mechanisms of electroless metal plating

1112 Y. Electrochem. Sot . : ELECTROC H EMI C A L SCIENCE AND TECHNOLOGY May 1983

Fig. I. An SEM print (2500N) of the augmentative replacement process. Left to right: zinc-nickel metallized epoxy, surface just after immersion into the plating solution, contiguous copper conductor after 5 m|n.

conventional autocatalytic electrroless plat ing tech- niques, our method is extremely rapid: Figure 2 is a cross-sectional view o~ one screen pr in ted sample. The thickness of the epoxy base is estimated at 50 ~m. The copper conductor is f rom 5-10 ~m thick. The conductor was deposited in only 4 rain! It would take up- wards of 1 hr to do this via conventional electroless plating.

Fig. 2. Cross-sectional view (1000X) of the copper coated epoxy. The thickness of the epoxy base is ca. 50 .~m; the copper conductor (region A) is ca. 8 ~m thick.

Although our process is "electrodeless," it more closely resembles electrodeposit,ion of copper in quan- t i ty per uni t time. This can be explained by the micro- bat tery action obtained from dissimilar metals. The process involves a replacement of the electronegative metals nickel and zinc by the electropositive copper.

A s the zinc dissolves, the l iberated electrons reduce the copper ions contained in the bath.1 The reduction occurs readi ly on all conductive surfaces. In the plate shown in Fig. 2 this would include both the nickel flakes and the deposited copper. This has been verified by moni tor ing both the surface potential of the metallized polymer and the resul tant current as the aug- menta t ive replacement process occurs (3).

In summary, we report a new method for producing contiguous copper conductors on a variety of substrates having: surface resistivities r ival ing those of conven- t ional thick film noble metal conductors, excellent surface adhes.ion, and solderabil i ty without leaching. Although the process is "electrodeless", plat ing occurs rapidly, within 2-7 rain. Fur thermore, the process costs substant ia l ly less than al ternat ive thick film methods. Possible future applications are manifold.

1 Although the nickel also dissolves, it does so at a much s lower rate under the conditions used. A more detailed discussion will be in a future publication.

REFERENCES 1. C. W. Eichelberger and R. J. Wojnarowski, Patents

submitted. 2. H. Lee and K. Neville, in "Handbuok of Epoxy

Resins;" McGraw-Hill , New York (1967). 3. In preparation.

Mechanisms of Electroless Metal Plating I. Application of the Mixed Potential Theory

Perminder Bindra* and Judith Tweedie

IBM, T. J. Watson Research Center, Yorktown Heights, New York 10598

The Wagner and Traud (1) theory o.f mixed poten- tials postulates that the rate of a faradaic process is independent of other faradaic processes occurring si- mul taneous ly at the electrode and, thus, depends only on the electrode potential. Hence, the polarization curves for independent anodic and cathodic processes may b e added to predict the overal l rates and poten- tials which exist when more than one reaction occurs s imul taneously at an electrode. More recently, it has been suggested (2) that the same concept applies to electroless plat ing processes and that plat ing mechanisms can be predicted from the polarization curves for the part ial processes. This has given rise to some controversy (3, 4) as the two part ial reactions in electroless plat ing processes are not ent i re ly independent of each other.

~ Electrochemical Society Act ive Member. Key words: e lectroless plating, mixed potential , rotating disk

electrode.

During the course of measurements to establish the val idi ty or otherwise of the mixed potent ia l theory for electroless plat ing processes, we have used a technique which allows determinat ion of the polarization curve for one of the part ial reactions dur ing electroless plating, i.e., while the other par t ia l reaction is occurring simultaneously. This technique is s imilar to a technique used earl ier by Makri,des (5) to s tudy the dis- solution of iron in sulfuric a,cid. It is a very simple technique and involves observation of the mixed potential of the plat ing system as a funct ion of agitation and as a function of the concentrat ion Of the reductant Or oxidant. It is based on the classification of electroless plat ing processes according to their overall mechanisms. Electroless plat ing of metals invar iab ly involves a reaction proceeding at a rate l imited by diffusion. For example, the plat ing rate of copper in a copper-for- maldehy,de bath is determined, to a large extent, by the rate of diffusion of copper to the plat ing surface

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(6). The technique descr ibed here al lows a c lear dis- t inct ion be tween those react ions whose ra te is con- t rol led by the ra te of diffusion of reac tants to the p l a t - ing surface, and those react ions whose ra te is l imi ted by some slow e lec t rochemical step.

Since each pa r t i a l react ion is under diffusion control or unde r e lec t rochemical (act ivat ion) control, the over - al l react ion scheme consists of four possible combina- tions. This is a simplif ied view of the s i tua t ion as it is pe r fec t ly possible for a pa r t i a l react ion to be under mixed control, i.e., unde r diffusion plus ac t iva t ion control. In this pape r however , the case in which the ca th- odic pa r t i a l react ion is en t i r e ly diffusion controlled, and the anodic pa r t i a l react ion to ta l ly act ivat ion con- t ro l led is described. This case appl ies to the copper- fo rma ldehyde p la t ing bath.

Theore t ica l Aspects We assume tha t the electroless p la t ing react ion is

composed of the fol lowing pa r t i a l react ions

Ro-> R z+ + Z e " (anodic) [1] and

M "+ + Z e - -+ M o (cathodic) [2]

When these two react ions are at s t eady state, the W a g n e r - T r a u d pos tu la te appl ies and the p la t ing rate , ip, is g iven b y

ip = iR = ]iMI [3]

where iR and iM are the anodic and cathodic pa r t i a l cur rents (wi th opposi te s igns) . The potent ia l associ- a ted wi th this equ i l ib r ium condit ion is the mixed potent ia l EMp. The cur ren t due to the diffusion of meta l ions to .a ro ta t ing disk is given by (7)

iM = BM' (C~ ~ -- CM~)X/~ [4]

where CM ~ iS the bulk concentration of metal ions, CM a the surface concentration, and BM' is a diffusion p a r a m e t e r g iven b y (8)

BM' --- 0.62 nMFDM2/3~,-1/eA [5]

In Eq. [5], DM : diffusion coefficient of the meta l ions; v ---- k inemat ic viscosity; nM -- n u m b e r of electrons involved in the meta l deposi t ion react ion; F -- Fa raday , and A -- a rea of the disk. For d i f fus ion-control led cathodic pa r t i a l r e a c t i o n , CM a = 0, and the l imi t ing cu r ren t iMD iS independen t of po ten t ia l and takes the form

iM D -- BM'CM ~/~ [6]

When the anodic pa r t i a l reac t ion is under e lec t rochemical control, the cu r r en t -vo l t age curve is descr ibed b y the equat ion

E = ER o - b a l n n R F k R - bRlnCR ~ -~ b R l n i R [7]

whe re kR iS the appa ren t ra te constant; na is the num- ber of e lectrons involved in the anodic reaction, and the Tafel slope bR -- aT~(1 -- ~R)nRF. Then combin- ing Eq. [6] wi th Eq. [7] by means of Eq. [3] gives

EMp -- ER ~ - - bRln nRFkR -- bRin CR ~~

ba In "~- ~- BYL'2~ -{- ba 111 CM r [8]

Equ,ation [8] descri:bes the dependence of EMp on ex - pe r imen ta l pa rame te r s such as ~ and CM ~o for the case in which the cathodic pa r t i a l react ion is diffusion con- t ro l led and anodic p a r t i a l reac t ion is ac t iva t ion con- t rol led. I t is c lear th,at EMp is a l inear function of In and In CM ~o. This case is represen ted g raph ica l ly in the symbol ic d i ag ram of Fig. 1.

Results The measurements were pe r fo rmed in a copper

E D T A - f o r m a l d e h y d e p la t ing solution which consisted

E L E C T R O L E S S M E T A L P L A T I N G 1113

of 4 X 10-~M CuSO4, 1.2 X 10-1M EDTA, and 0.0784M formaldehyde . The opera t ing t empera tu re was 70~ and the solution pH was ma in ta ined at a va lue of 11.7 wi th NaOH. In order to achieve condi t ions of con- t ro l led mass t ransfer , the measurmen t s were pe r - fo rmed with a ro ta t ing disk electrode. The da ta obta ined is shown in Fig. 2 and 3. I t is c lear that the expe r imen ta l points in each one of these plots group a round s t ra igh t lines. The slopes of the s t ra igh t lines were de te rmined by least squares analysis. We note that the mixed potent ia l increases with both the rota t ion ra te ~ and the meta l ion concentra t ion CM oo whi le it decreases wi th the concentra t ion Ca ~ of fo rmaldehyde in the p la t ing solution. This behav ior meets the cr i ter ia of a d i f fus ion-control led cathodic par t ia l react ion coupled with an ac t iva t ion-cont ro l led par t i a l anodic react ion as indica ted in Eq. [8]. F u r t h e r verif icat ion for this overa l l mechanism was obta ined by compar ing the measured slope of the ro ta t ion ra te dependence dEMp/d In ~ with the concentra t ion dependences dEMp/d In CM'~ and dEMp/d In Ca ~. The expe r imen ta l slopes have

$;

w I w 2 w 3 w 4

/

EMp ",'----" - v | POTENTIAL I - bJ

-__g:: gl: f . )

Fig. 1. Symbolic representation of the overall reaction scheme for electroless metal deposition in which the cathodic partial reaction is diffusion controlled and the anodic partial reaction activation controlled.

-0.68 U D~

d -o. 7o >

N - 0 . 7 2

M

Iz N - 0 . 7 4

O

N N ~ - 0 . 7 6

| e / ' |

1 i

1.0 1.5 2.0

l o g

Fig. 2. Plot of the mixed potential of the plating solution as a function of rotation rate. Temperature ~ 70~ pH ~_ 11.7.



1114 J. Electrochem. Sock: E L E C T R O C H E M I C A L SCIE N CE A N D T E C H N O L O G Y May 1983

>

A

v

H

H

-0.67

-0.69

-0.71

-0.73

-0.75

I ' I ' I ' I

, J

-1.8

, I , I ~ I -1.6 -1.4 -1.2

log [CuSO 4 ]

Fig. 3. A plot of the mixed potential of the plating solution against the logarithm of the CuSO4 concentration. Temperature 70~ pH ~ 11.7.

the re la t ionship

dEMp dEMp dEMp - = 2 X , [ 9 ]

d l n C n ~ d l n CM~ d l n ~

which is also pred ic ted by Eq. [8]. A Tafel slope of 170 mV/decade for fo rma ldehyde oxida t ion obtained in the p la t ing solut ion is s l ight ly lower than the value obta ined in the anoly te alone. This is due to the in te r - dependence of the two pa r t i a l react ions in the p la t ing solution. Nonetheless, a Tafel slope of 170 mV/decade corresponds to a va lue of 0.41 for the anodic t ransfer coefficient. Deviat ion of the t ransfer coefficient f rom 0.5 is known to occur when the reac t ing species are specifically adsorbed, Such behavior is character is t ic of ca ta ly t ic reactions.

Manuscr ip t submi t ted Sept. 7, 1982; revised m a n u - scr ip t received Nov. 15, 1982.

Any discussion of this pape r wi l l appear in a Dis- cussion Section to be publ i shed in the December 1983 JOURNAL. Al l discussions for the December 1983 Dis- cussion Sect ion should be submi t ted by Aug. 1, 1983.

Publication costs o$ this article were assisted by IBM T. J. Watson Research Center.

REFERENCES 1. C. Wagner and W. Traud, Z. Electrochem., 44, 391

(1938). 2. M. Paunovic, Plating, 51, 1161 (1968). 3. S. M. E1-Raghy and A. A. Abo-Sa l ama , This Journal,

126, 171 (1979). 4. F. Donahue, ibid., 119, 72 (1972). 5. A. C. Makrides , ibid., 107, 869 (1960). 6. F .M. Donahue, ibid., 127, 51 (1980): 7. V. G. Levich, "Physicochemical Hydrodynamics ,"

Pren t ice-Hal l , Englewood Cliffs, NJ (1962). 8. A. C. Riddiford, "Advances in Elec t rochemis t ry and

Elect rochemical Engineer ing," Vol. IV, In tersc i - ence, New York (1966).

Time-Resolved Laser Interferometric Investigation of the Growth of Diffusion Layer during an Electrodeposition of Pyrrole Polymer

R. N. O'Brien

Department of Chemistry, University of Victoria, Victoria, British Columbia~ Canada V8W 2Y2

and K. S. V. Santhanam

Tata Institute of Fundamental Research, Bombay, India

Laser in t e r f e romet ry has been shown to be a useful technique for the invest igat ions of the growth of diffusion layers at the e lec t rode-so lu t ion interface dur ing e lect rochemical processes such as meta l ion deposi- t ion (1, 2), bu t i ts app l i cab i l i ty to the s tudy of organic e lec t rode processes such as po lymer deposi t ion has not yet been explored. Po lymer deposi t ion on an elec- t rode involves severa l steps including, for example , format ion of f ree radicals , p ropaga t ion and t e rmina - t ion of the po lymer ic chains. A few po lymers such as po lypyr ro l e exhib i t the p rope r ty of electronic conduc- tion and are being considered as rep lacements of con- vent ional e lect rode mater ia l s (3, 4). We wish to re - por t in this communicat ion on the growth of the di f - fusion l aye r dur ing the e lect rodeposi t ion of po lypy r ro le on a gold e lect rode and the use of such a po lymer as an e lec t rode ma te r i a l in the inves t igat ion of e lec t rochemical oxidat ion and reduction.

The e lect rodeposi t ion of po lypy r ro l e was carr ied out ga lvanos ta t ica l ly f rom an acetoni t r i le (anhydrous) ba th conta ining 0.1M (C4H9)4NC104 and 60 mM freshly

Key words: interferometry, diffusion, polypyrrole, polymer coated, nonaqueoUs.

dis t i l led pyrrole . The e lec t rochemical cell consisted of two go ld -p la ted copper blocks ( length 11 mm, th ick- ness 1.78 mm) separa ted by ei ther 7.5 or 5.0 mm fitted into a Teflon cell holder. A few exper iments were also carr ied out using go ld -p la ted glass e lec t rode pieces (deposi ted by vacuum deposi t ion) ( length 30.0 ram, thickness 5.0 mm) separa ted by a fixed distance of 6.60 mm. The cell i tself acted as a F a b r y - P e r o t in te r - fe romete r (5). The gold p la t ing of the copper blocks and glass pieces was done by the sput te r ing technique or by vacuum deposi t ion for a m a x i m u m per iod of 5 min. In al l cases the electrodes were in the deep ve r - t ical configuration (V) so tha t the grea te r dimension ( length) is the depth of solut ion and the lesser d imen- sion ( thickness) is the solut ion thickness the laser l ight t raverses . The in te r fe romet r ic mul t ip le beam fringes were formed using d ie lec t r ic -coa ted flats separa ted by the thickness of the e lectrodes (1.78 mm) having a finesse of about 7. The l ight source was a nomina l 1 m W He-Ne laser. The fringes were a r r anged by su i tab ly t ightening the cell c lamps and the concen- t r a t i o n - p e r t u r b e d fr inge pa t t e rns were recorded at in terva ls wi th a 35 m m Nikon camera. The exper i -



mechanisms of electroless metal plating

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