A Curvable Silicon Retinal Implant
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1
Rostam Dinyari , Jim D. Loudin , Phil Huie , Daniel Palanker , Peter Peumans
Electrical Engineering Department, Stanford University, Stanford, California 94305, USA
2
Applied Physics Department, Stanford University, Stanford, California 94305, Stanford, California 94305, USA
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Ophthalmology Department, Stanford University, Stanford, California 94305, USA
1
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
recorder/projector system, can restore vision. Fig. 1 shows
a schematic of the system [2]. A miniature camera captures
video that is processed by a pocket computer before being
projected into the eye at a near-infrared wavelength (λ =
900 nm) onto the silicon implant located in the subretinal
space in front of the retinal pigment epithelium (RPE). The
implant consists of a two-dimensional (2D) array of
photovoltaic pixels. The projected image is provided in
pulsed fashion and each pixel element consists of up to
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
inner nuclear layer [3]. Working at λ = 900 nm prevents
confusion by preventing stimulation of the remaining
functional photoreceptor cells.
Figure 1. System design. The implant is surgically inserted
between the retina and retinal pigment epithelium (RPE) and
does nor require a power or data connection. Vision is
achieved by projecting an image recorded by the
google-counted camera into the eye at a near-infared
wavelength in pulsed fashion [2]. The pulses are interpreted
as vision by the visual cortex.
Scanning electron micrographs (SEMs) of the implants are
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.
(a)
(b)
(c)
(d)
(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.
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shown in Fig 2. Usage of three series-connected subpixels
per pixel (Fig. 2d) improves the impedance matching to the
surrounding tissue and enhances the injected current per
pixel allowing for higher resolution implants. Curving the
implant (Fig. 1a and b) is advantageous since the complete
implant is in focus, resulting in optimum quality of vision
perceived.Curved implants can also be substantially larger
than planar implants and can hence cover a larger part of
the field of view. A further advantage is that in the case of
flat implants, retinal cells migrate to fill the empty space
between the implant and RPE which leads to a
reorganization of multiple layers of retinal cells. A curvable
implant stays in close proximity to the RPE, as shown in the
optical coherence tomography in Fig. 3.
spray-coated resist (Fig. 4f). A conformal SiO2-coat /
anisotropic dry etch sequence is then used to protect the
pixel and flexure sidewalls, provide an antireflective (AR)
coating, cover the metal interconnects, expose the
electrodes regions, and expose the handle wafer at the
bottom of the trenches (Fig. 4g and h). A forming gas
anneal is performed. A sputtered Iridium oxide film (SIROF)
is deposited and patterened using a second lift-off with
spray-coated resist to cover the electrode areas (Fig. 4i).
The IrOx coating is biocompatible and know to have a high
charge-injection capacity into saline [5]. A XeF2 silicon etch
releases the arrays from the wafer (Fig. 4j).
(a)
(f)
(b)
(g)
(c)
(h)
(d)
(i)
(e)
(j)
Figure 3. Optical Coherence Tomography (OCT) image of a
curvable silicon array embedded in the subretinal space of a pig
eye.
Fabrication
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
(resistivity > 2 kΩ.cm) <100> SOI substrates with 30
μm-thick device layer (Fig. 4a). BBr3 and POCl gas diffusion
was used to define the p+ and n+ contact regions (Fig. 4b
and c). An SiO2 anti-reflection coating is grown which also
functions as surface passivation. It is opened over the
contact regions (Fig. 4d). The pixels are subsequently
isolated from each other using a deep reactive ion etch
(DRIE) step to the buried oxide that leaves the individual
pixels connected in a two-dimensional array using
~300nm-thick silicon flexures (Fig. 4e `and Fig. 2c). The
same DRIE step also electrically isolates the three
subpixels from each other while they are mechanically
connected via resistive silicon bridges (Rbridge> 10MΩ, see
Fig. 2d) resulting in a series connection via the patterned
metal layer. The silicon flexures allow the pixels to move
and rotate such that the array conforms to spherical
surfaces with a radius of curvature as small as 5mm (Fig.
2a and b). The flexure resistance (Rflexure>100MΩ)
electrically isolates the pixels to prevent pixel cross-talk.
The buried oxide is then opened. A Ti (20nm) / Pt (100nm)
metal layer is deposited and patterned using lift-off with
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Figure 4. The fabrication process. (a) FZ SOI wafer with 30 um
device layer. (b) p+ diffusion. (c) n+ diffusion. (d) Antireflection
coating and contact openings. (e) DRIE. (f) Ti/Pt deposition and
lift-off. (g) LTO oxide deposition. (h) Removal of oxide from bottom
of the trenches and over the electrodes followed by forming gas
anneal. (i) Injection electrode deposition and lift-off. (j) XeF2
release.
Results
SEMs of fabricated arrays are shown in Figs. 2 and 5.
Detailed views of pixels are shown in Fig. 2c, 2d, 5b, and 5d.
Opto-electrical measurements of a single-diode (1D) and
three-diode (3D) pixel under illumination are shown in Figs.
6 and 7. The responsivity of the 1D pixels is R1D = 0.30 A/W
in air and R1D = 0.35 A/W in DI water for λ=904nm laser
illumination, corresponding to an internal quantum yield
(IQE) > 96% despite the large surface area represented by
the vertical sidewalls. In the current design, 43% of the
incident radiation at 904nm is absorbed limited by the
not-optimal SiO2 AR coating thickness. The responsivity of
the 3D subpixels is identical to that of 1D pixels. When
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measured in saline, the 3D pixels inject 1.4 mC/cm
2
compared to 0.3 mC/cm for 1D pixels (for a 0.5 ms pulse
2
duration, 1 mW/mm average irradiance at λ = 900 nm) due
to the higher voltage generated. This corresponds to 70,
17.5, and 4.4 nC per optical pulse for different sizes of 3D
pixels which is sufficient for retinal stimulation [5]. The open
circuit voltage (VOC) generated by the 1D diodes is 0.57 V
(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
electron-hole pair recombination sites. Their effect is most
noticeable at high electron and hole concentrations,
resulting in a decreased VOC but largely unaffected IQE.
Current (μA)
200
0 μW
13 μW
44 μW
64 μW
(a)
100
0
-1.5
-1.0
-0.5
(a)
0.0
0.5
1.0
Voltage (V)
(b)
Short Circuit Current (μA)
20
15
10
Measurement Data
Responsivity = 0.30 A/W
5
0
(b)
0
20
40
60
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.
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Current (μA)
600
1 diode
2 diodes
3 diodes
(a)
Conclusions
By structuring an array of miniature solar cells into separate
pixels connected by silicon flexures, curvable silicon
photovoltaic retinal implants were fabricated. Each pixel
contains up to three subpixels that are connected in series.
The pixel bodies were isolated using DRIE-defined deep
trenches. Despite the deep-etched trenching to define the
silicon flexures and isolate the subpixels, the photovoltaic
diodes exhibit an internal quantum efficiency of 96%. The
sub-optimal SiO2 AR coating limits the responsivity to ~0.30
A/W. Measurements in saline indicate that the charge
injected per optical pulse at safe optical intensities is
sufficient to stimulate the retina. The implants do not require
an electrical power or data-connection and may be able to
restore high-acuity vision to patients suffering from AMD or
RP. The achievable spatial resolution, field-of-view and
absence of scarring make this approach a good alternative
to existing approaches.
400
200
0
-3
-2
-1
0
1
2
3
Voltage (V)
150
0 μW
14 μW
33 μW
57 μW
(b)
Current (μA)
100
References
[1] E. Zrenner, Science, 295, 1022-1025 (2002).
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[2] J. D. Loudin, D. M. Simanovskii, K. Vijayraghavan, C. K.
Sramek, A. F. Butterwick, P. Huie, G. Y. McLean, and D.
V. Palanker, Journal of Neural Engineering, 4, S72-S84
(2007).
0
-1.0
-0.5
0.0
0.5
[3] J. Dowling, Eye, 22, 1-7 (2008)
1.0
[4] R. Dinyari, S. Rim, K. Huang, P. B. Catrysse, and P.
Peumans, Applied Physics Letters, 92, 091114 (2008).
Voltage (V)
Short Circuit Current (μA)
20
[5] S. Cogan, Annual Review of Biomedical Engineering, 10,
275-309 (2008).
(c)
15
10
Measurement Data
Responsivity = 0.30 A/W
5
0
0
20
40
60
Beam Power (μW)
Figure 7. (a) Current vs voltage of 1, 2, and 3 diodes connected in
series of the same 3D pixel. The deep trenches and high
resistance bridges effectively isolate the three subpixels. (b)
Current vs. voltage as a function of light intensity measured on
one subpixel of a 3D pixel. All subpixels have identical
performance. (c) Short-circuit urrent vs light intensity
measurements on a subpixel of a 3D pixel, showing a responsivity
of 0.31 A/W in air.
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