Nonvolatile Memory Block Screening via Hard Off-Cell Detection
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Solution Overview
Problem
In nonvolatile memory devices, detecting defective memory cell blocks is challenging due to the small voltage difference between leakage and normal word lines during gate induced drain leakage (GIDL) erase operations, making it difficult to identify memory cell blocks with leakage word lines.
Innovation Solution
A method is introduced to detect defective memory cell blocks by performing a read operation on memory cells using an off-cell detection voltage different from the read reference voltage, counting hard off-cells with higher threshold voltages, and identifying defective blocks based on the number of hard off-cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If GIDL erase operation is performed, then erase efficiency is improved, but detection precision of defective memory cell blocks deteriorates due to small voltage difference between leakage and normal word lines
Solution Approach 1:
The patent applies preliminary action by performing an off-cell detection operation before the main erase operation. This detection operation uses a detection voltage that is higher than the read reference voltage to identify potential defective blocks in advance. By detecting hard off-cells (memory cells with abnormally high threshold voltages) before erasure, the system can flag suspicious blocks for further investigation or exclusion, thereby improving the overall detection precision without compromising the efficiency of the GIDL erase operation on healthy blocks.
2Ease of operation
If read reference voltage is used for detection, then operation simplicity is maintained, but detection precision of hard off-cells deteriorates
Solution Approach 1:
The patent applies parameter changes by introducing an off-cell detection voltage that is specifically set higher than the read reference voltage. This voltage parameter change enables the detection operation to distinguish hard off-cells (with abnormally high threshold voltages) from normally erased cells. The detection voltage is carefully selected to be high enough to activate hard off-cells but not so high as to cause false positives, thereby achieving precise detection while maintaining operational simplicity through automated voltage generation.
Data Source
AI summary
A method of detecting, by a nonvolatile memory system, a defective memory cell block from among memory cell blocks, includes performing, after performing an erase operation, a read operation on at least some memory cells included in a target memory cell block based on an off-cell detection voltage that is different from a read reference voltage that distinguishes an off-cell on which no data is written from an on-cell on which data is written; counting a number of hard off-cells having a higher threshold voltage than the off-cell detection voltage from among the memory cells based on a result of performing the read operation; and identifying whether the target memory cell block is a defective memory cell block based on the number of counted hard off-cells.