Langmuir probe diagnostic studies of pulsed hydrogen plasmas in planar microwave reactors

Abstract

Langmuir probe techniques have been used to study time and spatially resolved electron densities and electron temperatures in pulse-modulated hydrogen discharges in two different planar microwave reactors (f microwave = 2.45 GHz, t pulse = 1 ms). The reactors are (i) a standingwave radiative slotted waveguide reactor and (ii) a modified travelling-wave radiative slotted waveguide reactor, which generate relatively large plasmas over areas from about 350 cm 2 to 500 cm 2 . The plasma properties of these reactor types are of particular interest as they have been used for basic research and for plasma processing, e.g. for surface treatment and layer deposition. In the present study the pressures and microwave powers in the reactors were varied between 33 and 55 Pa and 600 and 3600 W, respectively. In regions with high electromagnetic fields shielded Langmuir probes were used to avoid disturbances of the probe characteristic. Close to the microwave windows of the reactors both the electron density and the electron temperature showed strong inhomogeneities. In the standing-wave reactor the inhomogeneity was found to be spatially modulated by the position of the slots. The maximum value of the electron temperature was about 10 eV and the electron density varied between 0.2 and 14×10 11 cm -3 . The steady state electron temperature in a discharge pulse was reached within a few tens of microseconds whereas the electron density needed some hundreds of microseconds to reach a steady state. Depending on the reactor the electron density reached a maximum between 80 and 200 µs after the beginning of the pulse.2

Elevating the Precision of Plasma Probe Diagnostics by Elimination of Bare Probe Protective Shields’ Influence

“…To determine radial distributions for T e measurement corrections, we inserted equations (2)-(5) into the equation (6), which resulted in the following expressions: (7), (8) and (9), we obtained similar measurement correction dependencies for the remaining three plasma parameters, n e , V s , and j es as graphically shown in figures 5-7. According to these dependencies, errors in T e measurements reached about 7%, for n e -up to 14%, for V s -up to 20%, and for j es -up to 28 %.…”

“…Therefore, in this situation, plasma state could hardly feel the presence of a probe in plasma. That is why probe leads were usually surrounded by bare metal shields to protect them against RF interferences [5,6]. According to our previous works, where the behavior of large conducting bodies immersed in plasma was analyzed qualitatively, it was found that such bodies self consistently acquired single plasma floating potential at the point close to the minimal floating potential beside the body.…”

“…Usually real scale of this phenomenon was considered to be rather low when metal shields shortcircuited rather small plasma potential differences and collected currents that were much less than the discharge currents which could hardly influence plasma state. That is why probe leads are usually protected by bare metal shields against RF interferences [5,6].…”

Plasma Parameter Analysis During Nano-Crystalline Diamond Growth Using Automated Langmuir Probe

“…It was reported that ordinary probes containing metal tip inside dielectric capillary do not provide reliable results in microwave plasma since they were strongly perturbed by the electromagnetic field of the microwave. 19 Therefore, shielded probes, made from solid coaxial rod that was comprised of an external copper layer on a floating potential surrounding its inner 2-mm radius dielectric and a 1-mm radius metal core, were utilized. The metal tip of the probe that was exposed to the plasma was made from a nickel-chromium (Ni-Cr) alloy spot welded to the inner metal core of the solid coaxial rod.…”

Plasma diagnostics and device properties of AlGaN/GaN HEMT passivated with SiN deposited by plasma-enhanced chemical vapour deposition

“…Plasma diagnostic, allowing to identify transient species, as well as stable products and other properties involved in plasma processing, has become a powerful method to understand the mechanisms which ultimately lead to the growth of films of the most diverse characteristics [19][20][21]. The diagnostic is mainly carried out with spectroscopic techniques like visible emission of excited species [6,[22][23][24][25] or infrared absorption of molecular bands [7,[26][27][28][29][30]; by mass spectrometry of neutrals [1][2][3]6,31,32] and ions [33][34][35], and with Langmuir probes [36,37].…”