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Benzocyclobutene-Based Intracortical Neural Recording Probes

Charles Carter1, Haixin Zhu2, Jiping He1

1 Arizona State University, Department of Bioengineering, Tempe, AZ 852872 Arizona State University, Department of Electrical Engineering, Tempe, AZ 85287

For years, scientists have been using electrodes to record electrical signals from the central and peripheral nervous systems of living organisms.(Schmidt, Bak and McIntosh, 1976) In more recent times, this type of neuronal recording has found application in medical therapies designed to restore limited mobility or a means of communication to tetrapalegics.(Kennedy et al., 2000) While this type of brain-machine interface holds promise for a variety of additional medical applications, the recording electrodes utilized in this neural interface design inevitably lose their functionality after long periods of time in vivo. Two reasons for this include the harshness of the biological environment as well as implant isolation due to the organism’s foreign body response.(Edell et al., 1992) In recent times, researchers have applied microfabrication techniques to the development of sophisticated recording arrays designed to overcome the biocompatibility challenges associated with their precursors. These arrays can be designed to enhance biocompatability by being flexible as well as by incorporating wells to be seeded with bioactive agents.(Rousche et al., 2001) In addition, these new arrays may obtain better signal integrity by incorporating on-chip signal processing elements. Such features may allow this new generation of electrodes to obtain higher quality neuronal recordings for long periods of time. Our research focuses on the development of one such next-generation flexible neuronal recording device utilizing 3022-63 Benzocyclobutene (BCB) as the insulating material. We are currently in the stages of refining the fabrication process for our neuronal recording probe in order to make it more robust. The process begins with a standard silicon wafer coated with 5000Å thermal oxide. The wafer is spin coated with 20µm 3022 BCB resin which is then partially cured in an inert atmosphere at 210°C for 40 minutes. A metal evaporator is then used to sequentially deposit 200 Å Cr, 2000 Å Au, and 200 Å Cr. Photolithography is then used to pattern electrical traces on the polymer. A second layer of BCB resin is then deposited 10µm thick and partially cured at 210°C for 40 minutes. 3000 Å of Al is then electron beam evaporated onto the surface, patterned and etched as a protective mask for reactive ion etch. The sample is then exposed to reactive ion etch (5sccm CF4, 20sccm O2) for 25 minutes. The aluminum mask is then patterned and

etched again to expose the BCB covering the 20 µm by 20 µm neural recording sites as well as the connectors of the device. The sample is then etched for another 15 minutes in reactive ion etch. The aluminum hard mask is then removed and the sample is fully cured at 250°C for 60 minutes. The device is then removed from the substrate with a 49% hydrofluoric acid solution.

Figure 1: BCB-based neural recording array on the face of a penny.