Technique for Controlling Thio Compound Concentration in Electroless Plating Baths N. Feldstein and J. A. Weiner RCA Corporation, David Sarnoff Research Center, Princeton, N . J .
USE OF THIOUREA and its derivatives in electroless metal plating baths has been the subject of several investigations (1-3). Although the mechanism by which these materials influence plating rate is not completely understood, they are widely used in commercial electroless plating solutions for the following reasons: in low concentration, they tend to accelerate the plating rate; they tend to stabilize the plating bath against homogenous decomposition; and the resulting deposits have been reported to be brighter. There are, however, certain disadvantages in the use of such materials stemming from their depletion during plating. This depletion may be due either to a hydrolysis reaction in solution or to incorporation (2) of the compound in the deposit during plating. Precise control of the concentration of these additives in working baths is needed to ensure reproducible plating conditions and deposit characteristics. It should be noted that since these materials are present in very low concentrations, conventional analytical procedures are quite tedious. In this paper, a simple potentiometric technique is proposed for controlling the concentration of a thio compound in an electroless plating bath in which that compound is one of the components present. In recent studies (4, 5) on the mechanism of electroless plating of nickel and cobalt, it was demonstrated that the deposition rate and the steady state potential change in a parallel fashion. Furthermore, although the behavior of thiourea is not exactly the same from one bath composition to another, for any given bath there is a reproducible variation in potential as the concentration of thiourea is increased. It is this observed correlation between potential and concentration of thiourea that forms the basis for the potentiometric technique described in this note. If thiourea is known to be present in the bath initially, it will be in a concentration smaller than that at which the abrupt potential decrease and the drop in plating rate occurs. By titrating a fresh bath with a thiourea solution of known concentration, using the abrupt potential drop as an end point, a calibration is obtained. For a replenished bath equal to the fresh bath, except in thiourea (and phosphite) Concentration, the amount of thiourea needed can be found by a second titration and a comparison with the previously determined calibration value.
Table I. Electroless Plating Bath Compositions Bath 1B (3) Bath 1A (3) None CoClz.6Hz0 30.0 g/l. 30.0g/l. None NiS04.6Hz0 50.0 g/l. 50.0 g/l. NHiCl
NaHzPOz.H20 Na3CaH6O7. 2Hz0 Temperature pH at 80 "C Bath 2 (1) NiClz.6Hz0 30.0 g/l. NaHzPOz HzO 10.0 g/l. NaCHaCOZ 20.8 g/l. Temperature 75 "C pH at 75 "C (adjusted with HCI)-4.0
8.0
20.0 g/l. 100.0 g/l. 80 "C 8.0
grade of K & K Laboratories, Plainview, N. Y. The plating baths employed are shown in Table I. The first is based on citrate as a complexing agent whereas acetate is the complexing medium employed in the second bath. All titration experiments were carried out using a conventional potentiometric analysis procedure. The titrants employed were 0.020 g/l. thiourea and a phenylthiourea solution of about 0.25 g/l. As indicator electrodes, freshly deposited nickel (or cobalt) were plated on 1-inch diameter ceramic wafers. A commercial saturated calomel electrode was used as the reference electrode. Potential measurements were carried out using a differential voltmeter (John Fluke Mfg. Co., Model 825A). Temperature control was within h0.5 "C and solution pH was maintained within hO.l pH unit at the operating temperature. RESULTS AND DISCUSSION
EXPERIMENTAL
Most of the chemicals employed in this investigation were reagent grade, and the water used was deionized and double distilled. In specific, ammonium chloride, cobalt chloride, nickel chloride, sodium acetate, sodium citrate, and thiourea were "Baker Analyzed" Reagent (J. T. Baker Chemical Co., Phillipsburg, N. J.). The phenylthiourea used was technical (1) C. H. DeMinjer and A. Brenner, Plating, 44, 1297 (1957). (2) J. S . Sallo, J. Kivel, and F. C. Albers, Jr., J. Electrochem. Soc., 110, 890 (1963).
20.0 g/l. 100.0 g/l. 80 "C
(3) J. Kivel and J. S . Sallo, ibid., 112, 1201 (1965). (4) N. Feldstein, T. S. Lancsek, and J. A. Amick, ANAL.CHEM., 42, 945 (1970). ( 5 ) N. Feldstein and T. S. Lancsek, J. Electrochem. SOC.,in press.
Figure 1 shows the results of titrating electroless cobalt and nickel plating baths with thiourea. In the case of the cobalt bath, upon addition of thiourea the potential changes from an initial level of about -920 mV us. SCE to a final level of about -760 mV us. SCE. For the nickel bath the potential changes from about -900 mV t o about -700 mV. These initial and final potentials correspond, respectively, to the mixed potential of the operating bath and to the potential of the bath in the absence of hypophosphite. Apparently, the oxidation of the hypophosphite on the electrode surface is inhibited upon the addition of thiourea (6). Furthermore, according to Figure 1, the sharp drop in potential for the cobalt bath takes place at a thiourea concentration of about 0.37 mg/l. This value seems to be in excellent agreement with the published work by Kivel et a/. (3), in which the plating rate for this bath was monitored as a function of thiourea concentration. Figure 1 also shows the effect of thiourea o n a n alkaline nickel bath (3) (bath 1B). The potential dependency for this medium is also in good agreement with the published (3) kinetic characteristics. ( 6 ) N. Feldstein and P. R. Amodio, ibid., 117, 1110 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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Figure 1. Electrode potential us. concentration of thiourea for cobalt and nickel-alkaline baths
In a study (2) on the effect(s) of thiourea in electroless plating media, data on the adsorption of thiourea and its time dependency were presented suggesting that the adsorption of thiourea is time dependent. In the course of the present work, it was found necessary t o standardize the titration so that the titrant was added t o a given sample at a slow rate, e.g., one milliliter per minute. The potential was recorded 45 seconds after the addition and mixing of titrant. Following this procedure, repeated titrations from baths 1A and 1B gave end-point values within *lox of those shown in Figure 1. Figure 2 shows the potential variations that occur for an acidic nickel bath (Bath 2) during titration with thiourea. As seen, the end point occurs at about 1.27 mg/l. of thiourea, and repeated runs showed a reproducibility of *8x from this average value. It has been shown (7) that phenylthiourea
is a stabilizer for electroless plating baths. In Figure 3, the results of titration with phenylthiourea are shown. In an attempt to verify whether the titration is reversible, a n acidic nickel bath (Bath 2) containing thiourea at about 2.2 mgjl. was diluted with the same bath in which the thiourea was absent. Results showed that in order to obtain the anticipated mixed potential following dilution, a fresh nickel electrode was required. An electrode which was exposed to solutions containing higher thiourea concentrations did not return to the original potential upon dilution of the bath. It would thus seem that the adsorption of thiourea is an irreversible process under the conditions used here. In a previous publication (4), it was shown that the amount of stabilizer required to produce total inhibition in an electroless bath depends on bath reactivity as modified by changes in the concentration of the reducing agent, and also by changes in the concentration of the other reactants. This same behavior has been noted for the thiourea titrations described above. It is therefore necessary to replenish a given sample with respect to reducing agents, metal ion, and p H before the concentration of a stabilizer is determined. The procedure proposed in this paper consists of the following sequence. An aliquot of a given electroless bath containing stabilizer (e.g., thiourea) is titrated with a known concentration of the same stabilizer until a predetermined potentiometric end point is reached. The end point for a given thio compound in a given bath is first determined for a fresh bath. The difference between the end points for the fresh bath and for the replenished bath gives the concentration of stabilizer needed to restore the replenished bath to the correct operating point. For those stabilizers which exhibit slow adsorption, it is suggested that the titration be carried out at a slow, constant addition rate. In so doing, excellent reproducibility for the end point is achieved. This same technique can be applied to other aqueous media containing thio compounds by converting the medium into an electroless plating bath having a known composition. Other materials capable of adsorbing chemically at an electrode-solution interface should also be determinable in a similar manner.
(7) F. Pearlstein and R. F Weightman, Plating, 54, 714 (1967).
RECEIVED for review March 8, 1971. Accepted April 20,1971.
Figure 2. Electrode potential us. concentration of thiourea for a nickel-acid bath
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