Plasma influence in tantalum sputtering

The angular distribution function (ADF) and the sputtering yield have been measured in a triode sputtering system where a dense ionic current (35 mA/cm2) drawn from an argon plasma confined by a magnetic field impinges upon a tantalum target biased at 1.8 keV. The system was placed in a vacuum chamber at a pressure on the order of 10-4 Torr. The results are the following for the two phases of Ta deposited. For α-Ta the ADF is Lambertian but the associated yield is lower than the theoretical one; for β-Ta the ADF curve lies below the Lambertian cosine curve. We suggest the following interpretation based on the influence of impurities. For α-Ta there are many impurities which can be ionized and contribute to the current. Also, these ions destroy the Ar* metastables and, therefore, lower the Ta-Ar* collision probability. For β-Ta there are no impurities. The current is due only to ionized argon and Ar* metastables are not destroyed (i.e., the collision probability is much higher). In such a way, we are able to explain the discrepancies observed in these two cases in terms of both the sputtering yield and the ADF