Modifying the electrical characteristics of magnetron sputtering sources using hollow cathode structured targets

Preliminary findings are presented on how small changes in the geometry of the cathode target in an unbalanced magnetron can affect their electrical, current-voltage (I-V), characteristics. Using a high magnetic field-strength device, (B at the surface of similar to 0.055 T), flat cathodes of Cu and Ti in Ar at 2-6 m Torr gave I-V characteristics consistent with the power law; I = k(V-V-0)(2). However as a "race-track" sputter trench evolved with time, deepening, but only partially widening, a dramatic change in the I-V response occurred, with the discharge current increasing greatly (e.g. x 2) at low and intermediate cathode voltages (less than or similar to 600 V). At higher cathode voltages, the effective plasma impedance was negative and the discharge current decreased with V until ifs value met approximately the original flat target discharge "load line'. The current enhancement and "kink" effect was more pronounced at higher pressures and for thinner targets, and was believed to arise from the onset of a hollow cathode type process, in which electrons were trapped in a deep potential well created by the formation of shallow trench walls. It is apparent that a specific, almost flat, magnetic field geometry is required, in which some field lines must link opposite walls, intersecting them in positions where substantial secondary electron emission occurs. Electrons then oscillate between the cathode sheaths, guided by the magnetic field through the negative glow. Machining structures into the planar cathode surface, (e.g. hollow cathodes of 2.5 mm diameter and 1 mm depth), enhanced further modification of the I-V characteristic. By introducing artificial wall-sides in the race-track region, discharge currents up to 1.8 times the usual value for conventional planar cathodes were achieved at 6 m Torr pressure. This current multiplication increased with discharge pressure, but decreased with target potential. The latter effect is considered to arise from the sheath thickness increasing with bias, and filling the features, thereby extinguishing the hollow cathode effect. Possible plasma and geometric conditions are suggested for achieving these novel effects, with some recommendations for further studies and improving magnetron design. (C) 1998 Elsevier Science Ltd. All rights reserved.