Study of microdroplet generation from vacuum arcs on graphite cathodesMunther Kandah and Jean-Lue MeunierPlasma Technology Research Center (CRTP), Department of Chernical Engineering, McGill University,Montreal H3A 2A7, Canada

(Received 26 August 1994; accepted 29 April 1995)

The emission of microdroplets from the cathode surface in the vacuum arc ion plating depositiontechniqle is the major drawback to the technique's industrial use. The generation of these particlesfrom graphite cathodes is studied in this article and correlated to the local thermal load in thecathode spot area. A pulse discharge was used for a precise control of this load. Increases in the arccurrent level, arc duration time, and, more generally, the local temperature of the cathode werefound to increase the number and the average size of the emitted particles. Particles under theseconditions also show an increase in the width of their size distributions. Increasing the distancebetween cathode and substrate was found to decrease the number density of particles observed onthe substrate according to the solid angle covered. The microdroplets show a graphite structure anddiameters between 0.2 and 2.0 p,m. Conditions needed to decrease the number of particles emittedto the substrate are given. @ 1995 American Vacuum Society.

I. INTRODUCTION

In recent years, great interest in the deposition of dia-mondlike fllms for industrial coatings has emerged. Dia-mondlike carbon (DLC) is an amorphous carbon materialwith a high degree of sp3 bonding. The predominance ofsp3 bonding in the diamondlike coatings gives these filmsmany of the desirable properties of diamond such as highhardness, low coefficient of friction, high packing density,good visible and infrared transparencies, and chemical inert-NCSS.

DLC films are made primarily using plasma enhancedchemical vapour deposition (PECVD) or ion beam tech-niques with low substrate temperatures"t-6 One of these ionbeam techniques of industrial interest is the vacuum arc ionplating [also called cathodic arc plasma deposition (CeeO;,vacuum arc deposition (VAD), and cathodic arc evapora-tion]. This technique uses a vacuum electric arc to generateemissions from the cathode consisting of ions, neutral va-pour, and microdroplets. From 30Vo to 1007o of the emiftedvapour is ionized and associated with high kinetic energyfrom 10-100 eV depending on the cathode material.T Theseenergetic ions are deposited onto the substrate to form thefilm. The major problem in using arc ion plating is the co-deposition of micron sized particles on the substrate that maydegrade the film's properties. The formation of these par-ticles in vacuum arc deposition systems is not limited tographite cathodes; it is seen also for practically any metallicelectrode. A review of microdroplet emission in vacuum arcdischarges is given by Johnson.S Films free,of particles wereachieved successfully by Aksenov et al.,g'ro who used acurved metal tube and magnetic field to guide and deflect thepath of ihe ions to the substrate. McKcnzie etal.ll andMartin et al.1z also produced films free of particles using thefiltered arc technique. This technique involves an in flightseparation of the particles from the ion beam before reachingthe substrate, Control of particle emissions at the cathodesource could increase the flexibility of this deposition tech-nique. To this end, a better knowledge of the particle emis-sion mechanisms at the cathode spot is needed. In this ar-

ticle, the parametric dependency of particle emissions fromgraphite cathodes as well as their structure and size distribu-tions are investigated for graphite cathodes. Films containingthese microdroplets were previously analyzed using Ramanspectroscopy and Auger electron spectroscopy (ens).13 ttrespatial resolution of these measurements did not permit dif-ferentiation between the film and the particle structure. Re-sults show, however, some presence of microcrystals withthe diamond structure imbedded in a matrix composed ofamorphous carbon with graphitic domains. Scanning electronmicroscopy (SEM) of the fllms reveals some resemblance tothe imbedded microdroplets with the surface morphology ob-served in chemical vapor deposition (CVD) growth of dia-mond when a "cauliflower-like" appearance is induced athigh C/H ratio. This led to the belief that the particles gen-erated by yacuum arc on graphite cathodes could have a

structure similar to this diamond film "cauliflower-like"structure.14 The results obtained by Douyon de Azevedo andMeunier,l3 Coll et a1.,1 and Rother et a1.15 showed that a

change in the deposition parameters strongly affects thestructure of the deposited fllm. The effect of the arc param-eters on the particle structure is not clear yet. High spatialresolution AES of the particles was performed in this work tomore precisely evaluate the crystal structure of these par-ticles.

This study provides preliminary results on the generationmechanism of the particles found on the substrate under lowpressure. Measurements of the size and population of theseparticles are given. The size distributions and number of par-ticles found on the substrate for different operating condi-tions correlate with the total electrical power delivered to thearc. This simple parameter is used to characterize the heatload in the cathode spot area.

II. EXPERIMENT

This experimental work was performed in a cylindricalvacuum chamber 25 cm in diameter and75 cm in length. Thepressure inside the chamber was kept constant during thedeposition at 1.33 x 10-2 Pa for all the samples, The arc

2444 J. Vac. Sci. Technol. A'13(5), Sep/Oct 1995 0794-2101195113(5y2444n$6,00 @1995 American Vacuum Society 2444

2445 M. Kandah and J.-L. Meunier: Study of microdroplet generation from vacuum arcs

COz Laser

2445

2g NO 2St6 262

KNETIC ENEBGY, cV

Frc. 2. Typical AES spectrum for a particle deposited on silicon in vacuum

with l:14 ms, 0:100 C and cathode-substrate distance at 7-5 cm. Inset:

spectra for diamond (D), graphite (G), and arnorphous carbon (AC) (Ref.

t7).

cm. The spatial resolution of these measurements was wellbelow the sizes of the particles, providing a good differen-

tiation between the particles and the background film struc-

ture. This spectrum is typical of a graphite structure (see

inner part of Fig. 2).17 All microskuctural analyses of the

particles deposited on silicon wafers at different deposition

conditions indicated a graphitic structure. Hence, changes inthe deposition parameters over the ranges used here did not

change the structure of the microdroplets, but they didmodify the structure of the fi1m.13 The morphology of par-

ticle containing films was analyzed by SEM. One example

that produced using the poorest deposition conditions, yield-

ing a very high particle content, is shown in Fig' 3. The

experiments were performed with very small source-

substrate distance to optimize particle collection and analy-

sis. The aim is not to represent diamondlike coating possi-

bilities of the technique, but rather to chatacteize the particle

F\c. 3. SEM recording showing particles deposited on silicon in vacuum

with ,:14 ms, d:7.5 cm, and Q=140 C.

Iarc

EzEe

Shunt

Frc. 1. Experimental setup (not to scale): C capacitor, PS power supply, and

Th thl,ristor triggering/shutoff circuit.

was ignited using a CO2-TEA laser pulse producing an in-frared radiation at 10.6 pm and a maximum power of 1 MWwith arc duration time of 200 ns. The laser beam was focused

onto the tip of the cathode as shown in Fig. 1. The arc cur-

rent was generated by the discharge of a capacitor bank of 38

mF and was measured using both a shunt and a Pearson

pulse current transformer. The electrical circuit shown in Fig.

1 uses thyristor switching in order to adjust the arc duration

time / between 10 ps and 100 ms and to obtain constant and

reproducible arc current pulses of a square shape (see Ref.

16). For experiments involving a constant electric charge

through the cathode, discharge times were varied between

2.5 and 14 ms. A relatively large time interval of 15-20seconds was used between the pulses in order to maintain a

constant charge of the capacitor bank for each pulse, and to

assume a constant cathode temperature at the beginning ofevery discharge in a given sequence. Arc current 1 varied

between 44 and 110 A. The cathode used in this work was a

graphite rod 12.7 mm in diameter arrd 25.4 mm in length.

The anode was a graphite circular disk of 50.8 mm in diam-

eter with a hole in the centre of 12.7 mm in diameter. The

anode thickness was 9.5 mm and the electrodes' gap was

constant and equal to 5 mm. Small magnets were attached at

the back of the anode to increase the amount of deposition on

the substrate when studying diamondlike carbon films. The

cathode to substrate distance d was in the range 7.5-13 cm.

Scanning electron microscopy (SEM) and Auger electron

spectroscopy (AES) were used to arralyze the morphology

and the structure of both the particles and the films, respec-

tively. Image analyzer software is used to analyze the size

and the number of the particles.

I!I. RESULTS

A. Structure of emitted particles

Figure 2 shows the typical AES spectrum of particles

found in a carbon film deposited on a silicon wafer at a

vacuum pressure of 1.33 x 10-2 Pa. with an arc duration time

of 14 ms and a distance between cathode and substrate of 7.5

JVST A - Vacuum, Surfaces, and Films

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2t146 M. lGndah and J.-L. Meunier: Study ol micmdroplet generatton from vacuum arce

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Frc. 4. Number density of particles as a function of electric charge through'cathode for I:44 and 110A, d:7.5 cm, and pulse discharge time r=14 ms.

emission characteristics of the cathode source. Films withlow particle content were obtained by control of depositionparameters and an increase of the subshate distance to cath-ode, as will be discussed.

B. Effect of arc parameters on particle emissaon

1. Electric charge passing through the cathodeThe total electric charge passing through the cathode dur-

ing a complete deposition experiment is used here as a pa-rameter. The erosion rate of vacuum arc cathodes is usuallyexpressed in the literature in terms of grams per coulomb,with the value for carbon being 17 p,glc.l8 A silicon wafer12,7 mm in diameter and 0.53 mm in thickness was used asthe substrate. The number of particles emitted by the cathodeand deposited onto the substrate was evaluated for each sub-strate over five different locations each having a 1681 p,mzarea (four close to the edge comers and one at the center ofthe wafer) using an image analyzer software connected to theSEM. The substrates were always located on the main axis ofthe discharge with their surface perpendicular to thecathode-anode axis. The data collection was adjusted to re-ject particles smaller than 0.1 1trm and those larger than 10

7um. The number density of the particles was evaluated bytaking the average of the number of particles over the flvelocations for each substrate divided by the area of the obser-vation window. For two values of the arc current and forfixed arc pulse duration time and cathode-subsffate distance,Fig. 4 shows that the number density of particles increaseslinearly with the total electric charge passing through thecathode. This total charge was varied by changing the num-ber of arc pulses in a given deposition sequence.

2, Arc current level and pulse duration timeFigure 5 shows the numher density of particles observed

in the films as a function of arc pulse duration for constantvalues of the total electric charge passing through the cath-

J. Vac. Sci. Technol. A, Vol. 13, No. E, Sep/Oct 1995

0.00 o '^*.rnrf(.") e 12

Ftc. 5. Number density of particles 4s 4 funstisn of the arc pulse durationtime for a total electric charge Q of 50 C and current I-44, 7?.5, and 110 A,at d=7.5 cm.

ode and the cathode-substrate distance of 7.5 cm. For agiveu arc pulse duration time, the number density of particlesis seen to inerease with increasing arc current.

Klyarfel'd et al.le found that the rate of erosion for agiven material has a limited dependence on the axc currentfor most materials studied. Rondeel2o investigated the ero-sion rate for Cu, Cr, and Cd cathodes and found that thecathode erosion rate was essentially dependent on the totalcharge passing through the cathode. Kimblin2l also studiedthe erosion rates of Cd, Zn, Ag, Cr, Fe, Ti, C, Mo, and W andfound that, for each material studied, there is a specific ero-sion rate value that depends on the total charge that passedthrough the cathode. Daalde?2,z3 finally found that the ero-sion rate is not only dependent on arc current and arc time,but also on cathode size.

NI

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trtn o.ol2EIo0lE]ED2 0.00 0 t 2 t ,t 5 a 7 A e tOtl t2t3

HEAT FLUX INPUT (Joule)

Frc. 6, Number density of particles as a function of the total arc energy perpulse.

M. Kandah and J.-L. Meunier: study of microdroplet genermon tronr uurln rE3

According to these authors, the total mass loss from agiven cathode geometry should essentially correlate with the

Iotal amount of electric charge passing through the cathode'

A large part of the mass loss is created by the ion current

emitted from the cathode' This ion current was found by

many authors to represent a constant proportion of the total

arc current, with the ratio being |l'Vo fot carbon cathodes

according to Kimblin.2l Hence, imposing a fixed value of the

arc currcnt for each discharge pulse as well as a given total

electric charge (i.e., a sum of all individual pulse charges)

passing through the cathode for a set of experiments should

induce a constant erosion throughout these experiments'

while also keeping the ion component of this erosion con-

stant. One would then expect the mass loss carried by the

macrodroplets to remain relatively constant throughout these

experiments. To verify this assumption, the arc duration time

was varied keeping the arc current, the cathode-substrate

distance, and the total electric charge through the cathode

constant. The arc duration time for each pulse was varied

between 2.5 and 14 ms and the total number of arc pulses

was adjusted to keep the total electric charge constanl at 50

C. The arc pulse duration remained constant during one ex-

periment carried over the 50 C charge. Figure 5 shows that

an increase in the arc duration time while keeping other pa-

rameters constant increases the numbcr d€nsity d pillicles

deposited onto the subsfiate. this result questions the use ofelecftic charge as a parameter to normalize the erosion rate'

C. Cathode spot temperature cohsiderations

The above parametric dependencies suggest the impor-

tance of the power input at the cathode affecting the tempera-

ture in the cathode spot area. Particle emissions were corre-

lated with an estimated heat load 4 in this zone' The

objective is to obtain a scaling of the temperature evolution

in the cathode spot area with varying arc discharge param-

eters. The heat load in the cathode spot area is approximated

simply by the axc power that is essentially delivered in this

zone during an arc duration time ,:

o: l" rrdr- (l)-' J,o

where /s is the arc triggering time, /1 the arc shutoff time of

an indiviAua arc pulse, 1 the arc current, and V the total

voltage drop.All experimental results on particle number density mea-

sured on the substrate arc grouped together using this main

parametric dependency. The total electric charge is adjusted

o.4 0.30

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Frc. 7. particle size disEibution for estimati heat flux values of (a) 0.48, (b) 3.02, (c) 6.05'

tro u." glr"n for each case. (O): Original data; (-): Gaussian fitted distribution'

JVST A - Vacuum, Surfaces, and Films

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2448 M. Kandah and J.-L. Meunier: Study of microdroplet generation from vacuum arcs

Frc. 8. SEM of a deposit on silicon in vacuum with t:14 ms showing

particles agglomerated form larger particles.

to the constant value of 50 C by controlling the number ofarc ignitions. The correlation of particle density with thisheat flux is shown in Fig. 6. One can see the number density

of particles increasing linearly with an increase in the heat

1oad. These results suggest that the generation mechanism ofparticles emitted by vacuum arcs on graphite is driven bylocal thermal effects in the cathode spot zone.

D. Particle size distributions

The size distribution of the particles deposited on the sub-

strate was analyzed at four of the heat flux values shown inFig. 6. Figure 7 gives the distributions observed as well as

the mean particle size $, and the standard deviation o, ofthese distributions. The solid lines represent Gaussian fitted

curyes used to evaluate d o and o, . The average particle size

increases from 0.2 to about 2.0 pm when the cathode spot

temperature is varied by modifying the arc parameters. The

results show that an increase of the heat load in the cathode

spot area increases the number density of the emittedparticles as well as their mean particle size and the

width of the size distribution. The total mass of particles ofthe substrate per surface area unit corresponding to the data

of Figs. 7(ai)-7(dt are 3.6x l0-4, 2.l3xl0-3, 6.25xlo-3,and 8.26x10-3 y,g/ y,m2, respectively. The very large par-

ticles may be formed by agglomeration of some of the larger

particies upon impact as shown in Fig. 8. Also, the number

of small particles observed in the distributions corresponding

to high heat flux may reflect the poorer mechanical integrityof large emitted particles that break upon impact as shown inFig. 9. Agglomeration and breaking upon impact reveal an inflight cooling time that is not long enough for solidificationof these larger sized particles.

It is very interesting to note at this point that the vast

majority of particles observed have a spherical morphology.

This indicates melting of the surface at the cathode spot,

melting being confirmed by the surface morphology in the

cathode spot area. Such an observation would not cause any

surprise on metallic electrodes. On graphite, however, a ther-

mionically emitting material, the occurrence of a liquidphase in this high pressure/high temperature zone may give a

J. Vac. Sci. Technol. A, Vol. 13, No. 5, Sep/Oct 1995

2448

10 pm

Frc. 9. SEM of a deposit on silicon in vacuum with t:5 ms showing large

particles dissociated into several small particles upon impact.

clue as to the actual local physical properties. Since graphite

is a porous material, it was expected that a latge portion ofthe observed particles would have irregular shapes. These

particles would have been emitted from the cathode because

of thermal shock effects around the spot. Such irregularlyshaped particles were, in fact, very seldom observed on the

substrates, with the vast majority of the particles having a

spherical shape. Rother et a1.24 have reported coatings con-

sisting of a diamondlike matrix in which fragmentlike and

agglomerated particles having a graphitic structure are em-

bedded. The size and lateral distribution of these fragment-

like particles did not depend upon the variation of deposition

parameters. Rother et al. do not state values of the cathode

spot residence times in their 125 A dc arc coating device. We

believe, howevel their conditions should correspond to the

high local heat load conditions inducing thermal shock emis-

sions and particle agglomeration.

E. Effect of cathode-substrate distance on particlecollection

Figure 10 shows the relation between the number density

of particles as a function of electric charge passing through

the cathode at two cathode-substrate distances and constant

arc current. A decrease in the number density of particles is

observed with an increase in the cathode-substrate distance

from 7.5 to 13 cm. Figure 11 shows that the number density

of particles normalized to the solid angle of the substrate

viewed by the cathode remains approximately constant at

different distances between cathode and substrate. The ionflux intensity was measured by Douyon de Azevedo and

Meunier 25 as a function of the distance to the cathode in the

axial direction of the cathode surface normal under the same

geometry and similar experimental conditions- In vacuum

the ionic flux also decreases with distance as the solid angle

covered by the probe. The flux was found to decrease withdistance R to the cathode from R-2 in vacuum down to

nl 0.2I I=44A

O d= 130 mmO d=75 mm

Frc. 10. Number density of particles as a function of electric charge throughthe cathode for cathode-substrate distarces of 7.5 and 13 cm with ,= 14 ms

and I:M A.

ft-s at 4 Torr hydrogen, as shown in Fig. 12. Hence, a com-promise has to be made between film growth rate and lowparticle content.

rv. DlscussroNIt was speculated by many researchers that some of the

particles emitted by vacuum arcs on graphite cathodes mayhave diamond structures. Although the films produced willshow diamondlike characteristics under proper depositionconditions, all the particles analyzed in the present study

show a graphite structure and no particles showing signs ofsp3 bonding in the AES sigrtal were found, in agreement

with results presented by Rother et ql.2a

The number density of particles on the target is found toincrease linearly with arc current, arc pulse duration, and

.+0 E0 120 t 60ELECTBIC CHARGE THROUGH CATI{ODE. C

Fro. 1 l. Number density of panicles per unit of solid angle of the substrate

as viewed by the cathode for arc currents of 44 and ll0 A at cathode-substrate distances of 7.5 and 13 cm.

JVST A - Vacuum, Surfaces, and Films

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M. Kandah and J.-L. Meunier: S:tudy of microdroplet generation from vacuum arcs

1000

2449

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Probe-Cathode distanee, mm

Ftc. 12. Axial distribution of the ionic flux at various hydrogen pressues(Ref. 2s).

electric charge passing tbrough the cathode. These evolutionsare to be expected; the cathode erosion is normally given interms of weight loss per integrated arc electric charge, Theexperimental results presented here concerning arc currentand pulse duration were, however, measured while keeping

the total electric charge through the cathode constant. Thenumber of arc discharges during each test was used to con-trol this total charge. Therefore, the evolution of the numberof emitted particles with arc current and arc discharge dura-tion is not consistent with an erosion process in which par-

ticle generation is always a fixed portion of the total cathode

erosion. The cathode mass loss in the form of particles mayvery well contribute to different fractions of the total erosion

rate with varying discharge parameters. The relative impor-tance of particle emission on erosion rate data is furtherstressed if one considers the erosion in the form of ions tomaintain a constant ion current to arc current ratio, and a

constant average ion charge for varying arc parameters.2l

Increasing the arc current may lead to a greater number ofspots or to a spot size increase.26 The maximum culTent re-quired to form more than one spot on a graphite cathode is

around 200 A.2r The arc crurent used in this work was al-ways below this value, therefore no change should be ex-

pected in the number of spots under the experimental condi-tions used. For a given value of the total electric charge, allparticle collection data conelate with a change in the heated

volume and the estimated heat load input in the cathode spot

zone. Cathode spots on graphite show very limited motioncompared to those on metal electrodes. It is interesting tonote that modifying the arc culrent intensity or the arc pulse

duration to obtain a given thermal load will result in the

same particle emission characteristics. Further evidence ofthe thermal nature of particle emission on graphite cathodes

is given by the particle size distributions. The mean particlesize and the width of the distributions correlate with the ther-

mal load. An increase in the cathode spot surface temperature

should produce a larger melted volume in the spot region,giving rise to larger particles for a given plasma pressure

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2450 M. Kandah and J.-L. Meunier: Study of miclodroplet generation from vacuum arcs

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0 1000 2000 3000 40@ 5000 60@

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FIc. 13. Phase diagram for carbon (Ref. 29). The shaded region corresponds

to possible temperatures in the cathode spot area for melting to occur as-

suming a plasma pressure of 106 Pa (Refs. 27 and 28).

above the cathode spot. It is thus important in view of coat-

ing applications to characterize the process not only in termsof mass loss of the cathode per unit charge, but to considerthe local heat load induced by cathode spots having a givenresidence time over a specific region of the cathode surface.

Although we cannot presently quantify the two effects, itappears that particle emission from graphite cathodes is es-

sentially composed of melted particles from the spot area and

not of thermal shock emission. This is particularly interestingin view of the possible control of particle emission, espe-

cially considering that the short pulse experiment shouldhave ia fact induced stronger thermal shock effects.

As stated in Sec. III D, the graphite particles seem to be

formed from a melted carbon pool on the cathode spot sur-

face. This may set a lower limit to the graphite cathode spot

surface temperature. According to lvanov et a1.27 and

Jiittner,2s the plasma pressure above the cathode spot evalu-

ated for metallic electrodes should be of the order of 106 Pa

with a plasma density of 1025 m-3. From the phase diagramfor carbon given in Fig. 13,2e such a plasma pressure leads toa minimum surface temperature for the onset of a liquidphase of roughly 4600 K.

V. CONCLUSION

Much progress has been made during recent years in pro-ducing films using vacuum arcs that, while not completelyfree of particles, are a significant improvement over the filmsthat were produced earlier.

The structure of the particles generated in the present

study was found to be graphite for all operating conditions.The claim that the particles emitted from a graphite cathodemight have diamond characteristics is not verified in thiswork. The average particle size ranged between 0.2 and 2.0pm in diameter depending on the temperature in the cathodespot area. Thermal shock particle emissions were found to besmall compared to emissions of melted microdroplets. Thenumber density of the particles emitted from the cathodesurface and observed on the coated substrate is found to cor-relate with the heat load in the cathode spot zone. The num-ber of microdroplets can hence be decreased by lowering the

J. Vac. Sci. Technol. A, Vol. 13, No. 5, Sep/Oct 1995

2450

temperature in the cathode spot volume through control ofthe arc current level and/or arc residence time at a given site

on the cathode surface. This is an important result and indi-cates that good control of the thermal flux in the cathode spotarea is essential if one aims at achieving a low particle con-

tent in the films. Also, increasing the distance between the

cathode and the substrate, i.e., decreasing the solid angle ofthe substrate as viewed by the cathode, results, as expected,in a decrease in the number density of particles reaching the

subsffate surface. In this work, no lower limit to particleemission by a graphite cathode was found within the arc

parameter ranges studied. Finally, particle morphology, indi-cating the presence of a liquid phase in the carbon cathode

spot, gives indications of the temperatures involved in thiszorle.

ACKNOWLEDGMENTS

The support of the Jordan University of Science and Tech-

nology (JUST), the Natural Science and Engineering Re-

search Council of Canada (NSERC), and the Fonds pour laFormation des Chercheurs et l'Aide i la Recherche (FCAR)

for sponsoring this research is gratefully acknowledged.

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