Artificial retina using thin film transistor report

B. Ramakrishna (11621A0409) AEC, Bhongir
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1. INTRODUCTION
Advantages of TFTs are large-areal fabrication, low cost, substrate flexibility, etc. In
particular, since poly-Si TFTs have high performance comparable with bulk-Si transistors, any
kind of electronic circuits can be composed. However, conventional integrated drivers utilize
only peripheral area. If in-pixel and inter-pixel operations are executed in pixel area, SOP will
become more effective. In this presentation, an artificial retina using thin-film photodiodes
(TFPDs) and poly-Si TFTs is proposed, which achieves edge enhancement, one of the functions
of living retinas. The artificial retina is an improvement of SOP in which the in-pixel and inter-
pixel operations are executed. Moreover, if this artificial retina is regarded as an initiative
development of artificial organs, since it can be fabricated on flexible, harmless and organic
substrates, it is expected to be suitable for living bodies.
1.1 Thin-film photodiode and thin-film transistor:
The TFPD is fabricated using the same fabrication processes as poly-Si TFTs and
consists of a PIN diode. The actual dependence of photo-induced current (Iphoto) on photo-
illuminance (Ephoto) with a variation in reverse voltage (Vreverse) is shown in Fig. 1.1. It is
found that dark current exists even under no illuminance, which is due to thermal generation of
electron-hole pairs via trap states at the oxide interface and grain boundaries. The transformation
efficiency from the photons to the electron hole pairs is as low as 10 %, which is due to the
recombination also via trap states. In any case, the dependence of Iphoto on Ephoto is linear.
The actual transfer characteristic of the poly-Si TFT is shown in Fig. 2.1. The poly-Si TFTs
have high performance, but they are still inferior to bulk-Si transistors. First, their transistor
mobility is not very high and their threshold voltage is not very low. Second, their saturation
region is not flat in the output characteristic, which might be a problem in analog circuits, such
as the artificial retina. Therefore, circuit simulation is necessary to develop them.

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Fig 1.1: Structure and characteristic of TFPD.
Fig1.2: Structure and characteristic of poly-Si TFTs.
The design of the retina pixel is based on an elementary current mirror, but some
improvements are added by considering the characteristics of the TFPDs and poly-Si TFTs and
operation of an artificial retina. Although the part for the generation of mirror current (Imirror)
consists of two p-type TFTs, the part for the load resistance consists of two n-type TFTs.
Sensitivity can be controlled by both bias voltage (Vbias) and adjust voltage (Vadjust). The

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scales for the TFPD and all TFTs are optimized. The simulated dependences of output voltage
(Vout) on Ephoto with a variation in Vbias are shown in Fig 1.3.
Fig1.3 Design and characteristic of retina pixel.
It is found that the sensitivity can be controlled by Vbias once a suitable voltage is
applied to Vadjust. The simulated edge enhancement of the retina array are shown in Fig. 1.4.
Ephoto to the left half of the retina pixels are stairs-like different from the right half. A suitable
voltage is applied to Vadjust to control the total sensitivity of the retina array. Vout is not only
the output signal but is also applied as Vbias in adjacent pixels. When a pixel is highly
illuminated, its Vout is high. When a high voltage is applied as Vbias in an adjacent pixel, Vout
in the adjacent pixel is decreased, and vice versa. It is found that edge enhancement can be
achieved. The level of Ephoto for the edge enhancement can be controlled by Vadjust.

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Fig 1.4: Network and edge enhancement of retina array.
 ARTIFICIAL retinas are necessary to recover the sight sense for sight handicapped
people
 Electronic photo devices and circuits substitutes for deteriorated photoreceptor cells in
eyes.
 Artificial retinas can be implanted in inside surface of the living retina at the back part
of the eyeball so that the stimulus signal can be directly conducted to neuron cells and
that living retinas are not seriously damaged.

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2. HUMAN EYE
2.1 Working of human eye:
 The eye is one of the most important organs of the body. Before we learn about ASR,
it is important to know the working of natural retina.
 The light coming from an object enters the eye through cornea and pupil and forms
inverted image on the retina.
 The light sensitive cells of the retina gets activated with the incidence of light and
generate electric signals. These electric signals are sent to the brain by the optic nerves
and the brain interprets the electrical signals in such away that we see an image.
Fig 2.1: Structure of the eye

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The Structure of Retina
Fig2.2: Structure of the retina
2.2 Damage related to eye:
Blindness is the condition of poor visual perception. Various scales have been developed
to describe the extent of vision loss.
Blindness can be temporary or permanent. Damage to any portion of the eye, the optic
nerve, or the area of the brain responsible for vision can lead to blindness. There are numerous
(actually, innumerable) causes of blindness. The current politically correct terms for blindness
include visually handicapped and visually challenged.

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2.2.1 Causes of blindness
Damage to:
• Clear structures in the eye, that allow the light to pass through.
• The nerves within the eye.
• The Optic nerve
• Brain
Fig. 2.3 Blind person with stick
2.2.2 MajorDiseasesofEye
1. Retinitis Pigmentosa
 Hereditary genetic disease.
 Degeneration of the retina
 Gradually progress towards center of eye.
 Spares the foveal region.
 Tunnel vision results

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Fig 2.4: Damaged retina
2. Macular Degeneration
 Genetically related.
 Cones in macula region degenerate.
 Loss or damage of central vision
 Common among aged people.
 Peripheral retina spared.
3. Age-RelatedMacular Degeneration
Age-related macular degeneration (AMD) is the physical disturbance of the center of the
retina called the macula Macular degeneration, often age-related macular degeneration, is a
medical condition that usually affects older adults and results in a loss of vision in the center of
the visual field (the macula) because of damage to the retina.

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Fig 2.5: Macular degeneration.
2.2.3 Artificial Thin-Film Transistor Retina
 The first application of an implantable stimulator for vision restoration was developed
by Dr S. Brindley and Lewin in 1968.
 Recovers the sight sense for sight-handicapped people.
 Electronic Photo devices and circuits substitutes deteriorated photoreceptor cells.
 Implanted inside the eyes.
 Implanting classified into two types: Epiretinal implant and Subretinal implant.
 Thin-Film Transistors, fabricated on transparent and flexible substrates.

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 Implantable microelectronic retinal prostheses
 Externally worn digital camera which samples the wearer’s visual environment.
Artificial retina is developed for the sight handicapped people, so that they can recover
their sight. It was developed with the help of external photo device (cameras) that worked as a
sight stimulus for the brain through the electrodes. The device was used as a substitute to the
retinal cells of the eye. They were formulated using large scale third dimension integration, so
that a person can receive the exact image of an object that is in front of them.

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3. Implantation of Artificial Retina
The artificial retinas work with the help of thin film transistors and capacitors. This
device is implanted near the retinal blind spot, so that the deteriorated photo receipting can be
replaced, with this advanced device. This device is implanted inside the human eye in two ways
which are:
Fig 3.1 Working of the implant
3.1 Retinal Implantation:
A retinal implant is a biomedical implant technology currently being developed by a
number of private companies and research institutions worldwide. The first application of an
implantable stimulator for vision restoration was developed by Drs. Brindley and Lewin in 1968.

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The implant is meant to partially restore useful vision to people who have lost their vision. There
are two types of retinal implants namely epiretinal implant and subretinal implant.
3.1.1 Epiretinal Implant
When the artificial retina is implanted using Epiretinal Implant then, the implanted retina
is placed at the inner surface of the blind spot that connects optic nerve with the retina. The
benefit of this implant is that, it works very well for those persons who are suffering from retinal
diseases and even works beyond the affected retina. The processing units of these artificial
retinas are composed with TFT’s and retina matrix molecules.
Epiretinal implants sit in the inner surface of the retina. They are advantageous as they
bypass a large portion of the retina. It could provide visual perception to individuals with retinal
diseases extending beyond the photoreceptor layer. The implants receive input from a camera
and processing unit (E.g. on glasses). Electrodes from the implants electrically stimulate the
ganglion cells and axons at the start of the optic nerve.
Fig 3.2: Epiretinal implant

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Fig 3.3: Eye structure after implant
3.1.2 Subretinal Implant
This is just the opposite of Epiretinal implant, where the retina is placed at the outer
surface. It is a one of a kind implant because here, the implanted retina has to work just like the
normal retina by depending upon the process conceived middle and the inner eye. This device
holds 1000s of light sensitive micro photodiodes, which start functioning through the
stimulation protruded by the electrode. In order to activate these retinas, all you have to do is
look at a light reflecting object and the retina will start doing its job.
Subretinal implants sit on the outer surface of the retina, between the photoreceptor layer
and the retinal pigment epithelium, directly stimulating retinal cells and relying on the normal
processing of the inner and middle retinal layers. It has a simpler design .It replace damaged
rods and cones by Silicon plate carrying 1000s of light-sensitive micro photodiodes each with a
stimulation electrode. Light from image activates the micro photodiodes, the electrodes inject
currents into the neural cells. Among the above implant methods, the epiretinal implant has
features that the image resolution can be high because the stimulus signal can be directly

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conducted to neuron cells and that living retinas are not seriously damaged. Trade of for the two
types is that, Subretinal Implant uses the entire retina (except the rods/cones). Epiretinal Implant
does not; it must replace the function of entire retina and convert light to neural code. But the
input to the Epiretinal Implant is more easily controlled (external camera).
Fig 3.4: Subretinal implant

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4. ARTIFICIAL RETINAUSING THIN FILM TRANSISTORS
4.1 Operation
Artificial Retina using Thin-Film Transistors (TFTs) is fabricated on transparent and
flexible substrates; it uses the same fabrication processes as conventional poly-Si TFTs and
encapsulated using SiO2, in order to perform in corrosive environments. Although the artificial
retina is fabricated on the glass substrate here to confirm the elementary functions, it can be
fabricated on the plastic substrate. The artificial retina using TFTs is shown in Figure 4.1
Fig 4.1: Artificial retina using TFTs
The retina array includes matrix-like multiple retina pixels. Although large contact pads are
located for fundamental evaluation, a principal part is 27 300 cm2, which corresponds to 154
ppi. The retina pixel consists of a photo transistor, current mirror, and load resistance. The photo
transistor is optimized to achieve high efficiency, and the current mirror and load resistance are
designed by considering the transistor characteristic of TFTs. The photosensitivity of the
reverse-biased p/i/n poly-Si phototransistor is 150 pA at 1000 lx for white light and proper

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values for all visible color lights .The field effect mobility and the threshold voltage of the n-
type and p-type poly-Si TFT were 93 cm2 V -1s-1 , 3.6 V, 47 cm2 V -1s-1 and -2.9 V,
respectively. First, the photo transistors perceive the irradiated light (Lphoto) and induce the
photo-induced current (Iphoto). Next, the current mirror amplifies I photo to the mirror current
(Imirror). Finally, the load resistance converts Imirror to the output voltage (Vout).
Consequently, the retina pixels irradiated with bright light output a higher Vout, whereas the
retina pixels irradiated with darker light output a lower Vout.
Fig 4.2: working of the implant
Electronic photo devices and circuits are integrated on the artificial retina, which is
implanted on the inside surface of the living retina at the back part of the human eyeballs. Since
the irradiated light comes from one side of the artificial retina and the stimulus signal goes out
of the other side, the transparent substrate is preferable. The concept model of the artificial retina
fabricated on a transparent and flexible substrate and implanted using epiretinal implant is
shown in Figure 4.2.

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4.2 Fabrication of thin film phototransistors
Low temperature poly-Si TFTs have been developed in order to fabricate active matrix
LCDs with integrated drivers on large glass substrates. For integrated drivers, CMOS
configurations are indispensable. Self-aligned TFTs are also required because of their small
parasitic capacitance which can realize high speed operation. Since ion implantation is one of
the key factors in fabricating such as TFTs and CMOS configurations, several non-mass-
separated I/D techniques are proposed. These techniques, however, are not suitable for
conventional poly-Si TFT processes and cannot be applied to large glass substrates, especially
those over 300 mm square.
4.2.1 ION Doping Techniques
Figure 4.3 shows a schematic diagram of the new I/D system which is one of the non-
mass separated implanters. 5 percent PH3 or 5 percent B2H6 diluted by hydrogen is used for
the doping gas and an RF plasma is formed in the chamber by RF power with a frequency of
13.56 MHz Ions from discharged gas are accelerated by an extraction electrode and an
acceleration electrode and are implanted into the substrate. Main features of this system are:
1) A large beam area (over 300 mm square)
2) A high accelerating voltage (maximum: 110 KeV)
Fig 4.3 ion doping system

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With this system, impurities can be implanted over the entire 300 mm square substrate
with a maximum accelerating voltage of over 110 KeV which is sufficient for implanting
impurities through the 150nm SiO2 gate insulator. On the other hand, the conventional non-
mass-separated I/D techniques are severely limited in beam area, which is about 150 mm in
diameter. Furthermore, they are incapable of implanting impurities through the gate insulator
since the accelerating voltages are less than 10 KeV. Consequently, the gate insulator must be
removed prior to implantation, which can result in failure from surface contamination or
breakdown between gate electrodes and source and drain regions.
4.2.1.1 Self Aligned structure and TFT characteristics
S/A TFTs and non-S/A TFTs with 25 nm thick as-deposited channel poly-Si r31 were
fabricated on the glass substrates, and the new I/D technique was used to achieve a self-aligned
structure. Schematic cross sectional views of a S/A TFT and a non-S/A TFT are illustrated in
Figure 4.4(a) and 4.4(b), respectively. Since the parasitic capacitance between the gate
electrode, source and drain regions of a S/A TFT is estimated to be only about 2 -5 percent that
of a non-S/A TFT, high speed operation can be expected.
The characteristics of S/A TFTs are compared with those of non-S/A TFTs. The
comparisons in the n-channel and the p-channel TFTs are shown in Figure 4.5 and Figure 4.6,
respectively. In these experiments, it is found that the characteristics of S/A and non-S/A TFTs
are similar, and mobility of the n-channel TFTs are around 5 cm2/V-sec while those of the p-
channel TFTs are around 3 cm2/V.sec. It should be noted that no degradation can be observed
as a result of using the new I/D technique.

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Fig 4.4 Cross sectional views of (a) a self-aligned (S/A) TFT and
(b) a non-self-aligned (non-S/A) TFT
Fig 4.5: N-channel poly-si TFT
characteristics of self-aligned and non-self-
aligned structure
Fig 4.6: p-channel poly-si TFT
characteristics of self-aligned and non-self-
aligned structure

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4.2.2 New MaskingTechnique and CMOS Process
A non-resist-masking process, however, is required when the CMOS configuration is
fabricated using the new I/D technique, since the temperature of the substrate reaches about 300
degree Celsius due to the high accelerating voltage. In order to solve this problem, a new
masking technique is also proposed. In this process, n-channel gate electrodes and p-channel
gate electrodes are formed separately in a sequential manner. In the process sequence for the
CMOS configuration, An SiO2 buffer layer is deposited on the glass .substrate to protect TFTs
from contamination from components of the glass. Then, pad poly-Si patterns are formed for
source and drain regions, which are made of a 150 nm poly-Silicon film. A 25 nm channel poly-
Si layer is deposited by low pressure chemical vapor deposition (LPCVD) at 600 degree Celsius.
Thinner poly-Si film gives better electrical characteristics such as high ON current, low OFF
current and low photo-current. After patterning of the channel poly-Si layer, a 150 nm SiO2
gate insulator is deposited by electron cyclotron resonance chemical vapor deposition (ECR-
CVD) at 100 degree Celsius in a vacuum. Then, a Cr film is deposited at 180 degree Celsius.
First, only p-channel gate electrodes are formed. The next step is to form source and drain
regions of p-channel TFTs by the new I/D technique. Boron ions are implanted through the gate
insulator with a dose of 5 x 1015 cm-2 at energy of 80 keV. N-channel gate electrodes are also
formed and phosphorus ions are implanted with a dose of 3x1015 cm-2 at energy of 110 keV
by the new I/D technique Impurities are activated by a XeCl excimer laser.
4.3 Device Characterization of p/i/n Thin-Film Phototransistors for Photo
Sensor Applications
Thin-Film photo devices are promising for photo sensor applications, such as ambient
light sensors, image Scanners, artificial retinas etc. Here thin-film photo devices are integrated
with low-temperature poly-Si thin-film transistors. The p/i/n TFPT is shown in Figure. 4.7. The
p/i/n TFPT is fabricated on a glass substrate using the same fabrication processes as TFTs which
were discussed earlier. First, an amorphous-Si film is deposited using low-pressure chemical-
vapor deposition of Si2H6 and crystallized using XeCl excimer laser to form a poly-Si film,

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whose thickness is 50 nm. Next, a SiO2 film is deposited using plasma-enhanced chemical-
vapor deposition of tetraethylorthosilicate to form a control-insulator film, whose thickness is
75 nm. A metal film is deposited and patterned to form a control electrode. Afterward,
phosphorous ions are implanted through a photo resist mask at 55 keV with a dose of 2 1015
cm-2 to form an n-type anode region, and boron ions are also implanted through a photo resist
mask at 25 keV with a dose of 1.5 1015 cm-2 to form a p-type cathode region. Finally, water-
vapor heat treatment is performed at 400 degree Celsius for 1 h to thermally activate the dopant
ions and simultaneously improve the poly-Si film, control-insulator film, and their interfaces.
The p/i/n TFPT must be illuminated from the backside of the glass substrate because the control
electrode is usually formed using an opaque metal film. Therefore, the other LTPS TFTs are
also illuminated when the p/i/n TFPT is integrated with them. However, the photo leakage
current in the LTPS TFTs can be negligible by appropriately designing them, i.e., the gate width
should be wide for the p/i/n TFT, whereas narrow for the LTPS TFTs.
Fig 4.7: p/i/n TFPT
4.3.1 Electro optical Measurement
The electro optical measurement is shown in Figure.4.8. The p/i/n TFPT is located on a
rubber spacer in a shield chamber and connected via a manual prober to a voltage source and
ampere meter. White light from a halogen lamp is formed to be parallel through a convex lens,
reflected by a triangular prism and irradiated through the glass substrates to the back surfaces
of the p/i/n TFPT. Although the light from a halogen lamp includes the light from 400 to 750
nm with a peak around 600 nm and is therefore reddish despite a built-in infrared filter, the
conclusion in this research is generally correct. The electric current between the n- and p-type
regions is detected with changing the applied voltage and irradiated illuminance. The electro

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optical characteristic is shown in Figure.4.9. First, it is found that the dark current, Idetect when
Lphoto = 0, is sufficiently small except when Vctrl and Vapply are large.
Fig 4.8: Electro optical measurement
The reason is because the p/i and i/n junctions steadily endure the reverse bias. This
characteristic is useful to improve the S/N ratio of the p/i/n TFPT for photo sensor applications.
Next, Idetect increases as Lphoto increases. This characteristic is also useful to acquire
fundamental detectability. Finally, Idetect becomes maximal when Vctrl Vapply. This reason is
discussed below:
When Vctrl < 0, since Vctrl < in the entire intrinsic region, a hole channel is induced,
and a pseudo p/n junction appears near the anode region. Since a depletion layer is narrowly
formed there, where carrier generation occurs due to light irradiation, Idetect is small. When
Vctrl is approximately equal to 0, although a hole channel is still induced, since Vctrl is
approximately equal to near the cathode region, the hole density is low there, which is similar
to the pinchoff phenomena in the saturation region of MOSFETs. Since another depletion layer
is widely formed there, Idetect is large. When 0 <Vctrl < Vapply, since Vctrl > on the side of
the cathode region, an electron channel is induced there. At the same time, since Vctrl < on the
side of the anode region, a hole channel is still induced there. Since the depletion layer is widely
formed between the electron and hole channels, Idetect is large.

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When Vctrl is approximately equal to Vapply, although an electron channel is further
induced, since Vctrl is approximately near the anode region, the electron density is low there.
Since the depletion layer is widely formed there, Idetect is large. Since generated carriers are
transported through the electron channel with high conductance instead of the hole channel,
Idetect becomes maximal. When Vapply < Vctrl, since Vctrl > in the entire intrinsic region, an
electron channel is further induced, and a pseudo p/n junction appears near the cathode region.
Since another depletion layer is narrowly formed there, Idetect is small. The anomalous
increases of Idetect when Vctrl and Vapply are large may be caused by the impact ionization
and avalanche breakdown in the depletion layers. The asymmetric behavior, for example,
comparing Vctrl = 2 and + 5 V for Vapply =3 V, may be occasioned by the difference of electric
field because the hole density when Vctrl = 2 V and donor density.
Fig 4.9 Electro optical characteristics

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5. WIRELESS POWER SUPPLY USING INDUCTIVE COUPLING
5.1 Introduction
Many implanted electrical power to function; be it in the form of an implanted battery or via
wireless power transmission. It is often advantageous to develop methods for wireless power
transmission to an implant located deep inside the body as replacement of batteries which
requires additional surgery is undesirable. An example of this is a retinal prosthesis. A retinal
prosthesis can create a sense of vision by electrically stimulating intact neural cells in the visual
system of the blind. Such prosthesis will require continuous power transmission in order to
achieve real-time moving images. Efficient transmission of power is a performance limiting
factor for successful implementation of the prosthesis. We estimate that a high density electrode
Array with more than 1000 electrodes will consume about 45 mW of power. This includes 25
mW to operate the electronics on the chip and an additional 20 mW for neuronal stimulation
with a 3.3 V stimulation threshold. The latter is calculated based on 64 simultaneously operating
electrodes each requiring a maximum of 0.3 mW at 60 Hz image refresh rate.
Fig 5.1: shows location of coil

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Inductive coupling of magnetic field is an efficient way for transmitting energy through
tissue. This is because electrical energy can be easily converted to magnetic energy and back
using conductive coils. Traditionally, a pair of inductive coils; a primary (transmit) and a
secondary (receive) coils, are used. The secondary coil can be located within the eye and the
primary coil external to the eye. However, several problems will arise if we implement this
method. The first problem is difficulty in placing a large receive coil inside the eye. This will
require complicated surgical procedure, often a major challenge in implementing a wireless
power solution. The other problems we face are large separation between the coils and the
constant relative motion between the primary and secondary coils. The latter problems result in
reduction in power transfer to the device. In order to overcome these problems we propose the
use of an intermediate link between the primary and secondary coil as shown in Figure 3.1. In
this figure we show the possible locations for one-pair coils and a two pair coils system which
consists of an additional intermediate link made out of a pair of serially connected coils. In this
method, the secondary coil is located under the sclera (eye wall) and is connected to the
implanted device via electrical wires which are embedded under the wall of the eye. By placing
these components under the sclera, we avoid having a permanent wire breaching through the
eye wall. The transmit coil is placed on the skin of the head at an inconspicuous location, for
example at the back of the ear. The intermediate coils are positioned with one end on the sclera
over the receive coil and the other end under the skin beneath the transmit coil. The advantage
of this method is immunity to variation in coupling due to rapid movements of the eye as relative
motion between adjacent coils is restricted. It also has the potential to increase the power transfer
efficiency compared to a one-pair coil system.
5.2 Working
The wireless power supply using inductive coupling is shown in Figure 3.2.The right graph in
Figure 3.2. is a measured stability of the supply voltage. This system includes a power
transmitter, power receiver, Diode Bridge, and Zener diodes. The power transmitter consists of
an ac voltage source and induction coil. The Vpp of the ac voltage source is 10 V, and the
frequency is 34 kHz, which is a resonance frequency of this system. The material of the
induction coil is an enameled copper wire, the diameter is 1.8 cm, and the winding number is

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370 times. The power receiver also consists of an induction coil, which is the same as the power
transmitter and located face to face. The diode bridge rectifies the ac voltage to the dc voltage,
and the Zener diodes regulate the voltage value. The Diode Bridge and Zener diodes are discrete
devices and encapsulated in epoxy resin. Although the current system should be downsized and
bio-compatibility has to be inspected, the supply system is in principle very simple to implant it
into human eyeballs. As a result, the generated power is not so stable as shown in Figure 3.2.,
which may be because the artificial retina is fabricated on a insulator substrates, has little
parasitic capacitance, and is subject to the influence of noise. Therefore, it is necessary to
confirm whether the artificial retina can be correctly operated even using the unstable power
source.
Fig 5.2: wireless power supply using inductive coupling

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6. SUMMARY
The artificial retina using poly-Si TFTs and wireless power supply using inductive coupling are
located in a light-shield chamber, and Vout in each retina pixel is probed by a manual prober
and voltage meter. White light from a metal halide lamp is diaphragmmed by a pinhole slit,
focused through a convex lens, reflected by a triangular prism and irradiated through the glass
substrate to the back surfaces of the artificial retina on a rubber spacer. The real image of the
pinhole slit is reproduced on the back surface. Figure. 6.1 shows the detected result of irradiated
light. It is confirmed that the Lphoto distribution can be reproduced as the Vout distribution
owing to the parameter optimization of the wireless power supply system even if it is driven
using the unstable power source, although shape distortion is slightly observed, which is due to
the misalignment of the optical system or characteristic variation of TFTs.
Fig 6.1: Result of irradiated light

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It was found that the Lphoto profile can be correctly detected as the Vout profile even if it is
driven using unstable power source generated by inductive coupling, Diode Bridge, and Zener
diodes. In order to apply the artificial retina to an actual artificial internal organ, we should
further develop a pulse signal generator appropriate as photo recepter cells, consider the
interface between the stimulus electrodes and neuron cells, investigate the dependence of Vout
on Lphoto, which realizes grayscale sensing, etc. However, the above result observed, shows
the feasibility to implant the artificial retina into human eyeballs.

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7. CONCLUSION
The artificial retina using the TFPDs and poly-silicon TFTs was proposed, which is an
improvement of the SOP where the in- pixel and pixel to pixel operations are executed using
the poly-si TFTs .The device characteristics of the TFPD and poly-silicon TFTs were
measured, and they were modeled into the circuit simulation. The circuit configurations of the
retina pixels and retina array were invented with some improvements, and they were designed.
It was confirmed that the artificial retina can operate and achieve the edge enhancement. The
behavior tolerance against the characteristics deviations of the poly-silicon TFTs and photo-
sensitivity control of the edge enhancement were evaluated.

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8. REFRENCES
 Yuta Miura, Tomohisa Hachida, and Mutsumi Kimura, Member, IEEE, “Artificial
Retina Using Thin-Film Transistors Driven by Wireless Power Supply” IEEE
SENSORS JOURNAL, VOL. 11, NO. 7, JULY 2011.
 M. Kimura, Y. Miura, T. Ogura, S. Ohno, T. Hachida, Y. Nishizaki, T. Yamashita, and
T.Shima, “Device characterization of p/i/n thin-film phototransistor for photosensor
applications,” IEEE Electron Device Lett., vol. 31, no. 9, pp. 984–986, 2010.
 Satoshi Inoue, Minoru Matsuo, Tsutomu Hashizume, Hideto Ishiguro, Takashi
Nakazawa, and Hiroyuki Ohshima, “LOW TEMPERATURE CMOS SELF-
ALIQNED POLY-Si TFTS AND CIRCUIT SCHEME UTILIZING NEW ION
DOPING AND MASKING TECHNIQUE “
 David C. Ng, Chris E. Williams, Penny J. Allen, Shun Bai, Clive S. Boyd, Hamish
Meffin, Mark E. Halpern, and Efstratios Skafidas “wireless power delivery for retinal
prosthesis” , 33rd Annual International Conference of the IEEE EMBS Boston,
Massachusetts USA, August 30 - September 3, 2011
 T. Tokuda, K. Hiyama, S. Sawamura, K. Sasagawa, Y. Terasawa, K. Nishida,
Y.Kitaguchi, T. Fujikado, Y. Tano, and J. Ohta, “CMOS-based multichip networked
flexible retinal stimulator designed for image-based retinal prosthesis,” IEEE Trans.
Electron Devices, vol. 56, no. 11, pp. 2577–2585, 2009 .

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