Large area, vacuum deposition equipment using tubular rotating targets has become the predominant method to deposit thin films on large area substrates such as e.g. window glass. In these installations, material is sputtered away from the rotating target by means of a plasma. The throughput through these huge installations determines their economic profitability. This throughput is—next to other factors such as uptime—determined by the physical deposition speed of material on the substrate. On its turn, this deposition speed depends on a number of factors such as the material that is to be deposited, the line speed, but above all it depends on the electrical voltage and current that is supplied to the target. Indeed, the positive ions of the plasma are accelerated towards the target by a high voltage where they eject material when impinging on the surface of the target. Because this voltage determines the energy by which these ions impinge on the target and the current supplied to the target is proportional to the number of ions impinging, the power supplied (i.e. the product of current and voltage) to the target will greatly influence the deposition rate. Hence, the higher the electric power, the faster the target material is eroded and deposited on the substrate.
While in the first rotating target installations (see U.S. Pat. No. 4,356,073) the electric supply to the target was a direct current, it became soon apparent that this DC supply had some serious drawbacks when depositing materials in a reactive atmosphere: dielectric reaction products tended to cover—to ‘poison’—the target and the positively biased vacuum installation walls—acting as an anode—and impede further sputtering. Due to the dielectric formed on the target, ‘arcing’ became a major problem. ‘Arcing’ is the occurrence of sparks at the target surface between the negatively biased uncoated region and the—due to the impact of positive ions—positively charged poisoned area. It was soon discovered that the use of an alternating current supply between two separate targets could alleviate this ‘poisoning’ problem and associated arcing problem (see U.S. Pat. No. 5,169,509) while maintaining a stable plasma as the walls of the vacuum enclosure were no longer electrically active.
While AC sputtering allowed for a further increase in power supplied to the target, it also led to new problems. The typical frequencies that are used are between 10 and 100 kHz at a current drawn of 300 A. Regime voltage is typically 300 to 500 V. At these frequencies, most of the current flows through the outer regions of a current conductor, a phenomenon that is well known as the skin effect. The current density drops by a factor 1/e (‘e’ being the natural logarithm base number) within a thickness ‘δ’ (in m) that is given by:δ=(πνμσ)−1/2 wherein:                ν is the frequency of the current (in Hz),        μ is the magnetic permeability (in H/m),        σ is the conductivity of the material (in S/m)        
For copper this leads already to a skin depth of ab. 670 μm at 10 kHz leading to very high local current densities and joule heating of the outer surface of conductors when used in a sputtering apparatus of the kind envisaged.                Mainly the rotatable electrical contact between current supply and target turns out to be a critical spot. Outside this region, the use of multifilament current leads can greatly overcome the problem. While many rotary electrical connections are known in the art, for examples in electrical motors and generators, the requirements that are asked from a rotary electrical connection for an end-block are quite different. As already mentioned, the currents and voltages used are relatively large given also the fact that a typical end-block is contained in only 20 cm×15 cm×15 cm. Also the frequency is large compared to the known rotary connections. On the other hand the number of revolutions at which the rotary connection must work is rather moderate: at the most 100 rotations per minute. Hitherto a couple of solutions have been suggested in order to overcome this joule heating and to counter its effects in rotatable sputtering target end-blocks.        US 2003/0136672 A1 describes the use of four semi-cylindrical stationary contact brushes that are forcedly enclosing the shaft that is rotating with the target. Current travels through a path from the brush block through the shaft to the target tube. Water flows through the shaft thus limiting the temperature rise due resistive heating by the current.        US 2003/0173217 A1 describes the use of the coolant conduit itself as a conducting member disposed within the rotary cathode device. Stationary brushes, immersed in the coolant, connected to this conducting member spread the current over the whole rotary cathode. As the coolant flows through the conducting member, the effects of the resistive heating are countered.        
Although the above prior-art may alleviate the consequences of the joule heating, they do not reduce the root cause of the problem, namely the restriction of the current carrying area due to the skin effect.