The Unbalanced Magnetron
Stronger magnets on the outside of an imbalanced magnetron cause the plasma to expand away from the target’s surface and towards the substrate. The imbalanced magnetic field’s effect is to capture secondary electrons that are travelling quickly and escape from the target surface. The quantity of ions and additional electrons produced in the vicinity of the substrate by these electrons’ ionising collisions with neutral gas atoms farther from the target surface significantly increases the substrate’s ion bombardment. In reality, a secondary plasma is created near the substrate. Ions from this secondary plasma are driven to the substrate and bombard it when a negative bias is provided; this ion bombardment is employed to control the numerous properties of the growing film. Go to coating nucleation and growth to learn more about the growing film’s characteristics.
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.