Introduction:: Thin-film dielectric materials are commonly used to encapsulate bioelectrical interfaces. Long-term stability is an ongoing limitation of neural interface devices, which affects their ability to consistently record neural activity and stimulate neurons. Amorphous silicon carbide (a-SiC) is a promising encapsulation material for its chemical inertness, high hardness, and good thermal stability. Our previous study with electrical accelerated aging of 500-nm a-SiC soaked in saline demonstrated failures influenced by film defects. This present study aims to investigate the electrical stability and failure modes of a-SiC for neural stimulation applications.
Materials and Methods:: Microfabricated interdigitated electrodes (IDEs) were used as test structures to assess a-SiC stability under electrical stress. IDEs were fabricated on 100-mm Si wafers (100) through photolithography. A layer of 1-μm thermal oxide by low pressure chemical vapor deposition was used to isolate the IDEs from the Si substrate. A 1-μm a-SiC film was deposited by plasma-enhanced chemical vapor deposition to initialize the a-SiC bilayer. Metallization layer consists of 210-nm Ti/Au/Ti (1:5:1) deposited by electron-beam evaporation. Another a-SiC film was deposited to complete the bilayer and device encapsulation. Exposure of vias and device singulations were made by reactive ion etching. Leakage currents were produced and measured by picoammeters with DC voltage sources. Biphasic current pulsing was applied to IDEs and resulting voltage transients were taken to evaluate the effect of rapid electrical stress. Experiments were submerged in phosphate-buffered saline at 37°C and programmed with MATLAB for instrument automation, electrical measurements, and statistical analyses.
Results, Conclusions, and Discussions:: The resistance of IDEs calculated from I-V measurements increased proportionally with a-SiC thickness, thus reducing the impact of surface conduction in air and minimizing localized breakdown at defects in saline. The breakdown window of a-SiC in saline was found to expand with thicker film compared to our previous study with ±3 V. Progressive-stress of incrementing voltage at smaller time step size required larger voltage levels to induce capacitive-faradaic transition for onset of dielectric breakdown. In our previous study, 500-nm a-SiC broke down approximately between 2 to 3 V for 1- to 5-min time steps. Pinhole development on 500-nm a-SiC were observed and characterized as breakdown of defects. When 2-μm a-SiC was held at 3-V bias for over 500 h, no breakdown was observed. For 2-μm a-SiC, smaller time steps of 5 to 10 s induced transitions approximately between 4 to 6 V. The voltage transients from pulsing with 500-μs phase width at 100 μA showed no change in capacitance—stimulator saturated at 13 V—after over 30 million pulses. This study demonstrates that a-SiC with appropriate thickness can provide sufficient electrical stability and can be used for deep brain stimulation, which typically requires up to 10 V.
Our study demonstrates that increasing the thickness of a-SiC encapsulation can effectively reduce localized breakdown at defects and expand the breakdown window in saline. Our previous methods of electrical testing are still useful for characterizing material electrical stability in saline environment. Furthermore, the use of smaller time steps for progressive-stress testing is more suitable for characterizing material electrical stability in neural stimulation applications. However, defects still play a role in formation of pinholes, cracking, and delamination. Overall, our study provides important insights into the electrical properties of a-SiC and its promising potential as a reliable and durable encapsulation material for neural interfaces.
Acknowledgements (Optional): : This research was funded under NIH SBIR Grant 5R44DC018261-03 awarded to Blackrock Neurotech.
