Development of Lithography and Stencil Mask Alignment Processes for Manufacturing High Density Thin Film Electrodes on Optical Fiber Substrates



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Neural probes with micron-scale electrodes are capable of recording action potential waveforms from single neurons with spatial and temporal resolution beyond the capability of other technologies (e.g. electroencephalography, electrocorticography). For the isolation of individual neurons, the determination of the frequency and strength of neural synapses is essential. Connectivity between neurons can be inferred by electrical or chemical stimulation of a cluster of neurons. It is however, difficult to determine specific synaptic locations, since a large number of neurons is indiscriminately stimulated. The isolation and stimulation of a specific set of neurons can be accomplished by opto-genetics, where genetically modified neurons are stimulated by light. This requires implantable probes with reliable light delivery for stimulation of neurons and integrated conductor wires for obtaining laminar recordings from different cortices of the brain. Optical fibers make good substrates for neural probe technology, since they have the strength and stiffness required to target deep brain structures and the ability to deliver multi-spectral light, essentially without coupling losses. In this dissertation, we report the development of i) an atom beam lithography (ABL) tool used for proximity printing on cylindrical substrates and ii) a mechanical alignment process for printing high-density (32 electrodes) micron-scale electrodes on optical fiber substrates. This tool features a high-brightness source of 50 keV helium atoms with a 0.5 nm penumbral blur for a 5-micron proximity gap and a flux density of 1.25×10^13 particles/s-cm2. An advanced alignment process is reported where the fibers are held in anisotropically etched silicon v-grooves, which are nearly atomically straight. Longitudinal and lateral alignment of stencil mask patterns was done using high-precision fiber stops and cubic beads, respectively. The longitudinal and transverse overlay accuracies were 0.8 + 0.39 micrometers and 0.1 + 0.05 micrometers, respectively.



Neural Probes, Atom Beam Lithography, Fabrication Process, Micro-Electrodes.


Portions of this document appear in: V. S. Jonnalagadda, N. S. Randhawa, A. Awale, P. Motwani, and J. C. Wolfe, “A high-brightness source of energetic He atoms,” J. Vac. Ssci. Technol. B, 38, 012601 (2020).