The planar diode is the simplest sputter source. The cathode is is typically in the shape of a disk about 5 to 10 cm in diameter, consisting of the material of interest. The target is usually thermally bonded (e.g. solder or conducting epoxy) to a cooled backing plate, or directly water cooled the use of a vacuum/water O-ring seal. A ground shield is used to with sputtering of the sides and support structure undesirable of the source body. A detailed treatment of the glow discharge is given in Chapter II. 1.