Fabrication of a Monolithic Implantable Neural Interface from Cubic Silicon Carbide

Mohammad Beygi; John T Bentley; Christopher L Frewin; Cary A Kuliasha; Arash Takshi; Evans K Bernardin; Francesco La Via; Stephen E Saddow

doi:10.3390/mi10070430

Fabrication of a Monolithic Implantable Neural Interface from Cubic Silicon Carbide

Micromachines (Basel). 2019 Jun 29;10(7):430. doi: 10.3390/mi10070430.

Authors

Mohammad Beygi¹, John T Bentley², Christopher L Frewin³, Cary A Kuliasha⁴, Arash Takshi¹, Evans K Bernardin², Francesco La Via⁵, Stephen E Saddow^{6

7}

Affiliations

¹ Department of Electrical Engineering, University of South Florida, Tampa, FL 33620, USA.
² Department of Medical Engineering, University of South Florida, Tampa, FL 33620, USA.
³ NeuroNexus Technologies, Inc., Ann Arbor, MI 48108, USA.
⁴ Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL 32611, USA.
⁵ CNR Institute for Microelectronics and Microsystems, Catania, Sicily 95121, Italy.
⁶ Department of Electrical Engineering, University of South Florida, Tampa, FL 33620, USA. saddow@ieee.org.
⁷ Department of Medical Engineering, University of South Florida, Tampa, FL 33620, USA. saddow@ieee.org.

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⁺ 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 µm² in phosphate buffered saline (PBS) solution. The measurements showed an impedance magnitude of 165 kΩ ± 14.7 kΩ (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 μA_rms. 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: 3C-SiC; MEA; SiC; amorphous SiC; doped SiC; electrochemical characterization; epitaxial growth; microelectrode array; n-type; neural implant; neural interface; neural probe; p-type.