G. W. Lewis

The mechanisms of etch pit and ripple structure formation on ion bombarded Si and other amorphous solids

Abstract Quasi-dynamic studies of the ion bombardment induced surface morphological changes on Si single crystals substrates have been made using a miniature ion accelerator located in the target chamber of an SEM. 10 keV Ar+ ions were employed over the fluence range 1016–1020 cm−2 and following incremental increases in fluence, direct observations of individual surface features were made. For high incidence angles to the normal the first features to appear were etch pits which, with increasing fluence interact to produce corrugated facet structures and ripples. Feature development is interpreted in terms of preferential sputtering at native or irradiation induced defect complexes, which initiate etch pits.

The development of sputter-induced pits and pyramids on ion bombarded (11 3 1) surfaces of face centred cubic metal single crystals

Abstract It is shown that energetic bombardment of (11 3 1) surfaces of copper single crystals with either Ne + , Ar + , Cu + , Kr + or Xe + ions results in the formation of regular geometric sputter-etch pits from which regular pyramids emerge during the subsequent pit enlargement. Similarly, energetic Ar + bombardment of (11 3 1) surfaces of single crystal nickel, palladium, silver, iridium and gold produces regular pits and pyramids. It is shown that, within certain parameters of ion bombardment, crystallography is the all important condition necessary for production of pits and pyramids. The presence of impurity is unnecessary.

The effect of ion species on morphological structure development of ion bombarded (1131) single crystal Cu

Abstract Carefully prepared samples of (1131) oriented Cu single crystals were bombarded with 40 keV ions to fluences of the order of 10 19 cm −2 . The ion species chosen were Ne, Ar, Kr, Xe, N and Cu, and thus represented a range of light to heavy, chemically inert species, a more chemically active species and the self ion species. Post-irradiation observation was performed by scanning electron microscopy at Copenhagen and subsequently at Salford. The results may be summarized as follows: 1. (1) All ion species lead to characteristic morphological surface changes including etch pit, and frequently, pyramid generation. 2. (2) The lighter ion species tend to produce lower pit densities and less well-defined pit habits and fewer, sometimes no pyramids. 3. (3) The heavier ions (including Cu + ) produce denser arrays of all features and better geometrically defined features. 4. (4) N + , Ne + and Ar + ions produce a zone of surface blisters outside but surrounding the irradiated area but none within the irradiated area. 5. (5) Some signs of pyramid bending occur from one to a subsequent examination but there are no clear correlations with ion species and surface zone. 6. (6) The as-formed pyramids are in dense crystallographic array after Cu, Kr and Xe irradiation. These results are explained in terms of preferential sputtering of native and irradiation induced defect structures and differential atomic mobility of different ion species in the Cu. The Cu + irradiation results exhibit the lack of necessity of occluded gas for feature development whilst the N + results indicate that, for Cu, improved impurity depth profiling accuracy by sputter sectioning with such a chemically active species is unlikely.

The effect of incidence angle on ion bombardment induced surface topography development on single crystal copper

Abstract The fluence dependence of development of microscopic surface features, particularly etch pits, during 9 keV Ar + ion bombardment of (11, 3, 1) oriented Cu single crystals has been studied employing quasi-dynamic irradiation and observation techniques in a scanning electron microscope-accelerator system. 9 keV ions are observed not to produce crystallographic pyramids under all irradiation conditions for this surface, a very different result from our earlier studies with higher energy ions. The bombardment does elaborate etch pits however, the habits and growth kinetics of which depend upon both polar and azimuthal angles of ion incidence to the surface. The results are explained in terms of differential erosion of crystal planes modified by the presence of pre-existing and irradiation induced extended defects.

Sputtering erosion of stratified media and by time dependent ion bombardment

Abstract The theory of the evolution of surface morphology resulting from ion bombardment induced sputter erosion (and other processes) is generalized to include description of spatio-temporally variable ion flux, and substrate inhomogeneity. This allows discussion of effects associated with rotated or translated ion beams and substrates and of striated or stratified substrates such as occur in polycrystals or in the neighbourhood of extended defects or impurities in single crystals. Geometric reconstructions of several two-dimensional surface sections are undertaken in order to illustrate the analytical methods.

Dynamic study of ion etching in a high resolution SEM

On-line studies of surface topographical development have been made by mounting a saddle-field ion source into a standard scanning electron microscope. Preliminary results obtained during operation in both dynamic and static modes are presented.

The effect of ion species on bombardment induced topography during ion etching of silicon

Abstract The topography induced by 7 keV particle bombardment of Si substrates has been measured dynamically as a function of particle fluence with a B11 source coupled into a scanning electron microscope. Distinctive differences are observed according to particle species used (Ar, Kr or O) and flux incidence angle to the surface. Strategies for morphology development minimization are discussed and the relevance of the study to sputter profiling and lithographic etching outlined.

Topography evolution in sputtered stratified media and in spatio-time variable ion flux conditions

Abstract Earlier theoretical studies on the development of topography on solids sputtered by spatially uniform and spatially non-uniform ion fluxes are extended to a general treatment including spatially non-uniform (e.g. stratified) solids and time varying ion fluxes. It is shown that the former case is relevant to the sputter erosion of, for example, polycrystalline media, dislocated solids and surface contaminant protection situations. Predictions of feature development in these three cases is shown to correspond very well with experimental studies, and offers convincing explanation of the different etch pit elaboration processes, associated with dislocations, depending upon incident ion beam and rotated target situations and the relevance of the theory to practical situations of (1) substrate thinning and polishing and (2) controlled surface contour evolution are outlined. Some simulation studies with sand blasting, which is, in many ways, a macroscopic analogue of ion bombardment sputter erosion are presented also.

The formation of striations on oblique boundaries during sputtering

Abstract The results of studies of the production of striations on boundaries inclined obliquely to incident sputtering ion flux are described. It is shown that two processes probably operate, one resulting from local shadowing of the substrate from ion flux by protection layers, the other resulting from the complex nature of the sputtering process in damaged crystalline solids.

The development of surface morphology by sputter etching of stationary and moving substrates

Abstract The development of surface features by ion bombardment induced or enhanced sputter etching may be treated, theoretically, as a special case of a wavefront propagating in an inhomogeneous, anisotropic medium. This approach is employed to extend our earlier studies of sputter etching induced surface morphological changes to cases where either or both the radiation flux or the substrate are moving, as would occur with ion beam pattern writing or substrate polishing and thinning. It is shown, rigorously, that incident flux and substrate motion are reciprocal processes and that the resulting morphological development is best treated in a framework moving with the ion flux or substrate. Special attention is paid to the role of spatially limited ion flux (ion beam writing) situations since they represent a class of surface discontinuity propagation phenomena which, although time dependent, are anologous to etching situations at media interfaces (e.g. resist on substrate).