IUVSTA Highlight Seminar

The 1991 Binnig, Rohrer et. al. Physical Review Letter on STM imaging of silicon kicked-started the present era with emphasis on the science and technology of nanometer structures. Sixteen years later, the science base has expanded dramatically. Our ability to analyze and fabricate nanometer sized structures has expanded in scope and sophistication.

We are now approaching the time frame associated with the development of new products from science innovation. Does the frequently used word nanotechnology mean very small amounts of technology, or is there evidence for imminent technology based on nanometer structures?

In my opinion, the science of nanometer structures will have the most dramatic impact in three areas: electronics / optoelectronics; molecular biology, biotechnology and medicine; and materials affordability. Let me show evidence for imminent technology for the first two by citing examples taken largely from the Naval Research Laboratory and the Office of Naval Research..

Electronic / optoelectronic devices have depended on nanometer dimensions for over a decade - thin films and superlattice structures require dimensional tolerance perpendicular to the interface at the nanometer and, increasingly, atomic level. One straightforward, but important, technology contribution has come from the proximal probes which enabled the visualization of surface topographies. Near real time imaging can lead to spectacular improvements in the nucleation/growth of semiconductor surfaces; at NRL we have demonstrated this for the growth of InAs on GaAs. But the power of STM/S goes beyond establishing topology. In a combined experimental and theoretical tour de force, Whitman et al (1) have established guidelines for understanding the vicinal surface structures on silicon crystals. The surface reconstructions are influenced by a delicate balance between the energies associated with dangling bonds and with strained bond angles/separations. The pristine surface reconstructions for planes between Si (001) and Si (111) are now reasonably well understood, and work-to-date on the effects of hydrogen chemisorption on those reconstructions shows no surprises. There are significant technology ramifications to this science because heteroepitaxy plays an important part in evolving electronic devices; the effect of substrate structure on the growth process is crucial.

Lyding and Hess (2) have contributed another technologically important piece of science involving H and silicon. Lyding, among several others, has demonstrated that a proximal probe can be desorb H from a H compensated Si (100) surface with near atomic precision. While investigating the desorption mechanism, he noted D desorption was far more retarded than expected from simple models of mass effects. Hess and Lyding realized that this observation could have dramatic impact on MOS devices. Hydrogen is presently diffused into the Si / SiO interface of MOS structures to tie off trap states which degrade the device performance characteristics. A principle failure mechanism for the devices has been electron induced disruption of those Si-H bonds. The substitution of D for H has been shown to dramatically improve the lifetime of those devices. This improvement in reliability allows the device designer to reduce the device dimensions; the result will be more rapid progress toward higher clock frequencies.

Three dimensional nanostructured electronics is still largely a dream. However, if continued miniaturization of electronic devices is to continue beyond the year 2015, this dream must be converted into a reality. Proximal probes have demonstrated the ability to fabricate structures atom-by-atom. The timeframe for a entire 10 cm semiconductor wafer surface fabrication, atom-by-atom, would be centuries - a bit long for most commercial markets. A single proximal probe suffers from this constraint of serial processing. However, proximal probes can be microfabricated and Quate (3) has now constructed 50 individually addressable proximal probes on a single Si wafer. In a demonstration for the NANO IV conference in China, his group wrote out that word using two separate tips for the first and last three letters.

Membrane functionality, the molecular recognition basis of immunoresponse, DNA and protein structure - all involve nanometer structures. It is on this basis I contend that biotechnology and medicine will be dramatically impacted by nanoscience. A particularly illustrative example is found in the work of Lee (4) who has used proximal probes to measure the forces associated with disruption of single molecule antibody/antigen bonds and pairing between complementary DNA strands. While this work has provided key insights which complement the traditional thermodynamic approaches to macromolecular folding , it also has stimulated what might become a revolution in immunoassay technology (5). Immunoassays have coupled the selectivity associated with the molecular recognition and the sensitivity associated with either radio tracer or fluorescent detection. The proximal probe provides a microfabricated sensor which is capable of detecting a single event. Lee has modified the proximal probe approach into a more robust sensing technology which has already demonstrated an order of magnitude more sensitivity than the present technologies.

1. A. A. Baski, S.C. Erwin and L.J. Whitman, submitted to Surface Science.

2. "Deuterium Processing Stabilizes Silicon Semiconductor Devices," Chemical and Engineering News, March 18. 1996, page 25.

3. S. C. Minne, S. R. Manalis, A. Atalar and C. F. Quate, JVSTB14, 2456 (1996).

4. G.U. Lee, L.A. Chrisey and R.J. Colton, Science 266, 771 (1994); "Getting Physical with DNA," Science News 151, 256 (1997).

5. D.R. Baselt, G.U. Lee, K.M. Hansen, L.A. Chrisey, and R.J. Colton, Proceedings of the IEEE 85, 672 (1997).

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