Large area smoothing of surfaces by ion bombardment: fundamentals and applications

Abstract

Ion beam erosion can be used as a process for achieving surface smoothing at microscopiclength scales and for the preparation of ultrasmooth surfaces, as an alternative tonanostructuring of various surfaces via self-organization. This requires that in the evolutionof the surface topography different relaxation mechanisms dominate over the roughening,and smoothing of initially rough surfaces can occur. This contribution focuses on the basicmechanisms as well as potential applications of surface smoothing using low energy ionbeams.In the first part, the fundamentals for the smoothing of III/V semiconductors,Si and quartz glass surfaces using low energy ion beams (ion energy:≤2000 eV) are reviewed using examples. The topography evolution of these surfaces with respectto different process parameters (ion energy, ion incidence angle, erosion time, samplerotation) has been investigated. On the basis of the time evolution of differentroughness parameters, the relevant surface relaxation mechanisms responsible forsurface smoothing are discussed. In this context, physical constraints as regardsthe effectiveness of surface smoothing by direct ion bombardment will also beaddressed and furthermore ion beam assisted smoothing techniques are introduced.In the second application-orientated part, recent technological developments related to ionbeam assisted smoothing of optically relevant surfaces are summarized. It willbe demonstrated that smoothing by direct ion bombardment in combinationwith the use of sacrificial smoothing layers and the utilization of appropriatebroad beam ion sources enables the polishing of various technologically importantsurfaces down to 0.1 nm root mean square roughness level, showing great promise forlarge area surface processing. Specific examples are given for ion beam smoothingof different optical surfaces, especially for substrates used for advanced opticalapplications (e.g., in x-ray optics and components for extreme ultraviolet lithography).