In this kind of sputtering apparatus there is provided a magnet assembly for forming a tunnel-shaped magnetic flux in front of the target (on the side of the surface to be sputtered). At the time of sputtering the target by charging negative DC voltage or AC voltage to the target, the electrons ionized in front of the target or the secondary electrons generated by the sputtering are arrested to thereby enhance the electron density in front of the target. By thus increasing the probability of collision of these electrons with the gas molecules of an inert gas to be introduced into a vacuum chamber, the plasma density is increased. As a result, there is an advantage in that the film forming speed can be improved without bringing about a remarkable increase in temperature in, e.g., the process substrate, and the like. The art is often used in forming a metallic film and the like on the process substrate.
As the magnet assembly to be assembled into this kind of sputtering apparatus, there is known one which is constituted by: a supporting plate (yoke) which is provided in parallel with the target; a central magnet which is disposed substantially in the upper center of the supporting plate along the longitudinal direction thereof and a peripheral magnet which is disposed so as to enclose the central magnet while changing the polarity on the target side (Patent Document 1).    Patent Document 1: JP-A-2004-115841 (see, e.g., the column of Prior Art)