High rate reactive magnetron sputtering
2012-09-27T07:52:11Z (GMT) by
Glow discharge sputtering has been used for many years to produce thin films but its commercial applications are severely limited by low deposit ion rates. The DC planar magnetron, developed a decade ago, allows much higher deposition rates and its commercial use has expanded rapidly. Non-reactive magnetron sputtering of metallic thin films is well understood and utilized. However when a reactive gas is introduced the process becomes harder to control and can switch between two stable modes. Often films are produced simply by using one of these stable modes even though this does not lead to optimum film properties or high deposition rates. This work gives a model of reactive magnetron sputtering and verifies experimentally its predictions. A 0.5 m long magnetron was designed and built specifically to allow reactive sputtering onto A4 rigid substrates. This magnetron has a variable magnetic field distribution which allows plasma bombardment of the substrate during film growth. This was shown to activate reactions at the substrate. The target lifetime was extended in our design by broadening the erosion zone and increasing the target thickness. The reactive sputtering process was shown to be inherently unstable and a control system was designed to maintain the magnetron in an unstable state. Light emission by the plasma at metal line emission wavelengths changes across the instability and so with this control signal a feedback system was built. The accuracy of control was shown experimentally and theoretically to depend on the delay time between measurement, action and effect. In practice this delay was limited by the time constant of the gas distribution manifold. The time constant of such manifolds was measured and calculated. Using our controller high quality films were produced at high rates in normally unstable deposition systems. Conducting indium oxide was produced at 6 nm/s with a resistivity of 6 x 10-6 ohm. metres onto A4 glass sheets. Tin oxide was produced at increased rates onto 2.5 m by 3 m substrates.