Adaptive tip-withdrawal control for reliable microfabrication by localized electrodeposition

The electrode tip withdrawal velocity relative to the deposit surface growth rate is found to play a major role in process repeatability, deposit characteristics, and geometry confinement of high aspect ratio microstructures fabricated by localized electrodeposition. The effect of the tip and deposit-surface relative velocities is understood through experimental investigation of deposit characteristics for the three possible cases where the tip withdrawal is; slower than, relatively equal to, and faster than the deposition rate. Best results were obtained when the tip withdrawal is relatively of the same magnitude of the deposition rate. For automation purposes, an adaptive control scheme is proposed to track the deposit surface acceleration, rather than its velocity, by monitoring the tip current gradient, and accordingly withdraw the tip at the same acceleration. This eliminates the need for the intervention of a skilful experimenter, avoids short circuit contact resulting between the tip and deposit surface, and thus greatly improves the repeatability of the process. Critically important to the proposed adaptive tip-withdrawal technique is the initial approach of the tip toward the substrate to assure proper tracking of deposition current. Automation and optimization of the initial tip positioning above the substrate is achieved by tracking deposition current variation with the tip-substrate spacing, thus leading to a better geometry confinement of deposited structures.