Global Shutter Pixel Charge Loss Reduction
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Solution Overview
Problem
Global shutter pixel cells in imaging devices suffer from charge loss and parasitic charge accumulation, leading to reduced efficiency and image quality issues such as vertical shading and object smearing due to the storage of photo-generated charge over time and parasitic charge generation.
Innovation Solution
The implementation of a global shutter pixel cell design that includes a deeper and angled photodiode covering the storage node, a high voltage potential applied to the photodiode, a high dose p-type implant below the storage node, a reduced distance between the metal shield and storage node, and the use of less light-transparent dielectric materials and black materials to limit light and reflections, along with modifications to the pixel structure to enhance charge transfer and reduce parasitic charges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If photo-generated charge is stored in the storage node over time for global shutter operation, then the shutter efficiency and image quality deteriorate due to charge loss and parasitic charge accumulation, but global shutter capability is achieved
Solution Approach 1:
The patent applies preliminary anti-action by introducing an anti-blooming region that actively prevents parasitic charge accumulation before it degrades image quality. The anti-blooming drain is positioned to intercept and redirect parasitic charges away from the storage node during the charge storage period, counteracting the harmful effects before they manifest as vertical shading or smearing artifacts
Solution Approach 2:
The patent uses an intermediary approach by introducing a dedicated anti-blooming region as a mediator between the storage node and the substrate. This intermediary structure captures and redirects parasitic charges that would otherwise accumulate in the storage node, allowing global shutter operation to proceed without the harmful charge accumulation effects
2Reliability
If photo-generated charge is stored in the storage node over time for global shutter operation, then the shutter efficiency and image quality deteriorate due to parasitic charge generation, but global shutter capability is achieved
Solution Approach 1:
The patent applies preliminary anti-action by introducing an anti-blooming region that actively prevents parasitic charge accumulation before it degrades image quality. The anti-blooming drain is positioned to intercept and redirect parasitic charges away from the storage node during the charge storage period, counteracting the harmful effects before they manifest as vertical shading or smearing artifacts
Solution Approach 2:
The patent uses an intermediary approach by introducing a dedicated anti-blooming region as a mediator between the storage node and the substrate. This intermediary structure captures and redirects parasitic charges that would otherwise accumulate in the storage node, allowing global shutter operation to proceed without the harmful charge accumulation effects
3Ease of operation
If a mechanical shutter is used to control exposure time, then the exposure control is achieved, but the cost and physical bulkiness of the camera increase
Solution Approach 1:
The patent replaces the mechanical shutter system with an electronic global shutter mechanism implemented at the pixel level. The storage node electronically holds photo-generated charge for the required exposure duration, eliminating the need for moving mechanical parts. This substitution reduces camera bulkiness, lowers cost, and simplifies the overall device structure while maintaining precise exposure time control
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 significantly improves global shutter pixel efficiency by minimizing charge loss and parasitic charge accumulation, resulting in improved image quality and reduced vertical shading and smearing artifacts.
Implementation Method 1
a photodiode (PD) implanted deeper and under an angle covering space under a storage node (SN)
Implementation Method 2
Each pixel includes a photosensor, which is implemented as a pinned photodiode (PD)
Data Source
AI summary
A global shutter pixel cell includes a serially connected anti-blooming (AB) transistor, storage gate (SG) transistor and transfer (TX) transistor. The serially connected transistors are coupled between a voltage supply and a floating diffusion (FD) region. A terminal of a photodiode (PD) is connected between respective terminals of the AB and the SG transistors; and a terminal of a storage node (SN) diode is connected between respective terminals of the SG and the TX transistors. A portion of the PD region is extended under the SN region, so that the PD region shields the SN region from stray photons. Furthermore, a metallic layer, disposed above the SN region, is extended downwardly toward the SN region, so that the metallic layer shields the SN region from stray photons. Moreover, a top surface of the metallic layer is coated with an anti-reflective layer.