Abstract
Capacitively coupled radio frequency plasmas operated in an electronegative gas (CF4) and driven by voltage waveforms composed of four consecutive harmonics are investigated for different fundamental driving frequencies using PIC/MCC simulations and an analytical model. As has been observed previously for electropositive gases, the application of peak-shaped waveforms (that are characterized by a strong amplitude asymmetry) results in the development of a DC self-bias due to the electrical asymmetry effect (EAE), which increases the energy of ions arriving at the powered electrode. In contrast to the electropositive case (Korolov et al 2012 J. Phys. D: Appl. Phys. 45 465202) the absolute value of the DC self-bias is found to increase as the fundamental frequency is reduced in this electronegative discharge, providing an increased range over which the DC self-bias can be controlled. The analytical model reveals that this increased DC self-bias is caused by changes in the spatial profile and the mean value of the net charge density in the grounded electrode sheath. The spatio-temporally resolved simulation data show that as the frequency is reduced the grounded electrode sheath region becomes electronegative. The presence of negative ions in this sheath leads to very different dynamics of the power absorption of electrons, which in turn enhances the local electronegativity and plasma density via ionization and attachment processes. The ion flux to the grounded electrode (where the ion energy is lowest) can be up to twice that to the powered electrode. At the same time, while the mean ion energies at both electrodes are quite different, their ratio remains approximately constant for all base frequencies studied here.
Highlights
Plasmas have been used for various surface processing applications for many decades [1, 2]
The selective and anisotropic etching of semiconductors as well as the deposition of functional coatings on large area substrates are performed in capacitively coupled radio frequency (CCRF) plasmas
D − sg, max ions ni,+(z) ions ni,−(z in electronegative plasmas to account for all charged heavy species, ni,+(z) and ni,−(z), which correspond to the density of the positively charged and the negatively charged ions, respectively. (Note that these expressions are valid for multiple ionic species, but we assume that all types of ions are singly charged.) In low-pressure electropositive plasmas, it has been found that the ratio of the mean net charge densities in the two sheaths differs from unity for a driving voltage waveform described by equation (1) due to the stronger acceleration of ions in one of the sheaths, thereby causing a self-amplification of the electrical asymmetry effect (EAE) [37]
Summary
Plasmas have been used for various surface processing applications for many decades [1, 2]. The effect of the base frequency on the discharge symmetry in electronegative plasmas, where the electron and ion dynamics are very different from the electropositive case, has not been studied. In this work we show that, in contrast to electropositive plasmas, the symmetry control via the EAE is significantly enhanced by choosing lower base frequencies This is an extremely important finding, since it shows that in many reactive gas mixtures used in processing applications, lower base frequencies should yield more control of process performance based on the EAE due to an improved control of the ion flux-energy distributions at the surfaces via.
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