Device for detecting overflow of charges for backside illumination pixel
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Solution Overview
Problem
Existing image sensors with backside illumination struggle to quickly and efficiently detect charge overflow phenomena, which is crucial for accurate image capture, due to the presence of a potential barrier that blocks the transfer of photogenerated carriers from the photosensitive region to the collecting region.
Innovation Solution
The image sensor incorporates a 'detection acceleration' transistor and a well structure with opposite conductivity types, allowing the transistor to weaken the potential barrier during charge overflow detection, facilitating rapid transfer of charges to the sense node and enabling quick detection of overflow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a potential barrier is applied to block charge transfer during exposure phase, then charge accumulation in photosensitive region is improved, but detection speed of charge overflow is reduced
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: a photosensitive region for charge generation, a charge-collecting region for charge accumulation, and a detection node for overflow detection. The transfer gate is positioned between these regions to control charge flow. This segmentation allows the potential barrier to be applied selectively at the transfer gate without affecting the charge accumulation process in the photosensitive region, while enabling rapid detection when overflow occurs.
Solution Approach 2:
A detection node is introduced as an intermediary element between the charge-collecting region and the readout circuitry. This detection node specifically monitors for charge overflow conditions without interfering with the normal charge transfer process. When charges overflow the potential barrier, they are detected at this intermediate node, enabling fast overflow detection while maintaining the integrity of the charge accumulation mechanism.
2Measurement precision
If transfer gate controls charge transfer, then charge transfer precision is improved, but detection stage gain is reduced
Solution Approach 1:
The detection node is designed with specific local properties optimized for overflow detection. It includes a capacitance structure that is particularly sensitive to charge accumulation changes. The transfer gate uses a specific doping profile and geometric configuration (extending vertically around the charge-collecting region) that creates a controlled potential barrier. This localized optimization of the detection node's electrical characteristics enhances the detection stage gain specifically for overflow conditions without compromising the overall charge transfer precision controlled by the transfer gate.
3Area of stationary object
If pixel size is reduced, then sensor integration is improved, but charge overflow detection capability is reduced
Solution Approach 1:
The transfer gate is configured to extend vertically around the charge-collecting region, utilizing the vertical dimension to create an effective potential barrier. This vertical arrangement allows the transfer gate to control charge transfer effectively without requiring excessive horizontal space. The detection node is also positioned strategically in three-dimensional space to optimize its sensing capability. This use of the vertical dimension enables efficient charge overflow detection within a compact pixel footprint.
Solution Approach 2:
The detection node is nested within or adjacent to the charge-collecting region, with the transfer gate surrounding the charge-collecting region. This nested arrangement allows multiple functional elements to share the same spatial volume, maximizing the use of available pixel area. The detection node can monitor charges that have overflowed from the charge-collecting region without requiring separate dedicated space, thereby maintaining detection capability in reduced pixel sizes.
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 allows for rapid and amplified detection of charge overflow, reducing the size constraints of the pixel and enhancing the detection stage's gain, while maintaining the potential barrier to control charge transfer during conventional reading phases.
Implementation Method 1
there is a photoconversion or integration phase during which the back side of the substrate is illuminated and electron-hole pairs are generated and carriers, either photogenerated electrons or photogenerated holes, accumulate in the photosensitive region
Implementation Method 2
The clamping is implemented by the inversion layer of biased capacitive deep isolation trenches (CDTI—'capacitive deep trench isolation')
Implementation Method 3
Applying a suitable potential to the transfer gate makes it possible to form a potential barrier in a region located inside the vertical transfer gate, between the photosensitive region and the collecting region
Data Source
AI summary
An image sensor provided with a pixel including a photosensitive region formed in a semiconductor substrate and surrounded by a peripheral isolation trench; a sense node formed on a charge collecting region; a charge transfer gate around the sense node; a well; the pixel being provided with a so-called “detection acceleration” transistor configured to, during a so-called “charge overflow detection” operation, be switched on so as to weaken a potential barrier generated by the transfer gate and thus to favour an overflow of photogenerated charges to the sense node of the photosensitive region and to accelerate detection of this overflow.