A plasma sputtering reactor in which a magnet is linearly scanned over the back of the sputtering target to enhance the sputtering. The magnet's linear scan is extended to beyond the wafer processing area. When the magnet reaches that point, conditions are changed within the reactor to cause particles otherwise trapped by the magnet to fall into an area of the reactor where they do not fall on the substrate being processed. The changed conditions may include extinguishing the plasma, reducing or reversing the target voltage, positively charging walls of the trap area, or pulsing gas through the plasma. Also, according to the invention, the plasma is ignited with the magnet positioned over the trap area so that particles generated in the ignition process are not immediately deposited on the wafer or the walls of the processing area, and they tend to stay in the trap area.