Integrated Neural Interface
The overall goal of this project is to develop and test (in-vivo) a chronically implantable neural recording array and provide the device to the neuroscience community upon completion of the initial technical development phase for experimental use and evaluation.
In order to accomplish this task, existing and new technologies are being merged. The chronic recording array will consist of a high density flip-chip integrated stack of modules: a thinned Utah Electrode Array (UEA), a CMOS based signal processor (low noise amplifier, peak detector) and communications module (forward and reverse telemetry link, RF module) and a high efficiency thin film coil for inductive power coupling. Silicon carbide and Parylene are intended for use as hermetic encapsulating material.
In the Microsystems Lab, focus is on the fabrication, characterization and testing of the neural arrays. Some of the works under this research are described below.
A novel method to fabricate high throughput, three dimensional microelectrode arrays with a mask-less process is developed. Wafer level etching allows not only a low cost process but also reduced lead time. The concept is based on the use of dicing saw and wet isotropic silicon etchant to define pen/fork-shaped electrodes. The etching of is a two-step process “dynamic” and “static”. The “dynamic” etching uniformly etches the electrodes at all heights into a square with rounded corners, while “static” etching etches the electrodes preferentially at the top of the column with respect to the base until a final column shape resembles a microneedle. A custom made stirring system is developed, in which the wafer is placed in a recessed Teflon holder over an O-ring, which completely seals the back-side of the wafer from the wet etchant.
Contact: Rohit Sharma
Electrode Tip Metalization: Sputtered Iridium Oxide Film
To improve the performance of the device, the microelectrodes are coated with material that can efficiently convert electrical to ionic current and vice versa. The charge delivery capacity (CDC, mC/cm2) of an electrode material is the ability of the material to transfer electrical charge to ionic charge in vivo. Iridium oxide (IrOx) ﬁlms have received considerable attention as a coating for neural stimulation electrode due to their high charge delivery and corrosion resistance. In this research, the influence of sputtering conditions (e.g. pressure, atmosphere) on the IrOx films are examined. The sputtered IrOx ﬁlms (SIROF) are characterized by surface analysis methods (e.g. scanning electron microscopy, atomic force microscopy, energy dispersive X-ray spectrometry, X-ray diffraction), four-point probe method, and electrochemical techniques (e.g cyclic voltammetry and electrochemical impedance spectroscopy). SIROF damage thresholds and degradation factors are also studied.
Contact: Layne Williams
A conformal and chronically stable dielectric encapsulation is required to protect the neural interface device from the harsh physiological environment and localize the active electrode tips. Chemical vapor deposited Parylene-C films are studied as a potential implantable dielectric encapsulation material using impedance spectroscopy and leakage current measurements. Etching processes are compared for removing the Parylene-C insulation to expose the active electrode tips. Also, the relationship between tip exposure and electrode impedance is determined. The conformity and the uniformity of the Parylene-C coating are assessed using optical microscopy, and small thickness variations on the complex 3-D electrode arrays are observed.
Contact: Xianzong Xie
Integrated Wireless Neural Interface
An integration concept for the neural interface is developed, and the materials and methods used are investigated. A multi-level hybrid assembly process utilizing the Utah Electrode Array (UEA) as a circuit board is implemented. The signal processing IC is flip-chip bonded to the UEA using Au/Sn reflow soldering, and includes amplifiers for up to 100 channels, signal processing units, an RF transmitter, and a power receiving and clock recovery module. An under bump metallization (UBM) using
potentially biocompatible materials is developed and optimized, which consisted of a sputter deposited Ti/Pt/Au thin film stack. After flip-chip bonding, an underfiller is applied between the IC and the UEA to improve mechanical stability and prevent fluid ingress in in vivo conditions. A planar power receiving coil fabricated by patterning electroplated gold films on polyimide substrates is connected to the IC by using a custom metallized ceramic spacer and SnCu reflow soldering. The SnCu soldering is also used to assemble SMD capacitors on the UEA.
Ultra High Aspect Ratio Electrode Arrays
Ultra high aspect ratio microelectrodes are designed and fabricated to record and stimulate neural signals from deeper areas of the brain and nerves and also to provide a
new research tool to the neuroscience community. A fabrication process to build ultra high aspect ratio silicon-based microelectrode neural arrays is developed. The µ-wire electrical discharge machining (µ-WEDM) process enables machining electrodes from highly conductive bulk silicon. The electrodes are electrically isolated near their base by glass. Thin, needle-shaped and smooth silicon microelectrodes are realized with an optimized chemical etching process.
Contact: Prashant Tathireddy