Imaging Device Impurity Regions Reduce Dark Current
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Solution Overview
Problem
Laminated imaging devices face challenges in reducing dark current, which degrades image quality due to leak current generated at pn junctions, making it difficult to distinguish signal charge from noise.
Innovation Solution
The imaging device incorporates a novel configuration with a semiconductor substrate having specific impurity regions and transistors, where the impurity concentrations are strategically controlled to reduce the electric field at the pn junctions, thereby suppressing leak current. This includes forming p-type and n-type impurity regions with varying concentrations and positions to minimize the electric field strength and prevent minority carrier flow into the floating diffusion region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the photoelectric conversion film is stacked on the semiconductor substrate to form a laminated structure, then the integration density is improved, but the dark current increases due to leak current at pn junctions
Solution Approach 1:
The patent applies local quality by creating impurity regions with different concentrations at specific locations. The first impurity region has a higher impurity concentration than the second impurity region, which is positioned to overlap with the gate electrode in plan view. This localized variation in impurity concentration reduces the electric field strength at the pn junction interface, thereby suppressing leak current while maintaining the laminated structure's integration density.
Solution Approach 2:
The patent changes the impurity concentration parameter to resolve the contradiction. By setting the first impurity region's concentration higher than the second impurity region's concentration, the electric field at the pn junction is reduced. This parameter change effectively suppresses dark current generation at the interface between the photoelectric conversion film and semiconductor substrate, allowing the laminated structure to maintain high integration density without excessive dark current.
2Measurement precision
If the impurity concentration is increased to improve charge collection, then the signal charge detection is improved, but the leak current increases due to stronger electric field at pn junctions
Solution Approach 1:
The patent uses local quality by positioning the first impurity region with higher concentration away from the gate electrode overlap area, while the second impurity region with lower concentration is positioned to overlap with the gate electrode. This spatial differentiation allows charge collection to be maintained in non-overlap regions while leak current is suppressed in the overlap region where the electric field would otherwise be strongest.
Solution Approach 2:
The patent applies parameter changes by varying the impurity concentration across different regions. The first impurity region has higher concentration for charge collection, while the second impurity region has lower concentration to reduce electric field strength and leak current. This graduated parameter change optimizes both signal charge detection and dark current suppression.
3Productivity
If the electric field strength is increased to improve charge separation, then the photoelectric conversion efficiency is improved, but the minority carrier flow into floating diffusion region increases
Solution Approach 1:
The patent changes the electric field strength parameter by controlling impurity concentrations. The first impurity region maintains sufficient field for charge separation, while the second impurity region reduces the field strength at the pn junction interface to prevent minority carrier flow into the floating diffusion region. This parameter optimization balances photoelectric conversion efficiency with dark current suppression.
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 reduces leak current, enhancing image sensor performance by minimizing thermal noise and allowing for the capture of high-resolution images by reducing the electric field and impurity concentration around the floating diffusion region.
Implementation Method 1
a photoelectric converter located above the semiconductor substrate; Charge generated in the photoelectric conversion film as a result of the photoelectric conversion
Data Source
AI summary
An imaging device including a unit pixel cell comprising: a semiconductor substrate including a first conductivity type region of a first conductivity type, a first and second impurity regions of a second conductivity type provided in the first conductivity type region; a photoelectric converter located above the semiconductor substrate; and a first transistor including a gate electrode and at least a part of the second impurity region as a source or a drain. The first impurity region is at least partially located in a surface of the semiconductor substrate and electrically connected to the photoelectric converter. The second impurity region is electrically connected to the photoelectric converter via the first impurity region and has an impurity concentration lower than that of the first impurity region. The second impurity region at least partially overlaps the gate electrode in a plan view.