Current–pressure–voltage characteristics in a planar magnetron discharge

An experimental study of the dependence of current intensity with pressure, at different constant voltages, $Va⩽450 V,$ in the pressure range between 3.3 and 66.7 Pa, is made for an argon–copper planar magnetron discharge. At $Va⩽360 V,$ the current increases with pressure, until approximately 12 Pa, where a maximum is observed and then decreases leading the discharge almost to extinction. At intermediate voltages, the relative current maximum, at approximately the same pressure, is followed by a local minimum after which a linear increase is observed. At higher voltages, the structure of maxima and minima tends to fade out. An empirical relation with two terms is proposed for the current intensity dependence on pressure and voltage. The first term suggests an electron scattering effect. The second term is linearly dependent on pressure. This empirical model is discussed and compared with a theoretical model from literature. From this analysis, the first term is interpreted as the contribution of electrons generated inside the cathode dark space, while the second term represents the contributions to the current of ions striking the target and of secondary electrons. Results indicate the occurrence of significant ionization in the cathode dark space for the pressure range from 3 to 30 Pa. For higher pressure values and $Va>360 V,$ the cathode sheath is relatively thin and most ionization takes place outside of it. Using both models, theoretical calculations, and experimental parameters, the thickness of the dark space as a function of pressure at several discharge voltages is obtained.