Vertically Stacked Pixel Sensor Layout for Higher Light Sensitivity
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Solution Overview
Problem
As CMOS image sensors shrink in size, the horizontal arrangement of photodiode and floating diffusion regions leads to reduced light sensitivity due to smaller photodiode areas, and increased noise from light leakage.
Innovation Solution
A vertically stacked arrangement of photodiode and floating diffusion regions, with the transfer gate surrounding at least a portion of both, increases the photodiode area for light collection and enhances gate switching control, reducing switching delay and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If photodiode and floating diffusion regions are arranged horizontally, then device layout is simple, but light sensitivity is reduced due to smaller photodiode area
Solution Approach 1:
The patent transitions from a horizontal planar arrangement to a vertical stacked arrangement of the photodiode region and floating diffusion region. This dimensional change allows the photodiode region to be positioned directly above the floating diffusion region, enabling the photodiode to achieve a larger effective light-sensitive area without increasing the lateral footprint of the pixel sensor, thereby resolving the contradiction between photodiode area and device layout simplicity.
2Loss of time
If photodiode and floating diffusion regions are arranged horizontally, then manufacturing is easier, but switching delay increases
Solution Approach 1:
By stacking the floating diffusion region vertically above the photodiode region, the patent creates a shorter and more direct charge transfer path compared to horizontal arrangement. This vertical configuration reduces the distance that photogenerated carriers must travel and be transferred, thereby decreasing switching delay and charge transfer time while maintaining manufacturability through standard semiconductor fabrication processes.
3Area of stationary object
If pixel sensor size is reduced, then device density increases, but light sensitivity decreases due to smaller photodiode area
Solution Approach 1:
The vertical stacking architecture enables the photodiode region to extend in the vertical dimension above the floating diffusion region, effectively increasing the light-sensitive volume and area without increasing the lateral pixel sensor footprint. This allows smaller pixel sensors to maintain or enhance light sensitivity by utilizing the third dimension for photodiode region expansion, thereby collecting more photons within a compact area.
4Reliability
If transfer gate does not surround floating diffusion region, then device complexity is lower, but photocurrent transfer efficiency is reduced
Solution Approach 1:
The transfer gate is configured to surround or enclose the floating diffusion region in a nested arrangement, where the gate structure envelops the diffusion region. This surrounding configuration creates an effective electric field control zone that enhances the transfer of photocurrent from the photodiode region to the floating diffusion region by applying gate voltage that influences charge carriers across the interface, thereby improving transfer efficiency while adding moderate structural complexity.
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 configuration enhances light sensitivity and quantum efficiency while allowing for a reduction in overall pixel sensor size, improving performance by increasing the area for photon collection and shielding the photodiode from noise.
Implementation Method 1
a photodiode region configured to convert photons of incident light into a photocurrent of electrons
Data Source
AI summary
A pixel sensor may include a vertically arranged (or vertically stacked) photodiode region and floating diffusion region. The vertical arrangement permits the photodiode region to occupy a larger area of a pixel sensor of a given size relative to a horizontal arrangement, which increases the area in which the photodiode region can collect photons. This increases performance of the pixel sensor and permits the overall size of the pixel sensor to be reduced. Moreover, the transfer gate may surround at least a portion of the floating diffusion region and the photodiode region, which provides a larger gate switching area relative to a horizontal arrangement. The increased gate switching area may provide greater control over the transfer of the photocurrent and/or may reduce switching delay for the pixel sensor.