Nonvolatile Memory Control Unit Threshold Voltage Drift
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Solution Overview
Problem
In NAND type nonvolatile semiconductor memory devices, data read operations can inadvertently alter the threshold voltage of unselected memory cells, leading to potential mistaken writes during resumed data writes, especially when data is suspended and then resumed on pages within a block.
Innovation Solution
The control unit determines the influence of read operations by counting memory cells with threshold voltages higher than a reference voltage and adjusts data writing by either writing additional data to the same block or shifting it to a different block, depending on the number of read operations and voltage influences, to minimize mistaken writes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of information
If data read operations are performed on selected pages, then data can be retrieved, but threshold voltage of unselected memory cells varies due to read voltage influence
Solution Approach 1:
The patent applies preliminary action by performing a compensation write operation before the main data write. The control unit writes compensation data to pages that were subjected to read operations, adjusting their threshold voltages in advance to counteract the cumulative effect of multiple read operations. This preliminary compensation prevents threshold voltage drift from causing mistaken writes when data is later written to these pages.
2Reliability
If additional data is written to compensate for threshold voltage changes, then mistaken writes are prevented, but writing time increases
Solution Approach 1:
The patent applies local quality by selectively applying compensation operations only to specific pages that meet certain conditions. The control unit identifies pages that have been read multiple times or show significant threshold voltage drift, and performs compensation write operations only on those pages rather than on all pages in the block. This selective approach reduces the overall time penalty while maintaining reliability for the most vulnerable pages.
Solution Approach 2:
The patent applies parameter changes by dynamically adjusting the compensation strategy based on read operation frequency and threshold voltage drift magnitude. The control unit monitors the number of read operations on each page and the extent of threshold voltage changes, then adapts the compensation write parameters (such as write voltage levels and which pages receive compensation) accordingly. This adaptive approach optimizes the balance between preventing mistaken writes and minimizing additional writing time.
Data Source
AI summary
A nonvolatile semiconductor memory device according to an embodiment comprises a control unit, in a data write, determining at least one of: a first requirement that the number of times of a data read on first through n1-th pages (where n1 is an integer of 1 to N-1) of a target block executed after the most recent data erase on the target block, is less than a reference number of times; and a second requirement that the number of memory cells whose threshold voltage is higher than a reference voltage, of a plurality of memory cells of a reference page of n1+1-th through N-th pages of the target block, is less than a reference number, and when the determined requirement is satisfied, writing additional data to the n1+1-th through N-th pages of the target block.