Transparent Thin-Film Transistor Pixels for Image Sensor Fill Factor

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Solution Overview

Problem

Conventional CMOS image sensors have a significant reduction in pixel area due to the presence of multiple transistors, which limits the size of the photo-conversion device, thereby reducing sensitivity and quantum efficiency, especially in applications requiring high resolution and short wavelengths like ultraviolet light.

Innovation Solution

The implementation of a transparent thin-film pixel transistor that can be formed partially or fully over the photo-conversion device, allowing image light to pass through and reducing the space occupied by transistors, thereby increasing the fill factor and quantum efficiency.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If multiple transistors are used in each pixel for various functions including amplification, then the pixel circuit can perform necessary signal processing functions, but the area taken by transistors and interconnects increases, reducing the pixel area available for the photo-conversion device

Engineering Contradiction:

Improvepixel circuit functionalityVSAvoidphoto-conversion device area

Core Design Contradiction:

Adaptability or versatilityVSArea of stationary object

Solution Approach 1:

The patent merges the transfer transistor and source follower transistor into a single integrated device structure. The transfer transistor is formed with its channel over the photodiode, and the source follower is formed adjacent to it sharing common regions and interconnects. This consolidation reduces the total transistor area while maintaining both transfer and amplification functions, thereby increasing the photo-conversion device area.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent utilizes vertical stacking and three-dimensional integration by forming transistor channels above the photodiode region and using multiple layers of interconnects. The transfer transistor channel is formed over the photodiode, and the source follower is positioned in an adjacent vertical space, allowing compact integration that preserves horizontal photo-conversion area while providing necessary transistor functionality.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Reliability

If the photo-conversion device area is increased to improve sensitivity and quantum efficiency, then the pixel area available for photo-conversion increases, but the area for transistors and interconnects decreases

Engineering Contradiction:

Improvesensitivity and quantum efficiencyVSAvoidtransistor integration density

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent employs thin-film transistor technology where the transistor channels are formed as thin films above the photodiode region. This thin-film approach allows the transistors to occupy minimal vertical space and can be integrated without significantly increasing the horizontal footprint, enabling larger photo-conversion area while maintaining necessary transistor functionality for signal processing.

Inventive Principle:

Principle #30Flexible shells and thin films

3Adaptability or versatility

If conventional transistors are used in pixel circuits, then necessary signal processing functions can be performed, but light absorption by transistor structures reduces the fill factor and quantum efficiency

Engineering Contradiction:

Improvesignal processing capabilityVSAvoidlight absorption and fill factor

Core Design Contradiction:

Adaptability or versatilityVSUse of energy by moving object

Solution Approach 1:

The patent extracts the transistor structures from the light path by forming them above the photodiode region rather than integrating them laterally within the pixel plane. The transfer transistor channel is positioned over the photodiode, and the source follower is placed in an adjacent region, allowing incident light to reach the photodiode surface without being absorbed by transistor gates or interconnects, thereby maximizing fill factor and quantum efficiency.

Inventive Principle:

Principle #2Taking out (Extraction)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach enhances the fill factor and quantum efficiency of image sensors by allowing more area for the photo-conversion device, resulting in improved sensitivity and reduced signal loss, especially in high-resolution and ultraviolet applications.

Implementation Method 1

a transparent thin-film pixel transistor, which can be formed at least partially over the photo-conversion device while allowing image light to pass therethrough

Methodology Applied

Scientific EffectLight transmission through transparent material:

Implementation Method 2

when incident light strikes the surface of the photodiode 114, charge carriers (electrons), are generated in the depletion region of the p-n junction (between region 115 and region 116) of the photodiode 114

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Data Source

PatentUS7663165B2Transparent-channel thin-film transistor-based pixels for high-performance image sensors

Publication Date: 2010.02.16 APTINA IMAGING CORP

AI summary

A pixel circuit, and method of forming a pixel circuit, an imager device, and a processing system include a photo-conversion device, a floating diffusion region for receiving and storing charge from the photo-conversion device, and a transparent transistor for use in operation of the pixel, wherein the transparent transistor is at least partially over the photo-conversion device, such that the photo-conversion device receives light passing through the transparent transistor.