Sheath structure and ion current onto a patterned surface immersed in a plasma at floating potential

Using a two-dimensional hybrid simulation scheme, we have studied the processes occurring above a patterned surface immersed in a plasma at floating potential, focussing on the structure of the two-dimensional plasma wall sheath and the uniformity of the ion current onto the wall in steady state. The surface consists of a periodic alternation of a trench and a ridge. We discuss the sheath structure by comparing it to that in front of a planar target. It is found that the sheath is not conformal since the trench widths studied are not large compared to the sheath width. Thus, the electrical potential and the ion and electron densities in the trench show characteristic differences to those of the planar sheath. We show that, in all cases, the ion dose is strongly non-uniform on the target surface. This non-uniformity is traced back to the enhanced electrical fields at the convex corners of the ridge and to the aspect ratio of the surface structure.