Isolated Floating Diffusion Pixels for Low-Noise Image Sensors
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in reducing 1/f noise and random telegraph noise due to capacitive coupling between the floating diffusion and transfer gate, limiting the minimum settling time and increasing readout noise, especially in low-light and photon-counting applications.
Innovation Solution
Incorporating a doped pinning region adjacent to the floating diffusion to reduce effective capacitive coupling between the floating diffusion and the transfer gate, and optionally the reset gate, to create a pinned transfer gate and/or reset gate, which isolates the charge transfer path from the surface region, thereby reducing overlap parasitic capacitance and feed-through voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CMOS image sensors are used with standard floating diffusion structures, then the device complexity is low and manufacturing is easier, but the readout noise increases and noise reduction capability deteriorates due to capacitive coupling between floating diffusion and transfer gate
Solution Approach 1:
The pixel structure is segmented by introducing an isolation region that physically divides the floating diffusion from the transfer gate, separating the capacitive coupling path and reducing noise interference
Solution Approach 2:
An isolation region is introduced as an intermediary element between the floating diffusion and transfer gate, acting as a mediator to reduce direct capacitive coupling while maintaining device functionality
2Productivity
If the settling time is reduced to improve readout speed, then the productivity increases, but the noise from 1/f and random telegraph noise increases because there is insufficient time for noise filtering
Solution Approach 1:
The isolation region is pre-configured in the pixel structure to reduce capacitive coupling before readout operations begin, enabling faster settling times without sacrificing noise reduction capability
3Measurement precision
If the capacitive coupling between floating diffusion and transfer gate is reduced, then the feed-through voltage decreases and conversion gain improves, but the device structure becomes more complex requiring additional isolation structures
Solution Approach 1:
The isolation region is strategically placed only in the critical area where capacitive coupling occurs between the floating diffusion and transfer gate, providing local noise reduction without requiring complete structural redesign
Solution Approach 2:
The isolation region extends into the substrate depth dimension, utilizing the vertical space to reduce capacitive coupling without increasing lateral footprint or complicating planar device layout
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 enables shorter correlated double sampling times, lower noise, and higher conversion gain by minimizing feed-through voltage, leading to improved low-noise and high-speed image sensing capabilities.
Implementation Method 1
capacitive coupling between the floating diffusion and transfer gate
Implementation Method 2
reduce overlap parasitic capacitance
Data Source
AI summary
According to some embodiments, an image sensor pixel includes a floating diffusion, a transistor gate configured to transfer charge with respect to the floating diffusion, and a doped pinning region disposed between the floating diffusion and the transistor gate, to reduce or eliminate the effective capacitive coupling between the floating diffusion and the transistor gate. The transistor gate may be an in-pixel transfer gate configured to selectively transfer photocharge from an in-pixel charge accumulation region to the floating diffusion. Alternatively, or additionally, the transistor gate may be an in-pixel reset gate configured to selectively reset the electrostatic potential of the floating diffusion.