Microfabrication, Coil Characterization, and Hermetic Packaging of Millimeter-Sized Free-Floating Neural Probes

This paper presents a new micromachining (MEMS) fabrication, microassembly, and hermetic packaging process for free-floating neural probes ( $1.3\times1.3$ mm2 bath-tub shaped micromachined silicon die, which serves as a substrate that supports all passive components in non-plated through-silicon cavities (TSCs) plus four $\varnothing 81~\mu \text{m}$ tungsten electrodes embedded in $\varnothing 100~\mu \text{m}$ non-plated through-silicon holes (TSHs), and a 6-turns bonding wire wound coil (WWC) around the die. The current passive probe prototype houses a $1.2\times1.2$ mm2 mock-up integrated circuit (IC) in a bath-tub cavity created in the passive micromachined die. The probe is hermetically sealed with a $5~\mu \text{m}$ thick parylene-C film except for the tip of the electrodes, and covered with an additional layer of polydimethylsiloxane (PDMS) for physical protection and reduction of mechanical mismatch with brain tissue. WWCs around the micromachined silicon die are used for power or data transmission and were electrically characterized by theoretical analysis, simulation, and measurements in air and lossy tissue medium. This way, the impact of the inner silicon substrate, packaging, and surrounding tissue on the inductance ( ${L} _{s}$ ) and Q-factor ( ${Q} _{s}$ ) of the WWC are studied and related to the resonance frequency ( ${f} _{0}$ ) and power transmission efficiency (PTE). The lifetime of the current probe prototypes is estimated by using a customized wireless hermetic failure monitoring tool under an accelerated condition (1 atm, 100% RH, and 80 °C).