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Langmuir Probe Diagnostics with Optical Emission Spectrometry (OES) for Coaxial Line Microwave Plasma

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Langmuir Probe Diagnostics with Optical Emission Spectrometry (OES) for Coaxial Line Microwave Plasma applied sciences Article Langmuir Probe Diagnostics with Optical Emission Spectrometry (OES) for Coaxial Line Microwave Plasma Chi Chen , Wenjie Fu * , Chaoyang Zhang , Dun Lu , Meng Han and Yang Yan School of Electronic Science and Engineering and Terahertz Science and Technology Key Laboratory of Sichuan Province, University of Electronic Science and Technology of China, Chengdu 610054, China; [email protected] (C.C.); [email protected] (C.Z.); [email protected] (D.L.); [email protected] (M.H.); [email protected] (Y.Y.) * Correspondence: [email protected] Received: 28 October 2020; Accepted: 12 November 2020; Published: 16 November 2020 Featured Application: Plasma diagnostics with a dirty Langmuir Probe. Abstract: The Langmuir probe is a feasible method to measure plasma parameters. However, as the reaction progresses in the discharged plasma, the contamination would be attached to the probe surface and lead to a higher incorrect electron temperature. Then, the electron density cannot be obtained. This paper reports a simple approach to combining the Langmuir probe and the optical emission spectrometry (OES), which can be used to obtain the electron temperature to solve this problem. Even the Langmuir probe is contaminative, the probe current–voltage (I–V) curve with the OES spectra also gives the approximate electron temperature and density. A homemade coaxial line microwave plasma source driven by a 2.45 GHz magnetron was adopted to verify this mothed, and the electron temperature and density in different pressure (40–80 Pa) and microwave power (400–800 W) were measured to verify that it is feasible. Keywords: plasma diagnostics; Langmuir probe; optical emission spectrometry; electron temperature; electron density 1. Introduction In industrial plasma processing, such as microwave plasma chemical vapor deposition (PECVD) [1–3], it would be useful to directly control plasma properties such as electron temperature and density [4]. For plasma diagnostics, the Langmuir is widely used [5,6]. A Langmuir probe means a probe placed in charged plasma. Though measuring the current–voltage (I–V) curve, the plasma’s electron temperature and density can be estimated. A lot of Langmuir probe structures have been researched by a lot of institutions around the world to monitor various plasmas. However, when the Langmuir probe is used to measure plasma for a long time, especially in PECVD, as the plasma reaction proceeds, the Langmuir probe will adsorb impurities and become dirty. Moreover, if the electron temperature and density are obtained by calculating this dirty Langmuir probe (I–V) curve, the electron temperature and density would be incorrect [4,7–9]. The contamination on the probe surface is very common in various probe designs. The contamination has become a significant problem that limits the application of the Langmuir probe. The optical emission spectrometry (OES) is a non-intrusive plasma diagnostics method, which is easy to performed, and it is also frequently used in studies of plasma [10–12]. Therefore, the plasma reaction cannot bring contamination to the OES. During the plasma reaction, OES can stably measure reliable electron temperature. The Langmuir can measure both the electron temperature and electron Appl. Sci. 2020, 10, 8117; doi:10.3390/app10228117 www.mdpi.com/journal/applsci Appl.Appl. Sci. Sci.2020 2020, 10, 10, 8117, x 2 2of of 11 11 reliable electron temperature. The Langmuir can measure both the electron temperature and electron density. Both electron temperature and electron density are very important for diagnosing plasma. density. Both electron temperature and electron density are very important for diagnosing plasma. However, as the plasma reaction progresses, the contamination is attached to the probe surface. Most of However, as the plasma reaction progresses, the contamination is attached to the probe surface. Most these clean probe methods need to interrupt the plasma reaction. However, in the experiment, the result of these clean probe methods need to interrupt the plasma reaction. However, in the experiment, the shows that the contamination on the Langmuir probe has little effect on the ion saturation current, result shows that the contamination on the Langmuir probe has little effect on the ion saturation ascurrent, reported as inreported other articles in other [7 ,8articl,13].es In [7,8,13]. other words, In other the words, ion saturation the ion saturation does not change. does not Therefore, change. ifTherefore, the electron if the temperature electron temperatureTev can be measured Tev can be by measured optical emission by optical spectrometry emission spectrometry (OES), the electron (OES), densitythe electron can be density calculated can be from calculated the I–V from curve the of I–V the curve probe of with the contamination.probe with contamination. Therefore, combiningTherefore, thecombining Langmuir the probe Langmuir and OES, probe both and electron OES, both temperature electron temperature and density and would density be measured, would be measured, even if the probeeven hasif the been probe contaminated. has been contaminated. In other words, In other the wo cleaningrds, the interval cleaning of interval the Langmuir of the Langmuir probe would probe be significantlywould be significantly extended. extended. BasedBased onon the above above proposal, proposal, a veri a verificationfication experiment experiment is carried is carried on and on presented and presented in this paper. in this paper.The experiment The experiment has been has done been in done a homemade in a homemade coaxial coaxialline microwave line microwave plasma plasmasource driven source by driven 2.45 byGHz 2.45 magnetron GHz magnetron and using and argon using as argon feeding as gas. feeding The gas.results The show results that showthe contaminative that the contaminative Langmuir Langmuirprobe I–V probe curve I–V with curve the OES with can the give OES the can approxim give the approximatelyately electron temperature electron temperature and density. and Besides, density. Besides,by measuring by measuring the electron the electron temperature temperature and anddensity density under under different different pressures pressures (40–80 (40–80 Pa) Pa) and and microwavemicrowave power power (400–800 (400–800 W), W), it it is is proved proved that that thisthis methodmethod isis feasible.feasible. 2.2. Experiment Experiment Setup Setup TheThe schematic schematic diagram diagram ofof thethe microwave-generatedmicrowave-generated plasma system used in in the experiment is is shownshown in in Figure Figure1. 1. FigureFigure 1. 1. SchematicSchematic diagramdiagram ofof thethe coaxialcoaxial microwave-drivenmicrowave-driven plasma with Langmuir probe probe and and emissionemission spectroscopy. spectroscopy. The magnetron was used to generate continuous-wave signals at a fixed output frequency of The magnetron was used to generate continuous-wave signals at a fixed output frequency of 2.45 GHz. The output power of the magnetron varied from 0–800 W. The output wave was launched 2.45 GHz. The output power of the magnetron varied from 0–800 W. The output wave was launched intointo the the WR340 WR340 waveguide waveguide using using TE10 TE10 modemode toto transfer.transfer. AnAn isolationisolation circulator with with a a water water load load in in anan isolated isolated way way was was connected connected to to the the magnetron.magnetron. AA waveguide coaxial connecter was was used used for for waves waves toto enter enter the the coaxial coaxial plasma plasma reaction reaction chamber chamber fromfrom thethe WR340WR340 waveguide.waveguide. An An automated, automated, three three stobs stobs tunertuner was was used used to to match match the the impedance, impedance, soso thatthat thethe maximummaximum amountamount of power could be be coupled coupled withwith the the plasma. plasma. ThisThis reaction reaction cavity cavity used used a coaxial a coaxial waveguide wavegu foride microwave for microwave transmission. transmission. The external The diameterexternal ofdiameter the coaxial of the waveguide coaxial waveguide was 150 mm, was and150 itsmm, inner and conductorits inner conductor was 8 mm. was A 8 hollow mm. A glass hollow rod glass was usedrod towas surround used to thesurround coaxial the inner coaxial diameter. inner The diameter inner. diameter The inner of diameter the glass tubeof the was glass 29 tube mm, was and the29 externalmm, and diameter the external was 33diameter mm. Using was 33 two mm. O-ring Using seals two to O-ring form aseals vacuum-sealed to form a vacuum-sealed cavity in the middlecavity ofin the the glass middle rod’s of the outer glass wall, rod’s both outer ends wall, of theboth glass ends rod of werethe glass connected rod were to connected an O-ring to seal an toO-ring form aseal vacuum-sealed to form a vacuum-sealed cavity between cavity the between glass rod’s the gl outerass rod’s wall outer and thewall reaction and the reaction cavity inner cavity surface. inner Appl. Sci. 2020, 10, 8117 3 of 11 Appl. Sci. 2020,, 10,, xx 3 of 11 Theresurface. were There several were KF several flanges KF on flanges the outer on the wall outer of the wall cavity, of the and cavity, the Langmuir and the Langmuir probe was probe mounted was onmounted a KF flange, on a KF which flange, is shown which inis detailshown in in Figure detail2 in. Figure 2. Figure 2. TheThe structure of the coaxial reacti reactionon cavity and the Langmuir probe.probe. InIn thethe
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