When collimated beams of low energy ions are used to bombard materials, the surface often develops a periodic pattern or “ripple” structure. Different types of patterns are observed to develop under different conditions, with characteristic features that depend on the substrate material, the ion beam parameters, and the processing conditions. Because the patterns develop spontaneously, without applying any external mask or template, their formation is the expression of a dynamic balance among fundamental surface kinetic processes, e.g., erosion of material from the surface, ion-induced defect creation, and defect-mediated evolution of the surface morphology. In recent years, a comprehensive picture of the different kinetic mechanisms that control the different types of patterns that form has begun to emerge. In this article, we provide a review of different mechanisms that have been proposed and how they fit together in terms of the kinetic regimes in which they dominate. These are grouped into regions of behavior dominated by the directionality of the ion beam, the crystallinity of the surface, the barriers to surface roughening, and nonlinear effects. In sections devoted to each type of behavior, we relate experimental observations of patterning in these regimes to predictions of continuum models and to computer simulations. A comparison between theory and experiment is used to highlight strengths and weaknesses in our understanding. We also discuss the patterning behavior that falls outside the scope of the current understanding and opportunities for advancement.

Making waves: Kinetic processes controlling surface evolution during low energy ion sputtering

We derive a stochastic nonlinear continuum equation to describe the morphological evolution of amorphous surfaces eroded by ion bombardment. Starting from Sigmunds theory of sputter erosion, we calculate the coefficients appearing in the continuum equation in terms of the physical parameters characterizing the sputtering process. We analyze the morphological features predicted by the continuum theory, comparing them with the experimentally reported morphologies. We show that for short time scales, where the effect of nonlinear terms is negligible, the continuum theory predicts ripple formation. We demonstrate that in addition to relaxation by thermal surface diffusion, the sputtering process can also contribute to the smoothing mechanisms shaping the surface morphology. We explicitly calculate an effective surface diffusion constant characterizing this smoothing effect and show that it is responsible for the low temperature ripple formation observed in various experiments. At long time scales the nonlinear terms dominate the evolution of the surface morphology. The nonlinear terms lead to the stabilization of the ripple wavelength and we show that, depending on the experimental parameters, such as angle of incidence and ion energy, different morphologies can be observed: asymptotically, sputter eroded surfaces could undergo kinetic roughening, or can display novel ordered structures with rotated ripples. Finally, we discuss in detail the existing experimental support for the proposed theory and uncover novel features of the surface morphology and evolution, that could be directly tested experimentally. � 2002 Published by Elsevier Science B.V.

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Morphology of ion-sputtered surfaces

Ion beam erosion can be used as a process for achieving surface smoothing at microscopic length scales and for the preparation of ultrasmooth surfaces, as an alternative to nanostructuring of various surfaces via self-organization. This requires that in the evolution of the surface topography different relaxation mechanisms dominate over the roughening, and smoothing of initially rough surfaces can occur. This contribution focuses on the basic mechanisms as well as potential applications of surface smoothing using low energy ion beams. 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 respect to different process parameters (ion energy, ion incidence angle, erosion time, sample rotation) has been investigated. On the basis of the time evolution of different roughness parameters, the relevant surface relaxation mechanisms responsible for surface smoothing are discussed. In this context, physical constraints as regards the effectiveness of surface smoothing by direct ion bombardment will also be addressed and furthermore ion beam assisted smoothing techniques are introduced. In the second application-orientated part, recent technological developments related to ion beam assisted smoothing of optically relevant surfaces are summarized. It will be demonstrated that smoothing by direct ion bombardment in combination with the use of sacrificial smoothing layers and the utilization of appropriate broad beam ion sources enables the polishing of various technologically important surfaces down to 0.1 nm root mean square roughness level, showing great promise for large area surface processing. Specific examples are given for ion beam smoothing of different optical surfaces, especially for substrates used for advanced optical applications (e.g., in x-ray optics and components for extreme ultraviolet lithography).

http://iopscience.iop.org/article/10.1088/0953-8984/21/22/224026/pdf

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

An overview is given of the possibilities and limitations of secondary ion mass spectrometry as an analytical tool in the investigation of near-perfect, i.e. almost atomically sharp, dopant and impurity distributions. The operating principles of the technique and the various quantification schemes are briefly presented. The most elaborate discussion pertains to the factors that determine the attainable depth resolution and what can be done to improve things, both from an experimental and from a theoretical point of view. Emphasis is placed on semiconductors and other brittle target materials, but the implications for metals are indicated.

https://iopscience.iop.org/article/10.1088/0034-4885/58/10/004/pdf

Ultra shallow doping profiling with SIMS

Surface roughening of Si under low-energy (0.5–2.0 keV) O2+ bombardment at incidence angles between 45° and 80° has been studied. Surface roughening occurred already at an erosion depth of only a few tens of nanometers. It was found that there were distinctly two angular ranges for sub-keV beams where roughening was strong, and two ranges where it was insignificant. The transition between the different ranges can be very sharp. These observations cannot be explained by the current models for surface roughening. Instead, it is believed that it is the combined sputtering rate dependence on both the surface topography and the oxygen content that determines the occurrence of roughening.

The complex formation of ripples during depth profiling of Si with low energy, grazing oxygen beams

The oxygen ion‐beam‐induced ripples observed during depth profiling of the GaAs surface by secondary‐ion mass spectrometry has been studied by scanning tunneling microscopy. Under the O+2 primary‐ion bombardment with energy of 10.5 kV at an incident angle of 37°, ripples of wavelength of 230 nm with the ridge lines perpendicular to the beam direction are formed, and they are grown to ∼80 nm in amplitude on the sputtered surface. Secondary‐ion intensity changes abruptly in accordance with the development of ripples when they reach an amplitude of 30–50 nm. The phenomenon occurs only when the primary beam direction is perpendicular to the ridge orientation. It suggests that the ion yield enhancement is effected by increase of local concentration of oxygen implanted onto the rippled surface. Dependence of the enhancement on sputtered elements also seems to agree with the proposed mechanism.

A study of the secondary‐ion yield change on the GaAs surface caused by the O+2 ion‐beam‐induced rippling

Roughening procedures including the early stage of the O 2 + -induced ripple formation of GaAs were studied quantitatively by scanning tunnelling microscopy (STM). Detailed examinations of the beginning of topography development using fast Fourier transform of the STM images revealed that the ripple formation was not caused by any accidental defects, particles or original irregularity on the substrate, but solely by the conditions of the ion beam. A systematic investigation of the rippled GaAs surface produced under various O 2 + bombardment conditions was conducted. The ripple wavelength and the transition depth were almost exactly proportional to E P cos θ, where E P and θ are the energy of the ion beam and the incident angle, respectively. For GaAs, the secondary ion yield transition occurs when the slope of ripples facing the incident O 2 + beam reaches a saturation angle of 20-30° from the macroscopic surface plane. Topography change on other III-V semiconductors was also examined for comparison. There was no ripple generation observed for GaP; InP gave a ripple-like structure without secondary ion yield change. A relatively rough surface resulted on GaSb and InAs at a much shallower depth than for GaAs. Rippling and ion yield changes during depth profiling have been suppressed successfully by sample rotation even in a magnetic sector-based instrument.

Quantitative investigation of the O2+‐induced topography of GaAs and other III–V semiconductors: An STM study of the ripple formation and suppression of the secondary ion yield change by sample rotation

The Se-based photoconductor is widely used in photoconductive printing. Depth profiling of elements of major components and also low concentration dopants and impurities is desirable to understand the photoconductive behavior of the materials. However, until recently, use of SIMS on chalcogen photocon-ductors has been minimal because of the charging problem encountered in the measurement.

Depth Profiling of Evaporated Se-Te Films with SIMS

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A study of the secondary‐ion yield change on the GaAs surface caused by the O+2 ion‐beam‐induced rippling

Quantitative investigation of the O2+‐induced topography of GaAs and other III–V semiconductors: An STM study of the ripple formation and suppression of the secondary ion yield change by sample rotation

Depth Profiling of Evaporated Se-Te Films with SIMS