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Insulation and passivation of three-dimensional substrates by plasma-CVD thin films using silicon-organic compounds
380
Materials Science and Engineering, A139 ( 1991 ) 380-384
Insulation and passivation of three-dimensional substrates by plasma-CVD thin films using silicon-organic compounds D. Peters, J. Miiller and T. Sperling TU Hamburg-Harburg, Eiflendorfer Str. 42, W-2100 Hamburg 90 (F.R.G.)
Abstract A reactor design developed for coating three-dimensional substrates is presented. Cathodic deposition was investigated to obtain highly cross-linked insulating films. The properties of films deposited from hexamethyldisilazane and tetraethylorthosilicate sources are discussed and compared.
I. Introduction
The properties of thin films deposited by plasma polymerization from the silicon-organic compounds hexamethyldisilazane (HMDS-N) and tetraethylorthosilicate (TEOS) can be varied across a wide range. Depending on the deposition conditions polymer-like soft or glass-like hard films of high density are formed [1, 2]. The polymer films can, for example, be used as gas diffusion membranes [3], or hard films for isolation and passivation applications. The composition of such a - S i - C - N - O - H films can be adjusted between carbon rich and SiO2 by adding different amounts of oxygen to the process atmosphere. The former are of high hardness, whereas the latter are useful as insulators with high breakdown voltages and excellent passivation properties. We report on a reactor design developed for coating three-dimensional substrates. In order to attain highly cross-linked insulating films cathodic deposition was investigated. The films can be deposited at high growth rates and low substrate temperatures. Their good step coverage guarantees the insulation even of nonplanar surfaces.
strate geometries. The substrate holder can be changed from run to run. The substrate is r.f. powered at 13.56 MHz up to 1 kW. In the case of metallic substrates the substrate forms the cathode by itself. Otherwise a substrate holder is HMDS-N
TEOS,
i powered
(a) I
~
l
&
vacuum
insula~on collar
feed
powersupply
~ ~AAAAAA~AA~A "
0921-5093/91/$3.50
J
(b)
2. Plasma reactor
Figure 1 shows a schematic drawing of the plasma reactor, which differs from the usual parallel-plate chambers by the strong asymmetry of the electrode arrangement. Such a configuration is very flexible with regard to different sub-
reservoir
l grounded
~X~ C0~ °-
1. (a) Schematic diagram of the plasma chemical vapour deposition reactor. A small heated plate electrode is mounted in the reactor. (b) Arrangement with a mounted rod assembly for coating cylindrical substrates. (c) Schematic diagram of the feed-through in more detail. Fig.
© Elsevier Sequoia/Printed in The Netherlands
381
used that is adapted to the substrate geometry as closely as possible. The large areas of the grounded parallel plates and chamber walls form the counter electrode (anode). The details of the advantages of this feature are as follows. A small electrode retains the high power density of the r.f. glow discharge that is necessary to dissociate complex monomer gases. The very high ratio of the anodic and cathodic areas results in a high sheath voltage at the substrate [4]. Consequently the growth is accompanied by a highly energetic ion bombardment which increases the degree of cross-linkage [5] and thus the density of the film. In addition, the radical density at the substrate is high because the power density of the plasma is concentrated close to the cathode region. The probability that activated specimens, in particular the highly active radicals generated in this region, reach the substrate surface is high. This arrangement allows the outer coating of three-dimensional substrates such as cylinders or other rotationally symmetric or hemispherical substrates. In the latter case the electrode is constructed as a rod assembly (see Fig. l(b)). Other tools can also be mounted. Two features provide a concentration of the plasma close to the substrate resulting in a uniform coating: a small cathode area in relation to the grounded areas and a large distance between the powered and any grounded surface. For the same reason parasitic depositions on reactor walls or contamination of the pump system are completely avoided. To supply the electrode with 13.56 MHz r.f. power a coaxial transmission line, consisting of an inner conductor with a grounded outer flexible metal tube, is fed through the side wall of the chamber to its centre (see Fig. l(c)). The electrical feed-through of the butt end is formed as a socket, to which different electrodes can be connected. To monitor the temperature a thermocouple is installed in the socket. In order to avoid a glow discharge between the electrode and the grounded surfaces of the transmission line, the feed-through is insulated by a ceramic or Teflon sleeve. When a small plate electrode is used, it is insulated to ground by a quartz plate. Such an arrangement ensures that the plasma is concentrated close to the substrate with optimum power density. The vapour pressure of the liquid HMDS-N with respect to the process pressure is high enough at room temperature (5 Pa) to adjust the
flow with a needle valve. Because of the lower vapour pressure the TEOS source has to be heated. To avoid condensation the short feeding pipe has to be heated too. For a finer control of the flow fluid, flowmeters for both monomers are installed. The pump system consists of a roots pump and a foreline pump. The amount of polymerized remains reaching the pumps is low and causes no problems. The following deposition conditions were used for the films investigated: controlled process pressure, 1-10 Pa; r.f. power, 2 - 1 0 0 0 W (1500 W is possible for a short time); substrate temperature, up to 250 °C; monomer flow rate, 1-4 g h-l; additional gases, 02, N 2, Ar. The power supplied corresponds to an average energy density of 1 W cm 3, i.e. 5 W cm 2 related to the substrate area.
3. Film properties For the characterization of the properties of the deposition and the deposited films, a small heatable plate electrode 12 cm in diameter was used. Figure 2 shows the mass deposition rates and the density of the deposited films as functions of the power supplied of the HMDS-N coating with the gas composition and substrate temperature as constant parameters. Results are shown for deposition with and without the addition of oxygen on substrates heated to 200°C and on an unheated substrate in a n H M D S - N - O 2 plasma. For low power and low temperature the density of the films is always low and rather soft. Their infrared spectra (Fig. 3) reveal a high content of hydrocarbons. At high power densities (2-5 W cm -2) and with heated substrates amorphous films are deposited which as their infrared spectra show contain no methyl groups (Fig. 3). As the power is increased further the density of the films decreases again as atomic hydrogen ions are incorporated [2] which saturate dangling bonds. In addition polymerization in the gas phase occurs and degrades the quality of the growth. Films deposited without any oxygen contain a large amount of carbon. Increasing the oxygen partial pressure causes oxidation of both silicon and carbon resulting in dense SiO2 and volatile carbon oxides. Secondary ion mass spectrometry (SIMS) analysis also shows this increasing SiO2 and decreasing carbon content. Such SiO2-1ike
382 deposition rate
0.80
[ ~g s-Icm-2 ]
0.60
0.70
~
O.SO
0.40
J
0.30 0.20
~
/
!H llH
JSiH
I
0.10 .OS .I power density [
.5
I
W/cm
S
CHe.s
10
SiICH~I, /
2 ]
4-.0 rate
deposition
/
[ nm : 1 ]
3.0
/
SiSi-0-Si -O-C I 1/x
[ella
HMDS-N
-1]
<+
f
3000
30 seem
'
'
'
~
0 2, 1 0 W ,
f
'
"
'
f
"
"
"
t'
"
"
"
800
1600
200°C
(a) O. .0S power
' .I density
, , .S [ W/era
,, 1 2 ]
...... S
i 10
2.S density
[ g
cm -3
] ,o
-
1.0 .OS power
.1 density
.S [
1
S
10
W/cm 2 ]
Fig. 2. Characteristics of H M D S - N cathodic deposition: =, H M D S - N + 4 standard cm 3 min -I A r (no O2), substrate heated to 200 °C; A , H M D S - N + 30 standard cm 3 rain- ~ Oe, substrate heated to 200 °C; @, H M D S - N + 30 standard cm 3 m i n - ~ O2, substrate unheated.
1/X
. . . . . , . . . . [ e n 1 - 1 ] <- 3000
HMDS-N
+ 30 seem
t ' 2000
0 2, S 0 0 W ,
'
'
( ' 160(I
"
"
I " 120o
'
"
I " 800
'
"
200°C
(b) films of high cross-linkage are useful, e.g. as insulation and passivation layers. In contrast to HMDS-N the T E S S molecule contains a n SiS 4 core which is easy to dissociate from the ethyl groups. Therefore oxide-like films of high density are deposited on heated substrates even at low power, as is shown in Fig. 4. Adding small amounts of oxygen increases the density by oxidizing the hydrocarbons. At r.f. powers beyond 5 W cm -2 gas phase reaction occurs which reduces the quality of the growing films. The only way to get soft films by T E S S decomposition is to reduce the substrate temperature (Fig. 4). The lack of thermal energy results in a loose deposition of H - C groups, as infrared spectra confirm (Fig. 5). HMDS-N and T E S S films deposited at high additional oxygen flow exhibit electrical resistances of 10 Tf2 and 20 Tf2 respectively (at a thickness of about 1.5 Bm) which were near the
Fig. 3. Infrared spectra of H M D S - N cathodic deposition: (a) H M D S - N + 3 0 standard cm 3 m i n - i Oe ' 10 W, 200 °C; (b) H M D S - N + 30 standard cm 3 rain J 0 2 , 5 0 0 W, 200 °C.
limit of our measurement equipment, because for example humidity can falsify the correct value of these high resistivities. These films can be considered as insulators. Films deposited without added oxygen exhibit a weak intrinsic conductivity. The electrical breakdown fields of the oxidelike HMDS-N films are 1.5x 1 0 6 W cm -1 a n d 8 × 106 V cm-1 respectively for the T E S S films. Voltages up to 1.2 kV were used. Besides a high electrical resistivity of the layers a low density of pinholes is required. In determining the pinhole density a salt water solution with reduced surface tension was applied. Using a test voltage up to 100 V a resistance of more than 25 Mr2 on an area of 25 mm 2 on oxide-like
383 4# deposition
rate
x/ //~ /11-.
O.S0 0.40
•
[ ~tg s -1 cm -2 ]
//
\
ii
0.30
0.10 O.
t
W' /
0.20 •
'
.-H Sill
.jk/ -Lff
cH~
I
.OS .1 .5 1 ~ower d e n s i t y [ W / c m z ]
' S
:
NH
CH3
3.0 2.5 deposition
rate
/"
2.0 [ nm s-~ ]
0.S 0.
"
•
//
/ 1\
,
i
/
1.S 1.0
e"
/
,/
II -
'k\ ~\
,,-~
l\ ",,,
/"
l/x [cm -t]
<- sooo
20oo
t~oo
12oo
TEOS, 20 W, u n h e a t e d
',\
k/
(a) .OS .1 .S 1 ,ower d e n s i t y [ W / c r n 2 ]
S
10
2.4 ~'~--~
density
[ g cm-3 ]
2.0
---~_~ .... i__~ ~ /
',
e
4
'\
//
1
/
1.6
CH3
i
~*~" .0S .1 .S 1 power density [ W/cm z ]
S
I
10
Fig. 4. Characteristics of T E O S cathodic deposition: m TEOS without additions, substrate heated to 200 °C; A , T E O S + 10 standard cm 3 min-~ O2, substrate heated to 200 °C; , , TEOS with no additions, substrate unheated.
Si-O-Si
!si-c i
,
I
i
Si-O-Si /Si-O-C 1/x [cm - l ] ~- 3000 TEOS, 20OW, 200°C
zooo
16oo
lzoo
800
(b) HMDS-N films and of 250 Mr2 on 25 mm 2 on TEOS films was obtained for layers 1.5 y m thick. To evaluate the passivation properties and thin film process compatibility the layers were exposed to several typical chemicals. The a - S i - C - N - O - H films investigated were absolutely resistant against the following chemicals at room temperature: hydrochloric acid, phosphoric acid, sulphuric acid, ammonium fluoride, aqua regia, gold etchants, aluminium etchants, and acetone. No wet process for etching the layers was found. In a plasma etching process using SF6 and 02 low etch rates (lower than 0.1 k(m min- 1) were obtained. The oxide-like films were also tested at temperatures up to 1100 °C with no change in their basic properties. A problem of the predominantly covalentbonded films is their low adhesive strength on metals, especially on steel. Mechanical stress and
Fig. 5. Infrared spectra of TEOS cathodic deposition: (a) TEOS, 20 W. unheated; (b) TEOS, 200 W. 200 °C.
thermal fluctuations may peel off parts of the film. The adhesion on aluminium is much better than on steel, because the thin native aluminium oxide film provides good adhesion of the coating. Therefore a thin aluminium intermediate layer (about 0.1 ~m), e.g. sputtered on steel, improved the adhesion considerably. 4. Conclusion Plasma decomposition of HMDS and TEOS produces coatings with good insulating and passivating properties at high temperatures. The cathodic deposition, a special feature of this reactor, provides an adaptation of the plasma to the substrate geometry by using suitable electrodes. The electrode assembly allows a large variety of different substrate shapes.
384
References K. W. Gerstenberg, in E. Broszeit, G. K. Wolf, W. D. Munz, H. Oeehsner and K.-T. Rie (eds.), Proc. 1st Int. Conf. on
Plasma Surface Engineering, 1988, DGM, Oberursel, 1989,
Garmisch-Partenkirchen,
2 K. W. Gerstenberg and W. Beyer, J. AppL Phys., 62 (5) (1987) 1782. 3 J. Weichert and J. Miiller, Prog. Colloid Polym. Sci., in the press. 4 A. Sherman, Thin Solid Films, 113(1984) 135. 5 R. Szeto and D. W. Hess, J. Appl. Phys., 52 (2) ( 1981 ) 903.
Materials Science and Engineering, A139 ( 1991 ) 380-384
Insulation and passivation of three-dimensional substrates by plasma-CVD thin films using silicon-organic compounds D. Peters, J. Miiller and T. Sperling TU Hamburg-Harburg, Eiflendorfer Str. 42, W-2100 Hamburg 90 (F.R.G.)
Abstract A reactor design developed for coating three-dimensional substrates is presented. Cathodic deposition was investigated to obtain highly cross-linked insulating films. The properties of films deposited from hexamethyldisilazane and tetraethylorthosilicate sources are discussed and compared.
I. Introduction
The properties of thin films deposited by plasma polymerization from the silicon-organic compounds hexamethyldisilazane (HMDS-N) and tetraethylorthosilicate (TEOS) can be varied across a wide range. Depending on the deposition conditions polymer-like soft or glass-like hard films of high density are formed [1, 2]. The polymer films can, for example, be used as gas diffusion membranes [3], or hard films for isolation and passivation applications. The composition of such a - S i - C - N - O - H films can be adjusted between carbon rich and SiO2 by adding different amounts of oxygen to the process atmosphere. The former are of high hardness, whereas the latter are useful as insulators with high breakdown voltages and excellent passivation properties. We report on a reactor design developed for coating three-dimensional substrates. In order to attain highly cross-linked insulating films cathodic deposition was investigated. The films can be deposited at high growth rates and low substrate temperatures. Their good step coverage guarantees the insulation even of nonplanar surfaces.
strate geometries. The substrate holder can be changed from run to run. The substrate is r.f. powered at 13.56 MHz up to 1 kW. In the case of metallic substrates the substrate forms the cathode by itself. Otherwise a substrate holder is HMDS-N
TEOS,
i powered
(a) I
~
l
&
vacuum
insula~on collar
feed
powersupply
~ ~AAAAAA~AA~A "
0921-5093/91/$3.50
J
(b)
2. Plasma reactor
Figure 1 shows a schematic drawing of the plasma reactor, which differs from the usual parallel-plate chambers by the strong asymmetry of the electrode arrangement. Such a configuration is very flexible with regard to different sub-
reservoir
l grounded
~X~ C0~ °-
1. (a) Schematic diagram of the plasma chemical vapour deposition reactor. A small heated plate electrode is mounted in the reactor. (b) Arrangement with a mounted rod assembly for coating cylindrical substrates. (c) Schematic diagram of the feed-through in more detail. Fig.
© Elsevier Sequoia/Printed in The Netherlands
381
used that is adapted to the substrate geometry as closely as possible. The large areas of the grounded parallel plates and chamber walls form the counter electrode (anode). The details of the advantages of this feature are as follows. A small electrode retains the high power density of the r.f. glow discharge that is necessary to dissociate complex monomer gases. The very high ratio of the anodic and cathodic areas results in a high sheath voltage at the substrate [4]. Consequently the growth is accompanied by a highly energetic ion bombardment which increases the degree of cross-linkage [5] and thus the density of the film. In addition, the radical density at the substrate is high because the power density of the plasma is concentrated close to the cathode region. The probability that activated specimens, in particular the highly active radicals generated in this region, reach the substrate surface is high. This arrangement allows the outer coating of three-dimensional substrates such as cylinders or other rotationally symmetric or hemispherical substrates. In the latter case the electrode is constructed as a rod assembly (see Fig. l(b)). Other tools can also be mounted. Two features provide a concentration of the plasma close to the substrate resulting in a uniform coating: a small cathode area in relation to the grounded areas and a large distance between the powered and any grounded surface. For the same reason parasitic depositions on reactor walls or contamination of the pump system are completely avoided. To supply the electrode with 13.56 MHz r.f. power a coaxial transmission line, consisting of an inner conductor with a grounded outer flexible metal tube, is fed through the side wall of the chamber to its centre (see Fig. l(c)). The electrical feed-through of the butt end is formed as a socket, to which different electrodes can be connected. To monitor the temperature a thermocouple is installed in the socket. In order to avoid a glow discharge between the electrode and the grounded surfaces of the transmission line, the feed-through is insulated by a ceramic or Teflon sleeve. When a small plate electrode is used, it is insulated to ground by a quartz plate. Such an arrangement ensures that the plasma is concentrated close to the substrate with optimum power density. The vapour pressure of the liquid HMDS-N with respect to the process pressure is high enough at room temperature (5 Pa) to adjust the
flow with a needle valve. Because of the lower vapour pressure the TEOS source has to be heated. To avoid condensation the short feeding pipe has to be heated too. For a finer control of the flow fluid, flowmeters for both monomers are installed. The pump system consists of a roots pump and a foreline pump. The amount of polymerized remains reaching the pumps is low and causes no problems. The following deposition conditions were used for the films investigated: controlled process pressure, 1-10 Pa; r.f. power, 2 - 1 0 0 0 W (1500 W is possible for a short time); substrate temperature, up to 250 °C; monomer flow rate, 1-4 g h-l; additional gases, 02, N 2, Ar. The power supplied corresponds to an average energy density of 1 W cm 3, i.e. 5 W cm 2 related to the substrate area.
3. Film properties For the characterization of the properties of the deposition and the deposited films, a small heatable plate electrode 12 cm in diameter was used. Figure 2 shows the mass deposition rates and the density of the deposited films as functions of the power supplied of the HMDS-N coating with the gas composition and substrate temperature as constant parameters. Results are shown for deposition with and without the addition of oxygen on substrates heated to 200°C and on an unheated substrate in a n H M D S - N - O 2 plasma. For low power and low temperature the density of the films is always low and rather soft. Their infrared spectra (Fig. 3) reveal a high content of hydrocarbons. At high power densities (2-5 W cm -2) and with heated substrates amorphous films are deposited which as their infrared spectra show contain no methyl groups (Fig. 3). As the power is increased further the density of the films decreases again as atomic hydrogen ions are incorporated [2] which saturate dangling bonds. In addition polymerization in the gas phase occurs and degrades the quality of the growth. Films deposited without any oxygen contain a large amount of carbon. Increasing the oxygen partial pressure causes oxidation of both silicon and carbon resulting in dense SiO2 and volatile carbon oxides. Secondary ion mass spectrometry (SIMS) analysis also shows this increasing SiO2 and decreasing carbon content. Such SiO2-1ike
382 deposition rate
0.80
[ ~g s-Icm-2 ]
0.60
0.70
~
O.SO
0.40
J
0.30 0.20
~
/
!H llH
JSiH
I
0.10 .OS .I power density [
.5
I
W/cm
S
CHe.s
10
SiICH~I, /
2 ]
4-.0 rate
deposition
/
[ nm : 1 ]
3.0
/
SiSi-0-Si -O-C I 1/x
[ella
HMDS-N
-1]
<+
f
3000
30 seem
'
'
'
~
0 2, 1 0 W ,
f
'
"
'
f
"
"
"
t'
"
"
"
800
1600
200°C
(a) O. .0S power
' .I density
, , .S [ W/era
,, 1 2 ]
...... S
i 10
2.S density
[ g
cm -3
] ,o
-
1.0 .OS power
.1 density
.S [
1
S
10
W/cm 2 ]
Fig. 2. Characteristics of H M D S - N cathodic deposition: =, H M D S - N + 4 standard cm 3 min -I A r (no O2), substrate heated to 200 °C; A , H M D S - N + 30 standard cm 3 rain- ~ Oe, substrate heated to 200 °C; @, H M D S - N + 30 standard cm 3 m i n - ~ O2, substrate unheated.
1/X
. . . . . , . . . . [ e n 1 - 1 ] <- 3000
HMDS-N
+ 30 seem
t ' 2000
0 2, S 0 0 W ,
'
'
( ' 160(I
"
"
I " 120o
'
"
I " 800
'
"
200°C
(b) films of high cross-linkage are useful, e.g. as insulation and passivation layers. In contrast to HMDS-N the T E S S molecule contains a n SiS 4 core which is easy to dissociate from the ethyl groups. Therefore oxide-like films of high density are deposited on heated substrates even at low power, as is shown in Fig. 4. Adding small amounts of oxygen increases the density by oxidizing the hydrocarbons. At r.f. powers beyond 5 W cm -2 gas phase reaction occurs which reduces the quality of the growing films. The only way to get soft films by T E S S decomposition is to reduce the substrate temperature (Fig. 4). The lack of thermal energy results in a loose deposition of H - C groups, as infrared spectra confirm (Fig. 5). HMDS-N and T E S S films deposited at high additional oxygen flow exhibit electrical resistances of 10 Tf2 and 20 Tf2 respectively (at a thickness of about 1.5 Bm) which were near the
Fig. 3. Infrared spectra of H M D S - N cathodic deposition: (a) H M D S - N + 3 0 standard cm 3 m i n - i Oe ' 10 W, 200 °C; (b) H M D S - N + 30 standard cm 3 rain J 0 2 , 5 0 0 W, 200 °C.
limit of our measurement equipment, because for example humidity can falsify the correct value of these high resistivities. These films can be considered as insulators. Films deposited without added oxygen exhibit a weak intrinsic conductivity. The electrical breakdown fields of the oxidelike HMDS-N films are 1.5x 1 0 6 W cm -1 a n d 8 × 106 V cm-1 respectively for the T E S S films. Voltages up to 1.2 kV were used. Besides a high electrical resistivity of the layers a low density of pinholes is required. In determining the pinhole density a salt water solution with reduced surface tension was applied. Using a test voltage up to 100 V a resistance of more than 25 Mr2 on an area of 25 mm 2 on oxide-like
383 4# deposition
rate
x/ //~ /11-.
O.S0 0.40
•
[ ~tg s -1 cm -2 ]
//
\
ii
0.30
0.10 O.
t
W' /
0.20 •
'
.-H Sill
.jk/ -Lff
cH~
I
.OS .1 .5 1 ~ower d e n s i t y [ W / c m z ]
' S
:
NH
CH3
3.0 2.5 deposition
rate
/"
2.0 [ nm s-~ ]
0.S 0.
"
•
//
/ 1\
,
i
/
1.S 1.0
e"
/
,/
II -
'k\ ~\
,,-~
l\ ",,,
/"
l/x [cm -t]
<- sooo
20oo
t~oo
12oo
TEOS, 20 W, u n h e a t e d
',\
k/
(a) .OS .1 .S 1 ,ower d e n s i t y [ W / c r n 2 ]
S
10
2.4 ~'~--~
density
[ g cm-3 ]
2.0
---~_~ .... i__~ ~ /
',
e
4
'\
//
1
/
1.6
CH3
i
~*~" .0S .1 .S 1 power density [ W/cm z ]
S
I
10
Fig. 4. Characteristics of T E O S cathodic deposition: m TEOS without additions, substrate heated to 200 °C; A , T E O S + 10 standard cm 3 min-~ O2, substrate heated to 200 °C; , , TEOS with no additions, substrate unheated.
Si-O-Si
!si-c i
,
I
i
Si-O-Si /Si-O-C 1/x [cm - l ] ~- 3000 TEOS, 20OW, 200°C
zooo
16oo
lzoo
800
(b) HMDS-N films and of 250 Mr2 on 25 mm 2 on TEOS films was obtained for layers 1.5 y m thick. To evaluate the passivation properties and thin film process compatibility the layers were exposed to several typical chemicals. The a - S i - C - N - O - H films investigated were absolutely resistant against the following chemicals at room temperature: hydrochloric acid, phosphoric acid, sulphuric acid, ammonium fluoride, aqua regia, gold etchants, aluminium etchants, and acetone. No wet process for etching the layers was found. In a plasma etching process using SF6 and 02 low etch rates (lower than 0.1 k(m min- 1) were obtained. The oxide-like films were also tested at temperatures up to 1100 °C with no change in their basic properties. A problem of the predominantly covalentbonded films is their low adhesive strength on metals, especially on steel. Mechanical stress and
Fig. 5. Infrared spectra of TEOS cathodic deposition: (a) TEOS, 20 W. unheated; (b) TEOS, 200 W. 200 °C.
thermal fluctuations may peel off parts of the film. The adhesion on aluminium is much better than on steel, because the thin native aluminium oxide film provides good adhesion of the coating. Therefore a thin aluminium intermediate layer (about 0.1 ~m), e.g. sputtered on steel, improved the adhesion considerably. 4. Conclusion Plasma decomposition of HMDS and TEOS produces coatings with good insulating and passivating properties at high temperatures. The cathodic deposition, a special feature of this reactor, provides an adaptation of the plasma to the substrate geometry by using suitable electrodes. The electrode assembly allows a large variety of different substrate shapes.
384
References K. W. Gerstenberg, in E. Broszeit, G. K. Wolf, W. D. Munz, H. Oeehsner and K.-T. Rie (eds.), Proc. 1st Int. Conf. on
Plasma Surface Engineering, 1988, DGM, Oberursel, 1989,
Garmisch-Partenkirchen,
2 K. W. Gerstenberg and W. Beyer, J. AppL Phys., 62 (5) (1987) 1782. 3 J. Weichert and J. Miiller, Prog. Colloid Polym. Sci., in the press. 4 A. Sherman, Thin Solid Films, 113(1984) 135. 5 R. Szeto and D. W. Hess, J. Appl. Phys., 52 (2) ( 1981 ) 903.