U-Shaped Common-Body NAND Flash Cell String
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Solution Overview
Problem
Existing vertical NAND flash devices face challenges such as high resistance in source lines and inefficient erase operations due to gate-induced drain leakage, which can be addressed by employing a U-shaped common-body NAND flash device with a pocket p-well and junctionless transistors.
Innovation Solution
The U-shaped common-body NAND flash device features a U-shaped flash cell string built directly on a substrate, with perpendicular and parallel portions including string and ground select transistors, and a bottom pass transistor, allowing for a single common electrical node during erase operations and reducing manufacturing complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional vertical NAND flash devices are used, then manufacturing processes are complex, but erase operation efficiency is poor due to high resistance and gate-induced drain leakage
Solution Approach 1:
The cell string is divided into distinct segments: a first vertical portion containing string select transistors, a second vertical portion containing ground select transistors, and a horizontal portion connecting them. This segmentation allows independent optimization of each segment's function, improving erase efficiency while maintaining manageable manufacturing complexity through modular construction
Solution Approach 2:
The cell string transitions from a purely vertical configuration to a three-dimensional U-shaped structure that incorporates horizontal portions parallel to the substrate surface. This dimensional change enables the creation of a common-body architecture that eliminates gate-induced drain leakage during erase operations, significantly improving erase efficiency without substantially increasing manufacturing complexity
2Reliability
If conventional cell string designs are used, then source line resistance is high, but the structure is simpler
Solution Approach 1:
The cell string adopts a U-shaped curved configuration instead of a straight vertical design. The horizontal portions extend parallel to the substrate surface, creating a curved path that reduces resistance by providing multiple conductive pathways and optimizing current flow distribution, thereby improving reliability while accepting increased structural complexity
Solution Approach 2:
The horizontal portions of the cell string serve multiple functions: they connect the string select and ground select transistor portions, provide alternative current pathways to reduce resistance, and establish a common-body architecture. This multi-functionality improves source line resistance characteristics while the modular design keeps manufacturing complexity manageable
Data Source
AI summary
A flash device comprising a well and a U-shaped flash cell string, the U-shaped flash cell string built directly on a substrate adjacent the well. The U-shaped flash cell string comprises one portion parallel to a surface of the substrate, comprising a junctionless bottom pass transistor, and two portions perpendicular to the surface of the substrate that comprise a string select transistor at a first top of the cell string, a ground select transistor at a second top of the cell string, a string select transistor drain, and a ground select transistor source.