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Glow discharge polymer deposition in a triode system
Thin Solid Films, 23 (1974) $ 4 5 - $ 4 8 © Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
$45
Short Communication
Glow discharge polymer deposition in a triode system K. ANDO and M. AOZASA
Faculty of Engineering, Osaka City University, Osaka (Japan) (Received June 12, 1974; accepted June 21, 1974)
Thin polymer films are becoming of increasing importance to the electronics industry and a number of techniques have been used for their production I. A m o n g these techniques, glow discharge polymerization has attracted the greatest interest.At present two methods of polymer deposition by glow discharge are used -- a diode system and an electrodeless system 2 -4. In the diode system, films are produced either on the electrode directly (the "direct method") or on a substrate placed in the discharge space (the "indirect method") 3. The electrodeless system has the advantage of freedom from contamination by the electrodes but the deposition rate is relativelylow compared with the direct method in the diode system. Connel and Gregor s have used a d.c. magnetic field in this system to improve the deposition characteristicsand have obtained a deposition rate as high as 1200 A/min. In the case of the diode system there are some differences in deposition characteristics between the direct and indirect methods: in the former the deposition rate is high b u t erosion easily occurs by violent ion bombardment; although the deposition rate is small in the latter method, comparatively less erosion occurs. In this communication we propose a m e t h o d o f glow discharge polymer deposition in a triode system which combines the merits of b o t h the direct and indirect methods in the diode system. The apparatus is shown in Fig. 1. The reaction tube, 9.4 cm in diameter and 33 cm long, contains three electrodes. The aluminium anode and cathode, 6.5 cm in diameter, are 25.4 cm apart and the third electrode, 2 cm in diameter, is placed halfway between them. A primary glow discharge is maintained between the anode and cathode b y a d.c. p o w e r supply with a stabilizing resistor. A r.f. voltage is applied through a capacitor to the third electrode, on which the substrate is mounted, by a r.f. generator connected between the anode and the third electrode. Polymer films can be produced under widely variable conditions because, in addition to the choice of the primary discharge condition, the frequency and nmgnitude of the r.f. voltage applied to the third electrode can also be varied, Consequently, wide control of the operating conditions is possible. Moreover, because a
$46
Exhaustionpump
f~--~
Monomer source
l Needle~ (<~ Cathode
valve ~ m a r y
discharge
I I1~An°de III / I rldllfor prima~y discha~g~ III I] I~1~ Substrate ~ IJ--I
Third fill I electrodel~ I ~ l -
~Glass
cover Stabilizing
Fig. 1. Apparatus for glow discharge polymer deposition in the triode system.
r.f. voltage is used, the substrate does not have to be o f conducting material and this method can be used for the production of thin polymer films on various kinds of substrate m a d e of conducting, semiconducting or insulating material. A n example of the polymerization of styrene m o n o m e r on a glass substrate is shown in Fig. 2. The styrene was distilled under reduced pressure and degassed before use for polymerization. The film thickness was measured by means of multiple b e a m interferometry. W h e n the r.f. voltage was not used, a growth rate of about 200 A/min was obtained. In this case our method is no different from the indirect method in a diode system, since the third electrode is rendered electrically floating because of the coupling capacitor. W h e n a r.f.voltage of 100 kHz and 150 V was applied to the third electrode, the growth rate became about 500 A/rain, which is more than twice that w h e n no r.f. voltage was applied. W h e n the r.f. voltage is zero, the third electrode assures a wall potential such that the electron and ion currents flowing are of the same magnitude. W h e n the r.f.voltage is applied, the d.c. potential of the electrode becomes more negative than the wall potential s . The actual depression of the d.c. potential of the third electrode as a result of the application of an increasing r.f.voltage is shown in Fig. 3 (the d.c. potentials of the electrode were measured with respect to the anode). The result is interpreted as follows. U p o n increasing the r.f.voltage the number of electrons which flow in the third electrode in a positive half cycle increases, and in order to keep the m e a n current flowing into the electrode zero during one
847 4000
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3000
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-200
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-I00
g c
~
1000
-300
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U
0 0
2
4
6
"o
-400
Time (rain) -500
0
I
I
100
200
,
I
300
r.f. V o l t a g e (V)
Fig. 2. T i m e dependence o f polymer deposition: vapour pressure o f the styrene monomer 0.1 mmHg; primary discharge current 15 mA; frequency of the r.f. voltage applied to the third electrode 100 kHz.
Fig. 3. Depression o f the d.c. potential of the third electrode as a result of the application o f increasing r.f. voltages: vapour pressure of the styrene monomer 0.1 mmHg; primary discharge current 15 mA; frequency of the r.f. voltage applied to the third electrode 100 kHz.
cycle more ion current is needed and in consequence the d.c. potential becomes more negative s . Therefore the number of ions impinging on the substrate per unit time increases as the r.f. voltage increases. From this point of view the increase of the growth rate when a r.f. voltage is applied to the third electrode is considered to be the result of an increase in the rate of impinging ions. The mechanism of film formation in the triode system is t h o u g h t to be the same as that suggested by Williams and Hayes 2 for the case of the direct method, i.e. the impinging ions which form the active species on the substrate play an important role in the polymerization. As mentioned above, the new m e t h o d enables us to control the ion flow which causes polymerization at the substrate, and the ratio 2
4=
number of adsorbed molecules number of ions striking the surface
which affects the properties of the deposited polymer film can easily be controlled by the r.f. voltage without changing the primary discharge conditions. Furthermore, the energy o f the ions bombarding the surface of the substrate, which is though to have a strong effect on the properties of the film, can also be controlled continuously by the r . i voltage over a wide range, from the low values obtained in the indirect m e t h o d to the high values of the direct method. Such a wide range of energy control cannot be attained using other methods. These features result from the separation of the electrical circuits for film formation and plasma generation. The present investigation clearly indicates t h a t this new m e t h o d is
$48
useful for the production of m a n y kinds of polymer films. Further detailed studies on formation characteristics and film properties are in progress. 1 2 3 4 5 6
L. V. Gregor, I B M J . Res. Develop., March (1968) 140. T. Williams and M. W. Hayes, Nature (London), 209 (1966) 769. P. J. Ozawa, IEEE Trans., PMP-5 (1969) 112. H. Yasuda and C. E. Lamaze, J. Appl. PolymerSci., 15 (1971) 2277. R. A. Connel and L. V. Gregor, J. Electrochem. Soc., 112 (1965) 1198. H. S. Butler and G. S. Kino, Phys. Fluids, 6 (1963) 1346.
$45
Short Communication
Glow discharge polymer deposition in a triode system K. ANDO and M. AOZASA
Faculty of Engineering, Osaka City University, Osaka (Japan) (Received June 12, 1974; accepted June 21, 1974)
Thin polymer films are becoming of increasing importance to the electronics industry and a number of techniques have been used for their production I. A m o n g these techniques, glow discharge polymerization has attracted the greatest interest.At present two methods of polymer deposition by glow discharge are used -- a diode system and an electrodeless system 2 -4. In the diode system, films are produced either on the electrode directly (the "direct method") or on a substrate placed in the discharge space (the "indirect method") 3. The electrodeless system has the advantage of freedom from contamination by the electrodes but the deposition rate is relativelylow compared with the direct method in the diode system. Connel and Gregor s have used a d.c. magnetic field in this system to improve the deposition characteristicsand have obtained a deposition rate as high as 1200 A/min. In the case of the diode system there are some differences in deposition characteristics between the direct and indirect methods: in the former the deposition rate is high b u t erosion easily occurs by violent ion bombardment; although the deposition rate is small in the latter method, comparatively less erosion occurs. In this communication we propose a m e t h o d o f glow discharge polymer deposition in a triode system which combines the merits of b o t h the direct and indirect methods in the diode system. The apparatus is shown in Fig. 1. The reaction tube, 9.4 cm in diameter and 33 cm long, contains three electrodes. The aluminium anode and cathode, 6.5 cm in diameter, are 25.4 cm apart and the third electrode, 2 cm in diameter, is placed halfway between them. A primary glow discharge is maintained between the anode and cathode b y a d.c. p o w e r supply with a stabilizing resistor. A r.f. voltage is applied through a capacitor to the third electrode, on which the substrate is mounted, by a r.f. generator connected between the anode and the third electrode. Polymer films can be produced under widely variable conditions because, in addition to the choice of the primary discharge condition, the frequency and nmgnitude of the r.f. voltage applied to the third electrode can also be varied, Consequently, wide control of the operating conditions is possible. Moreover, because a
$46
Exhaustionpump
f~--~
Monomer source
l Needle~ (<~ Cathode
valve ~ m a r y
discharge
I I1~An°de III / I rldllfor prima~y discha~g~ III I] I~1~ Substrate ~ IJ--I
Third fill I electrodel~ I ~ l -
~Glass
cover Stabilizing
Fig. 1. Apparatus for glow discharge polymer deposition in the triode system.
r.f. voltage is used, the substrate does not have to be o f conducting material and this method can be used for the production of thin polymer films on various kinds of substrate m a d e of conducting, semiconducting or insulating material. A n example of the polymerization of styrene m o n o m e r on a glass substrate is shown in Fig. 2. The styrene was distilled under reduced pressure and degassed before use for polymerization. The film thickness was measured by means of multiple b e a m interferometry. W h e n the r.f. voltage was not used, a growth rate of about 200 A/min was obtained. In this case our method is no different from the indirect method in a diode system, since the third electrode is rendered electrically floating because of the coupling capacitor. W h e n a r.f.voltage of 100 kHz and 150 V was applied to the third electrode, the growth rate became about 500 A/rain, which is more than twice that w h e n no r.f. voltage was applied. W h e n the r.f. voltage is zero, the third electrode assures a wall potential such that the electron and ion currents flowing are of the same magnitude. W h e n the r.f.voltage is applied, the d.c. potential of the electrode becomes more negative than the wall potential s . The actual depression of the d.c. potential of the third electrode as a result of the application of an increasing r.f.voltage is shown in Fig. 3 (the d.c. potentials of the electrode were measured with respect to the anode). The result is interpreted as follows. U p o n increasing the r.f.voltage the number of electrons which flow in the third electrode in a positive half cycle increases, and in order to keep the m e a n current flowing into the electrode zero during one
847 4000
/
3000
g 2000
-200
@
u
LE E
-I00
g c
~
1000
-300
h
U
0 0
2
4
6
"o
-400
Time (rain) -500
0
I
I
100
200
,
I
300
r.f. V o l t a g e (V)
Fig. 2. T i m e dependence o f polymer deposition: vapour pressure o f the styrene monomer 0.1 mmHg; primary discharge current 15 mA; frequency of the r.f. voltage applied to the third electrode 100 kHz.
Fig. 3. Depression o f the d.c. potential of the third electrode as a result of the application o f increasing r.f. voltages: vapour pressure of the styrene monomer 0.1 mmHg; primary discharge current 15 mA; frequency of the r.f. voltage applied to the third electrode 100 kHz.
cycle more ion current is needed and in consequence the d.c. potential becomes more negative s . Therefore the number of ions impinging on the substrate per unit time increases as the r.f. voltage increases. From this point of view the increase of the growth rate when a r.f. voltage is applied to the third electrode is considered to be the result of an increase in the rate of impinging ions. The mechanism of film formation in the triode system is t h o u g h t to be the same as that suggested by Williams and Hayes 2 for the case of the direct method, i.e. the impinging ions which form the active species on the substrate play an important role in the polymerization. As mentioned above, the new m e t h o d enables us to control the ion flow which causes polymerization at the substrate, and the ratio 2
4=
number of adsorbed molecules number of ions striking the surface
which affects the properties of the deposited polymer film can easily be controlled by the r.f. voltage without changing the primary discharge conditions. Furthermore, the energy o f the ions bombarding the surface of the substrate, which is though to have a strong effect on the properties of the film, can also be controlled continuously by the r . i voltage over a wide range, from the low values obtained in the indirect m e t h o d to the high values of the direct method. Such a wide range of energy control cannot be attained using other methods. These features result from the separation of the electrical circuits for film formation and plasma generation. The present investigation clearly indicates t h a t this new m e t h o d is
$48
useful for the production of m a n y kinds of polymer films. Further detailed studies on formation characteristics and film properties are in progress. 1 2 3 4 5 6
L. V. Gregor, I B M J . Res. Develop., March (1968) 140. T. Williams and M. W. Hayes, Nature (London), 209 (1966) 769. P. J. Ozawa, IEEE Trans., PMP-5 (1969) 112. H. Yasuda and C. E. Lamaze, J. Appl. PolymerSci., 15 (1971) 2277. R. A. Connel and L. V. Gregor, J. Electrochem. Soc., 112 (1965) 1198. H. S. Butler and G. S. Kino, Phys. Fluids, 6 (1963) 1346.