Storage Device Read Level Adjustment via Off-Cell Count
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Solution Overview
Problem
Nonvolatile memory devices in storage devices, such as SSDs, face reliability issues due to deterioration over time, leading to variations in read voltage requirements across memory cells connected to different word lines, necessitating precise adjustments to maintain driving reliability.
Innovation Solution
A storage device with a nonvolatile memory cell array and an OTP memory cell array that calculates an off-cell count value from adjacent word lines to determine a threshold voltage shift, allowing for precise adjustment of read levels for memory cells, thereby improving reading accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If read voltage is adjusted according to deterioration, then driving reliability is improved, but reading accuracy deteriorates due to variations across different word lines
Solution Approach 1:
The patent applies local quality by determining read levels separately for each word line based on its specific deterioration characteristics. The controller calculates off-cell count values and threshold voltage shifts specific to each word line, then generates customized read levels for each word line rather than using a uniform adjustment across all word lines. This localized approach ensures that each word line's read operation is optimized for its individual state, resolving the contradiction between maintaining driving reliability and preserving reading accuracy.
2Measurement precision
If precise adjustment of read voltage is implemented, then reading accuracy is improved, but device complexity increases
Solution Approach 1:
The system implements self-service by automatically determining off-cell count values and calculating threshold voltage shifts for each word line without requiring external intervention or complex manual calibration. The controller autonomously generates appropriate read levels based on the calculated parameters, and this information is stored in the OTP memory cell array for future reference. This self-adjusting mechanism achieves precise reading accuracy while minimizing the need for complex external control systems.
Solution Approach 2:
The patent applies preliminary action by pre-calculating and storing threshold voltage shift information in the OTP memory cell array during manufacturing or initial operation. This pre-stored information is later used to quickly determine appropriate read levels during normal operation, avoiding the need for complex real-time calculations and reducing control complexity while maintaining high reading accuracy.
3Ease of manufacture
If uniform read voltage adjustment is applied to all word lines, then manufacturing simplicity is maintained, but reading accuracy deteriorates due to differential deterioration
Solution Approach 1:
The patent resolves this contradiction by implementing local quality through word line-specific read level determination. Rather than applying a uniform adjustment to all word lines, the system calculates individual off-cell count values and threshold voltage shifts for each word line, then generates customized read levels. This approach maintains manufacturing simplicity by using automated calculations while significantly improving reading accuracy by accounting for differential deterioration patterns across different word lines.
Data Source
AI summary
A storage device including a nonvolatile memory device that includes a nonvolatile memory cell array including a string including first and second memory cells stacked sequentially, and an OTP memory cell array that stores reference count values, the first and second memory cells respectively connected to first and second word lines; a controller including a processor that generates a read command for the first memory cell; a read level generator including a counter that receives the read command and calculates an off-cell count value of memory cells connected to the second word line, and a comparator that receives a first reference count value from the OTP memory cell array, compares the off-cell count value with the first reference count value to determine a threshold voltage shift of the second memory cell, and determines a read level of the first memory cell based on the threshold voltage shift.