A magnetron sputtering source which is capable of very low pressure operation is disclosed. The source comprises a dish-shaped sputter target behind which is mounted a primary magnet for confining a plasma discharge adjacent to the front surface of the sputter target. A bucking magnet, positioned adjacent to the perimeter of the sputter target and, preferably, at the same level as the target, is used to prevent the magnetic field created by the primary magnet from spreading out at near the edge of the sputter target. This enables the source to operate at very low pressure and reduces the impedance of the discharge.