Complex transients in power modulated inductively-coupled chlorine plasmas

Abstract

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.

Transients using low-high pulsed power in inductively coupled plasmas

“…One of the origins of the E-H transition in electronegative ICPs and its associated instabilities is the need to reignite the plasma at the beginning of each period in pulsed plasmas. One strategy to circumvent the E-H transition while also modulating power is to use a high-low power scheme [23]. Using this method, the pulsed power format consists of a high power portion of the pulsed period followed by a low power portion of the pulsed period-that is, non-zero power.…”

Gas temperature measurements in a pulsed, low-pressure inductively coupled plasma in oxygen

“…This is slightly longer than the transition time of ∼0.5-1 ms that can be observed for pulsed plasmas with higher duty cycles. [9][10][11] The difference due to a longer afterglow period and therefore lower electron density at the start of the pulse. Rotational temperatures during the first 1-2 ms are found to be higher for higher powers (at the same pressure) and lower pressure (at the same power), both of which are in line with electron density trends during plasma ignition.…”

“…The transition time of 1-3 ms is slightly longer than what is typically reported in literature after a change in power, or start of a plasma pulse. In most situations, the plasma reaches steady-state operation in about 0.5-1 ms. [9][10][11] However, this is likely due to the low duty cycle (15%) and repetition rate (5 Hz), compared to the experiments in literature. This means that in our case, during the off-time the electron density drops more significantly, requiring a longer "ignition phase" during which the electron density builds up again at the start of the next pulse.…”

“…7) In addition, power-modulation allows treatment of heat-sensitive materials, for example plastics, with high-power, highdensity plasma processes. 8) In practice, plasma pulsing introduces transient effects when the plasma is switched on, or off, [9][10][11] which are not fully understood, making pulse optimization often an empirical process. Temporal evolution of electron properties is regularly reported, e.g.…”

“…Temporal evolution of electron properties is regularly reported, e.g. [9][10][11] however, evolution of gas temperature during a plasma pulse is not often studied or taken into account in modeling. 12,13) Duten et al reports gas temperature measurements in a pulsed, medium pressure microwave discharge in hydrogen, 7) while Macko and Veis measured gas temperatures in a pulsed glow discharge in oxygen.…”