2.1 Reactive pulsed DC-MSIP
Sputtering of pure metallic coatings usually takes place using argon as an inert process gas. In this process, the inert argon atoms pass through the process of ionization and recombination with electrons several times and are thus frequently reused before they are captured by the vacuum pumping unit. If a reactive gas, such as oxygen or nitrogen, is also introduced, it chemically combines with the target plasma and forms an oxide in the case of oxygen or a nitride in the case of nitrogen. For homogeneous reactive layer formation, a uniform and controllable supply of the reactive gas is essential. The reactive gas is introduced in close range to the target. Usually, tubes with small holes are placed along the target for a homogeneous supply of the process and reactive gases. Similarly to inert gas, the reactive gas becomes ionized and is accelerated onto the target. Therefore, adding to the reaction with the target plasma also helps to contribute to the sputtering process. Since the reactive gas can also be ionized, it is possible to sputter without inert process gas in a pure reactive gas atmosphere. However, this method is rarely used in practice, and generally, a constant flow is maintained with the process gas while the reactive gas is added in a pressure-controlled manner. The control of the reactive gas via the partial pressure can be used as a manipulated variable for the stoichiometry of the layer formation [10, 11].
Three main processes are involved in a reactive deposition, whereby the main process of deposition is sputtering on the target surface. Simultaneously, when reactive gas is introduced into the deposition chamber, getter processes occur on the target surface, the substrate surface, and the chamber walls of the deposition equipment. Getter processes occur when layers are formed by the interaction of reactive gas atoms and they take place in various forms on all surfaces in a coating system. Getter processes that take place on the target surface are also known as "target poisoning." As the supply increases, the reaction with reactant gas causes the sputtering yield to decrease because of the increasing occupancy of the target. The secondary electron yield thus increases and the plasma impedance decreases, resulting in a reduced cathode voltage at the magnetron. The sum of the events leads to a hysteresis behavior in the process of reactive magnetron sputtering. While the stoichiometry in the deposition of nitrides can generally be relatively easily controlled via the partial pressure of the reactive gas, the process of target poisoning in the reactive sputtering of oxides behaves much more dynamically and is therefore more difficult to control with conventional DC-MSIP. As the target surface is also covered by an oxide layer during the deposition process, the resulting lower electrical conductivity of the oxide layer leads to increased arcing, which in turn results in an extremely unstable sputtering process, creating undesired droplets of molten target material, and can even destroy the sputtering target. The use of a pulsed DC source for magnetron sputtering can overcome these disadvantages as the pulsed mode of the magnetron with frequencies ranging from 100 kHz to 350 kHz avoids arcing on the target surface, since it is covered with an oxide layer.