The effect of the substrate on the extracellular neural activity recorded micromachined silicon microprobes

The influence of a highly-doped silicon substrate on the neural activity recorded by thin-film microelectrode arrays has been explored using top, back, and double-sided recording sites. Probes having shank widths from 25-50 microm and site spacings (in depth) from 20-40 microm were used. The realization of back-looking and double-sided sites requires one mask in addition to the normal 8-mask passive probe process. Back-looking sites record as well as top sites even though separated by only a few microns from the surrounding silicon substrate, indicating that the substrate acts as an insulator and does not shunt local current from the extracellular space. On wide substrates, back-looking sites can thus be used to ensure a spherical recording field. As the substrate width is scaled to dimensions of a few microns, it should leave the extracellular field relatively undisturbed while providing mechanical support and suppressing crosstalk.