Solid-State Image Sensor Dark Current Reduction via High-Impurity Barrier
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Solution Overview
Problem
In solid-state image pickup elements, electrons injected from the well contact into the photodiode cause dark current due to diffusion into the n-type region, degrading image quality.
Innovation Solution
A higher impurity concentration region is formed between the well contact and the photodiode, acting as a potential barrier to prevent electron flow into the photodiode, thereby reducing dark current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a well contact is formed to supply electric potential to the well region, then the well region can be fixed to a given electric potential, but electrons are injected from the well contact into the photodiode causing dark current
Solution Approach 1:
An impurity region with higher impurity concentration is introduced as an intermediary between the well contact and the photodiode. This impurity region acts as a mediator that allows the well contact to supply electric potential to the well region while simultaneously blocking the injection of electrons into the photodiode, thus resolving the contradiction between electric potential stability and dark current generation.
Solution Approach 2:
The impurity concentration is locally increased in the region between the well contact and the photodiode, creating a localized high impurity concentration region. This local quality change enables the specific function of blocking electron injection while maintaining the overall structure and electric potential stability of the well region.
2Reliability
If electrons diffuse into the n-type region of the photodiode, then the well contact can supply electric potential, but dark current is generated degrading image quality
Solution Approach 1:
The high impurity concentration region serves as an intermediary barrier that prevents electron diffusion from the well contact into the n-type region of the photodiode. This intermediary structure maintains the electric potential supply function while eliminating the harmful electron injection that degrades image quality.
Solution Approach 2:
The impurity concentration parameter is changed locally in the region between the well contact and photodiode, creating a high impurity concentration region. This parameter change fundamentally alters the electrical properties of the region, transforming it from an electron-conducting path to an electron-blocking barrier, thus preventing dark current while maintaining electric potential stability.
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 solution effectively suppresses the generation of dark current, leading to improved image quality and higher definition imaging.
Implementation Method 1
The photodiode PD converts a light made incident thereto into electrons (or holes) by photoelectric conversion to accumulate therein the resulting electric charges (electrons or holes).
Implementation Method 2
A higher impurity concentration region is formed between the well contact and the photodiode, acting as a potential barrier to prevent electron flow into the photodiode
Data Source
AI summary
A solid-state image pickup element including: a photoelectric conversion region; a transistor; an isolation region of a first conductivity type configured to isolate the photoelectric conversion region and the transistor from each other; a well region of the first conductivity type having the photoelectric conversion region, the transistor, and the isolation region of the first conductivity type formed therein; a contact portion configured to supply an electric potential used to fix the well region to a given electric potential; and an impurity region of the first conductivity type formed so as to extend in a depth direction from a surface of the isolation region of the first conductivity type in the isolation region of the first conductivity type between the contact portion and the photoelectric conversion region, and having a sufficiently higher impurity concentration than that of the isolation region of the first conductivity type.