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Ž .Thin Solid Films 390 2001 192196

 Anode mode in cathodic arc deposition apparatus with variouscathodes and ambient gases

R. Miyano, T. Saito, K. Kimura, M. Ikeda, H. TakikawaU, T. Sakakibara

 Department of Electrical and Electronic Engineering, Toyohashi Uni   ersity of Technology, Toyohashi, Aichi 441-8580, Japan

 Abstract

The anode mode of a vacuum arc in a cathodic arc deposition apparatus was observed as a function of ambient gas pressureŽ . Ž .ranging from 0.01 to 300 Pa. The chamber 400 mm in diameter and 600 mm in length made of stainless steel SUS304 acted as

the anode. The arc was operated at a relatively low constant current of 50 A. The cathode materials used were Al, Ti, Fe, Ni, andŽ .Cu, and ambient gases were He, Ne, Ar, H , N , O , and CH . The principal results are as follows. 1 As the pressure was

2 2 2 4

Ž .increased, the anode mode changed from diffuse-arc to footpoint to plane luminous to anode-spot mode. 2 The anode modeŽ .and resultant arc voltage increase were strongly dependent on gas species, and weakly on the cathode material. 3 Comparing

Ž . Ž .diatomic and polyatomic H , N , O , and CH with mono-atomic molecule gases He, Ne, and Ar , the onset pressure of the2 2 2 4

anode mode transition in the former was lower, the arc voltage higher, and the footpoints more numerous, smaller, and clearer.Both the dependence of the ambient pressure and the influence of the cathode materials and gas species on the anode modechanges were explained by the ion deficiency theory. 2001 Elsevier Science B.V. All rights reserved.

 Keywords: Cathodic arc deposition apparatus; Anode mode; Pressure dependence; Cathode material; Ambient gas

1. Introduction

The cathodic vacuum arc is known to be the simplestmetallic or carbon ion source and is industrially applied

w xto the preparation of thin solid films 1 . A variety of films of metals, nitrides, oxides, carbides and carbona-ceous materials can be fabricated using a cathodic vacuum arc. The technique is called vacuum arc depo-

Ž .sition, cathodic arc deposition, or vacuum arc ion

plating. The authors have prepared various nitrides andw xoxides by this method to date 2 .

On the other hand, a vacuum arc can also be observedin the vacuum bulb of an electrical circuit breaker.Such arcs are known to present a variety of anodeappearances, depending on arcing conditions. In orderto understand the basic characteristics of an arc in the  vacuum bulb, and to improve bulb performance, a

UCorresponding author.

Ž .  E-mail address: takikawa@eee.tut.ac.jp H. Takikawa .

number of studies on anode phenomena of vacuumarcs have been carried out and the results are summa-

w xrized in some reviews 35 . The anode mode is usuallymapped as a function of the arc current and electrodegap length, and sometimes of the arc current andpressure. The anode mode is characteristically classi-fied as follows:

1. Diffuse-arc mode: the anode is inert, acting merely

as a collector of particles emitted from cathodespot.

2. Footpoint mode: one or more luminous points existon the anode surface.

3. Anode-spot mode: one large or several small anodespots appear on the anode surface. The anode spotis active and erodes the surface.

4. Intense-arc mode: the anode spot is very active andseverely erodes the surface.

However, the anode mode at currents as low as thatused for cathodic vacuum arc deposition has not been

0040-6090r01r$ - see front matter 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 1 0 0 9 1 8 - X

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( ) R. Miyano et al. r Thin Solid Films 390 2001 192196 193

studied very well. The authors have observed the foot-point and the plane luminous modes in cathodic arcdeposition apparatus for TiN film preparation at a

w xrelatively high pressure 6,7 . Therefore, it is importantto reveal the influences of the cathode material andambient gas species on the anode mode. In the presentstudy, the appearance of the anode surface in a ca-thodic vacuum arc deposition apparatus was observedfor various cathode materials and ambient gases aspressure ranged from medium to soft vacuum. The arc voltage was also measured simultaneously.

2. Experimental

Fig. 1 depicts the vacuum arc deposition apparatus.Ž  A cylindrical vacuum chamber 400 mm in diameter

. Ž .and 600 mm in length made of stainless steel SUS304Žserves as a grounded anode. A cathode 64 mm in

.diameter is placed at the end. The substrate is usuallylocated at the center of the chamber. An insulation

Ž .plate made of ceramic Zr O is placed around the2 3

cathode in order to expand the plasma in the substratedirection. The arc is ignited by a mechanical triggeringsystem.

In the present study, Al, Ti, Fe, Ni, and Cu wereused as cathode materials, and the ambient gases used were He, Ne, Ar, H , N , O , and CH . The chamber2 2 2 4

 was first pumped down to less than 0.01 Pa. Then thegas, at a flow rate of 40 mlrmin, was introduced intothe chamber, and the main valve of the pumping sys-tem was closed. The arc was started at 0.01 Pa. Theanode surface was observed, and the arc voltage wasmeasured with a pen recorder, while the pressure wasgradually increased to 300 Pa. A DC power supply with

Ž .a constant voltage 500 V was used, and the arccurrent was regulated at a constant of 50 A using a variable resistor bank.

3. Results

 3.1. Aspects of the anode surface

Fig. 2 shows photographs of the arcs with a Ticathode in N gas at various pressures. The angle2

recorded was illustrated in Fig. 1. The diffuse-arc mode was observed at lower pressure. A typical image of thediffuse-arc is shown in Fig. 2a, indicating that theanode is inert. A few small luminous footpoints ap-peared on the anode surface at approximately 30 Pa.  As the pressure was increased, the number of foot-points increased and then coalesced, so that a plane

w xluminous layer appeared on the anode surface 6 . Athigher pressure, the plasma stream was pinched, and ahighly luminous position appeared on the anode sur-

Fig. 1. Experimental apparatus for the vacuum arc.

face, which is considered to be the anode-spot mode, asshown in Fig. 2d.Such a transition in anode mode relative to the

Žpressure diffuse-arc, footpoint, plane luminous, and.anode-spot modes was the same for other gases. How-

ever, the mode-change pressures were different, ac-cording to the gas species. Some unique and interestinganode aspects of a Ti cathode arc are shown in Fig. 3.For a mono-atomic molecule gas, the footpoints arerelatively large, few in number and blurred, as shown inFig. 3a,b for He and Ar, respectively. For a diatomicmolecule gas, the footpoints tended to be small,numerous and clear. Especially for H , small, brilliant2

footpoints with a sharp outline were distributed in abeautifully uniform array at regular intervals, as shownin Fig. 3c. For O , very fine footpoints with strong2

radiation were observed, as shown in Fig. 3d. These gaseffects were the same for other cathode materials.

 3.2. Arc ¨  oltage

  Arc voltages for various cathodes and gases as afunction of pressure are shown in Fig. 4. With a Ticathode as shown in Fig. 4b, the arc voltage was ap-proximately 20 V for N gas at low pressure. As the2

pressure was increased, the voltage increased, with theongoing appearance of footpoints. The voltage abruptlyincreased at approximately 60 Pa, corresponding to thetransition to the anode-spot mode. For other gases, the voltage was also 20 V in diffuse-arc mode. The pres-sures at which the footpoints first appeared were 70"10, 60"10, 70"10, 7"2, 30"10, 5"2, and 7"2 Pafor He, Ne, Ar, H , N , O , and CH , respectively. The2 2 2 4

onset pressure of mode-change and the voltage in-crease obviously occurred at lower pressure for dia-tomic and polyatomic than for mono-atomic moleculegases. In addition, the arc voltage had fluctuations of 

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( ) R. Miyano et al. r Thin Solid Films 390 2001 192196194

Ž . Ž . Ž . Ž . Ž .Fig. 2. Photographs of arc between the cathode and anode for Ti cathode and N gas: a 5 Pa diffuse mode ; b 40 Pa footpoint mode ; c 602

Ž . Ž . Ž .Pa plane luminous mode ; and d 150 Pa anode-spot mode .

approximately "2, "5 and "10 V for the diffuse,footpoint and anode-spot modes, respectively, for allgases.

For other cathodes, the trend mentioned above wasthe same, although the mode-change pressures wereslightly different, as shown in Fig. 4a,ce.

Ž . Ž . Ž . Ž . Ž . Ž . Ž .Fig. 3. Photographs of the unique anode aspects of Ti cathode arc with various gases: a He 100 Pa ; b Ar 90 Pa ; c H 30 Pa ; and d O2 2

Ž .20 Pa .

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Ž . Ž . Ž . Ž . Ž .Fig. 4. Pressure dependence of arc voltage for various cathode materials and gas species: a Al; b Ti; c Fe; d Ni; and e Cu.

4. Discussions

4.1. Pressure dependence of anode mode

In general, provided that the anode is inert, the voltage of a vacuum arc is nearly equal to the cathodedrop, because the anode and arc column drops arealmost zero. When the anode surface becomes active,the arc voltage increases. The mode changes, as well asthe arc voltage increases, are explained based on theformation of an ion deficiency region near the anodew x35,8 . The ion deficiency region is formed when ionsemitted from a cathode spot collide with other particlesand do not reach the anode surface. Then the iondeficiency must be compensated for by ions providedfrom the anode andror ambient gas near the anode.

This compensation causes an anode drop andror elec-tric field, thus increasing the arc voltage. The electricfield occurrence in a footpoint mode arc was actuallyconfirmed in the same experimental apparatus by means

w xof Langmuir probe measurement 6 . Therefore, whenthe pressure rises, it is readily understandable that theanode mode changes.

4.2. Influence of gas species and cathode material on anode mode

 As shown in Section 3.2, the anode mode stronglydepended on gas species and slightly on the cathodematerial. This fact may be qualitatively understood bythe occurrence of ion deficiency, i.e. banishment of ions emitted from the cathode spot. The collision

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cross-sections of metal iongas molecule are evaluatedbased on the polarization potential of the gas molecule.

Ž .The order of cross-section is Ne-He- Ar- N ,O2 2

Ž y19 y19-CH -H approx., 1.0=10 , 1.3=10 , 1.5=4 2

10y19 , 1.7=10y19 , 1.7=10y19 , 2.6=10y19 , and 3.7=10y19 m2, respectively, for all metal ions in the present

.study . Thus, qualitatively, the mode-change pressuresare lower for diatomic and polyatomic than for mono-atomic molecule gases. However, the order of mode-change pressure is not strictly consistent with thecross-section order. We suppose that the process of gasdissociation and ionization may be further involved inthe gas species dependency.

With regard to cathode material dependency, it isconsidered as follows. The diameters of Al, Ti, Fe, Ni,

Žand Cu atoms are similar 0.294, 0.286, 0.248, 0.230,.and 0.256 nm, respectively , so that the collision cross-

section between these metal ions and the gas moleculesis not varied. Therefore, it would appear that the ion

deficiency forms at a similar pressure, and thus theanode mode changes at a similar pressure. Nonethe-less, it is a fact that there is small dependence on thecathode material. This is considered to be due to notonly the size of the ions, but also their charge, energyand density.

5. Conclusions

In the present study, the pressure dependence of anode modes was revealed at the low current of 50 A,using a cathodic vacuum arc deposition apparatus. The

anode aspect was observed and arc voltage was mea-sured, while the pressure was increased from 0.01 toŽ .300 Pa for various cathodes Al, Ti, Fe, Ni, and Cu and

Ž .ambient gases He, Ne, Ar, H , N , O , and CH . The2 2 2 4

principal results are summarized as follows.

1. The anode mode changed from diffuse-arc to foot-point to plane luminous to anode-spot mode as thepressure was increased.

2. The anode mode and resultant voltage increase were strongly dependent on gas species and weaklyon the cathode material.

Ž3. Comparing diatomic and polyatomic H , N , O ,2 2 2

. Žand CH with mono-atomic molecule gases He,4

.Ne, and Ar , the onset pressure of the anode modetransition in the former was lower, the arc voltagehigher, and the footpoints were more numerous,smaller, and clearer.

Both the dependence of the ambient pressure andthe influence of the gas species and cathode materialson the anode mode changes can be qualitatively inter-preted by the ion deficiency theory and the differenceof collision cross-section. However, further investiga-tion is required for qualitative and precise understand-ing. In addition, new and unique features of the regularformation of footpoints for diatomic molecule gases arealso a subject to be investigated.

 Acknowledgements

The present work was partly supported by a Grant-in-Aid for Scientific Research from The Ministry of Education, Science, Sports and Culture of Japan.

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