Abstract: Sputtering apparatus and method employing an auxiliary magnetic structure situated between the substrate holder and target of a plasma sputtering chamber to control-the lateral extent of the plasma. The auxiliary magnetic structure, possessing a lower field strength in the plasma region than the principal magnet or magnets, is situated immediately outside of and around a circumference of the chamber's anode shield. The principal magnets maintain the plasma in a ring adjacent to the sputtering target. The auxiliary magnetic structure causes the plasma ring to expand toward the edge of the target or contract away from the edge depending on the magnetic strength and polarity of the structure and its position relative to the target.