The transient sheath expansion around square and cross-shaped targets
is simulated numerically with a two-dimensional fluid model. The angul
ar distribution of the ions impinging on the target surface and the no
nuniformity of the incident ion dose are calculated. The incident ion
dose peaks near, but not at, the convex corner and has a minimum at th
e concave corner. The dip of the dose profile at the convex corner is
shown to be caused by the product of a decreasing normal velocity prof
ile and an increasing ion density profile along the target surface fro
m the center to the corner. (C) 1995 American Institute of Physics.