Relation between cathode-fall and ion-sheath thicknesses in multi-electrode-type magnetron discharges

Spatial distributions of floating potential were investigated by a superposition method of alternating voltage in magnetron discharges for diagnostics of processing plasmas. Phase differences of floating potential to a reference signal of alternating voltage superposed to a d.c. cathode voltage were measured with a capacitive probe. The fluctuation of floating potential propagated with a time-lag in the presheath of the cathode-fall region. An ion-sheath thickness, evaluated by the Child-Langmuir law in magnetron discharges, was explained with the superposition method. The ion-sheath region was much thinner than the cathode-fall region. The relation between the cathode-fall and ion-sheath thicknesses was found to be important for self-organized stability in magnetron discharges.