stable glow plasma at atmospheric pressure

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Stable glow plasma at atmospheric pressure

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1988 J. Phys. D: Appl. Phys. 21 838

(http://iopscience.iop.org/0022-3727/21/5/028)

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J. Phys. D: Appl. Phys. 21 (1988) 836-840. Printed in the UK

S Kanazawa M Kog om a T Moriwaki and S OkazakiFaculty of Science and Technology, S ophia University, Kioi-cho 7-1, Chiyoda-ku,Tokyo 102, JapanReceived 23 February 1988

Abstract. A stable glow plasma at atmospheric pressure has been achieved forplasma treatment under selected conditions, for exam ple the structure ofelectrodes, the kind of dilute gas, and the frequency of power. The surfacefluorination of PET film and carbon thin-film deposition by such a plasma isdescribed here. The surface energy can be controlled by plasma treatment ofvarious concentra tions (O,/CF,/He) in the same way as the lower pressu remethod that has been reported by Kogoma and co-workers. Active species inplasma are identified by emission spectroscopy.

Thin-film deposition (of, e.g. , am orp hou s carb on ilms)and surface modification (e. g. wettability controls) byplasma processes are employed for manypractical pur-poses. Most of these processes occur at low pressures(a few torr) except for the process employing coronadischarge (Rouzbehi et af 1985). If the same processcan bearriedutttmosphericressure,hatmethod would be much m ore advantageous from theviewpoint of the cost of apparatus and the constructionof aarge-scaleystem. However,ischargetatmospheric ressureendsoranslateohermalplasma (cf anarcdischarge). So it is im portan t oachieve table nd ontinual glow dischargeorplasma processes at atmospheric pressure (Kanazawaet af 1987 .In hisLetter we shall report he necessarycon-ditionsand hestructure of the appa ratus neede d oestablishaplasmaprocess with a glow dischargeatatmo sphe ric pressu re. The results of the surface fluo-rination of PET (polyethylene erephthalate) f ilm andthe deposition of carbon thin film by that process a rereportedandcomp ared with the results obtain ed bythe lower pressure plasmas. The two kinds of plasma,at atmospheric pressure and at lower pressure, are ana-lysed an d compare d by emission spectroscopy.A glow disc harge w as created in a P yrex glass reac-tor (see figure 1 at atmospheric pressure. The lowerelectrode was heated and i ts tempe rature was measuredby a thermocouple.The requirements for a stable glow discharge are asfollows. (i) Helium is used as dilute gas. (ii) It is most

imp ortant that an insulating plate be set on the lowerelectrodelatefigure 1 . The kind of insulator

materialmustbeselected by considering heat resist-ance :kapton for 3000 Hz an d m ica or quartz glass forRF (13.56 MH z). ( ii i) Th e brush-style electrode is betterfor the upp er ele ctro de. It consists of 25 fine wires ofstainless or tungsten metal. (iv) Th e continuous stabledischarge is created by using 3000 H zor RF. Whenusing 50 Hz , the discharge was not stable. The surface

V

G a s

* HeaterFigure 1. The experimental apparatus.0022-37271881050838 + 03 02.50 @ 1988 IOP Pub lishing Ltd


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Letter to the Editor

3 6T r e a t m e n tl m em l n l

Figure 2. The relationship of plasma treatment time withcontact angle. Curves: A the atmospheric pressuremethod, He 1950 ml min- l, CF, 56.4 ml min- l, 3000 Hz,16W; B , the lower pressure method, CF, 5Torr, RF 50 W ;C, as curve A but also washed by Daiflon solvent.

fluorination of PET was carried out by only 3000 Hzglow becaus e R F glow plasma created a high temp era-ture, which the P E T could not resist, on the electro desurface. A carbon thin f ilm was obtained from plasmadeposition by R F power, which could provide sufficientenergy.A glow discharge sustained in CF 4 + O + He cankeep i tstabilityorongime.When usingC H 4 + H2+ H e, black product graphite) wasdeposited at the endof the wire of the upp er ele ctro de,so when a discharge was continued for a long time, itgradually lost stability.Figure 2 shows he contact angle as a function oftreatment time for the surface fluorination of PET filmsby He + CF4 . The frequ ency of power was 3000 Hzand he ource gas was CF4 (2.65 ) .CurveA wasobt aine d by surface luorination by theatmosphericpressure method and curve B was obtained when PETwas treated by the ower pressuremethod.CurveCindicates the valueof the conta ct angle after the surface

100 L

O , / O , C F IFigure 3 The surface treatment by various concentrationsof CF, + O2+ He. Curves: A the atmospheric pressuremethod, He 4900 ml min , CF, 68 ml min , O 2 0-68ml min , 3000 Hz, 23 W , 5 min; B, the lower pressuremethod, total gas pressure 3 Torr, R F 50 W, 5 min.was wash ed by Daiflon olvent C2C13F3 ) to removeany oligomers. By the atmospheric pressure method , ahigherequilibriumvalue was achieved orashortert ime of treatmen t than by the lower pressure method.Afterwashing, hecontact anglebecame omewhatsmaller, but the surface fluorination was confirmed bycomparisonwith hecont act angle of theuntreatedsurface.The surfaceodification of PET film byH e + CF, + O was carried out as follows. Th e PETsurfacewasreate d in plasma with variouson-centrations of O with CF 4. Th e contac tanglecouldbe varied by the 0 2 / 0 2CF 4) ratio (Kogoma et a11987 , as is demonstrated in figure 3. Amonotonicdecre ase of the conta ct angle occurs on increasing the0 2 / 0 2CF4) ra t io by e i ther method . However , the

Table l Carbon film deposition in He-CH,-H2. Total gas flow rate: 4600-4900 ml mi t ' , power:200 W , treatment time: 30 min.CH,IHe H2ICHI Thickness (,urn) Properties of film State of discharge0.037 1.26 cannot measure0.79 0.170.41 0.171.65 0.07

0.021

0.014

1.040.540.191.450.750.270.410.25

_ ~ ~ _0.190.670.35cannot measurecannot measurecannot measurecannot measure

_ ~ ~ _ ~ ~ _

-

non-uniform transform to arcblack, uniform dark blue, unstableblack, soft dark blue, stablenon-uniform transform to arcblack, uniform dark blue, stableblack, uniform dark blue, stable ~black,soft dark blue, stablenon-uniform transform to arcblack transform to arcblack dark blue, stableno products light blueblack dark blue

~ ~ ~ ~ - - - - -- -

- - - - - _ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

839


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Letter to the Editor

H eHa

LOO 5 6 7Wavelength ( l

Figure 4. The emission spectrum obtained by theatmospheric pressure method. He 4500 ml min, CH,130 m1 min, H2 70 m1 min, RF 200 W.

lower pressure method acquires a higher 0 2 / 0 2C F 4ratio for the same value of th e contact angle than theatmospheric pressure method.The aging effect at a treated surface was observ ed.We will report on the details of this in a future pape r.Carbon hin f ilms were prepared on the Pyrex orquartz glass substrates by decomposition ofC H 4 + H z + He in the glow discharge by theatmospheric pressure method. Variousmixing ratios ofCH,/H e or H2/C F4 are shown in tab le 1. A carbonfilm with betterprope rties was prepa red t mixingratios bound by the broke n line in table 1.Thearb on films with betterropertiesweredeposited at about400 C. The carbonilms which w eredeposited at 300 C tended to be oft and no n-uniform.A t 500 600 C,deposi tedproductswe re black ndwere composed of small particles.Th e emission s pectra during carbon film deposition,in the wavele ngth region 4000-7000 A are shown infigures 4 and 5. The analysis of the emissionspectrahasshown the presence of emission inesandbandsoriginating from electronic excited states of H , C 2 ndC H .Th e am e kinds of speciesweredentified in

1W

-

1 1LOO0 5000 6

Wavelength 1 )

J7

Figure 5. The emission spectrum obtained by the lowerpressure method. CH4 0.04 Torr, H2 0.06 Torr; RF 100 W .

plasma by both meth ods. By comp aring he intensityof the C2 band head i th tha t o f the CHband head, wefind that the C 2 band head o btainedy the atmosphericpressuremethod was strong,but hat by the lowerpressure method was relatively weak .The autho rs wish to thank The Mu rata Science Foun-dation for partial financial support for this wo rk.ReferencesKanazawa S , Kogoma M, Moriwaki T and Okazaki S 1987Proc. ISPC 8 1839Kogoma M, Kasai H, Takahash i K, Mor iwaki T andOkazak i S 1987 J . Phys . D pp l Phys. 20 147Rouzbehi F, Arefi F, Cato i re B. Goldm an M andAm ouroux J 1985 Proc. ISPC 7 2 485

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stable glow plasma at atmospheric pressure

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