Electrocomponent Science and Technology1976, Vol. 3, pp. 13-20

(C) Gordon and Breach Science Publishers Ltd., 1976Printed in Great Britain

A SYSTEM FOR DEPOSITING TUNGSTEN COATINGSSUITABLE FOR AN ELECTRICAL CONTAC]? APPLICATION

R. CARPENTERAllen Clark Research Centre, The Plessey Company Limited, Caswell, Towcester, Northants, U.K.

(Received August 7, 1975)

A method was needed for depositing smooth coatings of tungsten at relatively high rates onto the electrodes of a reedswitch. A literature survey indicated that the most suitable technique was based on the vapour phase hydrogen reduction oftungsten hexafluoride. A reactor was designed for this purpose on the basis of safety, optimum utilisation of tungstenhexafluoride, and possible adaption to higher throughput. Experience with this reactor indicated that precautions werenecessary before good quality coatings could be deposited and suggested that films of an acceptable thickness variationcould be produced by using a flow reversal technique.

1 INTRODUCTION

A contact material was required for the developmentof a reed switch. The switch of novel design employedmagnetic coupling through the contact material itselfand needed to handle currents of a few amperes atopen circuit potentials in excess of 100 volt. Tungstencontacts sealed in an oxygen-free environment offeredthe best solution, but restrictions on the permissiblethickness were imposed by the coupling requirement.There was a maximum thickness which could be usedand yet still give a sufficient closure force with anacceptable number of ampere-turns in the energisingcoil. On the other hand, too thin a contact wouldlead to considerable shortening of contact life. Theserestrictions led to an acceptable band of thickness ofbetween 30/am and 50/am.

Discrete contacts as thin as 50/am were not readilyavailable in the right form, as they needed to be cutfrom swaged rod to give a grain orientation to resistspalling. The other possibility, and the one examinedin detail, was to deposit a coating of tungsten on theswitch electrode (substrate) in such a way that thedeposition on non-contact areas was minimised. Thecontact area was half of one face of a rectangular soft-iron substrate 20 mm x 5 mm.

Bearing in mind possible future development, itwas estimated that in any experimental depositionsystem at least one hundred substrates would have to becoated in a batch at a rate of 5/am to 10/am per min.A brief survey of possible deposition techniques wasmade to find the one most suitable for this applica-tion. Having done this, an experimental system fordepositing tungsten was constructed.

2 TECHNIQUES FOR DEPOSITING TUNGSTENCOATINGS

There are very few suitable methods. Vacuum deposi-tion techniques did not seem promising on account ofthe temperature required to give the high depositionrate. Published work -a using laboratory scaleequipment for metallisation refers to rates below0.05/am/min. A source temperature exceeding3,300 K would be required for evaporation andalthough this should be possible with an electron-bombardment heated source, the cost of developing asuitable system would be considerable. Similarly, lowrates have been reported for tungsten sputtering4

although recent advances in sputtering systemss offerthe prospect of considerable improvement.

Electrodeposition techniques appeared to beequally inapplicable. Early work by Davis6 showedthat it was impossible to deposit tungsten from anaqueous solution, as the electrode potential neededlies far beyond that needed by hydrogen so that wateris always decomposed first of all. Deposition fromorganic solutions of tungsten salts also proved to beimpossible. However, the metal was electrodepositedfrom mixed borate baths fused at 1150-1200 K, butsevere corrosion problems were encountered with thesalts and their reaction products which had to beovercome by shielding the exposed metal parts with aflow of nitrogen. Rates of up to 25/am/min. anddeposits up to 500/am thick were quoted. Otherworkers7,8 have reported on electrodeposition frommolten salts, but Brenner9 has cast some doubt onthe thickness uniformity and surface finish of thetungsten coatings obtained.

14 R. CARPENTER

Arc-plasma spraying techniques for depositingtungsten have been developed on a commercial basis,but little detailed information has been published o

and no firm assessment of the viability of coatings forcontacts could be made. The method involvesblowing tungsten powder through an arc struck in anon-oxidising environment and the granularity of thedeposit depends on the mesh size of the powder. Veryfine powders tend to be scattered easily from the jetstream and without recycling the powder the processbecomes inefficient. However, recycling leads to anincrease in the impurity level of the deposit. Thesefactors would all be important for the contactapplication.

Chemical vapour deposition (C.V.D.) is used todeposit a variety of films. General articles have beenpublished by Feist and Amick 2, the latter withparticular reference to films used in microelectronics.A number of tungsten-forming reactions are possible.The metal can be deposited directly by pyrolyticallydecomposing the carbonyl3 at 1,200 K, but thedeposition rate is low and any attempt to raise thetemperature leads to a gas-phase reaction and a looseporous deposit. The other C.V.D. techniques dependon reactions of tungsten halides. A de-Boer typereaction has been used with the bromides 4 in whichbromides were formed from the elements at 1,100 Kand deposited at 1,800 K. The pressure was reducedto promote the tungsten-forming reaction. Providedthat care was taken to exclude oxygen, uniformadherent deposits up 6 mm thick could be produced.A similar reaction can be used with the iodidesbut the substrate temperature needed is impracticablyhigh. In the case of the chloride 6 the halide formingreaction occurs at only 700 K, but the substratetemperature of 1700 K to 1900 K is too high for ironsubstrates. A much easier technique is to employ thehydrogen reduction of the hexachloride 7, wheredense adherent deposits are formed with the sub-strates at 850 K to 950 K. Glaski 8 has usedchlorination of tungsten followed by hydrogenreduction to achieve the same result, but with bothreactions occurring in one plant. The tungsten chipswere cleaned in situ by firing in hydrogen beforechlorination at 1300 K.

The hexafluoride, instead of the hexachloride, canbe used as the starting material to considerableadvantage.9 The reaction

WF6 + 3H W + 6HF

has been used in most recently r,eported work ontungsten vapour deposition19 2 and there isgeneral agreement that it offers advantages over the

other techniques described above. The substratetemperature required to achieve good coherentdeposits is only 800 K and deposit rates of a fewmicrons per minute are obtained easily. The reactioncan be carried out at atmospheric pressure and thelow boiling point, 292.5 K, of the fluoride meansthat it is easy to maintain a sufficient supply ofvapour to the reactor. An advantage cited byBerkeleyz is the absence of more stable lowerfluorides to interfere with the homogeneity of thetungsten deposit, although Glaski 3 has reportedmass spectrometric evidence for the presence oftungsten tetra-fluoride (B.P. 900 K) in the tungsten.The deposited tungsten shows a columnarstructure2O,24 with major grain boundaries runningperpendicular to the substrate; this type ofstructure is most resistant to spalling of thetungsten which can occur following temporarywelding of the switch contacts. A possible limitationis the adhesion of the film to iron substrates, sinceaccording to Bryantz6 the behaviour is inconsistent.However, since tungsten adheres well to both copperand nickel, this problem could be overcome byelectroplating the contacts first of all.

Of all the methods considered for tungstendeposition the easiest and most well documented wasevidently the C.V.D. fluoride technique and as thisalso seemed adaptable to the contact application itwas selected for the experimental study.

3 DESIGN OF C.V.D. SYSTEM

In the design of the experimental system and itspossible scaling to a production sized facility, twoconsiderations needed particular attention. Foremostof these was that of safety, since both tungstenhexafluoride and the reaction by-product hydrogenfluoride are obnoxious and are mixed with potentiallyexplosive hydrogen. Secondly, although tungstenfluoride is readily available commercially, the cost ofthe tungsten is 2 to 4 times that of the equivalentmetal contained within it.

Initially a choice had to be made between a staticand a continuous flow deposition system. Two factorslimit the amount of tungsten available for depositionin a static system, the mass of the tungsten fluorideand the equilibrium of the reaction. According toBerkely, the reaction in a static volume is inhibitedby a hydrogen fluoride concentration above 25% ofthe total. Typically rather more than the stoichio-metric volume ratio is used, so that it is readily shownthat to deposit a 30/m coating on the contact area of

DEPOSITION OF TUNGSTEN COATINGS 15

Susc.._..ep_to..rVitonseals

Reference plane forsubstrate arrangements \-14"

Substrate platform Thin wall J-/ minimize axial heat loss "-"1"

4x IKw Inlet and outlet gas

Rigid Fire rods flow pipes

Assembly clamp fWater / , ,Section throu.ghsusc..___eptor

/ cooling Note close fitof surrounding*- wateriacket

Stainless steel susceptorfor minimal chemical attack

Water cooled and low heat transferSliding 0’ rinq seal to allow for flange Slot to reduce Narrow gap to increase impedence

axial expansion peripheral temperature and minimize gas flowHoles to reduce susceptor mass

and cut down conduction

FIGURE

Gas flowpipes

Apparatus for C.V.D. of tungsten from WF

50 mm2 with an efficiency of 50% requires in excessof 100 cm3 of gas. Hence the reactor volumenecessary to coat upward of hundred contacts in astatic system is excessively large and would beawkward to fill and empty. It appeared to be muchmore advantageous to use a continuous flow system,where the concentrations of the reactants can beeasily controlled.

Published results2 o indicate that the optimumdeposition temperature is 800 K, since highertemperatures lead to a gas-phase reaction and theformation of a loose non-adherent deposit. Allsurfaces except the contact areas need to be kept coolfor maximum tungsten utilisation, designs in whichthe whole deposition chamber is heated are thereforeuneconomic. Direct and indirect radio-frequencyheating of the substrates were considered butrejected; the former on account of the difficulty ofsimultaneously coupling power into a large numberof substrates, the latter since an R.F. heatedsusceptor offered no significant advantage over asubstrate holder heated with resistive elements.

The most convenient reactor geometry to use witha continuous flow system is one based on cylindricalsymmetry. Ideally, the substrates should be mountedaround the inside of a heated tube, along which the

reactants pass, to minimise deposition on the non-contact areas. Alternatively, the substrates could bemounted around the periphery of a heated core.

Mounting the substrates so as to maximisedeposition on the contact areas presents difficultieswhichever method is chosen and a compromisebetween these possibilities and an arrangement whichwould simplify loading was adopted.

This design, which consists of a heated coreenclosed within a cooled jacket, is illustrated inFigure 1. Besides enabling the substrates to be loadedeasily, the arrangement gives reasonable temperatureuniformity along the substrate-bearing surface whilstmaintaining the other surfaces as cool as possible. Allthe main parts are fabricated in stainless steel to offerthe best corrosion resistance and to provide highthermal impedance.

The substrate-bearing surface is formed by millingthe central part of a cylindrical bar to a semicircularsection. Two kw cartridge heaters are inserted ateach end into two holes drilled paraxially, close tothe substrate-bearing surface. Each pair of heaters iscontrolled independently. A number of other paraxialholes serve to reduce the thermal mass and increasethe radial thermal impedance besides permittingforced-air cooling to be used to cool the outer

16 R. CARPENTER

Hydroqen

Silica- Molecularael Sieve ’,way

Flow-

Flush hydrogen sste,m system

De-oxo

jMoI.Sieve(pH/edsCqr:n’line

Pump

Reactor

Chamber

pressure

way By pass

ter-FlowJvJ

Warmed delivery pipes- Jbath MeterllWF Source Subsidiaryenclosure

Enclosure vented to outside atmosphere

FIGURE 2 Overall layout of deposition system

To outsideatmosphere

Bubbler

wo VacuUmpump

surfaces of the bar. A thermocouple inserted throughone of these holes into the centre of the bar providesa convenient method of establishing a temperaturereference. At the ends, the wall thicknesses arereduced to improve thermal isolation of the core. Ademountable and close fitting water-cooled jacketconcentric with the core forms the reactor volume.Flanges welded to the core and water-cooled jacket inconjunction with a ’Viton’ O-ring form a hermeticseal at one end; while a standard sliding seal, formedby an O-ring sealing on the periphery of the core andretained by a free flange and flange welded to thewater-cooled jacket, allows for axial expansion andsubsequent contraction at the other. The close fit ofthe water-cooled jacket surrounding the core restrictsthe gas flow and hence extraneous tungsten depositionon the periphery.

This construction has certain other advantages.Both the gas and electrical connections can be madeentirely through the flanged end of the core, so thatthe jacket can be completely removed to give accessfor loading and unloading the substrates. The heatersand their associated electrical connections arecompletely isolated from the reactor volume itselfand the chance of explosion from accidental electrical

discharge is eliminated. Furthermore, since thevolume of gas exposed at any instant within thereactor is small, the hazard from explosion is kept toa minimum.

Safety is also a primary requirement for the gasdistribution system. Proprietary stainless steel fittingsand tubulation are consequently used throughout inthe reactant gas lines. Figure 2 shows a schematicdiagram. The flows of both hydrogen and tungstenhexafluoride are monitored and can be adjusted withneedle valves. The hydrogen is obtained from acylinder situated well away from the reactor. Thehigh-purity cylinder gas is passed through a deoxounit and then into series.connected drying towerscontaining self-indicating silica.gel and molecularsieves. The tungsten hexafluoride is obtained from alecture bottle, which is supported in a thermostati-cally controlled low-temperature water-bath. Thecontrol valve and flowmeter are positionedimmediately adjacent to the. water-bath and in thesame enclosure to keep their temperatures above293 K and prevent liquefaction of the hexafluoridegas. All pipework containing tungsten hexafluoride isheld at a temperature above 293 K by heaters. Theseare widely spaced turns of nichrome wire insulated

DEPOSITION OF TUNGSTEN COATINGS 17

from the pipes and held in position with P.T.F.E.tape and are operated from appropriate secondarytappings of a low-voltage heater transformer.

The hydrogen and tungsten hexafluoride mixtureis normally fed directly into the reactor and thereaction products are then passed into the absorptiontower to remove fluorides. The tower contains acontinuously recirculating solution of potassiumhydroxide, which is sprayed onto the top of closelypacked lengths of P.V.C. tubing through which thegases diffuse upward. It is designed to preventaccidental suck-back of the solution into the hotreactor. Finally, the hydrogen is vented directly intothe atmosphere outside the laboratory.

The entire system can be flushed out with argonto expel all traces of reactants and their gaseousproducts. Like the hydrogen, the argon is thoroughlydried before use. Capsule-type pressure gauges arefitted both to the hydrogen line and to the chambervolume to ensure that no excessive pressures arebuilt up. A facility is also provided to enable roughevacuation of the chamber after the reactor has beenexposed to the atmosphere; this assists in the removalof traces of water.

Finally the reactor and all the gas lines arecontained within a fume cupboard, which is formedin perspex and is fitted with a ’blow out’ panelsituated away from the operator. The extractor fanfor the fume cupboard is interlocked with the reactorheaters to ensure that it is always operating duringtungsten deposition.

4 DEPOSITION PROCEDURE

The nickel-coated iron substrates are arranged on thesubstrate platform (Figure 1) so that the contactsurfaces alone are exposed to the main gas stream.This is achieved by using an echelon arrangementwith each succeeding substrate masking the noncontact area of the preceeding one. Five rows ofsubstrates, stacked close together to minimise gasflow to the under-surface and each containing 25substrates, are used. The plane of each substratesubtends an angle of "--10 with the substrate plat-form and the exposed contact surfaces face towardthe gas inlet. After assembling the water-cooledjacket, the entire system is flushed out with argon.The reactor is next isolated and evacuated with therotary pump, while the heaters are set to a prede-termined level to raise the core temperature to 500 K.The pump is isolated, the heater power increased anda hydrogen-argon flow introduced into the reactor.

The argon is added through the tungsten hexafluorideline and serves to reduce the thermal conductivity ofthe gas. When hydrogen is used by itself, the rapidrise in temperature occurring on introducing tungstenhexafluoride leads to non-reproducible depositionconditions. By choosing the right flow for the argonit is possible to ensure that little or no change in thereactor temperature occurs when tungsten hexa-fluoride is admitted. After the temperature hasstabilised at about 750 K, the argon supply is shut offand tungsten hexafluoride is introduced. Typicallythe flow rates are 1.2 1/min for hydrogen and0.15 1/min. for tungsten hexafluoride with 62.5 cm2

of contact surface. After the required thickness hasbeen deposited, both the tungsten hexafluoridesupply and the reactor heaters are switched off. Thehydrogen flow through the reactor is maintained topromote rapid cooling. When the temperature hasfallen below 400 K, the tungsten hexafluorideremaining in the pipeline is flushed through thereactor by-pass into the absorption tower while thehydrogen is kept flowing to prevent a back-flow intothe reactor. Subsequently, the entire system isflushed through with argon to remove all traces oftungsten hexafluoride and hydrogen fluoride beforethe reactor is opened. Using this procedure,contamination-free tungsten films are produced.

5 RESULTS

The parameters of particular interest are the thicknessand quality of the tungsten coatings and the utilisa-tion of the tungsten hexafluoride.

Early experiments confirmed the observations ofBryant27 that tungsten films deposited directly oniron were generally non-adherent. However, with anickel intermediate layer, the adhesion is high andthe quality good as long as the temperature is below800 K. Coatings deposited at rates up to 5/.tm/min.are generally of excellent quality. The effects ofimperfections in the underlying nickel layer areexaggerated by the mode of growth of the tungstenand it is essential for the underlying layer to be ofgood quality. As the coating thickness increases, thesurface of the tungsten becomes rougher as a result ofpreferential grain growth.

Examination of coatings was carried out bypreparing metallographic sections for viewing with anoptical microscope and by means of a scanningelectron microscope (Stereoscan). Selective etching ofpolished metallographic sections was achieved byusing a nitric acid-methanol mixture for iron and a

18 R. CARPENTER

(a)

Section oftypical coatincj

(b)

Tungsten blue

layer

at interface

(c)

Nodular growth

produced by

underlayer detect

(d)

Nodule

at surface

(e)

Typical

topography

(f)

Edge ofetched coating

revealing

nickel underlayer

40 #m

FIGURE 3 Tungsten coatings

potassium hydroxide-potassium ferricyanide solutionfor tungsten. Figure 3 shows examples of the variouscoatings together with some of the faults encountered.Figure 3(a) is a section through a thick tungstencoating and illustrates preferred grain growth andprogressive grain enlargement. Figure 3(b) shows thepresence of tungsten oxides (tungsten blue) formed atthe nickel interface when the precautions taken to dryout the system before admitting tungsten hexa-fluoride were inadequate. The effect of an imper-fection in the nickel underlayer which leads to thenodule formation on the surface of the tungsten is

illustrated in Figure 3(c). Nodule formation can alsobe seen in the Stereoscan micrograph of Figure 3(d),while 3(e) shows the surface of a typical contactcoating. Figure 3(f) is an edge of the coating pro-duced by covering part of the deposit with a protect-ive layer and then removing the unprotected portionwith an anodic etch. The columnar structure of thefilm is clearly seen.

Two methods were used to estimate coatingthickness. A time-consuming technique was to etchthrough parts of the coating to the substrate and thenmake direct measurements with a ’Talysurf’ surface

DEPOSITION OF TUNGSTEN COATINGS 19

O’10

0.2

Flow

Synthesised curve

curve

xXx"x...,,,x

Substrate position

FIGURE 4 Substrate weight increase as a function of position.

profile monitor. A more convenient method was toweigh individual films. This was meaningful sinceidentical areas of the substrates were exposed and’Talysurf’ measurements indicated that thicknessvariations over individual substrates were less than 5%.

The thickness of tungsten deposited on individualsubstrates depended on the substrate position as wellas the deposition parameters. Within a batch,consisting of 5 rows each containing 25 substrates,thickness variations could be minimised to -+ 15% byusing low temperatures and long deposition times.This minimised the depletion of tungsten hexa-fluoride in the gas stream, but yielded very lowutilisation efficiencies. Economically, this would beunacceptabie for a production technique. A moreprofitable approach is to maximise the tungstenutilisation, so that the coating thickness diminishesalong the reactor. This is shown in Figure 4, wherethe weight of the coating deposited on each of 25substrates is plotted against axial position. Eachsubstrate is five times the width of those used forthe contact application and its position is numberedfrom the gas inflow. A way of producing a moreuniform thickness distribution is to arrange to reversethe direction of gas flow midway through thedeposition process. However, this experiment couldnot be carried out with the existing reactor. Theuniformity which might be expected from flowreversal can be inferred and is also plotted in Figure 4;the synthesised curve was obtained by adding theexperimental curve to its mirror image in a planethrough the centre and shows variations of thicknesswithin -+ 10%. In practice it may be impossible toachieve this degree of uniformity without manipula-ting the substrates, because of the asymmetry relativeto gas flow of echelon stacking. The tungsten utilisa-tion efficiency was calculated from the total weight

of the tungsten coatings on the contact areas and themeasured weight loss of the tungsten coatings on thecontact areas and the measured weight loss of thetungsten hexafluoride source cylinder. Expressed aspercentages, 40% of the tungsten was deposited onthe contact areas and 7% on the backs and edges ofthe substrates. The tungsten conversion efficiency wasdeduced by determining the tungsten residue wasfound by oxidising the fluid with hydrogen peroxideand acidifying with hydrochloric acid to precipitatetungsten oxide. The result indicated a conversionefficiency of 90%. Thus about 40% of the tungsten iswasted through deposition on the hot parts of thereactor.

6 CONCLUSIONS

The literature survey indicated conclusively that themost satisfactory method for depositing thicktungsten f’rims at high rates for the required contactapplication involved the use of the hydrogenreduction of tungsten hexafluoride.A system designed to produce small numbers of

contacts for evaluation purposes has proved to beremarkably simple to use and could be scaled up toproduce a larger throughput without too muchdifficulty. In designing equipment for this purpose,the safety and utility of tungsten were aspects whichwere recognised initially. The experimental workindicated the need to start off with a smooth sub-strate surface, since any irregularity is emphasised bythe growth mode, and the necessity of removing alltraces of moisture.

The particular configuration of the contact doesnot lend itself to highly efficient tungsten utilisation.However, the results indicated that by using a system

20 R. CARPENTER

in which the direction of tungsten hexafluoride flowcould be reversed it should be possible to achievetungsten utilisation efficiencies of up to 40%.

The method yields contacts both of acceptablesmoothness for thicknesses up to 50/am and ofsuitable grain orientation, and could offer analternative to wrought tungsten contacts in someapplications.

7 ACKNOWLEDGEMENTS

The Author is indebted to Mr. G. V. Smith for the Stereoscanmicrographs and Mr. J. Rigby for the chemical analysis. Hewould also like to thank the Directors of the Plessey CompanyLimited for permission to publish thi paper.

REFERENCES

1. N.J. Maskalich and C. W. Lewis, Some Properties ofEvaporated Refractory Metal Films, Trans. 8th Am. Fac.Symp. 2, 874 (1961).

2. F. L. Schuermeyer, W. R. Chase and E. L. King, IonEffects During e-Beam Deposition of Metals, J. Fac. Sci.& Technol. 9, 330 (1972).

3. A.K. Sinha, T. E. Smith, T. T. Sheng and N. N. Axelrod,Control of Resistivity in Electron Beam EvaporatedTungsten Films, J. Fac. ScL & Technol. 10, 436 (1973).

4. W.W.Y. Lee, High Resistivity of d.c. Sputtered MetalFilms, J. Appl. Phys. 42, 4366 (1971).

5. R. L. Cormia, P. S. McLeod and N. K. Tsujimoto, HighRate Sputtering Achieved by Permanent Magnets,J. Electrochem. Soc. 121,110C (1974).

6. G. L. Davis and C. H. R. Gentry, The Electrodepositionof Tungsten, Metallurgia 53, 3 (1956).

7. F.X. McCawley, C. B. Kenahan and D. Schlain,Electrodeposition of Tungsten Coatings from MoltenSalts, J. Electrochem. Soc. 110, 180C (1963).

8. G.W. Mellors and S. Senderoff, The Electroforming ofRefractory Metals, Plating 51,972 (1964).

9. A. Brenner and W. E. Reid, Vapour Deposition ofTungsten, U.S. Patent No. 3,072,983 (1963).

10. A. R. Moss and W. J. Young, Arc Plasma Spraying,Science and Technology of Surface Coatings, ed. B. N.Chapman and J. C. Anderson (Academic Press 1974)p. 287.

11. W. M. Feist, S. R. Steele and D. W. Readey, ThePreparation of Films by Chemical Vapour Deposition,Physics of Thin Films, ed. G. Hass and R. E. ThunAcademic Press (1969) p. 237.

12. J. A. Amick and W. Kern, Chemical Vapour DepositionTechniques for the Fabrication of SemiconductorDevices, Proc. of 2nd Int. Conf. on C.F.D. (Electrochem.Soc. 1970) p. 551.

13. H. E. Carlton and W. M. Goldberger, FundamentalConsiderations of Carbonyl Technology, J. Met. 17, 611(1965).

14. R.M. Caves, Tungsten Coatings from the ThermalDeposition of Tungsten Bromides, Trans. A.LM.E. 224,267 (1962).

15. J. A. Moore and C. M. Jolly, Quartz Iodine Lamps,G.E.C. Journal 29, 99 (1962).

16. C. F. Powell, J. H. Oxley and J. M. Blocher, FapourDeposition Electrochem. Soc. 1966 chap. 10, p. 322.

17. E.J. Mehalchick and M. B. Maclnnis, Preparation ofVapour-Deposited Tungsten at Atmospheric Pressure,Electrochem. Technol. 6, 66 (1968).

18. F. A. Glaski, Controlling Grain Orientation in C.V.D.Tungsten, Proc. 2nd lnt. Conf. on C.F.D. (Electrochem.Soc. 1970) p. 839.

19. V. A. Nieberlein, Vapour Deposited Tungsten Coatingson Graphite, Am. Ceram. Soc. Bull. 44, 14 (1965).

20. J. F. Berkeley, A. Brenner and W. E. Reid, Jr., VapourDeposition of Tungsten by Hydrogen Reduction ofTungsten Hexafluoride, J. Electrochem. Soc. 114, 6(1967).

21. A. M. Schroff and G. Delval, Recent Developments inthe Chemical Vapour Deposition of Tungsten andMolybdenum, High Temp.-High Pressures 3,695(1971).

22. W. A. Bryant and G. H. Meier, Kinetics of the ChemicalVapour Deposition of Tungsten, J. Electrochem. Soc.120, 559 (1973).

23. F. A. Glaski, The Use of Oxygen Additive to ControlResidual Fluorine in Chemical Vapour DepositedTungsten, Record of 9th Thermionic SpecialistConference (LE.E.E. 1970) p. 72.

24. R. K. Chuzhko, I. V. Kirillov, Yu N. Golovanov andA. P. Zakharov, Texture of Tungsten Formed byDeposition from the Vapour, J. of Crystl. Growth 3,219 (1968).

25. C. J. Smithels, Tungsten (Chapman and Hall 1945)chap. 11, p. 229.

26. W. A. Bryant, The Adherence of Chemically VapourDeposited Tungsten Coatings, Proc. of 2nd lnt. Conf. onC.V.D. (Electrochem. Soc. 1970) p. 409.

27. A. I. Vogel, Quantitative Inorganic Analysis (Longmans1962) p. 567.

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