Adjustable return path magnet assembly and methods

A sputter deposition assembly comprising a sputtering chamber, a sputtering target, and a magnet assembly, the magnet assembly being mounted adjacent to a rear surface of the sputtering target, the magnet assembly comprising a magnetic backing plate and spaced-apart first and second permanent magnets, the first and second permanent magnets each having a proximal end and a distal end, the distal end being further from the sputtering target than is the proximal end, the magnet assembly creating a magnetic field comprising field lines that extend from the proximal end of the first magnet, through the sputtering target, along an arc located in front of the sputtering target, back through the sputtering target, to the proximal end of the second permanent magnet, through the second permanent magnet, along a return path, and to the distal end of the first permanent magnet, the return path passing through the magnetic backing plate of the magnet assembly, the magnetic backing plate being a plate-shaped wall formed of ferromagnetic or ferrimagnetic material, the plate-shaped wall extending between the first and second permanent magnets and defining a blind recess, the magnet assembly further comprising a magnetic control body mounted moveably relative to the magnetic backing plate, the magnetic control body having first and second positions, the magnetic control body when in the first position being disposed further into the blind recess of the magnetic backing plate than when in the second position.