Plasma dynamics in a discharge produced by a pulsed dual frequency inductively coupled plasma source

Abstract

Using a Langmuir probe, time resolved measurements of plasma parameters were carried out in a discharge produced by a pulsed dual frequency inductively coupled plasma source. The discharge was sustained in an argon gas environment at a pressure of 10 mTorr. The low frequency (P2 MHz) was pulsed at 1 kHz and a duty ratio of 50%, while high frequency (P13.56 MHz) was maintained in the CW mode. All measurements were carried out at the center of the discharge and 20 mm above the substrate. The results show that, at a particular condition (P2 MHz = 200 W and P13.56 MHz = 600 W), plasma density increases with time and stabilizes at up to ∼200 μs after the initiation of P2 MHz pulse at a plasma density of (2 × 1017 m−3) for the remaining duration of pulse “on.” This stabilization time for plasma density increases with increasing P2 MHz and becomes ∼300 μs when P2 MHz is 600 W; however, the growth rate of plasma density is almost independent of P2 MHz. Interestingly, the plasma density sharply increases as the pulse is switched off and reaches a peak value in ∼10 μs, then decreases for the remaining pulse “off-time.” This phenomenon is thought to be due to the sheath modulation during the transition from “pulse on” to “pulse off” and partly due to RF noise during the transition period. The magnitude of peak plasma density in off time increases with increasing P2 MHz. The plasma potential and electron temperature decrease as the pulse develops and shows similar behavior to that of the plasma density when the pulse is switched off.

Electron density modulation in a pulsed dual-frequency (2/13.56 MHz) dual-antenna inductively coupled plasma discharge

“…31 Langmuir probe study under the same set-up and similar discharge conditions has predicted a slow rise in the electron density during the 2 MHz pulse-on phase in order to reach to a quasisteady state value. 32 The plasma potential has shown a similar trend as measured by the emissive probe over a course of one pulse period, whereas the electron temperature confirmed the emissive probe measurements only when the density reached to a quasisteady state value and in the early afterglow. During the pulse-on phase and late afterglow, the electron temperature predicted an opposite trend, i.e., an increase in electron temperature with an increase in 2 MHz power level and attributed to capacitive to inductive transition.…”

“…These results are in contradiction with the previously published work in Ar plasma in the same setup. 32 In Ar discharge, the plasma density overshoots were observed at the beginning of early afterglow, which was attributed to the thin to thick sheath transition limiting the probe theory. In this case, the hairpin probe is used to measure the electron density which is a fully floating probe, i.e., not drawing any current from the plasma and therefore independent of the above mentioned effect.…”

“…56 MHz pulsing have been demonstrated. [31][32][33] In Argon discharge, pulsing 2 MHz and different combination of 2/ 13.56 MHz power levels has shown significant modulation in the plasma potential during the pulse-off phase. 31 From an estimate of electron temperature from plasma potential and floating potential measured by floating emmisive probe, it was found that the effect of increasing 2 MHz power level is to lower the electron temperature; however, increasing 13.56 MHz power increases the electron temperature.…”