ETRASECT’16

Artificial Retina Using Thin Transistor

Technology Driven by Wireless Power

SupplyPRESENTED BY

SHWETA KUMARIThird year (ece), JIET School of Engineering and Technology for Girls ,JIET Universe-342002

INTRODUCTION

HOW RETINA WORKS ?Normal vision is where light enters the eye and

falls on photosensitive cells that lie on the surface of the retina.

The "circuits" in the retina convert the light into a series of coded electrical signals or neural pulses, and pass them onto output cells called ganglion cells that transmit the coded pulses to the brain via the optic nerve at the back of the eye.

The brain understands the stream of coded neural pulses and translates it into meaningful images.

A common cause of blindness is when the retina is damaged by diseases that kill the photoreceptors, and/or destroy the circuits that create the coded neural pulses.

Blindness is caused by damage to clear structures in the eye that allow the light to pass through, the nerves within the eye, the Optic nerve and brain.

Various diseases of eye blindness are-

1. Retinitis Pigmentosa: It is a medical condition which may result in blured or no vision in the center of the vision field. Macular degeneration typically occur in older people. Genetic factors and smoking also play a role.

2. Macular Degeneration: This is also genetically related. Cones in macula region degenerate. Loss or damage of central vision. Common among aged people. People with RP experience a gradual decline in their vision because photoreceptor cells (rods and cones) die.

EYE BLINDNESS

NEED OF ARTIFICIAL RETINA Artificial Thin-Film Transistor Retina recovers the sight

sense for sight-handicapped people.

Electronic Photo devices and circuits substitutes deteriorated photoreceptor cells implanted inside the eyes.

We develop electronic artificial retina for restoration of sight to patients suffering from degenerative retinal diseases such as Retinitis Pigmentosa and Age-Related Muscular Degeneration.

• A retinal implant is a biomedical implant technology.• The first application of an implantable stimulator for vision

restoration was developed by Drs. Brindley and Lewis in 1968.• There are two types of retinal implants :- Epiretinal Implant- Sub retinal Implant

TYPES OF IMPLANTATION

Fig. Retinal Implantation

1. Epiretinal implant:

The epiretinal implants typically work in a two component system that consists of an extraocular and intraocular part.

The extraocular part contains-• Image sensor: It is responsible for catching visual images, • Artificial neural net: It can imitate the functions of different layers

of the retina• Transmitter.

The neural net transform the visual images control signals that the electrodes pick up and become stimulated.

The signal is passed on to the internal eye and then is picked up by the intraocular part of the implant.

The intraocular component consists of a receiver for the relayed signals and also power for the implant.

Radiofrequency links are being used currently and optical links may be used in the future.

There is an integrated circuitry component that decodes the signals for the electrodes and controls the stimulation array to produce action potentials in the upper ganglion cell layers to cause visual sensations.

Fig.1 Epiretinal implant

2. Sub retinal implant  Sub retinal implants sit on the outer surface of the retina.

Directly stimulates the retinal cells.

Replace damaged rods and cones by Silicon plate carrying 1000’s 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.

Fig. 2 Subretinal implant

FABRICATION OF ARTIFICIAL RETINA USING THIN FILM TRANISTOR TECHNOLOGY

1.New masking technology

2. Electro optical Measurement

1.NEW MASKING TECHNIQUE SiO2 buffer layer is deposited on the glass substrate.

Then, poly-Si patterns are formed for source and drain regions.

A 25 nm channel poly-Si layer is deposited by low pressure chemical vapor deposition (LPCVD) at 600 Degree Celsius.

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 of 80 keV. N-channel gate electrodes are also formed and phosphorus ions are implanted with a dose of 110 keV by the new I/D technique. Impurities are activated by a XeCl exciter laser.

2. ELECTRO-OPTICAL MEASUREMENTWhite light from a halogen lamp is reflected by a

triangular prism and irradiated through the glass substrates to the back surfaces of the 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.

Fig. 3 Setup of electro optical measurement

OPERATION• It uses the same fabrication processes as conventional

poly-Si TFTs and encapsulated using SiO2, in order to perform in corrosive environment.

• The retina array includes matrix-like multiple retina pixels.• 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 photo transistors perceive the irradiated light (Photo) and induce the photo-induced current (IPhoto).

• Next, the current mirror amplifies IPhoto to the mirror current ( I mirror).

• Finally, the load resistance converts I mirror to the output voltage (V out).

Consequently, the retina pixels irradiated with bright light output a higher V out , whereas the retina pixels irradiated with darker light output a lower V out.

• 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.

• 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.

FABRICATION OF THIN FILM PHOTOTRANSISTOR

ION-DOPING TECHNIQUEIons 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)

• 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 300oC due to the high accelerating voltage.

• 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, 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-Thinner poly-Si film gives better electrical characteristics such as high ON current, low OFF current and low photo-current.

NEW MASKING TECHNIQUE AND CMOS PROCESS

WIRELESS POWERLESS SUPPLY• Inductive coupling of magnetic field. Electrical energy can

be easily converted to magnetic energy and back using conductive coil.

• Traditionally, a pair of inductive coils are used. The secondary coil can be located within the eye and the primary coil external to the eye.

• 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. The transmit coil is placed at the back of the ear.

Fig. Wireless power supply using inductive coupling

WORKING• Wireless power supply using inductive coupling 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 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. Working of retinal implantation having wireless power supply

CONCLUSION• 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 diaphragm med 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 shows the detected result of irradiated light.

• The artificial retina is perfected using a wireless supply from the inductive coupling.

 

 

 

• There are just two main units needed for the wire supply which are power system and power transmitter.

• The units are made bio-compatible, so that they can be placed inside human eye.

• When the retina is implanted inside the eyeballs then the power generator is not stable during that period, the reason behind that is probably the fabrication, presence of parasite or the noise that is caused during the implantation.

• Hence, while operating the retina the surgeon should ensure that the retina can be placed under unstable power supply or not.

 

REFERENCES

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IV.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

V. Satoshi Inoue, Minoru Matsuo, Tsutomu Hashizume, Hideto Ishiguro, Takashi Nakazawa, and Hiroyuki Ohshima, LOW TEMPERATURE CMOS SELF-ALIQNED POLY-Si TFTS AND CIRCUIT SCHEMEUTILIZING NEW ION DOPING AND MASKING TECHNIQUE www.ieeexplore.ieee.org, 2011 2010. T.

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