Effect of radio-frequency power levels on electron density in a confined two-frequency capacitively-coupled plasma processing tool

Abstract

The plasma electron density ne in a symmetric confined capacitive-coupled plasma processing tool containing Ar∕O2∕C4F8 gas mixtures is studied as a function of two, combined radio frequency (2MHz+27MHz) powers. For measuring ne we have used a floating hairpin resonance probe. The results show a linear increase in ne with 27MHz power. Also the density is higher with an increase in 2MHz power, in contrast with published particle-in-cell simulation results in argon where the plasma density decreased with increases in low frequency voltage, for fixed high frequency current [P. C. Boyle et al., J. Phys. D 37, 697 (2004)]. Analyzing the relative phase between radio frequency current and voltage, we observe slightly lower 2MHz phase shifts at higher 2MHz voltage, which is attributed to an increase in the real component of the current through the sheath. This is possible due to the increase in secondary electron emissions arising from ion bombardment, which is favored by an increase in 2MHz voltage. We therefore conclude that the secondary electrons could play an important role in the discharge process.

Experimental and numerical investigations of electron density in low-pressure dual-frequency capacitively coupled oxygen discharges

“…In Fig. 7(a), when the HF power is fixed at 30 W the electron density increases with increasing LF power, 20,24 and, however, the electron density decreases with increasing LF power when the HF power increases up to 120 W. 19,44 Note that the simulation presents the analogous result as shown in Fig. 7(b).…”

Influence of plasma chemistry on oxygen triplets

“…In combination with the symmetric plasma system, the plasma potential (V p ) oscillates predominantly at the 2 MHz rf voltage with an associated high frequency oscillation at 27 MHz [22,23], with the mean time-averaged V p reaching several hundreds of volts above ground. The advantage of dual-rf-frequency power is that it provides independent control of the ion current and ion energy onto the boundary [24,25]. For the capacitive plasma configuration the high (low) frequency sees a low (high) impedance, resulting in high (low) current and low (high) voltage.…”

Direct measurement of spatial electron density oscillations in a dual frequency capacitive plasma

“…11 The quality of the hairpin resonance signal obtained in that experiment suggested significant oscillation in n e at the midplane of the narrow gap discharge. Oscillations of the high energy component of n e at the midplane of the discharge with both 2 and 27 MHz oscillations was also inferred from the recent measurements using phase-resolved-optical-emissionspectroscopy ͑PROES͒ in the same apparatus 8 but at different experimental conditions.…”