M. J. Nobes

Analytical modelling of sputter induced surface morphology

Abstract A review is presented of the mathematical treatment of the development of surface topography of amorphous materials bombarded by an energetic beam of particles. In the first section the relevant analytical equations are presented and extended to include surface diffusion. In general the complete analytical approach is shown to be intractable, but in the second section some of the equations are adapted to suit computer programmes which allow calculation of the development of a surface contour provided the original contour and the functional relationship between sputtering coefficient and angle (S(θ)) are specified. Finally, geometrical construction techniques, which allow the topographical development to be predicted, are discussed and a new cursor, which evolves from the appropriate S,θ relationship, is shown to be significantly simpler to use than the erosion slowness curve when making a geometrical construction of the evolving surface.

The growth of topography during sputtering of amorphous solids

Theories of topographic surface development during ion bombardment fall broadly into two categories, one based upon erosion of intersecting planes [1] and the other on a point by point erosion basis [2–4]. A recent paper by Barber et al [5] has shown how, in semiquantitative fashion, an earlier theory by Frank [6] on chemical dissolution or growth of crystals, can be developed to encompass the ion sputtering case. This theory itself is based upon the kinematic wave equation outlined by Lighthill and Whitham [7, 8] and applies to problems of river flooding [7] and traffic flow [8]. In the present communication, the earlier topographic development theories are shown to fit precisely and analytically into the Frank development of the kinematic wave treatment and it is also shown how the occurrence of sharp angled cones formed on surfaces can be analytically and unequivocally predicted.

EQUILIBRIUM TOPOGRAPHY OF SPUTTERED AMORPHOUS SOLIDS. II.

The sputtering of an amorphous solid is considered analytically and the equations of motion of the changing surface topography derived.

The development of cones and associated features on ion bombarded copper

Abstract Observations of ion-bombardment-induced surface modifications on crystalline copper substrates have been made using scanning electron microscopy. The delineation and development of grain boundary edges, faceted and terraced etch pits and small-scale ripple structure, together with the formation of faceted conical features, have all been observed on low and high purity polycrystalline substrates. In general, the density of such surface morphological features, although variable from grain to grain, is higher in the proximity of grain boundaries. In particular, cones are only found within regions where other surface erosional features are present and it would appear that the development of these other surface features is a pre-requisite to cone generation in high-purity crystalline substrates. We suggest the operation of a defect-induced mechanism of cone formation whereby sputter elaboration of bulk defects (either pre-existing or bombardment-induced) leads to the formation and development of surfa...

Ion bombardment induced ripple topography on amorphous solids

Abstract Earlier studies of the ion bombardment induced ripple morphology on the surfaces of amorphous solids when compared with geomorphological effects are shown to possess many similar features. The present study, with 40 keV Ar+ ion bombarded Si suggests that analogies are incomplete, however, and that greater similarities with the process of macroscopic sandblasting (corrasion) exist. It is shown that the genesis of wave like structures on Si is from isolated features, which have the appearance of ripple trains, which are faceted. It is suggested that these features result from particle flux enhancement processes near surface dimples generated by stress induced surface lifting.

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.

Theoretical assessments of major physical processes involved in the depth resolution in sputter profiling

Abstract Two of the more important physical processes which militate against high depth resolution capabilities for ion-induced sputter sectioning associated with compositional analysis techniques are surface topography development and recoil atomic mixing. This review describes earlier, simplistic, theoretical modelling of such processes and describes new approaches based upon empirical evidence of the nature and magnitude of these processes. It is shown that, in general, the depth resolution of the sputtering technique will be a complex function of the depth probed and that both “broadenings” and “shifts” in depth evaluation are to be expected.

The mechanism of ripple generation on sandblasted ductile solids

Abstract Studies of the erosion rate-incidence angle function and the characteristic parameters of wavelength, amplitude, facet angle and velocity of well-developed ripple patterns on sandblasted copper are reported. A microscope investigation of the morphology of individual impact craters and their integration to form initially a random collection of depressions and ridges and gradually a phase-locked repetitive ripple pattern is also presented. Both areas of study suggest that earlier theories of the erosion of ductile solids are open to doubt, and a new model of surface morphology generation based upon the production of embrittled ridges and wave crests which are induced by plastic flow and ablated by impact is proposed.

Sputtering induced topography development on f.c.c. metals

The generation and development of surface features by ion bombardment induced sputtering erosion of f.c.c. metals is reviewed. Studies with polycrystalline substrates reveal the plethora of individual features, both etch pits and cones and the repetitive features, such as ripple trains, which form differently on different crystallite surfaces. Studies with well defined single crystals clarify the origins of such features and the relative independence of their habit on ion species and substrate material.

A second-order erosion slowness theory of the development of surface topography by ion-induced sputtering

The spatial variation of energy deposited in a solid can lead to local variations in sputtering yield at points on the surface neighbouring the point of ion impact. An approximate theory is developed to describe this local sputtering yield variation in terms of the local morphology. It is then shown how, if this local variation merely moderates the standard sputtering yield-projectile incidence angle function by multiplication, an erosion slowness theory can be simply modified and generalized to allow prediction of the time development of sputtered surface morphology. Both transient and steady-state morphologies are explored.