Adaptive NAND Memory Verification for Unwritten Cells
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Solution Overview
Problem
Existing non-volatile memory systems, such as flash memory, face issues with false verification of memory states due to unwritten cells, leading to inaccurate reading and erasing of data, especially in blocks with significant unwritten space, which can result in false indications of erased conditions and misread data.
Innovation Solution
Implementing adaptive erase and read verification methods by determining the amount of unwritten space in a block and adjusting parameters accordingly, using either default or modified sets of parameters based on the amount of unwritten space, and performing unverified writes to raise threshold voltages of unwritten cells to ensure accurate erasure and reading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If default erase verification parameters are used in blocks with significant unwritten space, then the erase verification process is simple and fast, but false verification occurs indicating erased condition when cells are not properly erased
Solution Approach 1:
The patent implements dynamic adjustment of erase verification parameters based on the amount of unwritten space detected in the block. The system transitions from static default parameters to adaptive parameters that change according to block conditions, resolving the contradiction between verification accuracy and system complexity by making the parameter selection dynamic rather than fixed
Solution Approach 2:
The patent changes verification parameters (such as voltage levels, threshold values, or timing) based on the detected amount of unwritten space in the block. By modifying verification parameters according to block history, the system achieves accurate verification across different block conditions without requiring completely different verification mechanisms
2Measurement precision
If adaptive parameters are used for erase and read verification, then verification accuracy improves, but the operation time and complexity increase
Solution Approach 1:
The patent performs preliminary detection of unwritten space in the block before executing erase or read verification operations. By identifying blocks with significant unwritten space in advance, the system can proactively select appropriate verification parameters, avoiding the need for iterative verification attempts and reducing total operation time
Solution Approach 2:
The memory system performs self-diagnosis by detecting unwritten space in blocks and automatically adjusts verification parameters without external intervention. This self-adaptive mechanism eliminates the need for manual parameter tuning or complex external control, achieving high verification accuracy efficiently
3Productivity
If unwritten cells are not compensated before erasure, then the erase process is faster and simpler, but false verification occurs and data integrity is compromised
Solution Approach 1:
The patent performs preliminary compensation of unwritten cells by raising their threshold voltages before executing the erase operation. This pre-treatment ensures that all cells are in a suitable state for erasure, preventing false verification and data integrity issues while maintaining erase speed through efficient voltage adjustment
Solution Approach 2:
The patent applies preliminary anti-action by raising threshold voltages of unwritten cells to counteract their unwritten state before erasure. This compensatory action prevents the harmful effect of false verification during subsequent erase verify operations, ensuring data integrity without significantly impacting erase speed
Data Source
AI summary
In a nonvolatile NAND memory array, a NAND block may be falsely determined to be in an erased condition because of the effect of unwritten cells prior to the erase operation. Such cells may be programmed with dummy data prior to erase, or parameters used for a verify operation may be modified to compensate for such cells. Read operations may be similarly modified to compensate for unwritten cells.