Comparison of NO titration and fiber optics catalytic probes for determination ofneutral oxygen atom concentration in plasmas and postglowsMiran Mozeti, Andre Ricard, Dušan Babi, Igor Poberaj, Jacque Levaton, Virginie Monna, and Uroš Cvelbar

Citation: Journal of Vacuum Science & Technology A 21, 369 (2003); doi: 10.1116/1.1539082 View online: http://dx.doi.org/10.1116/1.1539082 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/21/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Comparison of fiber optics and standard nickel catalytic probes for determination of neutral oxygen atomsconcentration J. Vac. Sci. Technol. A 20, 189 (2002); 10.1116/1.1427893 Fiber optic catalytic probe for weakly ionized oxygen plasma characterization Rev. Sci. Instrum. 72, 4110 (2001); 10.1063/1.1409567 Determination of atomic oxygen density with a nickel catalytic probe J. Vac. Sci. Technol. A 18, 338 (2000); 10.1116/1.582189 Design of a cone-penetrometer-compatible probe and housing: The LLNL Raman probe Rev. Sci. Instrum. 70, 3735 (1999); 10.1063/1.1149985 In situ observation of infrared spectra of some molecular fragments and products in rf discharge plasmasgenerated in an absorption cell of a Fourier transform infrared spectrometer Rev. Sci. Instrum. 68, 2305 (1997); 10.1063/1.1148140

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Comparison of NO titration and fiber optics catalytic probesfor determination of neutral oxygen atom concentrationin plasmas and postglows

Miran Mozetica)

Institute of Surface Engineering and Optoelectronics, Teslova 30, 1000 Ljubljana, Slovenia

Andre RicardCPAT, Universite Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse, France

Dusan Babic and Igor PoberajUniversity of Ljubljana, Faculty of Mathematics and Physics, Jadranska 19, 1000 Ljubljana, Slovenia

Jacque Levaton and Virginie MonnaCPAT, Universite Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse, France

Uros CvelbarInstitute of Surface Engineering and Optoelectronics, Teslova 30, 1000 Ljubljana, Slovenia

~Received 28 May 2002; accepted 25 November 2002; published 17 January 2003!

A comparative study of two different absolute methods NO titration and fiber optics catalytic probe~FOCP! for determination of neutral oxygen atom density is presented. Both methods weresimultaneously applied for measurements of O density in a postglow of an Ar/O2 plasma created bya surfatron microwave generator with the frequency of 2.45 GHz an adjustable output powerbetween 30 and 160 W. It was found that the two methods gave similar results. The advantages ofFOCP were found to be as follows: it is a nondestructive method, it enables real time measuring ofthe O density, it does not require any toxic gas, and it is much faster than NO titration. Theadvantage of NO titration was found to be the ability to measure O density in a large range ofdissociation of oxygen molecules. ©2003 American Vacuum Society.@DOI: 10.1116/1.1539082#

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I. INTRODUCTION

In the past decade, technologies based on applicatiooxygen plasma have been successfully applied to diffebranches of science and industry. The technologies incthe nonisotropic plasma etching, plasma drilling, plasma odation, plasma cleaning, plasma ashing, and plasma suactivation.1–9 Different technologies require application oplasma with different parameters. For anisotropic etchand drilling, a plasma with a high degree of ionizationneeded, while for some other technologies, an oxyplasma with a low density of charged particles performs bter. For very delicate treatments of samples, for instanceing plasma ashing and selective plasma etching, a stategas with a negligible concentration of charged particshould be used. In such cases, it is much better to tsamples in postdischarges rather than in plasmas themseIn systems used for delicate plasma treatments, the mosportant parameter is the density of neutral oxygen atoms

The density of neutral oxygen atoms can be measureddifferent means including a variety of optical emission~acti-nometry! and absorption spectroscopy methods,10–12 NOtitration13–15,18and catalytic probes.16–29Spectroscopy methods usually give relative density values and are often cbrated with titration.11,26,30,31

Titration is the oldest method for quantitative measuments of the density of radicals in plasmas and postglow

a!Author to whom correspondence should be addressed; electronicmiran.mozetic@guest.arnes.si

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review of prior articles is given in the work of Wise anWood.18 Titration is obtained by producing a chemiluminecent reaction between the atoms coming from a flowplasma and a molecular gas~calledactinometer! introduceddownstream the plasma. The reaction isA1B2→AB* 1B,where A is the atom andB2 the actinometer. TheA atomdensity is determined by detection ofAB* radiative states.The observed luminescence is attributed to the radiative tsition of an electronically excited speciesAB* . The reactionis quite rapid, hence it can be employed to measure the ccentration of atomsA in a gas stream by simply introducinB2 actinometer at a known flow rate and observing thetensity of the light emission with a suitable photoelectdetector. The flow rate of actinometerB2 is usually increaseduntil saturation or extinction occurs. At that point, the florate of the actinometer is related to the density ofB2 which isthen used to determine the density ofA atom species. Titra-tion is a suitable method for measuring the density of ariety of radicals including H, N, and O. The main drawbaof the method is that it is both destructive and slow socannot be used for real time monitoring of radicals densi

The first method used to detect atoms in plasmascalorimetry. As early as 1937, Poole reported that caloretry could be used for a qualitative detection of hydrogatoms.32 The technique is based on heterogeneous recomnation of radicals on an appropriate surface. The energyleased on the surface due to catalytic recombinationheats the catalyst. The heating rate is proportional toconcentration of atoms in the vicinity of the catalyst. Meil:

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suring the temperature of the catalyst enables estimatiothe atom density. Calometry was used in several modesmost advanced being catalytic probes. The method wasways regarded semiquantitative due to unknown or unpdicted effects. A major drawback was an uncertain valuethe recombination coefficient. Different authors found diffeent values, sometimes varying for a order of magnitudesolution of a problem was to measure the recombinationefficient for exactly the same material one used for a calytic probe. A reliable way to do this has been published.24

Both NO titration and catalytic probes therefore detmine absolute values of the O atom density. In order to aldirect comparison of the NO titration and catalytic promethods, the results of extensive measurements performdifferent experimental conditions are presented in this artiExperiments were performed with a fiber optic catalyprobe~FOCP! since it was previously reported to be supercompared to other catalytic probes.29

II. EXPERIMENTAL RESULTS

A. Experimental setup

The experimental system is shown in Fig. 1. The systis pumped with a two stage oil rotary pump with a pumpispeed of 28 m3/h. The experimental chamber is a Pyrex cinder of the length of 20 cm and of the diameter of 15 cmPyrex tube of the diameter of 2 cm, and length of 20leads to the discharge that is inside a quartz tube of indiameter of 5 mm. The plasma is created in a 2450 Mmicrowave discharge. The microwave discharge can beduced either inside a waveguide connected to the microwsupply ~the device is calledsurfaguide!, or inside a cavitywhich is connected to the waveguide and power supplymeans of a coaxial cable~called surfatron!.33,34 At thepresent experiment the power in the surfatron cavity isjustable between 30 and 160 W. The surfatron is placedcm away from the junction of Pyrex and quartz tubesorder to prevent propagation of plasma into the Pyrex tuThe quartz tube is forced air cooled so that the tube temp

FIG. 1. The experimental setup.~1! rotary pump,~2! gate valve,~3! air inletvalve, ~4! catalytic probe,~5! Pyrex chamber,~6! vacuum gauge,~7! micro-wave generator,~8! optical spectrometer,~9! leak valve and flowmeter,~10!high pressure valve,~11! argon,~12! oxygen,~13! nitrogen,~14! argon12%NO.

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ture never exceeds 150 °C. Other parts of the system areat room temperature. Pressure is measured in the poscharge reactor with an Edwards vacuum gauge. Partial psures of different gases in the reactor are adjusted withphagas volume flow controllers. In the range between 20100 Pa, the pressure increases roughly linearly with increing total gas flow.

Experiments with the FOCP were performed at the cstant Ar flow of about 1000 ccm/min giving a gas pressureabout 200 Pa in the reactor, and different O2 flow rates from29 up to 248 ccm/min. At each oxygen flow rate we pformed measurements at different output power of micwave generator. Since titration experiments are time consing, those experiments were performed only at two oxygflow rates of 57 and 92 ccm/min, and at the power levels40, 80, 120, and 160 W. Once the O atom density was msured, the degree of dissociation of oxygen moleculescalculated using the relation:

h5nOkT/2pO2, ~1!

wherenO is the density of O atoms,k the Boltzmann con-stant,T the gas temperature andpO2

the oxygen partial pressure.

B. Results of titration

For measurement of N and O atom densities the comonly used actinometer gas is NO. Measurements invotwo steps. First, the method is calibrated with nitrogen poglow and then it is applied to measure the density of oxygatoms.

First, the N atom density was determined by NO titratiin a N2 postglow. NO was progressively introduced bmeans of an Ar-2% NO gas mixture into the N2 postglow byincreasing the Ar–NO flow rate. At low NO flow rate, theis a blue afterglow emission from NO~B! radiative stateswhich are produced by the reactions N1NO→N21O andN1O1N2→NO~B!1N2.

At high NO flow rates, there is a green emission comifrom NO2* molecules which are produced by the reaction1NO1N2→NO2* 1N2. The density of N atoms is deduceby measuring the NO flow rate at the extinction point btween the blue and green emissions where the N andflow rates are equal in quantity. The calibration of the sptrometer is performed by measuring the sloper 1 of NO2*intensity in the nitrogen afterglow versus the Ar-2% Nleaked into the chamber.

For experiments with Ar–O2 plasma, the Ar-2% NO wasleaked in the afterglow to detect oxygen atoms by the retion O1NO1(Ar–O2)→NO2* 1(Ar–O2).The intensity variation of NO2* with NO is with a sloper 2 .Finally the oxygen atom density in the postglow is giventhe following equation:

nO5nNr 2 /r 1 , ~2!

wherenN and r 1 have been previously measured in the N2

afterglow.

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The O density as determined by NO titration is plottedFig. 2 for two O2 ratios into the Ar–O2 gas mixtures~bvalues of 0.057 and 0.092!, whereb is the ration between O2and total gas flow,~i.e., b5@O2#/@Ar#1@O2#).

C. Results of FOCP

FOCP is a novel sensor developed recently for a real tabsolute measurements of oxygen atom concentrationweakly ionized plasmas and postglows.27 The main parts of aFOCP are an optical fiber, a small piece of catalyst me~nickel in our FOCP! attached to one of end of a fiber ashown in Fig. 3, and detection electronics with infrareddiation detector. Oxygen atoms in the plasma recombinethe surface of the catalyst metal into molecules releasmost of the energy to the metal causing its temperaturerise by up to several hundred degrees Celsius dependinthe concentration of oxygen atoms, pressure in a plachamber and probe construction. A fraction of thermal radtion emitted from a hot catalyst is guided with the opticfiber to a remote radiation detector. From the measurednal one can calculate the density of oxygen atoms by usin

FIG. 2. Density of neutral oxygen atoms in the postglow reactor vsmicrowave power for O2 ratios into Ar:b50.057 and 0.092 as determineby NO titration.

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suitable model of heat transfer from the catalyst to the sroundings and by knowing some material constants whcan be determined by independent measurements duFOCP calibration. A detailed description of FOCP constrution and operation was published elsewhere.27 A time re-sponse of the FOCP is approximately 1 s making a real timetracking of oxygen atom concentration variations possibThe whole measuring system has a high frequency interence immunity since the active area of the sensor is cpletely electrically isolated from the well shielded detectielectronics thus minimizing artifacts in the measured sigproduced by strong high frequency fields used for plasexcitation.

The FOCP tip was placed in the center of the experimtal chamber~Fig. 1! and the density of O atoms in its vicinitwas measured just before the NO injection. The concention of O atoms obtained by FOCP in the same experimeconditions as for NO titration is plotted in Fig. 4.

Since measurements by FOCP are fast, the O densityalso measured in a broad range of experimental conditioi.e., b values from 0.029 to 0.248 and different output powof microwave generator between 30 and 160 W. The res

eFIG. 4. Density of neutral oxygen atoms in the postglow reactor vsmicrowave power forb values as in Fig. 2, as determined by FOCP.

FIG. 3. Schematic of the fiber opticscatalytic probe.

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FIG. 5. Density of neutral oxygen vsthe microwave power forb values be-tween 0.029 and 0.248, as determineby FOCP.

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of these measurements are summarized in Fig. 5. Once tdensity was known, the degree of dissociation of oxygmolecules was calculated using Eq.~1!. The degree of dissociation as calculated from the measurements with NO tition and FOCP is plotted in Fig. 6. The degree of dissociatof oxygen molecules obtained at systematic measuremenO density with FOCP only is plotted in Fig. 7.

III. DISCUSSION

Let us first discuss the results of the systematic measments performed by FOCP only. Figure 5 represents thdensity as a function of microwave power, with the oxygpercentage~b value! as a parameter, while Fig. 7 is a plot

FIG. 6. Degree of dissociation of oxygen molecules vs the microwpower, forb values as in Fig. 2, calculated from NO titration and FOCP

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the resultant degree of dissociation. At a low output powethe microwave generator, the O density increases fairlyearly with increasing microwave power. At a low percentaof oxygen in argon~i.e., lowest curve in Fig. 5!, the curvebecomes less steep at the power of about 50 W. With furincrease of microwave power, the density of neutral oxygatoms keeps a constant value. Increasing the oxygen perage thenO5 f (P) curves are first similar with a saturatioshifted to higher powers. But at higher oxygen percenta~three uppermost curves in Fig. 5!, the O density keeps increasing with increasing power even at high microwapower and no saturation is observed.

The resultant degree of dissociation is plotted in Fig. 7.a low oxygen percentage, saturation is observed. In contence with Fig. 5, the saturation at a low oxygen flowreached at a low microwave power, while at higher perceages it is shifted to higher powers. At the three highest ogen percentages, no saturation is observed.

In order to explain the degree of dissociation in the poglow reactor as measured with the FOCP, let us considerbehavior of plasma in the discharge vessel. The thermonamically equibrilated gas mixture is leaked in the dischatube. The gas in the discharge is transformed into a nonelibrium plasma. Oxygen molecules are partially ionized adissociated, mainly at the collisions with fast electrons aslow metastable atoms. The charged particles are extensrecombined on the walls of the discharge chamber~the prob-ability is close to unity!, while the recombination of neutraoxygen atoms is rather poor due to a low recombinatcoefficient for the reaction O1wall→1/2O21wall on quartzsurface~the order of magnitude is 102418!. Since the gasphase recombination is negligible at the pressure of 1 mor less,18 the loss rate for oxygen atoms is rather low. T

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FIG. 7. Degree of dissociation of oxy-gen molecules vs the microwavpower forb values as in Fig. 5, calcu-lated from systematic measuremenof O density by FOCP.

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probability of dissociation for an oxygen molecule passthe discharge chamber depends on plasma parametersmost important parameters are the electron density (ne) andelectron temperature (Te). The concentration of Ar metastable atoms also depends onne and Te . In the surfatronmicrowave discharge at a given gas pressure,ne increaseswith increasing power. At a fixed oxygen flow rate, the dgree of dissociation should increase with increasing micwave power. At a high enough power level, the concentraof fast electrons and metastable atoms is so high than almall molecules are dissociated. The power, at which thiscurs, depends on the concentration of oxygen in argon.viously, the higher the oxygen concentration~thus oxygenpercentage! the higher the power needed for achievingmost full dissociation rates in the discharge.

Oxygen atoms are weakly recombined on the waytween the discharge tube and the Pyrex chamber. The recbination probability is low but finite. According to the articlof Sorli and Rocak24 the attenuation depth~i.e., the length atwhich the O density drops to 1/e of its initial value! in a tubemade of a borosilicate glass is about 50 cm. Our tubeshorter~20 cm! but narrower. The attenuation depth for otube was not measured but considering the results publisin Ref. 24 we can estimate that the degree of dissociatioour case should drop for about 20% between the dischtube and the Pyrex vessel.

Let us now reconsider Figs. 5 and 7. The results aregood agreement with the above discussion. As long asoxygen flow rate is low enough~in our case up to the oxygeflow of 91 ccm/min! saturation in degree of dissociationobserved. The saturated value at about 75% is lower tthat in the discharge—a fraction of O atoms is recombinon the way from the discharge tube to the Pyrex chamberhigh oxygen flows~three lowest curves in Fig. 7! no satura-tion is observed even at the highest microwave power ofW. A higher microwave power, probably well above 200 Wwould be needed to reach saturation.

Let us now compare the results obtained by the twoferent methods. For a better comparison, the degree of

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sociation obtained by NO titration and FOCP is plottedFig. 6. The first finding is that the results of both measuments are similar—the degree of dissociation in both caseof the same order of magnitude. The discrepancy is upfactor of 2, which is acceptable for this kind of measurments. There is, however, a detail not easy to explain.results obtained with FOCP are sound with the updiscussion—the degree of dissociation increases withcreasing microwave power until a saturation of about 75%reached. Atb values of 0.057 the saturation is reached atpower of 120 W, while atb values of 0.092 it is reached athe maximum power of 160 W. The result obtained by Ntitration, on the other hand, is different. The saturationobserved at different levels, for theb values of 0.057 thesaturation is at the degree of dissociation of about 60while for theb values of 0.092 it is at about 40%. This resudisagrees with the upper discussion. One would expectthe saturation, whatever its value is, always appears atsame level. Different saturation values found by NO titratican only be explained by different extent of recombinationoxygen atoms on the way between the discharge tube andPyrex chamber. Increasing the oxygen flow would meancreasing the recombination probability.

According to the article of Sorli and Rocak,24 the attenu-ation depth for O atoms passing a glass tube does not deon the density of O atoms as long as the oxygen parpressure is low enough to neglect the gas phase recombtion. In the case of Ref. 24 it was found to be true up tooxygen pressure of 100 Pa. In our case, the oxygen papressure is about 10 Pa so it is obvious that the effect ofgas phase recombination should not be taken into accoThe only other reason for obtaining different saturationdifferent oxygen flow rates would be increased surfacecombination. Recombination coefficients, however, dodepend on O atom arrival rate as long as it is reasonable~our case!. This discussion is also consistent with the claswork of Wise and Wood18 who reported that the recombination coefficient for heterogeneous recombination of O atoon a glass surface does not depend on the oxygen atom

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rival rate. The observed difference in saturation levelsdetermined by NO titration, therefore cannot be explaine

IV. CONCLUSIONS

Two different methods have been used to determinedensity of neutral oxygen atoms in a postglow reactor: Ntitration and fiber optics catalytic probe~FOCP!. It wasfound that both methods gave similar results. The discrancy between the results was up to a factor of 2, whichreasonable for this kind of experiments. Studying detailsthe results, however, showed that the FOCP gave resultscan be both quantitatively and qualitatively well explaine

Still, both methods have certain advantages. The madvantage of NO titration is its ability to measure thedensity at low concentration of O atoms—even belowdegree of dissociation of 1023. Namely, the FOCPs can measure the O density only as long as the degree of dissociais at least 1%.29,35At lower O density, the signal is too low tobe detected by FOCP. The advantages of FOCP are aslows: it is a nondestructive method, it enables real time msuring of the O density, it does not require any toxic gas ait is much faster than NO titration.

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