Non-volatile Memory Data Refresh via Adjacent Cell State Detection
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Solution Overview
Problem
Non-volatile memory systems face data retention issues due to charge leakage over time, leading to errors when reading data as threshold voltages unintentionally drift lower than programmed, causing incorrect data states.
Innovation Solution
A memory system that identifies memory cells with threshold voltages within an offset of a read compare voltage and adjacent cells in low data states, performs programming to refresh the data and maintain the original intended state, thereby preventing errors during read operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If charge is stored in memory cells for long-term data retention, then data can be retained without power, but charge leakage occurs over time causing threshold voltage to drift lower than programmed
Solution Approach 1:
The patent applies preliminary action by proactively identifying memory cells with threshold voltages within an offset of the read compare voltage before actual read errors occur. The system performs preliminary detection of cells approaching failure thresholds and refreshes them in advance, preventing data corruption before it happens. This is evident in the refresh operations that target cells based on predicted threshold voltage drift rather than waiting for actual read failures.
Solution Approach 2:
The patent implements feedback mechanisms by monitoring threshold voltage levels of memory cells and using this information to trigger selective refresh operations. The system continuously assesses the state of memory cells, compares threshold voltages against reference values, and adjusts refresh operations accordingly. This feedback loop enables the system to maintain data integrity by refreshing only those cells that show signs of degradation, optimizing the balance between data retention and reliability.
2Reliability
If selective refresh operations are performed on memory cells, then data integrity is maintained, but additional programming operations are required
Solution Approach 1:
The patent applies local quality by performing refresh operations selectively on specific memory cells or blocks that are identified as needing refresh, rather than refreshing the entire memory array. The system determines which particular cells have threshold voltages within the offset range and targets only those for refresh operations. This localized approach maintains data integrity where needed while avoiding unnecessary operations on cells that are still functioning correctly, thereby reducing overall programming complexity.
Solution Approach 2:
The patent segments the memory array into distinct regions or blocks, allowing independent assessment and refresh of individual segments. By dividing the memory space into manageable units, the system can apply refresh operations to only those segments that require it, based on local threshold voltage conditions. This segmentation strategy simplifies the control logic and reduces the complexity of coordinating large-scale refresh operations across the entire memory array.
3Loss of time
If threshold voltage offset is used to identify cells for refresh, then refresh timing is optimized, but additional read operations are needed to detect cell states
Solution Approach 1:
The patent applies partial action by performing read operations only on memory cells that are suspected of needing refresh, rather than reading the entire memory array. The system uses threshold voltage offset criteria to identify a subset of cells that are at risk of data loss and limits read operations to these specific cells. This partial monitoring approach optimizes refresh timing for at-risk cells while minimizing the impact on overall read throughput by avoiding unnecessary reads of healthy cells.
Data Source
AI summary
A memory system identifies memory cells connected to a common word line that have had their threshold voltage unintentionally drift lower than programmed by determining whether memory cells meet two criteria: (1) the memory cells have threshold voltages within an offset of a read compare voltage of a data state; and (2) adjacent memory cells (connected to word lines that are adjacent to the common word line) are in one or more low data states. For those memory cells meeting the two criteria, the memory system performs some amount of programming on the memory cells to refresh the data stored in those memory cells to be as originally intended.