Angle of Incidence Effects in Ion Beam Processing

Abstract

ABSTRACTThe angle of incidence of ion bombardment is an important processing parameter, which can strongly affect the shape, composition and microstructure of bombarded surfaces. We describe several phenomena directly related to the angle of ion incidence during ion beam etching and ion beam assisted deposition. First, the development of surface ripple topography during ion beam etching is modeled. Surface perturbations are shown to grow under ion bombardment, while surface selfdiffusion acts to select a characteristic wavelength. The orientation of these characteristic ripples changes by 90° as the angle of ion incidence is varied from near-normal to near-glancing angle. The second example is the effect of angle of incidence on the etching rate of Ta under mixed Ar-O2 ion bombardment. For pure Ar bombardment, the sputtering yield of Ta increases with angle of ion incidence slower than secθ, producing a maximum etch rate at normal incidence. Above a critical pressure of O2, however, the yield increases faster than secθ dependence, producing a maximum etch rate at a non-normal angle of incidence. The third example is the effect of angle of incidence on the preferential sputtering of Al relative to Cu in Al-5% Cu thin films. Films deposited by evaporation with simultaneous Ar ion bombardment at 500 eV show a depletion of Al relative to Cu. This composition change is enhanced by increasing the angle of incidence away from normal, resulting in a higher Cu concentration in a film deposited on a tilted surface. Finally, a mechanism is described for the generation of oriented microstructure in films deposited under simultaneous glancing-angle ion bombardment, demonstrated previously for Nb. Grain orientations are selected which allow channelling of the ion beam. These results show that the shape, composition and microstructure of films deposited under ion bombardment respond to changes in angle of incidence, and that these effects need further study and modeling.