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Characteristics of a thermionically assisted triode ion-plating system
Thin SolidFilms,
80 (1981)41-48
METALLURGICAL
AND PROTECTIVE
CHARACTERISTICS PLATING SYSTEM* A. MATTHEWS
41
COATINGS
OF A THERMIONICALLY
ASSISTED TRIODE ION-
AND D. G. TFZR
Department of Aeronautical (Gt. Britain)
and Mechanical
Engineering,
University
of Salford,
Salford
M5 4WT
There is increasing evidence to show that the properties of ion-plated coatings can be improved by increasing the specimen ion current density and by operating at low deposition pressures. This paper examines a technique of discharge support which facilitates such improvements. The characteristics of the discharge are presented and explanations are given for the effects observed. The recommended arrangement for maximized specimen ion currents utilizes a positive electrode and a thermionic source which is electrically floating so that the filament takes up a negative potential with respect to the earthed chamber. Direct biasing of the filament can also be used, obviating the need for the positive electrode.
1. INTRODUCTION
In ion plating and related techniques, it is now well accepted that the use of a gaseous discharge improves coating properties in several ways. Firstly, adhesion is improved owing to the benefits of sputtering on cleaning and interface formation’. Secondly, coating structures are improved for both metal.and ceramic fihn~~-~. Thirdly, improved reaction can be achieved for reactive deposition5,6. Further improvements are obtained if the discharge is intensified, i.e. if ionization is increased, particularly when the discharge is maintained at lower pressures than are normally possible using a conventional d.c. diode arrangement. There are several means by which the d.c. diode discharge can be enhanced and supported. The main methods used involve increasing the electron path within the discharge or providing an additional source of ionization. Sometimes the vapour generation source can assist in this respect. For example, the hollow cathode electron beam (EB) gun’,* and the magnetron sputter source’ both increase ionization in ion plating. R.f. electrodes within, or even external to, the chamber have been used to increase ionization and to extend the operating pressure rangelo. Possibly the simplest means of discharge support is to incorporate a positive electrode to give a triode configuration. This produces a useful increase in ionization *Paper presented at the 3rd International Amsterdam, The Netherlands, June 30-July 0040~6090/81/oooo-oooo/$02.50
Conference 2, 198 1.
on Ion and
Plasma
fQ Elsevier Sequoia/Printed
Assisted
Techniques,
in The Netherlands
42
A. MATTHEWS, D. G. TEER
and can lower the permissible operating pressure. The arrangement has similarities to triode sputtering1 1 though in that case it is the target, and not the specimen, which is biased negatively. Positive probe discharge support is particularly influential under evaporation conditions, when electrons emitted in the source region can be accelerated to increase ionization. A similar effect is achieved by biasing the melt negatively”. EB evaporators produce electrons with a range of energies and intensities. Hot cathode EB guns thermionically emit a beam of electrons which are accelerated to energies of several kiioelectronvolts. The melt emits the thermionic electrons usually associated with an incandescent source and also provides secondary electrons owing to the incident primary beam on the melt surface. Great control over ionization in triode ion plating can be achieved if a separate thermionic emitter is incorporated independent of the vapour source. This permits the ionization to be increased during the sputter-cleaning or pre-etch stage and allows low chamber pressures throughout the cycle. The first reported use of a thermionically assisted triode type of arrangement for ion plating was by Baumr3. He claimed four advantages for the technique: (1) greater control over the gas discharge; (2) improved stability due to the use of lower bias voltages; (3) lower pressure operation; (4) reduced substrate heating. The last point results from the reduced bias required to reach a desired specimen current and from the consequent reduction in discharge power input. We feel that the most important benefit of the thermionically assisted triode system is the capability of increasing the specimen current density even at low bias levels. This has been shown to be particularly desirable for reactive ion plating6. However, there does appear to be a lack of data available on the discharge effects occurring within such systems which in effect combine hot and cold cathode discharges. For this reason a study was undertaken to identify the characteristics of a typical thermionically assisted triode layout and to offer explanations for the effects observed. 2.
EXPERIMENTAL DETAILS
2.1. Equipment A schematic diagram of the circuit used for system evaluation is shown in Fig 1. The circuit incorporates a filament of tungsten wire 0.5 mm in diameter and 12 cm long. The hot cathode circuit can be operated in two basic states which are obtained by selecting position 1 or 2 on the common point earthing switch. Position 2 allows the filament and the positive probe circuit to float independently of the rest of the system, whereas position 1 imposes an earth datum at the common point. Extensive investigations were carried out into the influence of system parameters such as probe voltage, specimen bias, gun powder and chamber pressure on the specimen and probe currents. Some important conclusions from these studies are outlined below. 2.2. Results Figures 2 and 3 show the variations in current monitored to a specimen 100 cm2 in surface area under conditions in which there is no power input to the EB gun and
THERMIONICALLY ASSISTED TRIODE ION-PLATING SYSTEM
43
O-5 ICY dc
O-32
Cormnon point Fig. 1. Schematicdiagram
of the system used for discharge system trials.
180 160
120 100 Specimen Current I%!!
80 60 40 20
2 3 4 5 Specimen Bias c_kv7 Fig. 2. Specimen current against s_@en bias for a supported triode discharge (filament earthed) without a vapour source at pressures of 10 mTorr (A) and 0.5 mTorr (0): V,, = 9 V, V, = 300 V. 1
A. MATTHEWS,
1
2
4 Bias
v*
3. Specimen against specimen together) without vapour source 300v.
G.
5
@zq
for a pressures of
triode discharge mTorr (A) 0.3 mTorr
and filament V,, = V,
therefore no evaporation. Figure 2 shows the earthed (switch position 1) results and Fig. 3 shows the currents for the floating system (switch position 2). Several observations can be made about these graphs. It is evident that the filament tends to reduce the effect on the specimen current of pressure variations which are normally observed with the d.c. diode. In both cases the discharge is maintained at low pressures which do not normally support the cold cathode d.c. diode discharge. Furthermore, with the switch in position 1 a specimen ion current exists even in the absence of a specimen bias, as the specimen, although earthed, is a cathode to the positive probe. This may have important consequences regarding the ion plating of earthed samples. We noted that under discharge conditions with the switch in position 2 the floating 300 V supply tends to take up ground potential on the positive side. Hence the probe is no longer biased with respect to the chamber: the unbiased specimen is not then a cathode and it receives no ion current. When the EB gun filament is heated the EB power may not initially be adequate to vaporize the melt, though it can still influence the discharge. At higher gun powers there are several possibly conflicting mechanisms such as the passage of an intense EB through the region above the source, secondary and thermionic emissions from the melt and the formation of a metal vapour cloud; All these factors can influence ionization. Furthermore, the electron yield at the sample may be affected by the formation of a deposit. Thus the results obtained with the EB gun in operation were found to differ considerably from those in Figs. 2 and 3 which were obtained with the gun off. Firstly, consider Fig. 4 which shows the results for a triode system with no additional electron source other than the EB gun. It can be seen that the specimen current reduces as the gun power is increased. Such an effect has been reported previously for a diode system as evaporation commenced’4.
‘MBRMIONICALLY ASSISTED TRIODB ION-PLATING SYSTEM
Specimen Current
E4.l
45
60 0 45’ 30 * 15 ’
-d
e-c.---
__L----
I 8
6
4
2
Chamber Pressure
I’0 b
1’2
Torrl
Fig. 4. Specimen current against chamber pressure for a triode discharge with the specimen at -4 kV showing the effect of gun powder with no voltage across the support filament: V, = 300 V. Full curves represent the system with the filament earthed (gun power: q,O kW; ‘17,l kW; 0,2 kW; & 3 kW; 0,4 kW) and the broken curve is for the floating system (gun power, 0, 1,2,3 or 4 kW)
120 Specimen Current tzt!I
1 100 80
e I
60 40. 20’
I
1
4
6
ChemberPressure
a
m
ITr
E Tora
Fig. 5. Specimen current against chamber pressure for a supported triode discharge (filament and probe floating) with the specimen at -4 kV: V,, = 13 V; V, = 300 V. Gun power as in Fig. 4.
46
A. MATTHJWS, D. G. TEJ%
With the thermionic filament in operation, specimen currents under high EB gun powers are different from those shown in Fig. 4, as shown in Figs. 5 and 6. Explanation of the results observed is complicated by the fact that, as mentioned earlier, there are several possible influential mechanisms and these may be producing conflicting effects. However, it is evident that the floating filament and probe can to some extent prevent the reduction in specimen current which normally accompanies evaporation. This corresponds to a negative filament arrangement and is the recommended system for maximized specimen currents in ion plating. 160 , 140 120 100 Specimen Current
80
E!?I 60 40 . 20 .
2
4
6 $ Chamber Pressure
10 [ImTorrI
15
Fig. 6. Specimen current against chamber pressure for a supported triode discharge (filament earthed) with the specimen at -4 kV: V,, = 13 V; V, = 300 V. Gun powers as in Fig. 4.
3. DWXJSSION AND CONCLUSIONS The discharge effects in the thermionically assisted triode ion-plating system are not directly covered by conventional discharge theories. It is intended that the presentation below will help to clarify the situation and will provide a basis for further discussion. The tendency for the floating probe to take up earth potential in the discharge can be explained as being due to the tendency of the earthed chamber walls in contact with the plasma to cause the plasma potential to be near zero. The probe, being the anode to the plasma, will also be near earth potential. In order to arrive at a fuller explanation of the influence of the EB gun, it is necessary to consider the fundamental ionization and collision effects probably associated with it. McDaniel’ 5 gives the optimum electron energy for argon ionization as being near 50 eV. As the electrons from the gun filament are initially accelerated to 10 keV, the ionization as they pass through the gas will be low unless collisions reduce their energy. In fact it is known that some excitation does occur and this is displayed in the glow which emanates from the beam. Considering the secondary electron yield under electron impact on the source material (titanium in
THJZRMIONICALLY A!SSISTED TRIODE ION-PLATING
SYSTEM
47
this case), although the yield will be low under bombardment”, examination of the secondary electron energy distribution given by McDaniel shows that these electrons will have energies near the optimum for gas ionization. Secondary electron emission from the melt surface can therefore be expected to have an important influence on ionization. Thermionic emissions from this source will also be important, if, as with titanium, significant emission occurs below the evaporation temperature. Each of these effects can be magnified under evaporation as a high pressure region forms above the source, increasing the possibility of ionizing collisions. It is clear that the influence of the EB gun will depend on the interaction of the secondary and thermionic electrons with the plasma. The crucible from which these electrons originate is held at earth potential. It is therefore reasonable to expect, as observed, that their effect under floating conditions will be different from that under earthed conditions, when the plasma potential with respect to earth will also differ. In the earthed condition, the plasma associated with the positive probe will be at about 300 V with respect to earth, whilst that associated with the d.c. diode to the sample will be near earth potential. Electrons emitted from the melt will be accelerated to the probe resulting in high probe currents. As these electrons form a concentrated flux, only a limited proportion may undergo ionizing collisions. When the probe is floating, its potential tends to earth. This will also be the approximate potential of the plasma associated with the dc. diode to the sample. Though secondary electrons emitted from the .melt will now have no large potential differences through which to be accelerated, they do have sufficient intrinsic energy to cause ionization and this is confirmed by results reported earlier. Lower probe currents are recorded in this case. Examination of the electron and ion paths in the two basic systems covered in this analysis (i.e. with an earthed filament or a negative filament) shows that in the negative filament arrangement the sample electrode provides a much greater proportion of the available cathode area. This is because with the earthed filament the chamber is also a cathode in the hot cathode discharge circuit. The negative filament is therefore more efficient in increasing the specimen ion current, which is often the objective in ion plating. This system is now the one adopted in our laboratories. By using the negative filament system, ionization efficiences over 3% and increases in specimen current density from less than 0.3 mA crnm2 (with a single diode) to over 3 mA cm- 2 were obtained. These effects, coupled with the ability to deposit at pressures below 10m3 mTorr have been shown to produce considerable coating dens&&ion and improved reaction in ceramic filmsr6. Improvements in metal alloy film properties have also been obtained by this technique”. ACKNOWLEDGMENTS
The authors are grateful to the Scienc Research Council for providing financial support to one of us (A.M.). Professor Halling’s continuing encouragement is acknowledged. We should like to record our thanks to Dr. D. G. Armour who contributed considerably to many helpful discussions on this work.
48
A. MATTHEWS, D. G. TBBR
1 D. M. Mattox, Proc. Conf. on Ion Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 1. 2 D. G. Teer and B. C. Deelcea, Thin Solid Films, 54 (1978) 295. 3 P. A. Higham au! D. G. Teer, Thin SolidFilms, 58 (1979) 121. 4 A. Matthews and D. G. Teer, Proc. Conf. on Zon Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 11. 5 R. F. Bun&ah, Froc. Conf. on Ion Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 230. 6 A. Matthews and D. G. Teer, Thin Solid Films, 72 (1980) 541. 7 T. Sato, M. Tada, Y. C. Huang and H. Takei, Thin Solid Films, 54 (1978) 61. 8 C. T. Wan, D. L. Chambers and D. C. Carmichael, J. Vat. Sci. Technol., 8 (1971) 99. 9 M. I. Ridge, R. P. Howson, J. N. Avaritsiotis and C. A. Bishop, Froc. Conf. on Ion Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 21. 10 Y. Murayama, Jpn. J. Appl. Phys. Suppl. 2, Part 1(1974) 459. 11 L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970. 12 R. Dugdale, Trans. Inst. Met. Finish., 54 (1976) 61. 13 G. A. Baum, Dow Chemical Co. PI&. FRPd86, Colorado, 1967. 14 M. G. D. El-Sherbiney, Ph.D. Thesis, University of Salford, Salford, 1974. 15 E. W. McDaniel, Collision Phenomena in Ionised Gases, John Wiley, New York, 1964. 16 A. Matthews, Ph.D. Thesis, University of Salford, Salford, 1980. 17 E. Minni and H. A. Sundquist, Proc. 3rd. Int. Co& on Ion and Plasma Assisted Techniques, Amsterdam, 1981, in Thin SolidFilms, 80(1981) 55.
80 (1981)41-48
METALLURGICAL
AND PROTECTIVE
CHARACTERISTICS PLATING SYSTEM* A. MATTHEWS
41
COATINGS
OF A THERMIONICALLY
ASSISTED TRIODE ION-
AND D. G. TFZR
Department of Aeronautical (Gt. Britain)
and Mechanical
Engineering,
University
of Salford,
Salford
M5 4WT
There is increasing evidence to show that the properties of ion-plated coatings can be improved by increasing the specimen ion current density and by operating at low deposition pressures. This paper examines a technique of discharge support which facilitates such improvements. The characteristics of the discharge are presented and explanations are given for the effects observed. The recommended arrangement for maximized specimen ion currents utilizes a positive electrode and a thermionic source which is electrically floating so that the filament takes up a negative potential with respect to the earthed chamber. Direct biasing of the filament can also be used, obviating the need for the positive electrode.
1. INTRODUCTION
In ion plating and related techniques, it is now well accepted that the use of a gaseous discharge improves coating properties in several ways. Firstly, adhesion is improved owing to the benefits of sputtering on cleaning and interface formation’. Secondly, coating structures are improved for both metal.and ceramic fihn~~-~. Thirdly, improved reaction can be achieved for reactive deposition5,6. Further improvements are obtained if the discharge is intensified, i.e. if ionization is increased, particularly when the discharge is maintained at lower pressures than are normally possible using a conventional d.c. diode arrangement. There are several means by which the d.c. diode discharge can be enhanced and supported. The main methods used involve increasing the electron path within the discharge or providing an additional source of ionization. Sometimes the vapour generation source can assist in this respect. For example, the hollow cathode electron beam (EB) gun’,* and the magnetron sputter source’ both increase ionization in ion plating. R.f. electrodes within, or even external to, the chamber have been used to increase ionization and to extend the operating pressure rangelo. Possibly the simplest means of discharge support is to incorporate a positive electrode to give a triode configuration. This produces a useful increase in ionization *Paper presented at the 3rd International Amsterdam, The Netherlands, June 30-July 0040~6090/81/oooo-oooo/$02.50
Conference 2, 198 1.
on Ion and
Plasma
fQ Elsevier Sequoia/Printed
Assisted
Techniques,
in The Netherlands
42
A. MATTHEWS, D. G. TEER
and can lower the permissible operating pressure. The arrangement has similarities to triode sputtering1 1 though in that case it is the target, and not the specimen, which is biased negatively. Positive probe discharge support is particularly influential under evaporation conditions, when electrons emitted in the source region can be accelerated to increase ionization. A similar effect is achieved by biasing the melt negatively”. EB evaporators produce electrons with a range of energies and intensities. Hot cathode EB guns thermionically emit a beam of electrons which are accelerated to energies of several kiioelectronvolts. The melt emits the thermionic electrons usually associated with an incandescent source and also provides secondary electrons owing to the incident primary beam on the melt surface. Great control over ionization in triode ion plating can be achieved if a separate thermionic emitter is incorporated independent of the vapour source. This permits the ionization to be increased during the sputter-cleaning or pre-etch stage and allows low chamber pressures throughout the cycle. The first reported use of a thermionically assisted triode type of arrangement for ion plating was by Baumr3. He claimed four advantages for the technique: (1) greater control over the gas discharge; (2) improved stability due to the use of lower bias voltages; (3) lower pressure operation; (4) reduced substrate heating. The last point results from the reduced bias required to reach a desired specimen current and from the consequent reduction in discharge power input. We feel that the most important benefit of the thermionically assisted triode system is the capability of increasing the specimen current density even at low bias levels. This has been shown to be particularly desirable for reactive ion plating6. However, there does appear to be a lack of data available on the discharge effects occurring within such systems which in effect combine hot and cold cathode discharges. For this reason a study was undertaken to identify the characteristics of a typical thermionically assisted triode layout and to offer explanations for the effects observed. 2.
EXPERIMENTAL DETAILS
2.1. Equipment A schematic diagram of the circuit used for system evaluation is shown in Fig 1. The circuit incorporates a filament of tungsten wire 0.5 mm in diameter and 12 cm long. The hot cathode circuit can be operated in two basic states which are obtained by selecting position 1 or 2 on the common point earthing switch. Position 2 allows the filament and the positive probe circuit to float independently of the rest of the system, whereas position 1 imposes an earth datum at the common point. Extensive investigations were carried out into the influence of system parameters such as probe voltage, specimen bias, gun powder and chamber pressure on the specimen and probe currents. Some important conclusions from these studies are outlined below. 2.2. Results Figures 2 and 3 show the variations in current monitored to a specimen 100 cm2 in surface area under conditions in which there is no power input to the EB gun and
THERMIONICALLY ASSISTED TRIODE ION-PLATING SYSTEM
43
O-5 ICY dc
O-32
Cormnon point Fig. 1. Schematicdiagram
of the system used for discharge system trials.
180 160
120 100 Specimen Current I%!!
80 60 40 20
2 3 4 5 Specimen Bias c_kv7 Fig. 2. Specimen current against s_@en bias for a supported triode discharge (filament earthed) without a vapour source at pressures of 10 mTorr (A) and 0.5 mTorr (0): V,, = 9 V, V, = 300 V. 1
A. MATTHEWS,
1
2
4 Bias
v*
3. Specimen against specimen together) without vapour source 300v.
G.
5
@zq
for a pressures of
triode discharge mTorr (A) 0.3 mTorr
and filament V,, = V,
therefore no evaporation. Figure 2 shows the earthed (switch position 1) results and Fig. 3 shows the currents for the floating system (switch position 2). Several observations can be made about these graphs. It is evident that the filament tends to reduce the effect on the specimen current of pressure variations which are normally observed with the d.c. diode. In both cases the discharge is maintained at low pressures which do not normally support the cold cathode d.c. diode discharge. Furthermore, with the switch in position 1 a specimen ion current exists even in the absence of a specimen bias, as the specimen, although earthed, is a cathode to the positive probe. This may have important consequences regarding the ion plating of earthed samples. We noted that under discharge conditions with the switch in position 2 the floating 300 V supply tends to take up ground potential on the positive side. Hence the probe is no longer biased with respect to the chamber: the unbiased specimen is not then a cathode and it receives no ion current. When the EB gun filament is heated the EB power may not initially be adequate to vaporize the melt, though it can still influence the discharge. At higher gun powers there are several possibly conflicting mechanisms such as the passage of an intense EB through the region above the source, secondary and thermionic emissions from the melt and the formation of a metal vapour cloud; All these factors can influence ionization. Furthermore, the electron yield at the sample may be affected by the formation of a deposit. Thus the results obtained with the EB gun in operation were found to differ considerably from those in Figs. 2 and 3 which were obtained with the gun off. Firstly, consider Fig. 4 which shows the results for a triode system with no additional electron source other than the EB gun. It can be seen that the specimen current reduces as the gun power is increased. Such an effect has been reported previously for a diode system as evaporation commenced’4.
‘MBRMIONICALLY ASSISTED TRIODB ION-PLATING SYSTEM
Specimen Current
E4.l
45
60 0 45’ 30 * 15 ’
-d
e-c.---
__L----
I 8
6
4
2
Chamber Pressure
I’0 b
1’2
Torrl
Fig. 4. Specimen current against chamber pressure for a triode discharge with the specimen at -4 kV showing the effect of gun powder with no voltage across the support filament: V, = 300 V. Full curves represent the system with the filament earthed (gun power: q,O kW; ‘17,l kW; 0,2 kW; & 3 kW; 0,4 kW) and the broken curve is for the floating system (gun power, 0, 1,2,3 or 4 kW)
120 Specimen Current tzt!I
1 100 80
e I
60 40. 20’
I
1
4
6
ChemberPressure
a
m
ITr
E Tora
Fig. 5. Specimen current against chamber pressure for a supported triode discharge (filament and probe floating) with the specimen at -4 kV: V,, = 13 V; V, = 300 V. Gun power as in Fig. 4.
46
A. MATTHJWS, D. G. TEJ%
With the thermionic filament in operation, specimen currents under high EB gun powers are different from those shown in Fig. 4, as shown in Figs. 5 and 6. Explanation of the results observed is complicated by the fact that, as mentioned earlier, there are several possible influential mechanisms and these may be producing conflicting effects. However, it is evident that the floating filament and probe can to some extent prevent the reduction in specimen current which normally accompanies evaporation. This corresponds to a negative filament arrangement and is the recommended system for maximized specimen currents in ion plating. 160 , 140 120 100 Specimen Current
80
E!?I 60 40 . 20 .
2
4
6 $ Chamber Pressure
10 [ImTorrI
15
Fig. 6. Specimen current against chamber pressure for a supported triode discharge (filament earthed) with the specimen at -4 kV: V,, = 13 V; V, = 300 V. Gun powers as in Fig. 4.
3. DWXJSSION AND CONCLUSIONS The discharge effects in the thermionically assisted triode ion-plating system are not directly covered by conventional discharge theories. It is intended that the presentation below will help to clarify the situation and will provide a basis for further discussion. The tendency for the floating probe to take up earth potential in the discharge can be explained as being due to the tendency of the earthed chamber walls in contact with the plasma to cause the plasma potential to be near zero. The probe, being the anode to the plasma, will also be near earth potential. In order to arrive at a fuller explanation of the influence of the EB gun, it is necessary to consider the fundamental ionization and collision effects probably associated with it. McDaniel’ 5 gives the optimum electron energy for argon ionization as being near 50 eV. As the electrons from the gun filament are initially accelerated to 10 keV, the ionization as they pass through the gas will be low unless collisions reduce their energy. In fact it is known that some excitation does occur and this is displayed in the glow which emanates from the beam. Considering the secondary electron yield under electron impact on the source material (titanium in
THJZRMIONICALLY A!SSISTED TRIODE ION-PLATING
SYSTEM
47
this case), although the yield will be low under bombardment”, examination of the secondary electron energy distribution given by McDaniel shows that these electrons will have energies near the optimum for gas ionization. Secondary electron emission from the melt surface can therefore be expected to have an important influence on ionization. Thermionic emissions from this source will also be important, if, as with titanium, significant emission occurs below the evaporation temperature. Each of these effects can be magnified under evaporation as a high pressure region forms above the source, increasing the possibility of ionizing collisions. It is clear that the influence of the EB gun will depend on the interaction of the secondary and thermionic electrons with the plasma. The crucible from which these electrons originate is held at earth potential. It is therefore reasonable to expect, as observed, that their effect under floating conditions will be different from that under earthed conditions, when the plasma potential with respect to earth will also differ. In the earthed condition, the plasma associated with the positive probe will be at about 300 V with respect to earth, whilst that associated with the d.c. diode to the sample will be near earth potential. Electrons emitted from the melt will be accelerated to the probe resulting in high probe currents. As these electrons form a concentrated flux, only a limited proportion may undergo ionizing collisions. When the probe is floating, its potential tends to earth. This will also be the approximate potential of the plasma associated with the dc. diode to the sample. Though secondary electrons emitted from the .melt will now have no large potential differences through which to be accelerated, they do have sufficient intrinsic energy to cause ionization and this is confirmed by results reported earlier. Lower probe currents are recorded in this case. Examination of the electron and ion paths in the two basic systems covered in this analysis (i.e. with an earthed filament or a negative filament) shows that in the negative filament arrangement the sample electrode provides a much greater proportion of the available cathode area. This is because with the earthed filament the chamber is also a cathode in the hot cathode discharge circuit. The negative filament is therefore more efficient in increasing the specimen ion current, which is often the objective in ion plating. This system is now the one adopted in our laboratories. By using the negative filament system, ionization efficiences over 3% and increases in specimen current density from less than 0.3 mA crnm2 (with a single diode) to over 3 mA cm- 2 were obtained. These effects, coupled with the ability to deposit at pressures below 10m3 mTorr have been shown to produce considerable coating dens&&ion and improved reaction in ceramic filmsr6. Improvements in metal alloy film properties have also been obtained by this technique”. ACKNOWLEDGMENTS
The authors are grateful to the Scienc Research Council for providing financial support to one of us (A.M.). Professor Halling’s continuing encouragement is acknowledged. We should like to record our thanks to Dr. D. G. Armour who contributed considerably to many helpful discussions on this work.
48
A. MATTHEWS, D. G. TBBR
1 D. M. Mattox, Proc. Conf. on Ion Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 1. 2 D. G. Teer and B. C. Deelcea, Thin Solid Films, 54 (1978) 295. 3 P. A. Higham au! D. G. Teer, Thin SolidFilms, 58 (1979) 121. 4 A. Matthews and D. G. Teer, Proc. Conf. on Zon Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 11. 5 R. F. Bun&ah, Froc. Conf. on Ion Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 230. 6 A. Matthews and D. G. Teer, Thin Solid Films, 72 (1980) 541. 7 T. Sato, M. Tada, Y. C. Huang and H. Takei, Thin Solid Films, 54 (1978) 61. 8 C. T. Wan, D. L. Chambers and D. C. Carmichael, J. Vat. Sci. Technol., 8 (1971) 99. 9 M. I. Ridge, R. P. Howson, J. N. Avaritsiotis and C. A. Bishop, Froc. Conf. on Ion Plating and Allied Techniques, London, 1979, CEP Consultants, Edinburgh, 1979, p. 21. 10 Y. Murayama, Jpn. J. Appl. Phys. Suppl. 2, Part 1(1974) 459. 11 L. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970. 12 R. Dugdale, Trans. Inst. Met. Finish., 54 (1976) 61. 13 G. A. Baum, Dow Chemical Co. PI&. FRPd86, Colorado, 1967. 14 M. G. D. El-Sherbiney, Ph.D. Thesis, University of Salford, Salford, 1974. 15 E. W. McDaniel, Collision Phenomena in Ionised Gases, John Wiley, New York, 1964. 16 A. Matthews, Ph.D. Thesis, University of Salford, Salford, 1980. 17 E. Minni and H. A. Sundquist, Proc. 3rd. Int. Co& on Ion and Plasma Assisted Techniques, Amsterdam, 1981, in Thin SolidFilms, 80(1981) 55.