Experimental investigation of mode transitions in asymmetric capacitively coupled radio-frequency Ne and CF4 plasmas

Abstract

The dependence of the electron density and the emission intensity on external parameters during the transitions of the electron power absorption mode is experimentally studied in asymmetric electropositive (neon) and electronegative (CF4) capacitively coupled radio-frequency plasmas. The spatio-temporal distribution of the emission intensity is measured with phase resolved optical emission spectroscopy and the electron density at the discharge center is measured by utilizing a floating hairpin probe. In neon discharge, the emission intensity increases almost linearly with the rf voltage at all driving frequencies covered here, while the variation of the electron density with the rf voltage behaves differently at different driving frequencies. In particular, the electron density increases linearly with the rf voltage at high driving frequencies, while at low driving frequencies the electron density increases slowly at the low-voltage side and, however, grows rapidly, when the rf voltage is higher than a certain value, indicating a transition from α to γ mode. The rf voltage, at which the mode transition occurs, increases with the decrease of the driving frequency/the working pressure. By contrast, in CF4 discharge, three different electron power absorption modes can be observed and the electron density and emission intensity do not exhibit a simple dependence on the rf voltage. In particular, the electron density exhibits a minimum at a certain rf voltage when the electron power absorption mode is switching from drift-ambipolar to the α/γ mode. A minimum can also be found in the emission intensity at a higher rf voltage when a discharge is switching into the γ mode.

Experimental diagnosis of electron density and temperature in capacitively coupled argon plasmas: Triple-frequency discharges and two-dimensional spatial distributions

“…The similar axial distribution of electron density was also observed in the experiments [6,9] and simulations [17,18]. Due to the asymmetric discharge between two electrodes, the powered electrode exists a dc self-bias [8]. Compared with the grounded electrode, thus, the electron density is higher near the driven electrode, where the ionization maximum occurs.…”

“…Based on a synergetic use of floating double probe and two-dimensional self-consistent electrostatic fluid model, Liang et al [7] presented that the radial uniformity of plasmas is better at low RF power and large electrode gap. By using a hairpin probe, Liu et al [8] studied the axial distribution of electron density in neon plasmas driven at single frequency. They discovered that the electron density is more uniform in the bulk plasma when RF voltage is low.…”

Fundamental study towards a better understanding of low pressure radio-frequency plasmas for industrial applications

“…[57] The underlying physics of the mode transitions in electronegative CCPs is primarily associated with the variation of the electronegativity (defined as the ratio of the negative ion to electron density) as a function of the external control parameters. [57][58][59][60] A transition between the striation and DA modes can be observed by changing either the pressure or the RF voltage (see Fig. 13).…”

Current status and nature of high-frequency electronegative plasmas: basis for material processing in device manufacturing

“…The heating mechanism of electrons in a HF-CCP in CF 4 was studied by both measurements using time-and spaceresolved optical emission spectroscopy and a time-averaged hairpin probe, as well as by PIC/MCS modeling. 83,84) A comprehensive study was carried out in the pressure range of 20-90 Pa, with powers of 15-100 W, and applied frequencies of 13. 56, 27.12 and 40 MHz.…”

“…83) A similar work was recently conducted at frequencies of 4, 8, and 12 MHz in a periodic steady state. 84) Over the range of investigations, t á ñ m and t á ñ e of the electron are estimated as wt wt á ñ > > á ñ --1 m e 1 1 at 4, 8, and 12 MHz, except for pressure P < 30 Pa and 12 MHz. 16) As a result, it is possible for almost all electrons to drift a distance W NL during the half period.…”