Study on electron properties of atmospheric pressure rf discharges using continuum radiation-based electron diagnostics

Abstract

Information on electrons is particularly valuable in atmospheric pressure plasmas because most of the chemical reactions and discharge characteristics are governed by electron kinetics. Thus, in contrast to the heuristic approach by trial-and-error, accurate and sufficient data on electrons enable one to better understand and utilize the produced plasma. Despite the importance of electron information, diagnostics of electron properties, particularly electron density ($n_e$) and electron temperature ($T_e$) of low-temperature atmospheric pressure plasmas, e.g. capacitive glow discharge, is still challenging although there are some well-established diagnostics available such as laser Thomson scattering or other spectroscopic methods combined with complex plasma equilibrium models. For this reason, unexplained discharge phenomena and mechanisms still exist. Due to the lack of electron diagnostics, many authors who research atmospheric pressure plasma physics investigated basic principles using numerical models with relevant approximations. In this thesis, using electron diagnostics based on electron-neutral atom ($e-a$) bremsstrahlung in the ultraviolet and visible range, characteristics of atmospheric pressure capacitive discharge associated with electron properties is investigated. Since the spectral emissivity of the $e-a$ bremsstrahlung is determined by $n_e$ and mean electron energy representing Maxwellian electron energy distribution, their diagnostics is possible. Among the amount of different configuration for producing plasmas, radio-frequency (rf) capacitive discharge that is most widely used discharge type is selected because its simple geometry regarding as the one-dimensional problem make easy to interpret experimental results. In this plasma source, the influence of external parameters such as supplying gas, geometric parameters, driving frequency, and the background pressure on the electron properties are investigated, and then these parameters are categorized into two groups in terms of $n_e$ and $T_e$.

First of all, the gas dependence on plasma emission is figured out by supplying helium and argon. From emission spectra, it is noticed that $T_e$ in both cases are the same at 2.5 eV while $n_e$ in argon discharge is much larger than that in helium discharge. Spatiotemporally resolved measurements provide the fundamental electron properties of argon capacitive discharge. According to nanosecond-resolved spectra, $T_e$ is varied in the range 2.3 - 3.0 eV during one rf cycle whereas $n_e$ is not. Two-dimensional distribution of the time-averaged $T_e$ is demonstrated by using the combination of imaging devices and optical interference filters, and this result shows the electron heating structure in high collisional capacitive discharges. As expected, $n_e$ is linearly proportional to the discharge current and input power in abnormal $\alpha$-mode discharges, while $T_e$ is not affected

the rf input power is dissipated mainly for ionization for neutrals in high-collisional plasmas. It is also found that the dielectric material is not a decisive factor in determining electron properties in $\alpha$-mode discharge.

Two representative operation parameters in strong relevance to $T_e$ are the driving frequency (also called excitation frequency) and the gas pressure. Driving frequency is one of the most important parameters in low-temperature plasma operation due to its strong influence on reactivity and applicability of the plasma in processes such as etching or deposition. In comparison to the low-pressure plasma, the role of driving frequency, particularly dual-frequency, in electron and ion kinetics has not been much addressed in atmospheric pressure plasmas. The electrical characteristics and electron information of single- (4.52 MHz and 13.56 MHz) and dual-frequency (4.52$+$13.56 MHz) atmospheric pressure argon capacitive discharges were experimentally studied within the abnormal $\alpha$-mode regime. The results show that $n_e$ linearly increases with rf input power while $T_e$ is not influenced substantially in the single-frequency plasma. In contrast, independent control of time-averaged $n_e$ and $T_e$ was achieved in the dual-frequency plasma. As the low-frequency voltage increases with the constant high-frequency input power, $T_e$ decreases from 2.5 eV to 1.8 eV, whereas $n_e$ increases from $7.7\times 10^{11} cm^{-3}$ to $1.4 \times 10^{12} cm^{-3}$. In addition, the spatiotemporal evolutions of $e-a$ bremsstrahlung and $T_e$ are observed and clearlt show that the electron heating is noticeably diminished by the low-frequency voltage at certain phases. Another external factor that affect to electron properties significantly is the gas pressure. As the gas pressure deceases from 760 Torr to 200 Torr, the time-averaged $n_e$ increases from $7.5 \times 10^{11} cm^{-3} to 1.2 \times 10^{13} cm^{-3}$ while $T_e$ decreases from 2.5 eV to 1.2 eV at the same current density. As a spatiotemporal evolution of $n_e$ and $T_e$, the electron heating structure is changed with the gas pressure

$T_e$ increases uniformly throughout the plasma bulk region during sheath expansion and collapse at 760 Torr, but the electron heating with the sheath collapse weakens as the gas pressure decreases.