Pixel Cell Isolation Using Separate P-Well Barriers

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Solution Overview

Problem

Conventional CMOS imager devices face challenges in isolating charge storage regions effectively, leading to potential contamination by photoelectrons generated within the P-well, which reduces shutter efficiency.

Innovation Solution

A separate P-well is formed to encase and contact the storage node and/or floating diffusion region, providing an electrical barrier to prevent electron contamination, with additional P-wells having higher doping concentrations for enhanced isolation, potentially eliminating the need for isolation trenches.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a single P-well is used to contain both storage node and floating diffusion region, then device complexity is reduced, but charge storage integrity deteriorates due to electron contamination from photoelectrons generated within the P-well

Engineering Contradiction:

ImproveP-well structure complexityVSAvoidcharge storage integrity

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The single P-well is divided into two separate P-wells: a first P-well for the storage node and a second P-well for the floating diffusion region. This segmentation prevents photoelectrons generated in one region from contaminating the other, thereby maintaining charge storage integrity while managing device complexity through functional separation.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

An isolation trench is introduced as an intermediary structure between the first and second P-wells. This trench acts as a physical barrier that blocks photoelectron migration between the storage node and floating diffusion region, resolving the contamination issue while allowing both regions to function independently.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If isolation trenches are used to separate P-wells, then charge storage integrity is improved, but device complexity and manufacturing difficulty increase

Engineering Contradiction:

Improvecharge storage integrityVSAvoidisolation structure complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The isolation trench is implemented only where strictly necessary—specifically between the first and second P-wells—rather than throughout the entire device. This partial application of the isolation structure provides sufficient charge storage integrity while minimizing the increase in device complexity and manufacturing difficulty.

Inventive Principle:

Principle #16Partial or excessive action

3Object-affected harmful factors

If higher doping concentration is applied to P-wells, then electron contamination is reduced, but manufacturing precision requirements increase

Engineering Contradiction:

Improveelectron contaminationVSAvoiddoping concentration control

Core Design Contradiction:

Object-affected harmful factorsVSManufacturing precision

Solution Approach 1:

Different doping concentrations are applied locally to different P-wells based on their specific functional requirements. The first P-well (storage node) and second P-well (floating diffusion region) can have optimized doping levels tailored to their respective roles, reducing electron contamination while allowing manufacturing processes to work within achievable precision ranges for each region.

Inventive Principle:

Principle #3Local quality

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

The solution effectively isolates the storage and floating diffusion regions from electron contamination, improving charge storage integrity and reducing cross-talk between pixels, thereby enhancing the overall efficiency of the CMOS imager.

Implementation Method 1

photoelectrons generated within the P-well

Methodology Applied

Scientific EffectPhotoelectric Effect: Photoelectric Effect

Implementation Method 2

providing an electrical barrier to prevent electron contamination

Methodology Applied

Scientific EffectElectrical isolation: Electrical Resistance

Data Source

PatentUS7605440B2Pixel cell isolation of charge storage and floating diffusion regions using doped wells

Publication Date: 2009.10.20 APTINA IMAGING CORP

AI summary

A pixel having a well-isolated charge storage region or floating diffusion region may be obtained by providing a separate P-well around the storage region or floating diffusion region. In one embodiment, a separate P-well entirely encases the storage region and is in contact with the storage region. This P-well provides an electrical barrier for preventing electrons that are generated elsewhere in the pixel from contaminating the storage region. In another embodiment, a first separate P-well encases and is in contact with the storage region and a second separate P-well encases and is in contact with the floating diffusion region.