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Comparison of the structure of PVD-thin films deposited with different deposition energies
SURfACE
&GOAR'IiS
ELSEVIER
Surfaceand CoatingsTechnology 86-87 (1996)177-183
IlGHNOLDGY
Comparison of the structure of PVD-thin films deposited with different deposition energies E. Lugscheider *, C. Barimani, C. Wolff, S. Guerreiro, G. Doepper Materials ScienceInstitute, Aachen University of Technology, Templergraben 55, D-52056 Aachen, Germany
Abstract
Under the various available PVD processes, thin films can be deposited with higher or lower deposition energy, e.g., with or without ion bombardment. Related to this deposition energy the structure and consequently the properties of the deposited films can be directly influenced. The wide range of possible deposition parameters for the PVD-processes enables the use of materials in form of thin films in a large scope of applications, as a result of the different properties which can be achieved. Particularly high adhesion of thin films is always desired, which depends among other things on the microstructure of coatings. The aim of this paper is to compare three different PVD deposition processes: cathodic arc evaporation, magnetron sputtering and electron beam evaporation. These PVD processes are related to their film structure produced under three different conditions and studied in terms of their deposition energies. Structure and morphology of the coatings are compared at identicaltemperatures resulting in a model, which is suggested to explain how excitation of deposited films takes place. Besides condensation effects, the momentum transfer of ions caused by biasing the substrates is obviously important and leads to film densification without increasing the substrate temperature. Keywords: PVD; Structure; Deposition energy; erN-coatings
1. Introduction Physical vapor deposition (PVD) processes are increasingly used for the deposition of various coatings on tools and components, e.g., to improve wear behavior or corrosion resistance [1,2]. In addition to others, Chromium nitride (CrN) is one of the popular and universally used coating systems. The desired properties of such coatings mainly depend on their microstructure, which can be directly influenced by varying the level of deposition energy during the deposition process. Therefore, the presence of ion bombardment in addition to heating up the substrates plays an important role in thin films microstructure. Depending on the type of PVD process and the target materials, different degrees of ionization of the particles and different quantities of ions in the vapor phase during deposition have been reported [3-5]. However, several structure zone models have been developed [6-11] to
* Corresponding author. Rheinisch-Westfaelische Technische Hochschule Aachen, Lehr- und Forschungsgebiet Werkstoffwissenschaften, Augustinerbach 4-22, 52056, Germany. Tel.: 49 241 805329; fax: 49 241 8888264. 0257-8972/96/$15.00 © 1996ElsevierScience SA All rights reserved
PI! S0257-8972(96)03041-1
determine the important parameters which may influence the resulting microstructure of coatings, especially for sputtering and electron beam evaporation. As a main parameter the ratio (TITnJ between the deposition temperature (T, in K) and the coating melting point (Tm , in K) has often been reported. Nevertheless, no comparison of the basic PVD processes has been carried out up to now, considering the influence of ion bombardment. In this paper the resulting microstructure of thin films deposited by cathodic arc evaporation, sputtering and electron beam evaporation are compared related to the different predominating deposition energies, using erN as a modern coating system.
2. Experimental details Standard PVD processes, such as cathodic arc evaporation, sputtering and electron beam evaporation, have been used to deposit the coatings. Each of these deposition methods represents a special state relating to deposition energy, e.g., partial pressure, kinetic energy and momentum transfer of vapor and gas atoms, cherni-
178
E. Lugscheider et al./Surfaceand Coatings Technology 86-87 (1996) 177-183
cal reactions, quantity and ionization degree of ionized target atoms as well as intrinsic plasma motion. Table 1 gives an overview of typical parameters and the quantity of ionized target atoms of some PVD processes [3]. The arc PVD-thin films were deposited in a Multi Arc PVD 20" system equipped with a random arc source. The coating plant is a commercially established system, which is able to evaporate several metallic materials with up to four targets. The CrN coatings had been deposited in an reactive gas atmosphere (nitrogen). The arc PVD-thin films were first carried out, because this is the process showing highest energy caused by the high quantity of ionized vapor particles, about 80% for Cr [12], as well as the mean ionization degree of 2 [13], using 0, 50, 100 and 200 V d.c. bias voltage. The resulting temperatures were 250, 330, 400 and 500aC, measured by a thermocouple at the backside of the 2-mm thin samples. Prior to deposition the substrates had been bombarded with chromium ions, which enhanced the cleanness of the surface. In the case of magnetron sputtering, only 5% of the target material is ionized, whereas the rest is made up of inert gas (argon) and reactive gas (nitrogen) ions. Therefore the energy during deposition, especially for reactive sputtering, must be lower than in arc evaporation because the mean number of the participating ions show lower atomic masses which leads to a decrease of momentum transfer. The investigations were performed in a Leybold Heraeus Z 400 Magnetron Sputter facility (laboratory equipment). The substrates were coated under reactive conditions using a commercially available chromium target. The deposition mode was d.c., whereas the biasing of the substrates was practiced under r.f. conditions. To realize the same substrate temperatures and deposition conditions, first the reachable temperatures, which could be achieved only by biasing the substrates with the same voltage values as in the arcPVD experiments, were measured. Resulting temperatures were 115, 145, 175 and 21O aC, measured as described above. The differences in temperature were balanced by using a resistance heating system, which enabled the same temperature values as reported for the arc-PVD experiments. In the case of sputtering, the
substrates were cleaned by r.f-sputtering with argon ions. The electron beam evaporation was used as the deposition process with the most obvious lowest deposition energy. In this case the quantity of ionized vapor particles is reported as less than 1% [13]. The depositions were carried out in a modified CemeCon CCSOOE electron beam evaporation laboratory apparatus. A specific feature of this plant is a second electron beam gun for heating up the substrates directly with electrons. The substrates were only heated in this way up to the temperature values as reported above, and no bias voltage was applied. Therefore only a thermal excitement of the condensing material took place. The evaporation material was a commercially available Cr2N powder, which was reactively evaporated by adding nitrogen during deposition. Prior to coating, the surfaces of the substrates were electron bombarded. The substrate material was a typical HSS, namely S 6-5-2. The specimens were polished with a 6-llffidiamond paste. In this context it should be noted that the melting point of CrN is low enough (approximately 1740aC= 2013 K) to reach a zone 3 microstructure which is reported in Ref. [6]. Considering the temperature when the substrate S 6-5-2 starts loosing its hardness (560 aC= 833 K), with CrN as coating system, a ratio of T/Tm =0.41 can be realized when only thermal excitement takes place during condensation. This value marks the beginning of the above-mentioned zone 3. However, in the experiments we attempted to enable comparability of the applied PVD processes, although the apparatus geometry had a limiting effect. For example, the distance between target and substrate was, in the case of sputtering, only 40 mm, whereas for arc and electron beam evaporation it was about 190 mm. This will be considered in Section 3. Table 2 gives an overview of the various parameters. 3. Results and discussion The investigation of the microstructure was carried out by high resolution scanning electron microscopy.
Table 1 Typical parameters and quantity of target ions of different PVD processes [3] Parameters
Evaporation tool Phase transformation Geometry of target/cathode Quantity of ionized target atoms (%) Additional ionization Inert gases necessary Reactive deposition possible
Processes Magnetron sputtering
Anodic arc ion plating
Electron beam ion plating
Cathodic arc ion plating
sputter effect solid-vapor any is possible
electron beam solid-vapor limited
thermal arc solid-vapor any is possible
aimed yes yes
unusual no yes
electron beam liquid-vapor limited <1 aimed variable yes
1-5
5-40
50-100
not necessary no yes
179
E. Lugscheider et al. / Surface and Coatings Technology 86-87 ( 1996) 177-183
Table 2 Processing parameters of the used PVD processes Parameters
Processes EB-PVD
Quantity of ionized target atoms (%) Target to substrate distance (mm) Evaporation material Inert gas Reactive gas Film thickness (J.ll11) Resulting pressure during deposition (Pa)
<1 [13]
190
Cr 2N N2 3.5
0.2
The aim was to examine the dominant parameters which are mainly responsible for the different microstructures occurring, depending on the energy level of the condensing material; Fig. I and Fig. 2 show the observed microstructures. Each figure compares the resulting microstructures for electron beam (EB) evaporation, sputtering and arc evaporation directly with respect to the different deposition temperatures: 250, 330, 400 and 500°C. The SEM images are arranged in such a way as to show the deposition energy increasing downwards. On the SEM images it can be seen that, with increasing temperature for each PVD process, the microstructure of the thin films tends from a columnar form towards a more "dense and uniform" structure. Moreover, the surface of the coating becomes more and more smooth with increasing substrate temperatures. This effect is strongly developed for the arc-PVD coatings. At lower temperatures, embedded droplets lead to defects in film uniformity and preferential growth of crystallites, as can be seen in Fig. 1(c). The arrow marks the pinhole where a crystal was dislocated from its original growing place. With increasing temperature and ion energy, this preferential growth is suppressed and the surface becomes smoother. These observations were confirmed by SEM images with a high magnification, on which the internal microstructure of the thin films could be impressively noticed. This is shown in Fig. 3 where the microstructure of the electron beam and arc evaporated coatings confirm the supposition, while the substrate temperature was 500°C for both. Comparison of the different PVD processes for each temperature level shows a densification of thin films with increasing condensation energy. Whereas the electron beam evaporated coatings have generally columnar structures, the sputtered and arc evaporated coatings show a more fibrous and "dense" microstructure. Obviously, the method of exciting the films during deposition plays a more important role than substrate temperature. In the SEM image in Fig. 3(a) it can also be observed that the EB-PVD deposit shows a transition from a columnar structure to recrystallization effects. In addition to some columnar crystallites, crosswise growth
Sputtering
Arc-PVD
40
5 [3]
80 [12] 190
Cr
Cr
N2 2.5
N2 5 2
Ar 1.3
of grains across the columnar crystallite boundaries can be observed simultaneously. Compared with this, the microstructure of the arc-PVD thin films has no distinctive grain boundaries, although both coatings were deposited at the same temperature. However, it can be seen that there are different ways to realize an energetic excitement of PVD thin films during deposition. The qualitative presentation of the assumed excitement processes is suggested in Fig. 4. At the same temperature, the resulting energies and the deposition temperatures are shown for the different PVD processes. The different values may be read either from the resulting energy or the temperature scale, which is indicated by the arrows. The basic bars up to 250°C represent the influence on the temperature, whereas in the case of sputtering and arc evaporation, the bars above represent the effect on film densification. It has been considered that the ratio of both bars results in an approximated calculation (shown later in this paper). Although in Fig. 4 the deposition temperature, for example, is 250°C, the excitation processes are basically the same for each temperature value. In Fig. 4, it can be seen that, on one hand, there exists a thermal excitement, which can be found on temperature measurement. On the other hand, an energetic supply of the condensing films can be ascertained by investigating the resulting microstructure. The latter observation is suggested because the coatings show a "denser" microstructure caused by an increasein energy levelof the condensing materials at the sametemperature levels. As discussed above, the quantity of ionized coating particles, and therefore their atomic masses, are higher for arc evaporation, so that such deposited thin films show very "dense" structures. This should be seen in context to the lower energy PVD processes, sputtering and electron beam evaporation, where coatings show a more columnar structure. Therefore the effect of ion bombardment on the growing microstructure may be divided into two parts: on one hand thermal excitement caused by condensation effects, and on the other hand densification of coatings caused by momentum transfer of energetic ions.
E. Lugscheider et al.fSurface and Coatings Technology 86-87 ( 1996) 177-183
180
(a)
(b)
(c)
Fig. 1. Resulting microstructures at deposition temperatures of 250 and 330°C for (a) electron beam evaporation, (b) sputtering, (c) arc evaporation.
Thornton named the densification effect "forward peening" [14], which leads to enhanced adatom migration and bulk diffusion [15]. This is 0 bviously more effective when a high quantity of ionized particles of the coating material is available. The ratio between the atomic mass of the target ions and other ions participating in condensation, e.g., inert or reactive gases, appears to play an important role. This effect has been approximately
estimated with the following formula, showing the atomic mass ratio (r) of the participating ions during deposition: quantity of target ions' atomic mass + quantity of gas ions'
r=-----------------
atomic mass atomic mass of target material
E. Lugscheider et al.jSurface and Coatings Technology 86-87 (1996) 177-183
181
(a)
(b)
(c)
Fig. 2. Resulting microstructures at deposition temperatures of 400 and 500°C for (a) electron beam evaporation, (b) sputtering, (c) arc evaporation.
1
'ED-PVD= -=0.02 52 0.05· 52+0.75 ' 40+0.20 ' 28 'sputtering
I'l\rc-PVD=
52 0.8' 52+0.2' 28 52
=0,91
0.73
Atomic masses: Cr=52, Ar=40, N 2=28. Conditions : atom/ion ratio = 1; degree of ionization = 1 (single charge). In the case of arc evaporation this ratio is 0.91, because most of the participating ions are made up of chromium. The sputtering ratio isjust 0.73 because most of the ions are gas ions with less atomic mass, whereas for electron beam evaporation the ratio is nearly zero,
182
E. Lugscheider et al.lSurface and Coatings Technology 86-87 ( 1996) 177-183
T [OCI
E [any unit] Deposition temperature 250°C
5
400 (a)
4 3
.- '"
.- '"
.- .-
300
II
200
,
100
I
0
i
2
~
EB-PVD
Sputtering
Arc-PVD
mom en tum transfer of ions (film den sifica tion)
(b)
therm al excitement by healing thermal excitem ent by ion bombardment (condensation effects ) E ~ resulting ene rgy
Fig. 4. Schematic diagram of the influencing parameters for the energetic excitement of PVD thin films during deposition.
Fig. 3. Internal microstructure at a deposition temperature of 500°C for (a) electron beam evaporation, (b) arc evaporation.
since there is nearly no ionization. This is shown in Fig. 4 by the bars representing the film densification. The ratios of the different species were determined by measuring the partial pressures. Compared with the above-mentioned structure zone models, no distinctive analogies are observed, particularly in the case of sputtering and arc evaporation. One reason may be that the substrate temperature range in this investigation was to small. On the other hand, the ion bombardment may cause another form of energetic excitement of the reactive processes compared to only heated substrates. Beyond that, the microstructures of the sputtered films are nearer to the arc-PVD thin films, than to the electron beam evaporated coatings. This may also be explained by the above-suggested influence of the atomic mass ratio between the participating ions. A tendency from columnar crystallization towards a microstructure like bulk material can be reported, depending directly on the energetic excitement level of the condensing film because of the increasing ada tom mobility. In addition to the above-discussed observations,
differences in interface adhesion can be mentioned. The EB-PVD coatings show lower adhesion to the substrates than for arc evaporated layers. The best interface can be observed for sputtered thin films (see Fig. I and Fig. 2). Therefore, ion cleaning before deposition seems to play an important role for interface bonding; r.f.sputter cleaning is the best way to achieve a good adhesive interface zone. 4. Conclusions A comparison of the microstructure of thin films deposited by different PVD processes was carried out. At the same temperature levels, ion bombardment was added in the case of two PVD processes, to investigate the effect of energetic excitement on the forming of thin film microstructure. With increasing deposition temperature on the one hand and increasing quantity of ionized target atoms on the other, the microstructure tends from a columnar form to a structure like bulk material. This is accompanied by a smoothing of the surface coating. As an attempt at an explanation, the effect of ionization has been approximately estimated using a formula for the ratio of participating ions. The resulting ratio values were in good agreement with the SEM investigations,
E. Lugscheider et al.jSurface and Coatings Technology 86-87 ( 1996) /77-183
because the ratios for sputter- and arc-PVD are similar as are their resulting microstructures. In contrast to this, the energy supply of the EB-PVD thin films is obviously much lower than the determined ratio. It can be seen that there is a direct dependence on the method of exciting the deposited films. Two effects are suggested when ion bombardment is used. On one hand there is a thermal excitement caused by condensation effects, on the other hand the momentum transfer of ions is responsible for a densification of coatings, without resulting in an increase of deposition temperature. In the present work, the different effects could not be quantified. Further work needs to be carried out on the effects taking place during deposition.
Acknowledgement
The authors would like to thank Dr. W. Rehbach and N. Cammainadi, GfE-RWTH Aachen, for their support and performance concerning the high resolution scanning electron microscope images.
183
References [I] J. Vetter and A.J. Perry, Surf. Coat. Technol., 61 (993) 305-309. [2] O. Knotek, W.o. Munz and T. Leyendecker, 1. Vae. Sci. Technol., A 5(4) (1987) 2173. [3] C. StoBel, Doctorial thesis, Aachen University ofTechnology, 1991. [4] A. Plyutto, A.V.N. Ryzhkov and A.T. Kapin, Sou. Phys. JETP, 20(2) (1965) 328-337. [5] W.D. Davies and RC. Miller, 1. Appl. Phys., 40 (5) (1969) 2212-2221. [6] B.A. Movchan and A.V. Demshichin, Phys. Metals Metallogr., 28(4) (1969) 83-90. [7] J.A. Thornton, 1. Vac. Sci. Technol., A 4 (6) {Nov.j'Dec. 1986). [8] R. Messier, A.P. Giri and R.A. Roy, 1. Vue. Sci. Technol., A 2 (2) (1984) 500-503. [9] J.V. Sanders, in J.R. Anderson (ed.), Chemisorption on Metallic Films 1, London and New York, 1971, pp. 1-38. [10] M. Lardon, R. Buhl, H. Signer, H.K. Pulker and E. Moll, J. Vae. Sci. Technol., A 2(2) (1984) 500-503. [11] C.R.M. Grovenor, H.T.G. Hentzell and D.A. Smith, Acta Metall.,32 (1984). [12] H.J. Scholl, Doctoral thesis, Aachen University of Technology, Verlag Mainz, Aachen, 1993. [13] R.F. Bunshah (ed.), Deposition Technologies for Films and Coatings, Noyes Publications, Park Ridge, NJ, USA, 1982. [14] J.A. Thornton, Thin Solid Films, 171 (1989) 5-31. [15] T. Tagaki, 1. Vae. Sci. Technol., A 2(2) (1984) 382-388.
&GOAR'IiS
ELSEVIER
Surfaceand CoatingsTechnology 86-87 (1996)177-183
IlGHNOLDGY
Comparison of the structure of PVD-thin films deposited with different deposition energies E. Lugscheider *, C. Barimani, C. Wolff, S. Guerreiro, G. Doepper Materials ScienceInstitute, Aachen University of Technology, Templergraben 55, D-52056 Aachen, Germany
Abstract
Under the various available PVD processes, thin films can be deposited with higher or lower deposition energy, e.g., with or without ion bombardment. Related to this deposition energy the structure and consequently the properties of the deposited films can be directly influenced. The wide range of possible deposition parameters for the PVD-processes enables the use of materials in form of thin films in a large scope of applications, as a result of the different properties which can be achieved. Particularly high adhesion of thin films is always desired, which depends among other things on the microstructure of coatings. The aim of this paper is to compare three different PVD deposition processes: cathodic arc evaporation, magnetron sputtering and electron beam evaporation. These PVD processes are related to their film structure produced under three different conditions and studied in terms of their deposition energies. Structure and morphology of the coatings are compared at identicaltemperatures resulting in a model, which is suggested to explain how excitation of deposited films takes place. Besides condensation effects, the momentum transfer of ions caused by biasing the substrates is obviously important and leads to film densification without increasing the substrate temperature. Keywords: PVD; Structure; Deposition energy; erN-coatings
1. Introduction Physical vapor deposition (PVD) processes are increasingly used for the deposition of various coatings on tools and components, e.g., to improve wear behavior or corrosion resistance [1,2]. In addition to others, Chromium nitride (CrN) is one of the popular and universally used coating systems. The desired properties of such coatings mainly depend on their microstructure, which can be directly influenced by varying the level of deposition energy during the deposition process. Therefore, the presence of ion bombardment in addition to heating up the substrates plays an important role in thin films microstructure. Depending on the type of PVD process and the target materials, different degrees of ionization of the particles and different quantities of ions in the vapor phase during deposition have been reported [3-5]. However, several structure zone models have been developed [6-11] to
* Corresponding author. Rheinisch-Westfaelische Technische Hochschule Aachen, Lehr- und Forschungsgebiet Werkstoffwissenschaften, Augustinerbach 4-22, 52056, Germany. Tel.: 49 241 805329; fax: 49 241 8888264. 0257-8972/96/$15.00 © 1996ElsevierScience SA All rights reserved
PI! S0257-8972(96)03041-1
determine the important parameters which may influence the resulting microstructure of coatings, especially for sputtering and electron beam evaporation. As a main parameter the ratio (TITnJ between the deposition temperature (T, in K) and the coating melting point (Tm , in K) has often been reported. Nevertheless, no comparison of the basic PVD processes has been carried out up to now, considering the influence of ion bombardment. In this paper the resulting microstructure of thin films deposited by cathodic arc evaporation, sputtering and electron beam evaporation are compared related to the different predominating deposition energies, using erN as a modern coating system.
2. Experimental details Standard PVD processes, such as cathodic arc evaporation, sputtering and electron beam evaporation, have been used to deposit the coatings. Each of these deposition methods represents a special state relating to deposition energy, e.g., partial pressure, kinetic energy and momentum transfer of vapor and gas atoms, cherni-
178
E. Lugscheider et al./Surfaceand Coatings Technology 86-87 (1996) 177-183
cal reactions, quantity and ionization degree of ionized target atoms as well as intrinsic plasma motion. Table 1 gives an overview of typical parameters and the quantity of ionized target atoms of some PVD processes [3]. The arc PVD-thin films were deposited in a Multi Arc PVD 20" system equipped with a random arc source. The coating plant is a commercially established system, which is able to evaporate several metallic materials with up to four targets. The CrN coatings had been deposited in an reactive gas atmosphere (nitrogen). The arc PVD-thin films were first carried out, because this is the process showing highest energy caused by the high quantity of ionized vapor particles, about 80% for Cr [12], as well as the mean ionization degree of 2 [13], using 0, 50, 100 and 200 V d.c. bias voltage. The resulting temperatures were 250, 330, 400 and 500aC, measured by a thermocouple at the backside of the 2-mm thin samples. Prior to deposition the substrates had been bombarded with chromium ions, which enhanced the cleanness of the surface. In the case of magnetron sputtering, only 5% of the target material is ionized, whereas the rest is made up of inert gas (argon) and reactive gas (nitrogen) ions. Therefore the energy during deposition, especially for reactive sputtering, must be lower than in arc evaporation because the mean number of the participating ions show lower atomic masses which leads to a decrease of momentum transfer. The investigations were performed in a Leybold Heraeus Z 400 Magnetron Sputter facility (laboratory equipment). The substrates were coated under reactive conditions using a commercially available chromium target. The deposition mode was d.c., whereas the biasing of the substrates was practiced under r.f. conditions. To realize the same substrate temperatures and deposition conditions, first the reachable temperatures, which could be achieved only by biasing the substrates with the same voltage values as in the arcPVD experiments, were measured. Resulting temperatures were 115, 145, 175 and 21O aC, measured as described above. The differences in temperature were balanced by using a resistance heating system, which enabled the same temperature values as reported for the arc-PVD experiments. In the case of sputtering, the
substrates were cleaned by r.f-sputtering with argon ions. The electron beam evaporation was used as the deposition process with the most obvious lowest deposition energy. In this case the quantity of ionized vapor particles is reported as less than 1% [13]. The depositions were carried out in a modified CemeCon CCSOOE electron beam evaporation laboratory apparatus. A specific feature of this plant is a second electron beam gun for heating up the substrates directly with electrons. The substrates were only heated in this way up to the temperature values as reported above, and no bias voltage was applied. Therefore only a thermal excitement of the condensing material took place. The evaporation material was a commercially available Cr2N powder, which was reactively evaporated by adding nitrogen during deposition. Prior to coating, the surfaces of the substrates were electron bombarded. The substrate material was a typical HSS, namely S 6-5-2. The specimens were polished with a 6-llffidiamond paste. In this context it should be noted that the melting point of CrN is low enough (approximately 1740aC= 2013 K) to reach a zone 3 microstructure which is reported in Ref. [6]. Considering the temperature when the substrate S 6-5-2 starts loosing its hardness (560 aC= 833 K), with CrN as coating system, a ratio of T/Tm =0.41 can be realized when only thermal excitement takes place during condensation. This value marks the beginning of the above-mentioned zone 3. However, in the experiments we attempted to enable comparability of the applied PVD processes, although the apparatus geometry had a limiting effect. For example, the distance between target and substrate was, in the case of sputtering, only 40 mm, whereas for arc and electron beam evaporation it was about 190 mm. This will be considered in Section 3. Table 2 gives an overview of the various parameters. 3. Results and discussion The investigation of the microstructure was carried out by high resolution scanning electron microscopy.
Table 1 Typical parameters and quantity of target ions of different PVD processes [3] Parameters
Evaporation tool Phase transformation Geometry of target/cathode Quantity of ionized target atoms (%) Additional ionization Inert gases necessary Reactive deposition possible
Processes Magnetron sputtering
Anodic arc ion plating
Electron beam ion plating
Cathodic arc ion plating
sputter effect solid-vapor any is possible
electron beam solid-vapor limited
thermal arc solid-vapor any is possible
aimed yes yes
unusual no yes
electron beam liquid-vapor limited <1 aimed variable yes
1-5
5-40
50-100
not necessary no yes
179
E. Lugscheider et al. / Surface and Coatings Technology 86-87 ( 1996) 177-183
Table 2 Processing parameters of the used PVD processes Parameters
Processes EB-PVD
Quantity of ionized target atoms (%) Target to substrate distance (mm) Evaporation material Inert gas Reactive gas Film thickness (J.ll11) Resulting pressure during deposition (Pa)
<1 [13]
190
Cr 2N N2 3.5
0.2
The aim was to examine the dominant parameters which are mainly responsible for the different microstructures occurring, depending on the energy level of the condensing material; Fig. I and Fig. 2 show the observed microstructures. Each figure compares the resulting microstructures for electron beam (EB) evaporation, sputtering and arc evaporation directly with respect to the different deposition temperatures: 250, 330, 400 and 500°C. The SEM images are arranged in such a way as to show the deposition energy increasing downwards. On the SEM images it can be seen that, with increasing temperature for each PVD process, the microstructure of the thin films tends from a columnar form towards a more "dense and uniform" structure. Moreover, the surface of the coating becomes more and more smooth with increasing substrate temperatures. This effect is strongly developed for the arc-PVD coatings. At lower temperatures, embedded droplets lead to defects in film uniformity and preferential growth of crystallites, as can be seen in Fig. 1(c). The arrow marks the pinhole where a crystal was dislocated from its original growing place. With increasing temperature and ion energy, this preferential growth is suppressed and the surface becomes smoother. These observations were confirmed by SEM images with a high magnification, on which the internal microstructure of the thin films could be impressively noticed. This is shown in Fig. 3 where the microstructure of the electron beam and arc evaporated coatings confirm the supposition, while the substrate temperature was 500°C for both. Comparison of the different PVD processes for each temperature level shows a densification of thin films with increasing condensation energy. Whereas the electron beam evaporated coatings have generally columnar structures, the sputtered and arc evaporated coatings show a more fibrous and "dense" microstructure. Obviously, the method of exciting the films during deposition plays a more important role than substrate temperature. In the SEM image in Fig. 3(a) it can also be observed that the EB-PVD deposit shows a transition from a columnar structure to recrystallization effects. In addition to some columnar crystallites, crosswise growth
Sputtering
Arc-PVD
40
5 [3]
80 [12] 190
Cr
Cr
N2 2.5
N2 5 2
Ar 1.3
of grains across the columnar crystallite boundaries can be observed simultaneously. Compared with this, the microstructure of the arc-PVD thin films has no distinctive grain boundaries, although both coatings were deposited at the same temperature. However, it can be seen that there are different ways to realize an energetic excitement of PVD thin films during deposition. The qualitative presentation of the assumed excitement processes is suggested in Fig. 4. At the same temperature, the resulting energies and the deposition temperatures are shown for the different PVD processes. The different values may be read either from the resulting energy or the temperature scale, which is indicated by the arrows. The basic bars up to 250°C represent the influence on the temperature, whereas in the case of sputtering and arc evaporation, the bars above represent the effect on film densification. It has been considered that the ratio of both bars results in an approximated calculation (shown later in this paper). Although in Fig. 4 the deposition temperature, for example, is 250°C, the excitation processes are basically the same for each temperature value. In Fig. 4, it can be seen that, on one hand, there exists a thermal excitement, which can be found on temperature measurement. On the other hand, an energetic supply of the condensing films can be ascertained by investigating the resulting microstructure. The latter observation is suggested because the coatings show a "denser" microstructure caused by an increasein energy levelof the condensing materials at the sametemperature levels. As discussed above, the quantity of ionized coating particles, and therefore their atomic masses, are higher for arc evaporation, so that such deposited thin films show very "dense" structures. This should be seen in context to the lower energy PVD processes, sputtering and electron beam evaporation, where coatings show a more columnar structure. Therefore the effect of ion bombardment on the growing microstructure may be divided into two parts: on one hand thermal excitement caused by condensation effects, and on the other hand densification of coatings caused by momentum transfer of energetic ions.
E. Lugscheider et al.fSurface and Coatings Technology 86-87 ( 1996) 177-183
180
(a)
(b)
(c)
Fig. 1. Resulting microstructures at deposition temperatures of 250 and 330°C for (a) electron beam evaporation, (b) sputtering, (c) arc evaporation.
Thornton named the densification effect "forward peening" [14], which leads to enhanced adatom migration and bulk diffusion [15]. This is 0 bviously more effective when a high quantity of ionized particles of the coating material is available. The ratio between the atomic mass of the target ions and other ions participating in condensation, e.g., inert or reactive gases, appears to play an important role. This effect has been approximately
estimated with the following formula, showing the atomic mass ratio (r) of the participating ions during deposition: quantity of target ions' atomic mass + quantity of gas ions'
r=-----------------
atomic mass atomic mass of target material
E. Lugscheider et al.jSurface and Coatings Technology 86-87 (1996) 177-183
181
(a)
(b)
(c)
Fig. 2. Resulting microstructures at deposition temperatures of 400 and 500°C for (a) electron beam evaporation, (b) sputtering, (c) arc evaporation.
1
'ED-PVD= -=0.02 52 0.05· 52+0.75 ' 40+0.20 ' 28 'sputtering
I'l\rc-PVD=
52 0.8' 52+0.2' 28 52
=0,91
0.73
Atomic masses: Cr=52, Ar=40, N 2=28. Conditions : atom/ion ratio = 1; degree of ionization = 1 (single charge). In the case of arc evaporation this ratio is 0.91, because most of the participating ions are made up of chromium. The sputtering ratio isjust 0.73 because most of the ions are gas ions with less atomic mass, whereas for electron beam evaporation the ratio is nearly zero,
182
E. Lugscheider et al.lSurface and Coatings Technology 86-87 ( 1996) 177-183
T [OCI
E [any unit] Deposition temperature 250°C
5
400 (a)
4 3
.- '"
.- '"
.- .-
300
II
200
,
100
I
0
i
2
~
EB-PVD
Sputtering
Arc-PVD
mom en tum transfer of ions (film den sifica tion)
(b)
therm al excitement by healing thermal excitem ent by ion bombardment (condensation effects ) E ~ resulting ene rgy
Fig. 4. Schematic diagram of the influencing parameters for the energetic excitement of PVD thin films during deposition.
Fig. 3. Internal microstructure at a deposition temperature of 500°C for (a) electron beam evaporation, (b) arc evaporation.
since there is nearly no ionization. This is shown in Fig. 4 by the bars representing the film densification. The ratios of the different species were determined by measuring the partial pressures. Compared with the above-mentioned structure zone models, no distinctive analogies are observed, particularly in the case of sputtering and arc evaporation. One reason may be that the substrate temperature range in this investigation was to small. On the other hand, the ion bombardment may cause another form of energetic excitement of the reactive processes compared to only heated substrates. Beyond that, the microstructures of the sputtered films are nearer to the arc-PVD thin films, than to the electron beam evaporated coatings. This may also be explained by the above-suggested influence of the atomic mass ratio between the participating ions. A tendency from columnar crystallization towards a microstructure like bulk material can be reported, depending directly on the energetic excitement level of the condensing film because of the increasing ada tom mobility. In addition to the above-discussed observations,
differences in interface adhesion can be mentioned. The EB-PVD coatings show lower adhesion to the substrates than for arc evaporated layers. The best interface can be observed for sputtered thin films (see Fig. I and Fig. 2). Therefore, ion cleaning before deposition seems to play an important role for interface bonding; r.f.sputter cleaning is the best way to achieve a good adhesive interface zone. 4. Conclusions A comparison of the microstructure of thin films deposited by different PVD processes was carried out. At the same temperature levels, ion bombardment was added in the case of two PVD processes, to investigate the effect of energetic excitement on the forming of thin film microstructure. With increasing deposition temperature on the one hand and increasing quantity of ionized target atoms on the other, the microstructure tends from a columnar form to a structure like bulk material. This is accompanied by a smoothing of the surface coating. As an attempt at an explanation, the effect of ionization has been approximately estimated using a formula for the ratio of participating ions. The resulting ratio values were in good agreement with the SEM investigations,
E. Lugscheider et al.jSurface and Coatings Technology 86-87 ( 1996) /77-183
because the ratios for sputter- and arc-PVD are similar as are their resulting microstructures. In contrast to this, the energy supply of the EB-PVD thin films is obviously much lower than the determined ratio. It can be seen that there is a direct dependence on the method of exciting the deposited films. Two effects are suggested when ion bombardment is used. On one hand there is a thermal excitement caused by condensation effects, on the other hand the momentum transfer of ions is responsible for a densification of coatings, without resulting in an increase of deposition temperature. In the present work, the different effects could not be quantified. Further work needs to be carried out on the effects taking place during deposition.
Acknowledgement
The authors would like to thank Dr. W. Rehbach and N. Cammainadi, GfE-RWTH Aachen, for their support and performance concerning the high resolution scanning electron microscope images.
183
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