Experimental investigation of low-frequency waves propagating in a direct current planar magnetron plasma

Experimental observations of low frequency waves propagating azimuthally in a direct current planar magnetron sputtering plasma are presented. The measurements, performed with a circular array of electric probes measuring the floating potential fluctuations, show the presence of several Fourier modes with azimuthal periodicity m=3-7, at frequencies of the order of 100 kHz. The modes appear when the discharge current is above a threshold value in a wide range of neutral gas pressures. The wave frequency decreases with increasing pressure. The modes have been interpreted as drift waves destabilized by the combined effect of density gradient and electric field. A nonregular amplitude modulation of the modes, similar to the behavior of coupled oscillators, is observed.