Surprising threads in the story of thin film strength Shefford P. Baker, Cornell University, MET DMR-0311848

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Surprising threads in the story of thin film
strength Shefford P. Baker, Cornell University,
MET DMR-0311848
Thin films (10-1000 nm) are critical components
in nanofabricated devices (e.g. microelectronics)
and can be extremely strong compared to bulk
materials, leading to very high stresses and
failures. We use computer simulations and
modeling to understand how the motion of line
defects called dislocations, which would relax
those stresses, is prevented. We find that strong
interactions between threads (dislocations that
run from the top to the bottom of the film),
formerly thought to be rare, are quite common,
and that weak interactions between threads and
misfits (dislocations lying along film
interfaces) persist to much higher average
stresses than expected. We attribute these
observations to large inhomogeneities in the
stress field generated by the dislocations
themselves. Threads cluster into small regions
and interact, and thread-misfit interactions stop
dislocations in regions where the stress is low.
These parameter free models successfully
predict both strength levels and strain hardening
rates in thin films. A simple statistical model
has been developed to describe the relationship
between stress inhomogeneity and the probability
of interactions, and can be applied to problems
as diverse as leaves blowing in the wind and
traffic densities.
Above Representative dislocation structure in a
copper thin film. The inset indicates the
complex structure formed during deformation and
some of the interactions that stop dislocation
motion. Left Stresses as a function of position
at the midplane in a film with dislocation
structure like that above. Spikes appear at
threads. Stress inhomogeneities facilitate
dislocation interactions and lead to the observed
high stresses and strain hardening rates in films.
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Highlights of broader impacts of MET DMR-0311848
(Mostly) Students

• Interactive demonstrations on dislocations and
  strength of metals and the structure-properties
  paradigm of materials science to groups of
  underrepresented high school students, and high
  school girls
• Answered Ask a Scientist questions e.g. Why
  is gold such a soft material compared to
  something like iron? in local newspaper.
• Workshop on magnets for 2nd graders at Beverly
  J. Martin Elementary School (December 13, 2005).

Left Graduate student Ray Fertig explaining
magnetism to second graders at Beverly J. Martin
Elementary School. Right PI Shef Baker
explaining the Nanoscale Informal Education
(NISE) network to fellow MRS members at the
Materials Research Society 2006 Spring Meeting in
San Francisco
(Mostly) PI

• Project to bring 92 high-school and middle
  school teachers to the Materials Research Society
  2004 Fall Meeting to form/strengthen links
  between teachers and researchers. Included fund
  raising, program development, and evaluation.
• Worked with Museum of Science (Boston), Science
  Museum of Minnesota (St. Paul), Exploratorium
  (San Francisco), and Materials Research Society
  to develop and propose Nanoscale Informal Science
  Education (NISE) network