The Unbalanced Magnetron
An unbalanced magnetron possesses stronger magnets on the outside resulting in the expansion of the plasma away from the surface of the target towards the substrate. The effect of the unbalanced magnetic field is to trap fast moving secondary electrons that escape from the target surface. These electrons undergo ionizing collisions with neutral gas atoms away from the target surface and produce a greater number of ions and further electrons in the region of the substrate considerably increasing the substrate ion bombardment. Effectively a secondary plasma is formed in the region of the substrate. When a negative bias is applied to the substrate, ions from this secondary plasma are accelerated to the substrate and bombard it; this ion bombardment is used to control the many properties of the growing film. To find out more about the properties of the growing film click coating nucleation and growth.
An unbalanced magnetron, the outer North poles are stronger than the inner South poles therefore the field lines stretch further into the vacuum chamber
With the development of the unbalanced magnetron the substrate ion current that could be achieved and therefore the quality of the coatings increased dramatically but more was to come with the development of multi-magnetron geometry with magnetic field linkage.
Closed-field Magnetron Sputtering
The magnets within the magnetrons are arranged such that alternating poles are next to each other resulting in the linkage of field lines. This prevents electrons escaping to the chamber walls resulting in much higher ion current densities and dense, hard well adhered coatings.
A closed-field magnetic arrangement. The magnets form an electron trap to increase the level of ionization.
To find out exactly what a magnetron is click the link.