Balance and Unbalance

Magnetrons are commonly classified as 'balanced' or 'unbalanced'. Conventional magnetrons are usually of the balanced type. The ability of the electrons to escape from the magnetic trap is determined by the position of the null point in the plasma above the target. If the null point is high above the target, there is little chance of electrons escaping and the magnetron is balanced. In this case, there is low ion bombardment on the substrate (assuming the substrate is positioned above the null point). If the null point is close to the target surface, the electrons can escape more easily and the magnetron is unbalanced. Unbalanced designs can produce high ion bombardment of the thin film at the same time as deposition.

Window and Savvides were the first people to formally recognize and classify unbalanced magnetrons (see figure below). Type 1 is a balanced design and is characterised by the open field lines extending over and beyond the target surface. Type 2 is an unbalanced design and is characterized by the closed field lines that are confined above the target surface.

 
Source: B Window and N. Savvides, J. Vac. Sci. Technol. A4(2), 196 (1986)