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Magnetron sputtering, combined with an accurate control of process parameters and layer quality, has become one of the most important methods for depositing thin films. The technique involves bombarding a target surface, which is positioned on a magnetic tube, with an ionized gas. The gas causes metallic atoms to be ejected from the target and subsequently deposited on the substrate to be coated. In standard metallic sputtering, an inert gas, such as argon, is used. No chemical reaction occurs between the gas and the target particles, resulting in a coating on the substrate with a composition similar to the target material.
In a reactive sputtering process, at least one reactive gas (e.g., oxygen or nitrogen) is added. The reactive gas enhances the sputtering process on the target surface and also generates a chemical reaction with the target particles, forming a compound layer on the substrate. As a result, high-purity, uniform coatings can be achieved.
A device called the rotating cylindrical magnetron can overcome all of these problems. By implementing new improvements to the magnet configuration, the technology can achieve target utilization levels of more than 90%. Local tuning of the magnetic field strength allows the layer thickness to be accurately controlled over the substrate width, and new technological developments to overcome arcing and anode problems have also been implemented in this system. With rotating cylindrical magnetrons, companies can achieve controlled reactive sputtering on a wide variety of substrates.
Equipment DesignTwo rotating cylindrical magnetron designs have been used for both metallic and reactive sputtering processes. In the first concept, the cylindrical target tube is supported and powered by two end blocks—one block supplies the target cooling and electrical power to the target tube, while the other sustains the rotation. The latter end block has a spindle that can be retracted very easily, allowing the target tube to be quickly removed for maintenance 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 