In one embodiment, an apparatus to treat a substrate may include an extraction plate to extract a plasma beam from a plasma chamber and direct the plasma beam to the substrate. The plasma beam may comprise ions forming a non-zero angle of incidence with respect to a perpendicular to a plane of the substrate; and a gas outlet system disposed outside the plasma chamber, the gas outlet system coupled to a gas source and arranged to deliver to the substrate a reactive gas received from the gas source, wherein the reactive gas does not pass through the plasma chamber.