Roughening instability and ion-induced viscous relaxation of SiO2 surfaces

Abstract

We characterize the development of nanometer scale topography (roughness) on SiO2 surfaces as a result of low energy, off-normal ion bombardment, using in situ energy dispersive x-ray reflectivity and atomic force microscopy. Surfaces roughen during sputtering by heavy ions (Xe), with roughness increasing approximately linearly with ion fluence up to 1017 cm−2. A highly coherent ripple structure with wavelength of 30 nm and oriented with the wave vector parallel to the direction of incidence is observed after Xe sputtering at 1 keV. Lower frequency, random texture is also observed. Subsequent light ion (H, He) bombardment smoothens preroughened surfaces. The smoothing kinetics are first order 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 observations which includes roughening both by random stochastic processes and by development of a periodic surface instability due to sputter yield variations with surface curvature which leads to ripple development. Smoothing occurs via ion bombardment induced viscous flow and surface diffusion. From the smoothing kinetics with H and He irradiation we measure the radiation enhanced viscosity of SiO2 and find values on the order of 1–20×1012 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 mechanism accounts for the ripple formation, while inclusion of stochastic roughening produces the random texture and reproduces the observed linear roughening kinetics and the magnitude of the overall roughness.

Surface instability and pattern formation by ion-induced erosion and mass redistribution

“…For Ar on Si with E = 1 keV, I use g S;rad % 1 Â 10 23 Pa ions/cm 2 and for E = 250 eV g S;rad % 4 Â 10 23 Pa ions/cm 2 to obtain ripple wavelengths in agreement with experimental data. For 500 eV Ar on SiO 2 and h = 65°I use g rad % 2 Â 10 23 Pa ions/cm 2 , somewhat smaller than the estimated value g rad % 2:5 À 10 Â 10 23 Pa ions/cm 2 , based on the data of [53,54] and [40]. There are no literature data available on the radiation-induced viscosity of amorphous carbon.…”

“…Depending on the materials' properties and the relaxation processes (e.g. viscous flow or surface diffusion) [39,40] we should replace Eq. 1 by…”

Ion-beam sculpting at nanometre length scales

“…An enhanced collective motion, driven by a reduced surface viscosity and/or enhanced surfaces stress due to implantation effects or surface tension, causes the layer to relax. Such a model has been invoked by Mayer et al 20 for atomic transport involved in sputter induced rippling phenomena in silicon dioxide. Related considerations are discussed by Carter 21 and Brongersma et al 22 .…”

Nanostructures on fused silica surfaces produced by ion beam sputtering with Al co-deposition

“…Before analyzing the nanostructures formed on the Si surface, we compared the chemical components deposited on the smooth SiO 2 and Si by XPS measurements. 2 and Si surfaces upon nanostructure generation will be considered in the following subsection.…”

“…Low-energy ion beam sputtering (IBS) is a powerful bottom-up technology for generating diverse self-organized nanostructures, such as ripples and dots on different materials including amorphous SiO 2 [1][2][3][4][5][6][7], single crystalline Si [8][9][10][11][12], Ge [10,13] and Ag [14], as well as compound semiconductors GaSb [15] and InP [16]; the IBS technology offers the potential to achieve high throughput and fabrication of large areas [10,[17][18][19]. Ion beam parameters (species, incidence angle, energy, flux, etc) and substrate parameters (material, temperature, initial surface topography, etc) interact to generate the features of such nanopatterns.…”