Perpendicular Electric and Magnetic fields: In magnetron, electrons ideally don't even reach the anode but are trapped near the target, enhancing the ionization efficiency there. This is accomplished by employing a magnetic field oriented parallel to the target and perpendicular to the electric field. Practically this is achieved by placing bar magnets behind the target. Therefore, the magnetic field lines emanates first normal to the target, then bend with a component parallel to the target surface and finally return, completing the magnetic circuit. Electrons emitted from cathode are initially accelerated toward the anode, executing a helical motion in the process, but when they encounter the region of the parallel magnetic field, they are bent in an orbit back to the target. The chief reason of its success is high deposition rate of up to 1 µm/min for aluminium.

Figure 39.6: Schematic representation of magnetron sputtering gun assembly.

In magnetron sputtering, the plasma is concentrated directly in-front of the cathode by means of a magnetic field. The substrate is exposed only to the flow of sputter material with few secondary electron bombardment. The effect of magnetic field on the glow discharge is shown in figure 39.6.

Advantage: All of the sputtering methods (DC and RF) described so far suffer two drawbacks when compared to conventional evaporation: (i) low deposition rates and (ii) high thermal load of the substrate due to bombardment of secondary electrons. When compared to conventional sputtering the advantages of magnetron sputtering are

• increased sputtering rates (~ 5 - 10 times) due to high plasma density around target,

• Low discharge voltages of 300 to 1000 V due to the reduced plasma impendence resulting from high plasma density,

• Low thermal load of the substrate due to deflection of secondary electrons in the magnetic field.