ARTIFICIAL RETINA USING THIN FILM TRANSISTOR

INTRODUCTION

• The Artificial Retina made using Thin-Film Transistors, which can be fabricated on transparent and flexible substrates.

• Electronic photo devices and circuits are integrated on the artificial retina.

• Implanted on the inside surface of the living retina at the back part of the human eyeballs. Moreover, since the human eyeballs are curved, the flexible substrate is also preferable.

• Wireless power supply is used

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

vision restoration was developed by Drs. Brindley and Lewin in 1968.

• There are two types of retinal implants namely epiretinal implant and subretinal implant.

Operation• It uses the same fabrication processes as conventional

poly-Si TFTs and encapsulated using SiO2• The retina array includes matrix-like multiple retina

pixels• The retina pixel consists of a photo transistor, current

mirror, and load resistance.• The photosensitivity of the reverse-biased p/i/n poly-Si

phototransistor is 150 pA at 1000 lx for white light.• The retina pix-els irradiated with bright light output a

higher Vout, whereas the retina pixels irradiated with darker light output a lower Vout.

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.

• Ion implantation is one of the key factors in fabricating

ION DOPING TECHNIC

• 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 13.56MHz.

Main features of this system are:1) A large beam area (over 300 mm square)2)A high accelerating voltage (maximum: 110 KeV)

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.

• The new I/D technique was used to achieve a self-aligned structure.

• In these experiments, it is found that the characteristics of S/A and non-S/A TFTs are similar.

• No degradation can be observed as a result of using the new I/D technique.

New Masking technique 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 300oC due to the high accelerating voltage.

• In order to solve this problem, a new masking technique is also proposed.

• An SiO2 buffer layer is deposited on the glass substrate to protect TFTs from contamination from components of the glass.

Electro optical Measurement• 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.

WIRELESS POWER SUPPLY USING INDUCTIVE COUPLING

• Many implanted electrical power to function; be it in the form of an im-planted battery or via wireless power transmission.

• which requires additional surgery is undesirable.• An example of this is a retinal prosthesis.• continuous power transmission• Efficient transmission of power is a performance

limiting factor• High density electrode array with more than 1000

electrodes will consume about 45 mW of power.

• Chip-25mw• Neuronal stimulation-20mw(3.3vth)• Based on 64 simultaneously operating electrodes each

requiring a maximum of 0.3 mW at 60 Hz image refresh rate.

Disadvantages of power transmission• Difficulty in placing a large receive coil inside the eye.• We face are large separation between the coils• Reduction in power transfer to the device.

Overcome problems:• Intermediate link between the primary and secondary

coil• Which are embedded under the wall of the eye.

Working• The power transmitter consists of an ac voltage

source(10V,34khz) and induction coil.

PIN

• A PIN diode is a diode with a wide, undoped intrinsic

semiconductor region between a p-type

semiconductor and an n-type semiconductor region.

The p-type and n-type regions are typically heavily

doped because they are used for ohmic contacts. The

wide intrinsic region is in contrast to an ordinary

PNdiode.

CONCLUSIONS

•The articial retina using poly-Si TFTs and wireless power supply using inductive couplinare located in a light-shield chamber•Vout in each retina pixel is probed by a manual prober and voltage meter.•it is driven using unstable power source generated by induc-tive coupling, Diode Bridge, and Zener diodes.

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