a new technique for investigating the electrochemical behavior of electroless plating baths and the...

Vol. 118, No. 6 G O L D I S L A N D S O N C O P P E R - P L A T E D W I R E 869

The occurrence of copper dissolution is evidenced by an increase with t ime of the height of the cathodic wave for gold at 0.8V.

It was not possible to obtain a quant i ta t ive measure for the coverage of the copper substrate with gold because th in gold films, like that on sample G, dis- appear from the surface during the cycling. The I -U curve of sample G was traced about 20 sec after con- necting it to the potentiostatic circuit. During extended cycling, the cathodic peak at 0.8V becomes smaller and finally disappears. Then the I -U curve looks like curve b of the copper substrate in Fig. 2. The removal of the thicker gold film sample H was much slower than that of sample G. It is not known at present if the gold film is removed by anodic dissolution or by mechanical breakage because of anodic dissolution of the under ly ing copper substrate.

A quali tat ive confirmation of the preceding results is derivable from I -U curves measured at small sweep rates. The curves at 1 mV/sec have the form recently reported (10) for solid copper in 1M NaOH. The large anodie wave appearing at about 0.85V dur ing the anodic sweep masks any other effect. However, the height of this wave increases with decreasing average thickness of the gold deposit, indicat ing that more copper areas become exposed to the electrolyte and participate in the anodic copper dissolution.

Conclusions It has been demonstrated that periodic cur ren t -

potential curves can be used to obtain quali tat ive

informat ion on the relat ive surface areas of copper and gold on composite samples. Differences between gold on smooth or on rough copper substrates could not be observed because of the quali tat ive na ture of the measurements.

Acknowledgments The authors wish to thank R. Skoda and B. J. Drum-

mond for the preparat ion of the samples.

Manuscript submit ted Oct. 26, 1970; revised manuscript received Jan. 20, 1971.

Any discussion of this paper will appear in a Dis- cussion Section to be published in the December 1971 JOURNAL.

REFERENCES

1. J. S. Mathias and G. A. Fedde, IEEE Trans. Mag- netics, MAG-5, 728 (1969).

2. F. E. Luborsky, R. E. Skoda, and W. D. Barber, J. Appl. Phys., 42, in process (1971).

3. R. J. Morrissey, This Journal, 117, 742 (1970). 4. M. W. Breiter, Electrochim. Acta, 11}, 543 (1965). 5. M. W. Breiter, J. Phys. Chem., 69, 901 (1965). 6. A. Hickling and D. Taylor, Trans. Faraday Soc., 44,

262 (1948). 7. R. W. Ohse, Z. Physik. Chem. N. F., 21, 406 (1959). 8. M. W. Breiter, This Journal, 114, 1125 (1967). 9. D. T. Napp and S. Bruckenstein, Anal. Chem., 40,

1036 (1968). 10. B. Miller, This Journal, 116, 1675 (1969).

A New Technique for Investigating the Electrochemical Behavior of Electroless Plating Baths and the

Mechanism of Electroless Nickel Plating N. Feldstein* and T. S. Lancsek

RCA Corporation, David Sarnoff Research Center, Princeton, New Jersey 08540

ABSTRACT

The electrochemical potent ial of electroless nickel and electroless cobalt plat ing baths can be altered by the addit ion of a var ie ty of chemical com- pounds to the baths. At the same time, the metal - to-phosphorus ratio in the deposit is altered. This behavior has been studied for a series of baths using typical accelerators including thiourea, glycine, and formate; each of these represents a different type of accelerator characteristic. A Tafel- l ike behavior was observed to apply for the deposition of nickel (or cobalt) and also for the deposition of phosphorus. Therefore, given a sui table reference point, the plat ing rate and metal - to-phosphorus ratio can be predicted from the measured potential. A model based on a modified hydride mechanism is proposed for the electroless deposition of nickel-phosphorus. With the help of this model, the hydrogen evolved dur ing deposition can be quant i ta t ive ly accounted for.

Since the discovery of the electroless plat ing process by Brenner and Riddell (1), m a n y commercial proces- ses have been developed employing this plat ing technique. In particular, the deposition of nickel layers by electroless techniques is widely used in the electronics industry. Although the actual s t ructure (2-4) of nickel- phosphorus deposits formed with these baths is still unresolved, the phosphorus content in the deposit is of importance. Typical chemical and physical properties (5, 6) which are dependent on the phosphorus content are: resistivity, hardness, in terna l stress, corro- sion resistance, magnet ic moment, and electrical con- tacts to semiconductor material . It is thus impor tant to control the phosphorus content through control of

* Elec t rochemical Society Act ive Member . K e y words : electroless plat ing, e lec t rochemical behav ior and

mechanism, role of accelerators.

those parameters which affect its deposition rate. In examining prior l i terature on the phosphorus

content of nickel deposits, there is a general agreement on the following trends.

1. The per cent phosphorus increases with increas- ing tempera ture of operation.

2. The per cent phosphorus increases with increas- ing hypophosphite concentration.

3. The per cent phosphorus increases with decreasing pH of the bath.

Although considerable effort has been devoted to the s tudy of the role played by major components of the bath (buffers, complexing agents, reducing agents, etc.), little at tent ion has been focused on the possible effect(s) of stabilizers (7) and accelerators (8-10) on the phosphorus content of the deposit.

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.203.227.61Downloaded on 2014-07-14 to IP

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870 J. Electrochem. Soe.: E L E C T R O C H E M I C A L S C I E N C E J u n e 1971

In a recent invest igat ion (11), it was demonstrated that inhibi tors (stabilizers) al ter the electrochemical parameters at the electrode solution interface and at the same t ime change the nickel-phosphorus ratio in the deposit. In the present investigation, it was de- cided to examine the effect(s) associated with some typical accelerators, since the use of such additives is highly desirable in practical plating. The accelerators chosen in this investigation were typical, each repre- senting a specific class of accelerator.

Exper imenta l Procedures

Chemicals.-- .A_] l chemica ls used in p r e p a r i n g the plating solutions were reagent grade. The water used w a s deionized and then distilled.

Rate.--For comparison of the relat ive effect(s) of the various accelerators, s tandard 2 in. x 2 in. a lumina substrates (American Lava Corporation 614 plain) having a 40 #in. surface roughness were used. Volume of solutions was held at 200 cc unless otherwise stated. Electroless plat ing on these substrates was carried out using a convent ional t in -pa l lad ium sensitization and activation sequence; the weight gain dur ing plat ing was recorded. In most cases, in i t ia t ion of plat ing was apparent wi thin 30 sec; however, for solutions having the highest thiourea concentration, the ini t iat ion was sluggish. No agitat ion was provided dur ing the plating.

The plat ing t ime in all cases was chosen as 10.0 rain except in the case of bath C in which the deposition t ime was taken as 20.0 rain. All plat ing temperatures were main ta ined within •176 pH values were main- tained within • pH units at the corresponding operating temperature with the help of a Corning pH- controller, Model 10c. No agitation was provided dur - ing the plat ing cycle. In the current work, electroless Ni -P and Co-P plat ing baths shown in the Appendix were employed.

Subsequent to the plating, the deposits were ana- lyzed for their nickel and phosphorus (or cobalt and phosphorus) content employing both x - r ay fluorescence spectroscopy and convent ional wet chemical analysis. For the wet chemical analysis, a gravimetr ic technique based on dimethylglyoxime was used for nickel (or cobalt) , while the phosphorus was determined colori- metr ical ly with molybdenum blue. In the x - ray technique, the net x - r ay in tensi ty (counts per second) w a s monitored for PK~ and NiK~ lines, respectively. Hydrogen evolved in the course of plat ing was collect- ed by a simple displacement apparatus.

Potential measurements.--Potential measurements were carried out using a recording potentiometer (John Fluke Manufactur ing Company, Model 825A). All s teady-state potentials (mixed potentials) and equi l ibr ium potentials were obtained using freshly deposited nickel or cobalt electrodes against SCE at the operating temperatures. Potent ial measurements were made as the concentrat ion of the additives was varied. All reported values are wi th in +1 inV.

3 2 0

O_

2 8 0

~E 24C

zoc

16C

co 12C

4C

uJ 5E~3 ~J

co

520 uJ

uJ 5 0 0

48o <

46o e u~ I I i i i i i i i __

! 6 4 792 132 198 2 6 4 3 3 0 3 9 6 4 6 2 52.8 5 9 4 6 6 0 M O L A R C O N C E N T R A T I O N OF T H I O U R E A ( x l O 6 )

Fig. l. Effect of thiourea on a nlckel-plating bath (bath A)

J o ~r2o = g

~EIO0

Eso

w .~ 6o a:

~J 2(

/ x l /

/

05 I 0 115 210 215 310 M O L A R C O N C E N T R A T I O N OF G L Y C I N E ( x l O )

6 6 0 z

64o s

6 2 0

6 0 0 j

580 w

o 5 6 0 ~

540

s20 <

5 0 0 ~

Fig. 2. Effect of glycine on e nickel-plating bath (bath B)

Results and Discussion In examining the kinetic results shown in Fig. 1-4,

the effect(s) of the accelerators may be divided into three distinct classes.

AI class (accelerator-inhibitor).--The over-all behavior of this class is demonstrated in Fig. 1 and 4. Initially, there is an increase in plat ing rate leading to a maximum. Fur ther increases in additive concentrat ion lead to inhibi t ion of the plat ing rate, due to an adsorption-poisoning type mechanism. Typical ma- terials in this class are thiourea and thiocyanate.

API class (accelerator with partial inhibition).---For this class, the over-al l behavior is shown in Fig. 2. Following the ini t ial acceleration in deposition rate, there is an inhibi ted range. However, the degree of inhibi t ion is not complete; a plateau is reached at the higher accelerator concentrations. The plateau level achieved can either be below or above the init ial level in the absence of additive.

Fig. 3. Effect of formate on a nickel-plating bath (bath C).

60 r A F T T T l 1 1 % ~- _.0oF - : o

2 ~ 4 0 ^

X

E 3o

~ 20

~ ,o

0 0

( n - =550 ~>

540 z ~

530 0

' W

0 PLATING RATE 520 ~'~

x POTENTIAL >-"~ 5,o ~;

�9 HYDROGEN EVOLUTION ME

500 - - .05 .I .15 .2 .25 .3 .35 .4 .45 3 0

MOLAR CONCENTRATION OF SODIUM FORMATE

3 6 . ~

e r 30 ~

o 9

24 ~ .; ~ E

18.J~ g u W ~

z z

a z ~-o

o



Vol . 118, No. 6

o

92 ~j 46 L9

.o.. .0

-~ 76 ~X X 340~ 7 58 ~ = ~. 0 '~ 68 ~20 z 54 .~ N u~

60: ~oo~, ~ 3o ~

36 840 >" 18 ~% ' ~ o~

{n ) - ~ o_ c O /I.O 2 .o Boo } o :

MOLAR CONCENTRATION OF THfOUREA(xlO ~)

Fig. 4. Effect of thiourea on a cobalt-plating both (both D)

E L E C T R O L E S S P L A T I N G B A T H S 871

Table II. Effect of glycine on the nickel-phosphorus content

Net x - r a y in tens i ty c o u n t s / s e c

Molar Weight rec la imed b coneen- P K a b y a n a l y s i s (rag) t r a t i o n g lyc ine P K ~ NiK<x NiK<x N i p %po

O 1263 1177 1.O7 15.61 0.66 4.05 O.01 1613 2768 0.562 34.02 2.55 6.95 0.02 1497 3325 0,450 - - - - - - 0.03 1022 3737 0.273 -- ~ 0.04 802 4697 0.171 55.27 2.29 3.98 0.07 442 3631 0.122 O.lO 411 3293 o.12s 42~5 0.~4 1.% 0.30 447 2852 0.157 33.65 0.52 1.52

b W e i g h t r e c l a i m e d b y s t a n d a r d w e t c h e m i c a l analysis . c Va lues c a l c u l a t e d u s i n g w e i g h t s in p r e v i o u s t w o co lumns .

AO class (accelerator-only) . - -For this class, the over-al l behavior is i l lustrated in Fig. 3. Contrary to the behavior in the above two classes, the plat ing rate approaches a max imum value asymptotical ly as the additive concentrat ion is increased. It should be noted that the additives in this class general ly do not form complexes with the metallic cation in bu lk solution. Besides formate, similar behavior was noted with fluoride ion.

As reported by DeMinjer and Brenner (7), the in- corporation in the bath of thiourea in low concentrations gives an accelerating effect on the plating rate. Figure 1 (bath A) shows the manne r in which the plating rate and steady-state potential vary as the thiourea concentrat ion is modified. Variations in both the plating rate and steady-state potential are similar; i.e., corresponding increases or decreases in each take place as thiourea is added to the bath. It should be noted, however, that, al though thiourea is known to accelerate plat ing from some acidic electroless nickel baths, there are cases (12) (acid-type bath) in which this anticipated acceleration does not take place. Ex- aminat ion of the deposits with respect to their elemental composition (Table I) shows distinct acceleration in the formation rate of both the nickel and phosphorus. At the same time, the per cent phosphorus in the deposit decreases dur ing the acceleration part of the curve. Due to the high operat ing temperature, no collection of hydrogen was attempted.

In Fig. 2 (bath B), the effect of glycine on the plating rate and the steady-state potentials is demonstrated. The plat ing rate characteristics are in excellent agreement with the work by Holbrook and Twist (8). Fur thermore, they have reported that the molar ratio of hydrogen to nickel (nickel plus phosphorus) is constant over the entire glycine range, and this de- te rminat ion has not been repeated in the current work. It is fur ther demonstrated in Fig. 2 that the steady- state potential gradual ly increases toward more cathodic values upon the addit ion of glycine and leveling occurs at the higher glycine concentrations. As the glycine content varies, the equi l ibr ium potential for the nickel redox system is altered in a regular fashion. Specifically, --384 mV vs. SCE and --445 mV vs. SCE were measured for this system in the absence of hypophosphite for 0 and 0.3M glycine, respectively. Al- though the potential curves for the glycine system do

Table I, Effect of thiourea on the nlckel-phosphorus content

Mola r W e i g h t r e c l a i m e d b c o n c e n t r a t i o n b y ana lys i s (mg)

t h i o u r e a • 104 Ni P %pc

None 68.97 4.29 5.84 5.3 132.24 4.61 3.40

16.5 135.90 4,47 3.19 33 125.82 3.62 2.81 39.5 18.04 0.25 1.32

b W e i g h t r e c l a i m e d b y s t a n d a r d we t chemica l analys is . c Values ca l cu l a t ed u s i n g w e i g h t s in p r e v i o u s co lumns .

not follow the same characteristics as shown in Fig. 1, 3, and 4, this behavior is not surprising since glycine can complex the nickel with a pK value of about 5 (13). However, in view of the variations of steady- state potential with added glycine, it is not surprising to find (Table II) that both the nickel and phosphorus formation rates are altered. In addition, deposits having a different phosphorus-nickel ratio are obtained.

As seen from Fig. 3 (bath C), upon the addition of formate to the electroless nickel bath, acceleration in plating rate takes place. The plat ing rate, the rate of hydrogen evolution, and the s teady-state potential all increase gradual ly leading to a plateau. As demonstrated in Table III, both the nickel and phosphorus deposition rates increase with formate addition. There is also a net decrease in the phosphorus concentrat ion in the deposit as the formate concentrat ion increases. Furthermore, from the data of Fig. 3 and Table III, the molar concentrat ion of hydrogen to nickel was found to be 1.4 over the ent i re formate concentrat ion range examined.

Examinat ion of the equi l ibr ium potential for the nickel system in the absence of hypophosphite resulted in a constant value of --383 • mV. vs. SCE at 75~ and pH of 4 over the entire formate concentrat ion range. This result, along with absorption measurements in the uv-vis ible range, suggests that there is no complex formation be tween the formate and the nickel ions in solution.

While in Fig. 1-3 electroless nickel plat ing baths were examined, Fig. 4 (bath D) shows the influence of thiourea on a typical electroless cobalt bath. In examining the results of Fig. 4, it is apparent that ini t ia l ly there is an increase in the plat ing rate, the rate of hydrogen evolution, and the steady-state potential with added thiourea. Fur ther addition of the thiourea leads to a decrease in these dependent var i - ables and the eventual cessation of the plat ing process. This general course of behavior closely resembles the results shown in Fig. 1. In Table IV it is clearly shown that both the cobalt and phosphorus and their respec- tive plat ing rates are enhanced, however in a manne r in which the per cent phosphorus is decreased in the

Table Ill. Effect of sodium formate on the nickel-phosphorus content

Net x - r a y i n t e n s i t y Mola r c o u n t s / s e c

conecn- W e i g h t rec la imed b t r a t i o n of PKc~ by a n a l y s i s (rag)

s o d i u m f o r m a t e P K a N i K a N i K ~ Ni* P* %po

None 1425 1365 1,04 14.3 2.01 12.3 0.013 - - - - - - 22.1 3.00 11.9 0.025 1640 2048 0.800 24.6 3.06 11.1 0.050 1690 2485 0.680 29.8 - - - - O.10 1671 2885 0.579 33.8 4,00 10.6 0.50 1570 3240 0,485 45.8 5.31 10.4

b W e i g h t r e c l a i m e d by s t a n d a r d we t c h e m i c a l ana lys i s . c Values ca l cu l a t ed u s i n g w e i g h t s in p r e v i o u s two co lumns . * S a m p l e s used in th i s t ab l e are no t the same as e m p l o y e d in

Fig. 3.



872 J. EIectrochem. Soc.: ELECTROCHEMICAL SCIENCE June 1971

Table IV. Effect of thlourea on the cobalt-phosphorus content

m g of m g of Mola r conc of coba l t a p h o s p h o r u s a Per c e n t b t h i o u r e a x 10 ~ r e c l a i m e d r e c l a i m e d p h o s p h o r u s

None 15.86 0,Sl 4.86 0.50 28.69 1.41 ,I-.70 2 , 5 0 36 .83 1 .50 3 .92

" W e i g h t r e c l a i m e d by s t a n d a r d we t c h e m i c a l analys is . b Va lues ca Icu la ted u s i n g w e i g h t s in p r e v i o u s two co lumns .

acceleration part of the curve. Examinat ion of the equi l ibr ium potent ial for the cobalt system in the absence of hypophosphite resulted in a constant value of --726 ___ 2 mY vs . SCE for thiourea concentrat ions of 0-4.5 x 10-6M. This behavior is exactly analogous to that exhibited by the nickel system shown in Table I. Analysis of the gaseous product evolved as related to the cobalt deposited shows that in all cases the molar concentrat ion of the gaseous product to cobalt is in the range of 4:1 to 3:1 without any specific t rend with thiourea addition. Furthermore, it was found mass spectrometrically that a substant ia l portion of the gas phase is ammonia.

Replott ing the results of Fig. 1, 3, and 4 (and their corresponding tables) shows the m a n n e r by which the net plat ing rates for nickel and cobalt vary with the steady-state potentials (Fig. 5). Since the deposits formed in this work are general ly quite thick, it is reasonable to assume that composition is uniform throughout the bu lk of the cross section. Although in Fig. 5 the plat ing rate for the metals is plotted v s . potential, by suitable t ransformat ion (using Faraday 's law) these plat ing rates can be converted to "deposition currents." As seen in all three cases, the logarithm of the plat ing rate (or deposition current ) is a l inear function of the potential. This behavior is characteristic of Tafel plots (14). From relat ing the slopes of Fig. 5 to the theoretical Tafel slope, the t ransfer coefficients (a) were evaluated (assuming n ---- 2). The theoretical Tafel slope is

n ~ R T d l n i

F dE

where n ----number of electrons in charge transfer step

---- t ransfer coefficient R ---- gas constant

tOOO

BOO

600

400 "E

E

ZOO E

IO0 g ~ 8o

5 ~ 6o

uJ ~ 40

<

d. 2o

900 910 920 930 940 I .

O �9 �9

X REA BATHx (SxlO 2

/x �9 COBALT - TH IOUREA BATHx(SxIO ~)

O NICKEL-FORMATE BATNx(IO 3)

I00

80

60

40

Io 470 440 510 5130 5~0 5~0 540

STEADY STATE POTENTIAL vs S C E (mv)x( - I )

Fig. 5. Plating rate of nickel and cobalt vs. steady-state potential.

T ---- absolute tempera ture F : Faraday constant

Specifically, the following t ransfer coefficients were found: 0.5 for the n ickel - formate and cobal t- thiourea baths, and 0.4 for the nickel- th iourea case.

Since the Tafel type equation appears to fit the observed data, a charge t ransfer step can be assumed to be the slow step in the reduction of the metall ic cations. Furthermore, from earl ier work, this slow charge t ransfer step is probably due to a slow electron ex- change or a slow chemical change dur ing the course of the anodic oxidation of hypophosphite.

Figure 6 was prepared from data on the phosphorus deposition rate. As seen in all three cases, a l inear re- lationship exists which again resembles a Tafel plot. From the slopes of Fig. 6 and the theoretical Tafel slope (n assumed to be un i ty ) , the corresponding charge t ransfer coefficients were found

0.71 for cobal t - thiourea bath 1.1 for n ickel- th iourea bath 1.1 for n ickel- formate bath

It is thus concluded that the electrochemical reduction of hypophosphite to phosphorus is also charge transfer controlled and that the kinetics of deposition may be described by the Tafel equation. However, since by definition ~ may have values only in the range 0-1, the value of n must be greater than 1 in the electrochemical process leading to the formation of phosphorus.

It is thus postulated that reduct ion of hypophosphite itself is not involved in the formation of phosphorus. Rather, another in termediate having phosphorus in a form_~l valence state greater than one is probably involved. This species is probably formed from hypophosphite dur ing oxidation step (s). This intermediate species is postulated to be a metaphosphorus acid (HPO~), which, although short-l ived, could lead to the formation of either elemental phosphorus by reduct ion or orthophosphorus acid (H3PO3) by hydration.

Wieland and Winkler (15) postulated, on the basis of organic analogies, that a metaphosphorus acid is formed dur ing the catalytic dehydrogenat ion of hypo- phosphorus acid. Based on Wieland and Winkler ' s pos- tulate, Goldenstein e t aL (2) proposed metaphosphorus acid as an in termediate in the oxidation of hypophosphite. It is interest ing to note that in their work (2) the metaphosphorus acid is formed as par t of an atomic hydrogen t ransfer mechanism rather than the hydr ide- type mechanism accounted in this investigation.

890 900 910 920 930 940 930 I0

6,

4C : x

E o

~ 20J

~ O S

zoo4

g x

0 2 X NICKEL-THIOUREA BATHx(SxlO 2 ) �9 COBALT-THIOUREA EiATHx(SxlO z)

O NICKEL FORMATE BATHx(IO 3)

o~,~o ,,40 5to s~o ~o s~o 5~o STEADY STATE POTENTIAL vs SCE (mv)x(-J)

Fig. 6. Plating rate of phosphorus vs. steady-state potential



Vol. I18, No. 6 E L E C T R O L E S S P L A T I N G B A T H S 873

REACTION TYPE

1. H 2 P O z + O H - + X , [ X - - - H ] - + P O 2 + H 2 0 ELECTROCHEMICAL

2. PO 2 + O H - ~ H P O 3 C H E M I C A L

3. 2 [ X - - - H I - + M ++ - M + 2 ( X - - - H ) E L E C T R O C H E M I C A L

4. 2 ( X - - - H) ~ H 2 + 2 X C H E M I C A L

5. IX - - - H ] - + H 2 0 - - ~ H 2 + O H - + X E L E C T R O C H E M I C A L

6. 3 [ X - - - H J - + 2 H 2 0 + P O 2 , P + 3 ( X - - - H ) + 4 O H - E L E C T R O C H E M I C A L

7. H 2 P O 2 + O H - ~ H P O 3 + H 2 C H E M I C A L

Fig. 7. A modified hydride transfer mechanism. For alkaline condition, X represents a catalytic site, and M represents nickel or cobalt.

In view of the results of the current investigation, and the arguments presented by Lukes (16), it is believed that a modified hydr ide- type mechanism (16) best accounts for general characteristics of Ni-P and Co-P type baths. Figure 7 shows the basic steps which occur in this modified hydride t ransfer mechanism. Re- action 1 is the ra te-control l ing step result ing in the charge t ransfer control behavior observed for reactions 3 and 6. No at tempt is made in this investigation to distinguish between the two isomeric forms of hypophosphite (17). At the same time, the hydrogen evolved dur ing the course of plat ing is the sum total of reactions 4, 5, and 7 (Fig. 7). Reaction 7, however, should occur only at high temperatures and in alkal ine media (16). From the exper imenta l data for the hydrogen evolved from baths B, C, and from recently reported data on an alkal ine room-tempera ture bath (11), excellent agreement was found between the hydrogen measured and that expected, based on the composition of the deposit. Specifically, it was found that the following correlation holds fairly rigorously

d d d d-'T [H2] ---- -~ - [Nil + 1.5 ~ [P]

This correlation is consistent with the over-al l mechanism proposed in Fig. 7 and suggests that reaction 5 plays a minor role for the baths under examination. Furthermore, due to the competitive na ture of reactions 2 and 6 with respect to PO2-, it can be anticipated that reaction 2 wil l become more impor tant in alkal ine media. This is consistent with the observation of several investigators that the phosphorus content in the deposit is lower as the pH is increased.

Based on the results of this investigation and the consistency of the values for the t ransfer coefficient (~) for nickel (or cobalt) and phosphorus, the following practical results are envisioned, all leading to greater simplicity and better control of the composition of the deposit.

1. Based on (a) the value found for a ( t ransfer coefficient), (b) at least one deposition rate measure- ment, (c) the corresponding steady-state potential, and (d) the composition of the deposit, a complete deposition rate for nickel (or cobalt) and phosphorus as a function of potential may be calculated. The potential can be changed for a given bath by, e.g., adding an accelerator so that a complete plat ing rate vs. accelerator concentrat ion curve can be constructed (for class I and III only) . From these data, precise bath compositions may be selected to yield the desired alloy composition and plat ing rate.

2. Although in the current investigation an analysis for a b inary system (Ni-P or Co-P) was made, it is believed that a s imilar analysis can be made for ter- nary systems.

3. The use of thiourea in electroless plat ing is highly desirable since it accelerates, as well as stabilizes,

such baths. There is, however, a basic l imitat ion in the use of this mater ia l because its concentrat ion decreases with time, due to hydrolysis and sulfur co- deposition. It is thus highly desirable to have a simple analyt ical technique for low thiourea concentrations. Based on the potent ial variat ions (Fig. 1 and 4) with thiourea concentration, a simple potentiometric technique (19) has been conceived for the determinat ion of thiourea in electroless plat ing baths.

Conclus ions In the acceleration of electroless nickel and cobalt

deposition from hypophosphite-based media, three dis- t inct classes of accelerators are observed

AI (accelerator- inhibi tor) API (accelerator with par t ia l inhibi t ion) AO (accelerator-only)

In all cases, there are distinct potent ial changes for the bath and compositional variat ions in the deposit as the accelerator concentrat ion is increased. Detailed analysis of classes I and III accelerators shows that the nickel (or cobalt) and phosphorus deposition kinetics satisfy a Tafel- l ike equation. This correlation with the Tafel equations implies that the electroless deposition reactions may be treated by simple electrochemical principles. From our earlier work (11), it is known that the anodic oxidation of hypophosphite is the ra te- controll ing step.

The hydrogen evolved is due to several chemical reactions taking place simultaneously. In acid media (or low- tempera ture alkaline media) , we observe that ~he moles of hydrogen gas evolved are the sum total of the moles of nickel deposited plus 1.5 • the moles of phosphorus deposited. This leads to the hypothesis that the phosphorus originates from a short- l ived intermediate such as metaphosphorus acid, ra ther than from the parent hypophosphite molecule. These ob- servations are consistent with the modified hydride mechanism proposed to account for the findings of this investigation.

A c k n o w l e d g m e n t s The authors wish to thank E. P. Ber t in for his

assistance in the x - r ay fluorescence spectroscopy measurements and J. A. Amick for cri t ically reviewing this manuscript .

APPENDIX

Bath A (7) NaH2PO2 �9 H20, 9.45 • 10-~M NiC12 �9 6H20, 1.26 • 10-1M C2H303Na, 2.54 • 10-ZM pH, 4.2 Temperature, 91.5~ CH4N2S, Variable

Bath B (8) NaH2PO2.H20, 3.63 X 10-2M NiCI2 �9 6H20, 5.00 • 10-2M pH, 5.8 Temperature, 87~ C2H~O2N, Variable

Bath C (12) Ni + +, 0.32M NaH2PO2 �9 H20, 0.236M pH, 4.0 Temperature, 75~ HCOONa, Variable

Bath D (10) CoC1 �9 6H20, 0.126M NH4C1, 0.935M NaH2PO2 �9 H20, 0.189M Na citrate �9 2HoO, 0.340M pH (with NH4OH), 8 Temperature, 85~ CH4N2S, Variable

Manuscript submit ted Jan. 28, 1971; revised m a n u - script received Feb. 22, 1971. This paper was prepared



874 J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E J u n e 1971

for oral presentat ion at the Inst i tute of Metal Finish- ing, England, May 1971.

Any discussion of this paper will appear in a Dis- cussion Section to be published in the December 1971 JOURNAL.

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(1970). 12. N. Feldstein, Unpubl ished results. 13. J. Chem. Soc., Special Publ icat ion No. 17 (1964),

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Voltammetric and Coulometric Studies of the Mechanism of Electrohydrodimerization of

Diethyl Fumarate in Dimethylformamide Solutions W. V. Childs, *'1 J. T. Maloy} C. P. Keszthelyi, and Allen J. Bard*

Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712

ABSTRACT

The reduction of diethyl fumarate (DEF) in t e t r a - n - b u t y l a m m o n i u m iodide (TBAI) -d imethy l formamide (DMF) solutions at a p la t inum electrode has been studied by cyclic voltammetry, double potential step chronoamperometry, and controlled potential coulometry. The chronoamperometric response for several possible mechanisms of electrohydrodimerization has been obtained by digital s imulation techniques, and a method for dist inguishing among the mechanisms suggested. Results of double potential step chronoamperometr ic experiments strongly support a mechanism where the electrochemically gen- erated anion radicals undergo a second-order dimerization reaction. Controlled potential electrolysis results give evidence for a bulk polymerizat ion reaction in the absence of proton donor; protonation in the presence of hydroquinones; and good efficiency to the hydrodimer product in the presence of l i thium perchlorate tr ihydrate.

The study of electrohydrodimerizations (or electrolytic reductive couplings) of activated olefins and related substances, by an over-al l reaction shown in Eq. [1] has been the subject of numerous investigations, most recently especially

RI H RI J R2C -- CH2 X

2 C ---- C -f- 2e -}- 2H + --> [ [1] ~ R2C -- CH2 X

R., X R1 O II

X ---- e lec t ron-withdrawing group, e.g. - -CN, - - C - - O E t

by Baizer and co-workers [see (1-3) and references contained therein]. The reduction on acrylonitr i le (RI ---- R2 ---- H, X ---- CN) has been the subject of most

of the investigations because of the commercial importance of the hydrodimerized product, adiponitrile. Relatively few studies have been concerned with a

* Electrochemical Society Act ive Member. ' Present address: Phillips Pe t ro leum Company, Bartlcsville, Okla-

homa, 74004. =Present address: Depar tmen t of Chemistry, West Virginia Uni-

versity, Morgantown, West Virginia. K ey words: electrolytic hydrodimerizat ion, potential step ehrono-

amperomet ry , cyclic vo l t ammet ry , coulometry, computer simulation.

kinetic analysis of the mechanism of the process, however. The first papers, general ly on the basis of product distribution, viewed the process as occurring with an init ial two-electron reduct ion to the dianion, which then attacked the parent molecule to produce coupled products. Beck (4), based on an analysis of cur ren t -potential curves for the reduction of acrylonitrile, proposed a ra te -de te rmin ing step involving one electron and one water molecule to form a neutra l radical, which is immediately reduced fur ther to the proton- ated carbanion. Recently Petrovich, Baizer, and Oft (2, 3) carried out polarographic, cyclic voltammetric, and macroscale electrolysis studies of a number of di- activated olefins in N,N-dimethyl formamide (DMF) solutions and concluded that dimeric products were formed by either at tack of an electrochemically gen- erated anion radical on the parent unreduced otefin, followed by fur ther electroreduction and protonation, or by protonation of the anion radical, followed by fur ther reduction to an anion and subsequent attack on the olefin. An al ternate pa thway to the dimer, that of coupling of the anion radicals, was deemed less likely.

The research described here was under taken to in- vestigate the mechanism of the hydrodimerizat ion reaction using a var ie ty of electroanalytical techniques,



a new technique for investigating the electrochemical behavior of electroless plating baths and the...

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