Plasma Polymerization - American Chemical Society

6 Plasma Diagnostics of Polymerizing Benzene Plasma Plasma Diagnostics of a Tool for Controlling Plasma Reaction

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MASAHIRO NIINOMI and KENJI YANAGIHARA Japan Synthetic Rubber Co., Ltd., Tokyo Research Laboratory, 7569 Ikuta, Tama-ku, Kawasaki, Japan

ABSTRACT The electron temperature (T ), electron density (n ) and electron energy distribution function for a plasma sustained in an argon/benzene mixture were measured by double and triple plasma -probe methods. Each probe was heated up to 1000 Κ with a sheathed heater, which was inserted into the probe, in order to prevent polymer film from depositing on the surface of the probe. T and n were very sensitive to the state of plasma, and were thus useful for detecting an occurrence of abnormal reaction and/or deviation of the polymerization reaction from the standard route. By using the method as a monitoring tool, control of plasma polymerization reaction can be performed during reaction with high reliability. e

e

e

e

Benzene was plasma-polymerized i n order to prepare a reverse osmosis (RO) membrane on a porous membrane f i l t e r f o r s e p a r a t i o n of v a r i o u s mixtures of organic s o l v e n t s . Based upon thermodynamic estimates, a pressure r e s i s t a n c e of more than 100 kg/cm^ i s neces sary i n many cases. Under optimum r e a c t i o n c o n d i t i o n s of the plasma, a polymer f i l m with the r e q u i r e d pressure r e s i s t a n c e could be prepared. Although the r e a c t i o n s were c o n t r o l l e d by monitoring macroscopic r e a c t i o n c o n d i t i o n s such as pressure of the system, monomer flow r a t e and e l e c t r i c power supply, the p r o p e r t i e s of the polymer f i l m sometimes d e t e r i o r a t e d s i g n i f i c a n t l y . Polymer d e t e r i o r a t i o n may be a t t r i b u t e d to mixing of i m p u r i t i e s such as a i r i n t o the monomer, and/or v a r i a t i o n of monomer/carrier-gas composition i n the r e a c t o r during r e a c t i o n . The conventional monitoring method could not detect an occurrence of such unfavorable r e a c t i o n s . A more s e n s i t i v e monitoring method f o r c o n t r o l l i n g the plasma r e a c t i o n was r e q u i r e d . In the case of a non c h e m i c a l - r e a c t i o n plasma, the s t a t e of the plasma can be described by the e l e c t r o n energy d i s t r i b u t i o n f u n c t i o n , f ( s ) (1) . In the case of a polymerizing plasma, however, f ( e ) may not be s u f f i c i e n t . In other words, the s t r u c t u r e of the

0-8412-0510-8/79/47-108-087$06.75/0 © 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.

88

PLASMA

POLYMERIZATION

plasma-polymerized polymers may not always be the same even i f f ( e ) i s kept constant, s i n c e v a r i o u s kinds of r e a c t i v e s p e c i e s other than e l e c t r o n s are present and take p a r t i n the plasma p o l y m e r i z a t i o n process. I t i s not c l e a r at present which of these species c o n t r o l s the p o l y m e r i z a t i o n r e a c t i o n and determines the c h a r a c t e r i s t i c s of the r e a c t i o n product. The plasma r e a c t i o n i s , however, i n i t i a t e d by generation of e l e c t r o n s and p o s i t i v e i o n s , followed by t h e i r c o l l i s i o n with monomer molecules. The d i s t r i b u t i o n of f i r s t - g e n e r a t i o n r e a c t i v e s p e c i e s , which governs the s u c c e s s i v e process of the polymerizat i o n r e a c t i o n , i s determined by the d i s t r i b u t i o n of e l e c t r o n energies i n the system. E l e c t r o n s are more s i g n i f i c a n t than ions f o r chemical r e a c t i o n because of t h e i r extremely high average energy. I t may be s a i d that a plasma r e a c t i o n process (or a t l e a s t i t s f i r s t step) can a l s o be c h a r a c t e r i z e d by the c h a r a c t e r - * i s t i c s of the plasma e l e c t r o n s . Thus, the present study was undertaken to see i f the e l e c t r o n c h a r a c t e r i s t i c s of the plasma could be used as a monitoring parameter f o r c o n t r o l l i n g the plasma p o l y merization reaction. EXPERIMENTAL Polymerization The apparatus and the e l e c t r i c power supply system used i n t h i s study were e s s e n t i a l l y the same as those of Yasuda and Lamaze (2). A Pyrex g l a s s r e a c t o r , as shown i n F i g . 1, i s composed of a discharge chamber (diameter 2.5 cm, l e n g t h 14 cm) and a r e a c t i o n chamber (diameter 6.5 cm, l e n g t h 31 cm): the former was equipped with an argon gas i n l e t , an r f c o i l and a mass sampling p o r t ; and the l a t t e r , a benzene i n l e t , a connection to a P i r a n i gauge, a s u b s t r a t e stage, two s e t s of plasma probes at d i f f e r e n t p o s i t i o n s , and another mass sampling p o r t . Nuclepore ^ membrane f i l t e r s of 0.03 ym pore-diameter were used as s u b s t r a t e m a t e r i a l on which polymer f i l m was deposited. Vacuum d r i e d f i l t e r s were placed on the s u b s t r a t e stage at two d i f f e r e n t p o s i t i o n s , 10 cm and 15 cm downstream from the end of the r f c o i l ( P o s i t i o n 10 and P o s i t i o n 15, r e s p e c t i v e l y ) . Monomer feed l i n e and evacuation l i n e s of the r e a c t o r are shown i n F i g . 2. D i s t i l l e d benzene was t r a n s f e r r e d i n t o a f l a s k with a ground-glass j o i n t (1 i n F i g . 2). A f t e r connecting the f l a s k to the monomer feed l i n e , the whole l i n e (1-5) was evacuated, by a r o t a r y pump (6) to l e s s than 10~2 Torr as measured with a P i r a n i gauge (5), while keeping the benzene f r o z e n with a l i q u i d n i t r o g e n bath. Benzene i n the f l a s k was then allowed to evaporate slowly, keeping a l l the stopcocks c l o s e d except f o r that to the monomer-gas r e s e r v o i r (3). Since the bottom p a r t of the r e s e r v o i r had been cooled with another cryogen, benzene was t r a n s f e r r e d i n t o , and s o l i d i f i e d i n the r e s e r v o i r . About one t h i r d of the benzene was l e f t i n f l a s k (1), and d i s c a r d e d . By s i m i l a r o p e r a t i o n , one t h i r d of the o r i g i n a l amount of benzene was t r a n s f e r r e d i n t o f l a s k (2) and was d i s c a r d e d as a pre-cut d i s t i l l a t e . As a r e s u l t , one


6.

N i i N O M i AND Y A N A G i H A R A

Plasma

89

Diagnostics

t h i r d of the o r i g i n a l amount of benzene was stored i n the r e s e r v o i r (3) under vacuum, and was used as monomer. I t was found necessary for steady feed of the monomer vapor i n t o the r e a c t o r to keep the vapor pressure i n the feed l i n e constant. This was a t t a i n e d by keeping the temperature of the r e s e r v o i r constant with an i c e water/ NaCl bath, and by using a l a r g e volume r e s e r v o i r (about 4 liters). The r e a c t o r (9) c o n t a i n i n g degassed Nuclepore f i l t e r s (10) was evacuated, p r i o r to r e a c t i o n , to l e s s than 10"^ Torr by an o i l d i f f u s i o n pump. The vacuum was measured with an i o n i z a t i o n gauge (11). As soon as dry argon was f e d i n t o the r e a c t o r through a rotameter (8), the evacuation of the system was switched to a second r o t a r y pump (12). The vacuum i n the r e a c t o r was now measured with a second P i r a n i gauge (13). The use of ground-glass j o i n t s and Kover-to-glass s e a l s between parts enabled maintenance of a high vacuum. The monomer vapor was then fed through rotameter ( 7 ) . The t o t a l pressure i n the r e a c t o r was c o n t r o l l e d with a bellows-sealed vacuum v a l v e (14) and monitored with a P i r a n i gauge (13). An r f power a t 13.56 MHz was supplied to the c o i l (15) by a c r y s t a l c o n t r o l l e d generator with a power a m p l i f i e r ( E l e c t r o n i c Navigation Ind., A-300) through an LC p a r a l l e l - r e s o n a n c e type impedance matching network. The power and SWR were monitored with a through-line wattmeter (Leader Test Instrument, LPN-885). Pressure r e s i s t a n c e of polymer f i l m s prepared under v a r i o u s r e a c t i o n c o n d i t i o n s were examined by using a batch-wise reverse osmosis apparatus. H y d r o s t a t i c pressure was a p p l i e d by using a mixture of toluene/methanol as a pressure medium. F o u r i e r Transformation IR spectra o f the polymer f i l m were recorded on a D i g i l a b FTS-20BD. An ATR spectrum was obtained by accumulating 100 scans of an interferogram with a r e s o l u t i o n of 8 cm""l f o r each specimen. Since the recorded spectrum included the spectrum of the Nuclepore f i l t e r , i t was necessary to subtract the l a t t e r from the former. Probe Experiments The e l e c t r o n temperature ( T ) and e l e c t r o n d e n s i t y ( n ) of the plasma were measured by a heated double probe method (DPM). A conventional DPM (3) could not be used i n a polymerizing plasma system s i n c e the surface of the probe was covered with d e p o s i t i n g polymer f i l m which prevented plasma e l e c t r o n s from flowing to the probe. To cope w i t h t h i s problem, a l l the probes were heated up to about 1000 Κ by sheathed tungsten heaters which were i n s e r t e d i n t o the c y l i n d r i c a l probes as shown i n F i g . 3. The probes (15 mm length, 500 ym diameter) were made of N i s i n c e i t does not emit e l e c t r o n s a t t h i s temperature (see Appendix). The heater was 7 Ω, and 6 V (DC) was a p p l i e d to maintain the temperature. The sheath (AI2O3, 200 ym dia.) was necessary to prevent e l e c t r o n s , emitted from the heater, from flowing i n t o the probe. Using t h i s device, a probe measurement could be performed on a polymerizing plasma. The w i r i n g diagram f o r the probe system i s shown i n F i g . 3. e


e

P L A S M A POLYMERIZATION Bz inlet RF

Pirani G.

Probe I

Λ Ί ι

Ar •

9&

'

^

\Mass sampling port

·. \l V

C7

80

Position Figure

2

25

10

1.

25

ground fitting

15

Schematic

of electrodeless

plasma

reactor

Rotary pump(6)«

Rotary pump(l2)

Diffusion pump Figure

2.

Schematic

of monomer

feed line, reactor, and evacuation text for details.

system.

Id Probe (Ni;

0

D.C.

'

π

r Λ W

W

Function Generator

D.C. Heater

~ A1 0 — 2

3

Sheath in plasma Figure

3.

Wiring

out of p l a s m a diagram for a heated double

probe


See

NiiNOMi

6.

AND Y A N A G i H A R A

Plasma

91

Diagnostics

A f t e r the probes were s e t i n the plasma (at P o s i t i o n 10 and/or 15 i n F i g . 1), an t r i a n g u l a r A.C. ( V , 20V p-p, 0.01 to 0.1 Hz) v o l t a g e was a p p l i e d between the probes. The c i r c u i t current (1^) as w e l l as the v o l t a g e between probes (V^) were then recorded. T and n were c a l c u l a t e d from the r e l a t i o n between and 1^ according to the equivalent r e s i s t a n c e method developed by Dote (4), i n which i t i s assumed that the e l e c t r o n energy d i s t r i b u t i o n i s Maxwellian. The value of n was c a l c u l a t e d according to M a l t e r and Webster ( 5 ) . A c t u a l l y , f(e) of a polymerizing plasma i s expected to be nonMaxwellians, and i t i s thus necessary to measure the exact form ί(ε) i n order t o estimate the true v a l u e s of T and n . Determin a t i o n s o f ί(ε) were c a r r i e d out through the f l o a t i n g asymmetrical t r i p l e probe method (ATPM) developed by Okuda and Yamamoto (6). The c i r c u i t f o r t h i s method i s shown i n F i g . 4. Although each of the three probes was again equipped with a sheathed heater, the dimensions of probes I and I I were d i f f e r e n t from those shown i n F i g . 3. Probe I (30 mm l e n g t h , 1 mm d i a . ) i n F i g . 4 was much l a r g e r than probe I I (1 mm t i p length, 5 ym t i p d i a . ) . The ATPM allowed us to measure ί(ε) from a low energy l e v e l without causing a s i g n i f i c a n t disturbance to the plasma. A t r i angular A.C. v o l t a g e (V^) was a p p l i e d between probes I and I I . A d j u s t i n g a s l i d e r e s i s t o r (SR) to keep the e l e c t r o n current i n t o the probe I I I zero, the values of 1^, and d^I^/dV^j were recorded. The second d e r i v a t i v e gives the exact form of f ( ε ) / V. The d e r i v a t i v e was obtained d i r e c t l y by taking the second harmonic of 1^. For t h i s purpose (0.01 Hz, 20V p-p) was modulated a t 400 Hz A.C. (lower than 4V), and the second harmonic of I d , which i s p r o p o r t i o n a l to the second d e r i v a t i v e , was detected with a l o c k - i n a m p l i f i e r (Princeton A p p l i e d Research Corp., Model 5204), as shown i n F i g . 4. d

e

e

e

e

e

Mass spectroscopy Ionic and n e u t r a l species i n the benzene plasma were analyzed d i r e c t l y w i t h a quadrupole mass spectrometer (ULVAC Corp, MSQ-500) equipped with an e l e c t r o s t a t i c sampling assembly ( 7 ) . This assembly serves to focus the i o n c o l l e c t e d from the plasma i n t o the quadrupole s t r u c t u r e of the mass spectrometer and i s needed to achieve a measureable i o n s i g n a l . The assembly was composed of an o r i f i c e , a s e t of e l e c t r o s t a t i c lenses and a d i f f e r e n t i a l pumping system as shown i n F i g . 5. The sampling o r i f i c e was a 30 ym diameter hole which was chemically machined i n a 30 ym t h i c k 18 Cr - 8 N i s t a i n l e s s s t e e l sheet. The lenses were made of 2.5 cm diameter 18-8 s t a i n l e s s s t e e l tubing and were supported by open T e f l o n bushings. The s i z e of the lenses were the same as those r e p o r t e d l y V a s i l e and Smolinsky (7). The e l e c t r i c p o t e n t i a l of the o r i f i c e was the same as the f l o a t i n g p o t e n t i a l , which was estimated a t about -10V from the plasma p o t e n t i a l . P o t e n t i a l s on the c y l i n d e r lenses were adjusted to obtained maximum s i g n a l i n t e n s i t y , i . e . about -340V on the f i r s t l e n s , -50V on the second, and -80V on the t h i r d . The e l e c t r i c l i n e s of f o r c e generated i n t h i s case are a l s o shown i n F i g . 5. I t i s seen that the e l e c t r o s t a t i c lens system forms an imaginary convex


PLASMA

POLYMERIZATION

to Lock-in Amp cos out

Generator 400 Hz

ure 4.

Wiring

cos 2 art

diagram

for a heated asymmetrical details.

triple probe.

See text for


6.

NIINOMI AND YANAGIHARA

Plasma

93

DiagtlOStlCS

lens f o r p o s i t i v e i o n s . Thus, p o s i t i v e ions a t the o r i f i c e were a c c e l e r a t e d toward, and focused on the entrance to the mass spectrometer. N e u t r a l species i n the plasma was analyzed by usual o p e r a t i o n of the mass spectrometer, i n which the s p e c i e s were i o n i z e d a t the i o n i z a t i o n chamber. The e l e c t r o n energy i n the i o n i z a t i o n chamber was sometimes lowered to 10V i n order to reduce the fragmentation. Mass a n a l y s i s was c a r r i e d out up to 500 AMU f o r a l l the s p e c i e s . RESULT AND DISCUSSION Reaction c o n d i t i o n and plasma c h a r a c t e r i s t i c s As i n the case of ethylene and acetylene (8), plasma polymer i z a t i o n o f benzene produced e i t h e r a powder or f i l m depending on r e a c t i o n c o n d i t i o n s . A t y p i c a l c o n d i t i o n i n which t h i n f i l m with the r e q u i r e d property was produced (the RO membrane c o n d i t i o n ) i s shown i n Table 1, coded as C o n d i t i o n B, while that f o r poor q u a l i t y f i l m formation i s designated A. Conditions f o r powder formation are designated C and Ε i n the t a b l e . G e n e r a l l y speaking, f i l m formation was observed a t high benzene flow r a t e s , and powder formation was observed a t low pressures and low benzene flow r a t e s , as i n the case of ethylene and acetylene (8). However, the RO membrane c o n d i t i o n s do not correspond to e i t h e r a unique p o i n t on the pressure (P) versus benzene flow r a t e (Q(Bz)) plane nor do they correspond to the c o n d i t i o n s i n which a l o t of polymer was produced. This means that the q u a l i t y o f the f i l m cannot be c o r r e l a t e d d i r e c t l y to the macroscopic r e a c t i o n c o n d i t i o n s . T , n and f ( e ) were measured under v a r i o u s r e a c t i o n c o n d i t i o n s . F i g s . 6, 7, 8 and 9 show f ( e ) f o r t y p i c a l plasma c o n d i t i o n s as w e l l as argon plasma. A l l the data i n t h i s s e c t i o n were c o l l e c t e d at P o s i t i o n 15 i n the r e a c t o r . In each f i g u r e , Maxwell and Druyvesteyn d i s t r i b u t i o n s are shown f o r comparison. They were obtained mathematically assuming that they have the same t o t a l number o f e l e c t r o n s (N) and t o t a l k i n e t i c energy (E) as those of the corresponding observed d i s t r i b u t i o n . Namely, Ν and Ε a r e expressed as f o l l o w s i n the case of Maxwell d i s t r i b u t i o n : e

e

oo

Ν =

exp

(-ε/kT )de

3/2 .1/2 /2)(kT ) (C7T e

0 3/2 = 0.886C(kT )

3/2

Ε =

exp

(-ε/kT )άε

5/2 C-T(5/2)(kT )

0 = 1.33C(kT )


PLASMA

Cylinder

POLYMERIZATION

lenses

Orifice

1st

2nd

3rd

-10V

-350V

-60 V

-90V

Figure 5. Schematic of ion sampling unit, composed of a sampling orifice, a set of electrostatic lenses, and a differential pumping system. The electric lines of force under the applied potential are drawn schematically together with an imagi nary convex lens.

Table 1.

Typical Reaction Conditions for Polymer Formation

Pressure

Flow r a t e

1 )

Product

Code

(Torr)

A

0.5

250

500

Film

Β

0.5

500

150

Film

C

1.0

250

1 50

Powder

Ε

0.2

1 50

1 50

Powder

benzene

argon

2 )

1) STP cc/min 2) Excellent pressure resistibility



Plasma

Diagnostics

E l e c t r o n energy

(eV)

Figure 6. Electron energy distribution function in a plasma generated under Condition A (see Table 1). Maxwell and Druyvesteyn distributions were calcu lated under the assumption that the total energy and the total electron density were the same as those observed.

Electron Figure

7.

Electron

energy

energy distribution function (see Table 1 )

(eV) of a plasma

under Condition


Β

96

PLASMA

POLYMERIZATION

Bz/Ar

Druyvesteyn

< 10

Figure 8. Electron energy distribution function of a pfosma under Condition Ε (see Table 1 )

Figure 9. Electron energy distribution function of an argon plasma under the condition of 0.5 torr, 40 W, 500 STP mL/min

5

10

Electron energy

5

15 (eV)

10

Electron energy

15 (eV)


6.


Plasma

97

Diagnostics

where C i s a constant; ε i s e l e c t r o n energy: T i s e l e c t r o n tem perature and Γ i s gamma f u n c t i o n . Ν and Ε of the observed ί(ε) were obtained by a graphic i n t e g r a t i o n method, and equated to those of the Maxwell d i s t r i b u t i o n to evaluate C and T i n the equations. By using these v a l u e s , f(c) of Maxwell d i s t r i b u t i o n was c a l c u l a t e d mathematically, where e

e

1/2

f(ε) = Ce

exp (-ε/kT ) .

In the case of a Druyvesteyn d i s t r i b u t i o n , Ν and Ε a r e expressed as: 00 Ν = / C^

1 / 2

2

-exp

0.613C(kT ) e

(-ε /Μ )άε =

3

/

(C/2)·Γ(3/4)·(kT )

3/4

e

4

oo

•ε

3 / 2

2

exp (-ε /Μ )

Plasma Polymerization - American Chemical Society

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