enance or replacement. The end blocks are mounted inside a vacuum chamber along with the target. With this robust configuration, targets up to 13 ft (4 m) long can be mounted. Figure 1 shows a double end block set configuration for coating large-area glass, with two 152-in. tin (Sn) target tubes mounted.
In both concepts, the key component is the rotating target tube, which contains a stationary magnet configuration (defining a stationary plasma racetrack) facing the substrate. The magnet configuration can be optimized at the target ends to extend the width of the uniform coating. The magnetic field strength and the corresponding layer thickness can also be easily adjusted.
Arcing and Anode AdvantagesDuring the reactive sputter deposition process, the insulating compound layer is formed not only on the substrate, but also on the chamber walls, on the anodes and on the target surface next to the erosion zone. On the target surface, the insulating layer charges up by ionic bombardment; when the dielectric strength is exceeded, electrical breakdown occurs, leading to arc discharges (or arcing). Arcing can cause process destabilization and physical damage to the target, anodes and substrate.
Initially, lower power levels were used to reduce the influence of arcing. Power supplies were later equipped with arc detection and interruptible circuitry (i.e., “pulsing”) to limit the amount of energy dissipated into the arc. However, neither of these technologies has been capable of completely eliminating arcing in the racetrack area of the target.
The introduction of large rotating cylindrical targets has restricted the arcing zone to two ring-shaped areas at the cathode tube ends.1,2 Very little arcing occurs in the racetrack area of a rotating cylindrical target, since it is continuously cleaned by the plasma. In fact, as long as the target rotation speed is high enough, only a thin zone at both extreme ends of the target tubes gets contaminated with the insulating material. (If the speed is too low, contamination of the complete target surface can occur before it is sputter-cleaned again, leading to arc events.) A setting of 10 rpm is sufficient for most reactive processes.
Finally, rotating cylindrical magnetrons have also proven to be useful for applications that require AC sputtering from two targets. The AC switching technology was introduced to circumvent the problems caused by arc discharges (by periodically discharging the cathode) and the disappearing anode problem (by sputter cleaning the target, which becomes an anode in the next cycle). However, on planar magnetrons, the magnetic field above the target surface that is used to enhance the plasma density impedes the flow of electrons to the target, resulting in a limited anode functionally. In rotating cylindrical magnetrons, the magnetic field-free area and the total effective surface are much larger, resulting in enhanced anode functionality.
Bottom Line BenefitsFor planar magnetrons, the presence of a racetrack groove limits target consumption, thereby increasing the cost of operating the technology. Although a stationary plasma track is also present during sputter deposition with a rotating cylindrical target, no racetrack groove corresponding to the magnet configuration is formed in the rotating target. As a result, very high target material utilization can be achieved.
Although the target utilization of cylindrical magnetrons is several times higher than a planar magnetron, up to 40% of that utilization can be gained or lost by the target and magnet setup at the end of the target tube. Using a dog bone-shaped target rather than a straight target can help increase target consumption (see Figure 4). Additionally, by strategically placing additional magnets on the target tube to provide a “double racetrack,” in which the turns are slightly shifted, virtually complete target consumption (up to 90%) can be achieved (see Figure 5).
In addition to providing increased target consumption efficiency, rotating cylindrical magnetrons also enhance the production process in sev