CMOS Image Sensor Pixel Radial Transfer Gate Segmentation
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Solution Overview
Problem
Large pixel photodiodes face challenges with lag issues due to long charge readout paths and impaired light collection, as well as low noise interference from microlens design and active readout transistor connections in traditional CIS designs.
Innovation Solution
A circular photodiode design featuring a transfer gate surrounded by an active gate, with a plurality of microlenses directing light away from the transfer gate towards the photodiode, and channels between microlenses to minimize lag and enhance light collection, including segmented or interrupted transfer gates and strategically placed circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a large photodiode is used to increase sensitivity, then light collection area is improved, but charge readout path length increases causing lag issues
Solution Approach 1:
The transfer gate is divided into multiple segments arranged radially around the photodiode center. This segmentation creates multiple parallel charge transfer paths from different regions of the large photodiode to the readout node, reducing the effective charge transport distance and lag time while preserving the large photodiode area for light collection.
2Quantity of substance
If a large microlens is used to improve light collection, then light gathering capability is improved, but manufacturing complexity and alignment difficulty increase
Solution Approach 1:
The single large microlens is divided into multiple smaller microlenses arranged in an array over the photodiode. Each small microlens is easier to manufacture with standard photolithography techniques and can be precisely aligned to corresponding photodiode regions, while collectively they maintain high light collection efficiency across the entire large photodiode area.
3Reliability
If the transfer gate is made continuous to ensure complete charge collection, then charge transfer efficiency is improved, but lag time increases due to longer charge path
Solution Approach 1:
The continuous transfer gate is segmented into multiple radial sections that independently collect charge from different photodiode regions and converge at the readout node. This creates multiple shorter parallel charge paths that maintain complete charge collection while reducing the average charge transport distance and lag time.
Solution Approach 2:
The transfer gate structure transitions from a single-dimensional linear path to a two-dimensional radial arrangement of multiple segments. This dimensional change allows charge to be collected and transported simultaneously through multiple parallel paths, reducing the effective charge transport time while maintaining complete charge collection efficiency.
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 decreases lag time and improves light collection, achieving increased sensitivity and reduced noise in low light level image sensing applications.
Implementation Method 1
a plurality of microlenses, each microlens being disposed over a portion of the photodiode and directing light away from the transfer gate towards the photodiode
Data Source
AI summary
A system for low light level image sensing is provided having: A photodiode; a transfer gate disposed in a center of the photodiode; an active gate disposed surrounded by the transfer gate; a plurality of microlenses, each microlens being disposed over a portion of the photodiode and directing light away from the transfer gate towards the photodiode.