characterisation studies of the pulsed dual cathode magnetron …parsons/paper2.pdf · 2003. 12....

Ž .Surface and Coatings Technology 142�144 2001 621�627

Characterisation studies of the pulsed dual cathodemagnetron sputtering process for oxide films

J. O’Brien�, P.J. KellyCentre for Materials Research, Uni�ersity of Salford, Salford M5 4WT, UK

Abstract

Pulsed magnetron sputtering has become the leading industrial production process for large area thin film deposition due to itsversatility, low environmental impact and ability to provide uniform coatings across large substrate areas. Such applications

Ž . Ž . Ž .commonly employ oxides, including silica SiO , alumina Al O and titania TiO . Although all of these materials can be2 2 3 2produced by reactive DC-magnetron sputtering, until recently the deposition process was highly problematic. The industrialexploitation of the pulsing process is impeded by the fact that, during long-term deposition runs, eventually all surfaces will be

Ž .covered with the insulator. Once the chamber and anode are covered with insulating material, there can be no average DCcurrent flowing to the power supply leads. Moreover, if the anode�chamber becomes covered with the dielectric film it will cease

Ž . Žto operate as an effective ground anode . A more viable approach is to use two magnetrons in a dual bipolar arrangement dual.cathode . In this arrangement each magnetron acts alternatively as an anode and a cathode. The broad aim of this study is to

investigate the inter-relationship between ‘global’ parameters, pulse parameters, plasma parameters and in particular, filmproperties in relation to the dual cathode system. The spatial distribution of measured parameters will be considered. This paperdescribes the production of Al O films by dual cathode unbalanced reactive magnetron sputtering, in particular, the effects of2 3spatial distribution and pulse frequency on coating properties. In general, it was observed that, once hard arcs have beenremoved, all coating structures, coating surfaces and hardness show little variation. However, more variation was observed incritical loads during scratch adhesion testing for coatings deposited at different pulse frequencies. � 2001 Elsevier Science B.V.All rights reserved.

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1. Introduction

High quality functional films on large area substratesare becoming steadily more important and widespread.Applications include low emissivity and solar controlcoatings on architectural glass, anti-reflective coatingson automobile windscreens and flat-panel displays and

� �transparent films on food packaging 1 . In all cases,high rate, stable deposition conditions are required.Pulsed magnetron sputtering has become the leadingindustrial production process for large area thin film

� Corresponding author. Tel.: �44-161-745-ext4009; fax: �44-161-745-5108.

deposition, due to its versatility, low environmentalimpact and ability to provide uniform coatings across

Ž .large substrate areas up to 4 m in width . The coatingmaterials used in the applications listed above are most

Ž .commonly oxides, including silica SiO , alumina2Ž . Ž .Al O and titania TiO . Although all of these mate-2 3 2rials can be produced by reactive DC magnetron sput-tering, until recently the deposition process was highlyproblematic. The use of this technique for the deposi-tion of highly insulating materials is limited by theintrinsic problems of target poisoning, and the conse-

� �quent arcing and process instabilities 2 . Arc eventsduring reactive sputtering are a serious problem, be-cause they can cause defects in the coating structure;affect the composition and properties in the growing

0257-8972�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 1 0 1 0 5 8 - 1

( )J. O’Brien, P.J. Kelly � Surface and Coatings Technology 142�144 2001 621�627622

film; and lead to damage of the magnetron power� �supply 3 .

The use of pulsed DC power has transformed thedeposition of dielectric materials such as alumina. Thisprocess has been well described in the literature and

� �will only be reviewed here 1,4 . Arc events are sup-pressed and as a result, very significant improvementsin structure and hence, other properties have been

� �observed compared to DC sputtered films 5 . Inessence, during the pulsed sputtering process the pulse-on time is limited, so that the charging of the insulatinglayers on the target does not reach the point werebreakdown and arcing occur. The charge is then dissi-pated through the plasma during the pulse-off time.The industrial exploitation of the pulsing process is,however, impeded by the fact that, during long-termdeposition runs, eventually all surfaces will be coveredwith the insulator unless new conductive, i.e. metallic,material is brought into the system constantly. Oncethe chamber and anode are covered with insulating

Ž .material, there can be no average DC current flowingto the power supply leads. Moreover, if theanode�chamber becomes covered with the dielectricfilm it will cease to operate as an effective groundŽ .anode . This gradual shifting of the electric field shapecan generate serious changes in the deposition patternof the magnetron and possibly wreck the deposition

� �uniformity 6 .A more viable approach is to use two magnetrons in

a dual bipolar arrangement. In this case, both mag-netrons are connected to the same alternating powersupply and the target voltage is reversed during eachcycle. Thus, each magnetron acts alternatively as ananode and a cathode. The periodic pole changing pre-vents arcing, but also effectively maintains ‘clean’ tar-

Ž .get surfaces, allowing high rate, long term �300 h ,� �stable deposition conditions 7 . This system has al-

ready overcome some of the difficulties involved insputtering dielectric materials and is clearly a processwith great potential. However, much development isstill required before this system is optimised and theprocess can be considered routine.

The broad aim of this study is to investigate theŽinter-relationship between ‘global’ parameters, such as

magnetron configuration and orientation, degree of.unbalance and magnetic field strength ; pulse parame-

Ž .ters frequency, phase relationship ; plasma parametersŽ Ž . Ž .electron temperature T , ion density n , electrone i

Ž .. Ždensity n and in particular, film properties optical,e.structural physical in relation to the dual cathode

system. In addition, the use of a separate ion assistedsource will also be investigated. In all cases the spatialdistribution of measured parameters will be con-sidered. The present study looks at the production ofAl O films by dual cathode unbalanced reactive mag-2 3netron sputtering, in particular the effects of spatial

distribution and pulse frequency. Operating dual un-balanced magnetrons in a closed-field configuration

Žcan result in a high flux of relatively low energy �50. � �eV ions being drawn at the substrate 8 . It was

observed that varying pulse frequency can lead tonotable variations in both the time-averaged ion cur-rent at the substrate, and in the time-resolved wave-form.

2. Experimental

For this work a Teer Coatings Ltd. UDP450 vacuum� �coating rig 9 was used fitted with 2 Gencoa Vtech

variable magnetrons in the vertically opposed configu-ration. These magnetrons allow magnetic field strengthand degree of balance to be varied in situ. For thisstudy the Vtech inner and outer magnetic arrays werefixed in the fully forward position giving maximum field

Ž .strength �0.06 T at the target and a ‘mid range’degree of unbalance for these magnetrons.

Power to the targets was supplied via a dual channelAdvanced Energy Pinnacle Plus magnetron driver. Inthe dual cathode mode this unit allows the magnetrondischarge to be pulsed at frequencies from 100 to 350kHz, at 0.5 duty factor, at a nominal reverse voltage of10%. Pulsed voltages are often schematically repre-sented by a square wave format but in the case of thePinnacle Plus significant voltage overshoots areobserved in both directions. This is illustrated by theoscilloscope measurements shown in Fig. 1 and Fig. 2.These figures are voltage waveforms at one of the twotargets during sputtering of aluminium. They also showtypical substrate current waveforms when the substrateis floating, but attached to an MDX supply. Theseexamples, at 100 and 350 kHz, illustrate how the volt-age waveforms differ greatly, along with the resultingsubstrate current waveform, both in magnitude and

� �shape. In concurrent studies by Bradley et al. 10 it hasbeen observed that the electron temperature increaseswith pulse frequency. This is attributed to the largerovershoot spike at higher pulse frequencies, marked �and � on Fig. 1 and Fig. 2. The aim of this work is toestablish if any marked difference in coating propertiesaccompany these changes due to pulse frequency in thedual cathode mode. In the dual cathode mode targetvoltages run 180� out of phase with each other, oneacting as a cathode whilst the other is an anode andvice versa. The majority of work reported here hasbeen completed using this arrangement. Additionaldepositions at 200 kHz frequency have also been com-pleted in the synchronous mode, were both targets runin phase. Clearly this mode of operation does not havethe same advantage of the dual cathode system but isof interest due to the different resulting substratecurrent forms. The DC films were deposited using the

( )J. O’Brien, P.J. Kelly � Surface and Coatings Technology 142�144 2001 621�627 623

Ž .Fig. 1. The voltage at one target and substrate current waveforms for metallic deposition 100 kHz, 2 A, dual cathode .

same Pinnacle Plus power supply with the pulse fre-quency set to zero.

Alumina films were deposited by reactive sputteringin an Ar�O atmosphere, using an optical emissions2monitoring control system. Conditions for stoichiomet-ric Al O and high rate deposition were determined2 3

from previously compiled hysteresis curves for the sys-� �tem utilised 11 . Target current was fixed at 4 A for all

runs and coating pressure was fixed at 1.25 mtorrpartial pressure of Ar. Substrates were rotated betweentargets at a target-to-substrate separation of 110 mmand samples were positioned at several positions along

Ž .Fig. 2. The voltage at one target and substrate current waveforms for metallic deposition 350 kHz, 2 A, dual cathode .


Ž .Fig. 3. The voltage waveforms at both targets in dual cathode mode, with resulting substrate current 150 kHz, 4 A .

the length of the substrate holder. The main substrateused was glass; run time was selected to give a coatingthickness of approximately 2 �m. A full experimentalarray was completed, including repeats, looking at theeffect of pulse frequency on coating properties. Pulsefrequencies of 100�350 kHz were investigated in thedual cathode mode.

Coating structure was investigated using SEM andsurface roughness was measured by profilometryŽ . �Talysurf 10 . XRD Philips PW1729 X-ray generatorŽ .�Cu K was used to assess if the structure was amor-�

Žphous or crystalline and EPMA JEOL JXA 50A.equipped with WDAX determined the composition of

the films. Hardness was measured by a Fischerscopemicrohardness tester fitted with a Vickers indentor. Aload of 50 mN was used and hardness values weredetermined at 10% of the coating thickness. Single passscratch testing was completed using a Teer CoatingsLtd. ST3001 Tribotester fitted with an acoustic emis-sion module.

3. Results

All pulsed depositions ran stabily with no hard arcsdetected, the reactive sputtering system also gave verystable operation. Conversely, during DC deposition arc-ing occurred almost continuously, which caused somedegree of instability with the reactive system. Deposi-tion rates during reactive pulsing was found to be in

the region of 65% of that for the DC reactive processand 43% of that for the metal under the same pulsedconditions.

Fig. 3 and Fig. 4 illustrate the voltage waveforms at150 kHz pulse frequency at each target, for dual cath-ode and synchronous mode respectively, also illustratedare the substrate currents drawn.1 It can be seen thatthe substrate current waveforms differ greatly, mostpredominantly in that the current waveform in thesynchronous mode features a large current peak duringevery cycle.

The ion current drawn at the substrate was measuredusing the bias supply, an advanced energy MDX unit,was found to be between 70 and 90% of that achievedrunning in DC mode during dual cathode pulsed opera-tion. This is despite the fact that the duty factor is only0.5. The actual value was dependent on pulse fre-quency. It was also observed that ion-to-atom ratiovaried as pulse frequency increased. We estimate thereare approximately 1.6 times more ions per atom at 200kHz, where the ion-to-atom ratio is at maximum, thanduring DC deposition. This is due both to a slight dropin deposition rate and also to an increase in the aver-age ion current density from 100 to 350 kHz pulsefrequency. In the synchronous pulse mode the averageion current drawn at 0.5 duty factor is approximately40�60% of the DC value, again dependent on pulse

1At �100 V bias


Ž .Fig. 4. The voltage waveforms at both targets in synchronous mode, with resulting substrate current 150 kHz, 4A .

frequency and has an additional ion current spikeŽ .refer to Fig. 4 .

Structure did not vary over the substrate area. Allcoatings produced by the dual cathode and syn-chronous mode pulsed processes were found to havefully dense structures with no defects. Furthermore, thecoatings were found to be amorphous and highly trans-parent, although no optical properties have been de-termined as yet. No inclusions were observed uponinvestigation with SEM. The glass-like structure ofcoatings produced during pulsed deposition is well il-lustrated by Fig. 5; all pulsed coatings show a similarstructure. The DC coatings were again dense but con-tained a very high number of inclusions and ‘splats’Ž .one such feature is marked A on Fig. 6b , DC coatingstended to be ‘smoked’ in appearance and therefore,would have a reduced transparency. Figs. 6a and bshow a typical DC coating structure.

Fig. 7 shows profilometry traces at a 10 000� magni-fication. It can be seen that the surfaces of pulsedsamples are very smooth. No real variation with pulsingfrequency or substrate position was noted. Coatingsproduced by DC deposition exhibit a much increasedroughness. Pulsed coatings were found to have a sur-face texture parameter, Ra2, value of 0.004�0.008 �m,

2 The arithmetic mean of the departures of the roughness profilefrom the mean line.

whereas DC coatings gave an average Ra value of0.25�0.3 �m.

Composition also varied a little with pulse frequencyor spatial position. All coatings were stoichiometricAl O and all contain levels of included Argon. The2 3

rougher surfaces of DC coatings made an accuratemeasurement of composition difficult. The Ar could beincluded in the lattice in the place of Al or be held in

� �small pockets between the structure 12 . The DC coat-Ž .ings were found to give lower Al readings �34% .

This could be due to one or a combination of four

Fig. 5. The typical glass-like structure of Al O pulsed DC coatings2 3Ž .150 kHz, 4 A .


Ž . Ž .Fig. 6. a Fracture section of DC Al O coating showing defects. b2 3Top surface of DC Al O coating, featuring ‘splats’ marked A.2 3

Ž . Ž .reasons: a an actual lower percentage of Al; bincreased inaccuracy due to scattering from the much

Ž .rougher top surface; c an increased degree of ArŽ .inclusion; and d an increased number of voids in the

structure. Further studies will be carried out to try andascertain the main factor effecting the DC EPMAreading; however, it is most likely that the low DCvalues are due to both scattering from the surface,giving greater inaccuracy and greater Ar content.

Extensive Vickers microhardness tests revealed novariation in hardness over the substrate area, any varia-tions were well within the error of the technique. Inaddition, no real trend with pulse frequency was es-tablished, although a slight peak in hardness at 100kHz was noted. Using Vickers hardness the hardness of

Žpulsed coatings varied from 12.5 to 13.75 GPa ��1.Gpa with a value of approximately 13.4 GPa for DC

coatings.Initial scratch tests have been completed on a num-

ber of samples. The critical load for coating failure wasfound to decrease from 25 N, for coatings produced at

150 kHz pulse frequency, to 3.5 N for 350 kHz pulsefrequency coatings. It should be noted, however, that todate scratch tests have not been completed on 100 and200 kHz specimens and further work is in progress toreaffirm results obtained. DC specimens failed at acritical load between 50 and 60% of that for the bestpulsed specimens tested so far.

4. Discussion

Results would seem to imply so far that, under theconditions investigated, pulsing in the dual cathode orsynchronous mode produces highly transparent, defectfree, stoichiometric coatings that are produced by avery stable process. Results to date show no variationalong the substrate length in structure, composition,thickness, Ar inclusion or hardness. In addition, no realmajor trends in the same properties with pulse fre-quency or mode of pulse can be discerned despite thedifferences in pulsing signal leading to variation inplasma properties and the energy�flux of particles inci-dent at the substrate.

Trends have been established between pulse fre-quency and scratch test critical loads. A general de-crease is observed with increasing pulse frequency �although all values have not been collected to date.The decrease in the critical load with frequency may bedue to either decreased adhesion between the coatingand the substrate or increased stresses in the coatings.

Extensive optical property measurements and furtherscratch and wear tests may reveal more variationsbetween the coatings and these will be investigated inthe next portion of the work.

5. Conclusions

Highly transparent, defect-free alumina films havebeen grown in the dual cathode and synchronous pulsedsputtering modes. In both modes the deposition process

Ž .is stable, at least in the short term �10 h . The DCprocess, under the same conditions, leads to hard arcsand a less stable process. Coatings produced using thisprocess have a high defect density.

Little variation in coating structure, surface rough-ness or coating hardness was observed with pulse fre-quency or mode of pulse. A decrease in critical load, asdetermined with scratch tests, was found with increas-ing pulse frequency. Optimal pulse coatings showed upto a two-fold improvement on critical load over DCcoatings.


Fig. 7. The surface roughness profile of typical DC and typical pulsed coating surface.

References

� �1 G. Brauer, J. Szczyrbowski, G. Teschner, Surf. Coat. Tech.Ž .94-95 1997 658�662.

� �2 S. Schiller, K. Goedicke, J. Rescke, V. Kirchoff, S. Schneider,Ž .F. Milde, Surf. Coat. Tech. 61 1993 331�337.

� �3 W.D. Sproul, M.E. Graham, M.S. Wong, S. Lopez, D. Li, R.A.Ž . Ž .Scholl, J. Vac. Sci. Technol. A13 3 1995 1188�1191.

� � Ž .4 A. Belkind, A. Freilich, R. Scholl, J. Vac. Sci. Technol. A17 4 ,Jul�Aug 1999.

� �5 P.J. Kelly, O.A. Abu-Zeid, R.D. Arnell, J. Tang, Surf. CoatŽ .Tech. 86�87 1996 28�32.

� � Ž .6 R.A. Scholl, Surf. Coat Tech. 98 1998 823�827.� � Ž .7 J.C. Sellers, Surf. Coat. Tech. 98 1998 1245�1250.� � Ž . Ž .8 P.J. Kelly, R.D. Arnell, JVST A16 5 1998 2858�2869.� � Ž .9 P.J. Kelly, R.D. Arnell, Surf. Coat. Technol. 108-109 1998

317�322.� �10 J.W. Bradley, H. Baeker, P.J Kelly, R.D. Arnell, accepted for

publication in Surf. Coat. Tech.� � Ž . Ž .11 P.J. Kelly, R.D. Arnell, JVST A17 3 1999 945�953.� �12 P.J. Kelly, PhD Thesis, University of Salford, UK, 1997.

characterisation studies of the pulsed dual cathode magnetron …parsons/paper2.pdf · 2003. 12....

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