Spatial and temporal variation of ion flux in the presence of an instability in inductively coupled SF₆ plasmas

A two-dimensional array of planar Langmuir probes on a 200 mm diameter silicon wafer was used in an inductively coupled plasma reactor to follow the spatial and temporal variation of ion flux impinging on the wafer in the presence of an instability. Small amplitude low frequency (∼2 Hz) oscillations superimposed on a steady state ion flux distribution were observed in SF₆ plasmas. The magnitude and phase of the oscillations depend on position on the wafer and analysis of these variations reveals that these low frequency oscillations correspond to waves that move across the wafer.