A method of forming a semiconductor device includes forming a photoresist over a target layer, where the target layer includes a substrate. The photoresist is patterned to form a patterned photoresist. Scum remains between portions of the patterned photoresist. The substrate is tilted relative to a direction of propagation of an ion beam. An ion treatment is performed on the scum. A pattern of the patterned photoresist is transferred to the target layer.