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Discharge studies in a triode ion plating system
Vacuum/volume 42/numbers 10/11/pages 661 to 663/1991 Printed in Great Britain
0042-207X/91 $3.00+.00 (D 1991 PergamonPressplc
Discharge studies in a triode ion plating system K R G u n a s e k h a r and S M o h a n , Instrumentation and Services Unit, Indian Institute of Science, Bangalore560012, India received in final form 14August 1990
A triode ion plating system with a hot cathode has been described. The performance of the system is studied, by studying the discharge behaviour from the bias voltage and bias current point of view, at the substrate, for different anode currents, filament voltages and pressures. The observed substrate bias current for different operating parameters is not found to be normal. The behaviour is explained on the basis of ionisation at the respective electrodes. The studies have revealed the importance of inter-electrode spacing in the enhancement of ionisation, in ion plating systems, at lower pressures.
Introduction Ion plating ~is known to be a very good PVD technique to deposit thick films in the range of a few tens of microns, most useful for tribological and metallurgical applications. This is the result of various elementary processes involved during deposition -~. The most important and predominant elementary process is ion bombardment of the surface be[ore and during condensation of the film. This results in preparation of an atomically clean surface, creation of defects causing surface activation and increase in the surface temperature of the substrate. Added to the above, sputtering, scattering and recondensation of the substrate atoms in the initial stages of condensation leads to the formation of a graded interface. The presence of this interface has several advantages in tribology ~. This is also known to be one of the reasons for excellent adhesion of the film to the substrate. Ion bombardment during film deposition also results in disruption of columnar morphology, generally found in films with thickness greater than one micron 4. Continuous or intermittent ion bombardment brings about changes in morphology, structure, stoichiometry and physical properties of the growing films56. The physical properties of the ion plated films can be tailored by controlling the operating parameters like pressure, substrate bias current, substrate bias voltage, etc. In diode discharge, it is not possible to control any one of the parameters independent of the others. This disadvantage of inter-dependence can be overcome in the triode discharge 7 "'. In addition to this, in the triode discharge the ratio of ions to vapour atoms can be controlled independent of substrate bias, current and pressure. The behaviour of the system and the extent to which the operating parameters can be controlled depend mostly on the electrode geometry, size of the chamber and the spacing between the electrodes. In the present work, a triode ion plating system, which enables independent control of the deposition parameters, has been fabricated and characterised for its performance from the discharge point of view. The merits and demerits are discussed.
Experimental The triode ion plating system consists of a cylindrical stainless steel chamber, 300 mm dia and 200 mm height of 2.0 mm wall
thickness, with the top and bottom faces open [Figure l(a)]. The chamber is mounted on the base plate at the centre of which the pumping port is provided. An aluminium plate is used to close the top face of the chamber. A water cooled high tension electrode (substrate holder) is fixed in the centre of this aluminium plate. On the circumferential surface of the chamber, there are four ports welded diametrically opposite to one another, with 90' separation. These ports are used for mounting the filament,
--
--300 (-) H"[
/ T
CATHODE (substrat ~. . . . . . . . . holder)
50
j T O P PLATE ~j
,,oo
--
BELLJAR FILAMENT / PORT
===
200 ANODE BASE PLATE
(a)
TO PUMP ~ E V A P O R A T I O N SOURCE
Substrat¢ . . ~ . ~ . ~ Anode
Thermiomc fi[oment
(b)
~
AC supply 20 V, 50 A
-I-
Figure I. (a) Schematic of the triode ion plating chamber showing the location of the three electrodes; (b) electrical circuit diagram used in the discharge studies.
661
K R Gunasekhar and S Mohan: Discharge studies
anode, viewing window and hot cathode ionisation gauge head. The filament and anode are diametrically opposite to each other, as shown in the figure. The filament is made of two strands of tungsten wire, 0.4 mm dia each, wound in the shape of a helix. One end of the filament is earthed. The gauge head port and viewing port are perpendicular to the filament and anode ports. The pressure in the system is monitored with a hot cathode ionisation gauge ( V A R I A N millitorr gauge head with an 890A R control unit), capable of measuring pressu,e between 10 and 0.001 Pa. The pumping system consists of a 450 I s ~ turbomolecular pump backed by a 500 1 m ' rotary pump. To study the perlbrmance of the system, the chamber is evacuated to 10 4 Pa. Argon gas is admitted into the chamber through a fine control needle valve and the pressure is maintained at 2.0 Pa. The substrate (75 mm dia O F H C copper disc) is biased to 1.5 kV. The filament is given 8.0 V ac. Then, the anode voltage is increased steadily until 3.0 A of anode current is observed [Figure l(b)]. The system is left in this condition for nearly 1 h for degassing the chamber as well as sputter cleaning of the substrate. After 1 h, the gas flow is stopped, the potentials at the electrodes are switched offand the chamber is pumped down to its ultimate vacuum. This procedure is repeated until reproducible discharge conditions are observed in the chamber. After obtaining stable conditions, argon gas is regulated into the chamber, and one of the predetermined pressures is selected. The filament voltage is set at the required value. The exact emission current of the filament is not measured but the filament voltage itself is taken as a measure of the emission current. Then, by adjusting anode bias, the anode current is set. At zero amps of anode current the system works in diode mode. After setting the anode current, the cathode (substrate) is given a negative bias. The substrate bias is varied from 0.0 to 3.0 kV, in steps of 1.0 kV and the resulting bias currents are noted. The filament voltage is maintained at 7.5, 8.0, 9.0 and 10.0 V and the anode current is maintained at 0.0, 1.0, 2.0 and 3.0 A. The pressure is maintained at 7.0, 5.0, 3.0, 2.0, 1.5, 1.0, 0.7 and 0.5 Pa. Precautions are taken to eliminate the errors in the pressure measurement by precalibrating the ionisation gauge against the McLeod gauge as well as by taking into account the gas constant of argon. Also it is made sure that there is no error in the pressure measurement due to the interference of the triode discharge with the ionisation gauge.
Anode
120
(O)
current
(Amps)/
(b)
Anode current
3.0
E
(Amps)
100
FZ W
rr
80
U
< t.LI I-'-
<
60 40,
t--t/3 m
,
~ U3
20 0
1
0
I
I
I
I 2 0 1 2 5UBSTRATE BIAS VOLTAGE (kV)
Figure 2. Effect of substrate bias voltage on substrate bias current for different anode currents at 0.7 Pa pressure and for (a) 7.5 and (b) 8.0 V of tilament voltage.
at constant pressure. It is also possible to maintain constant ion current at different bias voltages. Figures 3(a)-(c) show the effect of filament voltage on the bias current. It is surprising to note that the bias current, instead of increasing with increasing filament voltage (emission current), tends to decrease significantly. In general with an increasing number of electrons in the discharge, the number of ionising collisions will increase. As a result, the current at the substrate is expected to increase. But when the anode current remains constant, the current at the substrate should at least stay constant for a given substrate bias voltage. For a given anode current and bias voltage, at constant pressure, by increasing the filament voltage it is possible to reduce the number of ions bombarding the substrate with a given ion energy. Figures 4(a) and (b) show the variation of bias current with pressure for different bias voltages. The trend is not steady, first the bias current decreases with decreasing pressure, in the
Results and discussion Figures 2(a) and (b) show the typical variation of bias current with bias voltage For different anode currents. As expected, the bias current increases with increasing bias voltage for a given anode current. With an increase in anode current thcre is a shift to a higher bias current for a given bias voltage. From I to 3 kV bias, the variation is linear. There is a minimum substrate bias current even when the substrate is not given any bias voltage. In this condition the substrate is floating in the discharge. Hence, it acquires a negative potential with respect to the discharge. The observed substrate bias current at zero substrate bias is due to this floating potential. N o substrate bias current is observed in the diode mode, i.e. in the absence of anode current at this pressure and below. This shows the ability of the triode discharge to operate in the pressure range where the diode discharge cannot be operated. It is possible to control the bias current for a given bias voltage independent of other parameters by varying the anode current 662
"z 120 uJ u
./
- (b)
Ca)
FiLament vo I t a g (a.c. v o l t s ) 7.5
100
80
60
Filarnent voltage (a.c volts) 7.5
o
g.O .0
un
9.0 20 0.0 f 1
I 2
r 3
I
SUBSTRATE
I
i
I
0 1 VOLTAGE ( kV)
2
3
I
BIAS
Figure 3. Effect of substrate bias voltage on substrate bias current for different (ilament voltages at 0.7 Pa pressure for (a) 1.0, (b) 2.0 and (c) 3.0 A of anode current.
K R Gtmasekhar and S Mohan." Discharge
f
Biasvottag¢ 3.0 X ~(kV) x
~E 40 (a)
studies
~
rx
2"0 ~ x " ~ x - - . _ _ x . ~ X
~- 20 z D k) v~
1 0.0
0
I
~
0
j/
~
I
x.~x..~x
(b)
80
.
Bias vottage(kV) x 3.0
~X~-x'~'x/
<
Q: 60
2.0
C13
~
40
o.I
~
~
'L 0
°'---"~"~-~.~_~.~o
20 0
&
i
I
I
l
I
I
Ill
1.o
I
0.0
I
1
I
I
I
I I
1o.o
PRESSURE(Pa) Figure 4. Effect of pressure substrate bias current at 7.5 V of filament voltage for different substrate bias voltages and for (a) 1.0 and (b) 2.0 A of anode current.
expected manner, reaches a minimum and then increases approaching near saturation limits. Below 0.5 Pa the discharge could not be sustained. It is interesting to note that for a given bias voltage, filament voltage and anode current there is more than one pressure where it is possible to get the same substrate bias current. The properties of the films deposited for the given bias voltage and bias current at these pressures cannot be the same. At the lower pressures the mean free path is higher and the number of ions striking the substrate with higher average energy will be higher. This kind of behaviour has not been observed by others 9'~° in similar studies, with pressure and filament voltage. The explanation for the observed, abnormal, behaviour of bias currents, as a function of filament voltage and pressure, can be offered based on the movement of negative glow with pressure near the filament and with respect to the location of the hot and cold cathodes. In the present system two independent discharges, cold and hot cathodes, are operating simultaneously. The architecture of the discharge between the hot cathode and anode is different to that of cold cathode discharge/~. The thermionic electrons cause ionisation by collisions when the applied voltage to anode exceeds the ionisation potential of the gas. As in the case of self sustained gas, the slow moving ions tend to accumulate in front of the filament and dark space is produced. This dark space consists of an electron sheath immediately in front of the filament followed by a positive ion sheath adjacent to the plasma. This sheath acts as though it were a virtual anode, very close to the filament, so that the electric field there becomes comparable with what would be obtained in high vacuum if much higher voltages were applied across the entire enclosure. The reason for the reduction of substrate bias current with increasing filament voltage, is due to the presence of this dark space.
When the filament voltage is increased, the electron emission also increases resulting in increased space charge in front of the filament. In order to compensate this charge build-up to maintain the neutrality of plasma, positive ions in the vicinity move immediately towards the filament. This reduces the number of ions available to the substrate, at constant anode current, thereby reducing the net positive ion current flow to the substrate (Figure 3). With decreasing pressure, the length of the cathode dark space increases. The discharge around the cathode, including negative glow (of the filament in particular), spreads to a larger volume. It is in the negative glow region that most of the ionising collisions take place (both in hot and cold cathode discharges). Considering the location of the substrate and filament [Figure l(a)], when the length of the cathode dark space increases, the negative glow shifts away from the filament, i.e. closer to the substrate. When this happens, the ions in the negative glow can come under the influence of the electric field at the substrate. Hence, the number of ions in the negative glow will drift towards the substrate. This has been confirmed by gradually varying the pressure and monitoring the movement of the cathode dark space and negative glow. It is observed that the increase in bias current at the critical pressure coincided with the approach of the edge of negative glow nearer to the negative glow of the substrate. Conclusions
These studies on the discharge characteristics in a triode ion plating chamber revealed that the substrate bias current is directly related to the bias voltage and anode current, whereas the filament voltage has an opposite influence. The discharge pressure in the system has an interesting effect on the substrate bias current. The substrate bias current keeps decreasing, depending on the anode current, and then starts increasing with decreasing pressure. This has been explained on the basis of the movement of negative glow with pressure and with respect to the location of the two cathodes. These studies have given deeper insight into the mechanism of discharge in a triode system leading to the possibility of a better control on the energy and the ratio of the ions to neutrals generated in the discharge. This is essential in controlling the properties of thick adherent coatings at high rates of deposition by ion plating. References
~D M Mattox, J Vac Sci Technol, 10, 47 (1973). 2S Schiller, H Heisig and K Goedicke, hm-Plating--A New Promising Vacuum Coating Process, 4th Int Electron Beam Processing Seminar, Long Island, New York, April (1976). 3D G Teer and F B Salem, Thin Solid Films, 45, 583 (1977). 4D M Mattox and G J Kominiak, J Vac Sei Technol, 9, 528 (1972). 5R D Bland, G J Kominiak and D M Mattox, J Vae Sci Technol, II, 671 (1974). C'D G Teer and B L Delcea, Thin Solid Films, 54, 295 (1978). 7T C Tisone and J B Bindel, J Vac Sci Technol, II, 519 (1974). XT C Tisone and P D Cruzan, J Vae Sci T~,ehnol, 12, 1058 (1975). '~A Mathews and D G Teer, Thin Solid Films, 80, 41 (1981). H~K Salmenoja, J M Olarius and A S Korhen, Thin Solid Films, 155, 143 (1987). J~L I Maissel and R Glang, Handbook ~[' Thin Film Technology, pp 4-7. McGraw-Hill, New York (1970).
663
0042-207X/91 $3.00+.00 (D 1991 PergamonPressplc
Discharge studies in a triode ion plating system K R G u n a s e k h a r and S M o h a n , Instrumentation and Services Unit, Indian Institute of Science, Bangalore560012, India received in final form 14August 1990
A triode ion plating system with a hot cathode has been described. The performance of the system is studied, by studying the discharge behaviour from the bias voltage and bias current point of view, at the substrate, for different anode currents, filament voltages and pressures. The observed substrate bias current for different operating parameters is not found to be normal. The behaviour is explained on the basis of ionisation at the respective electrodes. The studies have revealed the importance of inter-electrode spacing in the enhancement of ionisation, in ion plating systems, at lower pressures.
Introduction Ion plating ~is known to be a very good PVD technique to deposit thick films in the range of a few tens of microns, most useful for tribological and metallurgical applications. This is the result of various elementary processes involved during deposition -~. The most important and predominant elementary process is ion bombardment of the surface be[ore and during condensation of the film. This results in preparation of an atomically clean surface, creation of defects causing surface activation and increase in the surface temperature of the substrate. Added to the above, sputtering, scattering and recondensation of the substrate atoms in the initial stages of condensation leads to the formation of a graded interface. The presence of this interface has several advantages in tribology ~. This is also known to be one of the reasons for excellent adhesion of the film to the substrate. Ion bombardment during film deposition also results in disruption of columnar morphology, generally found in films with thickness greater than one micron 4. Continuous or intermittent ion bombardment brings about changes in morphology, structure, stoichiometry and physical properties of the growing films56. The physical properties of the ion plated films can be tailored by controlling the operating parameters like pressure, substrate bias current, substrate bias voltage, etc. In diode discharge, it is not possible to control any one of the parameters independent of the others. This disadvantage of inter-dependence can be overcome in the triode discharge 7 "'. In addition to this, in the triode discharge the ratio of ions to vapour atoms can be controlled independent of substrate bias, current and pressure. The behaviour of the system and the extent to which the operating parameters can be controlled depend mostly on the electrode geometry, size of the chamber and the spacing between the electrodes. In the present work, a triode ion plating system, which enables independent control of the deposition parameters, has been fabricated and characterised for its performance from the discharge point of view. The merits and demerits are discussed.
Experimental The triode ion plating system consists of a cylindrical stainless steel chamber, 300 mm dia and 200 mm height of 2.0 mm wall
thickness, with the top and bottom faces open [Figure l(a)]. The chamber is mounted on the base plate at the centre of which the pumping port is provided. An aluminium plate is used to close the top face of the chamber. A water cooled high tension electrode (substrate holder) is fixed in the centre of this aluminium plate. On the circumferential surface of the chamber, there are four ports welded diametrically opposite to one another, with 90' separation. These ports are used for mounting the filament,
--
--300 (-) H"[
/ T
CATHODE (substrat ~. . . . . . . . . holder)
50
j T O P PLATE ~j
,,oo
--
BELLJAR FILAMENT / PORT
===
200 ANODE BASE PLATE
(a)
TO PUMP ~ E V A P O R A T I O N SOURCE
Substrat¢ . . ~ . ~ . ~ Anode
Thermiomc fi[oment
(b)
~
AC supply 20 V, 50 A
-I-
Figure I. (a) Schematic of the triode ion plating chamber showing the location of the three electrodes; (b) electrical circuit diagram used in the discharge studies.
661
K R Gunasekhar and S Mohan: Discharge studies
anode, viewing window and hot cathode ionisation gauge head. The filament and anode are diametrically opposite to each other, as shown in the figure. The filament is made of two strands of tungsten wire, 0.4 mm dia each, wound in the shape of a helix. One end of the filament is earthed. The gauge head port and viewing port are perpendicular to the filament and anode ports. The pressure in the system is monitored with a hot cathode ionisation gauge ( V A R I A N millitorr gauge head with an 890A R control unit), capable of measuring pressu,e between 10 and 0.001 Pa. The pumping system consists of a 450 I s ~ turbomolecular pump backed by a 500 1 m ' rotary pump. To study the perlbrmance of the system, the chamber is evacuated to 10 4 Pa. Argon gas is admitted into the chamber through a fine control needle valve and the pressure is maintained at 2.0 Pa. The substrate (75 mm dia O F H C copper disc) is biased to 1.5 kV. The filament is given 8.0 V ac. Then, the anode voltage is increased steadily until 3.0 A of anode current is observed [Figure l(b)]. The system is left in this condition for nearly 1 h for degassing the chamber as well as sputter cleaning of the substrate. After 1 h, the gas flow is stopped, the potentials at the electrodes are switched offand the chamber is pumped down to its ultimate vacuum. This procedure is repeated until reproducible discharge conditions are observed in the chamber. After obtaining stable conditions, argon gas is regulated into the chamber, and one of the predetermined pressures is selected. The filament voltage is set at the required value. The exact emission current of the filament is not measured but the filament voltage itself is taken as a measure of the emission current. Then, by adjusting anode bias, the anode current is set. At zero amps of anode current the system works in diode mode. After setting the anode current, the cathode (substrate) is given a negative bias. The substrate bias is varied from 0.0 to 3.0 kV, in steps of 1.0 kV and the resulting bias currents are noted. The filament voltage is maintained at 7.5, 8.0, 9.0 and 10.0 V and the anode current is maintained at 0.0, 1.0, 2.0 and 3.0 A. The pressure is maintained at 7.0, 5.0, 3.0, 2.0, 1.5, 1.0, 0.7 and 0.5 Pa. Precautions are taken to eliminate the errors in the pressure measurement by precalibrating the ionisation gauge against the McLeod gauge as well as by taking into account the gas constant of argon. Also it is made sure that there is no error in the pressure measurement due to the interference of the triode discharge with the ionisation gauge.
Anode
120
(O)
current
(Amps)/
(b)
Anode current
3.0
E
(Amps)
100
FZ W
rr
80
U
< t.LI I-'-
<
60 40,
t--t/3 m
,
~ U3
20 0
1
0
I
I
I
I 2 0 1 2 5UBSTRATE BIAS VOLTAGE (kV)
Figure 2. Effect of substrate bias voltage on substrate bias current for different anode currents at 0.7 Pa pressure and for (a) 7.5 and (b) 8.0 V of tilament voltage.
at constant pressure. It is also possible to maintain constant ion current at different bias voltages. Figures 3(a)-(c) show the effect of filament voltage on the bias current. It is surprising to note that the bias current, instead of increasing with increasing filament voltage (emission current), tends to decrease significantly. In general with an increasing number of electrons in the discharge, the number of ionising collisions will increase. As a result, the current at the substrate is expected to increase. But when the anode current remains constant, the current at the substrate should at least stay constant for a given substrate bias voltage. For a given anode current and bias voltage, at constant pressure, by increasing the filament voltage it is possible to reduce the number of ions bombarding the substrate with a given ion energy. Figures 4(a) and (b) show the variation of bias current with pressure for different bias voltages. The trend is not steady, first the bias current decreases with decreasing pressure, in the
Results and discussion Figures 2(a) and (b) show the typical variation of bias current with bias voltage For different anode currents. As expected, the bias current increases with increasing bias voltage for a given anode current. With an increase in anode current thcre is a shift to a higher bias current for a given bias voltage. From I to 3 kV bias, the variation is linear. There is a minimum substrate bias current even when the substrate is not given any bias voltage. In this condition the substrate is floating in the discharge. Hence, it acquires a negative potential with respect to the discharge. The observed substrate bias current at zero substrate bias is due to this floating potential. N o substrate bias current is observed in the diode mode, i.e. in the absence of anode current at this pressure and below. This shows the ability of the triode discharge to operate in the pressure range where the diode discharge cannot be operated. It is possible to control the bias current for a given bias voltage independent of other parameters by varying the anode current 662
"z 120 uJ u
./
- (b)
Ca)
FiLament vo I t a g (a.c. v o l t s ) 7.5
100
80
60
Filarnent voltage (a.c volts) 7.5
o
g.O .0
un
9.0 20 0.0 f 1
I 2
r 3
I
SUBSTRATE
I
i
I
0 1 VOLTAGE ( kV)
2
3
I
BIAS
Figure 3. Effect of substrate bias voltage on substrate bias current for different (ilament voltages at 0.7 Pa pressure for (a) 1.0, (b) 2.0 and (c) 3.0 A of anode current.
K R Gtmasekhar and S Mohan." Discharge
f
Biasvottag¢ 3.0 X ~(kV) x
~E 40 (a)
studies
~
rx
2"0 ~ x " ~ x - - . _ _ x . ~ X
~- 20 z D k) v~
1 0.0
0
I
~
0
j/
~
I
x.~x..~x
(b)
80
.
Bias vottage(kV) x 3.0
~X~-x'~'x/
<
Q: 60
2.0
C13
~
40
o.I
~
~
'L 0
°'---"~"~-~.~_~.~o
20 0
&
i
I
I
l
I
I
Ill
1.o
I
0.0
I
1
I
I
I
I I
1o.o
PRESSURE(Pa) Figure 4. Effect of pressure substrate bias current at 7.5 V of filament voltage for different substrate bias voltages and for (a) 1.0 and (b) 2.0 A of anode current.
expected manner, reaches a minimum and then increases approaching near saturation limits. Below 0.5 Pa the discharge could not be sustained. It is interesting to note that for a given bias voltage, filament voltage and anode current there is more than one pressure where it is possible to get the same substrate bias current. The properties of the films deposited for the given bias voltage and bias current at these pressures cannot be the same. At the lower pressures the mean free path is higher and the number of ions striking the substrate with higher average energy will be higher. This kind of behaviour has not been observed by others 9'~° in similar studies, with pressure and filament voltage. The explanation for the observed, abnormal, behaviour of bias currents, as a function of filament voltage and pressure, can be offered based on the movement of negative glow with pressure near the filament and with respect to the location of the hot and cold cathodes. In the present system two independent discharges, cold and hot cathodes, are operating simultaneously. The architecture of the discharge between the hot cathode and anode is different to that of cold cathode discharge/~. The thermionic electrons cause ionisation by collisions when the applied voltage to anode exceeds the ionisation potential of the gas. As in the case of self sustained gas, the slow moving ions tend to accumulate in front of the filament and dark space is produced. This dark space consists of an electron sheath immediately in front of the filament followed by a positive ion sheath adjacent to the plasma. This sheath acts as though it were a virtual anode, very close to the filament, so that the electric field there becomes comparable with what would be obtained in high vacuum if much higher voltages were applied across the entire enclosure. The reason for the reduction of substrate bias current with increasing filament voltage, is due to the presence of this dark space.
When the filament voltage is increased, the electron emission also increases resulting in increased space charge in front of the filament. In order to compensate this charge build-up to maintain the neutrality of plasma, positive ions in the vicinity move immediately towards the filament. This reduces the number of ions available to the substrate, at constant anode current, thereby reducing the net positive ion current flow to the substrate (Figure 3). With decreasing pressure, the length of the cathode dark space increases. The discharge around the cathode, including negative glow (of the filament in particular), spreads to a larger volume. It is in the negative glow region that most of the ionising collisions take place (both in hot and cold cathode discharges). Considering the location of the substrate and filament [Figure l(a)], when the length of the cathode dark space increases, the negative glow shifts away from the filament, i.e. closer to the substrate. When this happens, the ions in the negative glow can come under the influence of the electric field at the substrate. Hence, the number of ions in the negative glow will drift towards the substrate. This has been confirmed by gradually varying the pressure and monitoring the movement of the cathode dark space and negative glow. It is observed that the increase in bias current at the critical pressure coincided with the approach of the edge of negative glow nearer to the negative glow of the substrate. Conclusions
These studies on the discharge characteristics in a triode ion plating chamber revealed that the substrate bias current is directly related to the bias voltage and anode current, whereas the filament voltage has an opposite influence. The discharge pressure in the system has an interesting effect on the substrate bias current. The substrate bias current keeps decreasing, depending on the anode current, and then starts increasing with decreasing pressure. This has been explained on the basis of the movement of negative glow with pressure and with respect to the location of the two cathodes. These studies have given deeper insight into the mechanism of discharge in a triode system leading to the possibility of a better control on the energy and the ratio of the ions to neutrals generated in the discharge. This is essential in controlling the properties of thick adherent coatings at high rates of deposition by ion plating. References
~D M Mattox, J Vac Sci Technol, 10, 47 (1973). 2S Schiller, H Heisig and K Goedicke, hm-Plating--A New Promising Vacuum Coating Process, 4th Int Electron Beam Processing Seminar, Long Island, New York, April (1976). 3D G Teer and F B Salem, Thin Solid Films, 45, 583 (1977). 4D M Mattox and G J Kominiak, J Vac Sei Technol, 9, 528 (1972). 5R D Bland, G J Kominiak and D M Mattox, J Vae Sci Technol, II, 671 (1974). C'D G Teer and B L Delcea, Thin Solid Films, 54, 295 (1978). 7T C Tisone and J B Bindel, J Vac Sci Technol, II, 519 (1974). XT C Tisone and P D Cruzan, J Vae Sci T~,ehnol, 12, 1058 (1975). '~A Mathews and D G Teer, Thin Solid Films, 80, 41 (1981). H~K Salmenoja, J M Olarius and A S Korhen, Thin Solid Films, 155, 143 (1987). J~L I Maissel and R Glang, Handbook ~[' Thin Film Technology, pp 4-7. McGraw-Hill, New York (1970).
663