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Gold lon PlatingA RECENTLY DEVELOPED COATING PROCESS

E. W. Williamsici Corporate Laboratory, Runcorn, England*

The availability of a new technique for the application of thin gold

coatings to a variety of substrate materials must be of interest to a

large number of gold users. Gold ion plating offers a number of

technical advantages over more traditional gold coating processes and

it ts to be expected that it will compete with such processes to an

increasing extent.

While electroplating was developed over acentury and a half ago(1), ion plating, whichwas invented in 1963 by Mattox (2), is a very newprocess. It is therefore only very recently thatindustries involved in the application of surfacecoatings have begun to realise the full potential

of this technique.

Interest in ion plating, especially outside thelaboratory, remained at a low ebb until the FirstEuropean Conference on Ion Plating and AlliedTechniques (IPAT) was held in Edinburgh inTune 1977 (3). Since that time, interest in Europeand North Amercia has been increasing at arapid rate and products in the manufacture ofwhich gold ion plating is now being used rangefrom decorative coatings, through corrosion re-sistant films to optical filters.

The Ion Plating Process

Ion plating basically derives from the con-ventional evaporation and sputtering techniquesand is best described in comparison with them.

Figure 1 is a schematic representation of theconventional evaporation (a), D.C. sputtering (b)and ion plating (c) processes. Conventionalevaporation is used for the deposition of thinfilms of gold onto metallic substrates and providedthe coat is lens than 1 pm in thickness, a degreeof adhesion of the gold film to the substrate isachieved. However, for conventionally evaporatedgold coatings on a wide range of non-metallicsubstrates, such as glass, plastics and ceramics,adhesion is very poor and the popular 'stickytape test' usually completely removes the coating.The vacuum required for the evaporation of most

*The author Is now witti Evi[, Centra! Research Laboratories. FIayes, England

(a) CONVENTIONAL (b) Q.C. SPUTTERING (c) D.C. ION PLATING

EVAPORATION

Fig. 1 Schematic comparison of three related gold deposition processes: (a) conventional evaporation (h) D.C.sputtering (c) D.C. ion plating

30

metals must be 0.001 Pa or lower so pumpdown times are quite long with most of the smaltbench top units. In the case of gold, because itresists oxidation at high temperatures the evacu-ation conditions need not be so stringent and apressure of 0.01 Pa is adquate. The gold wire orfoil is melted in the low voltage tungsten filamentbasket source until the gold wets the sourcewire. During this melting process a shutter,which is not shown in the diagram of Figure1(a), remains in place to prevent gold particlesand impurities from reaching the target. Oncemelting is complete, the shutter is removed andthe evaporation commences by raising further thetemperature of the source. Evaporation ceaseseither when all the gold is used up or as theoperator reduces the current through the sourcewhen a film thickness monitor indicates that therequired coating thickness has been deposited.

Conventional evaporation equipment is rela-tively simple and for the same size of vacuum unitand belt jar is always cheaper than that for sput-tering or ion plating. It is also has the advantagethat the substrate on which the film is to bedeposited can be suspended from a clamp fittedto the base of the unit, and does not require avacuum 'feedthrough'. The smaller number ofvacuum feedthroughs means that there are fewerplaces where leaks can occur.

Although sputtering and ion plating processesare normally more expensive, the coating adhesionobtained is much stronger than with conventionalevaporation for the vast majority of coating andsubstrate material combinations. The main reasonfor this is that the energy of the incident metalatoms is higher. This is clearly shown in Figure 2which indicates the metal atom or ion energyrange in the three processes of Figure 1 and alsothat for ion implantation. For evaporation theenergy ranges from 0.1 to 1 eV compared tosputtering where it ranges up to 30 eV. With ionplating, on the other hand, it can go up to severalthousand eV.

Only a very brief outline of sputtering will begiven and the reader is referred to Maissel andGlang (4) for a review of sputtering methods.Figure 1(b) shows a simple sputtering rig. Thesource or target in the foren of a gold disc isfixed onto a cathode. The substrate is positionedjust below the source on an earthed electrode.After evacuating the belt jar in the normai way to apressure of about 0.001 Pa, argon is admitteduntil the pressure reaches about 100 Pa. A dis-charge is then produced by establishing a cathodevoltage above 1 000 volts. Positive argon ions inthe plasma are accelerated towards the cathodeand on striking the target they eject gold ions

100K

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Fig. 2 Metal atom or ion energy ranges for evapo-ration, sputtering, ion plating and ion implantation.(Logarithmie scale)

and secondary electrons. These ions are thenaccelerated towards the earthed substrate anddeposited onto it. Those of the secondary electronsthat strike argon atoms cause ionization and main-tamn the plasma. Some of the electrons reach thesubstrate and raise its temperature to well over100 °C. An earthed shield around the side of thecathode contains the plasma in front of the source.

D.C. ion plating has recently been reviewed byTeer (5). The process, as shown in Figure 1(c)has some of the features of both conventionalevaporation and sputtering. As in sputtering,argon is admitted to the jar until the pressurereaches about 100 Pa and a discharge is struckbetween the earthed source and the substratecathode. The argon ion plasma is firstly used tosputter-clean the metal substrate onto which thegold is to be deposited. The time required forthe cleaning will depend on the metal but for steelabout 30 minuten is required. After cleaning, goldis evaporated in the manner described above andenters the plasma. Gold atoms and ions arescattered by the argon ions and are acceleratedtowards the substrate where they deposit. Asa result, excellent bonding is achieved betweenthe deposited gold and the substrate. Film ad-hesion is also enhanced by the sputter cleaningwhich goes on during the deposition and removesany material loosely bonded to the substrate; it isalso increased by diffusion in cases in whichalloys can form between the gold and the sub-strate. The diffusion takes place more rapidly ifthere is no water cooling of the cathode since

31

temperatures in excess of 400 °C are produced bythe ion bombardment. With the earthed shielddesigned to protect only the top part of thecathode, as the diagram shows, the plasma extendsto the back of the substrate so that gold platingoccurs on the back as well as the front. With thesubstrate stationary the gold film is only half asthick on the back as on the front, but by rotatingthe sample in the plasma a uniform thickness isobtained. Some gold ion plated samples are shownin the photograph in Figure 3.

Gold ion plating onto metals like steel, copperand aluminium gives excellent film adhesion inevery case but in the case of non-conductors likeplastics and ceramics problems occur because ofthe D.C. biasing or charge accumulation. Carefulcleaning and treatment before the plating isnecessary for most non-conductors and hand-ling after cleaning must be done with appropriateprecautions. The use of a conductive mesh cagein front of the non-conductive substrate has en-abled reasonably good adhesion to be obtainedwith gold on plastics like Polyethersulphone while`lightning' discharge and burning out of the surfacecoating have been avoided. Jones etal. have

described in some detail the use of D.C. ionplating for plastics (5, 6).

It is likely that in the near future radio-frequency (R.F.) biased ion plating will replaceD.C. biasing for coating of plastics and othernon-conductive substrates and some preliminarywork in this area has recently been reported (7).

It is obvious that ion plating is a higher energyprocess than sputtering when the two techniquesare compared as in Figure 1 In the case ofsputtering the process is a secondary one in thatthe target atoms have first to be sputtered offthe source and then only deposit onto the earthedsubstrate. With ion plating the process is a directone as thermal energy is supplied for the evapora-tion of the gold.

The energy of the metal atoms which depositonto the substrate depends on the magnitude ofthe bias. In contrast to ion implantation, in ionplating the impact energy can be made sufficientlylow so that crystalline substrates like silicon canbe plated without damage. Consequently, the areaof electronic device fabrication is beginning toopen up for ion plating. There is little doubtthat gold and gold-alloy ion plating will havean important part to play in the electronicsbusiness of the future.

The Advantages of Gold Ion Plating

In spite of being a new technique, with develop-ment work still in progress, there appear to be anumber of advantages attached to gold ion plating.Some of these advantages arise from the ex-ceptional properties of gold, whereas others arisefrom the nature of the ion plating process.

Economie Advantages

Because of the unreactive nature of gold, thevacuum conditions in the equipment used for ion

Fig. 3 The versutility of ionplating is illustrated byexamples of components ofvarious shapes and materialswhich were gold coated by theprocess. Clockwise, the ob-jeets in this photograph are:a piece of stainless steeltubing, a piece of polyimideplastic foil, a steel screw, avalve moutding of Polyether-sulphone (an ici plastic), apair of stainless steel tweetersand a part of a polypropylenesyringe. The gold coatings onall these objects are in therange 2000 to 5000 .1 thick

32

plating are not critical. This means that simplevacuum units, which are inexpensive to operateand maintain can be used. For instance, a smallbench top unit is quite suitable for the coatingof small jewellery items. Further, the requiredmoderate vacuum is normally achieved after veryshort pumping times. This, together with the factthat gold plated samples can be withdrawn fromthe unit when they are still hot without anydamage or detrimental oxidation, results in rapidrates of batch processing. It can be anticipatedthat when high rates of evaporation can be appliedwithout any Toss of adhesion, batch processingtimes will be a few minutes only.

Another consequence of the non-critical vacuumconditions required for gold ion plating is thatits application to continuous plating of metalfoil or plastic strip is simplified. With conventionalevaporation, reel to reel coating inside the chamberor alternatively travel through four vacuum-to-airseals is mandatory to maintain a high vacuuminside the chamber. With ion plating, a singleseal is sufficient and there is no limitation to thesize of the rolls of substrate as these are locatedoutside the coating unit.

Small ion plating units are simple and operatorscan be trained in a matter of hours. However,powerful units for high rate deposition, whichuse an electron beam or a magnetron sputtersource, are more complicated and operator trainingtime is longer.

The power consumption of the process islow, in part because the melting point of gold(1064 °C) makes for easy evaporation. Muchsmaller amounts of gold are used with ion platingthan with conventional evaporation to achieve thesame coating thickness. This resuits from theconcentrating effect of the plasma which directsthe gold onto the cathodic specimen. A shieldwith a small viewing port can be used inside thebell jar to collect most of the gold that does notgo into the plasma. This gold can periodically berecovered by peeling it off the shield.

Lastly, the process is entirely pollution-freewhich has a beneficial effect on both investmentand operating colts.

Properties of the Coatings

As recorded earlier, one of the main features ofgold coatings deposited by ion plating is theirexcellent adhesion to a variety of materials. Thisis normally achieved without particular difficultyon all metallic substrates and on ceramics with arough surface finish. If reasonable adhesion topolished ceramics or high surface quality plasticsis to be obtained, then careful cleaning is essentialprior to coating.

Provided that the right operating conditionsare used, films of high electrical conductivity areproduced. High surface reflectivity is also oneof the advantages exhibited by ion plated deposits,which have proven suitable for optical appli-cations. The surface finish obtained is certainlybetter than that achieved by conventional electro-plating on an identical substrate.

Substantially pinhole-free gold deposits can beprepared by ion plating. This is important whencoatings are to provide corrosion protection and isachieved by thinner coatings than with electro-plating.

Area of Application

The possibility of coating non-conductingmaterials by gold ion plating opens up a widerange of applications for the process. In additionto the virtually unrestricted choice of substrates,the high throwing power inherent in ion platingmeans that almost any shape can be coated.For example, a tube can be coated internally inone run provided that its length is not morethan four times the diameter. For longer tubes orcomplex shapes, special electrodes and/or samplerotation can be used.

Also, due to the excellent adhesion of thecoatings, ion plated components are particularlysuited for applications which involve subsequentbonding or demanding electrical properties.

Gold alloys and some compounds can be de-posited by ion plating. However, compositionalcontrol is poor with the standard filament evapor-ation or with an electron beam source. Improvedcontrol should be achieved with a sputter source.

Conclusion: Electroplating or Gold Ion

Plating ?

A comparison between ion plating and electro-plating has been tabulated (8). The main ad-vantages of ion plating over electroplating arethat it is a pollution-free process which giveshighly adherent gold coatings on a wide range ofconducting and non-conducting substrates and thefilms have high optical, electrical and non-porousqualities even when they are less than lpm inthickness.

Electroplating is a cheaper process and generallyis much simpler to operate. However, in theauthor's opinion, the advantages of gold ion platingand the new applications it is opening up in thefield of electronics and optical communications,optical instruments, decorative and corrosion pro-tection coatings, leave little doubt that it willbecome increasingly competitive with electro-plating in many areas.

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Table

A Comparison of the Electroplating and Ion Plating Processes

A Features of the process Electroplating Ion plating

Materials that can be deposited Pure metals, alloys Pure metals, alloys, someinorganic compounds

Non-conducting substrates Not possible without Yes — a wide range

autocatalytic preplating

Deposition rate (%/second) 0 to 10" 10 to 103

Throwing power Limited Good

Substrate temperature 20 to 100 C 50 to 400 C depending onpower levels and cathode cooling

Cost Moderate High

Simplicity of equipment Simple in most instances Simple only for smal) batchprocessing

Gold recovery Costly Relatively simple

Pollution Problems — especially with Nonecyanide

B Properties of the deposits Electroplated lon plated

Adhesion Moderate Excellent

Porosity Usually porous for thin coatings Generally non-porous if care istaken

Uniformity Good on flats but may have Good on the front surface but

non-uniformity on edges will be thinner on the back.Rotation ensures uniformity

Purity May contain gases and Limited only by purity of theinclusions of inorganic or source materialorganic nature

Resistance to tarnish and Excellent Excellent

corrosion

Electrical conductivity Good for thick layers GoodModerate for thin layers

Optical reflectivity Good Excellent

Acknowledgements

The author is grateful to D. G. Teer for many helpfuldiscussions on ion plating and to Professor Takagi for theinformation in Figure 2. Keith Jones provided invaluableexperimental assistance and produced all the samples shown inFigure 3.

References

1 L. B. Hunt, Gold Bull., 1973, 6, 16-272 D. M. Mattox, J. Appl. Phys., 1963, 34, 24933 `Proceedings of the IPAT 77 Conference', available Erom

CEP Consultants Ltd., 14a Henderson Row, EdinburghEH3 5DH

4 `Handbonk of Thin Film Technology', cd. by L. I. Maisseland R. Glang, McGraw-Hill. 1970

5 D. G. Teer, in `Proceedings of the IPAT 77 Conference',1977, 13

6 K. Jones, A. J. Griffiths and E. W. Williams, in `Proceedingsof the IPAT 77 Conference', 1977, 115

7 E. W. Williams, K. Jones, J. M. Bel! and D. G. Teer, in`Proceedings of the Advances in Surface Coating TechnologyConference', The Welding Institute, London, February 1978

8 A few of the comparisons shown in this table are takenfrom :R. I. Sims, Metal. Mater. Technol., 1976, 8, (1), 23-27N. J. Archer and L. C. Archibald, Chart. Mech. Eng., 1977,24, (2), 59-63

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