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Fundamental processes in vacuum arc deposition
t'a111mw!
ELSEVIER
Surface and Coatings Technology 74 75 (1995) 92 96
Fundamental processes in vacuum arc deposition P. Siemroth, B. Schultrich, T. Schtilke Fraunhofer-lnstitut J~ir Werkstoffphysik und Schichttechnologie Dresden, Helmholtzstr. 20, O1069 Dresden, Germany
Abstract
Notwithstanding its multiple technical applications, the fundamental processes of the vacuum arc technique have been insufficiently explored experimentally and theoretical discussions have involved certain contradictions. In particular, the main parameters and dimensions of the cathode spots have been under discussion for some time. To answer these current questions, a new system, the high speed framing camera (HSFC), was designed to combine a long distance observation with microscopic resolution and nanosecond time resolution. This camera was used to study the microscopic behaviour of cathode spots in a pulsed high current vacuum arc. The observations described reveal that a single cathode spot, as normally observed by optical means, consists of a number of simultaneously existing microscopic subspots, each with a diameter of about 15 ~tm, 30-50 ~tm apart and a microsecond lifetime. Keywords: Vacuum arc; Cathode spot; High speed; Microscopy; Subspots
1. Vacuum arcs and their problems
The term vacuum arc indicates that this kind of electrical discharge does not require an external gas atmosphere and can exist under vacuum conditions by producing the necessary conductive medium by its own action. For arc currents below several kiloamperes, the conductive medium will be produced exclusively by evaporation, ionization and acceleration towards the anode of the cathode material. This cathode plasma can be used for thin film deposition. For this application, the cathode of a low voltage discharge circuit is the evaporation target and the chamber wall represents the anode. This equipment is arranged in a vacuum chamber, but operation in an inert or reactive atmosphere is also possible. In comparison with other deposition methods, the vacuum arc process is distinguished by its high degree of ionization which allows the deposition of dense films with good adherence. The homogeneity is reduced by microparticles in the micrometre range splashed away by the highly dynamic processes on the cathode surface. Nevertheless, the vacuum arc deposition technique is now well established on an industrial scale, especially for the protection of tools by hard coatings. Notwithstanding these extended applications, the fundamental processes of the vacuum arc discharge have been insufficiently explored experimentally and theoretical discussions have involved contradictions. Obviously, the 0257-8972/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 2 5 7 - 8 9 7 2 ( 9 5 ) 0 8 3 4 6 - 4
key process in the vacuum arc discharge is the localized heating of the cathode producing the evaporation and electron emission necessary for maintaining the arc. There are still many open questions. Some of these are listed below. (l) What are the dimensions of the cathode spots? Are they of about 100 ~tm according to optical observations of bright spots [ 1,2], or about 10 lam according to the craters recognized afterwards [ 3 - 5 ] ? (2) Is the spot a single element or does it represent an entity composed of several subspots? (3) What is the lifetime of the individual spots? Is it below 100 ns [4] or in the microsecond range [2]? (4) What is the course of the spot? Is the individual spot broken down by internal instability and replaced by a new one developing in the surroundings, or is there an overlap of neighbouring subspots existing simultaneously? (5) What is the elementary step of the underlying diffusion-like process? How is it connected with the dimension and lifetime of the cathode spots? These questions show the principal uncertainties of the local physical conditions, e.g. the predicted current density varies by many orders of magnitude according to the different conceptions. Furthermore, the local dynamics, which are responsible for the splashing of arc melted material into the small, disturbing droplets, are obscured. Most of these uncertainties and contradictions are
P. Siemroth et al./Surface and Coatings Technology 74- 75 (1995) 92 96
caused by the experimental difficulties of observing these complex processes on the cathode and in the plasma with sufficiently high spatial and time resolution. In the following, some observations and conclusions are presented, obtained with apparatus especially designed for tackling these problems. This paper concentrates on the processes on the cathode surface; results for the arc plasma are presented in an accompanying contribution
I-6].
93
expand radially from the ignition point. Consequently, even with extremely small observation areas, arc spots can be caught with sufficient probability. In the experiments described here, the HSFC was used to observe cathode spots on a copper cathode with the highest magnification. The observation area of 900 pm x 700 pm is located at a distance of about 6 mm from the centre. 3. Cathode spot: former conceptions and new results
2. Experimental conditions Deeper insight into the processes underlying vacuum arc evaporation has been made possible by newly developed equipment especially designed for such problems. The high speed framing camera (HSFC) system [-7] consists of a long distance microscope, a beam splitter and four parallel electronic short time cameras allowing a minimum exposure time of 5 ns and a spatial resolution of several micrometres over a distance of about 1 m. The four channels can be independently controlled for exposure time, delay time, gain and spectral detection. The arc spot studies have been carried out at the cathode of a new high current pulsed arc evaporator [8]. A removable flat cylinder cathode (diameter, 30 mm) is mounted in the cathode holder. In its centre, there is a high voltage trigger electrode. The observations have been performed under high vacuum conditions (10-4_10 3 Pa). Sinusoidal current pulses (peak current, 3-6 kA; duration, 0.5-1 ms; repetition rate, from single pulses up to 300 s -1) are formed by a resonant pulse forming circuit (L = 35 pH, C = 3 mF). Due to the repelling forces, the cathode spots run radially from the ignition point to the rim of the cathode. Due to the high current, a large number of simultaneously existing spots
a) open shutter photograph resolution 80 lam
Notwithstanding the experimental uncertainties and open questions, the following picture of the cathode spots has been widely accepted [-5]. At currents at or below about 100 A, there is only a single cathode spot of about 10 pm and the corresponding current density is about 108 A cm -2. The spot undergoes rapid development and is extinguished after several tens of nanoseconds. By the repulsion of the emitted vapour pulse, the localized melt pool is splashed over the rim, leading to sharp tips where the new spot is created by the combination of field enhancement and runaway heating. Hence, random walking involves statistical nucleation on the border of the old spot and the elementary diffusional step corresponds to the spot radius. In this way, the cathodic arc consists (even under external d.c. conditions) of a succession of single spots appearing one after another with a frequency in the megahertz range. The in situ observation of luminous 100 pm regions then appears as an artefact of the experimental limitations: the insufficient lateral resolution of normal short time cameras and the insufficient time resolution of higher magnification instruments. By using the HSFC system, these limitations can be overcome. The information gained by this more sophisticated equipment is demonstrated in Fig. 1. To compare
b) intensified CCD-camera exposure 200 ns resolution 40 lam e) software zoom of b)
d) HSFC-photograph exposure time 100 ns resolution 1.2 lam/pix
h.~ i . . d
28 m m
14 m m
2.8 m m
0.28 m m
Fig. I. Comparison of traditional photography, high speed observation and HSFC investigation.
P. Siemroth et aL /SurJace and Coatings Technology 74-75 (1995) 92-96
94
conventional photographic observations with HSFC pictures, a series of arc spot images with three different temporal and lateral resolutions is shown in Fig. 1. In Fig. l(a), an open shutter photograph with a lateral resolution of about 80 jam shows a number of arc spot traces running from the ignition point in the centre to the rim of the cathode. In Fig. l(b), using a gated intensified CCD camera with a standard objective (exposure time, 200 ns; resolution, about 40 ~tm), a ring of simultaneously existing spots can be seen. Zooming in on this photograph, it can be seen that each spot has dimensions of about 100jam (Fig. l(c)), remarkably larger than the lateral resolution of the optical system used. In Fig. l(d), the HSFC picture shows the inner structure of the large spot; a number of small spots exist at the same time and are separated from one another by about 20-50 jam. The dimensions of the small subspots are of the order of 10-20 jam, i.e. similar to those of the microscopically
observable post-mortem craters [ 3 - 5 ] . The dimensions and observation of numerous parallel existing spots in our experiments are consistent with the absorption pictures taken by Anders et al. [9] by a side-on passage of a subnanosecond dye laser pulse through the plasma cloud in front of the cathodes. To determine the mean spot lifetime, the number of spots was counted on frames with different exposure times (20 ns, 100 ns, 1 jas, 10 jas), each interlocked in the middle of the next longest time frame. By statistical estimation, the mean lifetime of the small spots was deduced to be about 3.2 jas. As an example, the four frames of one typical shot are shown in Fig. 2. By only counting the number of spots, it cannot be demonstrated whether the spot exists at one place all of the time or moves randomly with a step width below the resolution limit. Therefore the brightness distribution of these spots was measured as a function of the exposure time. It was established that the width of this distribution
~iiib
Delay: 75.55 ~
Exposure: 10 ns
Delay: 75.50 Its Exposure: 100 ns
4iiDii~i i;~iii~i!i
Delay: 75.00 its Exposure: 1 !as
Delay: 70.0 ~
Exposure: 10
Fig. 2. F o u r frames of one typical shot taken with different exposure times, but the same middle time observation area (900 ~tm x 700 p.m).
P. Siemroth et aL/Surface and Coatings Technology 74 75 (1995) 92-96
160
140
il
Arrem ps BrnemExposure Exposure10lps ......... CrneanExposure100ns DrremExposure 10ns
Data: A mean (10 ps) Model:Lorentz Chi^ 2 =1,44886 y0 =81,7641 x c =- 1,04782E-6 w =25,8308 pm A =0,00244072 Data:B mean(tps) Model: L o r e n t z Chi^ 2 =9,0348 y0 =32,74 x c =-1,447E-6 w =26,51pm A =0,003813
loo
Data: C mean (100 ns) Model: Lorentz Chi ^ 2 =1,82906 y0 =8,902 xc =-8,867E- 7 w =19,15 pm A =0,001627
~jL"
7E
95
40
,./
20 I
-60,0pm
~
- - I
-,
I
-40,0pm -20,0pm
,
I
0,0m
,
I
20,0pm
,
40,01Jrn
Data: D mean (20 ns) Model: L o r e n t z Chi^2 =0,24934 y0 =3,839 x c =8,016E- 7 w =15,76 IJm A = 2,671E-4
distance from spot centre Fig. 3. Brightness distribution of arc spots, observed with different exposure times (20 ns, 100 ns, 1 gs, 10 Hs). Each curve consists of the mean value of ten single line scans.
is about 20 ___5 ~tm, nearly independent of the exposure time. As an example, in Fig. 3, the mean values of 16 line scans through the spots are fitted by a bell-shaped lorentzian curve. By numerical simulation [10], it was shown that the width of the brightness distribution should be proportional to the square root of the exposure time if a small spot is moving randomly. Therefore it can be concluded that the spot position remains stable over its lifetime of about 3 ~ts with an uncertainty of less than 2-3 ~tm.
4. Conclusions The direct observation of cathode spots at high resolution revealed the following main features. (1) For a certain time the cathodic spot area embraces a region of 100-200 ~m corresponding to the luminous zone observed with conventional optical methods. (2) This spot area is structured and consists of several (about ten) microspots of about 20 ~tm existing at the same time. (3) The microspots have a lifetime of several microseconds. (4) The single microspot represents a rather stable, immobile element.
(5) New microspots arise within or at the border of the spot area, but not necessarily adjacent to dying spots of the preceding generation. (6) The spot area represents a rather stable entity changing its form and position but preserving its dimensions. From these experimental results a new picture can be derived based on confirmed observations, but partially hypothetical in its extrapolations. The fundamental elements of the vacuum arc discharge are the microspots mentioned above (dimensions, 10-30 ~m; lifetime, 1-5 ~ts). They are nearly invariant during their lifetime and show only small deviations even under changed electrical conditions. It is assumed that a single microspot can only carry a current of about 10 A, corresponding to a current density of about l06 10-7 Acre z. Starting from ignition, a number of microspots, determined by the arc current, develop in the ignition area. They are connected by a common plasma front which approaches the surface to about 100 ~tm. Hence, new microspots preferentially originate in this confined plasma region, thus allowing a diffusion-like movement of the whole entity without spreading. Under suitable conditions of a high current per microspot, surface roughness and covering layers, microspots can be generated in the immediate neighbourhood to the spot area.
96
P. Siemroth et al./Surface and Coatings' Technology 74-75 (1995) 92 96
They may develop into a new family of microspots, i.e. a new cathode spot, which will move independently. According to these concepts, the splitting of the cathode spots at higher arc currents is not caused by the limited current carrying capability of the single spot, but by the increasing probability for escape of a microspot and its propagation. This splitting is favoured by the repulsive magnetic forces between different current paths.
[4]
[5] [6]
E7]
References [1] B.E. Djakov and R. Holmes, Cathode spot structure and dynamics in low-current vacuum arcs, J. Phys. D: Appl. Phys., 7 (1974) 569-580. [2] V.I. Rakhovsky, Experimental study of the dynamics of cathode spots development, IEEE Trans. Plasma Sci., 4 (1976) 81-102. [3] J.E. Daalder, Diameter and current density of single and multiple
[8] [9]
[10]
cathode discharges in vacuum, IEEE Trans. Pow. App. Syst., 93 (1974) 1747-1758. V.F. Puchkarev and A.M. Murzakayev, Current density and the cathode spot lifetime in a vacuum arc at threshold currents, J. Phys. D: Appl. Phys., 23 (1990} 26-35. E. Hantzsche and B. Jfittner, Current density in arc spots, IEEE Trans. Plasma Sci., 13 (1985) 230-234. T. Witke, A. Lenk, B. Schultrich and C. Schultheiss, Investigation of plasma produced by laser and electron pulse ablation, Surf Coat. Technol., X X (1995). P. Sicmroth, T. Witke, T. Schiilke and A. Lenk, Short-time investigation of laser and arc assisted deposition processes, Surf Coat. Technol., 68 (1994) 713. P. Siemroth and T. Scht~lke, High-current arc a new source for high rate deposition, Surf. Coat, Technol., 68 (1994) 314. A. Anders, S. Anders, B. Jfittner, W. B6tticher, H. Lt~ck and G. Schr6der, Pulsed dye laser diagnostics of vacuum arc cathode spots, IEEE Trans. Plasma Sci., 20 (1992) 466 472. S. Anders and A. Anders, Simulation of the brightness pattern of a moving cathode spot, Proc. X V t h Int. Syrup. Disch. El. Ins. ~bc., Darmstadt, 1992, pp. 294 298.
ELSEVIER
Surface and Coatings Technology 74 75 (1995) 92 96
Fundamental processes in vacuum arc deposition P. Siemroth, B. Schultrich, T. Schtilke Fraunhofer-lnstitut J~ir Werkstoffphysik und Schichttechnologie Dresden, Helmholtzstr. 20, O1069 Dresden, Germany
Abstract
Notwithstanding its multiple technical applications, the fundamental processes of the vacuum arc technique have been insufficiently explored experimentally and theoretical discussions have involved certain contradictions. In particular, the main parameters and dimensions of the cathode spots have been under discussion for some time. To answer these current questions, a new system, the high speed framing camera (HSFC), was designed to combine a long distance observation with microscopic resolution and nanosecond time resolution. This camera was used to study the microscopic behaviour of cathode spots in a pulsed high current vacuum arc. The observations described reveal that a single cathode spot, as normally observed by optical means, consists of a number of simultaneously existing microscopic subspots, each with a diameter of about 15 ~tm, 30-50 ~tm apart and a microsecond lifetime. Keywords: Vacuum arc; Cathode spot; High speed; Microscopy; Subspots
1. Vacuum arcs and their problems
The term vacuum arc indicates that this kind of electrical discharge does not require an external gas atmosphere and can exist under vacuum conditions by producing the necessary conductive medium by its own action. For arc currents below several kiloamperes, the conductive medium will be produced exclusively by evaporation, ionization and acceleration towards the anode of the cathode material. This cathode plasma can be used for thin film deposition. For this application, the cathode of a low voltage discharge circuit is the evaporation target and the chamber wall represents the anode. This equipment is arranged in a vacuum chamber, but operation in an inert or reactive atmosphere is also possible. In comparison with other deposition methods, the vacuum arc process is distinguished by its high degree of ionization which allows the deposition of dense films with good adherence. The homogeneity is reduced by microparticles in the micrometre range splashed away by the highly dynamic processes on the cathode surface. Nevertheless, the vacuum arc deposition technique is now well established on an industrial scale, especially for the protection of tools by hard coatings. Notwithstanding these extended applications, the fundamental processes of the vacuum arc discharge have been insufficiently explored experimentally and theoretical discussions have involved contradictions. Obviously, the 0257-8972/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 2 5 7 - 8 9 7 2 ( 9 5 ) 0 8 3 4 6 - 4
key process in the vacuum arc discharge is the localized heating of the cathode producing the evaporation and electron emission necessary for maintaining the arc. There are still many open questions. Some of these are listed below. (l) What are the dimensions of the cathode spots? Are they of about 100 ~tm according to optical observations of bright spots [ 1,2], or about 10 lam according to the craters recognized afterwards [ 3 - 5 ] ? (2) Is the spot a single element or does it represent an entity composed of several subspots? (3) What is the lifetime of the individual spots? Is it below 100 ns [4] or in the microsecond range [2]? (4) What is the course of the spot? Is the individual spot broken down by internal instability and replaced by a new one developing in the surroundings, or is there an overlap of neighbouring subspots existing simultaneously? (5) What is the elementary step of the underlying diffusion-like process? How is it connected with the dimension and lifetime of the cathode spots? These questions show the principal uncertainties of the local physical conditions, e.g. the predicted current density varies by many orders of magnitude according to the different conceptions. Furthermore, the local dynamics, which are responsible for the splashing of arc melted material into the small, disturbing droplets, are obscured. Most of these uncertainties and contradictions are
P. Siemroth et al./Surface and Coatings Technology 74- 75 (1995) 92 96
caused by the experimental difficulties of observing these complex processes on the cathode and in the plasma with sufficiently high spatial and time resolution. In the following, some observations and conclusions are presented, obtained with apparatus especially designed for tackling these problems. This paper concentrates on the processes on the cathode surface; results for the arc plasma are presented in an accompanying contribution
I-6].
93
expand radially from the ignition point. Consequently, even with extremely small observation areas, arc spots can be caught with sufficient probability. In the experiments described here, the HSFC was used to observe cathode spots on a copper cathode with the highest magnification. The observation area of 900 pm x 700 pm is located at a distance of about 6 mm from the centre. 3. Cathode spot: former conceptions and new results
2. Experimental conditions Deeper insight into the processes underlying vacuum arc evaporation has been made possible by newly developed equipment especially designed for such problems. The high speed framing camera (HSFC) system [-7] consists of a long distance microscope, a beam splitter and four parallel electronic short time cameras allowing a minimum exposure time of 5 ns and a spatial resolution of several micrometres over a distance of about 1 m. The four channels can be independently controlled for exposure time, delay time, gain and spectral detection. The arc spot studies have been carried out at the cathode of a new high current pulsed arc evaporator [8]. A removable flat cylinder cathode (diameter, 30 mm) is mounted in the cathode holder. In its centre, there is a high voltage trigger electrode. The observations have been performed under high vacuum conditions (10-4_10 3 Pa). Sinusoidal current pulses (peak current, 3-6 kA; duration, 0.5-1 ms; repetition rate, from single pulses up to 300 s -1) are formed by a resonant pulse forming circuit (L = 35 pH, C = 3 mF). Due to the repelling forces, the cathode spots run radially from the ignition point to the rim of the cathode. Due to the high current, a large number of simultaneously existing spots
a) open shutter photograph resolution 80 lam
Notwithstanding the experimental uncertainties and open questions, the following picture of the cathode spots has been widely accepted [-5]. At currents at or below about 100 A, there is only a single cathode spot of about 10 pm and the corresponding current density is about 108 A cm -2. The spot undergoes rapid development and is extinguished after several tens of nanoseconds. By the repulsion of the emitted vapour pulse, the localized melt pool is splashed over the rim, leading to sharp tips where the new spot is created by the combination of field enhancement and runaway heating. Hence, random walking involves statistical nucleation on the border of the old spot and the elementary diffusional step corresponds to the spot radius. In this way, the cathodic arc consists (even under external d.c. conditions) of a succession of single spots appearing one after another with a frequency in the megahertz range. The in situ observation of luminous 100 pm regions then appears as an artefact of the experimental limitations: the insufficient lateral resolution of normal short time cameras and the insufficient time resolution of higher magnification instruments. By using the HSFC system, these limitations can be overcome. The information gained by this more sophisticated equipment is demonstrated in Fig. 1. To compare
b) intensified CCD-camera exposure 200 ns resolution 40 lam e) software zoom of b)
d) HSFC-photograph exposure time 100 ns resolution 1.2 lam/pix
h.~ i . . d
28 m m
14 m m
2.8 m m
0.28 m m
Fig. I. Comparison of traditional photography, high speed observation and HSFC investigation.
P. Siemroth et aL /SurJace and Coatings Technology 74-75 (1995) 92-96
94
conventional photographic observations with HSFC pictures, a series of arc spot images with three different temporal and lateral resolutions is shown in Fig. 1. In Fig. l(a), an open shutter photograph with a lateral resolution of about 80 jam shows a number of arc spot traces running from the ignition point in the centre to the rim of the cathode. In Fig. l(b), using a gated intensified CCD camera with a standard objective (exposure time, 200 ns; resolution, about 40 ~tm), a ring of simultaneously existing spots can be seen. Zooming in on this photograph, it can be seen that each spot has dimensions of about 100jam (Fig. l(c)), remarkably larger than the lateral resolution of the optical system used. In Fig. l(d), the HSFC picture shows the inner structure of the large spot; a number of small spots exist at the same time and are separated from one another by about 20-50 jam. The dimensions of the small subspots are of the order of 10-20 jam, i.e. similar to those of the microscopically
observable post-mortem craters [ 3 - 5 ] . The dimensions and observation of numerous parallel existing spots in our experiments are consistent with the absorption pictures taken by Anders et al. [9] by a side-on passage of a subnanosecond dye laser pulse through the plasma cloud in front of the cathodes. To determine the mean spot lifetime, the number of spots was counted on frames with different exposure times (20 ns, 100 ns, 1 jas, 10 jas), each interlocked in the middle of the next longest time frame. By statistical estimation, the mean lifetime of the small spots was deduced to be about 3.2 jas. As an example, the four frames of one typical shot are shown in Fig. 2. By only counting the number of spots, it cannot be demonstrated whether the spot exists at one place all of the time or moves randomly with a step width below the resolution limit. Therefore the brightness distribution of these spots was measured as a function of the exposure time. It was established that the width of this distribution
~iiib
Delay: 75.55 ~
Exposure: 10 ns
Delay: 75.50 Its Exposure: 100 ns
4iiDii~i i;~iii~i!i
Delay: 75.00 its Exposure: 1 !as
Delay: 70.0 ~
Exposure: 10
Fig. 2. F o u r frames of one typical shot taken with different exposure times, but the same middle time observation area (900 ~tm x 700 p.m).
P. Siemroth et aL/Surface and Coatings Technology 74 75 (1995) 92-96
160
140
il
Arrem ps BrnemExposure Exposure10lps ......... CrneanExposure100ns DrremExposure 10ns
Data: A mean (10 ps) Model:Lorentz Chi^ 2 =1,44886 y0 =81,7641 x c =- 1,04782E-6 w =25,8308 pm A =0,00244072 Data:B mean(tps) Model: L o r e n t z Chi^ 2 =9,0348 y0 =32,74 x c =-1,447E-6 w =26,51pm A =0,003813
loo
Data: C mean (100 ns) Model: Lorentz Chi ^ 2 =1,82906 y0 =8,902 xc =-8,867E- 7 w =19,15 pm A =0,001627
~jL"
7E
95
40
,./
20 I
-60,0pm
~
- - I
-,
I
-40,0pm -20,0pm
,
I
0,0m
,
I
20,0pm
,
40,01Jrn
Data: D mean (20 ns) Model: L o r e n t z Chi^2 =0,24934 y0 =3,839 x c =8,016E- 7 w =15,76 IJm A = 2,671E-4
distance from spot centre Fig. 3. Brightness distribution of arc spots, observed with different exposure times (20 ns, 100 ns, 1 gs, 10 Hs). Each curve consists of the mean value of ten single line scans.
is about 20 ___5 ~tm, nearly independent of the exposure time. As an example, in Fig. 3, the mean values of 16 line scans through the spots are fitted by a bell-shaped lorentzian curve. By numerical simulation [10], it was shown that the width of the brightness distribution should be proportional to the square root of the exposure time if a small spot is moving randomly. Therefore it can be concluded that the spot position remains stable over its lifetime of about 3 ~ts with an uncertainty of less than 2-3 ~tm.
4. Conclusions The direct observation of cathode spots at high resolution revealed the following main features. (1) For a certain time the cathodic spot area embraces a region of 100-200 ~m corresponding to the luminous zone observed with conventional optical methods. (2) This spot area is structured and consists of several (about ten) microspots of about 20 ~tm existing at the same time. (3) The microspots have a lifetime of several microseconds. (4) The single microspot represents a rather stable, immobile element.
(5) New microspots arise within or at the border of the spot area, but not necessarily adjacent to dying spots of the preceding generation. (6) The spot area represents a rather stable entity changing its form and position but preserving its dimensions. From these experimental results a new picture can be derived based on confirmed observations, but partially hypothetical in its extrapolations. The fundamental elements of the vacuum arc discharge are the microspots mentioned above (dimensions, 10-30 ~m; lifetime, 1-5 ~ts). They are nearly invariant during their lifetime and show only small deviations even under changed electrical conditions. It is assumed that a single microspot can only carry a current of about 10 A, corresponding to a current density of about l06 10-7 Acre z. Starting from ignition, a number of microspots, determined by the arc current, develop in the ignition area. They are connected by a common plasma front which approaches the surface to about 100 ~tm. Hence, new microspots preferentially originate in this confined plasma region, thus allowing a diffusion-like movement of the whole entity without spreading. Under suitable conditions of a high current per microspot, surface roughness and covering layers, microspots can be generated in the immediate neighbourhood to the spot area.
96
P. Siemroth et al./Surface and Coatings' Technology 74-75 (1995) 92 96
They may develop into a new family of microspots, i.e. a new cathode spot, which will move independently. According to these concepts, the splitting of the cathode spots at higher arc currents is not caused by the limited current carrying capability of the single spot, but by the increasing probability for escape of a microspot and its propagation. This splitting is favoured by the repulsive magnetic forces between different current paths.
[4]
[5] [6]
E7]
References [1] B.E. Djakov and R. Holmes, Cathode spot structure and dynamics in low-current vacuum arcs, J. Phys. D: Appl. Phys., 7 (1974) 569-580. [2] V.I. Rakhovsky, Experimental study of the dynamics of cathode spots development, IEEE Trans. Plasma Sci., 4 (1976) 81-102. [3] J.E. Daalder, Diameter and current density of single and multiple
[8] [9]
[10]
cathode discharges in vacuum, IEEE Trans. Pow. App. Syst., 93 (1974) 1747-1758. V.F. Puchkarev and A.M. Murzakayev, Current density and the cathode spot lifetime in a vacuum arc at threshold currents, J. Phys. D: Appl. Phys., 23 (1990} 26-35. E. Hantzsche and B. Jfittner, Current density in arc spots, IEEE Trans. Plasma Sci., 13 (1985) 230-234. T. Witke, A. Lenk, B. Schultrich and C. Schultheiss, Investigation of plasma produced by laser and electron pulse ablation, Surf Coat. Technol., X X (1995). P. Sicmroth, T. Witke, T. Schiilke and A. Lenk, Short-time investigation of laser and arc assisted deposition processes, Surf Coat. Technol., 68 (1994) 713. P. Siemroth and T. Scht~lke, High-current arc a new source for high rate deposition, Surf. Coat, Technol., 68 (1994) 314. A. Anders, S. Anders, B. Jfittner, W. B6tticher, H. Lt~ck and G. Schr6der, Pulsed dye laser diagnostics of vacuum arc cathode spots, IEEE Trans. Plasma Sci., 20 (1992) 466 472. S. Anders and A. Anders, Simulation of the brightness pattern of a moving cathode spot, Proc. X V t h Int. Syrup. Disch. El. Ins. ~bc., Darmstadt, 1992, pp. 294 298.