1709
Langmuir 1990,6, 1709-1710
Notes Mechanism of Copper Deposition in Electroless Plating Tetsuya Ogura
We report below the results that we have obtained by the use of deuterated formaldehyde and deuterated paraformaldehyde as reducing agents in the electroless deposition of copper on a copper surface, from an alkaline solution of copper(I1)-EDTA.
Departmento de Quimica, Universidad Antonoma de Guadalajara, Guadalajara, Jalisco, Mexico
Experimental Section
Mark Malcomson and Quintus Fernando'
Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received March I , 1990. In Final Form: May 2, 1990
Introduction In the past 25 years, the importance of the electroless deposition of copper in the fabrication of printed circuit boards has been clearly established. As a result of the increasing interest in this copper deposition process, many investigations have been carried out on the kinetics of the reaction.I4 Because of the complexity of the reaction, these kinetic studies have yielded conflicting results, and no agreement has been reached on the detailed mechanism of the reduction process. Copper is deposited by the reduction of an alkaline solution containing copper(I1) ions stabilized by chelation with EDTA; the reducing agent that is employed is f~rmaldehyde.~ The overall stoichiometry of the reaction has been thoroughly investigated by several workers and is represented by6 CuY2- + 2CH,O
+ 40H-
-
Cuo + H,
-
Cuo + D,
+ 2HC00- +
Y4- + 2H,O (1) If deuterated formaldehyde is employed in this reaction, analysis of the gaseous reaction products should enable us to determine the manner in which the formaldehyde molecule participates in the reduction process at the surface of the copper metal. Only three possible results can be obtained from the analysis of the gaseous reaction products: (a) Only DZis evolved; in this case, the overall reaction may be represented by CuY2-+ 2CD,O
+ 40H-
+ 2DC00- + Y"+ 2H,O (2)
(b) Only HD is evolved; in this case, the overall reaction may be represented by CuY2- + 2CD,O
+ 40H-
-
+
+
Cuo + HD + HDO H,O Y4- + 2DC00- (3) (c) A mixture of D2 and HD is evolved; in this case, the overall reaction may be represented by an appropriate combination of reactions 2 and 3. (1)Schumacher, R.; Pesek, J. J.; Melroy, 0. R. J. Phys. Chem. 1985, 89, 4338. (2) W.iese, H.; Weil, K. G. Ber. Bunsen-Ges. Phys. Chem. 1987,91,619. (3) Bindra, P.; Roldan, J. J. Appl. Electrochem. 1987, 17, 1254. (4) Nichimura, K.; Machida, K.; Enyo, M. J. Electroanal. Chem. Interfacial Electrochem. 1988, 251, 103. (5) Goldie, W. Metallic Coating o f P l a s t i c s ; Electrochemical Publications Ltd.: Middlesex, England, 1968; Vol. 1. (6) Lukes, R. M. Plating 1964,51, 1066.
0743-7463/90/2406-1709$02.50/0
Materials. Deuterated paraformaldehyde (99.5 atom 5%) was purchased from MSD Isotopes, Montreal, Canada. D&O was freshly prepared just before use by thermal depolymerization a t 150 "C of the paraformaldehyde-dz stored in mineral oil? The gaseous product was absorbed in water with the aid of a slow stream of nitrogen, and the concentration of the resultant solution was determined iodimetrically. HD was obtained by the addition of CaH2 to DzO, and Dz was prepared by the reaction of Ca with D2O. Electroless Copper Deposition. The apparatus for the copper deposition consisted of a 125-mL Erlenmeyer flask fitted with a stopper through which was inserted a glass tube (5 cm in length, 15-mm 0.d.) packed with Drierite. One end of the glass tube was connected to a gas sample collector which consisted of a 10-cm glass tube (10-mm 0.d.) equipped with a stopcock a t each end. The gas evolved in the Erlenmeyer flask was passed through the gas sample collector, and the volume of the evolved gas was measured by collecting it over water in a graduated cylinder. One-hundred milliliters of a solution containing 0.04 M CuSO4 and 0.04 M EDTA, adjusted to pH 12 with NaOH, was introduced into the Erlenmeyer flask and heated to 70 OC. An aqueous formaldehyde solution (8 mmol) or solid paraformaldehyde was added to the flask, and the system was purged with a stream of nitrogen. A filter paper (5 cm in diameter) was immersed in a sensitizer solution which consisted of a mixture of 0.04 M SnClz and 0.5 M HC1 for 1 min, rinsed with water, and then immersed in an activator solution that was 10-3 M in PdC142- and 0.05 M in HCl, for 1 min. The filter paper was rinsed thoroughly and introduced into the Erlenmeyer flask in which an atmosphere of nitrogen was maintained. The reaction mixture in the flask was stirred continuously with the aid of a magnetic stirrer. After 80 mL of gas was evolved, a sample of the evolved gas was trapped in the gas sample collector by closing the stopcocks at both ends. In a separate experiment, a coiled copper wire (50 cm in length, 0.5 mm in diameter) was employed instead of the filter paper impregnated with the Sn(I1)-Pd(I1) catalyst, and a sample of the evolved gas was collected in the manner described above. Analysis of the Evolved Gas. A Hewlett-Packard Model 5710 gas chromatograph equipped with a thermal conductivity detector was used to analyze the evolved gas mixture. Ironcoated alumina (80-100 mesh)s was prepared by adding alumina granules to a solution of 1.8 M FeC13 and neutralizing the solution with 6 M NHs to pH 7 . A stainless steel column (180 cm long, 3.3-mm 0.d.) was filled with the iron-coated alumina granules. The gas samples that were collected were eluted through this column and maintained at liquid nitrogen temperature, with helium as the carrier gas. The Hz, HD, and Dz that were eluted were converted into H20, HDO, and D2O by passing the eluted gas over solid CuO heated to 550 OC and determined with the aid of a thermal conductivity detector. The retention times of Hz, HD, and Dz in the column packed with iron-coated alumina were 13.3, 16.3, and 23.4 min, respectively.
Results and Discussion The overall stoichiometry of the plating reaction that has been reported6 is (7) Walker, J. F. J . A m . Chem. SOC.1933, 55, 2821. (8) Shipman, G. F. Anal. Chem. 1962, 34, 877.
0 1990 American Chemical Society
1710 Langmuir, Vol. 6, No. 11, 1990 2CH,O
+ CuY" + 40H-
-
Notes
+ H,+ 2HC00- + Y'- + 2H,O
Scheme I
CuO
(1) This stoichiometry was confirmed in our study by the determination of the number of moles of formaldehyde consumed, the number of moles of Cuo deposited, and the number of moles of gas evolved. The same stoichiometry was also obtained by employing paraformaldehyde as the reducing agent. Our results showed t h a t the gas evolved during the copper deposition consisted of more than 99% HD when either formaldehyde-d2 or paraformaldehyde-do was employed and the reaction was catalyzed by Pd(I1)-Sn(11). The results were identical when the reaction was catalyzed by copper metal. The overall stoichiometry of the reaction can therefore by represented as CuY2- + 2D,CO
+ 40H-
-
+
+
CuO+ HD 2DC00HDO + Y.' + H,O (2) It has been established that a formalin solution contains primarily methylene glycol (I), the hydrated form of formaldehyde, and i t s oligomer^.^ Since t h e acid dissociation constant of methylene glyeol1Jo~"is lCr'3, only 10';) of the methylene glycol in solution is present as the deprotonated species (11) a t pH 12
I
.
QC,O I D
7
m\
Q\/H
H/'\oH
H l C \oH
I
11
The concentrations of the various oligomers of methylene glycol in the plating solution are not known. The plating reaction given below is, therefore, represented by eq 3 or eqs 4 and 5 in which the reductant is, for the sake of simplicity, assumed to be a dimer of methylene glycol
-+ + + +
CuY2- + D,C(OH)O(OH)CD, + 40H- CuO+ HD + 2DCO; HDO Yc + 2H,O
+
+
2CuY2- + (OH)D,C(OH)OCD, + 4 0 K 2Cu' HD 2DCO; i 2H,O HDO 2Y'- (4)
-
+
2cu+ + Y'cuo CuYZ(5) In the sequence of reactions 4 and 5, it has been assumed that Cu+ is formed as an intermediate and subsequently disproportionates in the presence of Y4- to CuOand the copper(I1) species CuY2-. The mechanism shown in Scheme I is proposed on the assumption that copper deposition occurs exclusively on
".
(9) Walker. J. F. Formldchvdo. 3rd 4. ACS : MonmaDh Series: Rp inhold Publishing Co.: New Ybrk; 1964. ' (10) Bell. R. P.; O n a d . D. P. Trona. Fomday Soc. 1%2,58. 1551. (11) Los. J. M.;Brinkman. A. A. A. M.:Wets", J. C. J. Electmami. Chem. Interfacial Electroehem. 1974.56, 181.
the copper surface. The reaction of four hydroxide ions with a dimer of methylene glycol (i.e., eqs 3 and 4) can be visualized in eq 6. The vertical lines to the copper atoms indicate that the dimers of methylene glycol and the anionic copper-EDTA complex, CuY", are adsorbed on the copper surface. Attack of the adsorbed methylene glycol by the hydroxide ions results in the formation of H20, HDO, and H D a molecule of HD is formed whenever a C-D bond in the methylene glycol is disrupted. Electron transfer to the adsorbed CuY2- species results in the deposition of copper metal (eq 7). The copper(1) formate that remains adsorbed on the surface disproportionates in the presence of the EDTA anion to copper metal and the copper(I1)-EDTA complex, CuY2-, and the formate anion (eq 8). This type of disproportionation reaction also o c c m in the gas phase. For example, copper(1) acetate or trifluoroacetate, when vaporized in vacuo, undergoes a slow disproportionation reaction in which pure copper metal is deposited on the walls of the container with the formation of copper(I1) acetate or trifluoroacetate.12
Acknowledgment. This work was supported in part by the University of Arizona Center for Advanced Studies in Copper Recovery and Utilization under Defense National Stockpile Center Grant No. DN-004. Registry No. C u V , 12276-01-6;CH~0,5000-0; Cu. 744050-8; paraformaldehyde. 30525-89-4. (12) Ogura, T.:Fernando. Q.Inor#. Chem. 1973.12, 2611.
