Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation

Abstract

The charged particle dynamics in low-pressure oxygen plasmas excited by odd harmonic dual frequency waveforms (low frequency of 13.56 MHz and high frequency of 40.68 MHz) are investigated using a one-dimensional numerical simulation in regimes of both low and high electronegativity. In the low electronegativity regime, the time and space averaged electron and negative ion densities are approximately equal and plasma sustainment is dominated by ionisation at the sheath expansion for all combinations of low and high frequency and the phase shift between them. In the high electronegativity regime, the negative ion density is a factor of 15-20 greater than the low electronegativity cases. In these cases, plasma sustainment is dominated by ionisation inside the bulk plasma and at the collapsing sheath edge when the contribution of the high frequency to the overall voltage waveform is low. As the high frequency component contribution to the waveform increases, sheath expansion ionisation begins to dominate. It is found that the control of the average voltage drop across the plasma sheath and the average ion flux to the powered electrode are similar in both regimes of electronegativity, despite the differing electron dynamics using the considered dual frequency approach. This offers potential for similar control of ion dynamics under a range of process conditions, independent of the electronegativity. This is in contrast to ion control offered by electrically asymmetric waveforms where the relationship between the ion flux and ion bombardment energy is dependent upon the electronegativity.

Electron power absorption dynamics in magnetized capacitively coupled radio frequency oxygen discharges

“…The simulations reveal that by increasing the magnetic field, that is parallel to the electrodes, a transition of the electron power absorption mode from the DA-mode at low magnetic fields to the α-mode can be induced in oxygen CCPs. This effect is based on the variation of the discharge electronegativity (the ratio of the negative ion density to the electron density) as a function of the applied magnetic field and is similar to the mode transitions induced by the variation of the driving voltage waveform, the gas pressure, and gap distance in oxygen CCPs [28,33,36,37]. We explain the reasons for the significantly decreased electronegativity of the discharge at stronger magnetic fields based on the dynamics of chemical reactions.…”

“…Transitions between these power absorption modes have been observed in different gases by changing external control parameters. For instance, in oxygen CCPs, a transition from the DA-mode to the α-mode has been found to be induced by changing the gas pressure [33,34], the driving frequency [35,36], the driving voltage waveform [7,28,[36][37][38], as well as the gap distance [34,39]. All of the above electron power absorption modes are based on electron motion along the direction perpendicular to the electrodes.…”

“…The reactions and cross sections we implemented in our code can be found in [55]. Metastable O 2 (a 1 ∆ g ) molecules have been found to play A c c e p t e d M a n u s c r i p t particle power absorption dynamics [37,56]. A high O 2 (a 1 ∆ g ) content in the discharge usually results in a low electronegativity.…”

Decoupling ion energy and flux in intermediate pressure capacitively coupled plasmas via tailored voltage waveforms

“…Enhanced control of ion dynamics in rf-CCPs at low pressure ( 67 Pa, 0.5 Torr) has been achieved through the application of multi-frequency 'tailored' voltage waveforms [14][15][16][17][18][19][20] . Tailored voltage waveforms comprise the superposition of two or more harmonics of a fundamental voltage frequency; by adjusting the relative phase or amplitude between harmonics, it is possible to introduce amplitude and slope asymmetries into the voltage waveform [21][22][23][24] .…”

Evaluation of oxygen species during E–H transition in inductively coupled RF plasmas: combination of experimental results with global model

“…Radio frequency (RF) plasmas at low pressure have been extensively investigated in experiment and modeling by many research groups over the past decades. In particular, progress was achieved in investigations of capacitively coupled RF plasmas (CCP), e.g., the dual or multi frequency RF plasmas [1][2][3][4][5] including consecutive harmonics for the electrical asymmetry effect [6][7][8] to control the plasma and reactive species density independently from the ion bombardment of the electrode in plasma surface processing, and the appearance of drift-ambipolar electric fields for electron heating in strongly electronegative plasmas [9]. Global/fluid models were applied for molecular CCP/ICP with large number of species [10][11][12][13][14][15], whereas the kinetic study of charged species in molecular RF plasmas was performed mostly by 1d3v particle in cell Monte Carlo collision (PIC-MCC) simulation of CCP [16][17][18][19].…”