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The chemistry of palladium—tin colloid sensitizing processes
The Chemistry of Palladium-Tin Colloid Sensitizing Processes1 R. L. COHEN AND R. L. M E E K Bell Laboratories, Murray Hill, New Jersey 07974
Received May 21, 1975; accepted November 11, 1975 We have used Mt~ssbauer spectroscopy to determine the chemical composition of layers deposited on graphite substrates by commercial Sn-Pd sensitizing systems, and to trace the changes produced in these layers by the succeeding process steps. In conjunction with other recent investigations of these systems, we conclude that the catalytic centers consist of colloidal particles of Pd-Sn alloy, and that the primary purpose of the "accelerator" step in the process is to remove stannic hydroxide deposits. These results are compared to other recent data obtained using plastic substrates.
INTRODUCTION Electroless plating of copper onto insulating substrates is a process widely used in the electronics industry for manufacture of printed wiring boards and in decorative plating of plastic trim parts for automotive and consumer appliance applications. Plastic surfaces must be specially prepared to catalyze deposition from the electroless bath, and two processes are in general use (1). The older process consists of treating the plastic surface first with a dilute solution of stannous chloride, and then with a solution of palladium chloride. In the newer process, a single bath is made up with tin and palladium chloride, and used to treat the plastic surfaces. Both techniques, although simple in execution, involve a complex series of chemical reactions including precipitation, colloid formation, and redox reactions, and only in recent years has basic research been performed to establish in detail the chemical mechanisms involved. The large number of recent publications (2-10) discussing the chemistry of these sensitizer systems demonstrates the importance of the subject. 1Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975.
About 2 years ago, results of M6ssbauer spectroscopy experiments on the "single bath" Sn-Pd system were reported (4). In conjunction with previously established phenomenology, that work showed that the vital elements of the sensitizing process were that: (1) the Sn-Pd bath contained extremely fine (colloidal) particles of Pd-Sn alloy, (2) these particles were adsorbed on the plastic surface during treatment in the sensitizing bath, and (3) the "accelerator" step removed surplus tin deposits on the surface. This work was not quantitative in terms of the amount of various elements deposited on the surface, and only semiquantitative in terms of particle sizes. More recently, detailed investigations of surface layers deposited from the sensitizing bath have been undertaken by electron microscopy (6) and Rutherford backscattering (7) (for elemental analysis). The results of these experiments have shown decisively that the catalytic centers are very fine particles (mostly smaller than 20 A diameter), and composed of Sn and Pd in about a 1:7 elemental ratio. Two other papers (8, 9) have also appeared recently, however, based primarily on electron diffraction analysis of the sensitizing layers, which controverted this model (4). Since most of the recent work was carried out using carbon
156 Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
Copyright O 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
SENSITIZING PROCESSES or graphite substrates, rather than those used in (4), we considered it worthwhile to repeat some of the MSssbauer experiments with samples prepared by techniques identical to those used in (6, 7). This combination of techniques, (electron microscopy for the particle size and shape information, Rutherford backscattering for the elemental analysis, and M6ssbauer spectroscopy for the determination of the chemical state of the tin involved) provides detailed and direct insight into the microscopic processes involved in sensitization with the Sn-Pd single bath system. EXPERIMENTAL PROCEDURES Two commercial sensitizer systems, Shipley 9F and MacDermid 9070, were used. The Shipley sensitizer was prepared by mixing 33 ml concentrate, 33 ml deionized H20, and 33 ml (37%) HC1 per the manufacturer's instructions. The MacDermid solution was mixed using 25 ml concentrate, 55 ml H20, and 20 ml HCI. Accelerator, where used, was 20 ml Shipley 19 in 100 ml H20, and the MacDermid 9070D electroless Cu plating solution was used. These conditions were identical to those used in (7), where the composition of the solutions is described in detail. Graphite flakes approximately 1 mm square X 25 #m thick were used as a substrate for deposition of the catalytic layer. These flakes were cleaved from the same type of graphite used in the work of (7). Thus, we have exactly duplicated the conditions (solutions and substrate) used in the earlier work, and the elemental analysis performed by Meek in that research should be applicable to the surface layers we have produced here. Samples were produced using: Shipley sensitizer and H20 wash; sensitizer, wash, accelerator, wash; and sensitizer, wash, accelerator, wash, electroless solution, wash. A similar set of samples was made using the MacDermid catalyst. The flakes were stirred into the catalyst solution, and agitated for 1 min. The mixture was then decanted onto a filter funnel, and the subsequent washes and treatments
157
were carried out in this funnel. Washes and electroless plating times were 2 min long, and the acceleration step was 5 min long. Each of the samples consisted of approximately 1.26 g of graphite flakes. The flakes were packed into teflon absorber holders of 15-mm diameter and 16-mm deep, sealed, and cooled to 78°K for the M6ssbauer measurements. The equipment used was standard for Sn n9 M6ssbauer measurements: The source was 10 mCi of Sn ngm in the form of BaSnO3 and was maintained at 78°K during the measurements. A 45 um Pd critical absorber was used to filter out the X rays, and the gamma rays were detected by a scintillation detector connected with standard nuclear electronics. Counting rates were typically ~ 3 0 000 cps in the single channel. A standard velocity drive was used to provide parabolic motion, and data were accumulated in a multichannel analyzer used in the up-down multiscaling mode. No smoothing, background subtraction, or other "massaging" was performed on the data. In addition to the graphite flake sensitized samples, "standards" of Pd-Sn alloys, the intermetallic Pd~Sn, and a stannic hydroxide colloid were made by standard techniques (12) and measured at 78°K. Spectra of the sensitizer-treated samples and standards are shown in Figs. 1 and 2. These spectra present the basic experimental results of the work. RESULTS All of the spectra of the sensitized surfaces contain only two visible components: a broad line at zero velocity, corresponding to stannic hydroxide, and a somewhat sharper line at 1.8 mm/sec, corresponding to the disordered fcc alloy (11) containing less than 25 at.°-/o Sn in Pd. In the discussion below, we have assumed that the recoil-free fraction (MSssbauer fraction, f) is the same for the stannic hydroxide and alloy phases, so that the relative areas of the absorption peaks reflect the relative numbers of tin atoms present in the two phases. Spectra of the Pd-Sn alloy standard and Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
158
COHEN AND MEEK
['see (4)] on the colloidal particles. Previous research (4) has shown that the amount of .980 • 2.0 tin in the stabilizing layer is approximately SENSITIZER ". ; the same as that in the particle cores. Since .960 : 4.0 : the quantity of tin observed in the stannic .'JM.O ; : 6.0 hydroxide layer (before acceleration) exceeds ..,j. the tin present in the particle cores by about a factor of 10, it greatly exceeds the quantity ~_,.oo . . . ~ o.o " of tin present in the stabilizing layer• Thus, -=9 9 8J- SENSITIZER X -' ~. d 12 the major source of the stannic hydroxide ACCELERATOR ° m must be the tin dissolved in the electrolyte. .99 " 0,4 As discussed in (7), the atomic Pd-Sn ratio i~ " ~ : ~v:• 1°''~ attained after acceleration is typically 2-3, and rises to 5-7 (via tin removal into the elecSENSIT,.. " "':" ;" " troless solution) during the electroless plating .99e s ~". o.2 ACCELERATOR ~,q'w~,' q process. As shown in our Figs. 1 and 2, a large ,99( ELECTROLESS ;" s" 0.4 fraction of the tin atoms (before plating), and a significant fraction after plating, are present " O.S '...; as stannic hydroxide. Thus, the actual ratio ' J ~ ' ,, of Pd-Sn in the alloy cores is at least 5 and -4 SOURCE VELOCITY(mml$£C) more usually 10. This excludes the possibility F16.1. M6ssbauer spectra (using SnuS) of sensitizing that the cores could consist of Pd3Sn, which layers produced on graphite flakes after treatment with would have a Pd:Sn ratio of 3. We have (top) Shipley sensitizer + rinse, (middle), sensitizer, recently found that the isomer shifts of Pd-Sn rinse, accelerator, rinse and (bottom) after initial stage of plating. The solid lines show the spectra obtained in separate experiments for stannic hydroxide and Pd0.8~Sn0.14alloy. 1.000
~ ' ~ X
j.~,.~-~,-~
o.0
"
stannic hydroxide have been superimposed on those of the sensitizing layers, and can be seen to fit very well. The spectra show that the initial sensitizer treatment and wash deposit a large amount of stannic hydroxide on the surface, and that this phase is removed by the accelerator. The early stage of electroless plating further decreases the amount of stannic hydroxide found on the surface. For the discussion below, it is important to note that (1) the Sn-Pd alloy line is present in all stages of the processing, and (2) that the amount of stannic hydroxide initially deposited is far in excess of amount of tin in the Pd-Sn alloy. There are two possible sources of tin ions for the production of stannic hydroxide: tin ions (chloride complexed) in the electrolyte, and the tin ions that form the stabilizing layer Journal o f Colloid and Interface Science, Vol. 55, No. 1, April 1976
~,ooo"./" ~". :-.-..-.. • . ,=_
.999
v'~ \'.t':", -x."
:. , ...
SENSITIZER %; SI ",'. ACCELERATOR
.
,..
..
P d - S n ALLOY
, :'~ ° °
HYDROXIDE .99e
"% " " "
..
,... ,.oooo...... -.:. "..•:...:...
•
.. -:...-.: .. ,-.
• ",.,:
.
.
.
.
".
.
.
SENSITIZER . . . . .9995
a "°" ACCELERATOR "
a
ELECTROLESS
• ..."
"" ?"
.
. °~ "
.." .v:
"?.:
.9990 t -4
o
2
I 4
SOURCE VELOCITY mm/SEC
FIG. 2. As in Fig. 1, but using MacDermid sensitizer.
SENSITIZING PROCESSES
159
alloys of the type studied in (4) can vary from 1.49 to 1.96 mm/sec depending upon the state of anneal and cold work history of the sample. Thus, the isomer shift is probably not a good indicator of the composition of the Pd-Sn colloidal particles.
drophobic surfaces has been previously commented on (13). Two other recent papers present results that appear to conflict with the model we present here. One of these papers (8) reported use of electron microscopy and electron diffraction to study sensitizer layers produced by Shipley 9F catalyst. DISCUSSION AND COMPARISON WITH The authors of (8) observed a dense coating OTHER RECENT RESULTS of fine particles after the sensitization step We will begin the discussion by emphasizing and water rinse, and a relatively sparse coating the consistent picture of Sn-Pd sol sensitiza- of fine (20-30 A) particles after processing in tion that can be assembled from this and other the accelerator. The material present after recent sensitizer work, and then describe how the sensitization step produced an "amorother recent results, which appear to be in phous" electron diffraction pattern, which was conflict with ours, in fact support them. We not further identified; the particles present will close with a comparison of the present after the acceleration step yielded a wellwork (on graphite substrates) with previous defined diffraction pattern from which six reresults on polyimide film. flections were indexed on an fcc structure correThe following model for sensitization using sponding to a lattice constant of 3.97 A (see Sn-Pd-HCI solutions can be arrived at from Table I). The authors considered this unequithe conclusions of (4, 6, 7) and the present vocal evidence for the presence of PdaSn, work: The catalyst is a suspension of colloidal which has been reported (11) to have fcc particles of Sn-Pd alloy, with an upper particle structure with the lattice constant 3.971 size limit of approximately 30 A diameter. 4-0.007 A. During sensitization, these Pd alloy particles Figure 3 shows a plot of lattice constant are adsorbed on the substrate. During the rinse versus Sn concentration for the Pd-rich end after catalysis, substantial additional stannic of the Sn-Pd system, taken from (11). It hydroxide is deposited over the Pd-Sn alloy can be seen that in the earlier work, the lattice particles. Most of this stannic hydroxide is constant obtained for pure Pd was too low by removed by the accelerator, which does not 0.02 A, and if the results are corrected (dotted change the chemical composition of the Sn-Pd line) for what must be presumed to be some alloy particles. The initial stage of electroless systematic error, the lattice constant of 3.97 A plating removes most of the renlaining stannic reported in (8) could have been obtained from hydroxide. the more dilute Pd-Sn alloy. This analysis incidentally explains another Combining Meek's elemental analysis results observation which we at first found puzzling: (7), which show about six atoms of Pd per Sn Cleaved graphite substrates are completely atom after the plating bath reaction, with the "wetted" after the sensitizer and wash steps M6ssbauer results, which show that 25-50% of processing, but then appear to be hydro- of the tin is in the form of stannic hydroxide phobic after the accelerator treatment. The at that processing stage, we see that the Pd results presented in (7) and here suggest that alloy particles themselves should have no more this apparent wetting is due to the heavy layer than --~10-15 at percent Sn. This would of (hydrophilic) stannic hydroxide material, suggest that t'd-Sn alloy, rather than PdaSn, and that when the hydroxide is removed in is the proper designation for the material. To put the issue of Pd-Sn alloy versus PdaSn the accelerator step, the graphite surface again shows its hydrophobic properties. The in perspective, it is appropriate to point out action of tin-based colloids in wetting hy- that arguments over precise lattice constant Journal of CoUoid and Interface Science, Vol. 55, No. 1, April 1976
160
C O H E N AND M E E K TABLE I
ComparisonofdSpacingsforFCC Lattice a Reference
FCC reflection ideal d spacing (A) sensitizer and substrate Treatment stage Shipley 9F Formvar
8) 9)
After accelerator
Precipitated reaction product
Soluble catalyst
9)
111 2.30
200 1.99
220 1.40
311 1.20
222 1.15
331 0.91
2.288
2.010
1.400
1.207
1.150
0.903
VS
S
S
S
2.39
2.02
1.42
1.21
VS 2.37
Aerobic catalyst
9)
VS 2.37
MacDermid etched ABS
(14)
MacDermid etched ABS
(14)
Catalyst (no rinse)
Catalyst (rinse) accelerator (rinse)
S
S
M
M
2.05
1.41
1.21
1.15
S
S
2.05
1.41
VS
S
S
S
M
2.37
2.05
1.41
1.21
1.15
S 2.25
M 1.95
M 1.41
a With a = 3.97 A_ with values previously reported for Pd-Sn sensitizer materials (VS = very strong, S =
strong).
values, order or disorder in similar phases, and precise values of composition, are of limited significance considering that the particles involved are only 20-30/~ in diameter.
,~ 4'00I uJ 3.97
....,..,..'"" '"'""
tll 0 k-1
3.90 3.89 :3.85 0
I0
20
50
ATOMIC % TIN
FIG. 3. Lattice constant verses composition for Pd-Sn alloys, after (11). The dotted line has been added by us to correspond to the known lattice constant of pure Pd, 3.89 _L
Thus, we do not consider the question of identity of the colloidal particles as being Pd-Sn alloy or Pd3Sn to be a significant one. A more interesting question is why no Pd-Sn alloy was observed in (8) after the treatment with 9F catalyst, but before acceleration. The M6ssbauer measurements show clearly the line for the Pd-Sn alloy at this stage. Consideration of the elemental analysis and M6ssbauer results shows that a heavy layer (--~100 A) of stannic hydroxide is present on the substrate at this stage of the process. In the electron diffraction work of (8), the pattern arising from this heavy coating of stannic hydroxide could have provided the "amorphous" pattern observed at that stage, and the scattering action of the stannic hydroxide layer would also have destroyed any pattern from the underlying Pd-Sn particles. Thus, when the stannic hydroxide is removed by the accelerator, the diffraction pattern from the underlying Pd-Sn alloy would become visible, as was observed. Results in (8, Fig. la and lb) support this analysis, since they show a dense layer of particles after the
Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
SENSITIZING PROCESSES catalysis step, and substantial reduction of this layer density after acceleration. Another recent article (9) reported on precipitates and catalytic material obtained from both simplified "model solutions" and compositions used commercially. In that work, X-ray diffraction lines (from "precipitated reaction product") and electron diffraction lines (from "soluble catalyst") corresponding to d-spacings of the Pd-Sn alloy were observed, but not identified as such by the authors (see Table I). Still another study of sensitizer deposits, on etched ABS, has recently been described (14). That article also reported (but did not identify) strong electron diffraction patterns corresponding to Pd-Sn alloy after catalyst treatment (see Table I), and then identified much weaker lines after the accelerator and rinse as arising from Pd metal, which has d spacings (see Fig. 3) about 2% less than those for the Sn-Pd alloy. Table I shows a compilation and summary of these results on X-ray and electron diffraction d spacings. It should be clear that regardless of substrate, bath makeup, or processing technique, all authors report observing lines corresponding to the fcc structure with a lattice constant in the range observed for Pd, Pd-Sn alloy, or Pd3Sn. Since none of the work reported provided accuracy estimates, it is difficult to tell whether the small (<20-/0) reported differences among that lattice constants are real or within the uncertainties of measurement. As we stated above, the elemental analysis results correspond to Pd-Sn alloy. Since the new results presented here have been obtained using graphite substrates, it is important to establish that they are relevant to metallization of plastic substrates which are commonly utilized. Some connection can be established with previous work (4) using MacDermid 9070 on polyimide substrates. Figure 4 shows data taken from (4), replotted to adjust for the different number of surface layers used in the two samples. In those earlier tests, one sample was made by sensitizing fresh Kapton polyimide film, and then washing
161
I Of3( %, 0998 SENSITIZER, ACCEL E RATOR AND RINSE
~ o
0996
~-
099~ 1000
....,~
~
0 998 ~" 4'
0 996
0994
SENSI [ IZE~ AND RINSE
~:~
SOURCE VELOCITY (mm/se&)
FIo. 4. M6ssbauer spectra of sensitizing layers on polyimidefilm,after (4). The data have been replotted so that the size of the absorption dip is proportional to the number of tin atoms per cm2in the corresponding phase. These data show that the role of the accelerator is to removestannic hydroxide (linenear zero velocity), but that additional weak lines produced by divalent tin (range of 2.5-4.5 mm/sec) also disappear after accelerator treatment. The spectra of sensitizerlayers on graphite substrates do not show any Sn~+components. in deionized H~O. The other was made by sensitizing, treating with fluoboric acid accelerator, and rinsing. No intermediate rinse was used between sensitizer and accelerator. The data show both similarities with, and differences from, the data obtained using the graphite flakes. The spectrum of the sensitized Kapton surfaces after treatment with the accelerator is essentially identical to that observed for the graphite surfaces studied here. Within the limit of accuracy, only lines for the stannic hydroxide and Pd-Sn alloy are observed. The spectrum of Kapton surfaces treated only with catalyst and water rinse, however, is slightly different from that observed for sensitized graphite surfaces, since a small amount of divalent tin (lines in the region 2.5-4.5 mm/sec) is deposited. This divalent tin is removed in the accelerator step, and thus is not present when the sensitized surface is placed in the electroless plating bath. It has been suggested (4) that this divalent tin is deposited in the form of a mixed-valence Journal of Colloid and Interface Science, V o l . 55, N o . 1, A p r i l 1976
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COHEN AND MEEK
colloid that forms in tin chloride solutions at low acidity and ionic strength (3). SUMMARY AND CONCLUSIONS We have reported here the results of M6ssbauer spectroscopy studies on sensitization of graphite surfaces with commercial S n - P d catalyst systems. These results, in combination with previously reported elemental analysis and electron microscopy studies, show clearly that the vital elements of the process are (1) adsorption of colloidal particles of P d - S n alloy from the sensitizing bath, (2) removal of excess stannic hydroxide during the acceleration treatment, and (3) removal of additional stannic hydroxide during the initial stage of electroless plating. Although these results conflict with some other recent studies of the sensitization process, we believe that the experiments carried out b y other researchers can actually be used to support the model presented here. ACKNOWLEDGMENTS We would like to thank K. W. West for assistance with the measurements and data reduction. REFERENCES 1. GOLDIE,W., "Metallic Coating of Plastics," Vol. 1, p. 39. Electrochemical Publications, Middlesex, 1968.
Journal of Colloid and Interface Science, Vol. 55, No. 1. April 1976
2. CorlEl% R. L., D'Amco, J. F., A~D WEST, K. W., Y. Electrochem. Soc. 118, 2042 (1971). 3. COrlEN, R. L., AIqDWEST, K. W., Y. Electrochem. So¢. 119, 433 (1972). 4. Com~r, R. L., AI~DWEST, K. W., J. Electroehem. Soe. 120, 502 (1973); Chem. Phys. Lett. 16, 128 (1972). 5. DE MINJER, C. H., ANDV.D. BOOM,P. F. J., J. Electrochem. Soc. 120, 1644 (1973). 6. MEEK, R. L., COHEn, R. L., ANDCULLIS,A. G., in "Proceedings of the Fifth Plating in the Electronics Industry Symposium," p. 146. New York, March 24-25, 1975. 7. MEEK,R. L., Y. Electrochem. Soc. 122, 1177 (1975). 8. FELDSTEIN, N., SCtILESINGER, M., HEDGECOCK, N. E., ANDCHOW,S. L., J. Electrochem. Soe. 121, 738 (1974). 9. RANTELL, A., AND HOLTZMAN,A., Plating, April 1974, p. 326; COHEN,R. L., AND WEST, K. W., Plating, November, 1974, p. 1053; AND MATIJEVId,E., Plating, November, 1974, p. 1051. 10. TSUKAHARA,M., KISHI, T., YAMAMOTO,H., AND BAGAI,T., J. Metal Finishing Soc. Japan 23, 83 (1972); KOSE, iV[., KISHI, T., YAMAMOTO,H., AND NAGAI, T., J. Metal Finishing Soc. Japan, 24, 203 (1973). 11. KNmI~T,J. R., AND RHYS, D. W., J. Less Comm. Metals 1, 292 (1959); Woo, O. T., REZEK, J., AND SCHLESINGER,M., Mater. S t . Eng. 18, 163 (1975). 12. IBRAIMOV,N. S., AND Kuz'mI% R. N., Soy. Phys. J E T P 21, 70 (1965); RAI, R. S. ANDGHOSrI,S., KoUoid Z. 153, 169 (1957). 13. KENNEY,J. T., TOWNSEND,W. P., AI~DE~rERSON, J. A., J. Colloid Interface S t . 42, 589 (1973). 14. RA~TELL, A., AND HOLTZMAN, A., Trans. Inst. Metal Finishing, 51, 63 (1973).
Received May 21, 1975; accepted November 11, 1975 We have used Mt~ssbauer spectroscopy to determine the chemical composition of layers deposited on graphite substrates by commercial Sn-Pd sensitizing systems, and to trace the changes produced in these layers by the succeeding process steps. In conjunction with other recent investigations of these systems, we conclude that the catalytic centers consist of colloidal particles of Pd-Sn alloy, and that the primary purpose of the "accelerator" step in the process is to remove stannic hydroxide deposits. These results are compared to other recent data obtained using plastic substrates.
INTRODUCTION Electroless plating of copper onto insulating substrates is a process widely used in the electronics industry for manufacture of printed wiring boards and in decorative plating of plastic trim parts for automotive and consumer appliance applications. Plastic surfaces must be specially prepared to catalyze deposition from the electroless bath, and two processes are in general use (1). The older process consists of treating the plastic surface first with a dilute solution of stannous chloride, and then with a solution of palladium chloride. In the newer process, a single bath is made up with tin and palladium chloride, and used to treat the plastic surfaces. Both techniques, although simple in execution, involve a complex series of chemical reactions including precipitation, colloid formation, and redox reactions, and only in recent years has basic research been performed to establish in detail the chemical mechanisms involved. The large number of recent publications (2-10) discussing the chemistry of these sensitizer systems demonstrates the importance of the subject. 1Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975.
About 2 years ago, results of M6ssbauer spectroscopy experiments on the "single bath" Sn-Pd system were reported (4). In conjunction with previously established phenomenology, that work showed that the vital elements of the sensitizing process were that: (1) the Sn-Pd bath contained extremely fine (colloidal) particles of Pd-Sn alloy, (2) these particles were adsorbed on the plastic surface during treatment in the sensitizing bath, and (3) the "accelerator" step removed surplus tin deposits on the surface. This work was not quantitative in terms of the amount of various elements deposited on the surface, and only semiquantitative in terms of particle sizes. More recently, detailed investigations of surface layers deposited from the sensitizing bath have been undertaken by electron microscopy (6) and Rutherford backscattering (7) (for elemental analysis). The results of these experiments have shown decisively that the catalytic centers are very fine particles (mostly smaller than 20 A diameter), and composed of Sn and Pd in about a 1:7 elemental ratio. Two other papers (8, 9) have also appeared recently, however, based primarily on electron diffraction analysis of the sensitizing layers, which controverted this model (4). Since most of the recent work was carried out using carbon
156 Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
Copyright O 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
SENSITIZING PROCESSES or graphite substrates, rather than those used in (4), we considered it worthwhile to repeat some of the MSssbauer experiments with samples prepared by techniques identical to those used in (6, 7). This combination of techniques, (electron microscopy for the particle size and shape information, Rutherford backscattering for the elemental analysis, and M6ssbauer spectroscopy for the determination of the chemical state of the tin involved) provides detailed and direct insight into the microscopic processes involved in sensitization with the Sn-Pd single bath system. EXPERIMENTAL PROCEDURES Two commercial sensitizer systems, Shipley 9F and MacDermid 9070, were used. The Shipley sensitizer was prepared by mixing 33 ml concentrate, 33 ml deionized H20, and 33 ml (37%) HC1 per the manufacturer's instructions. The MacDermid solution was mixed using 25 ml concentrate, 55 ml H20, and 20 ml HCI. Accelerator, where used, was 20 ml Shipley 19 in 100 ml H20, and the MacDermid 9070D electroless Cu plating solution was used. These conditions were identical to those used in (7), where the composition of the solutions is described in detail. Graphite flakes approximately 1 mm square X 25 #m thick were used as a substrate for deposition of the catalytic layer. These flakes were cleaved from the same type of graphite used in the work of (7). Thus, we have exactly duplicated the conditions (solutions and substrate) used in the earlier work, and the elemental analysis performed by Meek in that research should be applicable to the surface layers we have produced here. Samples were produced using: Shipley sensitizer and H20 wash; sensitizer, wash, accelerator, wash; and sensitizer, wash, accelerator, wash, electroless solution, wash. A similar set of samples was made using the MacDermid catalyst. The flakes were stirred into the catalyst solution, and agitated for 1 min. The mixture was then decanted onto a filter funnel, and the subsequent washes and treatments
157
were carried out in this funnel. Washes and electroless plating times were 2 min long, and the acceleration step was 5 min long. Each of the samples consisted of approximately 1.26 g of graphite flakes. The flakes were packed into teflon absorber holders of 15-mm diameter and 16-mm deep, sealed, and cooled to 78°K for the M6ssbauer measurements. The equipment used was standard for Sn n9 M6ssbauer measurements: The source was 10 mCi of Sn ngm in the form of BaSnO3 and was maintained at 78°K during the measurements. A 45 um Pd critical absorber was used to filter out the X rays, and the gamma rays were detected by a scintillation detector connected with standard nuclear electronics. Counting rates were typically ~ 3 0 000 cps in the single channel. A standard velocity drive was used to provide parabolic motion, and data were accumulated in a multichannel analyzer used in the up-down multiscaling mode. No smoothing, background subtraction, or other "massaging" was performed on the data. In addition to the graphite flake sensitized samples, "standards" of Pd-Sn alloys, the intermetallic Pd~Sn, and a stannic hydroxide colloid were made by standard techniques (12) and measured at 78°K. Spectra of the sensitizer-treated samples and standards are shown in Figs. 1 and 2. These spectra present the basic experimental results of the work. RESULTS All of the spectra of the sensitized surfaces contain only two visible components: a broad line at zero velocity, corresponding to stannic hydroxide, and a somewhat sharper line at 1.8 mm/sec, corresponding to the disordered fcc alloy (11) containing less than 25 at.°-/o Sn in Pd. In the discussion below, we have assumed that the recoil-free fraction (MSssbauer fraction, f) is the same for the stannic hydroxide and alloy phases, so that the relative areas of the absorption peaks reflect the relative numbers of tin atoms present in the two phases. Spectra of the Pd-Sn alloy standard and Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
158
COHEN AND MEEK
['see (4)] on the colloidal particles. Previous research (4) has shown that the amount of .980 • 2.0 tin in the stabilizing layer is approximately SENSITIZER ". ; the same as that in the particle cores. Since .960 : 4.0 : the quantity of tin observed in the stannic .'JM.O ; : 6.0 hydroxide layer (before acceleration) exceeds ..,j. the tin present in the particle cores by about a factor of 10, it greatly exceeds the quantity ~_,.oo . . . ~ o.o " of tin present in the stabilizing layer• Thus, -=9 9 8J- SENSITIZER X -' ~. d 12 the major source of the stannic hydroxide ACCELERATOR ° m must be the tin dissolved in the electrolyte. .99 " 0,4 As discussed in (7), the atomic Pd-Sn ratio i~ " ~ : ~v:• 1°''~ attained after acceleration is typically 2-3, and rises to 5-7 (via tin removal into the elecSENSIT,.. " "':" ;" " troless solution) during the electroless plating .99e s ~". o.2 ACCELERATOR ~,q'w~,' q process. As shown in our Figs. 1 and 2, a large ,99( ELECTROLESS ;" s" 0.4 fraction of the tin atoms (before plating), and a significant fraction after plating, are present " O.S '...; as stannic hydroxide. Thus, the actual ratio ' J ~ ' ,, of Pd-Sn in the alloy cores is at least 5 and -4 SOURCE VELOCITY(mml$£C) more usually 10. This excludes the possibility F16.1. M6ssbauer spectra (using SnuS) of sensitizing that the cores could consist of Pd3Sn, which layers produced on graphite flakes after treatment with would have a Pd:Sn ratio of 3. We have (top) Shipley sensitizer + rinse, (middle), sensitizer, recently found that the isomer shifts of Pd-Sn rinse, accelerator, rinse and (bottom) after initial stage of plating. The solid lines show the spectra obtained in separate experiments for stannic hydroxide and Pd0.8~Sn0.14alloy. 1.000
~ ' ~ X
j.~,.~-~,-~
o.0
"
stannic hydroxide have been superimposed on those of the sensitizing layers, and can be seen to fit very well. The spectra show that the initial sensitizer treatment and wash deposit a large amount of stannic hydroxide on the surface, and that this phase is removed by the accelerator. The early stage of electroless plating further decreases the amount of stannic hydroxide found on the surface. For the discussion below, it is important to note that (1) the Sn-Pd alloy line is present in all stages of the processing, and (2) that the amount of stannic hydroxide initially deposited is far in excess of amount of tin in the Pd-Sn alloy. There are two possible sources of tin ions for the production of stannic hydroxide: tin ions (chloride complexed) in the electrolyte, and the tin ions that form the stabilizing layer Journal o f Colloid and Interface Science, Vol. 55, No. 1, April 1976
~,ooo"./" ~". :-.-..-.. • . ,=_
.999
v'~ \'.t':", -x."
:. , ...
SENSITIZER %; SI ",'. ACCELERATOR
.
,..
..
P d - S n ALLOY
, :'~ ° °
HYDROXIDE .99e
"% " " "
..
,... ,.oooo...... -.:. "..•:...:...
•
.. -:...-.: .. ,-.
• ",.,:
.
.
.
.
".
.
.
SENSITIZER . . . . .9995
a "°" ACCELERATOR "
a
ELECTROLESS
• ..."
"" ?"
.
. °~ "
.." .v:
"?.:
.9990 t -4
o
2
I 4
SOURCE VELOCITY mm/SEC
FIG. 2. As in Fig. 1, but using MacDermid sensitizer.
SENSITIZING PROCESSES
159
alloys of the type studied in (4) can vary from 1.49 to 1.96 mm/sec depending upon the state of anneal and cold work history of the sample. Thus, the isomer shift is probably not a good indicator of the composition of the Pd-Sn colloidal particles.
drophobic surfaces has been previously commented on (13). Two other recent papers present results that appear to conflict with the model we present here. One of these papers (8) reported use of electron microscopy and electron diffraction to study sensitizer layers produced by Shipley 9F catalyst. DISCUSSION AND COMPARISON WITH The authors of (8) observed a dense coating OTHER RECENT RESULTS of fine particles after the sensitization step We will begin the discussion by emphasizing and water rinse, and a relatively sparse coating the consistent picture of Sn-Pd sol sensitiza- of fine (20-30 A) particles after processing in tion that can be assembled from this and other the accelerator. The material present after recent sensitizer work, and then describe how the sensitization step produced an "amorother recent results, which appear to be in phous" electron diffraction pattern, which was conflict with ours, in fact support them. We not further identified; the particles present will close with a comparison of the present after the acceleration step yielded a wellwork (on graphite substrates) with previous defined diffraction pattern from which six reresults on polyimide film. flections were indexed on an fcc structure correThe following model for sensitization using sponding to a lattice constant of 3.97 A (see Sn-Pd-HCI solutions can be arrived at from Table I). The authors considered this unequithe conclusions of (4, 6, 7) and the present vocal evidence for the presence of PdaSn, work: The catalyst is a suspension of colloidal which has been reported (11) to have fcc particles of Sn-Pd alloy, with an upper particle structure with the lattice constant 3.971 size limit of approximately 30 A diameter. 4-0.007 A. During sensitization, these Pd alloy particles Figure 3 shows a plot of lattice constant are adsorbed on the substrate. During the rinse versus Sn concentration for the Pd-rich end after catalysis, substantial additional stannic of the Sn-Pd system, taken from (11). It hydroxide is deposited over the Pd-Sn alloy can be seen that in the earlier work, the lattice particles. Most of this stannic hydroxide is constant obtained for pure Pd was too low by removed by the accelerator, which does not 0.02 A, and if the results are corrected (dotted change the chemical composition of the Sn-Pd line) for what must be presumed to be some alloy particles. The initial stage of electroless systematic error, the lattice constant of 3.97 A plating removes most of the renlaining stannic reported in (8) could have been obtained from hydroxide. the more dilute Pd-Sn alloy. This analysis incidentally explains another Combining Meek's elemental analysis results observation which we at first found puzzling: (7), which show about six atoms of Pd per Sn Cleaved graphite substrates are completely atom after the plating bath reaction, with the "wetted" after the sensitizer and wash steps M6ssbauer results, which show that 25-50% of processing, but then appear to be hydro- of the tin is in the form of stannic hydroxide phobic after the accelerator treatment. The at that processing stage, we see that the Pd results presented in (7) and here suggest that alloy particles themselves should have no more this apparent wetting is due to the heavy layer than --~10-15 at percent Sn. This would of (hydrophilic) stannic hydroxide material, suggest that t'd-Sn alloy, rather than PdaSn, and that when the hydroxide is removed in is the proper designation for the material. To put the issue of Pd-Sn alloy versus PdaSn the accelerator step, the graphite surface again shows its hydrophobic properties. The in perspective, it is appropriate to point out action of tin-based colloids in wetting hy- that arguments over precise lattice constant Journal of CoUoid and Interface Science, Vol. 55, No. 1, April 1976
160
C O H E N AND M E E K TABLE I
ComparisonofdSpacingsforFCC Lattice a Reference
FCC reflection ideal d spacing (A) sensitizer and substrate Treatment stage Shipley 9F Formvar
8) 9)
After accelerator
Precipitated reaction product
Soluble catalyst
9)
111 2.30
200 1.99
220 1.40
311 1.20
222 1.15
331 0.91
2.288
2.010
1.400
1.207
1.150
0.903
VS
S
S
S
2.39
2.02
1.42
1.21
VS 2.37
Aerobic catalyst
9)
VS 2.37
MacDermid etched ABS
(14)
MacDermid etched ABS
(14)
Catalyst (no rinse)
Catalyst (rinse) accelerator (rinse)
S
S
M
M
2.05
1.41
1.21
1.15
S
S
2.05
1.41
VS
S
S
S
M
2.37
2.05
1.41
1.21
1.15
S 2.25
M 1.95
M 1.41
a With a = 3.97 A_ with values previously reported for Pd-Sn sensitizer materials (VS = very strong, S =
strong).
values, order or disorder in similar phases, and precise values of composition, are of limited significance considering that the particles involved are only 20-30/~ in diameter.
,~ 4'00I uJ 3.97
....,..,..'"" '"'""
tll 0 k-1
3.90 3.89 :3.85 0
I0
20
50
ATOMIC % TIN
FIG. 3. Lattice constant verses composition for Pd-Sn alloys, after (11). The dotted line has been added by us to correspond to the known lattice constant of pure Pd, 3.89 _L
Thus, we do not consider the question of identity of the colloidal particles as being Pd-Sn alloy or Pd3Sn to be a significant one. A more interesting question is why no Pd-Sn alloy was observed in (8) after the treatment with 9F catalyst, but before acceleration. The M6ssbauer measurements show clearly the line for the Pd-Sn alloy at this stage. Consideration of the elemental analysis and M6ssbauer results shows that a heavy layer (--~100 A) of stannic hydroxide is present on the substrate at this stage of the process. In the electron diffraction work of (8), the pattern arising from this heavy coating of stannic hydroxide could have provided the "amorphous" pattern observed at that stage, and the scattering action of the stannic hydroxide layer would also have destroyed any pattern from the underlying Pd-Sn particles. Thus, when the stannic hydroxide is removed by the accelerator, the diffraction pattern from the underlying Pd-Sn alloy would become visible, as was observed. Results in (8, Fig. la and lb) support this analysis, since they show a dense layer of particles after the
Journal of Colloid and Interface Science, Vol. 55, No. 1, April 1976
SENSITIZING PROCESSES catalysis step, and substantial reduction of this layer density after acceleration. Another recent article (9) reported on precipitates and catalytic material obtained from both simplified "model solutions" and compositions used commercially. In that work, X-ray diffraction lines (from "precipitated reaction product") and electron diffraction lines (from "soluble catalyst") corresponding to d-spacings of the Pd-Sn alloy were observed, but not identified as such by the authors (see Table I). Still another study of sensitizer deposits, on etched ABS, has recently been described (14). That article also reported (but did not identify) strong electron diffraction patterns corresponding to Pd-Sn alloy after catalyst treatment (see Table I), and then identified much weaker lines after the accelerator and rinse as arising from Pd metal, which has d spacings (see Fig. 3) about 2% less than those for the Sn-Pd alloy. Table I shows a compilation and summary of these results on X-ray and electron diffraction d spacings. It should be clear that regardless of substrate, bath makeup, or processing technique, all authors report observing lines corresponding to the fcc structure with a lattice constant in the range observed for Pd, Pd-Sn alloy, or Pd3Sn. Since none of the work reported provided accuracy estimates, it is difficult to tell whether the small (<20-/0) reported differences among that lattice constants are real or within the uncertainties of measurement. As we stated above, the elemental analysis results correspond to Pd-Sn alloy. Since the new results presented here have been obtained using graphite substrates, it is important to establish that they are relevant to metallization of plastic substrates which are commonly utilized. Some connection can be established with previous work (4) using MacDermid 9070 on polyimide substrates. Figure 4 shows data taken from (4), replotted to adjust for the different number of surface layers used in the two samples. In those earlier tests, one sample was made by sensitizing fresh Kapton polyimide film, and then washing
161
I Of3( %, 0998 SENSITIZER, ACCEL E RATOR AND RINSE
~ o
0996
~-
099~ 1000
....,~
~
0 998 ~" 4'
0 996
0994
SENSI [ IZE~ AND RINSE
~:~
SOURCE VELOCITY (mm/se&)
FIo. 4. M6ssbauer spectra of sensitizing layers on polyimidefilm,after (4). The data have been replotted so that the size of the absorption dip is proportional to the number of tin atoms per cm2in the corresponding phase. These data show that the role of the accelerator is to removestannic hydroxide (linenear zero velocity), but that additional weak lines produced by divalent tin (range of 2.5-4.5 mm/sec) also disappear after accelerator treatment. The spectra of sensitizerlayers on graphite substrates do not show any Sn~+components. in deionized H~O. The other was made by sensitizing, treating with fluoboric acid accelerator, and rinsing. No intermediate rinse was used between sensitizer and accelerator. The data show both similarities with, and differences from, the data obtained using the graphite flakes. The spectrum of the sensitized Kapton surfaces after treatment with the accelerator is essentially identical to that observed for the graphite surfaces studied here. Within the limit of accuracy, only lines for the stannic hydroxide and Pd-Sn alloy are observed. The spectrum of Kapton surfaces treated only with catalyst and water rinse, however, is slightly different from that observed for sensitized graphite surfaces, since a small amount of divalent tin (lines in the region 2.5-4.5 mm/sec) is deposited. This divalent tin is removed in the accelerator step, and thus is not present when the sensitized surface is placed in the electroless plating bath. It has been suggested (4) that this divalent tin is deposited in the form of a mixed-valence Journal of Colloid and Interface Science, V o l . 55, N o . 1, A p r i l 1976
162
COHEN AND MEEK
colloid that forms in tin chloride solutions at low acidity and ionic strength (3). SUMMARY AND CONCLUSIONS We have reported here the results of M6ssbauer spectroscopy studies on sensitization of graphite surfaces with commercial S n - P d catalyst systems. These results, in combination with previously reported elemental analysis and electron microscopy studies, show clearly that the vital elements of the process are (1) adsorption of colloidal particles of P d - S n alloy from the sensitizing bath, (2) removal of excess stannic hydroxide during the acceleration treatment, and (3) removal of additional stannic hydroxide during the initial stage of electroless plating. Although these results conflict with some other recent studies of the sensitization process, we believe that the experiments carried out b y other researchers can actually be used to support the model presented here. ACKNOWLEDGMENTS We would like to thank K. W. West for assistance with the measurements and data reduction. REFERENCES 1. GOLDIE,W., "Metallic Coating of Plastics," Vol. 1, p. 39. Electrochemical Publications, Middlesex, 1968.
Journal of Colloid and Interface Science, Vol. 55, No. 1. April 1976
2. CorlEl% R. L., D'Amco, J. F., A~D WEST, K. W., Y. Electrochem. Soc. 118, 2042 (1971). 3. COrlEN, R. L., AIqDWEST, K. W., Y. Electrochem. So¢. 119, 433 (1972). 4. Com~r, R. L., AI~DWEST, K. W., J. Electroehem. Soe. 120, 502 (1973); Chem. Phys. Lett. 16, 128 (1972). 5. DE MINJER, C. H., ANDV.D. BOOM,P. F. J., J. Electrochem. Soc. 120, 1644 (1973). 6. MEEK, R. L., COHEn, R. L., ANDCULLIS,A. G., in "Proceedings of the Fifth Plating in the Electronics Industry Symposium," p. 146. New York, March 24-25, 1975. 7. MEEK,R. L., Y. Electrochem. Soc. 122, 1177 (1975). 8. FELDSTEIN, N., SCtILESINGER, M., HEDGECOCK, N. E., ANDCHOW,S. L., J. Electrochem. Soe. 121, 738 (1974). 9. RANTELL, A., AND HOLTZMAN,A., Plating, April 1974, p. 326; COHEN,R. L., AND WEST, K. W., Plating, November, 1974, p. 1053; AND MATIJEVId,E., Plating, November, 1974, p. 1051. 10. TSUKAHARA,M., KISHI, T., YAMAMOTO,H., AND BAGAI,T., J. Metal Finishing Soc. Japan 23, 83 (1972); KOSE, iV[., KISHI, T., YAMAMOTO,H., AND NAGAI, T., J. Metal Finishing Soc. Japan, 24, 203 (1973). 11. KNmI~T,J. R., AND RHYS, D. W., J. Less Comm. Metals 1, 292 (1959); Woo, O. T., REZEK, J., AND SCHLESINGER,M., Mater. S t . Eng. 18, 163 (1975). 12. IBRAIMOV,N. S., AND Kuz'mI% R. N., Soy. Phys. J E T P 21, 70 (1965); RAI, R. S. ANDGHOSrI,S., KoUoid Z. 153, 169 (1957). 13. KENNEY,J. T., TOWNSEND,W. P., AI~DE~rERSON, J. A., J. Colloid Interface S t . 42, 589 (1973). 14. RA~TELL, A., AND HOLTZMAN, A., Trans. Inst. Metal Finishing, 51, 63 (1973).