Embedded Transfer Gate Structure for CMOS Charge Return Prevention
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Solution Overview
Problem
Conventional CMOS solid-state imaging elements face challenges in modulating the potential in the semiconductor substrate effectively, leading to charge transfer failures due to potential peaks and locally deep regions, which result in charges returning to the photodiode instead of being transferred to the floating diffusion region.
Innovation Solution
A light receiving element with a semiconductor substrate, a photoelectric conversion unit, a charge holding unit, and a transfer transistor featuring a gate electrode with a pair of embedded gate units, which allows for improved potential modulation and reduced charge transfer failures by avoiding potential peaks and local deep regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a vertical transfer transistor with an embedded unit is used to modulate potential to a deep region in the semiconductor substrate, then charge transfer reliability is improved, but local potential peaks are generated causing charges to return to the photodiode
Solution Approach 1:
The gate electrode is divided into multiple independent gate units (first gate unit, second gate unit, third gate unit) positioned at different depths and locations. This segmentation allows each gate unit to independently modulate potential in its specific region, preventing the formation of localized potential peaks while achieving comprehensive potential control across the semiconductor substrate.
Solution Approach 2:
Different gate units are positioned at different locations and depths within the semiconductor substrate to provide localized potential modulation. The first gate unit is positioned deeper than the second and third gate units, allowing each unit to address specific regional potential requirements and avoid generating harmful local potential peaks.
2Reliability
If potential is modulated to a deep region in the semiconductor substrate, then charge transfer is prevented from failing, but charges remain in deep potential parts and return to the photodiode
Solution Approach 1:
The segmented gate electrode structure with multiple gate units at different positions and depths enables distributed potential control. This prevents the formation of single deep potential wells that would trap charges, while still achieving sufficient potential modulation to ensure complete charge transfer from the photodiode to the floating diffusion region.
Solution Approach 2:
The multiple gate units act as intermediaries that mediate the potential distribution in the semiconductor substrate. By positioning gate units at different depths (with the first gate unit deeper than the second and third), they create a graduated potential landscape that guides charges forward without creating deep traps that would cause charge return.
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 transfers charges from the photoelectric conversion unit to the charge holding unit without interruption, reducing charge transfer failures and enhancing the imaging process in CMOS solid-state imaging elements.
Implementation Method 1
a photoelectric conversion unit that is provided in the semiconductor substrate and converts light into charges
Data Source
AI summary
Provided is a light receiving element including a semiconductor substrate, a photoelectric conversion unit (PD) that is provided in the semiconductor substrate and converts light into charges, a charge holding unit (FD) that is provided in the semiconductor substrate and holds the charges, and a transfer transistor (VG) that transfers the charges from the photoelectric conversion unit to the charge holding unit, in which the transfer transistor includes a gate electrode having a pair of first embedded gate units embedded in the semiconductor substrate.