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Deposition of Ti-N compounds by thermionically assisted triode reactive ion plating
Thin Solid Films, 72 (1980) 541-549 © Elsevier Sequoia S.A., Lausanne--Printedin the Netherlands
541
DEPOSITION OF T i N COMPOUNDS BY THERMIONICALLY ASSISTED TRIODE REACTIVE ION PLATING* A. MATTHEWS AND D. G. TEER Department of Aeronautical and Mechanical Engineering, University of Salford, Salford M5 4 W T (Gt. Britain) (Received April 7, 1980; accepted April 24, 1980)
Details are given of the influence of the ion current density on the properties of T i N compounds deposited at low temperatures (< 600 °C) by a thermionically assisted triode reactive ion-plating technique. As the chamber pressure increases, greater specimen currents are necessary, both to prevent the deposition of powdery TiN and also to ensure reaction for the formation of Ti2N and TiN at lower partial pressures of nitrogen. The ionization efficiency, being related to both pressure and current density, is suggested as a suitable parameter for defining optimum conditions. An ionization efficiency of about 0.3% is identified as the minimum required for the deposition of cohesive TiN, whilst values above this improve the densification and the hardness.
1. INTRODUCTION Several investigators x-3 have shown that the discharge current density has a predominant influence in improving the properties of single-element coatings deposited by ion plating. Compound deposits, including those formed by reactive methods, also benefit from bombardment, as shown .for TiN deposition by Matthews and Teer 4 and by Dugdale 5. More recently Fleischer e t al. 6 and Yoshihara and Mori 7 have also obtained improved TiN coating morphology under enhanced discharge conditions. In each of these studies the conventional d.c. diode discharge was augmented by a thermionic source and an anode. The evolved system has similarities to the biased activated reactive evaporation technique reported by Suri e t al. s Other workers have used various techniques to improve ionization. The hollow cathode electron beam (EB) gun has been found to give greater ionization than the hot filament EB gun and has been used by Sato e t al. 9 for reactive TiN deposition. Murayama 1° has deposited TiN by using an r.f. coil electrode to enhance the d.c. diode discharge. The evidence of research therefore supports the view that increased ionization is vital for effective low temperature TiN reactive ion plating. This conclusion is confirmed by the lower success rate of ceramic coatings formed at low ionization and temperature 11,12. Thus while a review of the available literature indicates that increased ionization is necessary, little information is available on the exact level * Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 21-25, 1980.
542
A. MATTHEWS, D. G. TEER
required. The objective of this paper is to improve this situation by outlining the results of coating trials which have been conducted by the authors. 2. EXPERIMENTAL DETAILS
2.1. Equipment In our laboratory we have been studying for some time the potential of reactively ion-plated Ti-N compounds for wear reduction, particularly on metal forming tools. The requirement in this application is to deposit at a high rate (1 Ixm min-1) whilst avoiding excessive substrate heating. Furthermore, the equipment must be suitable for a production environment and must avoid the use of techniques and devices which necessitate specialist knowledge. Figure 1 shows the deposition arrangement adopted. O-$KV O-IA
WATER
FLOW THERHOCOUPLE~ t
T II
o-3zv
h
"T "
[ WEEDtE FLOW
'1 . I
V,L~S
h
k--1
i
'~' ILl ~SWITCH "eU~PS /
.E__TEnS
==0=--
'
''''°''
__L 0-,00v
I_.e!..l °'o'# ~ O-60V --F-
o-zs,
Fig. 1. A schematic diagram of the thermionically assisted triode coating system.
The system chosen for discharge enhancement employs a positive electrode (or "probe") which can extract electrons generated in the vapour source region to increase ionization. Arc suppression is incorporated in the probe supply in the form of an inductive resistance on the output. To improve controllability and to permit a smooth transition from the etch to the deposition stages, as well as to increase the ionization, a hot filament electron source is also incorporated. This type of system was first suggested for ion plating by Baum 13 and the arrangement is similar to that used in triode-assisted sputtering 14. Titanium is vapourized by a 225 ° bent beam EB gun. There are two chamber regions, each pumped by a separate diffusion pump, with a single backing rotary pump. This arrangement permits the operation of the EB gun in the lower chamber and allows ion-plating pressures of over 30 mTorr in the upper chamber. The specimen support cathode is water cooled and can carry industrial tools weighing over 20 kg. The source-to-specimen distance is nominally 30 cm.
2.2. Procedure The results to be reported here were obtained at a standard specimen bias of
DEPOSITION OF
TiN
COMPOUNDS BY REACTIVE ION PLATING
543
- 4 kV. Early trials used either an argon or an N2-Ar pre-etch followed by the evaporation of titanium in a nitrogen discharge. This procedure led to the formation at the interface of a dark layer with poor adhesion. In subsequent coatings, therefore, a thin (0.5 Inn) titanium layer was deposited in an argon discharge prior to bleeding nitrogen in at the desired rate. In view of the preferential removal of nitrogen it was important to maintain a constant pumping speed from run to run in order to make reactive gas flow requirements consistent; this was achieved by baffling back the pumping speed to maintain 15 m Torr at an argon flow of 6 cm 3 min-1. Gas flows were monitored with flowmeters which were calibrated for air at STP. The supply gauge pressure was 260 Torr and most of the pressure drop to vacuum occurs at the needle valves. The flow measurement system described is typical of that used in many laboratories and results will therefore be quoted as read on these meters. The chamber pressure was controlled by altering the argon flow. Mass spectrometry facilities were not available. Although a constant gun power of 4 kW was adopted throughout, it was found that the evaporation rate (as determined retrospectively by billet weight loss) varied, severely limiting the predictability of the process. This emphasized the need for real time evaporation rate monitoring for reactive ion plating. Typically a sputter-cleaning time of 30 min was used, with a pressure of 15 m T o r r and a current density of 0.5 mA cm-2. When intensification from the probe and thermionic filament was required to achieve this current density, e.g. when large specimens were being coated, the switch in Fig. 1 was used in position 1. Thus the filament was at earth potential and the anode at supply voltage. Under evaporation conditions, though, enhancement was invariably greater if the switch was in position 2. The cause for this appears to be related to the high electron concentration occurring when both the support filament and the EB gun are operating. In position 2, with the filament and probe electrically floating, the probe tends to take up earth potential. The whole chamber then acts as an anode to the filament. 3.
THEORETICAL CONSIDERATIONS
In this paper we shall use the concept of ionization efficiency in ion plating, i.e. the percentage of the atoms bombarding the cathode that are ionized. The ionization efficiency can be estimated by neglecting any secondary electron current from the cathode, converting the specimen current density to a unit charge flow and dividing by the total number of bombardments per unit area. The latter figure can be obtained from kinetic gas theory. It was found more convenient to use for this purpose the overall chamber pressure as measured with an'ionization gauge remote from the sample. An estimate of the magnitude of the effect of unaccounted depositing atoms can be made by neglecting sputtering and converting the deposition rate to an atom arrival rate. A deposition rate of 1 pm rain- 1 represents about 0.1 × 1018 atoms c m - 2 s- 1. For comparison, 10 mTorr and 1 mTorr represent 4.3 x 1018 and 0.43 x 1018 atoms cm -2 s -1 respectively is. Thus the potential error from using the overall chamber pressure for ionization efficiency calculations varies from about 2.39/0 at 10 mTorr to 239/0 at 1 m T o r r and 230~ at 0.1 mTorr. In fact at lower pressures the deposition rate defines the maximum ionization efficiency achievable at a fixed current density. The maximum efficiencies for a deposition rate
544
A. MATTHEWS,
D. G. TEER
of 1 ~trn m i n - 1 at 1, 2 and 3 mA c l r 1 - 2 would therefore be about 6%, 12% and 19% respectively. However, in the usual range of ion-plating pressures and deposition rates, the error due to making an estimate based on a reading remote from the sample is acceptable. If we adopt the pressure as a measure of surface bombardment, the variation in ionization efficiency at a constant specimen current density is as shown in Fig. 2.
z.0
o_ 1.0
3mAcro-z
I \ \~
/
S
2.At.-' I0
IS
20
eNMMSERPRESSURE[mTorr]
Fig. 2. The variation of ionization efficiency ~ t h pressure at constant current densities.
/ 1"4 NITROGEN-DEFICIENTTiREGION// 1"2
'~,o '~'
Ti2N
/
REGION (>0"6mACm-I / REQUIRED)
o.,
/
0-_
/
0., ~
/
TiN
/ / REGION /
/
/
"/(>0.4mAcm-Y RE*UIRE~
/
//.
.OESS N.RO~EN
R ,OTN,N-00,TIN0S F,E ,RG'E
-
O'4
0.2
0
I
I0
I
I
I
I
I
20 30 40 SO 60 .JTROGE" ,NttT FLOWRATE CsTP) [c.' . i . - ' ]
I
70
I
$0
Fig. 3. Summary of the compositions produced across the range of titanium evaporation rates and nitrogen inlet flow rates. The current density requirements refer to chamber pressures below 2 mTorr; higher pressures require an increasedcurrent density. 4. RESULTS After a series of calibration trials a clear pattern emerged of the nitrogen flow required for TiN formation at different titanium evaporation rates. Basically this
DEPOSITIONOF T i - N COMPOUNDSBY REACTIVEION PLATING
545
was an indicated 55 cm 3 m i n - 1 of nitrogen for each gram of titanium evaporated per minute. Figure 3 shows the range of compositions produced on either side of this value. The achievement of the balance required did not necessarily lead to the compound shown or even to a dense coating. These were both critically influenced by the specimen current density, in the manner outlined in the following. Within the region to the left of the bold line, at current densities insufficient for Ti2N or TiN formation, it was possible to deposit coatings which gave a titanium Xray diffractometer pattern. These had increasing hardnesses as the interstitial nitrogen content increased, over 1200 H K (Knoop) being possible. The preferred orientation was (002) and structures became more dense as the current density increased and as the chamber pressure decreased. In the region in Fig. 3 to the right of the bold line, powdery TiN deposits resulted, unless the ion current density exceeded a minimum level which increased with pressure, in which case dense TiN could be formed within the band shown. Examples will be given of the ionization enhancement required for the production of non-friable TiN deposits at various pressures. Coatings deposited at 2 mTorr required a minimum current density of 0.4 mA cm-2. These showed a microhardness of over 1800 H K ; their structure is shown in Fig. 4. They had a (111) preferred orientation. Increasing the current density to 2.2 mA cm -2 under otherwise similar conditions produced a harder more dense structure with a (200) texture (Fig. 5). At 7 m T o r r a minimum current density of 1.2 mA c m - 2 was required for nonpowdery deposition. Figure 6 shows a typical structure. The preferred orientation was (111). Increasing to over 2.0 mA c m - 2 again densified and hardened the coating, though maximum hardness ( > 2500 HK) was achieved above 2.5 mA cm-2. Ti2N was detected in the hardest of these films. Frequently, higher chamber pressures are required to improve coating coverage through gas-scattering effects. The highest
Fig. 4. Fracture sectionof TiN coating depositedat 2 mTorr and 0.4 mA cm- 2.
546
A. MATTHEWS, D. G. TEER
Fig. 5. Fracture section of TiN coating deposited at 2 mTorr and 2.2 mA c m - 2.
Fig. 6. Fracture section of TiN coating deposited at 7 mTorr and 1.2 mA cm -2.
pressure at which this was attempted for TiN deposition was 15 mTorr. A current density of 1.8 mA crn- 2 was required in this case. The preferred orientation was (200) and hardnesses over 1800 H K were obtained. The structure is shown in Fig. 7. In the region shown in Fig. 3 for Ti2N deposition a similar relationship between structure, hardness and ionization to that for TiN was found. For example, for Ti2N deposition at 1 mTorr the required current density was 0.6 mA cm-2, whereas at 13 mTorr over 1.9 mA c m - 2 was necessary. Again even higher current densities tended to produce harder and denser coatings. Figure 8 shows a TizN coating deposited at
DEPOSITIONOF T i - N COMPOUNDSBY REACTIVEION PLATING
547
2.6 mA c m - 2 and 2 mTorr. The preferred orientation is (002) and the hardness of the coating is over 2500 HK. Although this was deposited in the Ti2N region in Fig. 3, peaks of TiN were also observed. In fact TiN could be formed outside the band indicated in Fig. 3, though this required increased current density as the balance condition moved to the left of the bold line. Ti2N was always present in such coatings.
Fig. 7. Fracture section of TiN coating deposited at 15 mTorr and 1.8 mA cm-2.
Fig. 8. Fracture section of Ti2N plus TiN coating deposited at 2 mTorr and 2.6 mA c m - 2 . As stated earlier, the intended usage for coatings developed on this p r o g r a m m e is for metal-forming tools, which m a y soften at elevated temperatures. With this in
548
A. MATTHEWS, D. G. TEER
mind the trials were performed on a typical tool steel (AISI H13). Bulk hardness measurements and practical tests after coating showed that unacceptable softening had not occurred and therefore that temperatures had been kept below 600 °C; thermocouple readings also confirmed this. Low temperatures were achieved with the assistance of water cooling, which was particularly vital at current densities above 1.5 mA cm- 2 at a bias of - 4 kV. Tribological studies of coatings developed in this programme included pin-ondisc tests and simulated metal-forming operations 16. In the former test a hardened steel pin with a tip radius of 3.175 mm was rubbed against the coating on a mild steel substrate under a load of 0.5 kg at 100 rev m i n - 1 on a wear track 1 cm in diameter. No wear was detectable on optimized coatings after 12 000 passes, though the pin was worn by over 0.25 mm radially. Coating wear was evaluated using scanning electron microscopy on fracture sections of wear tracks. The true wear was thereby recorded, rather than the depression of the substrate due to loading. 5. DISCUSSION AND CONCLUSIONS
This work has shown that, provided the depositing species are correctly balanced, the pressure and the current density are the dominant variables in reactive ion plating (at a given bias voltage). As stated in Section 3, these combine to give the parameter termed the ionization efficiency. Across the pressure range used it was found impossible to form cohesive TiN at ionization efficiencies less than about 0.3%. An ionization efficiency of 0.3% was necessary again when, at lower nitrogen pressures, Ti2N was formed; though when the total pressure was low there was evidence that even greater ionization efficiencies were required for Ti2N. The deposits formed at ionization etticiencies less than the critical level consisted either of unreacted titanium with interstitial N2 or of powdery TiN, depending on whether the atomic ratio of Ti: N was to the left or the right of the bold line in Fig. 3. The effect of increasing the ionization efficiency further was to promote very dense (zone 3 type 17) structures and high hardnesses (near 3000 HK). The ionization efficiency therefore appears to offer a good criterion for process optimization in reactive ion plating. It may not, however, take full account of all variables. One of these is the specimen bias, which could be incorporated as the input power density to form a suitable criterion. However, this requirement would still vary with pressure. Another variable which needs to be taken into account is the deposition rate. Dugdale 5 sets as a criterion for dense deposits that almost all the condensing atoms should be resputtered, stating that at high deposition rates this necessitates increased bias power. He uses rates from about 0.5 ~tm m i n - 1 (with a sputter source) to over 1 ~tm m i n - 1 (with an EB gun source). In the context of our model, deposition rates within this range would not be expected to require a significantly greater ionization efficiency at the pressures above I0 mTorr that have been quoted. Dugdale's work therefore suggests that the deposition rate may have a more important influence than is evident from our researches. We may conclude that there is no single parameter which can be said fully to define reactive ion-plating requirements. The ionization efficiency, however, still has distinct advantages over most others, as it takes into account the total number of
DEPOSITION OF Ti-N COMPOUNDS BY REACTIVE ION PLATING
549
incident atoms (which in effect share the deposition energy) and in our range of operating variables it proved an effective criterion for predicting coating properties. Work to evaluate the effect of decreasing specimen bias at high current density, and therefore high ionization efficiency, is continuing. The results of Koboyashi and Doi 1s indicate that dense coatings can still be produced at low bias levels. They use extremely low gas pressures (<4.5 x 10 -4 Torr). Although ion currents are not given, the discharge support system and low pressure used suggest high ionization efficiencies. Moreover, the ion energy spectrum will include more ions near to the full acceleration energy than in higher pressure deposition. Evidence obtained at low pressure thus suggests that biases of several kV are not required, but rather that some much lower voltage is sufficient, provided that ionization can be increased by suitable means. This should permit deposition at low power input and therefore obviate the need for forced cooling. ACKNOWLEDGMENTS
The authors are grateful to the Science Research Council for funding this project and for providing one of us (AM) with financial support. Thanks are also due to Professor J. Hailing for continuing enthusiastic encouragement. Miss P. E. Bond provided assistance with scanning electron microscopy. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
D.G. Teer and B. L. Delcea, Thin Solid Films, 54 (1978) 295. J. A Thornton, Thin Solid Films, 40 (1977) 335. M Lardon, R. Buhl, H. Signer, H. K. Pulker and E. Moll, Thin Solid Films, 54 (1978) 317. A. Matthews and D. G. Teer, Proc. Ion Plating and Allied Techniques Conf., CEP Consultants, Edinburgh, 1979. R.A. Dugdale, Trans. Inst. Met. Finish., 54 (1976) 61. W. Fleischer, D. Schulze, R. Wilberg, A. Lunk and F. Schrade, Thin Solid Films, 63 (1979) 347. H. Yoshihara and H. Mori, J. Vac. Sci. Technol., 16 (1979) 1007. A . K . Suri, R. NimmagaddaandR. F. Bunshah, ThinSolidFilms, 64(1979) 191. T. Sato, M. Tada, Y . C . HuangandH. Takei, ThinSolidFilms, 54(1978)61. Y. Murayama, J. Vac. Sci. Technol., 12 (1975) 818. R.J. Hill, G. Scheuermann and R. Lucariello, Thin Solid Films, 40 (1977) 217. W.R. Stowell, Thin Solid Films, 22(1974) 111. G.A. Baum, Dow Chemical Co. Publication RFP-686, Colorado, 1967. T.C. Tisone and P. D. Cruzan, J. Vac. Sci. Technol., 12 (1975) 1058. L. Ward and J. P. Bunn, Introduction to the Theory and Practice o f High Vacuum Technology, Butterworths, London, 1967. A. Matthews and D. G. Teer, Proc. Int. Conf. on Metallurgical Coatings, San Diego, California, 1980, in Thin Solid Films, 73 (1980). B.A. Movchan and A. V. Demchishin, Fiz. Met. Metalloved., 28 (1969) 653. M. Koboyashi and Y. Doi, Thin Solid Films, 54 (1978) 67.
541
DEPOSITION OF T i N COMPOUNDS BY THERMIONICALLY ASSISTED TRIODE REACTIVE ION PLATING* A. MATTHEWS AND D. G. TEER Department of Aeronautical and Mechanical Engineering, University of Salford, Salford M5 4 W T (Gt. Britain) (Received April 7, 1980; accepted April 24, 1980)
Details are given of the influence of the ion current density on the properties of T i N compounds deposited at low temperatures (< 600 °C) by a thermionically assisted triode reactive ion-plating technique. As the chamber pressure increases, greater specimen currents are necessary, both to prevent the deposition of powdery TiN and also to ensure reaction for the formation of Ti2N and TiN at lower partial pressures of nitrogen. The ionization efficiency, being related to both pressure and current density, is suggested as a suitable parameter for defining optimum conditions. An ionization efficiency of about 0.3% is identified as the minimum required for the deposition of cohesive TiN, whilst values above this improve the densification and the hardness.
1. INTRODUCTION Several investigators x-3 have shown that the discharge current density has a predominant influence in improving the properties of single-element coatings deposited by ion plating. Compound deposits, including those formed by reactive methods, also benefit from bombardment, as shown .for TiN deposition by Matthews and Teer 4 and by Dugdale 5. More recently Fleischer e t al. 6 and Yoshihara and Mori 7 have also obtained improved TiN coating morphology under enhanced discharge conditions. In each of these studies the conventional d.c. diode discharge was augmented by a thermionic source and an anode. The evolved system has similarities to the biased activated reactive evaporation technique reported by Suri e t al. s Other workers have used various techniques to improve ionization. The hollow cathode electron beam (EB) gun has been found to give greater ionization than the hot filament EB gun and has been used by Sato e t al. 9 for reactive TiN deposition. Murayama 1° has deposited TiN by using an r.f. coil electrode to enhance the d.c. diode discharge. The evidence of research therefore supports the view that increased ionization is vital for effective low temperature TiN reactive ion plating. This conclusion is confirmed by the lower success rate of ceramic coatings formed at low ionization and temperature 11,12. Thus while a review of the available literature indicates that increased ionization is necessary, little information is available on the exact level * Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 21-25, 1980.
542
A. MATTHEWS, D. G. TEER
required. The objective of this paper is to improve this situation by outlining the results of coating trials which have been conducted by the authors. 2. EXPERIMENTAL DETAILS
2.1. Equipment In our laboratory we have been studying for some time the potential of reactively ion-plated Ti-N compounds for wear reduction, particularly on metal forming tools. The requirement in this application is to deposit at a high rate (1 Ixm min-1) whilst avoiding excessive substrate heating. Furthermore, the equipment must be suitable for a production environment and must avoid the use of techniques and devices which necessitate specialist knowledge. Figure 1 shows the deposition arrangement adopted. O-$KV O-IA
WATER
FLOW THERHOCOUPLE~ t
T II
o-3zv
h
"T "
[ WEEDtE FLOW
'1 . I
V,L~S
h
k--1
i
'~' ILl ~SWITCH "eU~PS /
.E__TEnS
==0=--
'
''''°''
__L 0-,00v
I_.e!..l °'o'# ~ O-60V --F-
o-zs,
Fig. 1. A schematic diagram of the thermionically assisted triode coating system.
The system chosen for discharge enhancement employs a positive electrode (or "probe") which can extract electrons generated in the vapour source region to increase ionization. Arc suppression is incorporated in the probe supply in the form of an inductive resistance on the output. To improve controllability and to permit a smooth transition from the etch to the deposition stages, as well as to increase the ionization, a hot filament electron source is also incorporated. This type of system was first suggested for ion plating by Baum 13 and the arrangement is similar to that used in triode-assisted sputtering 14. Titanium is vapourized by a 225 ° bent beam EB gun. There are two chamber regions, each pumped by a separate diffusion pump, with a single backing rotary pump. This arrangement permits the operation of the EB gun in the lower chamber and allows ion-plating pressures of over 30 mTorr in the upper chamber. The specimen support cathode is water cooled and can carry industrial tools weighing over 20 kg. The source-to-specimen distance is nominally 30 cm.
2.2. Procedure The results to be reported here were obtained at a standard specimen bias of
DEPOSITION OF
TiN
COMPOUNDS BY REACTIVE ION PLATING
543
- 4 kV. Early trials used either an argon or an N2-Ar pre-etch followed by the evaporation of titanium in a nitrogen discharge. This procedure led to the formation at the interface of a dark layer with poor adhesion. In subsequent coatings, therefore, a thin (0.5 Inn) titanium layer was deposited in an argon discharge prior to bleeding nitrogen in at the desired rate. In view of the preferential removal of nitrogen it was important to maintain a constant pumping speed from run to run in order to make reactive gas flow requirements consistent; this was achieved by baffling back the pumping speed to maintain 15 m Torr at an argon flow of 6 cm 3 min-1. Gas flows were monitored with flowmeters which were calibrated for air at STP. The supply gauge pressure was 260 Torr and most of the pressure drop to vacuum occurs at the needle valves. The flow measurement system described is typical of that used in many laboratories and results will therefore be quoted as read on these meters. The chamber pressure was controlled by altering the argon flow. Mass spectrometry facilities were not available. Although a constant gun power of 4 kW was adopted throughout, it was found that the evaporation rate (as determined retrospectively by billet weight loss) varied, severely limiting the predictability of the process. This emphasized the need for real time evaporation rate monitoring for reactive ion plating. Typically a sputter-cleaning time of 30 min was used, with a pressure of 15 m T o r r and a current density of 0.5 mA cm-2. When intensification from the probe and thermionic filament was required to achieve this current density, e.g. when large specimens were being coated, the switch in Fig. 1 was used in position 1. Thus the filament was at earth potential and the anode at supply voltage. Under evaporation conditions, though, enhancement was invariably greater if the switch was in position 2. The cause for this appears to be related to the high electron concentration occurring when both the support filament and the EB gun are operating. In position 2, with the filament and probe electrically floating, the probe tends to take up earth potential. The whole chamber then acts as an anode to the filament. 3.
THEORETICAL CONSIDERATIONS
In this paper we shall use the concept of ionization efficiency in ion plating, i.e. the percentage of the atoms bombarding the cathode that are ionized. The ionization efficiency can be estimated by neglecting any secondary electron current from the cathode, converting the specimen current density to a unit charge flow and dividing by the total number of bombardments per unit area. The latter figure can be obtained from kinetic gas theory. It was found more convenient to use for this purpose the overall chamber pressure as measured with an'ionization gauge remote from the sample. An estimate of the magnitude of the effect of unaccounted depositing atoms can be made by neglecting sputtering and converting the deposition rate to an atom arrival rate. A deposition rate of 1 pm rain- 1 represents about 0.1 × 1018 atoms c m - 2 s- 1. For comparison, 10 mTorr and 1 mTorr represent 4.3 x 1018 and 0.43 x 1018 atoms cm -2 s -1 respectively is. Thus the potential error from using the overall chamber pressure for ionization efficiency calculations varies from about 2.39/0 at 10 mTorr to 239/0 at 1 m T o r r and 230~ at 0.1 mTorr. In fact at lower pressures the deposition rate defines the maximum ionization efficiency achievable at a fixed current density. The maximum efficiencies for a deposition rate
544
A. MATTHEWS,
D. G. TEER
of 1 ~trn m i n - 1 at 1, 2 and 3 mA c l r 1 - 2 would therefore be about 6%, 12% and 19% respectively. However, in the usual range of ion-plating pressures and deposition rates, the error due to making an estimate based on a reading remote from the sample is acceptable. If we adopt the pressure as a measure of surface bombardment, the variation in ionization efficiency at a constant specimen current density is as shown in Fig. 2.
z.0
o_ 1.0
3mAcro-z
I \ \~
/
S
2.At.-' I0
IS
20
eNMMSERPRESSURE[mTorr]
Fig. 2. The variation of ionization efficiency ~ t h pressure at constant current densities.
/ 1"4 NITROGEN-DEFICIENTTiREGION// 1"2
'~,o '~'
Ti2N
/
REGION (>0"6mACm-I / REQUIRED)
o.,
/
0-_
/
0., ~
/
TiN
/ / REGION /
/
/
"/(>0.4mAcm-Y RE*UIRE~
/
//.
.OESS N.RO~EN
R ,OTN,N-00,TIN0S F,E ,RG'E
-
O'4
0.2
0
I
I0
I
I
I
I
I
20 30 40 SO 60 .JTROGE" ,NttT FLOWRATE CsTP) [c.' . i . - ' ]
I
70
I
$0
Fig. 3. Summary of the compositions produced across the range of titanium evaporation rates and nitrogen inlet flow rates. The current density requirements refer to chamber pressures below 2 mTorr; higher pressures require an increasedcurrent density. 4. RESULTS After a series of calibration trials a clear pattern emerged of the nitrogen flow required for TiN formation at different titanium evaporation rates. Basically this
DEPOSITIONOF T i - N COMPOUNDSBY REACTIVEION PLATING
545
was an indicated 55 cm 3 m i n - 1 of nitrogen for each gram of titanium evaporated per minute. Figure 3 shows the range of compositions produced on either side of this value. The achievement of the balance required did not necessarily lead to the compound shown or even to a dense coating. These were both critically influenced by the specimen current density, in the manner outlined in the following. Within the region to the left of the bold line, at current densities insufficient for Ti2N or TiN formation, it was possible to deposit coatings which gave a titanium Xray diffractometer pattern. These had increasing hardnesses as the interstitial nitrogen content increased, over 1200 H K (Knoop) being possible. The preferred orientation was (002) and structures became more dense as the current density increased and as the chamber pressure decreased. In the region in Fig. 3 to the right of the bold line, powdery TiN deposits resulted, unless the ion current density exceeded a minimum level which increased with pressure, in which case dense TiN could be formed within the band shown. Examples will be given of the ionization enhancement required for the production of non-friable TiN deposits at various pressures. Coatings deposited at 2 mTorr required a minimum current density of 0.4 mA cm-2. These showed a microhardness of over 1800 H K ; their structure is shown in Fig. 4. They had a (111) preferred orientation. Increasing the current density to 2.2 mA cm -2 under otherwise similar conditions produced a harder more dense structure with a (200) texture (Fig. 5). At 7 m T o r r a minimum current density of 1.2 mA c m - 2 was required for nonpowdery deposition. Figure 6 shows a typical structure. The preferred orientation was (111). Increasing to over 2.0 mA c m - 2 again densified and hardened the coating, though maximum hardness ( > 2500 HK) was achieved above 2.5 mA cm-2. Ti2N was detected in the hardest of these films. Frequently, higher chamber pressures are required to improve coating coverage through gas-scattering effects. The highest
Fig. 4. Fracture sectionof TiN coating depositedat 2 mTorr and 0.4 mA cm- 2.
546
A. MATTHEWS, D. G. TEER
Fig. 5. Fracture section of TiN coating deposited at 2 mTorr and 2.2 mA c m - 2.
Fig. 6. Fracture section of TiN coating deposited at 7 mTorr and 1.2 mA cm -2.
pressure at which this was attempted for TiN deposition was 15 mTorr. A current density of 1.8 mA crn- 2 was required in this case. The preferred orientation was (200) and hardnesses over 1800 H K were obtained. The structure is shown in Fig. 7. In the region shown in Fig. 3 for Ti2N deposition a similar relationship between structure, hardness and ionization to that for TiN was found. For example, for Ti2N deposition at 1 mTorr the required current density was 0.6 mA cm-2, whereas at 13 mTorr over 1.9 mA c m - 2 was necessary. Again even higher current densities tended to produce harder and denser coatings. Figure 8 shows a TizN coating deposited at
DEPOSITIONOF T i - N COMPOUNDSBY REACTIVEION PLATING
547
2.6 mA c m - 2 and 2 mTorr. The preferred orientation is (002) and the hardness of the coating is over 2500 HK. Although this was deposited in the Ti2N region in Fig. 3, peaks of TiN were also observed. In fact TiN could be formed outside the band indicated in Fig. 3, though this required increased current density as the balance condition moved to the left of the bold line. Ti2N was always present in such coatings.
Fig. 7. Fracture section of TiN coating deposited at 15 mTorr and 1.8 mA cm-2.
Fig. 8. Fracture section of Ti2N plus TiN coating deposited at 2 mTorr and 2.6 mA c m - 2 . As stated earlier, the intended usage for coatings developed on this p r o g r a m m e is for metal-forming tools, which m a y soften at elevated temperatures. With this in
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mind the trials were performed on a typical tool steel (AISI H13). Bulk hardness measurements and practical tests after coating showed that unacceptable softening had not occurred and therefore that temperatures had been kept below 600 °C; thermocouple readings also confirmed this. Low temperatures were achieved with the assistance of water cooling, which was particularly vital at current densities above 1.5 mA cm- 2 at a bias of - 4 kV. Tribological studies of coatings developed in this programme included pin-ondisc tests and simulated metal-forming operations 16. In the former test a hardened steel pin with a tip radius of 3.175 mm was rubbed against the coating on a mild steel substrate under a load of 0.5 kg at 100 rev m i n - 1 on a wear track 1 cm in diameter. No wear was detectable on optimized coatings after 12 000 passes, though the pin was worn by over 0.25 mm radially. Coating wear was evaluated using scanning electron microscopy on fracture sections of wear tracks. The true wear was thereby recorded, rather than the depression of the substrate due to loading. 5. DISCUSSION AND CONCLUSIONS
This work has shown that, provided the depositing species are correctly balanced, the pressure and the current density are the dominant variables in reactive ion plating (at a given bias voltage). As stated in Section 3, these combine to give the parameter termed the ionization efficiency. Across the pressure range used it was found impossible to form cohesive TiN at ionization efficiencies less than about 0.3%. An ionization efficiency of 0.3% was necessary again when, at lower nitrogen pressures, Ti2N was formed; though when the total pressure was low there was evidence that even greater ionization efficiencies were required for Ti2N. The deposits formed at ionization etticiencies less than the critical level consisted either of unreacted titanium with interstitial N2 or of powdery TiN, depending on whether the atomic ratio of Ti: N was to the left or the right of the bold line in Fig. 3. The effect of increasing the ionization efficiency further was to promote very dense (zone 3 type 17) structures and high hardnesses (near 3000 HK). The ionization efficiency therefore appears to offer a good criterion for process optimization in reactive ion plating. It may not, however, take full account of all variables. One of these is the specimen bias, which could be incorporated as the input power density to form a suitable criterion. However, this requirement would still vary with pressure. Another variable which needs to be taken into account is the deposition rate. Dugdale 5 sets as a criterion for dense deposits that almost all the condensing atoms should be resputtered, stating that at high deposition rates this necessitates increased bias power. He uses rates from about 0.5 ~tm m i n - 1 (with a sputter source) to over 1 ~tm m i n - 1 (with an EB gun source). In the context of our model, deposition rates within this range would not be expected to require a significantly greater ionization efficiency at the pressures above I0 mTorr that have been quoted. Dugdale's work therefore suggests that the deposition rate may have a more important influence than is evident from our researches. We may conclude that there is no single parameter which can be said fully to define reactive ion-plating requirements. The ionization efficiency, however, still has distinct advantages over most others, as it takes into account the total number of
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incident atoms (which in effect share the deposition energy) and in our range of operating variables it proved an effective criterion for predicting coating properties. Work to evaluate the effect of decreasing specimen bias at high current density, and therefore high ionization efficiency, is continuing. The results of Koboyashi and Doi 1s indicate that dense coatings can still be produced at low bias levels. They use extremely low gas pressures (<4.5 x 10 -4 Torr). Although ion currents are not given, the discharge support system and low pressure used suggest high ionization efficiencies. Moreover, the ion energy spectrum will include more ions near to the full acceleration energy than in higher pressure deposition. Evidence obtained at low pressure thus suggests that biases of several kV are not required, but rather that some much lower voltage is sufficient, provided that ionization can be increased by suitable means. This should permit deposition at low power input and therefore obviate the need for forced cooling. ACKNOWLEDGMENTS
The authors are grateful to the Science Research Council for funding this project and for providing one of us (AM) with financial support. Thanks are also due to Professor J. Hailing for continuing enthusiastic encouragement. Miss P. E. Bond provided assistance with scanning electron microscopy. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
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