BSI Global Shutter Pixel Structure for Floating Diffusion Shielding
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Solution Overview
Problem
Conventional back-side illuminated global shutter image sensors suffer from reduced efficiency due to undesired noise charges affecting exposure data during readout, leading to image signal distortion.
Innovation Solution
The implementation of a pixel structure that forms a strong electric field near the floating diffusion node using a high voltage terminal, or creates a structure such as a guard ring or deep trench isolation to shield additional electrons, preventing noise charges from reaching the floating diffusion node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If exposure data is stored in floating diffusion node during readout interval, then global shutter function is achieved, but noise charges pollute the exposure data causing image signal distortion
Solution Approach 1:
A guard ring structure is introduced as an intermediary element between the floating diffusion node and the incident light path. This guard ring acts as a mediator that intercepts and redirects photons before they can generate noise charges in the floating diffusion node, thereby protecting the stored exposure data while maintaining the global shutter function.
Solution Approach 2:
The harmful function of the floating diffusion node is extracted and separated from its harmful effect by introducing the guard ring structure. The guard ring selectively removes harmful photons from the light path before they reach the floating diffusion node, while allowing the node to continue performing its useful function of storing exposure data.
2Productivity
If readout time is extended to read all pixels, then all exposure data can be output, but sensing elements continue to generate noise charges during readout
Solution Approach 1:
The guard ring structure performs preliminary protection by blocking harmful photons before they can generate noise charges during the readout interval. This preliminary anti-action prevents the accumulation of noise charges over time, allowing extended readout periods without degrading image quality.
3Measurement precision
If floating diffusion node is exposed to incident light, then exposure data is captured, but noise charges are generated reducing shutter efficiency
Solution Approach 1:
The guard ring structure creates a localized modification in the pixel structure, applying light-blocking properties specifically in the region where noise charge generation occurs. This local quality change allows the floating diffusion node to maintain its light-sensitive properties for useful signal capture while the guard ring region provides selective protection against harmful light that generates noise.
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 effectively absorbs or blocks noise charges, thereby improving the shutter efficiency by preventing them from polluting the exposure data stored in the floating diffusion node.
Implementation Method 1
arranging a high voltage terminal of a device in a sensor pixel circuit at a position very close to a floating diffusion region to form at least one strong electric field that has the effect of shielding the floating diffusion region
Data Source
AI summary
There is provided a structure to improve BSI global shutter efficiency. In a sensor pixel circuit, at least one strong electric field is formed at the position of a floating diffusion region to accordingly have the effect of shielding the floating diffusion region. Or, the semiconductor material from the floating diffusion node toward a light incident direction is removed in the manufacturing process such that a depletion region cannot be formed in this direction. Or, a reflection layer or a photoresist layer is formed in the light incident direction to block the light. In these ways, charges generated by the undesired noises are reduced, and noise charges are difficult to reach the floating diffusion region thereby improving the shutter efficiency.