NAND Flash Read-Level Adaptation for Threshold Shift Tracking
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Solution Overview
Problem
NAND Flash memory devices experience read errors due to shifting threshold voltage distributions caused by retention charge loss and back pattern effects, which cannot be accurately tracked by pre-defined read voltage levels.
Innovation Solution
Implementing a dynamic automatic valley detection (AVD) scheme that dynamically adjusts read voltage levels by dividing memory cells into groups and using varying sense develop times to optimize read operations and compensate for distribution shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pre-defined read voltage levels are used, then device complexity is reduced, but read reliability deteriorates due to inability to track threshold voltage distribution shifts
Solution Approach 1:
The patent implements dynamic automatic valley detection (AVD) that automatically adjusts read voltage levels in real-time based on detected threshold voltage distributions. The system transitions from static pre-defined voltage levels to dynamic adaptive voltage selection, allowing the read voltage to dynamically track the shifting distributions caused by retention charge loss and back pattern effects, thereby maintaining high read reliability without requiring complex manual intervention
Solution Approach 2:
The patent employs feedback mechanisms where the peripheral circuit detects the actual threshold voltage distributions of memory cells and uses this information to automatically determine optimal read voltage levels. The feedback loop involves measuring the threshold voltage distributions, comparing them against predefined levels, and adjusting the read voltage accordingly to minimize read errors while maintaining system simplicity
2Reliability
If dynamic automatic valley detection is implemented, then read reliability is improved by tracking threshold voltage shifts, but device complexity increases
Solution Approach 1:
The patent implements self-service through automatic valley detection where the peripheral circuit autonomously detects threshold voltage distributions and determines optimal read voltage levels without external intervention. The system self-adjusts based on detected distribution characteristics, with the peripheral circuit automatically identifying valley points and selecting appropriate read voltages, thereby improving reliability while keeping the complexity increase manageable through automation rather than complex external control
3Reliability
If multiple read voltage levels are used to accommodate distribution shifts, then read reliability is improved, but measurement precision requirements increase
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting read voltage levels based on detected threshold voltage distributions. Instead of using fixed voltage levels, the system changes the voltage parameters adaptively to match the actual distribution characteristics. The peripheral circuit determines optimal read voltages by analyzing the detected distributions and selecting voltages that maximize reading accuracy for the current state of the memory cells
Data Source
AI summary
A memory device includes memory cells coupled to a same word line and bit lines, respectively, and a peripheral circuit coupled to the memory cells through the word and bit lines. Each memory cell is in one state. The peripheral circuit is configured to determine a first number of a first set of the memory cells and a second number of a second set of the memory cells in parallel. Threshold voltages of the first set of the memory cells are between a first voltage and a second voltage larger than the first voltage. Threshold voltages of the second set of the memory cells are between the second voltage and a third voltage larger than the second voltage. The peripheral circuit is also configured to determine a first read level corresponding to a first state based on the first number and the second number.