7.4.2 Reactive PVD
For a simple compound, such as a metal oxide or nitride, a modification to the sputter deposition technique can be used. The plasma is formed from a reactive gas, such as nitrogen, instead of the inert argon used for conventional PVD. A side effect of the plasma excitation is that chemical bonds are broken in the source gas to release copious unpaired atoms (radicals), which are extremely reactive and form a monolayer coating of nitride on the target surface. These can then be sputtered to release a shower of molecular fragments. The compound reforms on the wafer surface, with the atmosphere of radicals ensuring full nitridation as the film grows.
In this form, the technique is suitable only for a few electrically conducting compounds – TiN and TaN diffusion barrier layers are the commonest applications. It cannot be used for insulators, as the surface coating on the target will not transmit a current to drive the plasma. If we increase the voltage the coating breaks down in an arc, resulting in particles and very poor reproducibility.
A further modification is required for reactive sputtering to deposit insulators, and the clue to this comes from the technique of platen bias voltage generation. If we replace the DC power supply with an RF one, this will both sustain the plasma and build up a DC voltage on the target surface to accelerate the ions without having to drive a DC current through the insulating coating. Instead, this coating acts as a thin capacitor, across which RF power is transmitted, and which carries a negative static charge on its surface to produce the necessary voltage.
The limitation of RF reactive sputter deposition is the power supply – 10 kW RF power supplies are more difficult to come by than the equivalent DC supply, leaving a choice of lower power (and therefore lower deposition rate) or more expensive equipment. This RF technique has now mostly been supplanted by pulsed DC. A negative DC voltage is applied to the target and a charge builds up on the insulating coating on its front side, just as would happen in conventional PVD. Before this reaches the point of arcing, the polarity is rapidly switched to apply a short positive voltage pulse and electrons from the plasma flood in and discharge the surface. Pulsed DC sputtering in a reactive atmosphere is the technique of choice when depositing simple metal oxides and nitrides, especially when film purity is important.