Monitoring Cell in NAND Flash String for Read Disturb
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Solution Overview
Problem
Repetitive read operations in NAND flash memory devices cause read disturb, leading to increased monitoring overhead as weak word lines are monitored to prevent misidentification of on cells as off cells, which is inefficient and resource-intensive.
Innovation Solution
Incorporating a monitoring cell within each cell string that receives adjusted monitoring voltages based on the characteristics of adjacent memory cells and blocks, allowing for differential voltage application during read operations to detect cell deterioration and trigger reclaim operations when necessary.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple word lines showing weak characteristics are monitored during read operations, then read disturb is detected, but monitoring overhead increases
Solution Approach 1:
The patent extracts the monitoring function from multiple word lines and concentrates it in a single monitoring cell within each cell string. This monitoring cell is specifically designated to track read disturb conditions, allowing the system to monitor weak characteristics without having to monitor all word lines individually, thereby reducing monitoring overhead while maintaining reliability
Solution Approach 2:
The monitoring cell acts as an intermediary that represents the weak characteristics of multiple word lines. Instead of directly monitoring each weak word line, the system uses the monitoring cell as a mediator that reflects the cumulative read disturb effects, simplifying the monitoring process and reducing the number of monitored elements
2Reliability
If a monitoring cell is added to each cell string, then read disturb monitoring is improved, but cell string complexity increases
Solution Approach 1:
The monitoring cell serves multiple functions: it acts as a regular memory cell for data storage and simultaneously functions as a monitor for read disturb conditions. This multi-functionality allows the system to improve monitoring accuracy without adding separate dedicated monitoring structures, thereby limiting the increase in cell string complexity
Solution Approach 2:
The patent merges the monitoring function with the existing memory cell structure. The monitoring cell is integrated into the cell string alongside other memory cells, sharing the same physical structure and circuitry. This merging approach improves monitoring capability while avoiding the need for separate monitoring circuits that would increase complexity
3Measurement precision
If different monitoring voltages are applied based on cell characteristics, then monitoring precision is improved, but control complexity increases
Solution Approach 1:
The patent applies different monitoring voltages to different monitoring cells based on their specific characteristics and positions. Each monitoring cell receives a customized voltage level tailored to its local conditions, such as its position in the cell string and the characteristics of adjacent memory cells. This local quality approach improves identification accuracy while keeping the voltage control logic relatively simple by making decisions based on local cell properties
Data Source
AI summary
A nonvolatile memory device capable of minimizing monitoring overhead associated with read disturb is provided. The nonvolatile memory device includes a memory cell array which includes a first cell string comprising a plurality of memory cells connected in series, wherein the plurality of memory cells includes a first monitoring cell, a first memory cell, and a second memory cell, and a row decoder which provides a first read voltage to the first memory cell and a first monitoring voltage to the first monitoring cell when reading the first memory cell among the memory cells and provides the first read voltage to the second memory cell and a second monitoring voltage different from the first monitoring voltage to the first monitoring cell when reading the second memory cell.