Anisotropic scaling of ion-induced ripple morphologies on Si

It is well known that oblique low and medium energy (typically 0.1 - 100 keV) ion erosion of solid surfaces can lead to the formation of periodic ripple patterns with wavelengths ranging from 10 to 1000 nm. The ripples produced in this way are oriented either parallel or normal to the projection of the ion beam and their wavelength scales with ion energy. These structures were found on a large variety of materials, such as semiconductors, metals, and insulating surfaces [1].

The formation of the ripple patterns can be qualitatively explained by continuum models like the Bradley-Harper model [2] which attributes the formation of regular patterns as resulting from the interplay between roughening due to sputter erosion and smoothing by surface diffusion. A surface instability is induced by the curvature dependence of the sputter yield, as the local erosion rate is higher in depressions than on elevations. This leads to an amplification of initial surface modulations and, therefore, to roughening of the surface. The resulting linear continuum equation is able to reproduce the formation and early evolution of the ripple patterns [2]. However, at longer times nonlinear terms have to be taken into account, leading to nonlinear models based on the Kuramoto-Sivashinsky equation [3].

In this work, Grazing Incidence Small Angle X-ray Scattering (GISAXS) has been used to study the evolution of Si(100) surfaces in-situ during oblique sub-keV ion sputtering. The sputtering has been performed with 500 eV Ar ions at incident angles ranging from 65° to 70°. The observed surface morphologies are dominated by nanoscale ripple patterns at short lateral distances but exhibit kinetic roughening at larger scales. The dynamic scaling properties of the surfaces have been analyzed by evaluating the one-dimensional power spectral density (PSD) functions in the direction normal and parallel to the ripples, respectively, which are extracted from the recorded scattering diagrams. A transition from anisotropic to isotropic scaling is observed with increasing incident angle. In addition, the ripples exhibit wavelength coarsening which is only slightly affected by the angle of incidence.

REFERENCES

[1] Chan, W.L. and Chason, E. Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering, J. Appl. Phys. 101, 121301 (2007).
[2] Bradley, R.M. and Harper, J.M.E. Theory of ripple topography induced by ion bombardment, J. Vac. Sci. Technol. A 5, 2390 (1988).
[3] Makeev, M.A., Cuerno, R. and Barabási, A.-L. Morphology of ion-sputtered surfaces, Nucl. Inst. Meth. Phys. Res. B 197, 185 (2002).