B.M.Ocko and M. Weinert (Department of Physics, Brookhaven National Laboratory)
R.J. Randler and D.M. Kolb (University of Ulm, Germany)
I.K. Robinson (Department of Physics, Univ. of Ill.)

Understanding the configuration of thin films is important to emerging nanotechnologies.  Copper is particularly relevant since it is the metal of choice for high- speed semiconductor interconnects. Our studies show that the structure of thin copper films (10-20 layers) can behave very differently than bulk copper.  Whereas bulk copper forms a face centered cubic (fcc) crystallographic arrangement, the energy difference between it and the body centered cubic (bcc) is predicted to be very small (38meV/atom). What happens to copper under stress?  Can the bcc form be adopted?

X-ray scattering studies of epitaxial, electrodeposited copper films on gold and silver surfaces do indeed show that the bcc form can be stablilized for films thinner than a critical thickness of 10 layers. These studies were carried out at X22A at the NSLS using in an in-situ electrochemical cell.  For the “bcc phase”the in-plane lattice spacing is expanded by 12% and the interlayer spacing is compressed by 25% compared to the normal (100) face of fcc copper. Remarkably, the entire copper film restructures above this critical thickness by forming regions which are locally orthorhombic (see lower left).  The specular reflectivity splits into two peaks, one corresponding to the expanded regions and the other corresponding to the compressed regions. Modulation peaks (not shown) indicate that the period of the spatially modulated pattern varies between 60 and 75 Å. The predicted specular reflectivity is shown as the solid line.

B.M. Ocko, I.K. Robinson, M. Weinert, R.J. Randler, and D.M. Kolb, Physical Review Letters, 83, 780 (1999).