NIST team develops novel method for nanostructured polymer thin
films
All researchers at the National Institute of Standards and Technology
(NIST) wanted was a simple, quick method for making thin films of block
copolymers or BCPs (chemically distinct polymers linked together) in
order to have decent samples for taking measurements important to the
microelectronics industry. What they got for their efforts, as detailed
in the Sept. 12, 2007, Nano Letters, was an unexpected bonus: a unique
annealing process that may make practical the use of BCP thin films for
patterning nanoscale features in next-generation microchips and data
storage devices.
BCP thin films have been highly desired by semiconductor manufacturers
as patterns for laying down very fine features on microchips, such as
arrays of tightly spaced, nanoscale lines. Annealing certain BCP films
- a controlled heating process - causes
one of the two polymer components to segregate into regular patterns
of nanocylinder lines separated by distances as small as five
nanometers or equally regular arrays of nanoscale dots. Chemically
removing the other polymer leaves the pattern behind as a template for
building structures on the microchip.
(Top L.) Schematic of the NIST
'cold zone' annealing process for polymer thin films on a
semiconductor wafer. Experiment images are color-coded to show
regions with different cylinder orientations, as measured by
atomic force microscopy. Relatively rapid transit times (top r.)
leave a jumble of different regions that become largely
homogeneous at slower speeds (r.).
Image � by NIST
In traditional oven annealing the quality of
the films is still insufficient even after days of annealing. A
process called hot zone annealing - where the
thin film moves at an extremely slow speed through a heated region
that temporarily raises its temperature to a point just above that at
which the cylinders become disordered - has
previously been used for creating highly ordered BCP thin films with a
minimum of defects but little orientation control. For some polymer
combinations, the order-disorder transition temperature is so high
that it is virtually impossible for manufacturers to heat them
sufficiently without degradation occurring.
To eliminate the time and temperature restraints without losing the
order yielded by hot zone annealing, the NIST researchers developed a
�cold zone� annealing system where the polymers are completely
processed well below their order-disorder transition temperature.
Properly controlled, the lower-temperature processing not only works
with BCPs for which hot-zone annealing is impractical, but, as the
NIST experiments showed, also repeatedly produces a highly ordered
thin film in a matter of minutes. NIST researchers also discovered
that the alignment of the cylinders was controlled by the �cold zone�
annealing conditions. Because it is simple, yields consistent product
quality and has virtually no limitations on sample dimensions, the
NIST method is being evaluated by microelectronic companies to
fabricate highly ordered sub 30 nm features.
The next step, the NIST researchers say, is to better understand the
fundamental processes that make the cold zone annealing system work so
well and refine the measurements needed to evaluate its performance.
Further Information and Source:
-
B.C. Berry, A.W. Bosse, J.F. Douglas, R.L. Jones and A. Karim: Orientational order in block copolymer films zone annealed
below the order-disorder transition temperature. Nano Letters, Vol. 7, No. 9, pp. 2789-2794, (Sept. 12, 2007);
doi: 10.1021/nl071354s