Flash Memory Cell Grouping by Threshold Voltage Distribution
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Solution Overview
Problem
In hybrid mode flash memory, memory cells with varying reliability lead to inefficient use of capacity, as cells with good reliability are often restricted to storing single bits instead of multiple bits to maintain reliability, resulting in wasted capacity.
Innovation Solution
A memory management method that groups memory cells by their threshold voltage distributions into multiple groups, allowing cells to operate under different programming modes (M-level and N-level cell modes) based on their reliability, thereby optimizing the use of memory cells without sacrificing capacity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If all memory cells in a physical block are arranged to store one single bit per memory cell to increase reliability, then the reliability of the physical block is improved, but the data capacity is wasted because some memory cells with well reliabilities cannot be used for storing multiple bits
Solution Approach 1:
The patent divides the physical block into multiple virtual blocks, where each virtual block contains memory cells with similar reliability characteristics. This segmentation allows different virtual blocks to operate at different storage densities (MLC, SLC, or hybrid modes), thereby resolving the contradiction by enabling high-reliability cells to operate in SLC mode while high-capacity cells operate in MLC mode within the same physical block.
Solution Approach 2:
The patent applies local quality by assigning different operational modes (SLC, MLC, or hybrid) to different virtual blocks based on their specific reliability characteristics. Instead of uniformly treating all cells in a physical block, each virtual block is optimized locally according to its threshold voltage distribution and reliability metrics, allowing cells with well reliability to store multiple bits while maintaining overall block reliability.
2Quantity of substance
If some memory cells are used to store two or three bits per memory cell while others store one single bit, then the data capacity is improved, but the reliability management becomes complex due to varying reliability among cells in the same physical block
Solution Approach 1:
The patent segments memory cells into different virtual blocks based on reliability metrics such as threshold voltage distribution. This segmentation simplifies reliability management by allowing each virtual block to be managed independently with a uniform operational mode, rather than managing heterogeneous cells within the same physical block. The controller can track and manage each virtual block's reliability characteristics separately, reducing overall system complexity.
Solution Approach 2:
The patent changes the operational parameter (storage mode) of memory cells based on their reliability parameters. By monitoring threshold voltage distributions and reliability metrics, the system dynamically adjusts whether each virtual block operates in SLC, MLC, or hybrid mode. This parameter-based management approach simplifies reliability tracking compared to cell-by-cell management, as entire virtual blocks can be upgraded or downgraded in storage mode based on collective performance.
Data Source
AI summary
A memory management method, a memory storage device and a memory control circuit unit are provided. The method comprises: obtaining a first threshold voltage distribution of memory cells; grouping the first threshold voltage distribution to a plurality of first threshold voltage groups; obtaining a second threshold voltage distribution of the memory cells; grouping the second threshold voltage distribution to a plurality of second threshold voltage groups; allocating a memory cell among the memory cells to a virtual block if a threshold voltage pair of the memory cell belongs to a specific group of the first threshold voltage groups and a specific group of the second threshold voltage groups, such that the first memory cell is operated under a specific-level cell mode. Accordingly, the reliability of memory cells may be improved without significantly sacrificing the capacity of the rewritable non-volatile memory module.