Read Disturb Scan for Unprogrammed Wordlines in NAND Flash
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Solution Overview
Problem
Conventional memory systems face issues with read disturb, where unprogrammed memory cells are inadvertently changed due to read operations, leading to write-in errors and limited read disturb management, especially in NAND flash memory devices, which can result in data loss and reduced reliability.
Innovation Solution
A memory sub-system that performs a read disturb scan on unprogrammed memory cells using a controller to determine the extent of read disturb by performing test reads and evaluating threshold voltages, allowing for more accurate detection of disturbed cells and reducing unnecessary block folding, thereby enhancing data reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If read operations are performed on programmed memory cells, then data retrieval is enabled, but unprogrammed memory cells are inadvertently changed due to read disturb
Solution Approach 1:
The patent performs a preliminary test read on unprogrammed memory cells before actual programming operations. This preliminary action detects the extent of read disturb on unprogrammed cells adjacent to programmed blocks, allowing the system to identify and avoid using disturbed unprogrammed cells for future programming, thereby preventing write-in errors and improving data reliability
Solution Approach 2:
The patent introduces an intermediary measurement mechanism that quantifies read disturb by performing multiple test reads at different read voltages and calculating the extent of disturbance. This intermediary assessment acts as a mediator between the harmful read disturb effect and the programming operation, enabling informed decisions about whether to proceed with programming or fold the block
2Reliability
If block folding is performed to prevent read disturb, then reliability is improved, but memory block utilization is reduced due to premature folding
Solution Approach 1:
The patent applies local quality assessment by evaluating read disturb specifically on unprogrammed memory cells adjacent to programmed blocks, rather than treating all blocks uniformly. By measuring the extent of read disturb locally and making folding decisions based on actual disturbance levels rather than conservative thresholds, the system maintains reliability while improving memory block utilization and reducing premature folding
3Measurement precision
If conventional read disturb scan is performed, then some detection is achieved, but detection precision is limited leading to inaccurate assessment
Solution Approach 1:
The patent performs multiple test reads (excessive action) at different read voltages to accurately determine the extent of read disturb, rather than relying on a single conventional scan. This excessive measurement approach, performed selectively on unprogrammed cells adjacent to programmed blocks, achieves high detection precision while managing complexity through targeted application
Solution Approach 2:
The patent implements feedback by using the results of test reads to determine the extent of read disturb, then using this information to make informed decisions about block folding and programming operations. The feedback loop continuously monitors unprogrammed cells and adjusts system behavior based on measured disturbance levels, improving detection accuracy and operational reliability
Data Source
AI summary
A memory device to perform a read disturb scan of unprogrammed memory cells. In one approach, a test read is performed on unprogrammed memory cells in a first memory block of a storage media (e.g., NAND flash) to provide a test result. Based on the test result, a portion of the unprogrammed cells for which a threshold voltage is above a predetermined voltage is determined. A determination is made whether the portion of the unprogrammed memory cells exceeds a predetermined limit. In response to determining that the portion exceeds the predetermined limit, data is moved from the first memory block to a second memory block of the storage media.