Nanoscale Morphology Evolution Under Ion Irradiation

Overview

Many distinct morphologies self-organize on surfaces under uniform energetic ion irradiation (Fig. 1), including ripples, pits, hillocks, and ultrasmooth surfaces.  The length scales of these structures can range from hundreds of nanometers down to 7 nm (Wei et al., Chem. Phys. Lett. 452, 124 (2008)), depending on the material and system parameters.

Fig 1. Nanopatterns self-organized under ion irradiation.  A range of materials, pattern types, length scales, and degree of ordering can be seen.  Image courtesy of Dr. Bashkim Ziberi.

Due to the high degree of ordering in some of these patterns, and because there is in principle no limit to the size of a uniform ion beam, ion bombardment holds promise in the high-throughput fabrication of simple devices.  Metamaterial devices, which generally feature an array of repeating, simple structures, could be particularly well-suited to this technique.  Examples of metamaterial devices include optical antenna arrays (Fig. 2), and the split ring resonators used in negative refractive index materials and optical cloaking.

Fig 2. An optical antenna array, a metamaterial that could be quickly self-organized, given enough understanding and control over the processes involved in ion bombardment.  The length scale and simplicity of the patterns makes this structure a good candidate for the technique. Image from C. Rockstuhl et al., Optics Express 14, 8827 (2006).

Our research focuses on understanding the fundamental mechanisms involved in nanoscale morphology evolution under ion irradiation in order to ultimately control it as a processing tool.  We have conducted grazing-incidence small-angle x-ray spectroscopy (GISAXS) in order to study realtime, in-situ morphology evolution.  The resulting data have allowed comparison between various models and experiment, allowing us to prove the importance of mass redistribution as a driving effect for pattern formation [Ref. mja202].

We have also collaborated with Professor Scott Norris (Southern Methodist University) and Professor Kai Nordlund's group (University of Helsinki) to develop and test a crater function theory, which uses the results of parameter-free molecular dynamics simulations to predict surface stability/instability to pattern formation, predict wavelength coarsening of the resulting patterns, and probe the underlying mechanisms driving the process [Refs. mja192, mja200].

Relevant Publications:

mja202. C.S. Madi, E. Anzenberg, K.F. Ludwig, Jr., M.J. Aziz, "Mass Redistribution Causes the Structural Richness of Ion-Irradiated Surfaces", PRL 106 (2011) 066101.  In this paper, we show that the erosion-based mechanism of pattern formation that the community accepted for a quarter century doesn't explain the phase diagram for the Ar+/Si system, but mass redistribution explains it well instead.  Click here to see the paper.

mja193. C.S. Madi, H.B. George, M.J. Aziz, "Linear stability and instability patterns in ion-sputtered silicon", J. Phys.: Condens. Matter 21 (2009) 224010.  This paper presents an energy/angle phase diagram of Ar+/Si at energies below 1.1 keV.  However, a later paper [3] showed that the pattern formation in the low-energy, low-angle corner was actually formed by multiple scattering.  Click here to see the paper.

mja214. C.S. Madi, M.J. Aziz, "Multiple Scattering Causes the Low Energy-Low Angle Constant Wavelength Topographical Instability of Argon Ion Bombarded Silicon Surfaces", Appl. Surf. Sci. 258 (2012) 4112.  In this paper, we show that the pattern formation in the low-energy, low-angle corner of the Ar+/Si phase diagram reported in [2] is actually formed by multiple scattering, thus demonstrating that artifacts can form even in systems free of chemical impurities.  Click here to see the paper.

mja192. S.A. Norris, M.P. Brenner, M.J. Aziz, "From crater functions to partial differential equations: a new approach to ion bombardment induced nonequilibrium pattern formation", J. Phys. Condens. Matter 21 (2009) 224017.  Click here to see the paper.

mja200. S.A. Norris, J. Samela, L. Bukonte, M. Backman, F. Djurabekova, K. Nordlund, C.S. Madi, M.P. Brenner, M.J. Aziz, "Molecular dynamics of single-particle impacts predicts phase diagrams for large scale pattern formation", Nature Comm. 2 (2011) 276.  Click here to see the paper.

mja228. J.C. Perkinson, C.S. Madi, M.J. Aziz, "Nanoscale topographic pattern formation on Kr+-bombarded germanium surfaces", 31 (2013) 021405.  Click here to see the paper.

mja229. E.A. Anzenberg, J.C. Perkinson, C.S. Madi, M.J. Aziz, K.F. Ludwig, Jr., "Nanoscale surface pattern formation kinetics on germanium irradiated by Kr+ ions", PRB 86 (2012) 245412.  Click here to see the paper.