248 Chiang Mai J. Sci. 2015; 42(1)

Chiang Mai J. Sci. 2015; 42(1) : 248-257http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Spectroscopic Electron Temperature Measurement inMethane/helium Plasma During Diamond-like CarbonCoatingArtit Chingsungnoen*[a], Vittaya Amornkitbamrung [b][a] Department of Physics, Faculty of Science, Mahasarakham University,

Maha Sarakham 44150, Thailand.[b] Integrated Nanotechnology Research Center, Department of Physics, Faculty of Science,

Khon Kaen University, Khon Kaen 40002, Thailand.*Author for correspondence; e-mail: artit.c@msu.ac.th

Received: 15 August 2013Accepted: 29 January 2014

ABSTRACTThe electron temperature (Te ) was extracted from the capacitively-coupled radio

frequency (CCRF) plasma reactor by the spectral line intensity ratio method. The measuredintensity ratio of 587.6/706.5 nm from CH4/He plasma was used to calculate the electrontemperature under the steady state corona (SSC) approximation. Results were in good agreementwith Langmuir probe measurement. This analysis method was then used to determinethe electron temperature profile of CH4/He plasma. The increase in power density from~980 to ~1715 mW/cm2 resulted in a decrease in the electron temperature from ~0.93 eV to~0.65 eV. The highest electron temperature appeared at the discharge radial center and~4 mm from the RF powered electrode. From the radial electron temperature measurement,the temperature gradient inside the discharge region tended to decrease with the increase inpower density, while in the axial direction, the electron temperature changed sharply near thepowered electrode.

Keywords: electron temperature, CCRF reactor, steady state corona approximation

1. INTRODUCTIONMethane-containing plasmas are

nowadays generally used for the productionof amorphous carbon (a-C:H) films whichare mainly deposited as protective layers,on a variety of substrates [1]. Of the variouspossible geometries, capacitively-coupledplasmas (CCP) which are generally sustainedby radio-frequency (RF) excitation arepreferred because they allow the depositionof uniform films over large areas [2, 3].

One of the physical parameters defining thestate of a neutral gas in thermodynamicequilibrium is its temperature, which representsthe mean translational energy of the particlesin the system. In low-temperature RFdischarges, plasmas are mainly characterizedby the electron (Te ) temperature. The electrontemperature corresponds to the translatorykinetic energy of the electrons that can exciteatoms from the ground states to the excited

Chiang Mai J. Sci. 2015; 42(1) 249

states. This parameter plays a very importantrole for thin film growth mechanismsbecause the electron collisions with neutralgas molecules control most behavior ofplasma [4]. Therefore, techniques formeasuring the electron temperature are alsoessential for identifying the key chemicalmechanisms that can link to the surfaceprocesses. These could provide a betterunderstanding about the basic mechanismthat leads to film formation.

Optical emission spectroscopy (OES)techniques have been widely used formeasurement of electron temperature [5]because they can probe from a long distance,without disturbing the light source. The OEStechniques based on the spectral lineintensity ratio methods are often used sincethey require only measurements of relative lineintensity of spectral lines of the same atomicor ionic species. However, the appropriatemodels for extracting are generally dependenton the electron density. In the case when theelectron density is too low to complete localthermal equilibrium (LTE) and none of thelines is affected by self-absorption [6, 7] thesteady-state corona (SSC) model can be used.This means that all upward transitions can beconsidered collisional from ground stateand all downward transitions are radiative.As a result, densities in various excited statesare proportional to the products of statisticalweight and the exponentials of the ratio ofthe electron thermal energy and excitationenergy. Therefore, the electron temperatureis directly calculated from inversion of thelogarithm of the ratio of the total intensitiesof lines arising from different upper levels.

In the present work, line intensity

measurements are presented from RF-generated methane/helium plasmas. Thesteady-state corona model is used to evaluatethe electron temperature by using the lineintensities of He I (λ = 587.6 and 706.5 nm).Results are compared with the RF-compensated Langmuir probe measurement.Spatially resolved spectral line intensity ratiowas also measured to investigate the electrontemperature distribution in the dischargearea. The temperature profile along axial andradial directions could give useful informationabout the species concentration and thecorrelation with the thin film uniformity.

2. MATERIALS AND METHODS2.1 Apparatus

The setup used for recording the spectralline ratio and a Hiden Analytical RF-compensated Langmuir Probe inserted intothe middle of the plasma, are shown infigure 1. The geometry of the CCRF reactor(MDC Vacuum Products, SSAC-12) consistedof a stainless steel chamber of internaldiameter ~290 mm and height ~350 mmwith a quartz window 150 mm in diameter,used for optical diagnostics. The spacingbetween the parallel-plate stainless steelelectrodes was held at 20 mm for theexperiments reported here. The plasmadischarge chamber was pumped by aturbomolecular pump to a backgroundpressure of 10-5 Pa. The CH4 and He gaseswere introduced into the chamber and the13.56 MHz RF power was supplied to thelower electrode with the area of 103.8 cm2.The same-size upper electrode was groundedtogether with the wall of the reactor.

250 Chiang Mai J. Sci. 2015; 42(1)

Figure 1. Schematic diagram of CCRF system and apparatus involved.

In the present experiment, an opticalspectrometer (getSpec-3648-VIS) is usedfor the analysis of optical emission fromthe plasma. The detector is a 3648 pixelcharge-coupled device (CCD) array with75 mm focal length. It can be used formeasurements from 359-614 nm for channel1 and 586-808 nm for channel 2, with spectralresolution of 0.07 nm FWHM. The localemission intensities are collected via acollimator lens (focal length 100 mm,25.6 mm diameter) which is connected via afiber optic (600 μm diameter) to an entranceaperture having a 10m slit width. The CCDaccepts light transmitted through a singleoptical fiber and dispersed via a fixed1200 grooves/mm holographic grating.The wavelength scale was calibrated with astandard H2 light source and the spectralsensitivity of the whole system was measuredwith a spectral lamp of known colortemperature. The Abel inversion method [8]was used to transform the measured intensityinto the correct intensity.

2.2 Spectroscopic Electron TemperatureMeasurement

When considering the atoms in the excitedstate, which decay spontaneously to a levelunder the emission radiation, the intensityof irradiation (radiant flux , measured inWatts/(cm3-steradian)) is defined by

Iij = Aij Eij Nj , (1)

where Nj is the population density (cm-3) inthe state j, and Eij , Aij, are respectively theenergy of the quantum of radiation and theEinstein coefficient (the probability persecond) for spontaneous emission associatedwith the transition from upper to lower states.Therefore, the spectral line intensity ratio forthe selected two emission lines whichcorrespond to wavelengths λij and λlk ismeasured as

Iij Eij Aij Nj (2)Ilk Elk Alk Nk

=

Chiang Mai J. Sci. 2015; 42(1) 251

In low density plasmas and underthermodynamic equilibrium in which theplasmas stay in the steady-state corona (SSC)discharge, the population ratio can be subjectto the Boltzmann distribution [9]:

Nj gj Ejk

Nk gk Te

where gi , gk and Ejk are the statistical weightsand the energy difference of the state j andstate k in electron volts, respectively, and Te

is the electron temperature in electron volts.By putting (3) into (2) and taking the naturallogarithm, the electron temperature can beextracted from that expression. This is justifiedon the basis that the optical emissions of bothspectral lines are proportional to the groundstate population and the transitions are notradiation trapped.

2.3 Langmuir Probe DiagnosisThe compensated Langmuir probe was

a tungsten wire of radius 0.075 mm withexposed length of 10 mm inserted into theplasma at the middle of the electrodes forcollecting plasma current at the sweptpotential from -20 V to 50 V. In RF plasma,a usual problem in probe diagnostics is achange in the probe surface conditions dueto the continuous sputtering of the RFelectrode. As a result the probe sheath voltagemay not correspond to the applied probevoltage. To ensure that the plasma beingmonitored did not contaminate its surface,the probe was continually cleaned at regularintervals. In this experiment, havingintermediate-pressure and weakly ionizedplasma, the conventional probe theory basedon the thin collisionless sheath approximationwas used. Under the assumption that the freeelectrons have a Maxwellian velocitydistribution, the electron density and theaverage electron temperature can be obtained

from [10]

d(lnIe ) -1 ,

dV

where Ie is the electron current.

In the case where the sheath in a plasma islarger than the probe radius, the collection ofthe charge particles by an attracting cylindricalprobe is determined by their orbital motion[11]. For large sheaths and for large negativebias potential, a plot of the square of the ionsaturation current versus bias voltage may bea straight line with a slope S that is related tothe ion density (ni) by equation [12]

π Smi

Ape 3/2 2

where mi is the ion mass, Ap is the exposurearea of the probe tip, and e is the electroniccharge of electron. For a quasi-neutral plasma,an ionized gas in which the electron densityand ion density are present in approximatelyequal number, ne ≈ ni.

3. RESULTS AND DISCUSSIONThe overall discharge was investigated at

the center between the power and groundedelectrodes and emission spectra were recordedover the spectral range 400-750 nm. Spectrawere accumulated by using 100 ms integrationtime and 100 times averaging to providestability of line intensities. The relative intensityof each line is determined directly when thebackground intensity has been subtracted.Figure 2 shows the principle features ofhelium and methane/helium emission spectratogether with identification of the dominantspecies under discharge conditions of gaspressure of 400 Pa, power density of 980mW/cm2, and electrode gap of 20 mm. It isapparent that the most prominent helium

exp - , (3)

√

=

TeV ≈ (4)

ni = , (5)

252 Chiang Mai J. Sci. 2015; 42(1)

emission lines in the visible range weregenerated from transitions ending at themetastable levels 21S, 21P (singlet state) and23S, 23P (triplet state). One emission bandfrom CH radicals together with the atomichydrogen Balmer series can also be observed.Figure 5 shows the electron temperatureand electron density as a function of powerdensity obtained from CH4/He plasma.

As seen in figure 3, the ln(Ie ) versus Vcharacteristic has the property that thecontribution of Maxwellian electrons toIe is proportional to exp(-eV/TeV) forsufficiently positive V. Therefore, it ispossible to determine the bulk-electrontemperature Te by measuring the slope ofthe Maxwellian lines of free electrons inthe transition region. With the increase inRF power the slopes of Maxwellian linesalso increase. This means that the averageelectron energy decreases with the increaseof power consumption. A deviationfrom linearity of this semi-logarithmicplot also indicates deviation of EEDF

from Maxwellian, and in such a case it maybe misleading to refer to an electrontemperature. Under the assumption ofquasi-neutral plasma and using the OMLtechnique of Pletnev and Laframboise [13],the plasma electron density ne or iondensity ni are proportional to the squareroot of a slope S in the ion saturationregion, as shown in figure 4. With theanalysis shown in figures 3 and 4, the electrontemperature and electron density as afunction of power density can be obtained.The result is shown in figures 5. It is seenthat with the increase of power densityfrom 590 mW/cm2 to 1760 mW/cm2, thene increases from 1×1011 cm-3 to 3×1011 cm-3

while the Te decreases from 1.7 eV to 0.8 eV.The change in slope of the Te in the higherpower density regime indicates thatthe discharge maintenance is changedfrom the RF-oscillatory movement of thesheath to the electrons emitted from theelectrodes under ion bombardment.

Figure 2. Typical emission spectra obtained in pure helium and in CH4/He plasmas.

Chiang Mai J. Sci. 2015; 42(1) 253

Figure 3. Maxwellian line of the free electrons in a semi-logarithmic I-V plot.

Figure 4. A plot of the square of ion current versus probe voltage in the ion saturationregion.

Figure 5. The electron temperature and electron density as a function of power density.

254 Chiang Mai J. Sci. 2015; 42(1)

Figure 6. Spectral line intensities of (a) 412.1, (b) 443.8, (c) 471.3, (d) 504.8, (e) 667.8, and (f)728.1 nm as a function of radial (-57.5 ≤ R ≤ 57.5 mm) and vertical (0 ≤ Z ≤ 20 mm)positions.

Figure 7. Comparing the excitation temperature obtained from the SSC model of 587.6/706.5 nm ( ), with the electron temperature obtained from a probe method (◊) as a functionof power density for considered pressures.

From now on, the temperatures obtainedunder the SSC approximation and measuredby a single Langmuir probe will be assignedas Te

SSC and Teprob, respectively. For checking

the validity of the selected line ratios, thesethree temperatures were compared as a

function of power density with different gaspressures (0.13, 0.40, 0.67, 0.93, 1.12, and 1.47kPa). As shown in figures 7 (a) - (f), the Te

SSC

is in good agreement with a single Langmuirprobe method over the experimentalconditions used. It decreases as the power

Chiang Mai J. Sci. 2015; 42(1) 255

density increases, in agreement with theresults of Ray and Chaudhury [14]. The closerelation of Te

SSC and Teprob observed in this

experiment indicates that the electrontemperature obtained from SSC model of587.6/706.5 nm can be used in conditions forwhich the original excitation temperaturedominates.

The next purpose of this work is thequantitative determination of electrontemperature profile from the emission lineidentifications. With regards to Te

SSC, thespectral line intensity ratio of 587.6/706.5 nmwas then applied for measurement of theelectron temperature in CH4/He plasmas. Forobserving radial profile, the collimator lensor optical probe was scanned from the left

hand side (r = -57.5 mm) to the end of theother side (r = 57.5 mm) with step incrementof 5 mm. In the vertical axis, it was startedfrom mm to mm with the incremental stepsof 2 mm, where represents distance fromRF electrode. In order to investigate the roleof power density in the electron temperatureprofile, the power densities were increasedfrom 980 to 1715 mW/cm2 with the stepincrement of 245 mW/cm2. All otherdischarge parameters such as gas pressure,total gas flow, and electrode gap weremaintained constant (at 1.33×102 Pa, 60 sccmand 20 mm) during the entire experiment. Inorder to avoid missing data near the plasmasheath boundaries, sets of data at and mmwere omitted.

Figure 8. Excitation temperature profiles with power densities of (a) 980 mW/cm2, (b)1225 mW/cm2, (c) 1470 mW/cm2, and (d) 1715 mW/cm2.

As shown in figure 8, the highest intensityappears near the discharge center abovethe power electrode around z = 4 mm. Thisresult shows that the emission intensityprofile is generally stronger in the center ofthe electrode gap than at the electrode edgeand tends to appear nearer the power

electrode due to the asymmetrical capacitivelycoupled discharge in which a large, self-biasvoltage develops on the power electrode withrespect to the plasma. This potential gives ahigher electric field near the plasma/sheathboundary above the power electrode. As aresult, a greater discharge occurs around this

256 Chiang Mai J. Sci. 2015; 42(1)

area. The increase of power density leadsto a decreasing of the Te from 0.93 eV to0.65 eV. It decreases by more than 60% nearthe electrode edge and consequently hightemperature gradients occur near theseareas. The temperature gradient inside thedischarge region tends to decrease with theincrease of power density and/or electrondensity. This means that the temperatureuniformity is expandable which certainlyaffects the film growth since it will shortenthe distance between the growing filmsurface and the point where film precursorsare initially produced.

4. CONCLUSIONSIn conclusion, spectroscopic temperature

measurement has been performed in themethane/helium discharge by spectral lineintensity ratio method. The steady-statecorona model is used to extract the electrontemperature from the absolute line intensityratio of 587.6/706.5 nm. The electrontemperature obtained from this methodwas compared with the electrostatic probemethod. It is found that the spectral lineratio of 587.6/706.5 nm under the SSCapproximation gives the electron temperaturein good agreement with a single Langmuirprobe. The increase of power density leadsto a decrease in the electron temperature.This result can be explained by more RFpower generating more fast electrons. As aresult, many species will transit into excitedstates or ionic states depending on the energyof electron impact and the threshold energyof reaction. There are many new electronsthat can reduce the average absorbed energy.This is the reason why the electron energydetected by probe tends to decrease withpower.

For spatially resolved Te, we found thatthe highest appeared near the discharge centeraround 4 mm above the power electrode and

the temperature gradient inside the dischargeregion tends to decrease with the increase ofpower density; as a result the temperatureuniformity is enhanced and expanded alongthe radial direction. In the axial direction, theelectron temperature has to change sharplynear the power electrode due to a largenegative self-bias voltage developing on thepower electrode with respect to the plasma.The temperature gradient can give a pictureof plasma potential because this potentialtends to retain the electrons leaving theplasma by attracting them back into theplasma. The shape of the temperature profilewhich corresponds to the regions with thehighest density of electrons, having theappropriate minimum energy to generate aparticular species, gives an important indicationof the uniformity of coatings that will beproduced from methane-containing plasmas.

ACKNOWLEDGEMENTSThis work has been partially supported

by the Thailand Research Fund (MRG5280216) and the National NanotechnologyCenter (NANOTEC), NSTDA, Ministry ofScience and Technology, Thailand, through itsprogram of Center of Excellence Network.

REFERENCES

[1] Elidiane C.R., Nilson C.daC.,Milton E.K., Rita C.C.R., Nazir M. andSteven F.D., Optical and electricalproperties of polymerizing plasmas andtheir correlation with DLC filmproperties, Plasmas Polym., 2004; 9: 1-22.DOI 10.1023/B:PAPO.0000039813.33634.c6.

[2] Roth J.R., Industrial Plasma Engineering vol.1 Principles, IOP Publishing, 1995; 125-131.

[3] Smith D.L., Thin-Film Deposition: Principlesand Practice, McGraw-Hill, New York,1995; 454-462.

Chiang Mai J. Sci. 2015; 42(1) 257

[4] Catherine Y., Pastol A., Athouel L. andFourrier C., Diagnostic studies of speciesconcentrations in a capacitively coupledRF plasma containing CH4-H2-Ar,IEEE Trans. Plasma Sci., 1990; 18:923-929. DOI 10.1109/27.61504.

[5] Herman I.P., Optical diagnostics for thinfilm processing, Annu. Rev. Phys. Chem.,2003; 54: 277-305. DOI 10.1146/annurev.physchem.54.011002.103824.

[6] Ohno N., Razzak M.A., Ukai H.,Takamura S. and Uesugi Y., Validity ofelectron temperature measurement byusing Boltzmann plot method in radiofrequency inductive discharge in theatmospheric pressure range, Plasma FusionRes., 2006; 1: 281-289. DOI 10.1585/pfr.1.028.

[7] Podder N.K., Johnson J.A. III,Raynor C.T., Loch S.D., Ballance C.P.,and Pindzola M. S., Helium line intensityratio in microwave-generated plasmas,Phys. Plasmas, 2004; 11: 5436-5443.DOI 10.1063/1.1812535.

[8] Okada A. and Kijima K., Analysis ofoptical emission spectra from ICP ofAr-SiH4-CH4 system, Vacuum, 2002; 65:319-326. DOI 10.1016/S0042-207X(01)00437-7.

[9] Boivin R.F., Kline J.L. and Scime E.E.,Electron temperature measurement by ahelium line intensity ratio method inhelicon plasmas, Phys. Plasmas, 2001; 8:5303-5314. DOI 10.1063/1.1418020.

[10] Chen F.F., Evans J.D. and Arnush D., Afloating potential method for measuringion density, Phys. Plasmas, 2002; 9:1449-1455. DOI 10.1063/1.1462630.

[11] Sudit I.D. and Woods R.C., A study ofthe accuracy of various Langmuirprobe theories, J. Appl. Phys., 1994; 76:4488-4498. DOI 10.1063/1.357280.

[12] Chang J.S. and Laframboise J.G., Theoryof electrostatic probes in a flowingcontinuum plasma: Numerical solutionsfor cylindrical probes in cross flow,J. Phys. D: Appl. Phys., 1993; 26: 42-48.DOI 10.1088/0022-3727/26/1/008.

[13] Pletnev V., Laframboise J.G., Currentcollection by a cylindrical probe in a partlyionized collisional plasma, Phys. Plasmas,2006; 13: 1-19. DOI 10.1063/1.2208620.

[14] Ray P.P. and Chaudhuri P., Opticalemission spectroscopy used to studythe characteristics of the capacitiveradio-frequency discharge in argon,Czechoslovak J. Phys., 2003; 53: 229-234.DOI 10.1023/A:1022924930833.