Plasma Polymerization of Tetrafluoroethylene in a Capacitively

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1

N. MOROSOFF and H. YASUDA

Research Triangle Institute, P.O. Box 12194, Research Triangle Park, NC 27709

A capacitively coupled reactor designed to permit continuous coating of a moving substrate with plasma polymer has been described [1]. In this paper the results of a study of the plasma polymerization of tetrafluoroethylene in such a reactor presented. Plasma polymer has been deposited on aluminum electrodes as well as on an aluminum foil substrate placed midway between electrodes. The study particularly explores conditions in which deposition is minimized on the electrode. For this reason the chemical nature of the polymer formed in a low flow rate (F = 2 cm (S.T.P.)/min) and low pressure (p = 60 millitorr) plasma has been analyzed by the use of ESCA (electron spectroscopy for chemical analysis) and deposition rate determinations. This method combined with the unusual characteristics of TFE plasma polymerization (described below) has yielded information concerning the distribution of power in the inter-electrode gap. The effects of frequency (13.56 MHz, 10 KHz and 60 Hz), power and magnetic field have been elucidated. The properties of the TFE plasma polymer prepared in this apparatus are compared to those of the plasma polymer deposited in an inductively coupled apparatus [2,3]. 3

m

The present work i s not an exhaustive i n v e s t i g a t i o n o f TFE plasma p o l y m e r i z a t i o n as a f u n c t i o n o f the v a r i a b l e s mentioned. Rather i t s aim i s to uncover broad trends that can be used i n choosing c o n d i t i o n s f o r plasma p o l y m e r i z a t i o n and i n planning more i n t e n s i v e experimental work. A.

Unusual Aspects o f TFE Plasma P o l y m e r i z a t i o n

ESCA i s p a r t i c u l a r l y w e l l s u i t e d f o r the study o f plasma p o l y m e r i z a t i o n because i t y i e l d s information about the elementa l make-up of the surface t o a depth of 50 A or l e s s and i s 1

Current Address:

Department of Chemical Engineering U n i v e r s i t y of M i s s o u r i - R o l l a Rolla, Missouri 65401

0-8412-0510-8/79/47-108-163$05.00/0 © 1979 American C h e m i c a l Society

Shen and Bell; Plasma Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

P L A S M A POLYMERIZATION

164

not i n f l u e n c e d by the s u b s t r a t e p r o p e r t i e s . However, i t y i e l d s l i t t l e information about the f u n c t i o n a l groups present i n the polymer, except f o r some s p e c i a l cases where a l a r g e enough chemical s h i f t i s observed. Thus l i t t l e besides i n f o r mation concerning elemental composition can be obtained from an ESCA i n v e s i g a t i o n of the plasma p o l y m e r i z a t i o n of hydro carbons. However, f l u o r i n e does cause a l a r g e enough chemical s h i f t that the p o s i t i o n of the peak o f a carbon bonded to 3 f l u o r i n e s (CF ) can be d i s t i n g u i s h e d from that of one bonded to 2 f l u o r i n e s t C F > and both are c l e a r l y separated from the normal C^ peak, 1.e., one not bonded to any f l u o r i n e s [ 2 J . It i s c l e a r that the plasma p o l y m e r i z a t i o n of t e t r a f l u o r o ethylene i s p a r t i c u l a r l y amenable t o study by ESCA. The plasma polymerization of t e t r a f l u o r o e t h y l e n e (TFE) has been found to be unusual i n that the monomer i s s e n s i t i v e to discharge power [3]. Both d e p o s i t i o n r a t e data and ESCA a n a l y s i s of plasma p o l y m e r i z a t i o n i n i n d u c t i v e l y coupled systems demonstrate that f l u o r i n e poor polymers are formed when the e x c i t e d monomer passes through a high power density r e g i o n of the plasma. Because of the n o n - n e g l i g i b l e atmomic weight of f l u o r i n e t h i s f l u o r i n e a b s t r a c t i o n r e s u l t s i n lower d e p o s i t i o n r a t e s a t very high energy per u n i t mass of monomer feed. Deposition r a t e and ESCA r e s u l t s on blanks placed at v a r i o u s s i t e s i n the plasma r e a c t o r may t h e r e f o r e be used as a probe of the power density d i s t r i b u t i o n i n a given r e a c t o r with a glow discharge f e d by t e t r a f l u o r o e t h y l e n e . 2

g

B.

Plasma Polymerization

o f TFE i n I n d u c t i v e l y Coupled Systems.

We may compare r e s u l t s presented here with those obtained i n two types of i n d u c t i v e l y coupled r e a c t o r s [ . 2 , 3 ] . One i s the r e a c t o r we have used f o r many years [ 4 ] , i n which the p o r t i o n of the r e a c t o r i n s e r t e d i n t o the r . f . c o i l i s smaller than the main p o r t i o n of the r e a c t o r , i n which plasma polymer i s c o l l e c t e d . Monomer f l u x i s d i r e c t e d i n t o the main p o r t i o n of the r e a c t o r , not through the r . f . c o i l . E l e c t r o n bombardment of plasma polymer and s u b s t r a t e i s reduced i n t h i s way [ 5 J . A c t i v e species are formed mainly under the r . f . c o i l and are transported by d i f f u s i o n to the e n t i r e volume of the r e a c t o r . I n t e r a c t i o n of these non-polymerizable energy c a r r y i n g species (e.g. e l e c t r o n s , e x c i t e d atoms) with the monomer entering the r e a c t o r leads to plasma p o l y m e r i z a t i o n [ 5 J . The energy s u p p l i e d per gram of monomer feed may be given as W/FM, where W i s the power input, F i s the flow r a t e and M i s the molecular weight. I n i t i a l l y , the d e p o s i t i o n r a t e g r a d u a l l y increases with i n c r e a s i n g power but begins to decrease because of the above mentioned f l u o r i n e a b s t r a c t i o n occuring i n t h i s system when W/FM exceeds 4 . 7 χ 1 0 Joules/kg [2,6].


10.

MOROSOFF AND YASUDA

Capacitively

Coupled

Discharge

165

The plasma p o l y m e r i z a t i o n of t e t r a f l u o r o e t h y l e n e has a l s o been studied i n a s t r a i g h t tube r e a c t o r . Deposition rates and ESCA r e s u l t s were obtained as a f u n c t i o n of l o c a t i o n upstream from, w i t h i n , and downstream from the i n d u c t i o n c o i l [3]. It was found that f l u o r i n e poor polymer was formed downstream fro,m the c o i l even at the r e l a t i v e l y low power l e v e l of 1.9 χ 10 Joules/kg. F l u o r i n e poor polymer was formed at a l l l o c a t i o n s at 7.7 χ 10 Joules/kg. C.

Plasma Polymerization

i n C a p a c i t i v e l y Coupled Systems

In a c a p a c t i v e l y coupled system the flow r a t e i n t o the plasma i s not known as p r e c i s e l y as f o r the systems described above. This i s because not a l l monomer feed must d r i f t i n t o the i n t e r - e l e c t r o d e gap. On the other hand the flow i n t o the gap cannot be obtained from the knowledge of the f r a c t i o n of r e a c t o r volume the i n t e r - e l e c t r o d e gap represents, because the plasma polymerization process acts as a pump. Moreover the range of power or current that can be used f o r plasma polymer d e p o s i t i o n i s l i m i t e d f o r c a p a c i t i v e l y coupled glow discharges at low pressure. This i s because at low pressure and high powers, RF plasma tend to expand outside the i n t e r - e l e c t r o d e gap and a r c i n g occurs i n AF and AC plasma. The use of a magnetic f i e l d to confine the plasma allows the p o l y m e r i z a t i o n to be c a r r i e d out over a wide range of powers. In c o n t r a s t i n g the r e s u l t s obtained f o r the RF glow discharge with those f o r the AF and AC glow discharges, i t i s w e l l to bear i n mind the fundamental d i f f e r e n c e between the two types of glow discharges [7,8]. The AC and AF glow d i s charges may be considered to be DC glow discharges of a l t e r n a t ing p o l a r i t y . P o s i t i v e ions h i t t i n g the cathode give r i s e to secondary e l e c t r o n s . These are a c c e l e r a t e d through the Aston dark space, cathode glow and Crookes dark space gaining enough energy to i o n i z e n e u t r a l species i n the negative glow. These ions can then r e i n i t i a t e the process i n s u r i n g a s e l f - s u s t a i n i n g glow discharge. Free r a d i c a l s and other a c t i v e species are formed i n or near the negative glow and react y i e l d i n g a plasma polymer. I n t e r n a l conducting e l e c t r o d e s are necessary to s u s t a i n such a discharge as buildup of a p o s i t i v e charge on the cathode would r e p e l c a t i o n s thus s h u t t i n g o f f the r e q u i r e d source of secondary e l e c t r o n s . In an RF glow discharge, e l e c t r o n s o s c i l l a t e i n the f i e l d set up between (and/or around the e l e c t r o d e s ) g a i n i n g enough energy to form f r e e r a d i c a l s , ions and other a c t i v e species by random c o l l i s o n s . Conducting e l e c t r o d e s are t h e r e f o r e not r e q u i r e d . Experimental The apparatus used i n t h i s study i s s i m i l a r to that described i n reference 1. I t d i f f e r e d i n that pumping was



166

through 3/4 i n c h diameter g l a s s tubing and stopcocks and the apparatus had only one r e a c t i o n chamber. Magnetic enhancement of the glow discharge was achieved as described i n reference 1. The power s u p p l i e s f o r RF, AF, and AC have been described elsewhere [ 9 ] . In order to c o l l e c t polymers at the center of the i n t e r - e l e c t r o d e gap, an aluminum f o i l s u b s t r a t e , 8 cm i n width was suspended between e l e c t r o d e s as shown i n reference 1. D e p o s i t i o n r a t e s at v a r i o u s l o c a t i o n s on the s u b s t r a t e and e l e c t r o d e were obtained by measuring the weight gain of small pieces of aluminum f o i l (sampling blank) a f f i x e d to a given s i t e so that e l e c t r i c a l contact was maintained between the s i t e of i n t e r e s t and the sampling blank. This was accomplished by f o l d i n g the ends of a 0.5 cm χ 3 cm piece of aluminum f o i l around the ends of a 0.5 cm χ 2.2 cm p i e c e of g l a s s cover s l i p and a f f i x i n g the center of the g l a s s cover s l i p to the e l e c t r o d e or s u b s t r a t e with double coated scotch tape. Samples f o r ESCA a n a l y s i s were prepared i n the same manner and on the same sampling blanks as used f o r the deposi t i o n r a t e s t u d i e s . The f o l l o w i n g s p e c t r a l l i n e s were obtained from each blank: 0.^ , F^ , C and A l by using a DuPont model 650 spectrometer with a MgK X-ray source, and equipped with a microcomputer data a c q u i s i t i o n and processing system. Spectra are c o r r e c t e d f o r charging. D e t a i l s of the experimental procedure are d e s c r i b e d elsewhere [1Q]. I t may be noted that the procedure i n v o l v e s using a f r e s h sample f o r each element and i s such as to keep hydrocarbon contamination small and constant. l g

2 g

S

Results and

Discussion:

As shown i n the previous paper, the use of a magnetic f i e l d leads to an annular zone of intense glow, s l i g h t l y removed from the e l e c t r o d e and with a mean radius of 4 cm. from the e l e c t r o d e center a x i s . For t h i s reason plasma polymer was c o l l e c t e d on aluminum blanks placed on the e l e c t r o d e center a x i s and a t a radius of 4 cm. These blanks were placed on the e l e c t r o d e and on the aluminum substrate suspended between e l e c t r o d e s . For some of the glow discharges where no magnetic f i e l d was used, samples of plasma polymer were c o l l e c t ed only on the i n t e r s e c t i o n of the e l e c t r o d e center a x i s with the e l e c t r o d e s and s u b s t r a t e . ESCA s p e c t r a were c o l l e c t e d f o r 0 , ^ > i ^7s' A f t e r c o r r e c t i o n f o r the p h o t o - e l e c t r i c c r o s s - s e c t i o n s or these l i n e s , the peak areas i n d i c a t e the r e l a t i v e number of each element at the s u r f a c e . As w i l l be shown below, these r e s u l t s cannot be i n t e r p r e t e d i n terms of the F/C r a t i o i n the plasma polymer because bonding of f l u o r i n e to aluminum can a l s o occur. However, a measure of the F/C r a t i o i n the d e p o s i t ed plasma polymer can be obtained from the shape of the C. F

l g

c

s

a

n

d

s


10.

MOROSOFF AND YASUDA

Capacitively

Coupled

167

Discharge

spectrum. The C, spectrum obtained f o r a sample of plasma polymer c o l l e c t e d on the e l e c t r o d e center with a power of 10 watts i s shown i n F i g u r e 1. It has peaks at 293.5 eV, ( C F ) , 291.5 eV, (CF ) and the peak at 284.6 a t t r i b u t e d to hydrocarbon or g r a p h i t i c carbons. The peaks between 291.6 eV and 284.6 eV, are a t t r i b u t e d to carbons bound to one f l u o r i n e atom or other e l e c t r o n e g a t i v e species (e.g., 0 or N). Spectra obtained f o r sample c o l l e c t e d i n very a c t i v e regions of the plasma with a high power input show much l e s s prominent peaks i n d i c a t i v e of carbons bound to f l u o r i n e and a very prominent carbon peak at 284.6 eV. An obvious q u a l i t a t i v e numerical i n d i c a t o r of these c h a r a c t e r i s t i c peak shapes i s the r a t i o of the peak height at 291.5 eV (CF > to that at 284.6 eV. The use of t h i s r a t i o as a means of i d e n t i f y i n g trends, _i.€i. , the c o n d i t i o n s that w i l l y i e l d a f l u o r i n e r i c h polymer as opposed to those that w i l l y i e l d a f l u o r i n e poor polymer, i s c l e a r l y j u s t i f i e d . However, i t should not be thought of as being d i r e c t l y p r o p o r t i o n a l to CF /CH or F/C r a t i o s i n the polymer, i n part because no e f f o r t has been made to c o r r e c t f o r the shoulders of neighboring peaks and i n part because the peak height at 284.6 eV w i l l i n c l u d e a c o n t r i b u t i o n from contamination i n the ESCA i n s t r u ment. Our experimental procedure i s such as to keep the l a t t e r small and constant [10]. 3

2

2

2

2

The values o f such peak height r a t i o s obtained f o r v a r i o u s powers or current l e v e l s i n the RF, AF and AC plasmas are shown i n Figures 2,3 and 4 r e s p e c t i v e l y . I t may be noted that the use of s t r a i g h t l i n e s i n these graphs i s intended only as a v i s u a l a i d to connect p o i n t s r e p r e s e n t i n g i d e n t i c a l l o c a t i o n s i n the i n t e r e l e c t r o d e gap. No i n f e r e n c e concerning the a c t u a l value of the o r d i n a t e at i n t e r v e n i n g p o i n t s on the a b s c i s s a i s justified. I f we equate a f l u o r i n e r i c h polymer with proximity of the sampling s i t e to a low power d e n s i t y r e g i o n of the plasma and a f l u o r i n e poor plasma polymer w i t h p r o x i m i t y to a high power d e n s i t y r e g i o n of the plasma, the f o l l o w i n g observations may be made. The data i n F i g u r e 2 i n d i c a t e t h a t , as expected, plasma polymers g e n e r a l l y decrease i n f l u o r i n e content as the power i s r a i s e d from 10 to 100 watts. The one exception i s plasma polymer c o l l e c t e d at the e l e c t r o d e with no magnetic field. In t h i s case the peak-height r a t i o i n c r e a s e s with i n c r e a s i n g power at the e l e c t r o d e , although evidence of a h i g h l y a c t i v e plasma i s seen at the s u b s t r a t e . The evidence of a low power d e n s i t y at the e l e c t r o d e at a high power input i s i n l i n e with the model of c a p a c i t i v e l y coupled RF glow discharge discussed e a r l i e r [1]. According to t h i s model, the most a c t i v e zone of the glow discharge i s i n the center of the i n t e r e l e c t r o d e gap with a r e l a t i v e l y i n a c t i v e zone immediat e l y adjacent to the e l e c t r o d e . The extent of i n a c t i v e zone i s expected to increase with i n c r e a s i n g power. The use of a


168

PLASMA

291.4 289

2

8

7

>

POLYMERIZATION

2

U293.5

Figure 1. C (Is) spectrum of plasma polymer deposited on the electrode 4 cm from the electrode center with a flow rate of 2 (STP)mL/min, P = 60 mtorr, 19 m A AF current and no magnets M

E S C A . RF

Figure

2.

Plot of the 291.5 eV/284.6

eV C (Is) peak height ratios vs. RF


power

MOROSOFF AND YASUDA

Capacitively

Coupled

169

Discharge

ESCA. AF

C U R R E N T (m/A)

Figure

3.


eV C (Is) peak height ratios vs. AF

current

ESCA. A C

CURRENT (m/A)

Figure

4.


eV C (Is) peak height ratios vs. AC


current

170


magnetic f i e l d tends to move the a c t i v e zone c l o s e r to the e l e c t r o d e with the r e s u l t that a decrease i n f l u o r i n e content i s observed at both e l e c t r o d e and s u b s t r a t e as the power i s increased from 10 to 100 watts. In c o n t r a s t , the peak height r a t i o s observed f o r AF and AC, shown i n Figures 3 and 4, i n d i c a t e that the power density i s g e n e r a l l y greater at the e l e c t r o d e than at the s u b s t r a t e . When a magnetic f i e l d i s employed a f l u o r i n e poor polymer i s formed at a radius of 4 cm at a lower current than i s r e q u i r e d to form a f l u o r i n e poor polymer at the center of the e l e c t r o d e . This was expected i n l i g h t of the c o n c e n t r a t i o n of glow at a r a d i u s of 4 cm, near the e l e c t r o d e , when magnets are used. F i n a l l y i t may be noted that f o r AC l i t t l e change occurs i n the nature of the plasma polymer deposited at the center of the e l e c t r o d e on i n c r e a s i n g the current to 150 mA. In f a c t , the plasma polymer deposited at the center of the e l e c t r o d e deposited i n an AC glow discharge at 150 mA i s more f l u o r i n e r i c h than that deposited at the s u b s t r a t e . In t h i s case there appears to be more of a c o n t r a s t i n the type of the polymer deposited at l o c a t i o n s on the e l e c t r o d e than between that deposited on the e l e c t r o d e and the s u b s t r a t e . In summary the d i s t r i b u t i o n of power i n the RF plasma appears to be the opposite of that i n the AF and AF plasma. Power i s concentrated near the substrate i n the RF plasma, near the e l e c t r o d e f o r AF and AC. The magnetic f i e l d causes a l o c a l i z a t i o n of glow on a r i n g of 4 cm radius f o r the AF and AC glow discharges, r e s u l t i n g i n the most f l u o r i n e poor polymer at a radius of 4 cm on the e l e c t r o d e . The opposite i s the case f o r RF at low power, there i s no d i f f e r e n t i a t i o n between the two at high power. Between AF and AC, there appears to be more d i f f e r e n t i a t i o n between the center of the e l e c t r o d e and the 4 cm radius on the e l e c t r o d e f o r AC than AF. Finally i t may be noted that at low powers more f l u o r i n e appears to be present i n the RF polymer than i n the- AF or AC polymer suggesting that the m i l d e s t c o n d i t i o n s are found i n the RF plasma. The elemental r a t i o s obtained from the ESCA data are presented i n Figures 5,6, 7 f o r RF, AF and AC r e s p e c t i v e l y . I t may be noted that i n a l l cases the F/C r a t i o remains between 1 and 2 r e g a r d l e s s of frequency or power. As the Cspectra i n d i c a t e that a f l u o r i n e poor polymer i s achieved at high powers, i t i s c l e a r that under high power c o n d i t i o n s f l u o r i n e i s bonded to some other element, aluminum being the only l i k e l y candidate. I t may be a d d i t i o n a l l y observed that a high p r o p o r t i o n of aluminum and oxygen i s observed on the s u r f a c e under c o n d i t i o n s of high power d e n s i t y . This l a t t e r trend had been p r e v i o u s l y observed f o r TFE plasma p o l y m e r i z a t i o n i n an i n d u c t i v e l y coupled s t r a i g h t tube r e a c t o r [3]. However, the F/C r a t i o was somewhat more v a r i a b l e f o r t h i s i n d u c t i v e l y coupled r e a c t o r than f o r the present c a p a c i t i v e l y coupled r e a c t o r , presumably because of a s h o r t e r residence time f o r


MOROSOFF AND YASUDA

10

20

31

Capacitively

40

M

Coupled

M

70

Discharge

Μ

Ν

100

POWER (WATTS)

Figure

5.

Semilogarithmic

plot of the O/C, F/C, RF power

and Al/C

elemental

ratios

vs.

ratios

vs.

ESCA. AF

, 15

, 30

0

, V 40

op** 50

M

. ™

. »

. w

^ 100

C U R R E N T (mA)

Figure

6.

Semilogarithmic

plot of the O/C, F/C, AF current

and Al/C

elemental


172


gaseous byproducts ( f l u o r i n e ) i n the s t r a i g h t tube r e a c t o r . By equating high O/C and Ai/C elemental r a t i o s with h i g h power d e n s i t y , one a r r i v e s at the same conclusions regarding the d i s t r i b u t i o n of power d e n s i t y i n the i n t e r e l e c t r o d e gap as i n the d i s c u s s i o n of Cpeak height r a t i o s . In F i g u r e 5 the Ο / β r a t i o remains unchanged on i n c r e a s i n g power from 10 to 1 0 0 watts only f o r the e l e c t r o d e without magnets. In a d d i t i o n only f o r t h i s case i s no aluminum observed on the plasma polymer s u r f a c e . This confirms the presence of a low power d e n s i t y environment near the e l e c t r o d e f o r 1 0 0 watts RF power without a magnetic f i e l d . As shown below, (See F i g s . , 1 1 - 1 2 ) , no negative d e p o s i t i o n rates are observed f o r RF. Aluminum appears to be c o n s t a n t l y ablated and redeposited i n a high power d e n s i t y environment. The oxygen observed on the surface i s assumed to be formed a f t e r exposure of plasma polymer to the a i r . The trends observed f o r an AF plasma from the e v a l u a t i o n of the C^ peak height r a t i o s are confirmed i n the elemental r a t i o s i n Figure 6 . G e n e r a l l y the e l e c t r o d e O/C and AZ/C r a t i o s are higher than the correponding substrate elemental r a t i o s f o r the same s e t of c o n d i t i o n s . For the case of a magnetic f i e l d , high O/C and A&/C r a t i o s are observed at a lower c u r r e n t f o r an e l e c t r o d e radius of 4 cm than at the e l e c t r o d e center. I t may be noted that negative d e p o s i t i o n r a t e s are observed at the e l e c t r o d e at high c u r r e n t s (See F i g s . , 1 2 , 1 3 and 1 4 ) . For AC ( F i g . 7 ) , the marked l o c a l i z a t i o n of power density i n the presence of a magnetic f i e l d at the annular zone of r a d i u s , 4 cm, i s confirmed at a current of 1 5 0 mA. Again the elemental r a t i o s f o r the s u b s t r a t e are intermediate between those of the e l e c t r o d e at r a d i i of 0 and 4 cm. A d d i t i o n a l l y , h i g h O/C and A£/C r a t i o s are observed at an e l e c t r o d e r a d i u s of 4 cm even at a current of only 5 0 mA. An a d d i t i o n a l d i f f e r e n t i a t i o n between the e f f e c t s of RF and AF, AC plasma i s found i n the p o s i t i o n of the oxygen peak p o s i t i o n as shown i n Figures 8 - 1 0 . For RF, the oxygen binding energy i s l e s s than 5 3 2 eV at the s u b s t r a t e at 1 0 0 watts power, i n d i c a t i n g that oxygen i s bound to aluminum f o r t h i s case. The oxygen peak p o s i t i o n at the e l e c t r o d e without magnets stands out because i t remains unchanged on i n c r e a s i n g power from 1 0 to 1 0 0 watts, confirming conclusions drawn above. I t may be noted that much l e s s change i s seen i n oxygen peak p o s i t i o n as a f u n c t i o n of current f o r AF and AC ( F i g s . 9 and 1 0 ) . D e p o s i t i o n r a t e data were obtained at r a d i i of 0 , 2 , 4 and 6 cm on e l e c t r o d e s and s u b s t r a t e . There are reported i n d e t a i l and i n t e g r a t e d to % conversion of the monomer to plasma polymer elsewhere |J9J . The r e s u l t s are shown i n Figures 1 1 and 1 2 . For RF, i t i s c l e a r that conversion i s l e s s at the s u b s t r a t e than at the e l e c t r o d e . However the e f f e c t of a g


MOROSOFF AND YASUDA

Figure

7.

Semilogarithmic

Capacitively

Coupled

plot of the O/C, F/C, AC current

Discharge

and Al/C,

elemental

ratios

POWER (WATTS)

Figure

8.

Plot of Ο (Is) binding

energy vs. RF

power


vs.

174

PLASMA

ELECTRODE

Λ ΟΔ

POLYMERIZATION

SUBSTRATE

Ο

·

NO M A G N E T S

£

A

M A G N E T I C FIELD

0 4

00

Δ0 •

4 0.4 # 0 4

Oo

50

6 0

CURRENT ImAI

Figure

9.

0 . 4 · ° ^


energy vs. AF

φ

NO M A G N E T S

•


current

4

g 533L

« 5321

CURRENT ImAI

Figure

10.


energy vs. AC

current


MOROSOFF AND YASUDA

Capacitively

Coupled

Discharge

175

RF

Ο NO M A G N E T S

ELECTRODE

Δ

SUBSTRATE


15

r

Figure

11.

Plot of apparent conversion of monomer feed to plasma the electrode and substrate vs. RF power

NO M A G N E T S

polymer

at

MAGNETS

AF Ο

Δ

ELECTRODE

AC φ

g

SUBSTRATE

Figure 12. Plot of apparent conversion of monomer feed to plasma polymer at the electrode and substrate vs. AF or AC current. Symbols on the abscissa with arrows pointing down indicate negative deposition rates. Those with arrows pointing up and down indicate both negative and positive deposition rates at the electrode. No conversions were calculated for such cases.


PLASMA

176

POLYMERIZATION

AF PLASMA, MAGNETICALLY ENHANCED ELECTRODE

DISTANCE

Figure

—Q

,

5

m

A

FROM ELECTRODE CENTER AXIS (cm)

13. Deposition rate profiles on electrodes and substrate for AF plasma a flow rate of 2(STP)mL/min, T? = 60 mtorr with a magnetic field M


at

MOROSOFF A N D YASUDA

Capacitively

Coupled

Discharge

AC PLASMA, MAGNETICALLY ENHANCED ELECTRODES

Ο

50 mA

SUBSTRATES

Δ

150 mA

-500 |DISTANCE

FROM

ELECTRODE

CENTER

AXIS (cm)

Figure 14. Deposition rate profiles on electrode and substrate for an AC glow discharge at a flow rate of 2 (STP)mL/min, P = 60 mtorr with a magnetic field M


177

178


magnetic f i e l d i s to moderate the a b l a t i o n at the substrate to some degree. In Figure 12 the symbols on the a b s i c c a a x i s with arrows p o i n t i n g downward i n d i c a t e that no conversions were c a l c u l a t e d because of a negative d e p o s i t i o n r a t e . I t may be noted that negative d e p o s i t i o n rates were only observed on the e l e c t r o d e . Where a magnetic f i e l d was used both negative and p o s i t i v e d e p o s i t i o n rates were observed at d i f f e r e n t s i t e s on the e l e c t r o d e . The conversion on the e l e c t r o d e i s l e s s than that on the substrate f o r the AF but not f o r AC. The ESCA data above i n d i c a t e d greater d i f f e r e n t i a t i o n between the center and the 4 cm p o s i t i o n on the e l e c t r o d e f o r AC than between the e l e c t r o d e and s u b s t r a t e . For AF the greatest d i f f e r e n t i a t i o n was between e l e c t r o d e and s u b s t r a t e . These trends are confirmed i n Figures 13 and 14, p l o t s of d e p o s i t i o n r a t e versus d i s t a n c e from the e l e c t r o d e center a x i s f o r AF and AC, r e s p e c t i v e l y . The very high d e p o s i t i o n r a t e on the e l e c t rode at 0 cm may be noted f o r AC as compared to the corresponding d e p o s i t i o n rate f o r AF. Conclusions : The f o l l o w i n g conclusions may be drawn f o r plasmas^at a pressure of about 40 m i l l i t o r r , and a flow r a t e of 2 cm /min: 1. ) Plasma polymers s i m i l a r to those observed f o r i n d u c t i v e l y coupled plasmas are l a i d down i n a c a p a c i t i v e l y coupled plasma. 2> 3 ' S P d a l i p h a t i c or g r a p h i t i c carbons are present. An i n c r e a s e i n power or current g e n e r a l l y leads to a more f l u o r i n e poor polymer. 2. ) In an RF c a p a c i t i v e l y coupled plasma i n c r e a s i n g the power leads to an i n c r e a s i n g power d e n s i t y at the substrate but not at the e l e c t r o d e . A magnetic f i e l d tends to move the most a c t i v e r e g i o n of the plasma c l o s e r to the e l e c t r o d e . 3. ) For an AF and AC plasma the most a c t i v e region i s near the e l e c t r o d e . The use of a magnetic f i e l d confines the most a c t i v e r e g i o n to a zone of 4 cm radius c l o s e to the e l e c t r o d e f o r over set-up. This i s seen v i s u a l l y [1] and i s r e f l e c t e d i n the ESCA and d e p o s i t i o n r a t e data. 4. ) At high currents the greatest d i f f e r e n c e i n the d e p o s i t i o n r a t e and character of plasma polymer i s between the 4 cm r a d i u s and e l e c t r o d e center f o r AC, between e l e c t r o d e and s u b s t r a t e f o r AF. 5. ) The m i l d e s t c o n d i t i o n s were obtained with RF at low power i n p u t . Etching r e s u l t i n g i n negative d e p o s i t i o n rates was observed only f o r AF and AC, at high current l e v e l s . D e f i n i t e bonding of oxygen to aluminum was only obtained with RF at high power. CF

C F

C F

r o u

s

a n


10.

MOROSOFF AND YASUDA

Capacitively

Coupled

Discharge

179

Acknowledgment : This work was supported by the O f f i c e of Water Research and Technology o f the U. S. Department o f the I n t e r i o r under Contract Nos. 14-30-3301 and 14-34-0001-7537. Measurement of d e p o s i t i o n rates and plasma polymer p r e p a r a t i o n was c a r r i e d out by Barry H i l l whereas the ESCA s p e c t r a were obtained by E. S. Brandt under the s u p e r v i s i o n o f C. N. R e i l l e y at the Depart ment o f Chemistry o f the U n i v e r s i t y o f North C a r o l i n a a t Chapel H i l l .

Literature Cited: 1. H. Yasuda and N. Morosoff, "Tandem Plasma Polymer ization Apparatus for Continuous Coating of Fibers Films", in paper presented at Plasma Polymerization Symposium, ACS meeting Miami Beach, September 1978. 2. H. Yasuda, T. S. Hsu, E. S. Brandt, C. N. Reilley, J. Polymer Sci., Polymer Chem. Ed., 16, 415 (1978). 3. H. Yasuda, Ν. Morosoff, E. S. Brandt, C. N. Reilley, J. Appl. Polymer Sci., in press. 4. H. Yasuda and C. E. Lamaze, J. Appl. Polymer Sci., 17, 1519 (1973). 5. H. Yasuda and T. Hirotsu, J. Polymer Sci., Polymer Chem. Ed., 16, 313 (1978). 6. H. Yasuda and T. Hirotsu, J. Polymer Sci., Polymer Chem. Ed., 16, 743 (1978). 7. N. Morosoff, W. Newton, and H. Yasuda, J. Vac. Sci. Tech., in press. 8. L. Maissel, "Application of Sputtering to the Deposi tion of Films", in L. Maissel and R. Glang, eds. "Handbook of Thin Film Technology," McGraw Hill, N.Y.(1970). 9. N. Morosoff, H. Yasuda, E. S. Brandt and C. N. Reilley, J. Appl. Polymer Sci., in press. 10. H. Yasuda, H. C. Marsh, E. S. Brandt and C. N. Reilley, J. Polymer Sci., Polymer Chem. Ed., 15, 991 (1977). Received March 29, 1979.


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