| AVS 66th International Symposium & Exhibition | |
| MEMS and NEMS Group | Tuesday Sessions |
| Session MN-TuM |
| Session: | MEMS, BioMEMS, and MEMS for Energy: Processes, Materials, and Devices II |
| Presenter: | Md Ashiqur Khan, University of Houston |
| Authors: | M.A. Khan, University of Houston M. Gheewala, University of Houston V.S. Jonnalagadda, University of Houston T.A. Tisa, University of Houston M. Rao, University of Houston A. Awale, University of Houston P. Motwani, University of Houston N.S. Randhawa, University of Houston H. Sajedi, University of Houston W.-C. Shih, University of Houston J.C. Wolfe, University of Houston J.A. Dani, University of Pennsylvania P. Mauger, No Matching Affiliation |
| Correspondent: | Click to Email |
Electrical probes are used to stimulate spiking activity within a target population of neurons and monitor how these electrical signals propagate through the brain. This paper describes a simple fabrication process for multi-electrode neural probes on optical fiber substrates. It relies on neutral particle proximity lithography to achieve the required depth-of-field and freedom from charging artifacts but circumvents the complexity of membrane masks (complementary exposures, radiation resistant coatings, and fragility) and on-fiber alignment.
Fig. 1 of the supplementary document shows, conceptually, a probe with 4-channel thin-film sensor, a tetrode, on each of four sides of a fiber. It requires two masks; the first for the interconnect traces. The other for vias in the dielectric overcoat where the metal lines contact the brain.
As shown in Fig. 2, optical fibers are held in V-grooves etched into the top surface of a (100) Si wafer. A second set of V-grooves, etched from the opposite side of the wafer, forms open windows at the bottom of the upper grooves. When this mask is illuminated by 50 keV He atoms, transmitted beamlets transfer the stencil pattern to resist on the fibers. A negative-tone, plasma-deposited, resist is used to mask the gold interconnects. The vias are similar, but require a tone-reversal step. Rotational alignment of the 2 masks uses a high precision cubic bead glued to the end of the fiber to reference the rotational angle of the fibers to a precision-ground aluminum platform on the jig. Longitudinal alignment is achieved using a fiber-stop. These are high precision (Grade 5) 440C stainless steel ball bearings which are held in an anisotropically etched pocket at the tip-end of a V-groove by a rare earth magnet. Longitudinal and transverse positional errors of 1.0+0.6 mm and 0.3+.15 mm, respectively. A single-interconnect mask can be printed multiple times to build the probe of Fig.1. The offset is produced by tilting the mask relative to the beam. Fig. 3 shows two lines printed on a 300 mm fiber with 21.5 and 29.5 mm offsets in the longitudinal and transverse directions, respectively.
Fig. 4 is an in-vitro recording from a brain slice (mouse) after a battery of bench tests, including a) a 3-week soak phosphate buffered, b) repeated insertion in agar and a stainless steel cannula, d) disinfection in MetriCide-2.6% glutaraldehyde, and a 6 hour implantation in mouse brain). Impedance spectra were the same within the measurement error of the impedance bridge before and after these bench tests.
At this time the tone reversal process has not been fully optimized.