Fabrication of Multiheight Microneedle Electrode Array Using Combination Process of Reactive Ion Etching

3-D electrode arrays, with heights of several 100~\mu m, can be placed near target areas for localized measurements. However, their height poses challenges in regions with diverse nerve cells, like the retina, and their uniform height limits depth-specific measurements. To address these challenges, this article presents a fabrication method for a multiheight microneedle electrode array that can be easily produced by combining anisotropic and isotropic silicon etching processes. In particular, by adjusting the diameter of the micropillars and the spacing between the micropillar and dummies, multiheight microneedles can be fabricated, providing fine control of the height variations within a range of several tens of micrometers. In addition, electrode arrays of various shapes, such as concave and convex profiles, can be produced. The electrode structures of the proposed electrode array of multiheight microneedles feature exposed electrode surfaces only at the tips of the microneedles, with the remaining area covered by an insulating material, making it advantageous for electrical measurements. The fabrication of the exposed electrode array utilizes a self-aligned etching technique that relies on the shape of the microneedle tips, enabling precise exposure of the electrode at the microneedle tips. Finally, the multiheight microneedle electrode array is validated using cyclic voltammetry.