ELSEVIER Surface and Coatings Technology 98 (1998) 1411-1415

SURfACE&GOATIN6S

HCHNOJDGY

Determination of 0 atom density in Ar-02 and Ar-02-H2 flowingmicrowave discharges

T. Czerwiec a.*, J. Gavillet a, T. Belmonte a, H. Michel a, A. Ricard b

a Laboratoire de Science et Genie des Surfaces (Unite de Recherche Associee au CNRS 1402).Institut National Polytechnique de Lorraine, Ecole des Mines de Nancy. pare de Saurupt, 54042 Nancy, Cedex. France

b Laboratoire de Physique des Ga= et des Plasmas (Unite de Recherche Associee au CNRSj. Universite de Paris-sud. Bdtiment 212.914050rsay, France

Abstract

Ar-02 and Ar-02-H2 gas mixtures ( 1500 Pal with a high dilution of molecular gases in argon are studied both in a 2450 MHzmicrowave discharge and in post-discharge conditions. In the discharge. excited atoms are investigated by means of opticalemission spectroscopy (OES). Then, OES results are carefully analysed in order to give some information about atom and radicalchemistry. Axial distribution of excited species as well as the influence of H2 admission into Ar-02 are investigated. Results areanalysed by taking into account electron collisions and argon metastable excitation transfers. Heterogeneous reactions on the wallof the reactor can be dominant in the balance of atom production in the discharge. In Ar-02 and Ar-02-H2 post-discharges, theabsolute density of 0 atom is determined by NO titration. The high 0/02 density ratio obtained in Ar-02 post-discharges(0.5-0.6) is probably due to adsorbed impurities that inhibit surface recombination of 0 atoms. The effect of H2 addition in anAr-02 microwave discharge at 1500 Pa is to decrease the 0 atom density by homogeneous reactions. © 1998 Elsevier Science S.A.

Keywords: Plasma; Optical Emission Spectroscopy; Chemiluminescence

1. Introduction

Production of active species is studied in low pressure,out-of-equilibrium plasma discharges in N 2, O2 and H2reactive gases for surface treatments. A flowing micro•wave post-discharge with N 2 and Ar-N2 gas mixtureshas previously been studied at pressures from 5 x 103

to 6.5 x 104 Pa for steel surface nitriding [I]. ChemicalVapour Deposition (CVD) post-discharge treatmentswhere Nand ° atoms are interacting with the SiH4 gasintroduced near the substrate have been experimentedin microelectronics to obtained Si3N 4 and Si02 thinfilms [2]. In these flowing post-discharges, the Nand °atoms coming from the plasma discharge have beendetected as being the active species for surface treatment.Such a CVD process has also been developed withmetallic gaseous halides for ceramic thin film deposition(nitrides or oxides) [3]. In this case, the coatings areproduced in an Ar-02-H2 post-discharge by introducing

• Corresponding author. Fax: +33 83534764;e-mail: czerwiec@mines.u-nancy.fr

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metallic chlorides near a substrate. Hydrogen introduc•tion improves the adhesion of the films [3]. In thepresent paper, the production of oxygen atoms in flow•ing microwave Ar-02 plasmas is studied in dischargeand post-discharge conditions. Particular attention isdevoted to the effect of H2 admission into Ar-02 gasmixture since it has been previously found that weakamounts of molecular gases (H2 [4] and H 20 [4-7])increase the extent of dissociation of O2 molecules inelectrical discharges. In our microwave discharge, theexcited atoms are analysed by Optical EmissionSpectroscopy (OES), whereas in the post-discharge, theabsolute density of°atoms is obtained by NO titrationas previously demonstrated [7].

2. The experimental set-up

The microwave plasma investigated in this study iscreated by a surface wave at 2450 MHz in a quartz tubeof diameter 5 mm via a surfaguide with a transmitted

1412 T C=erwiec et al. / Surface and Coatings Technology 98 ( 1998) 1411-1415

Hence the ratio of intensities Ix/IAr (which will bereferred hereafter as relative intensity for species X) isproportional to the [X]/[Ar] concentration ratio if thecoefficient Cx remains constant throughout the experi•mental range. In Ar-02-H2 microwave discharges, theemission spectrum is characterized by Ar, Hand 0 linesand OH bands. The Ar I: A=842.5 nm (2ps-+ls4)' Hex:).=656.3 nm (n=3-+n=2) and 0 I: ).=844.7 nm(3p 3p-+3s 3S) lines and the OH: ).=309.3 nm(A 21:+, v"=O-+X 2n, v'=O) band have been selectedon the basis of previous studies [8-11]. Several condi•tions are needed to fulfil the requirement of Eq. (1).

Firstly, in the event of creation of species X from amolecular form (0 from O2 or H from H2), dissociativeexcitation leading to radiative states X* must be negligi•ble in comparison with direct excitation from the atom.By considering the shape of the excitation cross-sections[12], it could be deduced that a direct excitation prevailsif the population of energetic electrons is weak. In ourexperimental conditions, any argon ion lines can beobserved. This was already mentioned in 2450 MHzmicrowave discharge in argon above 600 Pa [13]. Wecan conclude that a low population of energetic electronsis present in the discharge and that the dissociativeexcitation is weak.

Secondly, the quenching processes relative to speciesX or to its parent molecule must be small, especially for

power of 130 W, a flow rate of 1000 seem and a gaspressure of 1500 Pa. The post-discharge runs into acylindrical reactor of diameter 2.8 cm at a distance of70 cm from the surfaguide, corresponding to a post•discharge time At'::::, 10- 2 s. Titration by NO is performedat the end of the small quartz tube (75 cm from thesurfaguide), in a mixing zone with a lateral exit of theflowing post-discharge (Fig. 1). The light emitted by themicrowave discharge or induced by chemiluminescentreactions in post-discharge is transmitted by opticalfibres to the entrance of a monochromator (Jobin-YvonHR 640) equipped with a 1200 groovesmm- 1 grating.The optical signal is measured by an intensified photodi•ode array detector (lRY/1024 G/B) and resolution of0.1 nm can be estimated with slits at 100 ~m width.

3. Excited states in Ar-Ol and Ar-Ol-H1 discharges

The emission intensities from electronically excitedstates of the plasma are not simply related to the groundstate density of the concerned species. However, atechnique known as actinometry allows the concen•tration of the ground state species to be monitored. Itis accomplished by an addition of a small amount of anoble gas (usually argon) to the plasma [8-11]. In ourmicrowave discharge, where there is high dilution ofmolecular gases in argon, this technique should beaccommodated. In the actinometry method, the emissionintensity of a species X is divided by the intensity of achosen argon line to eliminate the electron density

variations, it is written as:

Ix [X]-=Cx -•I Ar [Ar]

(1)

\ Gas inlct pipe4l=Smm

Upstream

Mass~ Ar now~

metersN2

II2

O 2

OplicalliberOptical liber

Rotary vacuwn pwnpSpectrometer: grating 12(x) gr./ mm

+OMA detector

Plasma sourcesurfaguidc2450 Mill

Fig. I. Experimental set-up.

T. Czerwiec et al. / Surface and Coatings Technology 98 (1998) 1411-1415 1413

1.2

Gas flow

z-axial distance from the gap (em)

leading to a zero relative intensity. It supports the abovediscussion on the excitation processes of 0* and H*.From Fig. 3, a decrease of the H relative intensitydownstream of the surfaguide centre (z=O) can beobserved. Similar H atom axial distributions have beenobtained by actinometry in pure H2 2450 MHz dis•charges at 133 Pa and low microwave power [17]. It canbe concluded that the hydrogen atom transport fromthe plasma is less efficient than for 0 atoms.

The influence of H2 on the relative intensity of 0 isplotted in Fig. 4 for an Ar-I.2%0z-y%H2 plasma. Itcan be seen that the H2 addition increases the a atomdensity especially at 10 cm downstream the gap and fory ~ 2%. This enhancement effect of [0] upon additionof H2 can result either from an increase in the creationterm of a or from a decrease in its loss term. Owing tothe high dilution of molecular gases used in theseexperiments, the variations of plasma parameters or theinfluence of homogeneous reactions must be weak. Apossible explanation is given by the introduction ofheterogeneous processes. The effect of H2 addition is topoison the wall of the discharge tube, making surfacerecombination of a atoms less likely. In other words,the oxygen atom recombination probability is stronglydecreased in the presence of H atoms that blocks thereactive sites for a atom recombination.

Fig. 3. Longitudinal profiles of the normalized relative intensities of°(A), H (<'» and OH (0) for an Ar-1.2%Or2.2%H2 microwave dis•charge at 1500 Pa, 1040 sccm and 130 W. The origin of the z-axis istaken at the gap.

2520

withoutH2

A A

IS10

'i:' 2<Q?;o l.S0;;;c~.::u.~;;u" 0.5

00

2.5

the 0* and H* radiative states. This was verified byapplying existing data [14,15] to the high dilution ofmolecular gases into argon. From this, it is concludedthat the major loss terms are quenching by argon atomsfor H (n = 3), radiative deexcitation for a (3p 3p) andboth quenching by argon atoms and radiative deexcita•tion for Ar (2ps)'

Variations of the relative intensity of a atoms arereproduced in Fig. 2 versus the z-axial distance for thedownstream part of Ar-1.2%02 and Ar~1.2%02•2.2%Hz discharges. It can be observed an almost con•stant signal along the plasma with an abrupt increasejust before the plasma end. Similar behaviour, with theexception of the abrupt increase, was observed in apure 0 2 433 MHz microwave discharge at 133 Pa [16].It is concluded that oxygen atoms are transported fromthe plasma to the flowing afterglow without significantlosses. Moreover, the relative intensity of 0 atoms isincreased upon the addition of H2 to an Ar-02 dis•charge. Fig. 3 represents the normalized relative inten•sities of H, a and OH as a function of the z-axialdistance for both upstream and downstream parts ofan Ar-1.2%0z-2.2%H2 plasma. All these signals areasymmetrical, increasing from roughly zero for 0* andH* in the upstream plasma to higher values at the gap.In contrast, OH relative intensity starts with a maximumin the upstream plasma. Granier et al. [16] have shownthat the actinometry signal (relative intensity in ourcase) must tend to zero in the upstream plasma whenthe direct excitation is dominant over dissociative excita•tion. As a matter of fact, as the cold molecular gasesenter the plasma, the atomic concentration is weak,

Ar - O~Gas flow

z-axial distance from the gap (cm)

Fig. 2. Downstream part longitudinal profiles of the relative intensityof ° for Ar 1.2% O2 (A) and Ar-I.2%Or2.2%H2 (0) microwavedischarge at 1500 Pa. 1040 scem and 130 W. The origin of the z-axisis taken at the middle of the surfaguide, hereafter referred as the gap.

4. 0 atom density in Ar-Oz-Hz post-dischargeconditions

The oxygen atom absolute density has been deter•mined in the late post-discharge by NO titration method

1414 T C=erwiec et al. / Surface and Coatings Technology 98 ( 1998) 1411 -1415

2

Fig. 4. Relative intensity of 0 as a function of hydrogen percentage(.1'%) for an Ar-1.2%Ory%H2 microwave discharge at 1500 Pa.1039:s;Q:s;IIOOsccm and 130W. Measurements are performed at4.5cm (0) and 10cm (A) from the gap.

as explained in Ref. [7]. A density ratio Co = [0]/[02]..where [02], denotes the density of O2with the dischargeoff, can thus be calculated assuming a gas temperatureof 300 K. As it is reproduced in Fig. 5, the oxygen atomdensity in the Ar-1.2%Ory%H2 post-discharge is ini•tially decreasing from [0]=(2.1±0.4)xI015 cm- 3

,

Co =0.48 for y(H2)=0 to (7±I)xlO14 cm- 3,

Co =0.16 for y(H 2)= 1%. The value obtained fory( H2) = 0 is considerably higher than those obtained in

pure oxygen microwave post-discharges which normallyranges around 0.01 and 0.1 [5,6]. However, when meas•urements are performed after cutting the hydrogenadmission and further afterglow signal stabilization, theo atom density decreases to [0]=(9±2)x 1014 cm -3,

Co =0.21 (Fig. 5). We have to note that this strikingphenomenon was not observed in our discharge meas•urements (see below). Hence it is surely producedbetween the end of the plasma and the titration pointin the post-discharge. It is probably due to hetero•geneous reactions on the walls of the reactor involvingadsorbed impurities such as water vapour or OH radi•cals. As a matter of fact, in an initial Ar-02 discharge,H20 or OH radicals can occupy a great number ofpossible recombination sites for 0 atoms leading to higho atom densities in the gas volume [6]. When H2 isintroduced in such contaminated Ar-02 plasma, partof the water vapour or OH radicals is removed fromthe walls of the reactor by heterogeneous reaction withH2or H leading to an increase of the possible recombina•tion sites for 0 atoms when H2 is removed from the gasmixture. By introducing H2 again into such a cleanAr-02 discharge, the occupation of active sites by Hatoms or hydrogenated species prevents the 0 atomrecombination as long as hydrogen percentage is low(y< 1%). In post-discharge, the effect of hydrogen on[0] is opposite as that observed in the discharge (seeFig. 2). The decrease of 0 atom density in introducinghigh H2 percentage (y> 1%) in the high pressureAr-02 gas mixture could also be explained by homogen•eous reactions [7].

652 3 4Hydrogen percentage (y%)

o0.4

~..4( Z = \OemQ 1.6~.~

c....5... 1.2.~;:;u

"0.8

Fig. 5. Relative O-atom density (Co) vs. oxygen percentage (.1'%) foran Ar-1.2%02-y%H2 microwave discharge at 1500 Pa.1039:s; Q:S; 1070 sccm and 130 W. The initial relative O-atom densityobtained for an initial Ar-02 gas mixture (.) is compared with thatobtained after H2 introduction and subsequent elimination from theAr-02 gas mixture.

Hydrogen percentage (y%)

0.5

Co

0.4

0.3

0.2

0.1

oo 2 3 4 5

5. Conclusion

Our OES study of Ar-02 and Ar-02-H2 gas mixtureswith high dilution of molecular gases into argon in2450 MHz microwave discharge and post-discharge con•ditions can be summarized as follows. The H2admissionin Ar-02 plasma increases the concentration of 0 atomsin the discharge. This can be explained by H atomadsorption on the discharge tube walls that inhibitssurface recombination sites for 0 atoms. On the con•trary, H2 admission in Ar-02 plasma decreases the °atom density [0] in the post-discharge. The effectobserved in the discharge is thus counterbalanced in thepost-discharge by homogeneous or heterogeneous reac•tions involving H atoms and OH radicals. Titrationsperformed in an Ar-02 post-discharge are very sensitiveto adsorbed impurities. However, an Ar-H2 dischargecan be used to clean a reactor but this is detrimental to[0]. As a matter of fact, the removal of H20 from thetube walls by hydrogenated species leads to more avail•able recombination sites for ° atom recombination. Itis concluded that heterogeneous reactions on the wallsof the reactor play a prominent role in the balance

T. Czerwiec et al. / Surface and Coatings Technology 98 (1998) 1411-1415 1415

of atom production in such remote plasma-assistedprocesses.

Acknowledgement

The authors wish to express their thanks to J.P. Prelotfor his technical support.

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