Analog Non-Volatile Memory Cell Screening for Read Stability
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Solution Overview
Problem
Non-volatile memory devices face instability during read operations due to random telegraph noise (RTN) caused by electron capture and emission at the gate oxide-channel interface, affecting accuracy and memory retention.
Innovation Solution
A read stabilization technique is implemented in the memory device controller to identify and screen out memory cells with excessive RTN by measuring and analyzing the control gate threshold voltage (Vtcg) fluctuations, using multiple measurements and voltage offset methods to distinguish stable from defective cells, and excluding defective cells from data storage operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If all memory cells are used for storage, then storage capacity is maximized, but read current stability deteriorates due to inclusion of unstable cells
Solution Approach 1:
The patent applies preliminary action by performing a read stability verification process during the formation read operation before the memory cells are fully formed. This preliminary screening identifies unstable memory cells early in the manufacturing process, allowing them to be excluded from normal storage operations while maintaining high storage capacity. The verification is conducted at a stage when cells are still being formed, enabling proactive identification and separation of defective cells.
Solution Approach 2:
The patent applies segmentation by dividing memory cells into two distinct groups: verified stable cells that are suitable for normal storage operations, and unverified or unstable cells that are excluded from regular use. This segmentation is achieved through the read stability verification process that tests individual cells during formation. The stable cells are then organized into valid storage regions, while unstable cells are either excluded or placed in separate handling categories, thereby improving overall read current stability without significantly reducing total capacity.
2Reliability
If read stability verification is performed on all memory cells, then read current stability is improved, but manufacturing time increases
Solution Approach 1:
The patent applies merging by combining the read stability verification operation with the existing formation read operation. Instead of adding a separate verification step that would increase manufacturing time, the patent integrates the stability check into the routine formation read process that is already performed during memory cell creation. This allows unstable cells to be identified and excluded without requiring additional time beyond what is already allocated for formation operations.
3Reliability
If unstable memory cells are excluded from storage, then read current stability is improved, but storage capacity decreases
Solution Approach 1:
The patent applies parameter changes by modifying the operational parameters of memory cells based on their verified stability characteristics. Stable cells that pass the read stability verification are assigned to normal storage operations with standard read and write parameters. Unstable cells that fail verification are excluded from regular storage or placed in separate handling categories with different operational parameters. This parameter differentiation allows the system to maximize the use of stable cells while minimizing the impact of excluding unstable ones, thereby maintaining high storage capacity with improved reliability.
Data Source
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AI summary
A memory device that includes a plurality of non-volatile memory cells and a controller. The controller is configured to erase the plurality of memory cells, program each of the memory cells, and for each of the memory cells, measure a threshold voltage applied to the memory cell corresponding to a target current through the memory cell in a first read operation, re-measure a threshold voltage applied to the memory cell corresponding to the target current through the memory cell in a second read operation, and identify the memory cell as defective if a difference between the measured threshold voltage and the re-measured threshold voltage exceeds a predetermined amount.