Magnetron sputtering.
It is well known that if an ionized gas
is subjected to a magnetic field, the ionic density is increased throughput
the area covered by the field. The Helmholtz coil, an external magnet
surrounding the sputtering system, uses this principal to enhance
ionization of the sputtering gas. This is due to the fact that the
electronic paths wind themselves around the field lines, increasing the
probability of collision, ionization. This phenomenon is shown
schematically in Figure. 6.4.
If the applied field, DC or RF, is concentrated near the target, and
oriented so that the field lines are parallel to the target surface, secondary
electrons formed during ionic collision concentrate in front of the target
and enhance the ion density. For a given voltage then, the number of
ions available for target bombardment increases, resulting in more atoms
being ejected from the target per unit time, another way of saying the
deposition rate increases.
Secondary electrons formed during diode sputtering are
accelerated to the anode and bombard the substrate. In the magnetron
system, these electrons are diverted to a nearby end shield or anode. An
important consequence of the effect of containing the electrons in the
neighborhood of the target during magnetron sputtering is the
considerable reduction of substrate heating. Table 7-2 compares the
deposition rates of some representative metals sputtered by both
conventional and magnetron methods, and Table 7-3 summarizes the
advantages of sputtering.