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8/19/2019 Artificial Retina Using TFT
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Artificial retina using thin film of photo transistors
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 conducted
to neuron cells and that living retinas are not seriously damaged. Trade of for the
two types is that( 'ubretinal #mplant uses the entire retina %e$cept the rods+cones&.
!piretinal #mplant does not, it must replace the function of entire retina and
convert light to neural code. But the input to the !piretinal #mplant is more easily
controlled %e$ternal camera&.
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Artificial retina using thin film of photo transistors
The retina array includes matri$"li2e multiple retina pi$els. Although
large contact pads are located for fundamental evaluation( a principal part is 3 4))
cm( which corresponds to 15 ppi. The retina pi$el consists of a photo transistor(
current mirror( and load resistance. The photo transis" tor is optimi7ed to achieve
high efciency( and the current mirror and load resistance are designed by
considering the transistor characteristic of TTs. The photosensitivity of the
reverse"biased p+i+n poly"'i phototransistor is 15) pA at 1))) l$ for white light and
proper values for all visible color lights. The field efect mobility and the threshold
voltage of the n"type and p"type
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Artificial retina using thin film of photo transistors
poly"'i TT were 94 cm 8 "1s"1 ( 4.6 8(
3 cm 8 "1s"1 and ".9 8( re"
spectively. irst( the photo transistors perceive the irradiated light %Lphoto& and
induce the photo"induced current %#photo&. e$t( the current mirror amplifies
#photo to the mirror current %#mirror&. inally( the load resistance converts #mirror
to the output voltage %8out&. :onse;uently( the retina pi$" els irradiated with bright
light output a higher 8out( whereas the retina pi$els irradiated with dar2er light
output a lower 8out.
!lectronic photo devices and circuits are integrated on the artificial retina(
which is implanted on the inside surface of the living retina at the bac2 part of the
human eyeballs. 'ince the irradiated light comes from one side of the artificial
retina and the stimulus signal goes out of the other side( the transparent substrate
is preferable. The concept model of the artificial retina fabricated on a transparent
and /e$ible substrate and implanted using epiretinal implant is shown in igure ..
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Artificial retina using thin film of photo transistors
2.2 Fabri"ation o# thin #ilm phototran$i$tor$
Low temperature poly"'i TTs have been developed in order to fabricate
active matri$ L:Ds with integrated drivers on large glass substrates. or in"
tegrated drivers( :
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Artificial retina using thin film of photo transistors
large glass substrates(
especially those over 4)) mm s;uare.
2.2.1 I!N %opin& Te"hni'ue$
igure .4 shows a schematic diagram of the new #+D system which is one
of the non"mass"separated implanters. 5 percent =>4 or 5 percent B>6 diluted by
hydrogen is used for the doping gas and an - plasma is formed
in the chamber by - power with a fre;uency of 14.56 7
#ons from discharged gas are accelerated by an e$traction electrode and an
acceleration electrode and are implanted into the substrate.
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Artificial retina using thin film of photo transistors
Department of Electronics and Communication Engineering, ALIET
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Artificial retina using thin film of photo transistors
since the accelerating voltages are less than
1) @e8. :onse;uently( the gate
insulator must be removed prior to implantation( which can result in failure from
surface contamination or brea2down between gate electrodes and source and drain
regions.
2.2.1.1 Sel# Ali&ne( $tru"ture an( TFT "hare"teri$ti"$
'+A TTs and non"'+A TTs with 5 nm thic2 as"deposited channel poly"'i
r41 were fabricated on the glass substrates( and the new #+D techni;ue was used to
achieve a self"aligned structure. 'chematic cross sectional views of a '+A TT and
a non"'+A TT are illustrated in igure .%a& and .%b&( respectively. 'ince the
parasitic capacitance between the gate electrode and source and drain regions of a
'+A TT is estimated to be only about "5 percent that of a non"'+A TT( high
speed operation can be e$pected.
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Artificial retina using thin film of photo transistors
The characteristics of '+A TTs are compared with those of non"'+A
TTs. The comparisons in the n"channel and the p"channel TTs are shown in
igure .5 and igure .6( respectively. #n these e$periments( it is found that the
characteristics of '+A and non"'+A TTs are similar( and mobility of the n"channel
TTs are around 5 cm+8"sec while those of the p"channel TTs are around 4
cm+8.sec. #t should be noted that no degradation can be observed as a result of
using the new #+D techni;ue.
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Artificial retina using thin film of photo transistors
., %e-i"e "hara"teriation o# p/i/n Thin0 #ilm
phototran$i$tor$ #or photo$en$or appli"ation$
Thin"ilm photo devices are promising for photo sensor applications( such as
ambient light sensors( image 'canners( artificial retinas etc. >ere thin"film photo
devices are integrated with low"temperature poly"'i thin"film tran" sistors. The
p+i+n T=T is shown in igure. .3. The p+i+n T=T is fabricated on a glass
substrate using the same fabrication processes as TTs which were discussed
earlier. irst( an amorphous"'i film is deposited us" ing low"pressure chemical"
vapor deposition of 'i>6 and crystalli7ed using e:l e$cimer laser to form a
poly"'i film( whose thic2ness is 5) nm. e$t( a 'i0 film is deposited using
plasma"enhanced chemical"vapor deposition of tetraethylorthosilicate to form a
control"insulator film( whose thic2ness is 35 nm. A metal film is deposited and
patterned to form a control electrode. Afterward( phosphorous ions are implanted
through a photo resist mas2 at 55 2e8 with a dose of 1)15 cm" to form an n"type
anode region( and boron ions are also implanted through a photo resist mas2 at 5
2e8 with a dose of 1.5 1)15 cm" to form a p"type cathode region. inally( water"
vapor heat treatment is performed at )) o: for 1 h to thermally activate the
dopant ions and simultaneously improve the poly"'i film( control"insulator film(
and their interfaces.The p+i+n T=T must be illuminated from the bac2side of
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Artificial retina using thin film of photo transistors
the glass substrate because the control
electrode is usually formed using an
opa;ue metal film. Therefore( the other LT=' TTs are also illuminated when the
p+i+n T=T is integrated with them. >owever( the photo lea2age current in the
LT=' TTs can be negligible by appropriately designing them( i.e.( the gate width
should be wide for the p+i+n TT( whereas narrow for the LT=' TTs.
2.,.1 Ele"troopti"al ea$urement
The electrooptical measurement is shown in igure... The p+i+n T=T is
located on a rubber spacer in a shield chamber and connected via a manual prober
to a voltage source and ampere meter. hite light from a halogen lamp is formed
to be parallel through a conve$ lens( re/ected by a triangu" lar prism and irradiated
through the glass substrates to the bac2 surfaces of the p+i+n T=T. Although the
light from a halogen lamp includes the light from )) to 35) nm with a pea2
around 6)) 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 electrooptical characteristic is shown in igure..9. irst( it is found that the
dar2 current( #detect when Lphoto C )( is sufciently small e$cept
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Artificial retina using thin film of photo transistors
w
hen 8ctrl and 8apply are large.
measurement .png measurement .pdf measurement .*pg measurement .mps
measurement .*peg measurement .*big measurement .*b measurement
.= measurement .=D measurement .E= measurement .E=!
measurement .EB# measurement .EB
The reason is because the p+i and i+n *unctions steadily endure the reverse
bias. This characteristic is useful to improve the '+ ratio of the p+i+n T=T for
photo sensor applications. e$t( #detect increases as Lphoto in" creases. This
characteristic is also useful to ac;uire fundamental detectabil" ity. inally( #detect
becomes ma$imal when 8ctrl 8apply. This reason is
discussed below?
hen 8ctrl F )( since 8ctrl F in the entire intrinsic region( a hole channel
is induced( and a pseudo p+n *unction appears near the anode region. 'ince a
depletion layer is narrowly formed there( where carrier generation occurs due to
light irradiation( #detect is small. hen 8ctrl is appro$imately e;ual to )( although
a hole channel is still induced( since 8ctrl is appro$imately e;ual to near the
cathode region( the hole density is low there( which is sim" ilar to the pinchof
phenomena in the saturation region of
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Artificial retina using thin film of photo transistors
is widely formed between the electron and hole channels( #detect is large.
hen 8ctrl is appro$imately e;ual to 8apply( although an electron channel is
further induced( since 8ctrl is appro$imately near the anode region( the electron
density is low there. 'ince the depletion layer is widely formed there( #detect is
large. 'ince generated carriers are transported through the electron channel with
high conductance instead of the hole channel( #detect becomes ma$imal. hen
8apply F 8ctrl( since 8ctrl G in the entire intrinsic region( an electron channel is
further induced( and a pseudo p+n *unction appears near the cathode region. 'ince
another depletion layer is narrowly formed there( #detect is small. The anomalous
increases of #detect when 8ctrl and 8apply are large may be caused by the impact
ioni7ation and avalanche brea2down in the depletion layers. The asymmetric
behavior( for e$ample( comparing 8ctrl C and H 5 8 for 8apply C4 8( may be
occasioned by the diference of electric field because the hole density when 8ctrl C
8 and donor density.
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Artificial retina using thin film of photo transistors
continuous power transmission in order to achieve real"time moving images.
!fcient transmission of power is a performance limiting factor for successful
implementation of the prosthesis. e estimate that a high density electrode array
with more than 1))) electrodes will consume about 5 m of power. This
includes 5 m to operate the electronics on the chip and an addi" tional ) m
for neuronal stimulation with a 4.4 8 stimulation threshold. The latter is calculated
based on 6 simultaneously operating electrodes each re;uiring a ma$imum of ).4
m at 6) >7 image refresh rate. #nductive coupling of magnetic field is an efcient
way for transmitting energy through tissue. This is because electrical energy can be
easily converted to magnetic energy and bac2 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 e$ternal to the eye. >owever( several problems will arise if we
implement this method. The first problem is difficulty in placing a large receive
coil inside the eye. This will re;uire complicated surgical procedure( often a ma" *or
challenge in implementing a wireless power solution. The other problems
we face are large separation between the coils and the constant relative mo"
tion between the primary and secondary coils. The latter problems result in
reduction in power transfer to the device. #n order to overcome these prob" lems we
propose the use of an intermediate lin2 between the primary and secondary coil as
shown in igure 4.1. #n this figure we show the possible locations for one"pair coils
and a two pair coils system which consists of an additional intermediate lin2 made
out of a pair of serially connected coils. #n this method( the secondary coil is
located under the sclera %eye wall& and is connected to the implanted device via
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Artificial retina using thin film of photo transistors
electrical wires which are embedded under the wall of the eye. By placing these
components under the sclera( we avoid having a permanent wire breaching through
the eye wall. The trans" mit coil is placed on the s2in of the head at an
inconspicuous location( for e$ample at the bac2 of the ear. The intermediate coils
are positioned with one end on the sclera over the receive coil and the other end
under the s2in 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 ad*acent coils is restricted. #t also has the potential to increase the
power transfer efciency compared to a one"pair coil system.
,.2 or*in&
The wireless power supply using inductive coupling is shown in igure 4..
The right graph in igure 4.. is a measured stability of the supply voltage. This
system includes a power transmitter( power receiver( Diode Bridge( and Iener
diodes. The power transmitter consists of an ac voltage source and induction
coil. The 8pp of the ac voltage source is 1) 8( and the fre;uency is 4 2>7(
which is a resonance fre;uency of this system. The material of the induction
coil is an enameled copper wire( the diameter is 1. cm( and the winding
number is 43) 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 Iener diodes regulate
the voltage value. The Diode Bridge and Iener diodes are discrete devices and
encapsulated in epo$y resin. Although the current system should be downsi7ed
and bio"compatibility has to be inspected( the supply system is in principle very
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Artificial retina using thin film of photo transistors
simple to implant it into human eyeballs. As a result( the generated power is
not so stable as shown in igure 4..( which may be because the artificial retina
is fabricated on a insulator sub" strates( has little parasitic capacitance( and is
sub*ect to the in/uence of noise. Therefore( it is necessary to confirm whether
the artificial retina can be correctly operated even using the unstable power
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Artificial retina using thin film of photo transistors
Chapter 4
SUAR
The artificial retina using poly"'i TTs and wireless power supply using
inductive coupling are located in a light"shield chamber( and 8out in each retina
pi$el is probed by a manual prober and voltage meter. hite light from a metal
halide lamp is diaphragmmed by a pinhole slit( focused through a conve$ lens(
re/ected by a triangular prism and irradiated through the glass substrate to the bac2
surfaces of the artificial retina on a rubber spacer. The real image of the pinhole slit
is reproduced on the bac2 surface. igure. .1 shows the detected result of
irradiated light. #t is confirmed that the Lphoto distribution can be reproduced as
the 8out distribution owing to the parame" ter optimi7ation 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 vari" ation of TTs.
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Artificial retina using thin film of photo transistors
#t was found that the Lphoto profile can be
correctly detected as the 8out
profile even if it is driven using unstable power source generated by induc" tive
coupling( Diode Bridge( and Iener diodes. #n order to apply the artificial retina to
an actual artificial internal organ( we should further develop a pulse signal generator
appropriate as photorecepter cells( consider the interface be" tween the stimulus
electrodes and neuron cells( investigate the dependence of 8out on Lphoto( which
reali7es grayscale sensing( etc. >owever( the above result observed( shows the
feasibility to implant the artificial retina into hu" man eyeballs.
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Artificial retina using thin film of photo transistors
Chapter 5
REFERENCES
J Kuta achida( and
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Department of Electronics and Communication Engineering, ALIET