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M. D. Gingerich, R. Akhmechet, D. B. Shire, J. L. Wyatt, J. F. Rizzo; Development of a Flexible High-Density Multi-Layered Metallization Interconnect Technology for a Subretinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3035.
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This work is related to the efforts of the Boston Retinal Implant Project to develop a sub-retinal prosthesis to restore vision to the blind. The specific purpose of this presentation is to describe our efforts to develop a high density polyimide-based flexible multi-layered metallization interconnect technology capable of interconnecting stimulation current sourcing electronics with hundreds or potentially even thousands of stimulating electrode sites. The hypothesis was that such a structure could be realized using standard microfabrication technology.
A comprehensive set of test structures was designed in CAD including structures to test: 1) conductor sheet resistance, 2) interconductor contact resistance, 3) conductor continuity over topographical steps, 4) inter- and intra- conductor capacitance, and, 5) physical flexibility of various device configurations. The test structures also included variations in conductor width from 25µm down to 3µm, contact sizes from 25µm down to 3µm, and overlap configuration. The test structures were fabricated by adapting and optimizing our standard polyimide fabrication process to repetitively stack and interconnect successive layers of lithographically patterned gold conductor metal each separated by a ~1.5µm-thick layer of polyimide. Measurements were made after major process steps to monitor the integrity and performance of the conductors as well as the integrity of the interlayer polyimide. Different conductor metal thicknesses were tested to reduce resistance while still maintaining reliable step coverage over underlying conductors.
A microfabrication process has been developed which can produce multilayered metallization encased in a flexible polyimide carrier. Up to four layers of conductor metal have been successfully fabricated with low resistance, good isolation and robust interlayer via structures. The overall thickness of a flexible structure with four layers of 750nm Au conductor metal was less than 12µm thick.
A means of fabricating a very thin flexible polyimide-based multi-layered metallization interconnect technology which can be implanted into the subretinal space has been developed. These structures may provide the basis for future high-density interconnects needed to drive hundreds or possibly even thousands of electrode sites.
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