"Atomic-Scale Microscopy Reveals the Mechanism
of Anisotropic Silicon Etching"
Melissa A. Hines, Dept. of Chemistry, Cornell University, Ithaca NY
The production of atomically perfect surfaces by simple solutions is both intrinsically fascinating and technologically important. For over half a century scientists have known that many aqueous bases — so-called “anisotropic etchants” — rapidly etch all silicon faces except one. As a result, a silicon sphere placed into one of these solutions becomes polyhedral. This type of highly precise but inexpensive chemical machining has found use in diverse applications ranging from the production of ink-jet nozzles to the fabrication of nanoscale electronics to the cleaning and polishing of silicon wafers. Twenty years ago, scientists were amazed to find that the etched surfaces are not just smooth, they are atomically flat and passivated by a single monolayer of H atoms. In spite of their technological and scientific importance, the chemical reactions that govern this behavior remain a source of controversy. Using a combination of chemical and morphological data, we resolve this controversy and give the first quantitative, atomic-scale understanding of anisotropic etching across all silicon surface — not just Si(111). More broadly, we show that etchants literally write an atomic-scale record of their reactivity into the etched surface — a record that can be quantitatively decoded into an atom-specific understanding of chemical reactivity using a STM and infrared spectroscopy.
After receiving her Ph.D. in chemistry from Stanford University in 1992, Hines was a postdoctoral staff member in the research division of AT&T Bell Laboratories. Since joining the Cornell faculty in 1994, her research has been aimed at understanding the fundamental mechanisms governing chemical reactions at semiconductor surfaces, inventing new techniques to probe surface reactivity, and using surface chemistry to fabricate novel nanostructures and control the behavior of nanomechanical devices. She has been named a Fellow of the American Association for the Advancement of Science, a Beckman Young Investigator, a Lily Teaching Fellow, and a Cottrell Scholar. She is also the recipient of a NSF Career Award and the Stephen and Margaret Russell Distinguished Teaching Award. She became the Director of the Cornell Center for Materials Research in the Fall of 2005.