Shared-String NAND Read Disturb Tracking Across Erase Blocks
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Solution Overview
Problem
Existing memory systems face challenges in managing read disturb among multiple erase blocks coupled to the same string, leading to increased bit error rates and data loss due to threshold voltage shifts caused by read and program verify operations.
Innovation Solution
Implementing a controller to monitor and track read disturb stress across erase blocks, adjusting counters based on read and program verify operations, and performing actions such as data refresh or scan operations when threshold criteria are met to mitigate the effects of read disturb.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple erase blocks are coupled to the same string, then memory density and storage capacity are improved, but read disturb increases causing threshold voltage shifts and bit error rates
Solution Approach 1:
The controller performs preliminary tracking and monitoring of read disturb counts for each erase block before data errors occur. By maintaining counters that track the number of read operations on aggressor blocks, the system can proactively identify blocks at risk and perform preventive refresh operations to mitigate threshold voltage shifts before they cause bit errors.
Solution Approach 2:
The system implements feedback by continuously monitoring read disturb counts and using this information to dynamically adjust refresh operations. The controller compares the tracked read disturb counts against threshold values and adjusts the refresh strategy accordingly, creating a closed-loop system that adapts to actual read disturb conditions to maintain data reliability.
2Productivity
If read and program verify operations are performed on aggressor blocks, then data retrieval and verification are improved, but read disturb stress increases on victim blocks
Solution Approach 1:
The controller monitors read disturb counts as feedback about the stress being imposed on victim blocks. This feedback information is used to dynamically adjust the refresh strategy, allowing the system to balance data retrieval operations with protective refresh operations to mitigate accumulated read disturb stress.
Solution Approach 2:
The system changes the parameter of refresh operation timing and frequency based on the monitored read disturb counts. By adjusting when and how often refresh operations are performed based on actual read disturb conditions, the system optimizes the balance between productivity and protecting against read disturb stress.
Data Source
AI summary
An apparatus can comprise a memory array comprising a plurality of strings of memory cells each comprising: a first group of memory cells coupled to a first group of access lines and corresponding to a first erase block; and a second group of memory cells coupled to a second group of access lines and corresponding to a second erase block. A controller is configured to determine a cumulative amount of read disturb stress experienced by the first erase block by monitoring read disturb stress experienced by the first erase block due to: read operations performed on the first erase block; read operations performed on the second erase block; and program verify operations performed on the second erase block. The controller can perform an action on the first erase block responsive to the cumulative amount of read disturb stress experienced by the first erase block meeting a criteria.