ROUGHENING INSTABILITY AND ION-INDUCED VISCOUS RELAXATION OF SIO2 SURFACES

We characterize the development of nanometer scale topography (roughne ss) on SiO2 surfaces as a result of low energy, off-normal ion bombard ment, using in situ energy dispersive x-ray reflectivity and atomic fo rce microscopy. Surfaces roughen during sputtering by heavy ions (Xe), with roughness increasing approximately linearly with ion fluence up to 10(17) cm-2. A highly coherent ripple structure with wavelength of 30 nm and oriented with the wave vector parallel to the direction of i ncidence is observed after Xe sputtering at 1 keV. Lower frequency, ra ndom texture is also observed. Subsequent light ion (H, He) bombardmen t smoothens preroughened surfaces. The smoothing kinetics are first or der with ion fluence and strongly dependent on ion energy in the range 0.2-1 eV. We present a linear model to account for the experimental o bservations which includes roughening both by random stochastic proces ses and by development of a periodic surface instability due to sputte r yield variations with surface curvature which leads to ripple develo pment. Smoothing occurs via ion bombardment induced viscous flow and s urface diffusion. From the smoothing kinetics with H and He irradiatio n we measure the radiation enhanced ViScoSitY of SiO2 and find values on the order of 1-20 X 10(12) N s m-2. The viscous relaxation per ion scales as the square root of the ion induced displacements in the film over the range of the ion penetration, suggesting short-lived defects with a bimolecular annihilation mechanism. The surface instability me chanism accounts for the ripple formation, while inclusion of stochast ic roughening produces the random texture and reproduces the observed linear roughening kinetics and the magnitude of the overall roughness.