ARTIFICIAL RETINA USING THIN FILMTRANSISTOR TECHNOLOGY

1.2 Retinal Implantation:

A retinal implant is a biomedical implant technology currently being devel-

oped 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. 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.

Epiretinal Implant :

Epiretinal implants sit in the inner surface of the retina. They are advanta-

geous as they bypass a large portion of the retina. It could provide visual

perception to individuals with retinal diseases extending beyond the pho-

toreceptor layer. The implants receive input from a camera and processing

7

unit (E.g. on glasses). Electrodes from the implants electrically stimulate

the ganglion cells and axons at the start of the optic nerve.

Subretinal Implant :

Subretinal implants sit on the outer surface of the retina, between the pho-

toreceptor layer and the retinal pigment epithelium, directly stimulating reti-

nal 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 stim-

ulation 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

conducted to neuron cells and that living retinas are not seriously damaged.

Trade off 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).

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Chapter 2

ARTIFICIAL RETINAUSING THIN FILMTRANSISTORS

2.1 Operation

Artificial Retina using Thin-Film Transistors (TFTs) is fabricated on trans-

parent and flexible substrates; it uses the same fabrication processes as con-

ventional 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

2.1.

9

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 transis-

tor 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 values for all visible color lights.

The field effect mobility and the threshold voltage of the n-type and p-type

10

poly-Si TFT were 93 cm2 V -1s-1 , 3.6 V, 47 cm2 V -1s-1 and -2.9 V, re-

spectively. First, the photo transistors perceive the irradiated light (Lphoto)

and induce the photo-induced current (Iphoto). Next, the current mirror

amplifies Iphoto to the mirror current (Imirror). Finally, the load resistance

converts Imirror to the output voltage (Vout). Consequently, the retina pix-

els irradiated with bright light output a higher Vout, whereas the retina

pixels irradiated with darker light output a lower Vout.

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 2.2.

11

2.2 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 in-

tegrated drivers, CMOS configurations are indispensable. Self-aligned TFTs

are also required because of their small parasitic capacitance which can re-

alize 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

12

large glass substrates, especially those over 300 mm square.

2.2.1 ION Doping Techniques

Figure 2.3 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)

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. Further-

more, they are incapable of implanting impurities through the gate insulator

13

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.

2.2.1.1 Self Aligned structure and TFT charecteristics

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 2.4(a) and 2.4(b),

respectively. Since the parasitic capacitance between the gate electrode and

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.

14

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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 2.5 and Figure 2.6, 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.

15

2.2.2 New Masking technique and CMOS Process

A non-resist-masking process, however, is required when the CMOS config-

uration is fabricated using the new I/D technique, since the temperature of

the substrate reaches about 300oC 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-Si film. A 25 nm

channel poly-Si layer is deposited by low pressure chemical vapor deposition

(LPCVD) at 600 oC. Thinner poly-Si film gives better electrical characteris-

tics 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 oC in a vacuum. Then, a Cr film is deposited at 180 oC. 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.

16

2.3 Device characterization of p/i/n Thin- film

phototransistors for photosensor 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 tran-

sistors. The p/i/n TFPT is shown in Figure. 2.7. 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 us-

ing low-pressure chemical-vapor deposition of Si2H6 and crystallized using

XeCl excimer laser to form a poly-Si film, 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 oC 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

17

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.

2.3.1 Electrooptical Measurement

The electrooptical measurement is shown in Figure.2.8. 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 triangu-

lar 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 electrooptical characteristic is shown in Figure.2.9. First, it is found

that the dark current, Idetect when Lphoto = 0, is sufficiently small except

18

when Vctrl and Vapply are large.

measurement .png measurement .pdf measurement .jpg measurement .mpsmeasurement .jpeg measurement .jbig2 measurement .jb2 measurement

.PNG measurement .PDF measurement .JPG measurement .JPEGmeasurement .JBIG2 measurement .JB2

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 in-

creases. This characteristic is also useful to acquire fundamental detectabil-

ity. 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 sim-

ilar 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

19

is widely formed between the electron and hole channels, Idetect is large.

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.

20

Chapter 3

WIRELESS POWER SUPPLY

USING INDUCTIVE

COUPLING

3.1 Introduction

Many implanted electrical power to function; be it in the form of an im-

planted 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 reti-

nal prosthesis can create a sense of vision by electrically stimulating intact

neural cells in the visual system of the blind. Such prosthesis will require

21

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 addi-

tional 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.

Inductive coupling of magnetic field is an efficient way for transmitting en-

ergy 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 ma-

jor challenge in implementing a wireless power solution. The other problems

22

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. In order to overcome these prob-

lems 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 trans-

mit 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.

23

3.2 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 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 sub-

strates, 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.

24

25

Chapter 4

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. 4.1

shows the detected result of irradiated light. It is confirmed that the Lphoto

distribution can be reproduced as the Vout distribution owing to the parame-

ter 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 vari-

ation of TFTs.

26

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 induc-

tive 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 photorecepter cells, consider the interface be-

tween 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 hu-

man eyeballs.

27

Chapter 5

REFERENCES

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 De-

vice Lett., vol. 31, no. 9, pp. 984986, 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 www.ieeexplore.ieee.org.

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David C. Ng, Chris E. Williams, Penny J. Allen, Shun Bai, Clive S.Boyd, Hamish Meffin, Mark E. Halpern, and Efstratios Skafidas wire-

less 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

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25772585, 2009.

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