CMOS Image Sensor Transfer Gate Impurity Gradient

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

CMOS image sensors with global shutter function face issues of charge leakage and image quality deterioration due to insufficient potential barriers in transfer gates, leading to mixed frames in motion images.

Innovation Solution

A solid state imaging device with a specific impurity concentration gradient under transfer gates, including a first transfer unit with a lower impurity concentration than the second and third transfer units, and a transfer assist region under the first transfer gate, to form effective potential barriers and prevent charge leakage.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If the first transfer gate uses a uniform potential barrier structure, then charge transfer is simplified, but charge leakage occurs when the gate is in off-state

Engineering Contradiction:

Improvetransfer gate structureVSAvoidcharge transfer accuracy

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent applies local quality by creating a non-uniform impurity concentration distribution under the first transfer gate, with a lower impurity concentration region positioned at a specific location. This localized variation in material properties forms an asymmetric potential barrier that selectively prevents charge leakage in the off-state while maintaining effective charge transfer in the on-state, thereby resolving the contradiction between structural simplicity and transfer reliability.

Inventive Principle:

Principle #3Local quality

2Productivity

If the potential barrier on the holding portion side is lower, then charge transfer efficiency is improved, but charge leakage into the holding portion increases

Engineering Contradiction:

Improvecharge transfer efficiencyVSAvoidcharge leakage

Core Design Contradiction:

ProductivityVSObject-generated harmful factors

Solution Approach 1:

The patent implements local quality by positioning a lower impurity concentration region at a specific location under the first transfer gate. This creates a localized potential well that asymmetrically modulates the potential barrier, allowing efficient charge transfer to the holding portion while simultaneously preventing leakage of charges that have already been transferred, thus resolving the contradiction between transfer efficiency and leakage prevention.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent applies preliminary action by pre-configuring the asymmetric potential barrier structure under the first transfer gate before charge transfer operations. The lower impurity concentration region is positioned in advance to create a potential landscape that guides charges efficiently to the holding portion during transfer while establishing preventive barriers against leakage, enabling both high efficiency and accuracy without requiring real-time adjustments.

Inventive Principle:

Principle #10Preliminary action

3Reliability

If the second transfer gate has a higher potential barrier, then charge retention is improved, but reverse flow of overflowed carriers to the photoelectric conversion portion occurs

Engineering Contradiction:

Improvecharge retentionVSAvoidreverse charge flow

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The patent applies local quality by creating a non-uniform impurity concentration distribution under the second transfer gate, with a lower impurity concentration region positioned at a specific location. This localized variation forms an asymmetric potential barrier that maintains high charge retention while preventing reverse flow of carriers to the photoelectric conversion portion, resolving the contradiction between reliable charge storage and prevention of harmful reverse currents.

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

This configuration ensures full charge transfer while preventing leakage, resulting in improved image quality and avoidance of residual images between frames.

Implementation Method 1

the impurity concentration of a second conductivity type in at least a part of a region under the first gate of the first transfer unit is lower than the impurity concentration of the second conductivity type in a region under the second gate of the second transfer unit and the impurity concentration of the second conductivity type in a region under the third gate of the third transfer unit

Methodology Applied

Scientific EffectPotential barrier formation: Electric Field

Data Source

PatentUS10848695B2Solid state imaging device, imaging system, and manufacturing method of solid state imaging device

Publication Date: 2020.11.24 CANON KK

AI summary

A solid state imaging device as an embodiment has a first transfer unit that includes a first gate and transfers charges from a photoelectric conversion portion to a holding portion; a second transfer unit that includes a second gate and transfers charges from the holding portion to a floating diffusion portion; and a third transfer unit that includes a third gate and drains charges from the photoelectric conversion portion to the charge draining portion. The impurity concentration of a second conductivity type in at least a part of a region under the first gate of the first transfer unit is lower than the impurity concentration of the second conductivity type in a region under the second gate of the second transfer unit and the impurity concentration of the second conductivity type in a region under the third gate of the third transfer unit.