**Fabrication of a Monolithic Implantable Neural Interface from Cubic Silicon Carbide**

#### **Mohammad Beygi 1, John T. Bentley 2, Christopher L. Frewin 3, Cary A. Kuliasha 4, Arash Takshi 1, Evans K. Bernardin 2, Francesco La Via 5 and Stephen E. Saddow 1,2,\***


Received: 13 May 2019; Accepted: 26 June 2019; Published: 29 June 2019

**Abstract:** One of the main issues with micron-sized intracortical neural interfaces (INIs) is their long-term reliability, with one major factor stemming from the material failure caused by the heterogeneous integration of multiple materials used to realize the implant. Single crystalline cubic silicon carbide (3C-SiC) is a semiconductor material that has been long recognized for its mechanical robustness and chemical inertness. It has the benefit of demonstrated biocompatibility, which makes it a promising candidate for chronically-stable, implantable INIs. Here, we report on the fabrication and initial electrochemical characterization of a nearly monolithic, Michigan-style 3C-SiC microelectrode array (MEA) probe. The probe consists of a single 5 mm-long shank with 16 electrode sites. An ~8 μm-thick p-type 3C-SiC epilayer was grown on a silicon-on-insulator (SOI) wafer, which was followed by a ~2 μm-thick epilayer of heavily n-type (n+) 3C-SiC in order to form conductive traces and the electrode sites. Diodes formed between the p and n<sup>+</sup> layers provided substrate isolation between the channels. A thin layer of amorphous silicon carbide (*a*-SiC) was deposited via plasma-enhanced chemical vapor deposition (PECVD) to insulate the surface of the probe from the external environment. Forming the probes on a SOI wafer supported the ease of probe removal from the handle wafer by simple immersion in HF, thus aiding in the manufacturability of the probes. Free-standing probes and planar single-ended test microelectrodes were fabricated from the same 3C-SiC epiwafers. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on test microelectrodes with an area of 491 μm<sup>2</sup> in phosphate bu ffered saline (PBS) solution. The measurements showed an impedance magnitude of 165 k Ω ± 14.7 k Ω (mean ± standard deviation) at 1 kHz, anodic charge storage capacity (CSC) of 15.4 ± 1.46 mC/cm2, and a cathodic CSC of 15.2 ± 1.03 mC/cm2. Current-voltage tests were conducted to characterize the p-n diode, n-p-n junction isolation, and leakage currents. The turn-on voltage was determined to be on the order of ~1.4 V and the leakage current was less than 8 μArms. This all-SiC neural probe realizes nearly monolithic integration of device components to provide a likely neurocompatible INI that should mitigate long-term reliability issues associated with chronic implantation.

**Keywords:** neural interface; neural probe; neural implant; microelectrode array; MEA; SiC; 3C-SiC; doped SiC; n-type; p-type; amorphous SiC; epitaxial growth; electrochemical characterization
