  Reactive magnetron sputtering enables the deposition of various thin films to
be used for protective as well as optical and electronic applications. However,
progressing target erosion during sputtering results in increased magnetic
field strengths at the target surface. Consequently, the glow discharge, the
target poisoning, and hence the morphology, crystal structure and stoichiometry
of the prepared thin films are influenced. Therefore, these effects were
investigated by varying the cathode current Im between 0.50 and 1.00 A, the
magnetic field strength B between 45 and 90 mT, and the O2/(Ar+O2) flow rate
ratio between 0 and 100%. With increasing oxygen flow ratio a
sub-stoichiometric TaOx oxide forms at the metallic Ta target surface which
further transfers to a non-conductive tantalum pentoxide Ta2O5, impeding a
stable DC glow discharge. These two transition zones (from Ta to TaOx and from
TaOx to Ta2O5) shift to higher oxygen flow rates for increasing target
currents. Contrary, increasing the magnetic field strength (e.g., due to
sputter erosion) mainly shifts the TaOx to Ta2O5 transition to lower oxygen
flow rates while marginally influencing the Ta to TaOx transition. To allow for
a stable DC glow discharge (and to suppress the formation of non-conductive
Ta2O5 at the target) even at a flow rate ratio of 100% either a high target
current (Im >= 1 A) or a low magnetic field strength (B <= 60 mT) is necessary.
These conditions are required to prepare stoichiometric and fully crystalline
Ta2O5 films. Our investigations clearly demonstrate the importance of the
magnetic field strength, which changes during sputter erosion, on the target
poisoning and the resulting film quality.
