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26.2 a Curvable Silicon Retinal Implant

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26.2 a Curvable Silicon Retinal Implant A Curvable Silicon Retinal Implant Rostam Dinyari1, Jim D. Loudin2, Phil Huie3, Daniel Palanker2,3, Peter Peumans1 1Electrical Engineering Department, Stanford University, Stanford, California 94305, USA 2Applied Physics Department, Stanford University, Stanford, California 94305, Stanford, California 94305, USA 3Ophthalmology Department, Stanford University, Stanford, California 94305, USA Abstract We have developed a curvable photovoltaic monolithic retinal implant that requires no electrical power or data connection. The implant consists of a two-dimensional network of miniature silicon solar cells that directly stimulate the retina when illuminated by a goggle system. A MEMS process isolates adjacent pixels and makes the arrays curvable allowing them to conform to the shape of the retina. Motivation and Approach In age-related macular degeneration (AMD) and retinitis pigmentosa (RP), two leading causes of blindness, the photoreceptor layer of the retina malfunctions while the other layers (inner nuclear layer and ganglion cells) remain fully functional [1]. To restore vision in such cases, we have developed a thin (30μm thick) photovoltaic silicon implant that, when combined with a goggle-mounted optical Figure 1. System design. The implant is surgically inserted recorder/projector system, can restore vision. Fig. 1 shows between the retina and retinal pigment epithelium (RPE) and a schematic of the system [2]. A miniature camera captures does nor require a power or data connection. Vision is video that is processed by a pocket computer before being achieved by projecting an image recorded by the λ google-counted camera into the eye at a near-infared projected into the eye at a near-infrared wavelength ( = wavelength in pulsed fashion [2]. The pulses are interpreted 900 nm) onto the silicon implant located in the subretinal as vision by the visual cortex. space in front of the retinal pigment epithelium (RPE). The implant consists of a two-dimensional (2D) array of Scanning electron micrographs (SEMs) of the implants are photovoltaic pixels. The projected image is provided in pulsed fashion and each pixel element consists of up to (a) (b) three series-connected photovoltaic cells such that the pixels deliver current pulses that are sufficiently strong to stimulate the remaining functional neural cells. The current pulses are interpreted as vision by the visual cortex. Placing the implant in the subretinal space (as compared to epiretinal) allows for utilization of the existing image-processing and data-compression functions of the (c) (d) inner nuclear layer [3]. Working at λ = 900 nm prevents confusion by preventing stimulation of the remaining functional photoreceptor cells. The novelty of the work reported here is the integration of photovoltaic devices in a MEMS process that allows the implant to conform to the natural curvature of the eye [4], while also providing isolation between the bodies of the three series-connected subpixels that make up each pixel. (e) (f) Figure 2. (a) and (b) SEM images of curvable silicon arrays conforming to the shape of the retinal pigment epithelium (RPE) of a pig. (c) SEM of a single diode (1D) and (d) three-diode (3D) pixel. The silicon flexures between pixels are 300 nm wide. The silicon bridges between subpixels in (d) are 200 nm wide. The pixels are 230 μm x 230 μm. SEMs of fully functional 1.775 mm x 1.775 mm arrays of 230 μm x 230 μm (e) 1D and (f) 3D pixels. 97-4244-5640-6/09/$26.00 ©2009 IEEE 26.2.1 IEDM09-595 shown in Fig 2. Usage of three series-connected subpixels spray-coated resist (Fig. 4f). A conformal SiO2-coat / per pixel (Fig. 2d) improves the impedance matching to the anisotropic dry etch sequence is then used to protect the surrounding tissue and enhances the injected current per pixel and flexure sidewalls, provide an antireflective (AR) pixel allowing for higher resolution implants. Curving the coating, cover the metal interconnects, expose the implant (Fig. 1a and b) is advantageous since the complete electrodes regions, and expose the handle wafer at the implant is in focus, resulting in optimum quality of vision bottom of the trenches (Fig. 4g and h). A forming gas perceived.Curved implants can also be substantially larger anneal is performed. A sputtered Iridium oxide film (SIROF) than planar implants and can hence cover a larger part of is deposited and patterened using a second lift-off with the field of view. A further advantage is that in the case of spray-coated resist to cover the electrode areas (Fig. 4i). flat implants, retinal cells migrate to fill the empty space The IrOx coating is biocompatible and know to have a high between the implant and RPE which leads to a charge-injection capacity into saline [5]. A XeF2 silicon etch reorganization of multiple layers of retinal cells. A curvable releases the arrays from the wafer (Fig. 4j). implant stays in close proximity to the RPE, as shown in the optical coherence tomography in Fig. 3. (a) (f) (b) (g) (c) (h) Figure 3. Optical Coherence Tomography (OCT) image of a curvable silicon array embedded in the subretinal space of a pig eye. Fabrication (d) (i) The implant fabrication flow is shown in Fig. 4. Retinal implants with 230 μm, 115 μm and 57.5 μm pixels containing 80, 40, and 20 μm diameter stimulation electrodes in a 2 mm x 2 mm array were fabricated on FZ (e) (j) (resistivity > 2 kΩ.cm) <100> SOI substrates with 30 μm-thick device layer (Fig. 4a). BBr3 and POCl gas diffusion Figure 4. The fabrication process. (a) FZ SOI wafer with 30 um was used to define the p+ and n+ contact regions (Fig. 4b device layer. (b) p+ diffusion. (c) n+ diffusion. (d) Antireflection and c). An SiO anti-reflection coating is grown which also coating and contact openings. (e) DRIE. (f) Ti/Pt deposition and 2 lift-off. (g) LTO oxide deposition. (h) Removal of oxide from bottom functions as surface passivation. It is opened over the of the trenches and over the electrodes followed by forming gas contact regions (Fig. 4d). The pixels are subsequently anneal. (i) Injection electrode deposition and lift-off. (j) XeF2 isolated from each other using a deep reactive ion etch release. (DRIE) step to the buried oxide that leaves the individual pixels connected in a two-dimensional array using Results ~300nm-thick silicon flexures (Fig. 4e `and Fig. 2c). The SEMs of fabricated arrays are shown in Figs. 2 and 5. same DRIE step also electrically isolates the three Detailed views of pixels are shown in Fig. 2c, 2d, 5b, and 5d. subpixels from each other while they are mechanically Opto-electrical measurements of a single-diode (1D) and Ω connected via resistive silicon bridges (Rbridge> 10M , see three-diode (3D) pixel under illumination are shown in Figs. Fig. 2d) resulting in a series connection via the patterned 6 and 7. The responsivity of the 1D pixels is R1D = 0.30 A/W metal layer. The silicon flexures allow the pixels to move in air and R1D = 0.35 A/W in DI water for λ=904nm laser and rotate such that the array conforms to spherical illumination, corresponding to an internal quantum yield surfaces with a radius of curvature as small as 5mm (Fig. (IQE) > 96% despite the large surface area represented by Ω 2a and b). The flexure resistance (Rflexure>100M ) the vertical sidewalls. In the current design, 43% of the electrically isolates the pixels to prevent pixel cross-talk. incident radiation at 904nm is absorbed limited by the The buried oxide is then opened. A Ti (20nm) / Pt (100nm) not-optimal SiO2 AR coating thickness. The responsivity of metal layer is deposited and patterned using lift-off with the 3D subpixels is identical to that of 1D pixels. When IEDM09-596 26.2.2 measured in saline, the 3D pixels inject 1.4 mC/cm2 compared to 0.3 mC/cm2 for 1D pixels (for a 0.5 ms pulse 2 λ 200 duration, 1 mW/mm average irradiance at = 900 nm) due (a) 0 μW to the higher voltage generated. This corresponds to 70, 13 μW μ 17.5, and 4.4 nC per optical pulse for different sizes of 3D 44 W 64 μW pixels which is sufficient for retinal stimulation [5]. The open A) circuit voltage (VOC) generated by the 1D diodes is 0.57 V μ 100 (64 μW) vs. 0.53 V for the 3D diodes (57 μW). This is attributed to the additional surface area of the isolation trenches between subpixels that provide additional Current ( electron-hole pair recombination sites. Their effect is most 0 noticeable at high electron and hole concentrations, resulting in a decreased VOC but largely unaffected IQE. -1.5 -1.0 -0.5 0.0 0.5 1.0 (a) Voltage (V) 20 (b) A) μ 15 10 Measurement Data Responsivity = 0.30 A/W 5 Short Circuit Current ( Current Circuit Short 0 (b) 0 204060 Beam Power (μW) Figure 5. (a) Current vs voltage as a function of light intensity and (b) short-circuit current vs light intensity measurements for 1D pixels in air. The responsivity is 0.30 A/W. Our future work will focus on in vitro and eventually in vivo experiments. This requires a final encapsulation in a conformal bio-compatible coating such as parylene. (c) (d) Figure 6. SEMs of fully functional 2 mm x 2 mm arrays of 115 μm x 115 μm (a) 1D and (b) 3D pixels. The flexures are 300 nm wide. (c) and (d) show detailed view of the pixels and subpixels. 26.2.3 IEDM09-597 600 1 diode (a) 2 diodes Conclusions 3 diodes By structuring an array of miniature solar cells into separate pixels connected by silicon flexures, curvable silicon 400 A) photovoltaic retinal implants were fabricated.
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