Electrochemical interfacial phenomena under microgravity: Part I. Anodic dissolution of copper in drop shaft

Abstract

As part of a series of physical modeling studies on the electrochemical etching of a printed wiring board, copper foil was anodically dissolved in a small cavity. Coupling of the ionic mass transfer rate with the undercutting phenomena must be precisely understood in order to finely control high aspect ratio anisotropic etching. Interferometry has been applied to the study of the ionic mass transfer rate accompanying electrochemical reactions, and it may help in the visualization of the concentration profile of the metal ion within a narrow cavity. As an idealistic case, the electrochemical dissolution of copper in a purely diffusional field, especially the Cu 2+ ion transfer rate itself, is now the focus of attention. The copper anode was galvanostatically dissolved in an aqueous solution (0.1M CuSO 4 -1M H 2 SO 4 ) contained in a quasi-two-dimensional electrolytic cell for 8 seconds during free fall in a drop shaft. A disk type of anode, 1 mm in diameter and 100 m in thickness, and a flat shaped ring cathode 20 mm in inner diameter were sandwiched by two sheets of slide glass. The electrolyte layer was 200 m in effective thickness. The interference fringe pattern accompanying the copper dissolution was measured in situ with a common path-type microscopic laser interferometer. Terrestrial experiments were also conducted with a horizontally installed cell under the same electrolytic conditions. No natural convection would be expected in the present electrolytic cell design with such a shallow and horizontal electrolyte layer. The distance between the anode surface and the location at which the outer interference fringe appeared was measured by the duration time. It increased linearly with the square root of time under microgravity, but started to deviate from this linearity a few seconds after starting the electrolysis in the terrestrial experiment. A significant difference in the growing distance between the environments had not been a priori anticipated for such a quasi-twodimensional cell. The local current density distribution along the smaller electrode surface may be influenced by induction of a kind of natural convection.

The concentration field during transient natural convection between vertical electrodes in a small-aspect-ratio cell

“…They used a Fabry-Perot interferometer to measure the concentration profile during electrodeposition of copper and zinc. Fukunaka's group used two-wave holographic interferometry to study the ion mass-transfer rate and concentration profile during the anodic dissolution of metals, such as zinc [15], lithium [16][17][18] and copper [19][20][21][22][23][24].…”

Electrochemical interfacial phenomena under microgravity: Part 2. Numerical analysis of the rate of ionic mass transfer accompanying anodic copper dissolution

“…[23] The maximum velocity in the At 1.4 A cm Ϫ2 , an increase in anodic overpotential starts at 0.2 seconds under both environments. It reaches 100 to 300-m-high anode is twice that of the 100-m-high anode.…”

“…In Part 1 of this article [23] the duration of electrolysis was Before beginning the numerical analysis, it is worth briefly describing the present electrochemical system. Solubility limited to 8 seconds, which is considerably shorter than the measured incubation time.…”

“…However, the experimental data reported in Part been employed in cell biochemistry. [3,4] Advances in indus-1 of this article, [23] clearly showed a difference between the trial electrochemical technology have led to progress in protwo environments, indicating that natural convection was cess productivity through operations that utilize natural inevitably induced even in such a horizontal and shallow convection phenomena. [5][6][7][8][9][10] electrolyte layer.…”

“…The reac-II. EXPERIMENTAL FINDINGS FROM tion mechanism is fairly simple, and the deposition or disso-PART 1 [23] lution rate can be easily controlled by changing the current density or potential. It enables physicists to study shape-…”