A substrate or semiconductor wafer processing apparatus has an agitator plate adjacent to an upper end of a process vessel. A workpiece holder holds a workpiece in the vessel at a processing position above the agitator plate. A vertical actuator assembly supporting the agitator plate oscillates the agitator plate vertically. The agitator plate may also rotate while oscillating vertically. In one design, the agitator plate has a spiral vane and a spiral slot. In a related metal plating apparatus, electrodes and a dielectric field shaping unit are in the vessel, below the agitator plate, and a shield ring is adjacent to the upper end of the vessel, above the agitator plate.