Dual-side and three-dimensional microelectrode arrays fabricated from ultra-thin silicon substrates

Abstract

A method for fabricating planar implantable microelectrode arrays was demonstrated using a process that relied on ultra-thin silicon substrates, which ranged in thickness from 25 to 50 μm. The challenge of handling these fragile materials was met via a temporary substrate support mechanism. In order to compensate for putative electrical shielding of extracellular neuronal fields, separately addressable electrode arrays were defined on each side of the silicon device. Deep reactive ion etching was employed to create sharp implantable shafts with lengths of up to 5 mm. The devices were flip-chip bonded onto printed circuit boards (PCBs) by means of an anisotropic conductive adhesive film. This scalable assembly technique enabled three-dimensional (3D) integration through formation of stacks of multiple silicon and PCB layers. Simulations and measurements of microelectrode noise appear to suggest that low impedance surfaces, which could be formed by electrodeposition of gold or other materials, are required to ensure an optimal signal-to-noise ratio as well a low level of interchannel crosstalk.

State-of-the-art MEMS and microsystem tools for brain research

“…where I i is the current of one point source, σ is the conductivity of the extracellular space, and r i is the distance from the source to the electrode (a dipole assumption results in a more complex isopotential and an amplitude proportional to 1/r 2 , but arguably the ratio is neither a monopole or dipole [28][29][30][31] ). Given the close spacing of tetrodes, multiple spike signals can be used to triangulate and localize a specific cell in space 32 .…”

“…This technique spread throughout the MEMS community and formed the basis for many other novel electromechanical structures. The planar lithography approach has evolved over the years to include integrated interconnects 38 , active electronics 35,39,40 , cochlear implants 41,42 , polytrodes 31 , and three-dimensional arrays 29,[43][44][45] . An important simplification for defining and releasing fine neural probe structures has been the use of silicon-on-insulator (SOI) wafers and deep reactive ion etching (DRIE) 46,47 that many groups have adopted.…”

“…Using MEMS-based arrays to improve software tools for neuroscientists also involves neuronal location information 9,24,29,31 and cell classification of neurons 4,83 , which are critical in brain mapping. The need for technologies to enable cell typing in brain research was highlighted as the number one priority by the U.S.…”

“…Some of most advanced silicon designs include double-sided, high-density arrays 29,102 , three-dimensional arrangements 43 , and integrated electronics 39,40,132 . Although the inherent brittleness of silicon is a concern for some clinical applications, its electrical, mechanical, and thermal properties offer the greatest design options and are supported by a wide range of commercial microfabrication tools.…”

Effects of Fabrication Process Variation on Impedance of Neural Probe Microelectrodes

“…[4] Especially, accurate positioning of each electrode in an array structure is important for studying functional connectivity of brain. Furthermore, more functions have been merged into MEMS neural probes including various stimulation modalities such as optical and chemical [5]. Among MEMS neural probes, probes with optical stimulation capability has recently received much attention because genetically targeted neurons can be selectively excited or inhibited by light without stimulating neighboring neuron networks [6].…”