Electrodeposition of metallic molybdenum films in

Electrochimica Acta51(2006)3776–3780Electrodeposition of metallic molybdenumfilms inZnCl2–NaCl–KCl–MoCl3systems at250◦CHironori Nakajima,Toshiyuki Nohira∗,Rika HagiwaraDepartment of Fundamental Energy Science,Graduate School of Energy Science,Kyoto University,Sakyo-ku,Kyoto606-8501,JapanReceived1July2005;received in revised form25October2005;accepted25October2005Available online2December2005AbstractMolybdenum metalfilm has been electrodeposited in ZnCl2–NaCl–KCl(0.60:0.20:0.20,in mole fraction)melt containing MoCl3at250◦C.In this melt,a densefilm was obtained by potentiostatic electrolysis at0.15V versus Zn(II)/Zn for3h.However,thefilm had a thickness of smaller than0.5␮m and was not adhesive.On the other hand,addition of4mol%of KF to the melt led to larger cathodic current in cyclic voltammogram, and gave a dense,adhesive and thicker metalfilm of ca.3␮m thickness in the same electrolysis condition as above.The present process is promising as a new method for molybdenum coating at low temperatures.©2005Elsevier Ltd.All rights reserved.Keywords:Electrodeposition;Molten salt;ZnCl2–NaCl–KCl;Molybdenum metalfilm;Molybdenum chloride;Fluoride ion1.IntroductionElectrodeposition of refractory metals such as tungsten, molybdenum,tantalum at low temperatures has many advan-tages from an engineering point of view.One of the promis-ing applications is the LIGA(Lithographie Galvanoformung Abformung,German abbreviation)process,a micro-fabrication technique consisting of lithography,electroforming and molding [1–6].Although many products have already been commercially produced with this method,applicable materials are limited to metals that are able to be electrodeposited in aqueous elec-trolytes such as copper,gold,nickel and Ni–Fe alloys[1–6]. Refractory metals are highly expected to be used for the elec-troforming step of LIGA process owing to their high hardness and high mechanical-strength.It is also very promising to coat the conventional LIGA parts with refractory metals by elec-trodeposition.However,electrodeposition of them has not been achieved except for chromium in aqueous solutions mostly due to the narrow electrochemical window of water.If the applica-tion of refractory metals to the LIGA process is realized,it would much contribute to improve the performance and reliability of the Micro-Electro-Mechanical Systems(MEMS)devices.∗Corresponding author.Tel.:+81757534817;fax:+81757535906.E-mail address:nohira@energy.kyoto-u.ac.jp(T.Nohira).Senderoff and Mellors have obtained refractory metals including molybdenum in alkali metalfluoride melts at 700–850◦C[7,8].Since then,refractory metals have been obtained in chloride,fluoride and oxide melts above500◦C [9,10].Katagiri et al.succeeded in obtaining metallic tungsten at 350–450◦C using ZnBr2–NaBr[11,12]and ZnCl2–NaCl melts [12,13].However,electrodeposition of smooth and dense refrac-tory metals becomes more and more difficult as the operation temperature decreases.To date,such electrodeposition of refrac-tory metals at temperatures below350◦C have not been reported except for the case of chromium.However,electrodeposition of metals must be made at and lower than250◦C for the LIGA process due to the limit of heat resistance of the resist sheet made of polymethylmethacrylate.For coating the LIGA parts, electrodeposition should be carried out at temperatures as low as possible to avoid the softening of the substrate materials. Thus,we have been studying the electrodeposition of refrac-tory metals in molten salts at around250◦C[14].It should be noted that metallic molybdenum deposit has not been obtained below550◦C[10].We selected a molten ZnCl2–NaCl–KCl eutectic(ZnCl2:NaCl:KCl=0.60:0.20:0.20,in mole fraction; mp:203◦C[15])as an electrolyte.The melts containing KF were also examined sincefine molybdenum deposits have been obtained influoride melts at high temperature in the previous works[9,10],which is possibly related to the formation offlu-oro complex ions.0013-4686/$–see front matter©2005Elsevier Ltd.All rights reserved. doi:10.1016/j.electacta.2005.10.041H.Nakajima et al./Electrochimica Acta 51(2006)3776–378037772.ExperimentalA schematic drawing of the experimental apparatus is shown in Fig.1.The experiments were conducted in an argon-filled glove box with a gas circulating purifier (MIW A MFG Co.,Ltd.).All the chemicals were anhydrous reagent grade.The eutectic mixture of ZnCl 2(99.9%,Wako Pure Chemical Industries,Ltd.),NaCl and KCl (99.5%each,Wako Pure Chemical Industries,Ltd.)was prepared (ZnCl 2:NaCl:KCl =0.60:0.20:0.20,in mole fraction),and then dried in a furnace under vacuum at 130◦C for 3days or more.After that it was melted at 250◦C in a glass beaker (Shibata Hario Glass Co.)installed in a four-necked sep-arable flask on a heating plate.A band shape heater was wound around the beaker to have uniform temperature distribution by heating.MoCl 3(99.5%,Alfa Acer)was added in the melt as a molybdenum ion source.KF (99%,Wako Pure Chemical Indus-tries,Ltd.)was added as a fluoride ion source.The working electrode was a nickel plate (99.7%,5mm ×10mm ×0.2mm,Furuuchi Chemical Corp.)that was fully immersed in the melt.The nickel plate was electrochemi-cally polished in a sulfuric acid then immersed in an acid cleaner containing NaHF2(Kizai Corp.,Kokeisan B)to remove surface oxides.The counter electrode was a glassy carbon rod (3mm,Tokai Carbon Co.,Ltd.,GC-20).A zinc wire (99.99%,0.5mm,Fig.1.Schematic drawing of experimental apparatus.(A)Thermocouple;(B)working electrode;(C)reference electrode;(D)counter electrode;(E)separable flask;(F)Pyrex beaker;(G)band heater;(H)ZnCl 2–NaCl–KCl melt;(I)heating plate.Nilaco Corp.)immersed in the melt was used for the refer-ence electrode.A chromel–alumel thermocouple was used for the temperature measurement.Cyclic voltammetry with positive feedback IR compensation and potentiostatic electrolysis were performed using electrochemical measurement systems (Hokuto Denko Co.,Ltd.,HZ-5000).After the electrolysis,the electrodes were immersed in acetone,then,rinsed with distilled water to remove adherent salts.The deposits were characterized by scanning electron microscopy (SEM,Hitachi,S-2600H),energy dispersive X-ray analysis (EDX,Horiba Co.,Ltd.,EMAX ENERGY EX-200),X-ray diffraction (XRD,Rigaku Industrial Co.,Ltd.,Multi Flex)with CuKa radiation and X-ray photoelectron spectroscopy (XPS,Shimadzu Co.,Ltd.,ESCA-3200).3.Results and discussion3.1.Electrodeposition of metallic molybdenum in ZnCl 2–NaCl–KCl–MoCl 3melt3.1.1.Cyclic voltammetryFig.2shows a cyclic voltammogram at the nickel plate elec-trode in ZnCl 2–NaCl–KCl–MoCl 3(0.05mol kg −1added)melt at 250◦C.A blank voltammogram obtained for the sameelec-Fig.2.(a)Cyclic voltammograms for nickel electrodes in ZnCl 2–NaCl–KCl and ZnCl 2–NaCl–KCl–MoCl 3(0.05mol kg −1added)melts at 250◦C.Scan rate:0.05V s −1.(b)The same voltammogram with a current density axis of smaller scale.3778H.Nakajima et al./Electrochimica Acta51(2006)3776–3780 trode in the melt without MoCl3is shown for comparison.TheseCVs were recorded after several cycles.In both the plots,acathodic current shoulder is observed at around0.03V as wellas an anodic current peak at around0.12V.They are attributedto the formation and dissolution of Ni–Zn alloy,respectively[14].Cathodic and anodic currents around0V in both theplots are attributed to the deposition and dissolution of metalliczinc,respectively.In the ZnCl2–NaCl–KCl–MoCl3melt,a newcathodic wave,C1,is also observed in the potential range of0.04–0.42V,suggesting the electrodeposition of molybdenum.The formations of Zn–Mo and/or Ni–Mo alloys are also takenaccount from the phase diagrams[16].3.1.2.Potentiostatic electrolysis and characterization of thedepositSince electrodeposition of molybdenum was suggested tooccur in the potential region of lower than0.42V,potentiostaticelectrolysis was performed at0.15V for3h,where deposition ofmolybdenum was expected whereas that of metallic zinc or theformation of Ni–Zn alloy was not.After the potentiostatic elec-trolysis,a blackfilm was deposited.However,only a small areaof the substrate was covered due to bad adhesion.Fig.3shows aspectrum of XPS of Mo3d for thefilm.Mo3d3/2and Mo3d5/2peaks corresponding to metallic molybdenum are observed at231and228eV,respectively[17].Thefilm is,thus,identifiedas metallic molybdenum.It is also confirmed that the Zn–Moand/or Ni–Mo alloys are not formed.EDX results showed thatchloride content in thefilm was lower than2at.%,indicating thatthe melt inclusion is not significant.On the other hand,an XRDpattern did not show any distinctive peaks corresponding to ametallic molybdenum crystal[18].This is likely to be caused bythe too small thickness and/or low crystallinity of thefilm.Fig.4shows a surface SEM image of thefilm,where sphericalparticles and microcracks are observed.The spherical particlesare observed as light gray because the oxidation by rinsing withthe water may have proceeded larger extent compared withtheFig. 3.An Mo3d XPS spectrum of a deposit obtained after potentio-static electrolysis at0.15V vs.Zn(II)/Zn for3h in ZnCl2–NaCl–KCl–MoCl3 (0.05mol kg−1added)melt at250◦C.Argon ion etching time:5000s.Fig.4.A surface SEM image of a deposit obtained after potentiostatic electrol-ysis at0.15V vs.Zn(II)/Zn for3h in ZnCl2–NaCl–KCl–MoCl3(0.05mol kg−1 added)melt at250◦C.flat part.The microcracks indicate that tensile stress is gener-ated on the depositedfilm.There are two possibilities for the tensile stress.One is the coefficient of thermal expansion(CTE) mismatching and the other is the deposition-induced internal stress.The CTE mismatching is excluded since CTE of nickel is larger than that of molybdenum,which gives rise to compressive stress.Thus,the microcracks would be produced by the deposi-tion induced tensile stress.The observation of theflakes of the film showed that it was dense but had a thickness lower than 0.5␮m.The metallic molybdenum is also obtained at0.40V, which was confirmed by XPS analysis.Hence,it is confirmed that cathodic currents at potentials lower than0.42V result from the electrodeposition of molybdenum.3.2.Electrodeposition of metallic molybdenum inZnCl2–NaCl–KCl–MoCl3–KF melt3.2.1.Cyclic voltammetryThe melts containing KF were examined sincefine molyb-denum deposits have been obtained influoride melts at high temperatures[9,10].Fig.5shows a cyclic voltammo-gram at a nickel electrode in ZnCl2–NaCl–KCl–KF(4mol% added)–MoCl3(0.05mol kg−1added)melt at250◦C.A curve for the melt added only KF to the blank melt is also shown for comparison.They were recorded after several cycles.Addi-tion of KF to the blank melt causes no distinctive change in the potential region examined in the present study.In the ZnCl2–NaCl–KCl–KF–MoCl3melt,a cathodic wave,C2,is observed in the potential region of0.04–0.45V.This current is possibly due to the electrodeposition of metallic molybde-num,which is approximately three times larger than those in the melt without KF.The shape change in the cyclic voltamme-try in the potential region of0–0.03V shows the suppression of both the deposition of metallic zinc and the formation of the Ni–Zn alloy,which coincides with the relatively large amount of molybdenum deposition at the electrode.In these potentialH.Nakajima et al./Electrochimica Acta 51(2006)3776–37803779Fig.5.Cyclic voltammograms for nickel electrodes in ZnCl 2–NaCl–KCl and ZnCl 2–NaCl–KCl–KF–MoCl 3(0.05mol kg −1added)melts at 250◦C.Scan rate:0.05V s −1.Starting potential for the ZnCl 2–NaCl–KCl–KF–MoCl 3melt:0.59V vs.Zn(II)/Zn.regions,the possibilities of the formations of the Zn–Mo and/or Ni–Mo alloys cannot be dismissed.3.2.2.Potentiostatic electrolysis and characterization of the depositSince electrodeposition of molybdenum was suggested in the potential region lower than 0.4V ,potentiostatic electrolysis was performed at 0.15V for 3h.A black film was deposited and its adhesion was better than that of the film obtained in the previous section.Fig.6shows an XRD pattern for the film.In this case,a diffract pattern is ascribed to metallic molybdenum [18].It is also confirmed by XPS analysis that no Zn–Mo and Ni–Mo alloys are formed.EDX results revealed that the chloride con-tent in the film is lower than 1at.%,indicating that the melt inclusion is also minimized in this case.Fig.7(a)shows a sur-face SEM image of the film.Although the surface was covered with spherical particles,the deposited molybdenum layer seems dense.The formation of the microcracks is also explained by the deposition-induced internal stress as the case for the melt without KF.However,the number and gaps of the microcracks markedly decreased with addition of KF.Fig.7(b)shows across-Fig.6.An XRD pattern of a deposit obtained after potentiostatic electrolysis at 0.15V vs.Zn(II)/Zn for 3h in ZnCl 2–NaCl–KCl–MoCl 3(0.05mol kg −1added)melt at 250◦C.Fig.7.(a)A surface SEM image and (b)a cross-sectional SEM image of a deposit obtained after potentiostatic electrolysis at 0.15V vs.Zn(II)/Zn for 3h in ZnCl 2–NaCl–KCl–KF (4mol%added)–MoCl 3(0.05mol kg −1added)melt at 250◦C.sectional SEM image of the film.Although the molybdenum film contains a few fine microcracks,it is almost dense and has the average thickness of approximately 3␮m.The improve-ment in adhesiveness,thickness and microcracks by addition of KF should be related to the change of complex structure of molybdenum species in the melt,such as the formation of fluoro or chloro–fluoro complex.This effect is now under investiga-tion by Raman spectroscopy.The microcracks will be reduced further by the other additives and/or the other electrolysis con-ditions such as the pulse technique.The electrodeposition rate observed,3␮m per 3h,is not large enough to apply this process directly to the electroforming step of the LIGA process.How-ever,further improvements on this process is expected because there are much room for increasing the electrodeposition rate,for example,by increasing the molybdenum composition in the melt,introducing the pulse electrodeposition technique,etc.The present method can be applied with minor improvements for3780H.Nakajima et al./Electrochimica Acta51(2006)3776–3780coating of a variety of substrates with molybdenum including the conventional LIGA parts.4.ConclusionMetallic molybdenumfilm was deposited by potentio-static electrolysis in ZnCl2–NaCl–KCl eutectic melt containing MoCl3at250◦C.In this melt,thefilm was dense,but was not adhesive.On the other hand,addition of4mol%of KF to the melt gave a dense,adhesive and thicker metal deposit in the same condition as above.The present process is promising as a new method for molybdenum coating at low temperatures such as the coating on the conventional LIGA parts.AcknowledgmentsThis study was supported by Industrial Technology Research Grant Program in2003from the New Energy and Industrial Technology Development Organization(NEDO)of Japan.We thank Sumitomo Electric Industries,Ltd.for theirfinancial and technical supports.References[1]E.W.Becker,W.Ehrfeld,P.Hagmann,A.M.D.Maner,Microelectron.Eng.4(1986)35.[2]A.Rogner,J.Eicher, D.M¨u nchmeyer,R.-P.Peters,J.Mohr,J.Micromech.Microeng.2(1992)133.[3]J.Hormes,J.G¨o 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