Artificial Retina Using TFT

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&.

Department of Electronics and Communication Engineering, ALIET


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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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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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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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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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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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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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., %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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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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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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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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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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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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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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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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#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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Chapter 5

REFERENCES

J Kuta achida( and


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Artificial Retina Using TFT