# Minimally-Invasive Neural Interface for Distributed Wireless Electrocorticogram Recording Systems
## 4 Microfabricated Flexible Electrode Array  
A thin and flexible microelectrode array designed and fabricated to be placed on top of the epidural layer in the brain. The electrodes of the device are made of Parylene C and Pt, both of which are FDA approved for bio-compatible materials. As shown in Figure 12a, there exist total 16 recording electrodes in addition to 4 reference electrodes and one large local ground electrode. The recording and reference electrodes have a diameter of \(\sim\)400 \(\upmu\)m with \(\sim\)3 mm and 7 mm spacing, respectively. The thicknesses of the electrode and routing metals are defined as 0.1 \(\upmu\)m and 0.5 \(\upmu\)m, respectively. The recording electrodes are exposed through the bottom surface of the device to be in contact with the dura mater, while the reference and the ground electrodes are exposed on the top surface to be in contact with the surrounding average potential.  
Figure 11: Intra-skin communication (ISCOM) configuration and current driver circuit.  
Figure 12: _Cont._  
The fabrication of the devices was performed in the Lurie Nanofabrication Facility at the University of Michigan. Simple, five-mask processes were used for fabrication. The process steps can be divided into five: (1) bottom parylene layer definition, (2) Pt electrode definition, (3) Au interconnect definition, (4) Au bonding pad definition, (5) top parylene layer definition, and (6) device release. Figure 12b illustrates the fabrication flow. First, a Cr/Au/Cr sacrificial layer for device release was deposited on a 4\({}^{\prime\prime}\) silicon wafer using the Enerjet electron beam evaporator. A parylene layer of 12.5-\(\upmu\)m in thickness was deposited over the sacrificial layer using the SCS PDS 2035 to form the bottom layer. This bottom parylene layer was then etched by oxygen plasma using the Plasmatherm 790 with a photoresist mask of 21-\(\upmu\)m thick AZ 9620. Pt electrodes for the bottom and top electrodes were then deposited and lifted off. A 2.5-\(\upmu\)m thick photoresist mask (SPR 220) was used for the lift-off mask. The 100-nm thick Pt layer was deposited using the Enerject electron beam evaporator with a Titanium adhesion layer of 10 nm and then lifted off defined in acetone. After the Pt electrode patterning, a 500 nm-thick Au layer was sputter-deposited using the Kurt J Kesker Lab 18 with a Cr adhesion layer of 5 nm. Gold was electroplated about 40 \(\upmu\)m, selectively on the contact pads for the external connection. Then, in order to use define the interconnection, the Cr/Au seed layer was wet etched in Au etchant followed by Cr etchant with a mask defined by AZ 5214 image reversal photoresist mask of 1.8-\(\upmu\)m in thickness. After defining the metal interconnection lines, the top parylene layer of 12.5-\(\upmu\)m in thickness was deposited using the SCS PDS 2035. The top parylene layer was then etched using the same process for the bottom layer definition, completing the device outline and the electrode/contact pad openings. After annealing the parylene layer in a hot (>150 \({}^{\circ}\)C) nitrogen chamber for 24 h, the devices were finally released from the wafers by submerging in a Cr etchant to remove the sacrificial layer. Finally, the Ti adhesion layer on top of Pt electrode was removed by a quick Ti etchant dip.