Method of and apparatus for reactively sputtering a graded surface coating onto a substrate

An apparatus for applying a graded solar selective surface coating to a substrate and which comprises:

• a first sputtering chamber,
• a second sputtering chamber which is located in series with the first sputtering chamber,
• means for advancing the substrate serially through the first and second sputtering chambers at a pre-determined feed-rate, a cathode located within each of the first and second sputtering chambers and extending lengthwise of the respective sputtering chambers in the direction of advancement of the substrate through the sputtering chambers, means for admitting a sputter supporting gas to the first and second sputtering chambers.
• means for directing a reactive gas into the second sputtering chamber at a rate such that the reactive constituent of the reactive gas is substantially entirely consumed in a reactive sputtering process in the second sputtering chamber, the means for directing the reactive gas into the second sputtering chamber being separate from the means for admitting the sputter supporting gas and including a feed point which is spaced from the second sputtering chamber in the direction of advancement of the substrate,
• and
• means for inducing a flow of the reactive gas into the cathode containing region of the second sputtering chamber in a direction counter to the direction of advancement of the substrate through the chamber.