Measurements of pulsed-power modulated argon plasmas in an inductively coupled plasma source

The behavior of pulsed-power (square wave) modulated argon plasmas generated by an inductively coupled plasma (ICP) source is experimentally investigated. The apparatus is an ICP source with a flat coil geometry equipped with a cylindrical Langmuir probe on the axis of the cylindrical chamber. The evolution of the plasma density is determined from the wave forms of ion saturation currents. The rise and fall of the density had a time scale of a few tens of microseconds. The time average plasma density is also measured as a function of pulse frequency and duty ratio, holding the average absorbed power constant. When the plasma is modulated, the density is larger than that for a continuous wave excitation of the same average power. Larger densities are obtained for smaller duty ratios. The density increases monotonically as the period is decreased down to 100 mu s. This agrees qualitatively with the modeling result, which accounts for the higher density by the difference of time scales for the generation and the loss of charged particles. The results for electron temperature measurements also show good agreement with the model quantitatively during the pulse ''on'' times. If the period is long enough, it is found that the electron temperature rises abruptly at the initial stage of power application, in agreement with the calculation. (C) 1996 American Vacuum Society.