Educational Background:

PhD, Stanford University, 1991 SB, Massachusetts Institute of Technology, 1984

Awards:

• Beckman Young Investigator • Cottrell Scholar • Lilly Teaching Fellow • NSF CAREER award

Research Description:

For reasons that are not completely understood, the electronic properties of nanoscale devices are often strongly influenced by their interface morphologies. For example, atomic-scale roughness can decrease the performance of transistors by a factor of 4!

In principle, a wide array of morphologies can be produced from chemical reactions that occur far from equilibrium; however, the evolution of surface morphology during etching is poorly understood. In general, our view of chemical reactivity is based on local, atomic-scale considerations, such as steric hindrance, relative electronegativity and structural rigidity, that have an effective range of no more than a few Ångstroms. In other words, surface chemistry is site-specific. Surface morphology, on the other hand, develops over the course of many such reactions - tens or hundreds of monolayers may need to be removed before the surface reaches steady state. Is the steady-state morphology, with its potentially long length scales, a simple consequence of many site-specific reactions, or do more subtle perturbations come to dominate the final surface? More importantly, can we tailor surface chemical reactions to produce surfaces of arbitrary morphology?

In our research, we are trying to answer questions such as these by combining experimental measurements of surface morphology, obtained with a scanning tunneling microscope (STM), with atomistic, kinetic Monte Carlo simulations of the etching process. Using these techniques, we have been able to extract microscopic information on surface reaction mechanisms. Over the past few years, we have investigated the production of ultra-flat surfaces, the chemical origins of hillock formation, and the effects of chemical additives.

Aqueous etching often has a dramatic effect on surface morphology. For example, the left panel of Figure 1 shows a STM image of a Si(111) wafer. By eye, this wafer had a perfect mirror finish. On an atomic scale, though, the wafer is very rough. Although each step is only a single atomic layer high, many atomic layers are exposed on this sample. In contrast, the right panel shows a similar surface that has been etched in an aqueous ammonium fluoride solution. During etching, all of the hills and valleys seen in the first image were removed, leaving behind a surface of near-perfection. Using computer simulations, we have been able to (quantitatively) show that ammonium fluoride selectively etches kink sites (i.e. corner sites on the steps). In effect, the steps "unzip" in a direction parallel to the step edge.

For a more up-to-date account of our group, please visit our home page.

Selected Publications:

Hines, M.A. In search of perfection: Understanding the highly defect selective chemistry of anisotropic etching. Ann. Rev. of Phys. Chem. 2003, 54, 29.

Garcia, S. P.; Bao, H.; Hines, M.A. Etchant anisotropy controls the step bunching instability in KOH etching of silicon. Phys. Rev. Lett. 2004, 93, 166102-1-4.

Wang, Y.; Henry, J. A.; Sengupta, D.; Hines, M.A. Methyl monolayers suppress mechanical energy dissipation in micromechanical silicon resonators. Appl. Phys. Lett. 2004, 85, 5736-8.

Wang, Y.; Henry, J. A.; Zehnder, A.T.; Hines, M.A. Surface chemical control of mechanical energy dissipation in micromachined silicon devices. J. Phys. Chem. B 2003, 107, 14270-77.

Garcia, S. P.; Bao, H.; Hines, M.A. Understanding the pH dependence of silicon etching: The importance of dissolved oxygen in buffered HF etchants. Surf. Sci. 2003, 541, 252.