A tantalum-on-sapphire microelectrode array

The design and fabrication are described of a new microelectrode array for the production of localized electrical excitation in nerve fibers, in particular in the auditory nerve. The sapphire substrate was chosen for its electrical (insulating) and mechanical properties, tantalum was chosen as the conductor metal because of its self-passivating characteristics, and platinum as the stimulation electrode material because of its resistance to electrolysis. Experimental measurements are presented to show the effectiveness of Ta
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O
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as an insulator in biphasic charge delivery systems, and the effectiveness of conformal dielectric overcoating of the Ta
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O
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in reducing capacitive signal leakage is demonstrated. The electrochemical properties of the platinum-electrolyte interface are shown to provide a satisfactory model on which to estimate the charge delivery capabilities of the electrodes. Finally, the sequence of process-compatible steps for fabricating the microelectrode array, from the first tantalum deposition to the final overcoating are detailed.