Uneven-Trench Pixel Cell for Blooming Path Control
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Solution Overview
Problem
Blooming artifacts degrade the quality of images captured by image sensors with vertical transfer gates due to optical crosstalk from excess photoelectrons in pixels exceeding saturation levels.
Innovation Solution
A blooming path is introduced as a leakage path that allows excess photoelectrons to travel from the photodiode to the floating diffusion region when the transfer gate is turned off, preventing blooming artifacts by detecting excess electrons during the integration period.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If vertical transfer gates are used to increase pixel density, then pixel density is improved, but blooming artifacts occur due to optical crosstalk from excess photoelectrons
Solution Approach 1:
The pixel structure is segmented into distinct depth zones using shallow and deep trenches. The shallow trench (extending to shallow bottom surface) and deep trench (extending to deep bottom surface) create spatial separation between the photodiode region and floating diffusion region, allowing independent control of electron transfer paths at different depths while maintaining high pixel density with vertical transfer gates
Solution Approach 2:
The uneven trench structure acts as an intermediary mechanism between the photodiode and floating diffusion region. By creating a controlled leakage path through the trench structure, excess photoelectrons are redirected to the floating diffusion region during integration, preventing blooming artifacts while maintaining the vertical transfer gate configuration for high pixel density
2Reliability
If transfer gate is turned off to prevent electron flow, then electron flow control is improved, but excess photoelectrons cannot be detected and blooming occurs
Solution Approach 1:
Different regions of the pixel structure are assigned different functional qualities: the shallow trench region provides a controlled leakage path for excess photoelectrons, while the deep trench region maintains the primary electron transfer path. This local differentiation allows the transfer gate to remain off during integration (preventing blooming) while still enabling detection of excess photoelectrons through the shallow trench leakage path
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 effectively prevents blooming artifacts by enabling the detection of excess photoelectrons, thereby improving image quality and reducing optical crosstalk in image sensors with vertical transfer gates.
Implementation Method 1
Light reaching the photodiode generates photoelectrons
Implementation Method 2
the transfer gate controls electron flow from the photodiode to the floating diffusion region and may be part of a field-effect transistor. Turning on the transfer gate forms a conducting channel that allows the accumulated photoelectrons to transfer or flow from the photodiode to the floating diffusion region
Implementation Method 3
When the transfer gate is pulsed to an off-state, the associated potential is lower than that of the photodiode raising a barrier in the corresponding energy band diagram to block electron flow from photodiode to floating diffusion region
Data Source
AI summary
An uneven-trench pixel cell includes a semiconductor substrate that includes a floating diffusion region, a photodiode region, and, between a front surface and a back surface: a first sidewall surface, a shallow bottom surface, a second sidewall surface, and a deep bottom surface. The first sidewall surface and a shallow bottom surface define a shallow trench, located between the floating diffusion region and the photodiode region, that extends into the semiconductor substrate from the front surface. A shallow depth of the shallow trench exceeds a junction depth of the floating diffusion region. The second sidewall surface and a deep bottom surface define a deep trench, located between the floating diffusion region and the photodiode region, that extends into the semiconductor substrate from the front surface. A distance between the deep bottom surface and the front surface defines a deep depth, of the deep trench, that exceeds the shallow depth.