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.

Highlights of broader impacts of MET DMR-0311848

• 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).

• 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

(Mostly) Students

(Mostly) PI

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