Experimental investigations of driving frequency effect in low-pressure capacitively coupled oxygen discharges

Abstract

The effect of driving frequency on the electron density is investigated in low-pressure capacitively coupled oxygen plasmas by utilizing a floating hairpin probe. The power absorbed by the plasma is investigated and it is found that the power lost in the matching network can reach 50% or higher under certain conditions. The effect of driving frequency on the electron density is studied from two aspects, i.e., constant absorbed power and electrode voltage. In the former case, the electron density increases with the driving frequency increasing from 13.56 to 40.68 MHz and slightly changes depending on the gas pressures with the frequency further increasing to 100 MHz. In the latter case, the electron density rapidly increases when the driving frequency increases from 13.56 to 40.68 MHz, and then decreases with the frequency further increasing to 100 MHz. The electron series resonance is observed at 40.68 MHz and can be attributed to the higher electron density. And the standing wave effect also plays an important role in increasing electron density at 100 MHz and 2.6 Pa.

The frequency dependence of the discharge properties in a capacitively coupled oxygen discharge

“…However, in their experiment the power was fixed as the frequency was increased, and we have to assume that the electrode voltage dropped with increasing driving frequency. This has been seen by Liu et al [6] who kept the absorbed power fixed while the driving frequency was increased from 13.56 to 40.68 MHz and the electrode voltage deceased with increasing frequency to a minimum and then increases again with further increasing the driving frequency. Also, Colgan et al [3,5] reported a decrease in the electrode voltage with increased driving frequency while the discharge current is kept fixed in an argon CCP.…”

“…However, it has to be kept in mind that at higher driving frequencies, standing wave and skin effects can limit process uniformity and power absorption in particular for large area processing in CCPs [8]. This was observed experimentally by Liu et al [6] in an oxygen CCP discharge, where under a constant electrode voltage condition the electron density increases with increased driving frequency in the frequency range 13.56-40.68 MHz and decreases as the frequency is increased further, due to lower power absorption as the frequency is increased above 60 MHz. The electron density has been suggested to scale roughly as the square of the driving frequency at constant rf voltage at fixed pressure and electrode separation [1,3,9].…”

“…The advantages of increasing the driving frequency in order to increase the plasma density have been pointed out throughout the years [1][2][3]. As the frequency is increased beyond 13.56 MHz, various plasma parameters change in a way that is beneficial for plasma materials processing, high plasma densities can be achieved at low applied voltages [3][4][5][6], the dissociation rate increases [7], the etch anisotropy is improved at a fixed pressure [5], and the uniformity improves across the electrode. This is because increasing the driving frequency decreases the sheath thickness, and ions experience fewer collisions in the sheath, and thereby increases ion directionality in the sheath at constant pressure.…”

Electron power absorption in capacitively coupled neon–oxygen plasmas: a comparison of experimental and computational results

“…Due to their high relevance in material processing, oxygen CCPs have been studied extensively, both experimentally and by simulations [13,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. In pure oxygen CCPs, transitions between the α-mode and the DA-mode have been found by changing the gas pressure [13,14], the gap distance [14,17], the driving frequency [13,47], the driving voltage waveform [13,15,18,46,51,52], and the external magnetic field [49].…”

Electron power absorption in capacitively coupled neon-oxygen plasmas: a comparison of experimental and computational results

“…As a consequence of this, it is more likely that the highly energetic SEs within the sheaths induce ionization rather than excitation. Due to their high relevance in material processing, oxygen CCPs have been studied extensively, both experimentally and by simulations [13,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. In pure oxygen CCPs, transitions between the α-mode and the DA-mode have been found by changing the gas pressure [13,14], the gap distance [14,17], the driving frequency [13,47], the driving voltage waveform [13,15,18,46,51,52], and the external magnetic field [49].…”