Recessed electrodes formed by laser ablation of parylene coated, micromachined silicon probes

A drawback of planar stimulating electrodes is the large current densities produced at the edge of the electrode. Such non-uniformity can be reduced by recessing the electrode. Research is presented that discusses the formation of recessed electrodes by laser ablation of parylene coating on silicon micromachined probes with thin-film iridium electrodes. Parylene was deposited conformal to the probe. A UV laser was used to remove the parylene over a 1750 μm² planar, rectangular electrode, forming a recess about the electrode. A residual parylene layer (resulting in an increased impedance), that could not be ablated without harming the thin-film metal, was removed both by inducing gas evolution at the metal-electrolyte interface and by using an oxygen plasma to etch the parylene. Removal of this layer restored the electrode impedance to near nominal values. Iridium oxide was electrochemically formed on the electrodes. Recessed electrodes attained a charge storage capacity equal to that of non-recessed electrodes. Impedance spectroscopy revealed that the access resistance increased with recess depth, from 5.2 kOhm (non-recessed) to 6.4 kOhm (8 μm recess). Finite element modeling predicts that both the depth and shape of the recess will affect the electrical properties of the electrode