The multi-aperture hollow target: a method for determining ion incidence angles in plasma immersion ion implantation

As in plasma immersion ion implantation ions are accelerated from a spherical plasma sheath edge which surrounds the target, a distribution of angles of ion incidence can be expected on a non-spherical target. The angle of ion incidence is of importance, as it determines the projected range of ions in the solid, their depth distribution and also the saturation dose, which is given by sputter limitations. In the present contribution, a simple but very effective method for determining the angles of ion incidence is described. A hollow cylindrical target with an upper cover plate with small drill holes is used. Ions from the plasma enter the target through these apertures and impinge onto the inner bottom where a cellulose sheet is placed which is blackened. By optically determining the positions of ion incidence on the bottom with respect to the positions of the aperture, the angles under which the ions enter the aperture, i.e. the angle of ion incidence on the plate, can be calculated. The obtained data show that in the center of the cylindrical target the ions impinge at right angles whereas in the outer parts smaller angles of incidence are found. The distribution of angles is given as a function of the process parameters such as pulse voltage and pulse duration.