Solid State Image Sensor Charge Bleeding Prevention
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Solution Overview
Problem
Miniaturization of CMOS image sensors to achieve higher pixel resolution and smaller camera modules leads to reduced photodetector size, resulting in decreased sensitivity and saturation signal strength, which degrades imaging performance, and poses challenges in maintaining signal linearity due to charge overflow and bleeding in multi-pixel sharing architectures.
Innovation Solution
Implementing a solid state image sensor with a horizontal overflow drain architecture where charge-to-voltage converters, reset gates, and amplifiers are shared among pixels, with the reset gate power supply set higher than the amplifier power supply, and resetting charge in photodetectors prior to readout to prevent charge bleeding, while maintaining signal linearity and saturation characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pixel size is reduced to achieve higher resolution and smaller camera modules, then pixel density and device compactness are improved, but photodetector size decreases resulting in reduced sensitivity and saturation signal strength
Solution Approach 1:
Multiple photodetectors share common charge-to-voltage converter, reset gate, and amplifier circuits. This merging of readout circuits among adjacent pixels reduces per-pixel circuit area, enabling smaller pixel sizes while maintaining sufficient signal processing capability and sensitivity through shared high-performance analog circuits.
Solution Approach 2:
The charge-to-voltage converter, reset gate, and amplifier are designed as multi-functional shared resources that serve multiple photodetectors. These universal circuits perform multiple functions including charge conversion, reset control, signal amplification, and overflow management for different pixels, reducing overall circuit complexity and area.
2Manufacturing precision
If photodetector size is reduced to fit more pixels, then pixel resolution is improved, but saturation signal strength decreases degrading imaging performance
Solution Approach 1:
Adjacent photodetectors share common charge-to-voltage converter and amplifier circuits, allowing each photodetector to maintain adequate signal processing capability despite size reduction. The shared analog circuits provide sufficient signal gain and conversion performance that would be difficult to achieve in highly miniaturized per-pixel circuits.
Solution Approach 2:
The patent extends the pixel array into multiple layers stacked vertically, with each layer containing photodetectors and shared readout circuits. This three-dimensional arrangement increases total pixel count and resolution without requiring proportional increases in individual photodetector size, as each layer's circuits serve only that layer's photodetectors.
3Area of stationary object
If multi-pixel sharing architecture is implemented to reduce per-pixel footprint, then device compactness is improved, but charge overflow and bleeding between pixels increases
Solution Approach 1:
Reset gates are positioned and activated to prevent charge overflow before it can bleed into adjacent pixels. The reset circuitry is designed to clear excess charge from photodetectors proactively, and the horizontal overflow drain architecture provides predetermined charge discharge paths that redirect overflow charge away from neighboring pixels through controlled potential barriers.
Solution Approach 2:
The charge-to-voltage converter serves as an intermediary between photodetectors and readout circuits, converting charge signals in a controlled environment. This intermediate conversion stage isolates photodetectors from direct electrical interaction with adjacent pixels' circuits, reducing charge bleeding while enabling efficient signal readout through the shared converter.
4Object-generated harmful factors
If horizontal overflow drain architecture is used in back-illuminated sensors to discard charge, then charge overflow is managed, but signal linearity may be affected due to charge passing under transfer gates
Solution Approach 1:
Different regions of the pixel array are assigned different overflow management strategies. Some pixels use vertical overflow drains discarding charge into the substrate, while others use horizontal overflow drains passing charge under transfer gates to shared charge-to-voltage converters. This localized differentiation optimizes each region's performance for its specific function, maintaining signal linearity where needed while managing overflow elsewhere.
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 approach allows for increased charge handling capacity of photodetectors, maintaining signal linearity and saturation characteristics, even with reduced pixel sizes, and prevents charge bleeding, thereby enhancing imaging performance without degrading it.
Implementation Method 1
each pixel includes a photodetector as well as a transfer gate configured to transfer charge photoelectrically converted by the photodetector
Data Source
AI summary
A solid state image sensor includes a pixel array, as well as charge-to-voltage converters, reset gates, and amplifiers each shared by a plurality of pixels in the array. The voltage level of the reset gate power supply is set higher than the voltage level of the amplifier power supply. Additionally, charge overflowing from photodetectors in the pixels may be discarded into the charge-to-voltage converters. The image sensor may also include a row scanner configured such that, while scanning a row in the pixel array to read out signals therefrom, the row scanner resets the charge in the photodetectors of the pixels sharing a charge-to-voltage converter with pixels on the readout row. The charge reset is conducted simultaneously with or prior to reading out the signals from the pixels on the readout row.