Smoother copper films for microchips

A surprising discovery in the growth behaviour of copper – an important ingredient in modern electronic devices – was made by a research group from Kiel University, Germany, in collaboration with staff from the ESRF. The team headed by Professor Olaf Magnussen found that minute quantities of chloride in the electrolyte solution have a decisive influence on the structure of the growing films, which in turn determines their electrical behaviour. These findings, just published in the renowned journal Physical Review Letters, will help engineers to better tailor the high-tech production processes for semiconductor microchips.

Copper with its superior electrical properties is the material of choice for the electric wiring in modern consumer electronics. Copper films on circuit boards as well as the ultra small connections on microchips are produced by discharging copper ions from solution with the help of an applied voltage. The research team studied how exactly the atoms attach to the growing surface during this electrodeposition process, using the brilliant X-ray radiation of the ESRF. They found that with increasing voltage the copper films grew smoother. “This actually was a big surprise”, Magnussen explains. “Other metals, for example gold, grow rougher at higher voltages and this is also what current theory predicts.” The scientists were finally able to pinpoint the anomalous behaviour of copper to the arrangement of the atomic layer of chloride ions present on the surface. Copper atoms moving across the surface have to push through this layer and become more mobile at higher voltage, where the chloride is more weakly bound.

Graphical representation of the copper film growth process. Deposited copper atoms move across the surface through a layer of chloride (Copyright: CAU, artwork: F. Golks).

Since reliable operation of microchips depends on high-quality electric connections, the control of the copper growth to optimise its properties is a subject of intense research. “People in industry have known for a long time that you have to have a bit of chloride in the solution to get good films, but nobody really knew why”, Magnussen points out. The new results may finally solve this mystery and help to improve the fabrication processes for Cu wiring in the semiconductor industry.

First author F. Golks adjusting a sample for X-ray studies (Copyright: CAU, photo: J. Stettner).

Text and images courtesy of Kiel University.

Reference
Anomalous potential dependence in homoepitaxial Cu(001) electrodeposition: an in situ surface X-ray diffraction study, F. Golks, J. Stettner, Y. Gründer, K. Krug, J. Zegenhagen, O.M. Magnussen, Physcal Review Letters 108, 256101 (2012).

Top image: Mounting the copper sample in the X-ray experiment (Copyright: CAU, photo: J. Stettner).