Electrostatic fluctuations in a direct current magnetron sputtering plasma

Electrostatic fluctuations (up to 500 kHz) have been studied in a planar dc magnetron device using Langmuir probes measuring the floating potential and the ion saturation current. Fluctuation levels as high as 30% have been found inside the magnetic trap. A two-point spectral analysis has shown that the fluctuations are due to coherent modes with a low azimuthal mode number. The modes are present only when the discharge power and the neutral gas pressure are above a threshold. Their frequency spacing decreases when the neutral gas pressure is raised, so that increasing the pressure leads to a more turbulent state. The modes have been interpreted as unstable ExB/density gradient modes. (C) 2001 American Institute of Physics.