Analog Memory Cell Retention Level Management
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Solution Overview
Problem
The reliability of analog memory cells, such as Flash cells, deteriorates over time due to prolonged retention in an erased state, leading to degradation and reduced storage reliability.
Innovation Solution
Implementing techniques such as setting memory cells to a retention programming level different from the erased state, applying preliminary storage operations like dummy erasure and programming cycles, and managing memory cell blocks in retention and erased pools to minimize prolonged erased state retention, thereby reducing reliability degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If memory cells are retained in an erased state for long periods, then storage capacity is maintained and cells are ready for new programming, but reliability deteriorates due to physical media degradation
Solution Approach 1:
The patent applies preliminary rejuvenation actions (dummy read operations and selective refresh cycles) to memory cells before they are programmed with new data. This preliminary action restores the physical media of cells that have deteriorated during long erased state retention, ensuring they meet storage quality levels before being reused, thus resolving the contradiction between maintaining storage readiness and preserving reliability
2Reliability
If memory cells are retained in a programmed state for rejuvenation, then reliability is improved, but storage operations are delayed
Solution Approach 1:
The patent applies partial rejuvenation actions selectively only to memory cells whose physical media has deteriorated below the storage quality level, rather than rejuvenating all cells. This selective approach minimizes the time loss while still improving reliability where needed, resolving the contradiction between reliability improvement and operation speed
3Reliability
If dummy read operations are performed on all memory cells, then reliability is improved, but processing time and energy consumption increase
Solution Approach 1:
The patent applies different operations to different memory cells based on their individual states. Dummy read operations are performed only on cells that require rejuvenation (those whose physical media has deteriorated), while cells in good condition are left untouched. This localized approach improves reliability where needed without unnecessarily increasing processing time and energy consumption for all cells
Data Source
AI summary
A method in a non-volatile memory, which includes multiple memory cells that store data using a predefined set of programming levels including an erased level, includes receiving a storage operation indicating a group of the memory cells that are to be retained without programming for a long time period. The memory cells in the group are set to a retention programming level that is different from the erased level. Upon preparing to program the group of memory cells with data, the group of memory cells is erased to the erased level and the data is then programmed in the group of memory cells.