Characterization of a magnetron sputtering discharge with simultaneous RF- and DC-excitation of the plasma for the deposition of transparent and conductive ZnO:Al-films

A general drawback of the RF-magnetron sputtering technique is the small deposition rate, compared to DC-sputtering. This is caused by the low DC-voltage (40-160 V for RF-powers of 10-100 W) that develops at the target, since the sputtering rate and therefore the deposition rate in the energy range below 1 keV depend linearly on the acceleration voltage in the cathode fall. In order to increase the target voltage, a simultaneous excitation of the plasma by RF (13.56 MHz) and DC has been used. The DC-excitation is about 1.5 times as effective concerning the deposition rate compared to an RF-excitation. The mass selected ion energy distributions measured on a floating substrate display the characteristic differences between the two discharge modes. While the RF-excitation shows broad energy distributions, caused by the oscillating movement of the plasma sheath, and high plasma potentials (45 V), the DC-ion energy distributions exhibit sharp peaks at much lower plasma potentials (3 V). By a combination of DC- and RF-excitation the plasma potential and therefore the ion energy at a floating substrate can be shifted continuously between low (DC) and high (RF) values. ZnO- and ZnO:Al-layers from ceramic targets were prepared by this new combination for magnetron sputtering. The influence of the different ion energies at a floating substrate on the film structure and the electrical parameters is shown. (C) 1998 Elsevier Science S.A.