Roughened polysilicon for low impedance microelectrodes in neural probes

Abstract

A polysilicon roughening process is developed to reduce the interface impedance of microelectrodes of neural chips. In developing micromachined neural interface systems, one of the basic requirements is to reduce the interface impedance of microelectrodes, because the neuronal signals generally have a very small amplitude and the increased impedance can cause the charge transfer capability of microelectrodes to decrease. The developed process involves forming metal microelectrodes on top of a low pressure chemical vapor deposition (LPCVD) polysilicon film, which is deposited on top of a heavy-phosphorous-content phosphosilicate glass film. The phosphorous inhibits LPCVD polysilicon nucleation and results in very large grains, and hence, very rough film surfaces. This process significantly increases the effective surface area, and the interface impedance can be significantly reduced without increasing the physical size of microelectrodes. By using this process, the interface impedance is significantly lowered. The impedances of conventional gold microelectrodes and the microelectrodes developed in this paper are measured and compared by using a scanning electron microscope, an atomic force microscopy and an impedance spectroscopy system. Experimental results show approximately 50 times lower interface impedance for the developed method.

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“…(16,(32)(33)(34)(35)(36)(37) 3D electrodes are expected to approach the target cells in the retina more closely than 2D electrodes. Also, the flexible polyimide substrate is adapted for conformal attachment to the spherical structure of the eye.…”

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“…Similar to the substrate material, the passivation layer must have a low dielectric constant while still being chemically inert and biocompatible. Passivation layers used in impedance platforms include silicon dioxide [123], [144], [145], silicon nitride [118], [122], [128], [133], [144], [146], a multilayer combination of silicon dioxide and nitride [121], parylene [143], polyimide [134], photoresist [41], [130], parafilm [138], polymer paste [142] and dry-film [126]. Other suggested passivation layers are silicon carbide, TEFLON, ceramic, or any kind of polymer or thermal plastic [147].…”

“…The electrode material must have a low impedance and also be compatible with lithography and patterning techniques. Gold [40], [41], [77], [107], [109], [112], [121], [122], [126], [129]- [132], [136], [138], [139], [141], [147], [148], platinum [125], [134], [142], indium tin oxide (ITO) [118], [124], [140], [146], iridium [120], [128], [147], polysilicon [149], and carbon [150] have all been used as electrode materials. The electrode material can be sputtered, evaporated, deposited by PECVD, or even screen printed.…”

“…This type of deposition has been found to decrease electrode impedance as much as two orders of magnitude as in the case of platinum black [151], [152]. Other surface techniques involve adding a porous conducting polymer [126], patterning the surface with micro-sized features [107], etching the surface to create a rough topography [122], as well as, chemical modifications like activated iridium [107].…”

“…presented data that electrodes smaller than 50 µm are definitely not suitable for the desired application. However, the interfacial impedance can be minimized by increasing surface area [179] and roughness [122] through electroplating [107], [128], [144], [180] and the incorporation of non-planar features [181].…”