nm. We are pursuing use of these structures
for templating the self-assembly of quantum dots, thus combining
top-down and bottom-up lithographic methods.
Students
Collaborators
- Prof. Vladimir Bulovic (MIT, EECS)
- Amanda Giermann (MIT, DMSE)
- Prof. Carl Thompson (MIT, DMSE)
- Dr. J. Aumentado (NIST, Boulder)
Additional
Information
"Robust
Shadow-Mask Evaporation via Lithographically-Controlled Undercut,"
B. Cord, C. Dames, J. Aumentado, K.K. Berggren, to appear
in J. Vac. Sci. Tech. B. Preprints available upon request.
Sponsors
- AFOSR (Air Force)
- ARO (Army)
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