Complex transients in power modulated inductively-coupled chlorine plasmas

Time-dependent studies of power-modulated chlorine inductively-coupled plasmas are presented. Power at 13.56 MHz applied to the plasma was modulated between high and low power states. Time-resolved optical emission, power delivery, and Langmuir probe measurements revealed at least two conditions upon switching from high to low power: a 'normal' mode in which electron temperature (T-e) remains constant, while electron and ion number densities (n(e )and n(+)) and optical emission spectroscopic (OES) intensities smoothly drop to a level roughly equal to the fractional drop in power, and an 'abnormal' mode in which n(e), n(+) and OES intensities plummet before the plasma re-ignites and these values rise to levels more commensurate with the drop in power. Whether the plasma operates in the normal or abnormal mode is sensitive to impedance matching conditions and is also a function of pressure and pulsing parameters. This ignition delays in the abnormal mode can be qualitatively understood in terms of a power balance model commonly used to explain instability-induced, self-modulation in highly electronegative plasmas, caused by the slower time response of negative ions compared with electrons. The study indicates that power modulation for added control in processes such as plasma etching will require careful measurement and possibly control of power with microsecond resolution.