The mechanism responsible for the very high plating rates at electrodes illuminated by a laser beam was investigated. Absorption of the laser energy by the electrode results in a localized increase in temperature at the metal‐solution interface. This leads to: (i) a shift in the rest potential, (ii) an increase in the charge transfer rate, and (iii) strong microstirring of the solution due to thermal gradients and, at high laser power densities, to strong local boiling. Verification of the first two effects was achieved by measuring the enhancement in plating rates as a function of overpotential, laser power, and substrate thickness and by comparing these results with measurements using solutions at various bulk temperatures. Observation of the cathode through a video monitor, as well as detection of bubble formation using a miniature microphone, verified that a correlation exists between the ejection of bubbles from the cathode and sharp increases in the current. Application of laser‐enhanced electroplating for maskless generation of patterns is also briefly discussed.
Hydrogen bubbles evolved cathodically under conditions encountered in electrochemical machining have been studied by stop‐motion photography. Constant current densities up to 150 A/cm2 and flow rates up to 2500 cm/sec have been employed with an experimental flow channel of 0.5 mm gap width. The observed bubble size decreased strongly with increasing flow rate and increased with increasing current density. At flow rates above 800 cm/sec, the bubble size was always below 20µ, the smallest diameter resolved by the optical arrangement used. Less gas was evolved in nitrate than in chloride electrolytes under otherwise identical conditions. The hydrogen bubbles were usually confirmed to a region near the cathode. Voltage oscillations and electric breakdown coincided with the appearance of a new type of bubble.
In order to maintain the dimensional accuracy of an x-ray mask, the residual stress of the absorber must be kept low (in the low range of 108 dyn/cm2 ). The absorber material studied in this paper is pure gold electroplated from a proprietary plating solution. Stress of the gold deposits is measured by holographic interferometry of bending disk/beam experiments. The effects of temperature, current density, thickness, pH, and pulse plating on the stress of the gold are investigated. The aging behavior of the plating solution is also discussed. The stress of electroplated gold does not show a strong dependence on the current density (1–5 mA/cm2 ). However, aging of the plating solution changes the stress of the gold deposits, especially for those obtained at a current density of ∼1 mA/cm2. Pulse plating produces stress levels similar to those given by dc plating, over a wide range of on-current-density, on-time, and off-time. Moreover, the pH of the solution does not affect the stress of gold when the current density is greater than 1 mA/cm2. Hence a plating solution and process has been found that produces low stress gold (≤4×108 dyn/cm2) over a wide range of operating conditions and shows no adverse effects upon aging over an extended period of time.
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