MASS-SPECTROMETRIC INVESTIGATION OF THE IONS IN AN AR-HEXAMETHYLDISILOXANE DISCHARGE

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MASS-SPECTROMETRIC INVESTIGATION OF THE IONS IN AN AR-HEXAMETHYLDISILOXANE

DISCHARGE

M. Schmidt, R. Seefeldt, G. Pohle

To cite this version:

M. Schmidt, R. Seefeldt, G. Pohle. MASS-SPECTROMETRIC INVESTIGATION OF THE IONS IN AN AR-HEXAMETHYLDISILOXANE DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-461-C7-462. �10.1051/jphyscol:19797224�. �jpa-00219206�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7- 461

MASS-SPECTROMETRIC INVESTIGATION OF THE IONS IN AN AR-HEXAMETHMDISILOXAM: DISCHARGE

M. Schmidt, R. Seefeldt and G. Pohle.

ZentraZinstitut fiir EZektronenphysik, V Aka&Zemie der Wissenschaften der DDR, DDR 22 Greifswald.

Introduction: In gas discharges with admixtures of gaseous organic components the formation of thin dielectric films is observed /1/. This glow polymerization is very complex and for an understanding of the film growth processes it is necessary to investigate the plasma and the thin films by many different methods. In this paper results are reported of the mass spectrometric examination of the ions in the positive column of a dc glow discharge in a mixture of argon and hexamethyldi-

siloxane (HMDS) using the method of ambi- polar effusion of charge carriers /2/.

First results of mass-spectrometric investigations of a rf discharge plasma with - silicon-organic admixtures (viwlki- methylsilane) are reported by Vasile and Smolinsky /3/.

Ex~erimental conditions: A diagram of the apparatus used in this study is shown in Fig. 1. The diameter of the diacharge tube was 2R=1,5 cm, th.e distance between the electrodes was 20 cm and that between ano- de and sampling orifice (diamet-40

r"'

was 4 om. The ions were analysed with the quadrupol mass spectrometer AbW 3, There- ference potential for the acceleration voltage was given by a floating wall PO-

be in the discharge tube opposite the sampling orifice. The argon pressure was 0.7 Torr, the pressure of the HMDS adrnix- ture varied from 1.10-~-5. lom2 Torr, the discharge current being 10 mA. To avoid digturbances of the mass-spectrometer by thin film formation, we did not use a flow but a closed system. The gas mixture was filled into the discharge tube and after the ignition of the discharge the ion mass spectrum wae examined for 20 minutes.

Experimental results and discussion: The investigation of the mass spectrum shows that in the Ar-HMDS-discharge many different kind of ions are found (Fig. 2). The relation be%ween the Ar* and Ar;-ions is similar to that in a pure argon discharge, but with increasing HMDS admixture the relative portion of Arh-ions decreases.

The A ~ H + ion current became greater than that of A'F ions, and also Ar2~+ ions were detected. Especially at low W S admixtures H;-ions and iona in the mass region of 12-18 and 27-32 were observed.The time developmet of these ions ebows that at first the concentration of the H$ ions increases and then stays nearly constant, but .the current of the other ions decrea- aea (Fig.3). For hlgher HMDS admixtures ions are observable with higher molecular weight (Fig.2). The intensity of these ions changes periodically in dependenceof the molecular weight. The distance between the maxima amounts to 14 mass units.

At these higher HlDS admixtures also the current of the iona with rnle.17 ( c H ~ ) be- comes relatively intensive.

This ion population is similar to that of a methane discharge /4,5/. An remarkable change in the intensity of the ions with even and odd mass numbers as shown in /4/

was not obeerved. This behaviour may be due to the relatively high. electron energy in the small diameter discharge tube, because Vasile and Smolinsky /5/ obtained the same results with an ion extraction from a rf discharge in methane throughthe electrode, where the ions were sampled from a region with higher electron energy, The observed ion population in the argon- HImS discharge mggests th.e following re-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797224

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action mechanism. After ignition thin film formation sets in. As shown through expe- riments in the flow system /6/ this pro-

cess comes to an end within the first se- cond. After this, there is an interaction of the plasma with tbe thin film on the inner surface of the discharge tube. The similarities of the ion mass spectrum to that of a methan discharge lead to the conclusion that from the thin film CH3- radicals or even greater particles are re- moved and ion molecule reactions lead to

the formation of the different kinds of ions observable in the ion mass-spectrum of the discharge. As discussed in /5/, for the formation of ions with 4 and more C atoms neutral particles with 2 C atoms are necessary. Mass-spectrometric investigation of the neutral components in an argon HMDS discharge have demonstrated in addition to CH the existence of C2H2 and C2H6/6/, so that condensation reactions 4

Leading to the formation of heavier particles are possible, The concentration of the organic molecules decreases, because these particles too lead to film formation.

Some of these reactions are connected with the formation of hydrogen which is accu- mulated in the volume.

References:

/I/ Millard, M., "Synthesis of organic polymer films in plasmasw in

"Techniques and Applications of Plas- ma Chemistryn, Ed. Hollahan, J.R., Bell, A.T., New York 1974.

/2/ Pahl, M., Z.Naturf. 2, (1957) 632.

/3/ Vasile, M.J., Smolinsky, G., Int. J.

Mass Spectrom. Ion Phys. %( 1973 1133, 147, 2 (1974) 381.

/4/ Drost, H., et a1 XIII. ICPIG 1978 Contr. Paper 1, 85

Drost, H., Physik und Technik des Plasmas 111, Suhl 1972, Proc. 2

Plasmachemie p. 87, Berlin 1972.

/5/ Vasile M.J., Smolinsky, G., Int. J.

Mass Spectrom Ion Phys. E(1975)179.

/6/ Schmidt M., Scbulz, KO-D., MaaB, Y., Proc. XIII. LCPIG, Contr.Paper 1,381.

/7/ Schmidt, M., Beitr. Plasmaphys. 13

(1973) 347.

manometer gas inlet ^I

A -

cat h o d e ' l r

I I "

gas inlet

sampling orifice

AMP 3

Fig.1. Experimental set up for the mass spectrometric investigations of the ions in the Ar-HMDS-discharge.

Pig.2, Mass spectrum of the ions in the Ar-HNLDS discharge, pAr=0,7 Torr,

~ ~ ~ ~ = 3 . 1 0 - ~ Torr, i=10 mA. For higher m/e values the sensitivity is increased by the factor 50.

Fig.3. Time development of the relative currents of the

~ 3 ,

^C+ ^{and C H+}₂₄

iona, pHMDS= 1.10-~ Torr.