Global Shutter Image Sensor Pixels with Centralized Charge Storage
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Solution Overview
Problem
Conventional CMOS image sensors with small pixel sizes face challenges in achieving global shuttering without exposure time skew, leading to image distortion, and require additional charge storage sites which increase pixel size and cost, especially in Back Side Illumination (BSI) applications where light shielding is less efficient.
Innovation Solution
A global shutter image sensor pixel design with a centrally located floating diffusion node and double reset circuitry, eliminating the need for a second pinned diode for charge storage, thereby optimizing area, voltage swing, and efficiency while minimizing kTC noise generation, and allowing back side illumination without light shields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple charge storage sites are provided in each sensor pixel to implement global shuttering, then exposure time skew is eliminated and distortion of moving objects is eliminated, but pixel size increases and manufacturing cost increases
Solution Approach 1:
The patent merges the charge storage function and charge detection function into a single centralized floating diffusion region. This eliminates the need for separate storage sites while maintaining global shuttering capability, thereby reducing pixel size without sacrificing reliability
Solution Approach 2:
The centralized floating diffusion region serves multiple functions simultaneously: it acts as both the charge storage site and the charge detection node. This multi-functionality eliminates the need for additional dedicated storage regions, reducing overall pixel area while maintaining global shutter performance
2Reliability
If a second pinned diode is used for charge storage in global shutter mode, then charge can wait for scanning without exposure time skew, but pixel area increases and voltage swing is reduced
Solution Approach 1:
The patent combines the charge storage diode and charge detection floating diffusion into a single centralized floating diffusion region. This eliminates the need for a second pinned diode while maintaining charge storage capability during the scanning period, thereby reducing pixel area
Solution Approach 2:
The patent extracts the charge storage function from the separate second pinned diode and integrates it into the centralized floating diffusion region. This eliminates the need for the second pinned diode structure, reducing pixel area while preserving the charge waiting capability for scanning
3Ease of operation
If reset transistor is used to drain charge from floating diffusion node, then pixel can be reset for subsequent integration cycle, but thermal kTC-reset noise is generated
Solution Approach 1:
The patent implements a feedback mechanism where the output of the source follower transistor is fed back to the floating diffusion node through a feedback transistor. This feedback action actively compensates for and cancels the kTC reset noise generated during the reset operation, allowing easy pixel reset while eliminating the harmful noise
4Ease of manufacture
If conventional 4T pixel circuit with pinned photodiode is used, then charge-to-voltage conversion is performed directly within pixels, but pixel size is larger compared to centralized floating diffusion design
Solution Approach 1:
The patent merges the pinned photodiode charge storage function and the floating diffusion charge detection function into a single centralized floating diffusion region. This integration maintains the direct charge-to-voltage conversion capability while significantly reducing the pixel area required for conventional 4T circuits
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 design achieves high global shutter efficiency, low dark current generation, and reduced pixel size, maintaining high quantum efficiency and dynamic range in BSI applications, while effectively eliminating kTC-reset noise.
Implementation Method 1
Typical complementary metal-oxide-semiconductor (CMOS) image sensors sense light by converting impinging photons into electrons that are integrated (collected) in sensor pixels
Data Source
AI summary
An image sensor may be provided with an array of image pixels formed on a substrate having front and back surfaces. Each pixel may have a photodiode that receives light through the back surface, a floating diffusion node, and a charge transfer gate. The floating diffusion node may be formed in the center of the photodiode and may be surrounded by the charge transfer gate at the front surface. The charge transfer gate may isolate the floating diffusion node from the surrounding photodiode. The pixel may include reset transistor gates, an addressing transistor gate, and a source follower transistor arranged about the periphery of the photodiode. By centering the floating diffusion node and charge transfer gate within the photodiode, the image pixels may have improved shutter efficiency and charge transfer efficiency relative to pixels having floating diffusion nodes at non-centralized locations.