Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Stephen E. Saddow, Ph.D.

Co-Major Professor

Ashok A. Kumar, Ph.D.

Committee Member

Christopher Frewin, Ph.D.

Committee Member

Michael Cai Wang, Ph.D.

Committee Member

Nathan Gallant, Ph.D.

Keywords

Carbon Materials, Michigan Style Neural Probes, Microfabrication, Silicon Carbide Biotechnology

Abstract

The use of neural modulation techniques to treat nervous system disorders, as well as neural recording to study brain function, continues to be of intense interest around the world. Due to the need to treat diseases such as Parkinson’s, dementia, depression and return normal function to patients suffering from nervous system damage and trauma, neural implants continue to be developed globally. One of these devices, the microelectrode implantable neural interface (mINI), has been rapidly developed, thanks to advanced semiconductor fabrication processes. However, the long-term reliability of present-day devices has still to be demonstrated and is delaying the application of this very promising technology for long-term human use. This is due to several factors, one of which is device reliability that is often caused by device material failure. In this dissertation, two pathways have been demonstrated which may offer a solution to this vexing problem: an all-silicon carbide (SiC) based mINI and a carbon-based mINI that relies on amorphous SiC (a-SiC) insulated pyrolyzed-photoresist-film (PPF) to form a carbon-based neural probe.

SiC is a wide bandgap semiconductor that is mechanically robust, chemically inert, as well as hemo- and bio-compatible. Therefore, it is a promising candidate for mINI applications. Building upon pioneering work in our group, a second-generation (Gen 2) all-SiC based Michigan-type neural probe constructed using 3C-SiC, is presented first. This Gen 2 probe contains 16 recording sites with a diameter of 20 μm on a shank of 5 mm in length. The device has a p-type 3C-SiC base layer with an n+-type 3C-SiC on top grown on a Si, or SOI, subtrate heteroepitaxially. The n+-type is semi-metallic and serves as both the conducting traces and recording sites. The p-njunction formed between p and n+ type serves as the substrate electrical insulation while the top insulation was realized using a thin layer of a-SiC. The Gen 2 mINI was characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on planar single-ended-electrodes. The results showed an impedance of 71.6 ± 29.1 kΩ with a phase angle of -91.9 ± 0.29° at 1 kHz, anodic and cathodic charge storage capacities (CSCa and CSCc) of 56.1 mC/cm2 and 18.2 mC/cm2, respectively. Comparing with the Gen 1 device, Gen 2 demonstrated a relatively low impedance at 1 kHz, with larger values of CSCa and CSCc while displaying lower leakage currents due to improved epitaxial material.

The second approach to developing a long-term, reliable neural implant is a carbon-based mINI whereby a-SiC encapsulates a carbon layer created using pyrolyzed photoresist film (PPF). This novel device was fabricated by patterning and pyrolyzing photoresist on an a-SiC coated Si wafer. The patterned carbon film was then conformally insulated with another layer of a-SiC. Thus the C (i.e., PPF) interlayer served as both the conducting traces and recording sites. This approach eliminates the need for noble metals and provides for a highly simplified fabrication process compared to other carbon-based electrodes. The a-SiC film was characterized via XPS and AFM, and the results indicated a very low oxygen content inside of a specular film with a surface roughness of 0.763 ± 0.17 nm. Electrochemical testing was performed on 1.9 kμm2 PPF electrodes and the results showed an impedance of 24.8 ± 0.4 kΩ with a phase angle -35.9 ± 0.6° at 1 kHz, and a CSCa of 8.9 C/cm2 and a CSCc of 5.26 C/cm2, respectfully.

This dissertation demonstrated the electrochemical ‘proof of concept’ for both the Gen 2 all-SiC and carbon (PPF) sandwiched in insulating a-SiC mINI’s. Several detailed process improvements have been proposed in the Future Work section of Chapter 5, including the completion of the Gen 2 all-SiC mINI probes (the RIE tool is currently under repair and has held up completion of these devices), the addition of an HF etch step to remove a surface oxide (SiO2) layer between the a-SiC base and cap layers of the carbon electrodes, along with the addition of better bonding pads for both device types via the addition of a Au overlayer to facilitate wire bonding and packaging. Finally, the need to prove both mINI devices in vivo in an animal model is suggested along with investigating the MRI compatibility of the carbon electrodes which, along with the proven MRI compatibility of the Gen 1 all-SiC devices at 7 Tesla, would further motivate investment into the development of both of these highly promising mINI concept.

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