Thin film deposition by sputtering
Permanent magnets increase the particle density
One thing that all sputtering methods have in common is the heavy ion bombardment, usually using argon, which continuously removes material from the target surface which will condense on the substrate surface.
The number of thermally excited ions in a vacuum especially at the room temperature is very low. Therefore, an increase in the ion density is needed to achieve the necessary high deposition rates of several 10 nm/min.
In the magnetron sputtering process (fig. left), permanent magnets are placed behind the sputtering target to increase the ion density in front of it: The magnetic field forces the free electrons into helical paths around its magnetic lines. This extends the electron path to the anode, thereby extending the time during which they can ionize Argon atoms. This elongation of path and ionization time is more noticeable where the magnetic field is perpendicular to the electric field. The argon ions generated are barely affected by the magnetic field because of their weight and they only accelerate in the direction of target, which they strike and remove material.
The pronounced effect of this increase in the ion density caused by the magnetic field can easily be seen on the used targets (fig. middle) in the form of a trench called as a "race track".
Another advantage of magnetron sputtering is the low process pressure (~ 5 x 10-3 mbar) which increases the mean free path up to around 20 mm. As a result, the atomic particle stream of the target exhibits negligible scattering and retains its high output energy on the substrate, which results in forming very dense layers (fig. right).