Lele Wang
Date: 05/22/2012
Time: 10:15 am (refreshment at 10:00am)
Location: Packard 202
Dissertation advisor: James Harris
Abstract:
Vision of patients blinded by retinal degeneration might be restored by bypassing the degenerated photoreceptors and delivering information to the visual system using patterned electrical stimulation of the remaining retinal neurons. In the photovoltaic approach to retinal prosthesis, the images captured by an external camera are projected onto the subretinal implant from video goggles, using pulsed near-IR light. Each pixel in the two-dimensional array contains photodiodes, which photovoltaically convert pulsed near-infrared light into bi-phasic current to stimulate nearby retinal neurons without wired power connections. We developed an integrated-circuit/MEMS process to fabricate a silicon photodiode array for subretinal stimulation. The device thickness is chosen to be 30 um to absorb a significant portion of the incident light while still being thin enough for subretinal implantation. Each pixel contains three diodes connected in series between the active and return electrodes, which confine current near each pixel. Electrodes are sputter coated with iridium oxide to enhance charge injection levels and provide a stable neural interface. Pixels are separated by 5 um-wide trenches to electrically isolate neighboring pixels and to improve nutrient diffusion through the device. For all of the three sizes of pixels fabricated – 280, 140 and 70 um, the reverse-bias dark current is sufficiently low for our application (<100 pA). The turn-on voltages of one, two and three series-connected photodiode structures are approximately 0.6V, 1.2V and 1.8V, respectively. The measured photo-responsivity per diode at 880 nm wavelength is ~0.36 A/W, at zero voltage bias.
The fabrication process of the silicon photodiode array and optoelectronic characterization of the fabricated devices will be presented. The initial electrophysiological results of retinal stimulation in-vitro and in-vivo show that all three sizes of pixels are able to reliably elicit retinal responses at safe near-infrared light irradiances, with good acceptance of the photodiode array in the subretinal space.
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