THICKNESS DEVIATIONS OF FILMS DEPOSITED BY ION-BEAM-ASSISTED DEPOSITION UNDER OFF-NORMAL ION INCIDENCE

Re-sputtering of already deposited material is a side effect of ion bombardment during film growth by ion beam assisted deposition (IBAD). This mostly undesirable effect leads to a reduction in film growth velocity. Because of this dynamic ion etching the actually deposited mass does not correspond to the evaporated mass as determined with a quartz crystal. This loss has to be accounted for by an increase in process time when a particular film thickness is required. The situation may become critical when the ion impact angle deviates from the surface normal, i.e. either when a plane substrate has to be tilted or when curved surfaces have to be coated. In the present study the deviations in film thickness are discussed for offnormal ion incidence IBAD. A mathematical expression is given which describes the dependence of the sputtering yield on the impact angle by fitting to experimentally obtained data. It is applied to calculate the dependence of film thickness deviations on the particle impact angle. As an example for coating components with a curved surface, the data for cylinders and spheres were calculated. Lengthening factors are listed for different IBAD conditions such as sputtering yield and arrival ratio of atoms and ions, which determine the excess in process time or in evaporated material necessary to coat a cylinder homogeneously with a film of a desired thickness.