3D NAND Drain Select Gate Electrode Segmentation
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Solution Overview
Problem
Three-dimensional NAND memory devices face issues with undesired drain select gate erase and voltage threshold upshift during Gate Induced Drain Leakage (GIDL) erase operations, particularly due to slow erase speed and threshold voltage shifts in drain select transistors.
Innovation Solution
The implementation of 'string-sharing' and 'string-selective' drain select gate electrodes in vertical NAND strings, where multiple strings share a common drain select gate electrode but have separate additional drain select gate electrodes not electrically connected, to prevent undesired erase operations and voltage shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a common drain select gate electrode is shared between multiple NAND strings, then device complexity is reduced and manufacturing is simplified, but undesired drain select gate erase and voltage threshold upshift occur during GIDL erase operations
Solution Approach 1:
The drain select gate electrodes are segmented into two types: common drain select gate electrodes shared by multiple NAND strings and separate drain select gate electrodes dedicated to individual strings. This segmentation allows the common electrodes to reduce complexity while separate electrodes prevent undesired erase and threshold voltage shifts during GIDL operations by providing isolated control for specific strings.
Solution Approach 2:
Different regions of the memory device have different drain select gate electrode configurations tailored to local requirements. The common drain select gate electrodes provide global control and simplify manufacturing, while separate drain select gate electrodes provide localized control to prevent threshold voltage instability in specific regions during erase operations.
2Reliability
If separate drain select gate electrodes are used for each NAND string, then threshold voltage stability is improved and undesired erase is prevented, but device complexity increases
Solution Approach 1:
The patent merges the functionality of drain select gate electrodes by implementing both common shared electrodes and separate dedicated electrodes. The common electrodes handle general select operations across multiple strings, while separate electrodes provide additional isolated control when needed, combining the benefits of simplified structure and enhanced reliability.
3Speed
If Gate Induced Drain Leakage (GIDL) erase operations are performed, then erase speed is achieved, but voltage threshold upshift occurs in drain select transistors
Solution Approach 1:
The separate drain select gate electrodes provide preliminary protective action by isolating specific NAND strings from undesired erase effects during GIDL operations. This pre-protection prevents voltage threshold upshift before it can occur, allowing fast GIDL erase operations to proceed without compromising threshold voltage stability in protected strings.
Data Source
AI summary
Systems, methods, and devices of the various embodiments provide both “string-sharing” drain select gate electrodes and “string-selective” drain select gate electrodes in vertical NAND strings. Various embodiments may provide two or more vertical NAND strings sharing a common drain select gate electrode while also having separate additional drain select gate electrodes not electrically connected across the two or more vertical NAND strings.