**4. Discussion**

A nearly monolithic SiC neural probe has been fabricated from epitaxial 3C-SiC films grown on SOI wafers. A combination of ethylene (C2H4) and trichlorosilane (SiHCl3) were used as precursor gasses in the epitaxial process. This produced a varying surface morphology with mean surface roughness of approximately ~21 nm (specular, edge region) to ~244 nm (rough, center region) [43]. It is possible this surface roughness contributed to complications in the fabrication process, such as with photolithographic patterning, and may have had an effect on the mechanical properties of the grown films to the detriment of probe function [44]. It is suspected that this contributed to the higher than desired leakage current (less than 8 μArms). By optimizing various parameters in the epitaxial process, such as gas composition and flow rates, temperature, and pressure [35,43], the process can be improved to reduce this surface roughness. Additionally, post-processing steps, such as mechanical or chemomechanical polishing, can be added to further improve the surface morphology; particularly to reduce surface roughness [19].

A major issue with the previous 4H-SiC probes was the difficulty in releasing the probe [33]. Essentially, much of the 4H-SiC substrate would have to be removed, and there was no effective etch stop to prevent over-etching. In order to effectively solve this issue, we used SOI wafers to provide an effective release layer by the simple process of wet etching the oxide. However, the SOI wafer used here possessed a relatively thick layer of silicon that remained on the backside of the probes, which was removed later via back-thinning using DRIE. This thick silicon layer can cause residual stress, due to mismatches in the coe fficients of thermal expansion and lattice parameters [45] at the interface between the SiC films and silicon, resulting in bowing or bending of the probes. The SOI wafer used in this work had an ~20 um thick top silicon layer and this may have been the cause of the bowing of the shank and some warping in the connector tab. The shank should be straight for a proper insertion trajectory into the neural tissue. Also, in order to maximize contact at the connector interface, the tab containing the contact pads should be as planar as possible. Using a SOI wafer with a thin silicon device layer may resolve this deformation problem and will be used in future all-SiC devices.

Epitaxial 3C-SiC thin films are ceramic-like materials with, relative to neural tissue, a high elastic modulus, measured to be 424 ± 44 GPa using microsample tensile testing [46] and 433 ± 50 GPa using nanoindentation [47]. Defects can reduce the value the Young's modulus of 3C-SiC [48] and doping may a ffect this value as well [49]. There is a trend towards utilizing softer materials, such as polymers, for implantable neural interfaces due to their potential to improve the interaction with neural tissue [50–52]. By decreasing the Young's modulus of the neural probe closer to the values of neural tissues, the harmful shear and normal stress applied from the shank to the tissue should decrease. However, it is really device sti ffness, which includes cross-sectional area, rather than just device modulus, that seems to matter the most [53]. Additionally, use of these softer materials introduce challenges with fabrication processes, scaling to higher channel-count systems, particularly with respect to interconnects, and can lead to insertion di fficulties. Once implanted, these materials face challenges with material stability and device reliability [54]. The hard, chemically inert nature, and ease of micromachining with traditional silicon processes means SiC neural probes may su ffer less from these limitations. Clearly, long-term in vivo studies in an animal model are needed to assess the performance of the all-SiC INI and are planned.

It has been demonstrated that once the implanted structure size is reduced to subcellular scale, i.e., less than ~10 μm, the foreign body response and associated neuron death is greatly reduced in a rat model [55,56]. With traditional silicon probes, reducing size increases the occurrence of probe fracture at high stress regions [57]. SiC is a much more robust material, with a reduced tendency to fracture at these desired smaller sizes, while maintaining the mechanical strength needed for proper penetration of neural tissue [24,27]. Therefore, SiC is an excellent material for developing a high electrode density neural interface, allowing for further reduction in size while greatly minimizing risk of fracture.

The heterogeneous composition of implanted neural interfaces that utilize metallic materials as electrode sites or conductive traces may increase the risk of delamination in chronic implantation, specifically, at regions under higher stress [57]. Delamination usually occurs at the interface between metal and semiconductor materials due to residual stress in the thin films. A homogeneous material composition can eliminate this residual stress, reducing the risk of delamination at the interfaces between di fferent materials in the probe by eliminating them.

The 3C-SiC is a wide-band-gap semiconductor with a high band energy of ~2.2 eV. This results in a higher turn-on voltage at the junction between <sup>n</sup>+- and p-type SiC. This higher turn-on voltage provides a wider voltage range to stimulate neurons while isolating individual channels via n-p-n junctions supporting simultaneous multichannel microstimulation and recording, as might be necessary for implementing a closed-loop system. The turn-on voltage for p-n junctions built from Si is ~0.7 V, which is low compared to ~1.4 V for SiC, and limits proper isolation via a n-p-n junction configuration. However, the higher leakage current in our all-SiC films may negatively a ffect the final device's functionality. Surface roughness is known to be associated with the density of crystal defects, thus a higher defect density may cause higher leakage current [43]. It is believed that the high surface roughness in this work, an indication of poor crystallinity, in conjunction with a high number of defects, may be the cause of the observed high leakage current. For reference, in our 4H-SiC devices with specular surface morphology, the leakage current was nA versus μA reported here [33]. A lower surface roughness via an optimized epitaxial growth process would be expected to improve both the mechanical properties and the leakage current [58].

The EIS results revealed that the doped, semi-metallic 3C-SiC conductors have impedance values approaching those of metals commonly used in implantable microelectrodes, such as gold, platinum, or tungsten, as well as highly doped polysilicon [59,60]. The average impedance for a surface area of 491 μm<sup>2</sup> was approximately 75% lower (165 kΩ vs. 675 kΩ at 1kHz) than previously reported for our 4H-SiC electrodes [33].

Both the charge balanced CV cycles and the negative phase angles from EIS measurements support a dominant capacitive charge transfer mechanism for 1 kHz and higher frequencies at the electrode-electrolyte interface, but faradaic currents may be present at lower frequencies. This differs from capacitive electrode materials like titanium nitride (TiN) [61], which has a phase closer to 90◦ at lower frequencies [62]. Compared to values previously reported for 4H-SiC, the charge values calculated from CV measurements reported here were approximately two orders of magnitude higher, with the average charge storage capacity (anodic: 15 mC/cm<sup>2</sup> vs. 0.41 mC/cm2; cathodic: 15 mC/cm<sup>2</sup> vs. 0.19 mC/cm2) and an average charge per phase (anodic: 75 nC vs. 2.0 nC; cathodic: 75 nC vs. 1.0 nC) using a Pt electrochemical window (−600 mV to +800 mV). It is possible that the greater surface roughness accounts for this large difference in electrochemical properties. It is also possible that there were more faradaic reactions at lower frequencies leading to more oxidation and reduction at the surface, which may be linked to defect sites in the SiC.

Current neural probe technology built from materials like silicon suffer from long-term reliability issues that reduces their lifetime considerably, resulting in loss of recording and microstimulation function when chronically implanted. This limits their use in medical applications for humans. Device-based modalities could become a more common alternative to pharmaceuticals for treatment of neurological trauma or disease if the issue of long-term reliability in implantable neural interfaces is properly addressed. After further refinement of the design and optimization of the material processing, the performance of the all-SiC neural probe will be evaluated with chronic *in vivo* experiments in rodent models to investigate its long-term safety and effectiveness in neural tissue. There is accumulating evidence [25–27,29,30,32,63] that SiC could be an appropriate material for the greatly needed implantable neural interface that functions for the lifetime of the recipient.
