- Email: [email protected]
Pulsed vacuum arc discharges on steered arc cathodes
Surface and Coatings Technology 116–119 (1999) 963–968 www.elsevier.nl/locate/surfcoat
Pulsed vacuum arc discharges on steered arc cathodes H. Fuchs a, *, K. Keutel b, H. Mecke a, Chr. Edelmann b a Faculty of Electrical Engineering, Institute of Electrical Engineering and Power Electronics, Otto-von-Guericke University of Magdeburg, Universita¨tsplatz 2, Magdeburg D-39106, Germany b Faculty of Natural Science, Department of Vacuum Science and Technology, Otto-von-Guericke University of Magdeburg, Universita¨tsplatz 2, Magdeburg D-39106, Germany
Abstract The processes d.c. arc, modified pulsed arc, d.c. steered arc and the combination of steered and pulsed arc were compared. Steered arc cathodes with different magnetic fields were used to obtain experimental results for spot movement, ion current, layer formation and droplet emission. The process combination of steered and pulsed arc was examined mainly. High-speed videos showed a clearly increased spot velocity and the enlargement of the eroded area. Measurements on total ion current verify the experiences of random pulsed arc. The angular distribution of ion current density led us to conclude a focused plasma column. In this case, the influence of the duty cycle is strongest. For example, the ion current density related to arc current in the perpendicular direction is increased by 35% for steered arc with pulses. Deposition experiments yielded an increased deposition rate and a reduced droplet density. Results available up to now need further confirmation. The influence of the magnetic field intensity has to be investigated systematically. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Droplets; Ion energy; Pulsed arc; Spot movement; Steered arc; Vacuum arc
1. Introduction The control of spot movement by external magnetic fields in vacuum arc deposition (steered arc) is a usual industrial procedure. One can make a distinction between low steered arcs which are only held on a certain cathode area and steered arcs with a clearly increased spot velocity on a defined track [2,8]. Some technological differences are caused by the varied emission characteristic compared to the random arc process. The emission of droplets is clearly reduced both in size and density [7]. In addition to the reduced ion energy one can determine a decreased layer deposition rate by about 30% [11]. Using movable magnetic systems there are advantages regarding cathode exploitation and the deposition of layer systems from segmented cathodes [5]. The modified pulsed arc process offers the possibility of increasing the spot velocity clearly [3]. A reduced droplet emission (about 15%, especially small droplets [4]) and an improved cathode exploitation goes along with this. The ion current density can be increased clearly by the pulsation of the arc current and the spatial * Corresponding author. Fax: +49-391-6712408. E-mail address: [email protected] (H. Fuchs)
plasma extension can be influenced by the pulse parameters (focused plasma column, dependence on pulse current) [6,9,10]. Measurements of the total ion current showed an increase of 30% (average) and 50% (maximum), respectively. By the use of high current pulses (1 kA, 100 ms) an ion charge increase by factor 1.7 and an ion velocity acceleration by 2–3 have been measured [6 ]. In comparison to the common steered arc process an increased ion energy and a deposition rate at an even more reduced build-in of droplets are expected by the combination of both processes, magnetically steered and modified pulsed arc. The adaptation to cathode geometry via magnetic field and pulse parameters represents a possible further technological advantage. In Ref. [7] for the steered arc a strong dependence of layer properties on arc current is remarked. The possibility of influencing the charge state and the formation of gas metal compounds by the magnetic field has been described by Anders and coworkers [1].
2. Experimental arrangement and arc mode The investigations were carried out in a commercial vacuum arc coating system with a cylindrical chamber
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 13 4 - 6
964
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968
Fig. 1. Experimental arrangement.
(diameter of 365 mm; height of 335 mm). The base pressure was less than 0.001 Pa. Nitrogen was used as reactive gas with a pressure of 1 Pa. Flat cylindrical arc cathodes (diameter of 65 mm, direct cooling, titanium with 99.8% purity) were mounted on the base plate of the chamber. The cathode was surrounded by a floating shield to prevent spots leaving the surface. Because of this arrangement and anode cooling, anode spots cannot exist and influence the ion current. For the investigations of the angular distribution of ion current density two square probes with an area of 1 cm2 were positioned at 0° and 45° with respect to the cathode normal direction. The distance to the cathode centre was 150 mm. For the measurements of the total ion current a sphere segment was used and the current was calculated afterwards assuming equal ion emission into a half-sphere above the cathode. All probes were negatively biased with respect to the anode (−150 V ). The spot movement was observed by use of a highspeed framing camera ( Kodak EktaPro HS 4540) with a maximum frame recording frequency of 40 500 pictures/s. Fig. 1 schematically shows the complete arrangement. Permanent magnets were inserted for the steered
movement of the cathode spots. Three cathodes with different shape and intensity of the magnetic field were prepared in this way. Fig. 2 shows the perpendicular component of the magnetic flux density for these three cathodes. A titanium cathode without magnets was inserted for the reference investigations of the random arc process. The known modified pulsed arc process (superimposition of a d.c. current with pulses) was used for the investigations. A newly developed electronic current source served as power supply which allowed the variation of the output parameters in a wide range (d.c. current 20–200 A; pulsed current 40–400 A, pulse frequency up to 20 kHz, current rise up to 160 A/ms). For the investigations presented here, especially the variation of pulse frequency and pulse duration (and duty cycle, respectively) has been carried out.
3. Results 3.1. Spot movement The high-speed videos were analysed with regard to the synchronous recorded arc current, arc voltage and ion current. The spots on the steered arc cathodes move on a track where the perpendicular component of the magnetic flux density (B) is zero (Fig. 2). Especially for cathode M3 a clearly defined circular path is visible. The high B gradient surrounding the B=0 area and the high maximum value of B (25 mT ) are supposed to be the reason. For M1 and M2 the spot behaviour corresponds to low steered arc (B =10 mT ). The spots are max able to cross the field centre. The pulsation of arc current causes an accelerated movement of the spots alternating around the ‘d.c. track’. The eroded area is expanded and the average velocity on the track increases.
Fig. 2. Perpendicular component of the magnetic flux density on the surface of the used steered arc cathodes.
965
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968 Table 1 Total ion current as a function of arc mode and process pressure (average arc current 120 A) Cathode
Arc process
Random
d.c. pulse
M1 steered
d.c. pulse
M2 steered
d.c. pulse
M3 steered
d.c. pulse
High vacuum (0.001 Pa) i
average max. (during the pulse) average max. (during the pulse) average max. (during the pulse) average max. (during the pulse)
For the modified pulsed arc process on random arc cathodes the undefined pulse start position is often criticised. The use of steered arc cathodes can be advantageous to this. 3.2. Ion current 3.2.1. Total ion current The total ion current (that means the ion current ratio of the arc current) increases by the pulsation of the arc current ( Table 1). No clear influence of pulse frequency and duty cycle could have been determined. The highest absolute values were measured for cathode M3. The average ion current ratio is 17% (average) and 56% (maximum, during pulse) for this. As a reference, about 10% can be given for the d.c. process. Due to the non-ideal probe arrangement (main probe area on the side of the cathode) some problems occurred
10.16 13.22 25.50 12.88 15.35 32.36 10.98 13.92 38.95 11.71 16.33 56.23
/i (%) ion arc
With N (1.0 Pa) i /i (%) 2 ion arc 9.40 10.22 16.85 10.57 12.73 28.50 9.40 12.06 33.66 12.03 17.44 55.12
(interference of pulse frequency and velocity of circulation of the spots). 3.2.2. Distribution of ion current density The reference for the investigations is the d.c. process with the same average arc current. Cathode M1 represented the steered arc process. No significant influence of current pulsation was observed in the 45° direction. However, there was a clear rise in the 0° direction (up to 27% for random arc and 35% for steered arc). This indicates a very focused plasma column. Fig. 3 shows the dependence of ion current density on the pulse repetition frequency. In the case of the random arc, the ion current density increases with higher pulse repetition frequency. For the steered arc no corresponding connection could have been determined. The increase of the duty cycle (and therefore the average arc current) leads to a rise of ion current density.
Fig. 3. Ion current density related to arc current as a function of pulse frequency (high vacuum, pulse duration: 500 ms).
966
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968
Fig. 4. Ion current density related to arc current as a function of pulse duration (high vacuum, period: 3 ms).
That is both for the random and steered arc (Fig. 4). At the steered arc a very high duty cycle causes saturation. The reason for this is still to come.
A long pulse duration causes the return to d.c. values. For the 45° direction no clear changes were found.
3.2.3. Time behaviour There are no differences in the time behaviour of the ion current between random and steered arc process (M1). The information given in Ref. [6 ] is also valid here. It is found there that the maximum values of ion current density in the 0° direction occur at the pulse beginning, i.e. in the phase of rising arc current ( Fig. 5).
3.3. Deposition Thin layers of TiN were deposited on samples of x silicon and stainless steel. Four different processes (d.c. random arc, pulsed random arc, d.c steered arc, pulsed steered arc) have been compared (deposition parameters according to Table 2; M1 served as steered arc cathode).
Fig. 5. Arc current and ion current density (0° and 45° probe) during one pulse (high vacuum).
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968 Table 2 Deposition parameters Parameter
Average arc current (A) D.c. current (A) Pulse current (A) Pulse frequency (Hz) Pulse duration (ms) Bias voltage ( V ) Nitrogen pressure (Pa)
967
The samples were analysed by REM. In doing this, no remarkable differences in layer structure were found. D.c. process 120 120 – – – −150 1
Modified pulsed arc process 120 80 280 340 500 −150 1
3.3.1. Deposition rate In Fig. 6 the deposition rates for the four processes are represented. One can determine a considerably increased deposition rate by use of the modified pulsed arc process. This value increases by 65% (steered arc) and 28% (random arc), respectively. The absolute values for the random arc process are lower than expected. A very thin cathode with the corresponding good cooling conditions seems to be the reason.
Fig. 6. Deposition rate (average arc current 120 A, p=1 Pa N ). 2
Fig. 7. Droplet density (average arc current 120 A, p=1 Pa N ). 2
968
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968
3.3.2. Droplets Here the absolute values are also under the influence of the low thickness of the random arc cathode. However, the pulsation of arc current results in a reduced droplet density ( Fig. 7) for both processes. Even for the steered arc process one can observe a reduction of droplet density by 13%. The subdivision into droplet size categories (not displayed here) results in a preferential reduction of small size droplets (diameter up to 1 mm).
4. Conclusions The combination of steered and pulsed arc process seems to be a promising variant to improve existing vacuum arc coating systems. Technological advantages are expected, in particular by increased deposition rate and reduced droplet density. But there are also some advantages for the modified pulsed arc process itself. There should exist the possibility of decreasing the d.c. part of the arc current. As a result, the droplet emission would be reduced again. The pulse energy could be concentrated in one cathode spot.
In addition, the adaptation of pulse parameters on cathode and field geometry is obviously possible. Further investigations are undoubtedly necessary to verify the results available now.
References [1] A. Bugaev, V. Gushenets, A. Nikolaev, E. Oks, A. Anders, in: Proc. 18th Int. Symp. on Discharges and Electrical Insulation in Vacuum, Eindhoven (1998) 256–259. [2] B. Schultrich, P. Siemroth, J. Vetter, O. Zimmer, Vakuum Forsch. Praxis 1 (1998) 37–46. [3] H. Fuchs, H. Mecke, in: Proc. 12th Int. Conf. on Gas Discharges and their Applications, Greifswald (1997) 22–25. [4] K. Keutel, H. Fuchs, H. Mecke, Chr. Edelmann, in: Proc. 18th Int. Symp. on Discharges and Electrical Insulation in Vacuum, Eindhoven (1998) 562–565. [5] H. Veltrop, in: Du¨nne Schichten (1990) 35–38. [6 ] H. Fuchs, H. Mecke, M. Ellrodt, Surf. Coat. Technol. 98 (1998) 839–844. [7] O. Knotek, C. Sto¨ßel, in; Du¨nnschichtechnologie, Du¨sseldorf (1990) 268–277. [8] M. Ellrodt, Doctoral Thesis, Otto-von-Guericke University of Magdeburg, 1997. [9] P. Siemroth, T. Schu¨lke, T. Witke, Surf. Coat. Technol. 68/69 (1994) 314–319. [10] M. Ellrodt, H. Mecke, Surf. Coat. Technol. 74/75 (1995) 241–245. [11] H.D. Steffens, M. Mack, K. Mo¨hwald, K. Reichel, Surf. Coat. Technol. 46 (1991) 65–74.
Pulsed vacuum arc discharges on steered arc cathodes H. Fuchs a, *, K. Keutel b, H. Mecke a, Chr. Edelmann b a Faculty of Electrical Engineering, Institute of Electrical Engineering and Power Electronics, Otto-von-Guericke University of Magdeburg, Universita¨tsplatz 2, Magdeburg D-39106, Germany b Faculty of Natural Science, Department of Vacuum Science and Technology, Otto-von-Guericke University of Magdeburg, Universita¨tsplatz 2, Magdeburg D-39106, Germany
Abstract The processes d.c. arc, modified pulsed arc, d.c. steered arc and the combination of steered and pulsed arc were compared. Steered arc cathodes with different magnetic fields were used to obtain experimental results for spot movement, ion current, layer formation and droplet emission. The process combination of steered and pulsed arc was examined mainly. High-speed videos showed a clearly increased spot velocity and the enlargement of the eroded area. Measurements on total ion current verify the experiences of random pulsed arc. The angular distribution of ion current density led us to conclude a focused plasma column. In this case, the influence of the duty cycle is strongest. For example, the ion current density related to arc current in the perpendicular direction is increased by 35% for steered arc with pulses. Deposition experiments yielded an increased deposition rate and a reduced droplet density. Results available up to now need further confirmation. The influence of the magnetic field intensity has to be investigated systematically. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Droplets; Ion energy; Pulsed arc; Spot movement; Steered arc; Vacuum arc
1. Introduction The control of spot movement by external magnetic fields in vacuum arc deposition (steered arc) is a usual industrial procedure. One can make a distinction between low steered arcs which are only held on a certain cathode area and steered arcs with a clearly increased spot velocity on a defined track [2,8]. Some technological differences are caused by the varied emission characteristic compared to the random arc process. The emission of droplets is clearly reduced both in size and density [7]. In addition to the reduced ion energy one can determine a decreased layer deposition rate by about 30% [11]. Using movable magnetic systems there are advantages regarding cathode exploitation and the deposition of layer systems from segmented cathodes [5]. The modified pulsed arc process offers the possibility of increasing the spot velocity clearly [3]. A reduced droplet emission (about 15%, especially small droplets [4]) and an improved cathode exploitation goes along with this. The ion current density can be increased clearly by the pulsation of the arc current and the spatial * Corresponding author. Fax: +49-391-6712408. E-mail address: [email protected] (H. Fuchs)
plasma extension can be influenced by the pulse parameters (focused plasma column, dependence on pulse current) [6,9,10]. Measurements of the total ion current showed an increase of 30% (average) and 50% (maximum), respectively. By the use of high current pulses (1 kA, 100 ms) an ion charge increase by factor 1.7 and an ion velocity acceleration by 2–3 have been measured [6 ]. In comparison to the common steered arc process an increased ion energy and a deposition rate at an even more reduced build-in of droplets are expected by the combination of both processes, magnetically steered and modified pulsed arc. The adaptation to cathode geometry via magnetic field and pulse parameters represents a possible further technological advantage. In Ref. [7] for the steered arc a strong dependence of layer properties on arc current is remarked. The possibility of influencing the charge state and the formation of gas metal compounds by the magnetic field has been described by Anders and coworkers [1].
2. Experimental arrangement and arc mode The investigations were carried out in a commercial vacuum arc coating system with a cylindrical chamber
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 13 4 - 6
964
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968
Fig. 1. Experimental arrangement.
(diameter of 365 mm; height of 335 mm). The base pressure was less than 0.001 Pa. Nitrogen was used as reactive gas with a pressure of 1 Pa. Flat cylindrical arc cathodes (diameter of 65 mm, direct cooling, titanium with 99.8% purity) were mounted on the base plate of the chamber. The cathode was surrounded by a floating shield to prevent spots leaving the surface. Because of this arrangement and anode cooling, anode spots cannot exist and influence the ion current. For the investigations of the angular distribution of ion current density two square probes with an area of 1 cm2 were positioned at 0° and 45° with respect to the cathode normal direction. The distance to the cathode centre was 150 mm. For the measurements of the total ion current a sphere segment was used and the current was calculated afterwards assuming equal ion emission into a half-sphere above the cathode. All probes were negatively biased with respect to the anode (−150 V ). The spot movement was observed by use of a highspeed framing camera ( Kodak EktaPro HS 4540) with a maximum frame recording frequency of 40 500 pictures/s. Fig. 1 schematically shows the complete arrangement. Permanent magnets were inserted for the steered
movement of the cathode spots. Three cathodes with different shape and intensity of the magnetic field were prepared in this way. Fig. 2 shows the perpendicular component of the magnetic flux density for these three cathodes. A titanium cathode without magnets was inserted for the reference investigations of the random arc process. The known modified pulsed arc process (superimposition of a d.c. current with pulses) was used for the investigations. A newly developed electronic current source served as power supply which allowed the variation of the output parameters in a wide range (d.c. current 20–200 A; pulsed current 40–400 A, pulse frequency up to 20 kHz, current rise up to 160 A/ms). For the investigations presented here, especially the variation of pulse frequency and pulse duration (and duty cycle, respectively) has been carried out.
3. Results 3.1. Spot movement The high-speed videos were analysed with regard to the synchronous recorded arc current, arc voltage and ion current. The spots on the steered arc cathodes move on a track where the perpendicular component of the magnetic flux density (B) is zero (Fig. 2). Especially for cathode M3 a clearly defined circular path is visible. The high B gradient surrounding the B=0 area and the high maximum value of B (25 mT ) are supposed to be the reason. For M1 and M2 the spot behaviour corresponds to low steered arc (B =10 mT ). The spots are max able to cross the field centre. The pulsation of arc current causes an accelerated movement of the spots alternating around the ‘d.c. track’. The eroded area is expanded and the average velocity on the track increases.
Fig. 2. Perpendicular component of the magnetic flux density on the surface of the used steered arc cathodes.
965
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968 Table 1 Total ion current as a function of arc mode and process pressure (average arc current 120 A) Cathode
Arc process
Random
d.c. pulse
M1 steered
d.c. pulse
M2 steered
d.c. pulse
M3 steered
d.c. pulse
High vacuum (0.001 Pa) i
average max. (during the pulse) average max. (during the pulse) average max. (during the pulse) average max. (during the pulse)
For the modified pulsed arc process on random arc cathodes the undefined pulse start position is often criticised. The use of steered arc cathodes can be advantageous to this. 3.2. Ion current 3.2.1. Total ion current The total ion current (that means the ion current ratio of the arc current) increases by the pulsation of the arc current ( Table 1). No clear influence of pulse frequency and duty cycle could have been determined. The highest absolute values were measured for cathode M3. The average ion current ratio is 17% (average) and 56% (maximum, during pulse) for this. As a reference, about 10% can be given for the d.c. process. Due to the non-ideal probe arrangement (main probe area on the side of the cathode) some problems occurred
10.16 13.22 25.50 12.88 15.35 32.36 10.98 13.92 38.95 11.71 16.33 56.23
/i (%) ion arc
With N (1.0 Pa) i /i (%) 2 ion arc 9.40 10.22 16.85 10.57 12.73 28.50 9.40 12.06 33.66 12.03 17.44 55.12
(interference of pulse frequency and velocity of circulation of the spots). 3.2.2. Distribution of ion current density The reference for the investigations is the d.c. process with the same average arc current. Cathode M1 represented the steered arc process. No significant influence of current pulsation was observed in the 45° direction. However, there was a clear rise in the 0° direction (up to 27% for random arc and 35% for steered arc). This indicates a very focused plasma column. Fig. 3 shows the dependence of ion current density on the pulse repetition frequency. In the case of the random arc, the ion current density increases with higher pulse repetition frequency. For the steered arc no corresponding connection could have been determined. The increase of the duty cycle (and therefore the average arc current) leads to a rise of ion current density.
Fig. 3. Ion current density related to arc current as a function of pulse frequency (high vacuum, pulse duration: 500 ms).
966
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968
Fig. 4. Ion current density related to arc current as a function of pulse duration (high vacuum, period: 3 ms).
That is both for the random and steered arc (Fig. 4). At the steered arc a very high duty cycle causes saturation. The reason for this is still to come.
A long pulse duration causes the return to d.c. values. For the 45° direction no clear changes were found.
3.2.3. Time behaviour There are no differences in the time behaviour of the ion current between random and steered arc process (M1). The information given in Ref. [6 ] is also valid here. It is found there that the maximum values of ion current density in the 0° direction occur at the pulse beginning, i.e. in the phase of rising arc current ( Fig. 5).
3.3. Deposition Thin layers of TiN were deposited on samples of x silicon and stainless steel. Four different processes (d.c. random arc, pulsed random arc, d.c steered arc, pulsed steered arc) have been compared (deposition parameters according to Table 2; M1 served as steered arc cathode).
Fig. 5. Arc current and ion current density (0° and 45° probe) during one pulse (high vacuum).
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968 Table 2 Deposition parameters Parameter
Average arc current (A) D.c. current (A) Pulse current (A) Pulse frequency (Hz) Pulse duration (ms) Bias voltage ( V ) Nitrogen pressure (Pa)
967
The samples were analysed by REM. In doing this, no remarkable differences in layer structure were found. D.c. process 120 120 – – – −150 1
Modified pulsed arc process 120 80 280 340 500 −150 1
3.3.1. Deposition rate In Fig. 6 the deposition rates for the four processes are represented. One can determine a considerably increased deposition rate by use of the modified pulsed arc process. This value increases by 65% (steered arc) and 28% (random arc), respectively. The absolute values for the random arc process are lower than expected. A very thin cathode with the corresponding good cooling conditions seems to be the reason.
Fig. 6. Deposition rate (average arc current 120 A, p=1 Pa N ). 2
Fig. 7. Droplet density (average arc current 120 A, p=1 Pa N ). 2
968
H. Fuchs et al. / Surface and Coatings Technology 116–119 (1999) 963–968
3.3.2. Droplets Here the absolute values are also under the influence of the low thickness of the random arc cathode. However, the pulsation of arc current results in a reduced droplet density ( Fig. 7) for both processes. Even for the steered arc process one can observe a reduction of droplet density by 13%. The subdivision into droplet size categories (not displayed here) results in a preferential reduction of small size droplets (diameter up to 1 mm).
4. Conclusions The combination of steered and pulsed arc process seems to be a promising variant to improve existing vacuum arc coating systems. Technological advantages are expected, in particular by increased deposition rate and reduced droplet density. But there are also some advantages for the modified pulsed arc process itself. There should exist the possibility of decreasing the d.c. part of the arc current. As a result, the droplet emission would be reduced again. The pulse energy could be concentrated in one cathode spot.
In addition, the adaptation of pulse parameters on cathode and field geometry is obviously possible. Further investigations are undoubtedly necessary to verify the results available now.
References [1] A. Bugaev, V. Gushenets, A. Nikolaev, E. Oks, A. Anders, in: Proc. 18th Int. Symp. on Discharges and Electrical Insulation in Vacuum, Eindhoven (1998) 256–259. [2] B. Schultrich, P. Siemroth, J. Vetter, O. Zimmer, Vakuum Forsch. Praxis 1 (1998) 37–46. [3] H. Fuchs, H. Mecke, in: Proc. 12th Int. Conf. on Gas Discharges and their Applications, Greifswald (1997) 22–25. [4] K. Keutel, H. Fuchs, H. Mecke, Chr. Edelmann, in: Proc. 18th Int. Symp. on Discharges and Electrical Insulation in Vacuum, Eindhoven (1998) 562–565. [5] H. Veltrop, in: Du¨nne Schichten (1990) 35–38. [6 ] H. Fuchs, H. Mecke, M. Ellrodt, Surf. Coat. Technol. 98 (1998) 839–844. [7] O. Knotek, C. Sto¨ßel, in; Du¨nnschichtechnologie, Du¨sseldorf (1990) 268–277. [8] M. Ellrodt, Doctoral Thesis, Otto-von-Guericke University of Magdeburg, 1997. [9] P. Siemroth, T. Schu¨lke, T. Witke, Surf. Coat. Technol. 68/69 (1994) 314–319. [10] M. Ellrodt, H. Mecke, Surf. Coat. Technol. 74/75 (1995) 241–245. [11] H.D. Steffens, M. Mack, K. Mo¨hwald, K. Reichel, Surf. Coat. Technol. 46 (1991) 65–74.