**7.2. Fabrication**

**Figure 12.** Typical medial gastrocnemius (ankle plantarflexor) EMG recording showing early, middle, and late respons‐

Because of the known spacing of the electrodes on the array (as compared with traditional fine-wire electrodes which do not have known interelectrode spacing), we were able, in addition, to determine whether electrode position had a significant impact on muscle recruit‐ ment. The appearance and magnitude of each of the EMG responses was indeed correlated with the choice of electrode position (Figure 13). This serves as evidence that position of stimulation is very important. With a one-dimensional array, it is difficult to assess whether a bilateral stimulation paradigm would also result in lateralization of response, but we strongly

**Figure 13.** Medial gastrocnemius EMG showing varying levels of activation due to stimulation at different rostrocau‐

es after stimulation of spinal cord with parylene MEA.

16 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

suspect that this would be the case.

dally located electrode sites.

Dual-metal-layer electrode arrays are fabricated as shown in Figure 14. Approximately 8 µm of parylene C is first deposited on a silicon wafer with the optional photoresist sacrificial layer, forming the underside of the electrode array. A platinum or titanium-platinum metal liftoff process is used to define traces with 16 µm pitch and 2000 Å to 3000 Å thickness. A second parylene deposition (~1 µm) forms the insulation between the two metal layers. At this point, 6 µm by 6 µm vias are patterned in the insulation layer over the ends of the traces using an O2 plasma RIE. A second step-coverage optimized liftoff process is used to define a second metal layer comprising electrodes and traces, while at the same time achieving electrical continuity between the underlying traces and the overlying electrodes. A final parylene coating approximately 7 µm thick forms the top insulation. The electrodes are exposed and the overall geometry of the implant is defined in a final set of O2 reactive-ion etches using a thick photoresist etch mask. Finally, the arrays are peeled from the wafer in a water bath or released through removal of the sacrificial photoresist in acetone. The process depends on optimal step coverage of the parylene sidewall during evaporation, which is aided, in part, by the slightly isotropic nature of the O2 plasma etch of parylene [47] as well as by the special design of the rotating wafer domes inside the e-beam evaporator, for which the angle of attack of the metal evaporant is adjusted for best coverage. This requirement conflicts marginally with those for successful metal liftoff, however, in this case, the liftoff technique is robust even under these step-coverage optimized conditions due to the choice of an LOR/positive photo‐ resist compound layer.
