applied sciences

Article Diagnostics with Optical Emission Spectrometry (OES) for Coaxial Line Microwave

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 . 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– (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. 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 experiment,experiment, firstly, firstly,firstly, aa vacuumvacuum pumppump was was used to pump the pressure below 10 Pa. Then, thethe argonargon flowflowflow rateraterate waswaswas adjustedadjustedadjusted tototo stabilizestabilizestabilize thethethe workingworking pressurepressure toto 8080 Pa.Pa. After enteringentering thethe microwave with 800 W power, thethe plasmaplasma waswas discharged,discharged, andand thethe lightlight could be observed from the observation window, asas shownshown inin FigureFigure3 3..

Figure 3. CoaxialCoaxial line plasma driven by 800 W microwave at fill fill pressure ofof 8080 Pa.Pa.

3. The Method of Electron Density and Temperature byby LangmuirLangmuir ProbesProbes For low-temperature plasmasplasmas ((<10<10 eV), eV), the the Langmuir Langmuir probes probes are generally used to measure the plasmaplasma density and electron temperature. A A Langmuir Langmuir probeprobe probe consistsconsists ofof of aa a barebare wirewire oror metal metal disk, disk, immersedimmersed inin thethe plasma,plasma, withwith aa sweep sweep voltage voltage toto collect collect electron electron andand ionsions [[5].[5].5]. InIn thisthis study,study, the the probe probe had a 1.0 mm diameter and was 20 mm long, suspended inin a 12 mm tube with the vacuum seal at the upper end. The probe tip was a tungsten rod 1 mm in diameterdiameter andand 88 mmmm long.long. When a probeprobe isis immersedimmersed in plasmas,plasmas, the current is dependentdependent on the ions current and the electron current.current. By loading a a sweeping sweeping voltage voltage on on thethe probe,probe, thethe I–VI–V curvecurve ofof thethe probeprobe isis obtained.obtained. AccordingAccording toto thethe didifferentfferent currentcurrent composition,composition, thethe I-VI-V curvecurve cancan be be divided divided into into three three regions regions [ 5[5]:]: (1)(1) IonIon saturationsaturation region.region. Appl. Sci. 2020, 10, 8117 4 of 11

(1) Ion saturation region. (2) Transition region. (3) Electron saturation region where ions are repelled, but electrons are attracted to the probe.

When a negative voltage is applied on the probe, the positive ions approach to the probe. However, there is an electronic shield on the probe surface, which is the electron Debye length. The ion saturation region means the probe voltage (Vp) on the probe is sufficiently negative to collect all the ions with the Bohm velocity. The ions are accelerated through a “pre-sheath” to the required energy. When Te T , the ion saturation current is determined by the electron temperature.  i Therefore, ion saturation current is, approximately, [7,14–16]: r kT I = 0.605A ne e (1) sat probe mi

Here, e is the electron charge, k is Boltzmann constant, Te is electron temperature, Aprobe is the surface area of the probe, mi is the ion mass. n is the plasma density in this paper, and we only consider the case where electron density and ion density are equal. In the transition region, the current consists of both electron current and ions current. Compared to electron current, the ion current is negligible. The electron current is expressed as follows:   e(V V  p s)  Ie = Ie0 exp −  (2)  kTe 

 1/2 kTe Ie0 = ne Aprobe (3) 2πme

Here, Vs is the space potential, Ie is the electron current, Ie0 is the saturation electron current. Equation (4) can be achieved by transforming Equation (2). Equation (4) shows that the slope of the (ln I)–Vp curve is exactly 1/TeV (which is convenient to write kTe/e as Te/V, the electron temperature in eV) and is a good measure of the electron temperature [5,7].

1   lnIe = Vp Vs + lnIe0 (4) Tev −

Bring the result of Tev into Equation (1), the plasma density (n) can be obtained. When the plasma is just charged, the I–V curve of the Langmuir probes immersed in coaxial microwave driven plasma is shown in Figure4, which is measured under the argon fill pressure Pa = 80 Pa and microwave power Mpower = 800 W. The applied voltage on Langmuir probes is AC 60 V-50 Hz. As shown in Figure4, when the voltage is positive, the voltage curve is consistent with the current curve. When the voltage is negative, the current curve first decreases with the voltage and then becomes smooth at around about 20 V. This is because +60 V does not make the probe current reach − the electron saturation region. Therefore, when the voltage increases from 0 V to 60 V, the current also increases exponentially. When the voltage is less than 20 V, the ion current variation is very small − (less than 0.005 mA), and it enters the ion saturation region. Combining the voltage curve and the current curve, the I–V curve of the Langmuir probes containing ion saturation region and transition region can be obtained. Appl. Sci. 2020, 10, x 4 of 11

(2) Transition region. (3) Electron saturation region where ions are repelled, but electrons are attracted to the probe. When a negative voltage is applied on the probe, the positive ions approach to the probe. However, there is an electronic shield on the probe surface, which is the electron Debye length. The ion saturation region means the probe voltage (Vp) on the probe is sufficiently negative to collect all the ions with the Bohm velocity. The ions are accelerated through a “pre-sheath” to the required energy. When ≫, the ion saturation current is determined by the electron temperature. Therefore, ion saturation current is, approximately, [7,14–16]:

kT I = 0.605A ne (1) mi

Here, e is the electron charge, k is Boltzmann constant, Te is electron temperature, A is the surface area of the probe, mi is the ion mass. n is the plasma density in this paper, and we only consider the case where electron density and ion density are equal In the transition region, the current consists of both electron current and ions current. Compared to electron current, the ion current is negligible. The electron current is expressed as follows:

e(V −V) I =Iexp[ ] (2) kT

kT ⁄ I =ne( ) A (3) 2πm

Here, Vs is the space potential, I is the electron current, I is the saturation electron current. Equation (4) can be achieved by transforming Equation (2). Equation (4) shows that the slope of the (ln I)–Vp curve is exactly 1/TeV (which is convenient to write kTe/e as Te/V, the electron temperature in eV) and is a good measure of the electron temperature [5,7]. 1 lnI = (V −V)+lnI (4) T Bring the result of Tev into Equation (1), the plasma density (n) can be obtained. When the plasma is just charged, the I–V curve of the Langmuir probes immersed in coaxial microwave driven plasma is shown in Figure 4, which is measured under the argon fill pressure Pa = 80 PaAppl. and Sci. microwave2020, 10, 8117 power Mpower = 800 W. The applied voltage on Langmuir probes is AC 60 V-505 Hz. of 11

Appl. Sci. 2020, 10, x 5 of 11 then becomes smooth at around about −20 V. This is because +60 V does not make the probe current reach the electron saturation region. Therefore, when the voltage increases from 0 V to 60 V, the current also increases exponentially. When the voltage is less than −20 V, the ion current variation is very small (less than 0.005(a) mA), and it enters the ion saturation region. Combining(b) the voltage curve and theFigureFigure current 4. 4. TheThe curve, I–V I–V curve curve the ofI–V of the the curve clean clean Langmuirof Langmuir the Langmu probe probe (ir (aa )) probesthe the voltage voltage containing and and current current ion curve, curve,saturation ( (bb)) I–V I–V region curve. curve. and transition region can be obtained. FromAsFrom shown the the I–V I–Vin curveFigure curve of of4, the thewhen Langmuir Langmuir the voltage probes, probes, is theposi the (lntive, (ln I− IVp) theVp) curve voltage curve slope slopecurve in intheis the consistenttransition transition regionwith region the is − 0.736,currentis 0.736, namely curve. namely Tev.probeWhenTev.probe the = voltage1.3578= 1.3578 eV.is eV.negative, In Inaddition, addition, the currentthe the average average curve current first current decreases in in the the ion ionwith saturation saturation the voltage current current and regionregion is is 0.0613 0.0613 mA, mA, namely namely IsatIsat == 0.06130.0613 mA. mA. Inserting Inserting the the parameters parameters in in Table Table 11 intointo Equation Equation (1), (1), itit can can be be found found that that the the density density of of the the plasma plasma is is n = 1.35691.3569 ×10 1016 m3. × − − TableTable 1. 1. ParametersParameters of of the the coaxial coaxial microwave microwave driven driven plasma plasma and and Langmuir Langmuir probes probes used used to to measure measure electronelectron temperature temperature and and density. density.

ParametersParameters SymbolSymbol Value Value Units Units Ion speciesIon species Ar+ Ar+ Ion massIon mass mi mi 6.76.7 ×10 10 26 kg kg × − MicrowaveMicrowave power power Mpower Mpower 800 W W PressurePressure PaPa 80 Pa Pa Probe diameter dprobe 1 mm Probe diameter dprobe 1 mm Probe length lprobe 8 mm Probe length lprobe 8 mm

TheThe Langmuir probeprobe isis a a feasible feasible method method to measureto measure the plasma.the plasma. However, However, as the plasmaas the plasma reaction reactionprogresses, progresses, there is graduallythere is gradually some phase some diff erencephase betweendifference the between voltage curvethe voltage and the curve current and curve, the currentand the curve, current and is decreasedthe current compared is decreased with compar the beginning,ed with the as shownbeginning, in Figure as shown5. in Figure 5.

FigureFigure 5. 5. TheThe voltage voltage and and current current curve curve of of the the Langmuir Langmuir probe probe with with contamination. contamination.

It is generally considered that hysteresis and current decrease are caused by surface contamination. These two features are well explained by the equivalent circuit model, reported by Hirao and Oyama (1972), as shown in Figure 6 [13]. Appl. Sci. 2020, 10, 8117 6 of 11

Appl. Sci. 2020, 10, x 6 of 11 It is generally considered that hysteresis and current decrease are caused by surface contamination. These two features are well explained by the equivalent circuit model, reported by Hirao and Oyama

(1972),Appl. Sci. as2020 shown, 10, x in Figure6[13]. 6 of 11

I Vp

I I Vp Vp RC

CC I Vp C R Figure 6. Equivalent circuit of the electrode contamination. CC

Figure 6 shows that the contamination layer on the probe surface might cause the capacitor Cc and the resister Rc. The CcFigure and 6. Rc Equivalent make the circuit hysteres of the is andelectrode current contamination. decreased. Therefore, if the power supply of the Langmuir probe is changed from AC to DC, the effect of the capacitor Cc can be counteracted.Figure6 6 shows showsLoading thatthat DC thethe supply contaminationcontamination on the Langmuir layerlayer onon theprobethe probeprobe and surface surfacesweeping mightmight the causecausevoltage thethe from capacitorcapacitor −60 V Cc Ccto +60and V, the the the resister resister I–V curve Rc. Rc. The can The Cc be Cc and obtained, and Rc Rcmake makeas theshown thehysteres in hysteresis Figureis and 7. current and current decreased. decreased. Therefore, Therefore, if the power if the powersupplyAlthough supplyof the oftheLangmuir the effect Langmuir of probe capacitance probe is changed is is changed eliminated, from from AC resistance ACto DC, to DC, thewill the effect still eff causeect of ofthe erroneous the capacitor capacitor information. Cc Cc can can be Itcounteracted. is well known Loading that the DC contamination supply on the attached Langmuir to probe the probe and sweepingsurface leads the voltageto a higher from incorrect60 V to counteracted. Loading DC supply on the Langmuir probe and sweeping the voltage from −−60 V to +electron+6060 V, thethe temperature. I–VI–V curvecurve cancan bebe obtained,obtained, asas shownshown inin FigureFigure7 .7. Although the effect of capacitance is eliminated, resistance will still cause erroneous information. It is well known that the contamination attached to the probe surface leads to a higher incorrect electron temperature.

Figure 7. The I–V curve of the clean Langmuir probe an andd the probe with contaminations driven by DC supply.

Although the effect of capacitance is eliminated, resistance will still cause erroneous information. FromFigure Figure 7. The I–V7, it curve can be of seenthe clean that Langmuirthere is no probe significant and the changeprobe with in ion contaminations saturation current. driven by Other It is well known that the contamination attached to the probe surface leads to a higher incorrect studiesDC on supply. probes also show that the contamination on the probe has little effect on ion saturation currentelectron [7,8,13]. temperature. When the voltage is less than −48 V, the ion current of the probe with contaminations From Figure7, it can be seen that there is no significant change in ion saturation current. Other entersFrom the ion-electron Figure 7, it cansaturation be seen region. that there The is average no significant current change in this regionin ion saturationis 0.0601 mA current. and is Other close studies on probes also show that the contamination on the probe has little effect on ion saturation tostudies the ion on saturation probes also of theshow clean that Langmuir the contaminatio probe. n on the probe has little effect on ion saturation current [7,8,13]. When the voltage is less than 48 V, the ion current of the probe with contaminations currentThe [7,8,13]. probe surface When thecleaning voltage can is beless done than in − 48several V, the ways, ion current such as of ion the bombardment, probe with contaminations warm probe, andenters emissive the ion-electron probe [17–19]. saturation Howeve region.r, most The of average these methods current inneed this to region interrupt is 0.0601 the plasma mA and reaction. is close Evento the the ion probesaturation has beenof the cleaned, clean Langmuir the probe probe. also would be contaminated. In the chemical reaction processThe suchprobe as surface PECVD, cleaning the cancontamination be done in several would ways, be insuch several as ion bombardment,minutes. From warm the probe,above and emissive probe [17–19]. However, most of these methods need to interrupt the plasma reaction. Even the probe has been cleaned, the probe also would be contaminated. In the chemical reaction process such as PECVD, the contamination would be in several minutes. From the above Appl. Sci. 2020, 10, 8117 7 of 11 enters the ion-electron saturation region. The average current in this region is 0.0601 mA and is close to the ion saturation of the clean Langmuir probe. The probe surface cleaning can be done in several ways, such as ion bombardment, warm probe, and emissive probe [17–19]. However, most of these methods need to interrupt the plasma reaction. Even the probe has been cleaned, the probe also would be contaminated. In the chemical reaction process such as PECVD, the contamination would be in several minutes. From the above presentation, the Isat can also be measured by the probe with contamination. The electron density n in Equation (4) can be calculated from the I–V curve of the probe with contamination, if the electron temperature Tev is measured by optical emission spectrometry (OES).

4. The Method of Electron Temperature by OES Because this coaxial line plasma is discharged in the enclosed cavity, and the gas flow rate is relatively low, it is usually considered to work at local thermodynamic equilibrium (LTE) [20]. In this case, electron temperature Te can be determined by the relative intensity of two or more spectral lines of the atoms, which have the same ionized stage. In LTE condition, the radiation energy density can be expressed as [21]: 1 ε = N A hv (5) mn 4π m mn mn where the subscripts m and n denotes the upper level and the lower energy level of the particles, respectively. εmn is the emission coefficients, Nm is the particles density of upper level, Amn is the transition probability. hvmn is the energy of the transition, where vmn is the frequency of the radiant wave. Using the speed of the light c, the wavelength of the emission light λmn is given by c λmn = (6) vmn

This leads immediately to 1 ε = N A hc/λ (7) mn 4π m mn mn

In complete LTE at the temperature Te (eV), Maxwell–Boltzmann statistics must be applied.   Nm g Em En = m exp − (8) Nn gn − kTe where g is the statistical weight of the level, E is the energy of the level in eV, k is the Boltzmann constant factor. The emission intensity measured by spectrometer can be written as

ε rη λ Ii = iVΩTi i i (9)

r where V is the volume of the measured plasma, Ω is the acceptance of solid angle, Ti is the transmittance of the plasma, ηi is the efficiency of the spectrometer. The emission coefficients of the two spectral lines can be expressed by the following formula according to Equation (10); hence, the spectral is measured with a fixed position and the same spectrometer [22].

I ε λ2A1g E E  1 = 1 = 1 exp 2 − 1 (10) I2 ε2 λ1A2g2 kTe

The subscript denotes the chosen spectrum lines. Take the natural logarithm of both sides of Equation (10) leads immediately to ! I1λ1A2g E E ln 2 = 1 − 2 (11) I2λ2A2g2 − kTe Appl. Sci. 2020, 10, 8117 8 of 11

Since multiple spectral lines are used to improve accuracy, taking only one spectral line, Equation (11) can be written as: ! λiIi 1 ln = Ei + C (12) Aigi −kTe where C is a constant. Through measurements and NIST atomic spectra database [23], all values except Te.oes are known. Substituting the adopted spectral lines into Equation (12), by fitting the curve’s slope, the electron temperature Tev.oes can be calculated. The spectrum of this coaxial line plasma is shown in Figure8, which is measured under the argon

Appl.fill pressure Sci. 2020, 10 Pa, x= 80 Pa and the microwave power Mpower = 800 W. 8 of 11

FigureFigure 8. The spectrum of the coaxial line plasma plasma driven driven by by 800 800 W W microwave microwave at at the the pressure pressure of of 80 80 pa. Pa.

The spectrum indicates two kinds of particles, nitrogennitrogen moleculemolecule (N(N22)) andand Ar.Ar. N2 comescomes from air residue. The The air residue is also one of the importantimportant causes of probe contamination. Compared with the nitrogen molecule, the peak of this spectrum of of Ar I is very obvious with high intensity, which improves the the accuracy accuracy of of measurement measurement of ofTev.Tev oes.oes. Although. Although there there are are many many spectral spectral lines lines of Ar, of this Ar, doesthis does not notmean mean that that all spectral all spectral lines lines are are suitable suitable for for calculating calculating electron electron temperature, temperature, because because some spectral lines have a superposition of other spectral lines with a different different wavelength, which is very close; besides, the presence of Stark broadening and and Doppler Doppler broadening broadening would would decrease decrease accuracy. accuracy. Therefore, those spectral lines with a smaller FWHM should be selected. The parameters required by the selected spectrum lines in this articlearticle areare givengiven inin TableTable2 2..

TableTable 2. Spectroscopic data of chosen lines for determinationdetermination of electron temperature [[23].23].

λi (nm) Ai1 (s−1) gi Eki (eV) λi (nm) Ai (s− ) gi Eki (eV) 706.7217 3.80 × 106 5 13.30223 706.7217 3.80 106 5 13.30223 763.5105 × 2.45 × 107 5 13.17178 763.5105 2.45 107 5 13.17178 × 7 772.4207 772.42071.17 1.1710 7× 10 3 313.32786 13.32786 × 6 801.4785 801.47859.30 9.3010 6× 10 5 513.09487 13.09487 × 6 866.7943 866.79432.43 2.4310 6× 10 3 313.15314 13.15314 × 7 912.2967 912.29671.89 1.8910 7× 10 3 312.90702 12.90702 × 6 922.4498 922.44985.00 5.0010 6× 10 5 513.17178 13.17178 × 978.4502 978.45021.47 1.4710 6× 106 5 513.09487 13.09487 × Using the peak value of the normalized intensity in Figure 9 and the parameters in Table 2, the valueUsing of the the specified peak value spectral of the line normalized can be calc intensityulated. inDraw Figure multiple9 and thepoints parameters and perform in Table linear2, regression;the value of the the line’s specified slope spectral can be used line canto calculate be calculated. the electron Draw temperature. multiple points The and result perform is presented linear inregression; Figure 10 the and line’s is determined slope can to be be used 1.452 to eV calculate in this theconfiguration. electron temperature. If the electron The temperature result is presented Tev.oes =in 1.452 Figure eV 10 obtained and is determinedby OES and tothe be ion 1.452 saturation eV in thiscurrent configuration. Isat = 0.0601 IfmA the obtained electron by temperature the probe Tev.oeswith contamination= 1.452 eV obtained is taken by into OES Equation and the ion(4), saturationthe electron current densityIsat can= 0.0601be obtained, mA obtained which byis = the probe1.2839 with × 10 contamination m. is taken into Equation (4), the electron density can be obtained, which is n = 1.2839 1016 m 3. × −

Figure 9. Liner fit for data of specified spectral lines driven by 800 W microwave at the pressure of 80 Pa. Appl. Sci. 2020, 10, x 8 of 11

Figure 8. The spectrum of the coaxial line plasma driven by 800 W microwave at the pressure of 80 pa.

The spectrum indicates two kinds of particles, nitrogen molecule (N2) and Ar. N2 comes from air residue. The air residue is also one of the important causes of probe contamination. Compared with the nitrogen molecule, the peak of this spectrum of Ar I is very obvious with high intensity, which improves the accuracy of measurement of Tev. oes. Although there are many spectral lines of Ar, this does not mean that all spectral lines are suitable for calculating electron temperature, because some spectral lines have a superposition of other spectral lines with a different wavelength, which is very close; besides, the presence of Stark broadening and Doppler broadening would decrease accuracy. Therefore, those spectral lines with a smaller FWHM should be selected. The parameters required by the selected spectrum lines in this article are given in Table 2.

Table 2. Spectroscopic data of chosen lines for determination of electron temperature [23].

λi (nm) Ai (s−1) gi Eki (eV) 706.7217 3.80 × 106 5 13.30223 763.5105 2.45 × 107 5 13.17178 772.4207 1.17 × 107 3 13.32786 801.4785 9.30 × 106 5 13.09487 866.7943 2.43 × 106 3 13.15314 912.2967 1.89 × 107 3 12.90702 922.4498 5.00 × 106 5 13.17178 978.4502 1.47 × 106 5 13.09487

Using the peak value of the normalized intensity in Figure 9 and the parameters in Table 2, the value of the specified spectral line can be calculated. Draw multiple points and perform linear regression; the line’s slope can be used to calculate the electron temperature. The result is presented in Figure 10 and is determined to be 1.452 eV in this configuration. If the electron temperature Tev.oes = 1.452 eV obtained by OES and the ion saturation current Isat = 0.0601 mA obtained by the probe Appl.with Sci.contamination2020, 10, 8117 is taken into Equation (4), the electron density can be obtained, which is9 of= 11 1.2839 × 10 m.

Appl. Sci. 2020, 10, x 9 of 11

5. Result and Conclusions To confirm the reliability of the Langmuir probe and OES, experiments were carried out at different microwave powers and different air pressures. The electron temperature was measured by the clean probe, the probe with contamination, and optical emission spectroscopy method. The electron density was measured by the clean probe, the OES(contribute the Tev.oes), and the probe Figure 9. Liner fit for data of specified spectral lines driven by 800 W microwave at the pressure of 80 Pa. withFigure contamination(contribute 9. Liner fit for data of specifiedthe Isat). spectralThe result lines is shown driven byin Figure 800 W microwave10. at the pressure of 80 Pa.

FigureFigure 10. 10.The The microwave microwave power power and pressureand pressure against agains the electront the electron density density and temperature and temperature measured bymeasured the probe by and the opticalprobe and emission optical emission spectrometry spectrometry (OES). ((OES).a,b) The (a,b microwave) The microwave power power changes changes at the pressureat the pressure of 80 Pa, of ( c80,d )Pa, the (c pressure,d) the pressure changes changes at the microwaveat the microwave power power of 800 of W. 800 W. 5. Result and Conclusions Figure 10 shows that the contamination attached to the probe surface leads to higher incorrect electronTo confirm . the reliability The electron of the temperatures Langmuir probe obtained and OES,by the experiments probe with werecontamination carried out are at diffabouterent 20 microwave eV higher powers than the and temperatures different air obtained pressures. by The the electron clean probe. temperature When the was higher measured incorrect by the cleanelectron probe, temperatures the probe with are used contamination, to calculate andthe densit opticaly, emissionthe result spectroscopy will be significantly method. less The than electron the densitycorrect waselectron measured density. by Comparing the clean the probe, temperature the OES(contribute obtained by OES, the theTev.oes clean), probe, and theand probethe probe with contamination(contributewith contamination, the temperature the Isat). The Tev.oes result obtained is shown by in OES Figure is much 10. closer to Tev.probe obtained by theFigure clean probe 10 shows than thatby probe the contamination with contamination. attached The to Tev.oes the probe is slightly surface higher leads than to higher that Tev.probe incorrect electronobtained temperatures. by the clean Theprobe, electron which temperatures is maybe caus obtaineded by the by OES the measurements probe with contamination only presenting are aboutthe 20temperature eV higher than of a thepart temperatures of electrons, especially obtained bythe thehigh-energy clean probe. electrons When [20]. the higherThe difference incorrect between electron these two electron temperatures decreases as the temperature drops. When the electron temperatures are lower than 1.5 eV, Tev.oes is basically equal to Tev.probe. Therefore, in the low-temperature plasma, especially below 1.5 eV, the electron temperature obtained by OES is accurate. For the high- temperature plasma, the electron temperature obtained may be higher than the correct electron temperature. Moreover, when the Tev.oes is brought into Equation (4), the slightly lower density is obtained in the same Isat. Using this electron temperature, Tev.oes can obtain the approximate electron density from the I–V curve of the Langmuir probe with contamination. Appl. Sci. 2020, 10, 8117 10 of 11 temperatures are used to calculate the density, the result will be significantly less than the correct electron density. Comparing the temperature obtained by OES, the clean probe, and the probe with contamination, the temperature Tev.oes obtained by OES is much closer to Tev.probe obtained by the clean probe than by probe with contamination. The Tev.oes is slightly higher than that Tev.probe obtained by the clean probe, which is maybe caused by the OES measurements only presenting the temperature of a part of electrons, especially the high-energy electrons [20]. The difference between these two electron temperatures decreases as the temperature drops. When the electron temperatures are lower than 1.5 eV, Tev.oes is basically equal to Tev.probe. Therefore, in the low-temperature plasma, especially below 1.5 eV, the electron temperature obtained by OES is accurate. For the high-temperature plasma, the electron temperature obtained may be higher than the correct electron temperature. Moreover, when the Tev.oes is brought into Equation (4), the slightly lower density is obtained in the same Isat. Using this electron temperature, Tev.oes can obtain the approximate electron density from the I–V curve of the Langmuir probe with contamination. Figure 10 also shows that the electron density is increased with improving microwave power; meanwhile, the electron temperature does not change significantly. Variation of electron density and temperature with pressure are also presented. The results show that when the pressure increases, both the electron density and temperature are decreased. This paper reports that a simple method to solve the contamination is attached to the probe surface, as the plasma reaction progresses. The electron temperature Tev and electron density n are important parameters in plasma diagnosis. The contamination on the probe surface leads to a higher electron temperature than the correct value. Then, the correct electron density cannot be obtained. The optical emission spectrometry (OES) can be used to solve this problem by obtaining the electron temperature. In addition, the electron temperature obtained by OES spectra can be used to calculate the electron density in the contaminative Langmuir probe I–V curve. The electron temperature and density in different pressure (40–80 Pa) and microwave power (400–800 W) are measured. It shows that this electron temperature and density obtained by this method are close to the clean probe. Therefore, the contaminative Langmuir probe with the OES can give the approximate electron temperature and density, and the cleaning interval of the Langmuir probe could be significantly extended.

Author Contributions: C.C. and W.F. contributed to the overall study design, analysis, and writing of the manuscript. C.Z., D.L., M.H., and Y.Y. provided technical support and revised the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Key Research and Development Program of China under 2019YFA0210202, the National Natural Science Foundation of China under Grant 61971097, the Sichuan Science and Technology Program under Grant 2018HH0136, and the Terahertz Science and Technology Key Laboratory of Sichuan Province Foundation under Grant THZSC201801. Conflicts of Interest: The authors declare no conflict of interest.

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