Image Sensor Charge Transfer via Stepped Potential Distribution
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Solution Overview
Problem
CMOS image sensors face challenges in charge transfer efficiency, particularly in low brightness conditions due to reduced photodiode sizes and decreased driving voltage, leading to degraded image quality.
Innovation Solution
The image sensor design includes a gate structure with multiple impurity regions and spacers formed through sequential ion implantation processes, creating a stepped potential distribution that enhances charge transfer efficiency by adjusting the depth and alignment of impurity regions and spacers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If photodiode size is reduced to increase pixel density, then integration scale is improved, but charge transfer efficiency deteriorates
Solution Approach 1:
The patent applies local quality by creating a non-uniform impurity concentration distribution within the photodiode. Specifically, the p-type impurity concentration is made higher near the transfer gate interface and lower deeper in the photodiode bulk. This localized variation in impurity concentration optimizes the electric field distribution at the critical charge transfer interface while maintaining adequate photodiode volume for charge generation, thus resolving the contradiction between miniaturization and charge transfer efficiency.
Solution Approach 2:
The patent changes the electrical parameters of the photodiode by controlling the impurity concentration profile. Through selective ion implantation, the p-type impurity concentration is adjusted to create a graded distribution that enhances charge transfer efficiency. This parameter change allows the photodiode to maintain effective charge transfer performance even as its physical dimensions are reduced for higher pixel density.
2Use of energy by moving object
If driving voltage is decreased for low power operation, then power consumption is improved, but charge transfer efficiency deteriorates
Solution Approach 1:
The patent changes the electrical parameters of the photodiode by controlling the impurity concentration profile. Through selective ion implantation, the p-type impurity concentration is adjusted to create a graded distribution that enhances charge transfer efficiency. This parameter change allows the photodiode to maintain effective charge transfer performance even as its physical dimensions are reduced for higher pixel density.
Solution Approach 2:
The patent applies local quality by creating a non-uniform impurity concentration distribution within the photodiode. Specifically, the p-type impurity concentration is made higher near the transfer gate interface and lower deeper in the photodiode bulk. This localized variation in impurity concentration optimizes the electric field distribution at the critical charge transfer interface while maintaining adequate photodiode volume for charge generation, thus resolving the contradiction between miniaturization and charge transfer efficiency.
3Device complexity
If simple photodiode structure is used to reduce manufacturing complexity, then device complexity is improved, but charge transfer efficiency deteriorates
Solution Approach 1:
The patent applies preliminary action by pre-forming the p-type impurity region with a specific graded concentration profile before subsequent processing steps. The impurity concentration is controlled to be higher near the transfer gate interface from the outset, which preliminarily establishes the optimal electric field distribution needed for efficient charge transfer. This preliminary structuring ensures high charge transfer efficiency without requiring complex additional processing steps.
Solution Approach 2:
The patent changes the electrical parameters of the photodiode by controlling the impurity concentration profile. Through selective ion implantation, the p-type impurity concentration is adjusted to create a graded distribution that enhances charge transfer efficiency. This parameter change allows the photodiode to maintain effective charge transfer performance even as its physical dimensions are reduced for higher pixel density.
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 design improves charge transfer efficiency by ensuring effective attraction and transfer of electrons to the floating diffusion region, enhancing image quality, especially in low brightness conditions.
Implementation Method 1
By performing an ion implantation process, a photodiode PD is formed in a predetermined portion of the semiconductor layer aligned with one side of the gate structure
Implementation Method 2
a fringing field applied on the n− region 15 of the photodiode PD increases when charges are transferred by applying a power voltage to the transfer gate Tx
Data Source
AI summary
An image sensor includes: a first impurity region of the first conductive type aligned with one side of the gate structure and extending to a first depth from a surface portion of the semiconductor layer; a first spacer formed on each sidewall of the gate structure; a second impurity region of the first conductive type, aligned with the first spacer and extending to a second depth that is larger than the first depth from the surface portion of the semiconductor layer; a second spacer formed on each sidewall of the first spacer; a third impurity region of the first conductive type aligned with the second spacer and extending to a third depth that is larger than the second depth from the surface portion of the semiconductor layer; and a fourth impurity region of a second conductive type beneath the third impurity region.