Kinetics and mechanism of electroless copper deposition at moderate-to-high copper ion and low-to-moderate formaldehyde
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1 INTRODUCTION
ELECTROLESS deposition of copper has been widely studied in relation to the solution composition, variation of substrate, and process kinetics.[1–17] The process is believed to proceed through the following reactions.[4,6,8] Anodic: 2HCHO 1 4OH2 → 2HCOO2 1 H2 1 2H2O 1 2e2
[1a]
Cathodic: Cu(L)2 1 2e2 → Cu 1 2L2
[1b]
Overall: 2HCHO 1 4OH 1 Cu(L)2 → Cu 2
1 2HCOO2 1 H2 1 2H2O 1 2L2
[1c]
Reactions [1a] and [1b] proceed simultaneously, in accordance with the mixed-potential theory. The Stern–Geary equation,[18] as well as the Wagner–Traud theory[19] of mixed potentials, have been employed to explain this process. The present authors have been working on the system[10,16,17] and, employing polarization studies, they suggested[10] changes in the mechanism of electroless copper deposition with changes in [Cu21] and [HCHO]. The rate-controlling mechanisms are anodic diffusion, activation, and cathodic diffusion, depending on the position of Em on the region of the respective half-cell polarization plots. The polarization data indicate the following four cases of varying formaldehyde and copper ion concentrations. Also, equations for Em and im have been deduced for all these cases by applying the principles of corrosion coupling (Table I). Interestingly, deposition rates reported by different authors[7] under different conditions have been found to follow this trend. The present authors have reported details about the kinetics and mechanism of the deposition process for the aforementioned cases 2[17] and 4.[16] A cathodic diffusion control mechanism at low-to-moderate [Cu12] and high [HCHO] values was observed,[16] whereas an anodic diffusion control mechanism was confirmed[17] at moderate
K.G. MISHRA, Research Associate (CSIR), and R.K. PARAMGURU, Scientist, are with the Regional Research Laboratory, Bhubaneswar 751 013, India. Manuscript submitted May 18, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
Solution Condition
Polarization Regions in Which Em Lies
Mechanism
(1) Cathodic linear and anodiclimiting current (2) Cathodic Tafel and anodiclimiting current (3) Cathodic and anodic Tafel (4) Cathodic-limiting current and anodic Tafel
[HCHO]
[Cu21]
anodic diffusion control
very low
anodic diffusion control
low
activation control
low to moderate to moderate high high low to moderate
cathodic diffusion control
—
moderate
[Cu21] and low [HCHO] levels. The present article attempts to study in detail the kinetics and mechanism of the electroless copper deposition process at low-to-moderate formaldehyde and moderate-to-high copper ion concentrations, which represent case 3. It will also examine case 1, which takes place at a very low formaldehyde concentration. Polarization measurements are employed, considering their suitability for the kinetic study of a reaction system which is electrochemical in nature. II. EXPERIMENTAL The details of sample preparation, activation, electrolyte selection, experimental setup, and procedures employed were given in a previous report.[10] The electrolytic-gr
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