A method for determining plasma power in rf-GDOES is presented. It is based on an effective resistance located in the inductive coil of the impedance matching. The amount of electrical power consumed in the matching system depends on the capacitive current flowing through the matching system, which depends on the applied voltage, the stray capacity, and the frequency. This correction method is experimentally evaluated and compared with the integral plasma power calculatio

Aeberhard, M.

Belenguer, P.

Guillot, P.

Michler, J.

Nelis, T.

Rohr, L.

Thérèse, L.

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TECHNICAL NOTEA simple method for measuring plasma powerin rf-GDOES instrumentsT. Nelis & M. Aeberhard & L. Rohr & J. Michler &P. Belenguer & P. Guillot & L. ThérèseReceived: 27 April 2007 /Revised: 11 July 2007 /Accepted: 12 July 2007 /Published online: 4 August 2007# Springer-Verlag 2007Abstract A method for determining plasma power in rf-GDOES is presented. It is based on an effective resistancelocated in the inductive coil of the impedance matching.The amount of electrical power consumed in the matchingsystem depends on the capacitive current flowing throughthe matching system, which depends on the appliedvoltage, the stray capacity, and the frequency. Thiscorrection method is experimentally evaluated and com-pared with the integral plasma power calculation.Keywords rf-GDOES . rf-Power control . Blind powerIntroductionRadio-frequency (rf) powered glow discharges (GD) in theGrimm type configuration are commonly used sources foroptical emission [1] and mass spectrometry [2]. Theanalytical figures of merit of a GD instrument dependstrongly on the possibility of controlling the dischargeproperties. The electrical conditions, voltage and current,have a significant effect on sputtering rate [3], emission,and ionisation yields [4]. Variations in the sputtering ratedue to changing plasma voltage and current are describedby Bouman’s equation [3]. The effect of the plasmaproperties on the emission yield has been discussed in arecent article by Nelis and Bengtson [4]. The emissionyields depend on voltage, current, and, to a significantlylesser extent, on the carrier gas pressure and, slightly, onplasma power. Quantification procedures developed forGDOES are based on the constant emission yield approach,requiring either the plasma conditions voltage and currentto be constant or the dependence of the emission yield onthese conditions to be known and controlled. Both thedischarge voltage and current must therefore be carefullycontrolled for quantitative analysis. Whereas the rf-excitationvoltage can be readily measured, the discharge current isdifficult to determine, because of the presence of a largecapacitive current. Different experimental procedures havebeen developed to measure [5, 6] the discharge current, butthey use a complex experimental set-up not alwayscompatible with equipment employed for routine analysis.The method suggested by Wilken at al. [6] uses a currentprobe surrounding the plasma chamber to directly determinethe plasma current. Measuring the electrical power suppliedto the plasma “Pp” is a possible replacement for directcurrent measurement. The plasma power can be estimatedfrom the difference of the rf-power emitted by the generator“Pg” and the blind power “Pl” consumed by the entire rf-supply circuit including the plasma source without plasmaoperation. This method was originally suggested by Canpont[7] for GDOES and was later described by Marshall [8]. Themethod is based on the fact that the blind power increases inproportion to the squared rf-voltage applied to the sample.The proportionality, however, depends strongly on thesample characteristics, in particular its size and shape.Anal Bioanal Chem (2007) 389:763–767DOI 10.1007/s00216-007-1509-3T. Nelis (*) :M. Aeberhard : L. Rohr : J. MichlerEMPA Materials Science and Technology,Feurwerkerstrasse 39,3602 Thun, Switzerlande-mail: thomas.nelis@empa.chP. BelenguerLaplace, UMR 5213, Université Paul Sabatier,118 route de Narbonne,31062 Toulouse cedex, FranceP. Guillot : L. ThérèseUniversité Jean François Champollion,Place de Verdun,81012 Albi cedex, FranceAlthough the blind power method has been implemented inmany commercially available rf-GDOES systems it hassome drawbacks. First, the proportionality factor must bedetermined for each sample to be analysed. Second thesetting of the impedance matching system must be keptconstant for the blind power measurement and the actualanalysis step. Because the load impedance necessarilychanges when the plasma is ignited, the method requires rfpower generators resisting a significant amount of reflectedpower [8].The method for determining the “lost” rf-power pre-sented in this work is based on an effective resistance of therf-circuit. Most parts, for example cables, connectors andcapacitors can be made in sufficiently high quality as toavoid measurable power losses. Inductive coils, however,operated at radio frequencies have a quality factor (Q=ωL/R)of the order of 250 and consequently a significant effectiveresistance. Payling [9] estimated the effective resistance ofthe inductive coil used in the automatic impedancematching system (L2 in Fig. 1) to be on the order of 1Ohm. The second inductive coil (L1 in Fig. 1) used in thisexperimental set-up is smaller and has, consequently, asmaller effective resistance. Assuming the inductive coilmakes the major contribution to power consumption(except for the plasma), the power loss is determined fromthe electrical current “IL” through the inductive coil L2(Eq. 1)Pl ¼ Pg  Pp ¼ RL  I2L ð1ÞThe plasma power can be determined from the differencebetween the generated power and the “lost” power. In thefollowing discussion we will use the term effectiveresistance “Reff” rather then the equivalent resistance RLof the inductive coil L2, as other components, not identifiedin this work, may contribute to the measured power lossand its functional dependence on the electrical currentmeasured at the exit impedance matching system.ExperimentalAll results presented in this work were obtained using aJY 5000 RF GDOES spectrometer (Horiba Jobin Yvon,Longjumeau, France). The layout of the rf supply circuit isdisplayed in Fig. 1. It has been described in detailpreviously [9].A current probe was made in our laboratory using aferrite ring having a cut off frequency of 40 MHz and aninsulated copper wire for the current probe coil (Fig. 2).The voltage generated between the two ends of the coil wasused as a measure of the current. Positioned at the exit ofthe automatic impedance matching system (Point 2 inFig. 1) and protected from the strong electric fieldsurrounding the rf-power-supply cable, it measures theelectrical current flowing through the coil L2.Fig. 1 Layout of the rf power supply and impedance-matchingsystem: 1, input of automatic impedance-matching system (6); 2, exitof automatic impedance; 3, co-axial power supply cable for source (5);ZP represents the plasma impedance, Cstray the stray capacities, L2 andL1 the inductive coils, and C1 and C2 the vacuum capacitors includedin the automatic matching system; RL represents the effectiveresistance of the coil L2; R0 is the internal impedance of the rf-powergenerator (7). Vr.f and Vdc refer to the potential difference measuredbetween the sample (4) and ground. IL is the current in the inductanceL2 flowing between points 1 and 2Fig. 2 Photograph of current probe, with coil (1) and electricalshielding (2)764 Anal Bioanal Chem (2007) 389:763–767Two different methods were used to verify the assump-tion that the power loss in the rf-power-supply circuit canbe reduced to the effect of one effective resistance.I ¼ Cstray dVrfdt ¼ CstraywVrf ð2ÞThe power loss depends on the stray capacity generatedby the source, including the sample and the voltage “Vrf”applied to the sample [8]. Equation 2 describes the linkbetween the displacement current, IL, the stray capacity,Cstray, and the rf-voltage applied to the sample “Vrf”,including its frequency. Samples of different size andmaterial were used for the experiment. The small sampleswere discs 2 cm in diameter and 5 mm thick. The largesamples were exactly the same specimen but backed with alarge rectangular (9 cm×17 cm) aluminium plate. Lowalloyed aluminium and steel were chosen because of theirsignificantly different secondary electron emission yields.In the first experiment the dc-bias voltage was increasedstepwise at constant pressure (800 Pa) for both small “Pgs,ILs” and large “Pgl, ILl” samples. The forward powerdelivered by the rf-generator had to be varied from 10 WFig. 3 Difference betweenpower for large and small sam-ples as function of squaredcurrent. The Al series was mea-sured using an aluminium sam-ple; Fe 1 to Fe 3 series arerepeated measurements with thesame low-alloy steel sampleTable 1 Equivalent resistance derived from blind power (withoutplasma ignition) and displacement current measurement for differentsample sizesPg=Pl=20 W Ieff a.u. Reff a.u. Vrf rms (V)Small sample 0.189 560 660Large sample 0.188 560 512Table 2 Comparison of plasma power derivedPg (W) Ieff a.u. Pp (W) Integral Pp (W) Vdc (V)Small sample sizeAl, 950 Pa20 0.072 17.1 17.3 31830 0.08 26.4 26.2 40040 0.095 34.9 35 460Fe, 550 Pa20 0.126 11.3 12.1 66030 0.15 17.4 18.5 80040 0.173 23 24.2 920Fe, 700 Pa20 0.107 13.6 14.2 54030 0.129 20.7 21.5 87040 0.151 27.2 28.5 920Fe 950 Pa20 0.083 16.1 16.4 38030 0.1 24.4 24.8 48040 0.117 32.3 33.1 550Large sample sizeFe 550 Pa20 0.144 8.4 8.17 58230 0.173 13.2 13.1 71140 0.201 17.4 17.2 806Fe 700 Pa20 0.126 11.2 11 50130 0.151 17.2 17.4 62040 0.176 22.7 22.5 705Fe 950 Pa20 0.099 14.5 14.3 36030 0.12 21.9 21.8 46040 0.139 29.2 29 530Anal Bioanal Chem (2007) 389:763–767 765to 90 W during the experiment. For each Vdc setting thedifference in required generator power was related to thedifference in the square of the current probe signal (Eq. 3),because for a given cathode material, chamber geometry,and constant plasma carrier gas pressure (density), theplasma power is a function of the discharge voltage only.ΔPl ¼ Pgl  Pgs ¼ Reff  I2LI  I2Ls  ð3ÞThe results are displayed in Fig. 3. The measurementuncertainty was estimated to be 1.5% of the generator powerreading. The measurement uncertainty is because of both theaccuracy of power reading and the spot-to-spot reproduc-ibility of the discharge conditions. The difference in slopeobserved for the different sample materials is thereforewithin the limit of experimental uncertainty. The experimen-tal results confirm the assumption of a single effectiveresistance. The power loss increases linearly with the squareof the capacitive current. The effect of sample size on powerloss, and its dependence on the excitation voltage, are inde-pendent of the sample material, i.e. the source impedance.This first experiment is close to routine analyticalapplication and supports the assumption of the effectiveresistance being responsible for the major part of the lostpower in GDOES. We nevertheless checked the assumptionusing a different approach.In this second experiment we compared the effective resis-tance approachwith the integral method suggested byBelenguer[5]. The effective resistance was calculated from blind powerand capacitive current measurements without plasma ignition.The experimental results are summarised in Table 1.Whereas the rf-voltage at the sample varies strongly withsample size, the capacitive current measured by the currentprobe (Fig. 2) is independent of the sample. This impliesthat the power consumption is directly linked to the capaci-tive current “IL”, but only indirectly, via the variable straycapacities, to the potential difference across the dischargesource “Vrf”. The effective resistance thus derived was thenused to derive the plasma power for a large range of dis-charge conditions. A comparison of the results with integralpower measurements is shown in Table 2. All measurementswere performed in the automatic impedance matching modeminimising the reflected power to a negligible value.The values derived by both methods are in goodagreement, within 1 W, independent of sample size, totalrf power, and source impedance.Analytical relevanceTo illustrate the relevance of the power transmission toanalytical routine work we analysed a computer hard disc,Fig. 4 Intensity–time profilesmeasured at 40 W and 700 Pafor samples of a computer harddisc of different sizes. Theresults for the small piece(squares) are identified byhigher intensities and faster ratesof erosion. The upper trace dis-plays the effect of sample sizeon the auto-bias voltage, in V.Small samples result in a higherauto-bias voltageTable 3 Line intensities on NiP layer of computer hard disk, on asmall sample and on the full hard diskHDD Size Set Pf (W) Pp (W) P (au) Ni (au) Vdc (V)Small 40 27.9 1.94 3.3 815Full 40 25.8 1.75 2.9 760Full 44 28.3 1.93 3.2 811Pf is the forward power delivered by the generator; Pp is the plasmapower derived from the capacitive current766 Anal Bioanal Chem (2007) 389:763–767first as a whole piece, a disc 9 cm in diameter and then as asmall sample (1 cm×2 cm) cut from the same specimen.These hard discs are often used as test samples in GDOESbecause they have a very reproducible layer structure andhomogeneous elemental composition [10]. The nickelphosphate layer and the aluminium–magnesium alloy basematerial have significantly different secondary electronemission yields, leading to different source impedance.The intensity–time profiles are compared in Fig. 4.The small sample results in higher intensities and fastererosion. The dc bias voltage decreases for both samplesfrom the NiP layer to the aluminium alloy base material.The average voltage is significantly higher for the smallsample. The difference between the detected line intensitiesis much stronger for the NiP layer and rather small for thealuminium alloy. Increasing the generator power from 40 Wto 44 W for the large sample is sufficient to compensate forthe reduced power transmission. Table 3 shows the emittedaverage intensities and dc bias voltages for the threeconfigurations; each value is derived from an average offour consecutive measurements. A general calibrationprocedure, based on the constant emission-yield approach,will necessary fail to supply accurate results. Both, a Vdccorrection or pressure regulation, used to compensate forthe effects of changes in the source caused by the varyingsecondary electron emission yield must fail, because thereason for the different auto-bias voltage is not thecomposition of the sample but its size. An analytical resultdepending on sample size is clearly not satisfactory.DiscussionThis work demonstrates that the power losses typicallyobserved in rf-GDOES are caused by the non-negligibleresistance of the inductive coil, which is part of theimpedance-matching system. The amount of electricalpower consumed in the matching system depends on thecapacitive current flowing through the matching system.The capacitive current depends on the excitation voltageand the stray capacity, which in turn depends on sampleshape and size. The analytical results obtained by rf-GDOES, therefore depend on the sample shape. Uncon-trolled variation of the plasma power directly affects theintensity–time profiles and renders control and/or correctionof the dependence of emission and ionisation yieldsimpossible and, consequently, significantly reduces theaccuracy of the results obtained.Two independent experimental proofs show that theresistance of the impedance matching system, i.e. theinductive coil, is the dominant source of power losses.Measurement of the capacitive current enables real-timedetermination of the power losses and, consequently,calculation of the plasma power.This correction method is independent of the samplecharacteristics, the discharge conditions, and the impedancematching settings. It is therefore compatible with automaticimpedance matching operation.In future work it is planned to integrate effectiveresistance correction into the spectrometer, enabling itsuse in routine analysis. We finally plan to verify the affecton analysis of non-conducting samples, for which powerlosses are even more important, because of the higherexcitation voltages [11] required to obtain suitable dis-charge conditions.The correction method is compatible with most com-mercially available rf-GDOES and MS instruments and caneasily be integrated into existing systems.References1. Nelis Th, Payling R (2004) A guidebook to glow discharge opticalemission spectroscopy. RSC2. Hohl M, Kanzari A, Michler J, Nelis Th, Fuhrer K, Gonin M(2006) Surf Interface Anal 38:292–2953. Payling R, Aeberhard M, Michler J, Authier C, Chapon P, NelisTh, Pitchford L (2003) Surf Interface Anal 35:334–3394. Bengtson A, Nelis Th (2006) Anal Bioanal Chem 385:568–5855. Belenguer Ph, Guillot Ph, Therèse L (2003) Surf Interface Anal35:604–6106. Wilken L, Hoffmann V, Uhlemann H-J, Siegel H, Wetzig K(2003) J Anal At Spectrom 18:646–6557. Canpont F (1993) PhD Thesis, Universitè Claude Bernard, Lyon8. Marshall KA, Casper TJ, Brushwyler KR, Mitchell JC (2003)J Anal At Spectrom 18:637–6459. Payling R, Bonnot O, Fretel E, Rogerieux O, Aeberhard M,Michler J, Nelis Th, Hansen U, Hartmann A, Belenguer Ph,Guillot Ph (2003) J Anal At Spectrom 18:656–66410. Nelis Th, Pallosi J (2006) Appl Spectrosc Rev 41:227–25811. Therese L, Ghalem Z, Guillot P, Belenguer P (2006) Anal BioanalChem 386:163–168Anal Bioanal Chem (2007) 389:763–767 767

A simple method for measuring plasma power in rf-GDOES instruments

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A simple method for measuring plasma power in rf-GDOES instruments

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