Sputter Deposition

Sputtering sources are usually magnetrons that utilize strong electric and magnetic fields to trap electrons close to the surface of the magnetron, which is known as the target. The electrons follow helical paths around the magnetic field lines undergoing more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. The sputter gas is inert, typically argon. The extra argon ions created as a result of these collisions leads to a higher deposition rate. It also means that the plasma can be sustained at a lower pressure. The sputtered atoms are neutrally charged and so are unaffected by the magnetic trap. Charge build-up on insulating targets can be avoided with the use of RF sputtering where the sign of the anode-cathode bias is varied at a high rate. RF sputtering works well to produce highly insulating oxide films but only with the added expense of RF power supplies and impedance matching networks. Stray magnetic fields leaking from ferromagnetic targets also disturb the sputtering process. Specially designed sputter guns with unusually strong permanent magnets must often be used in compensation. Ion-beam sputtering (IBS) is a method in which the target is external to the ion source. A source can work without any magnetic field like in a Hot filament ionization gauge . In a Kaufman source ions are generated by collisions with electrons that are confined by a magnetic field as in a magnetron. They are then accelerated by the electric field emanating from a grid toward a target. As the ions leave the source they are neutralized by electrons from a second external filament. IBS has an advantage in that the energy and flux of ions can be controlled independently. Since the flux that strikes the target is composed of neutral atoms, either insulating or conducting targets can be sputtered. IBS has found application in the manufacture of thin- film heads for disk drives. The principal drawback of IBS is the large amount of maintenance required to keep the ion source operating. Reactive sputtering refers to a technique where the deposited film is formed by chemical reaction between the target material and a gas which is introduced into the vacuum chamber. Oxide and nitride films are often fabricated using reactive sputtering. The composition of the film can be controlled by varying the relative pressures of the inert and reactive gases. Film stoichiometry is an important parameter for optimizing functional properties like the stress in SiNx and the index of refraction of SiOx. The transparent indium tin oxide conductor that is used in optoelectronics and solar cells is made by reactive sputtering. In ion-assisted deposition (IAD) the substrate is exposed to a secondary ion beam operating at a lower power than the sputter gun. Usually a Kaufman source like that used in IBS supplies the secondary beam. IAD can be used to deposit carbon in diamond-like form on a substrate. Any carbon atoms landing on the substrate which fail to bond properly in the diamond crystal lattice will be knocked off by the secondary beam. NASA used this technique to experiment with depositing diamond films on turbine blades in the 1980's. IAS is used in other important industrial applications such as creating tetrahedral amorphous carbon surface coatings on hard disk platters and hard transition metal nitride coatings on medical implants. High Target Utilisation Sputtering (HiTUS) is a process based upon the remote generation of a high density plasma. The plasma is generated in a side chamber opening into the main process chamber, containing the target and the substrate to be coated. To enhance adhesion/prepare substrate, volatile contaminants on the substrate surface are removed, by directing the plasma beam onto the substrate. Prior to deposition, the target is sputter cleaned in a pure argon plasma to remove oxides /contamination. As the plasma is generated remotely, and not from the target itself (as in conventional magnetron sputtering), the ion current to the target is independent of the voltage applied to the target. The process offers a multitude of advantages compared with traditional sputtering techniques such as: * 95% use of the target with no racetrack * Increased deposition rates especially for reactively sputtered dielectrics * Reactive sputtering process simply controlled – no feedback system required * Higher coating precision * Better control of film characteristics, with properties close to bulk * Better smoothness control * High levels of repeatability and reproducibility * Higher production speed * Possibility of in-line and roll to roll production line with multi-layer deposition * Stress is readily controllable, from compressive to tensile, with zero stress in between. * Low temperature process enabling deposition onto organic substrates * The process can be easily integrated into many existing sputtering set-ups.