Volume 56, Number 9, 2002 APPLIED SPECTROSCOPY 11470003-7028 / 02 / 5609-1147$2.00 / 0q 2002 Society for Applied Spectroscopy

Determination of Cyanide Species in Silver and Gold PlatingSolutions by Raman Spectroscopy

KEUNCHANG CHO, YONG SOON JANG, MYOUNG-SEON GONG, KWAN KIM,and SANG-WOO JOO*Cheonan Research Institute, Acqutek Semiconductor & Technology, Cheonan 330-300 Korea (K.C., Y.S.J.); Department ofChemistry, Dankook University, Cheonan 330-714 Korea (M.-S.G., S.-W.J.); and School of Chemistry, Seoul National University,Seoul 141-742 Korea (K.K.)

Raman spectra of silver and gold cyanide complexes have been in-vestigated in order to determine the speciation of aqueous platingsolutions. We have examined the ionic species in the concentrationranges of 0.003 ; 0.5 M for the metals and 0.1 ; 2.5 M for thecyanide ion. When the initial concentration of [Ag(CN)2]2 was below;0.1 M, [Ag(CN)4]32 did not appear prominent even at high CN2/Ag(I) ratios of 3 ; 10, differing from the case of previous infraredmeasurements. Concentrations of the metal cyanide complexes andCN2 ion as low as 0.001 ; 0.01 M could be determined with anerror of less than ;3% from the analysis of Raman peak areas.

Index Headings: Silver plating; Gold plating; Silver cyanide com-plexes; Gold cyanide complexes; Raman spectroscopy.

INTRODUCTION

The determination of cyanides in various metal com-plex forms is important for industrial processes as wellas for environmental investigations and monitoring.1 Thechemical analysis of plating solutions has been of con-siderable interest due to its practical applications in man-ufacturing semiconductor devices.2 Because of a highlyuniform quality during the deposition process, cyanidebaths have been extensively used in silver and gold plat-ing despite its high toxicity.

It has been previously reported that silver(I)–cyanideforms three complexes depending on the molar ratio ofAg(I) and CN2, whereas gold forms only the di-cyanidecomplex.3,4 Jones and Penneman3 studied infrared ab-sorption spectra of the silver cyanide complexes:[Ag(CN)2]2, [Ag(CN)3]22, and [Ag(CN)4]32. The averagedissociation constants for the tri- and tetra-cyanide com-plexes into the next complexes were determined to beK3,2 5 0.113 M and K4,3 5 3.18 M, respectively, at 218C. Chantry and Plane reported the relative Raman inten-sities and bond polarizabilities of [Ag(CN)4]32 and CN2

ions by Hg exciting lines.5 The depolarization ratios ofthe di-, tri-, and tetra-coordinated silver cyanide com-plexes were also reported.6

There have been numerous electrochemical methodsusing colorimetric titration,2 impedance analysis,7,8 cap-illary ion analysis,9,10 and electrochemical quartz crystalmicrobalance measurement11 to quantitatively determinethe concentrations during the plating process. Electros-pray mass spectrometry was employed to directly deter-mine the ionic species in solutions.12 Since numerousconstituents present in the baths make the single responseof the electrochemical signal less competent for a precise

Received 24 January 2002; accepted 22 April 2002.* Author to whom correspondence should be sent.

monitor of plating solutions, an effective spectroscopicmethod is necessary to overcome several problems ofelectrochemical analysis for a monitor of the constituentsin plating baths.

Raman spectroscopy has several advantages in theanalysis of the chemical species in aqueous solutions.13

Advances in compact spectrometer design and high sen-sitivity detection have enabled rapid developments in theapplication of Raman spectroscopy to process and mon-itor relevant samples and systems.14 Infrared and Ramanspectroscopy has been employed to determine the spe-ciation of transition metal complexes.15–19

Although the gold and silver cyanide complexes werepreviously studied using infrared and Raman spectros-copy, there has been no Raman data for a quantitativedetermination of the ionic species in aqueous solutions.In this paper we have reinvestigated Raman spectra ofsilver and gold cyanide complexes in aqueous platingbaths. Raman spectra at low silver concentrations of 0.01; 0.05 M and high cyanide concentrations of ;0.15 Mshowed somewhat different features from the infraredmeasurements.3 Referring to the Raman intensities andequilibrium constants, we managed to determine the rel-ative concentrations of each cyanide complex as well asthe free CN2 ion. The purpose of this study is to providea practical guideline for in situ Raman analysis of themetal cyanide complexes in aqueous plating baths. Thiswork will be useful in developing a Raman monitoringsystem of the plating solutions in the semiconductor in-dustry.

EXPERIMENTAL

Sample Preparations. KAu(CN)2 (99%), KAg(CN)2

(99.62%), and KCN (95%) were purchased from TechnicInc. We attempted to reduce the exposure to light duringthe experiment, since silver cyanides are sensitive tolight. The samples were constantly kept in a dark room.Other chemicals were reagent grade. Triply distilled waterwas used to make plating solutions. The plating solutionsconsisted mainly of silver cyanide (or gold cyanide) com-plexes, KCN, and phosphates.9,10 Alkaline conditionswere preferably used for the present cyanide plating bathfor fewer impurities.1 We did not add any buffer in theRaman test, since the pH remained alkaline at ;11 with-out much variation.

Raman Measurements. Each spectrum was taken im-mediately after mixing each constituent solution. Themethod of Raman measurements was described in theprevious report.20 Raman spectra were obtained using a

1148 Volume 56, Number 9, 2002

FIG. 1. CN stretching region in the Raman spectra of the two platingsolutions at different concentrations. (a) 0.0151 M of KAg(CN)2,0.00347 M of KAu(CN)2, 0.154 M of KCN, and 0.0574 M of phosphate(KH2PO4, Mw 5 174.18). (b) The same constituents as in (a) except ahigher KAg(CN)2 concentration of 0.0502 M.

Renishaw Raman system model 2000 spectrometerequipped with an integral microscope (Olympus BH2-UMA). The 514.5-nm radiation from a 20-mW air-cooledargon ion laser (Spectra Physics Model 163-C4210) wasused as the excitation source for the Raman experiments.Raman scattering was detected with 1808 geometry usinga peltier cooled (270 8C) charge-coupled device (CCD)camera (400 3 600 pixels). A glass capillary (Chase Sci-enti� c Inc.) with an inner diameter of ;0.9 mm was usedas a sampling device. The holographic grating (1800grooves/mm) and the slit allowed the spectral resolutionto be 1 cm21. The Raman band of a silicon wafer at 520cm21 was used to calibrate the spectrometer, and the ac-curacy of the spectral measurement was estimated to bebetter than 1 cm21. Since the Raman spectra were strong-ly dependent upon the laser � eld, special attention waspaid to minimizing deviation in focusing. The sample po-sition was adjusted with an accuracy of a few microme-ters by a microscope attached to the Raman spectrometer.We managed to focus a homogeneous sample solutionembedded in an ;0.9-mm-i.d. capillary without muchdif� culty. Also, the focused laser power was frequentlyexamined by the intensity of the silicon Raman band at520 cm21. Each scan for 10 s was accumulated threetimes to obtain the spectrum and the total data acquisitiontime was 30 s. The Raman spectrometer was interfacedwith an IBM compatible PC, and the spectral data wereanalyzed using Renishaw WiRE software v. 1.2 based onthe GRAMS/32C suite program (Galactic). All the Ra-man spectral measurements were taken at room temper-ature of ;20 8C. We have examined the ionic species inthe concentration ranges of 0.003 ; 0.5 M for the metalsand 0.1 ; 2.5 M for the cyanide ion.

For an analysis of overlapping peaks, the spectral lineswere � tted by a Gaussian deconvolution procedure. Todetermine the intensity, the peak area was used insteadof the peak height. For most Raman spectra, it was dif-� cult to choose the peak height as intensity, since quitea few peaks had different line widths. The accuracy inmeasuring peak areas was better than 1% in most anal-yses. In this Raman measurement, combined with a peakarea analysis, the reliable detectable limits of [Ag(CN)2]2

and [Au(CN)2]2 were as low as ;0.001 M. For CN2,[Ag(CN)3]22, and [Ag(CN)4]32, the accuracy worsened to0.005 ; 0.01 M, since the exact determination of con-centration was hampered by an error of spectral � tting.

RESULTS AND DISCUSSION

Raman Spectra of Electroplating Solutions. Figures1a and 1b show the CN2 stretching region between 2300and 1900 cm21 for the two plating solutions with thedifferent concentrations. The spectral features signi� -cantly changed by varying the ratio of the constituentsof KAg(CN)2, KAu(CN)2, and KCN. The concentrationsof silver and gold cyanides in actual plating baths areusually in the range of 0.01 ; 0.05 M and 0.003 ; 0.05M, respectively. The cyanide concentration was typicallyin the range of 0.1 ; 0.5 M. We did not add any addi-tional KCN to maintain a certain free cyanide concentra-tion. The constituents in Fig. 1a corresponded to 0.0151M of KAg(CN)2, 0.00347 M of KAu(CN)2, 0.154 M ofKCN, and 0.0574 M of phosphate (KH2PO4, Mw 5

174.18). The Raman peaks at 2164, 2142, 2110, and 2080cm 2 1 were ascribed to [Au(CN)2]2 , [A g(CN)2]2 ,[Ag(CN)3]22, and CN2, respectively. The Raman stretch-ing frequencies of CN2 and metal ion complexes werequite consistent with the previous reports.5,6 The constit-uents of Fig. 1b were the same as those of Fig. 1a exceptfor a higher KAg(CN)2 concentration of 0.0502 M. Wecould clearly see an increase of [A g(CN)2]2 and[Ag(CN)3]22 intensities, whereas the [Au(CN)2]2 peak re-mained almost unchanged. On the other hand, the CN2

peak decreased considerably with the increase ofKAg(CN)2 concentra tion . It is noteworthy that the[Ag(CN)4]32 peak did not look so conspicuous, even atthe high cyanide ion concentration of ;0.15 M, when theinitial [Ag(CN)2]2 concentration was as low as 0.01 ;0.05 M. This result seemed not to agree well with theprevious infrared study,3 which showed that the[Ag(CN)4]32 peak was most prominent at the CN2 /Ag(I)ratio of 5 ; 9. The weakness of the [Ag(CN)4]32 Ramanband was thought to be its relatively small Raman scat-tering intensity in comparison with the infrared molarextinction coef� cient. A more detailed discussion on theRaman intensity of metal cyanide complexes is presentedin the following section.

Relative Raman Intensities. In order to determine theconcentration of each ionic species, we tested the stan-dard solutions of each constituent. The peak areas ofKCN and KAg(CN)2 at a concentration of 1.00 M weretaken as a standard for determining the concentrations ofCN2 and [Ag(CN)2]2, respectively. For [Au(CN)2]2, weused the 0.500 M aqueous KAu(CN)2 solution as a stan-dard due to its relative low solubility. The relative Ramanscattering coef� cients were referred to the previous lit-erature.13,21,22 The concentrations of CN2 and [Ag(CN)2]2

could be determined by comparing the peak areas usingthe standard solutions of 1.00 M. The relative Raman

APPLIED SPECTROSCOPY 1149

FIG. 2. Raman spectra at different ratios of CN2 with respect to[Au(CN)2]2. The concentrations of CN2 ranged from 0.100 to 1.00 Mat the constant [Au(CN)2]2 concentration of 0.100 M. (a) 0.00 M, (b)0.100 M, (c) 0.200 M, (d ) 0.300 M, (e) 0.500 M, and (f) 1.00 M of theCN2 concentration.

scattering intensity values for [Ag(CN)2]2 and[Au(CN)2]2 with respect to the cyanide ion were mea-sured to be 5.37 and 6.99, respectively.

For the tri- and tetra-cyanide complexes, the concen-trations were determined by using the concentrations ofthe other two species, since we could not prepare thepurely isolated complexes. The complete dissociationconstants for the bi-, tri-, and tetra-cyanide complexeswere referred to the previous literature.3,23 Among theseveral values3,23–24 for the complete dissociation con-stants, K II

C 5 1.26 3 10221, K IIIC 5 1.58 3 10222, and

K IVC 5 2.54 3 102 21 were used for [A g(CN)2]2 ,

[Ag(CN)3]22, and [Ag(CN)4]32, respectively, in order todetermine each silver ion complex. The relative Ramanintensity factors could be obtained from the known con-centration values. The average values for [Ag(CN)3]22

and [Ag(CN)4]32 with respect to CN2 were 7.80 and 11.2,respectively. For [Ag(CN)4]32, the relative Raman scat-tering intensity of the CN stretching band with respect tothat of the cyanide ion was found to be somewhat smallerthan the infrared molecular absorptivity.3 This may ex-plain why the [Ag(CN)4]32 peak did not look so conspic-uous even at high ratios of CN2 /Ag(I) in Figs. 1a and1b. The relative Raman intensity for CN2 was found tobe smaller than that of the silver cyanide complex ions,as in the case of the infrared molar extinction coef� cientvalues.3 The Raman intensity factor for [Ag(CN)4]32 wasfound to be larger than that determined by Chantry andPlane,6 where only the isotropic part of the scattering wasconsidered for A1 symmetry vibrations. The [Au(CN)2]2

ion showed a little higher Raman intensity than that of[Ag(CN)2]2.

Raman Spectra of Gold Cyanide Complexes. Figure2 shows the Raman spectrum of a mixture of KAu(CN)2

and KCN. The concentration of CN2 ranged from 0.00to 1.00 M while the initial concentration of [Au(CN)2]2

was � xed to 0.100 M. As previously reported,3,15 our Ra-man spectra of the gold cyanide complex has shown onlythe single peak of [Au(CN)2]2. It is not absolutely certainwhy gold would not form a tri- or tetra-cyanide complex,differently from the case of silver cyanide complexes. Itis presumably due to the much higher stability constantof [Au(CN)2]2 than that of [Ag(CN)2]2.23,24 We could de-termine the [Au(CN)2]2 and CN2 concentrations by mea-suring their peak areas and taking a ratio with respect tothe standard solution. Initial concentrations were com-pared with those determined by spectral measurement.The error in determining the concentrations from our Ra-man spectral measurements was estimated to be less than3% in most cases.

It has been known that the gold dicyanide complex hasa propensity to adhere to glass surfaces, particularly atlow cyanide concentrations. For most cases of our Ramanmeasurements, the cyanide concentration was higher thanthat of the gold dicyanide complex. In order to avoidobtaining erroneous results, we attempted to take thespectra as quickly as possible. The adsorption of goldcyanide complex on glass seemed insigni� cant, as evi-denced in the Raman spectra of Fig. 2. Polypropylenecells are used in the actual plating process.

Raman Spectra of Silver Cyanide Complexes. Tocheck the spectral behavior of silver cyanide complexesin detail, we have taken a series of Raman spectra when

the initial concentration of [Ag(CN)2]2 was as high as0.100 and 0.500 M, as shown in Figs. 3 and 4, respec-tively. The concentrations in Figs. 3 and 4 were the val-ues before mixing the two solutions. Figure 3 shows thatthe [Ag(CN)4]32 peak at 2096 cm21 did not look so con-spicuous at the silver concentration of 0.100 M. Theseresults appeared to be consistent with the observations inFigs. 1a and 1b.

Since there was a considerable degree of overlappingin spectral lines, a Gaussian deconvolution procedure wasused to analyze the peaks corresponding to the three sil-ver cyanide complexes, [Ag(CN)2]2, [Ag(CN)3]22, and[Ag(CN)4]32. We had to depend on the method of Gauss-ian peak � tting to determine the concentration of eachionic species because we could not prepare a pure tetra-cyano complex. The [Ag(CN)4]32 peak at 2094 cm21

looked negligibly small at some concentration ranges,however, as shown in Figs. 1, 3, and 4. We should men-tion that [Ag(CN)2]2 and [Ag(CN)3]22 could have beenquantitatively determined by a least-squares � tting meth-od at speci� c concentration ranges. We plan to apply oth-er spectral analysis methods of determining the speciesin a more effective way depending on the concentrationranges.

Figure 4 showed that the [Ag(CN)4]32 became promi-nent when the cyanide ion ranged from 0.00 to 2.50 M

1150 Volume 56, Number 9, 2002

FIG. 3. Raman spectra at different ratios of CN2 with respect to[Ag(CN)2]2. The concentrations of CN2 ranged from 0.00 to 1.00 M atthe constant [Ag(CN)2]2 concentration of 0.100 M before mixing thetwo solutions. (a) 0.100 M, (b) 0.200 M, (c) 0.300 M, (d ) 0.500 M,and (e) 1.00 M of the CN2 concentration.

FIG. 4. Raman spectra at different ratios of CN2 with respect to[Ag(CN)2]2. The concentrations of CN2 ranged from 0.00 to 2.50 M atthe constant [Ag(CN)2]2 concentration of 0.500 M before mixing thetwo solutions. (a) 0.00 M, (b) 0.500 M, (c) 1.00 M, (d ) 1.50 M, and(e) 2.50 M of the CN2 concentration.

TABLE I. Determ ination of concentrations and equilibrium con-stants of silver cyanide complexes by spectral measurements.a

CN2

[Ag(CN)2]2

[Ag(CN)3]22

[Ag(CN)4]32 bK3,2

bK4,3

0.06460.1340.1820.2080.3560.5140.7770.9181.69

0.04940.03920.1420.03020.01770.06930.009500.03900.0152

0.02140.03650.2190.04540.05290.2810.05920.2700.204

···0.001830.01230.003070.005020.04210.01090.07860.153

0.1490.1440.1180.1380.1190.1270.1250.1320.126

···2.683.243.083.753.434.233.162.26

cAvg: 0.131(60.011)

cAvg: 3.23(60.61)

a Units in M. The concentrations were determined from the relative peakareas with respect to those of 1.00 M standard solutions by comparingthe Raman intensities.

b The de� nitions of the average dissociation constants K3,2 and K4,3 arethe same as in Ref. 3.

c Avg: Average values. The number in parentheses is the standard de-viation.

at 0.500 M of KAg(CN)2. The peak corresponding to[Ag(CN)2]2 at 2142 cm21 became much weaker with anincrease in the CN2 concentration. Since the relative Ra-man intensity factors were determined, it was possible tomeasure the concentrations of each complex ion and de-termine the average dissociation constants for the tri- andtetra-cyanide complexes into the next complexes. Asshown in Table I, the average values of the concentrationequilibrium constants determined from our Raman spec-tral measurements were K3,2 5 0.131 and K4,3 5 3.23 M,respectively, at 20 8C. These values showed an excellentagreement with the infrared absorbance measurements byJones and Pennemann3 actually used in determination ofrelative Raman intensities in our experiments. These re-sults indicated that Raman spectroscopy should be wellutilized to determine the concentrations of silver cyanidecomplexes. Furthermore, our Raman data will be usefulfor monitoring real plating systems since the concentra-tions of silver and gold cyanides are usually in the rangeof 0.01 ; 0.05 M.

Our recent study has shown that the plating qualitydepends on the molar ratio of the silver cyanides and thefree cyanide ion.25 This result implies that the ionic statesof silver cyanide complexes may affect the actual platingprocess. In this respect, monitoring not only the total sil-ver concentration but also the exact speciation of silverion complexes should be signi� cant in achieving a better

plating result. Our future research is to clarify the effectof each ionic species on the plating.

The present method cannot be employed to monitorthe plating baths containing both silver cyanides and cop-per cyanides, since their Raman bands appear in the sameregion. We should mention that the gold and silver cya-

APPLIED SPECTROSCOPY 1151

nides were exclusively used for the purpose of the presentelectroplating. Other electrochemical tests did not indi-cate any copper ions remaining in the plating bath asimpurities. For the on-line monitoring of a general in-dustrial process, several detection methods will be com-bined with the present Raman analysis to determine othermetal species simultaneously.

Although not presented in this paper, in order to obtainmore precise information on the metal–cyanide interac-tion in the plating baths, we are currently investigatingthe n(metal–CN) peaks21 at 300 ; 500 cm21, as well asthose of the n(CN) bands at 2000 ; 2200 cm21. In thisrespect, Raman spectroscopy has the advantage of ob-taining the information in a wide spectral range that FT-IR spectroscopy cannot provide simultaneously.

CONCLUSION

Raman spectroscopy combined with the peak areaanalysis has been successfully utilized to study the de-termination of each ionic species in the actual platingsolutions. We could determine the concentrations of thespecies as low as 0.001 ; 0.01 M with an error less than;3% from the analysis of its Raman peak areas in mostcases. Raman measurement at low silver concentrationshas shown somewhat different spectral features fromthose in the previous infrared spectroscopic studies onthe species of metal cyanides. To understand the specia-tion of the plating solutions, we need to obtain the rela-tive Raman intensity factors since different selection rulesshould be used in infrared and Raman spectroscopy. Thedissociation constants for silver cyanide complexes ob-tained from the Raman peak areas matched quite wellwith those from the previous infrared absorbance mea-surements. This monitoring of species in a metal cyanidecomplex is potentially useful to achieve a better platingquality.

1. F. A. Lowenheim, Electroplating (McGraw–Hill, New York, 1978).2. T. H. Irvine, The Chemical Analysis of Electroplating Solutions

(Chemical Publishing Co., New York, 1970).3. L. H. Jones and R. A. Penneman, J. Chem. Phys. 22, 965 (1954).4. R. A. Penneman and L. H. Jones, J. Chem. Phys. 24, 293 (1956).5. G. W. Chantry and R. A. Plane, J. Chem. Phys. 33, 736 (1960).6. G. W. Chantry and R. A. Plane, J. Chem. Phys. 35, 1027 (1961).7. F. A. Ludwig and R. P. Verdes, U.S. patent 4,631,116 (1986).8. O. A. Ashiru and J. P. G. Farr, J. Electrochem. Soc. 142, 3729

(1995).9. K. Hwang, S. A. Choi, and C. Cho, Plat. and Surf. Fin., 74 (October

1998).10. K. Hwang, S. A. Choi, and C. Cho, Plat. and Surf. Fin., 133 (May

1999).11. M. I. Jeffrey and I. M. Ritchie, J. Electrochem. Soc. 147, 3272

(2000).12. G. R. Agnes, I. I. Stewart, and G. Horlick, Appl. Spectrosc. 48,

1347 (1994).13. A. Weber, Raman Spectroscopy of Gases and Liquids (Springer–

Verlag, New York, 1979).14. J. Workman, Jr., K. E. Creasy, S. Doherty, L. Bond, M. Koch, A.

Ullman, and D. J. Veltkamp, Anal. Chem. 73, 2705 (2001).15. L. H. Jones, J. Chem. Phys. 27, 468 (1957).16. B. M. Chadwick and S. G. Frankiss, J. Mol. Struct. 31, 1 (1976).17. C. Kappenstein, R. Hugel, A. J. P. Alix, and J. L. Beaudoin, J.

Chim. Phys. 75, 427 (1978).18. G. C. Lukey, J. S. J. Van Deventer, R. L. Chowdhury, D. C. Shall-

cross, S. T. Huntington, and C. J. Morton, React. Funct. Polym. 44,121 (2000).

19. G. C. Lukey, J. S. J. van Deventer, S. T. Huntington, R. L. Chow-dhury, and D. C. Shallcross, Hydrometallugy 53, 233 (1999).

20. S. W. Joo, S. W. Han, H. S. Han, and K. Kim, J. Raman Spectrosc.31, 145 (2000).

21. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Co-ordination Compounds (John Wiley and Sons, New York, 1997),5th ed.

22. G. W. Chantry and R. A. Plane, J. Chem. Phys. 32, 319 (1960).23. E. Hogfeldt, Stability Constants of Metal-Ion Complexes: Part A.

Inorganic Ligands (Pergamon, New York, 1982).24. C. J. Hawkins, O. Mfnsted, and J. Bjerrum, Acta. Chem. Scand.

24, 1059 (1970).25. K. Cho, Y. S. Jang, and S.-W. Joo, unpublished data.