Memory Read Control Using On-Cell Counts for Threshold Drift
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Solution Overview
Problem
Nonvolatile memory devices experience reliability issues due to changing threshold voltage distributions over time, leading to increased error bits when reading data with fixed read voltages, which affects the accuracy of data retrieval.
Innovation Solution
A memory device and method that applies an equalizing voltage to all word lines, followed by a series of search voltages to selected word lines, counts the number of on cells at specific time points, and determines an evaluation period based on these counts to sense bit line voltages accurately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed read voltage is applied to read data from memory cells, then the read operation is simple and fast, but the reliability of data retrieval deteriorates due to changing threshold voltage distributions over time
Solution Approach 1:
The patent applies dynamic read voltages instead of fixed read voltages. The read voltage is adjusted based on the number of on cells counted during the read operation. This dynamic adjustment compensates for threshold voltage distribution changes over time, improving data read reliability while managing operational complexity through adaptive voltage selection.
Solution Approach 2:
The patent implements a feedback mechanism where the number of on cells is counted during the read operation, and this count is used to determine the appropriate read voltage for subsequent operations. The fail cell counter provides feedback information that guides voltage adjustment, creating a closed-loop system that improves reliability by continuously adapting to threshold voltage changes.
2Measurement precision
If the read voltage is changed to accurately obtain data from memory cells, then data retrieval accuracy improves, but the read operation time increases
Solution Approach 1:
The patent performs preliminary counting of on cells using search voltages before applying the final read voltage. This preliminary action provides information about the threshold voltage distribution state, allowing the system to select an appropriate read voltage in advance. This approach ensures accurate data retrieval while minimizing the time spent on voltage adjustments during the actual read operation.
Solution Approach 2:
The patent uses search voltages to quickly assess the threshold voltage distribution state and determine the appropriate read voltage without performing a complete read operation at each voltage level. This skipping approach allows the system to rapidly identify the optimal read voltage, reducing the overall time penalty associated with adaptive voltage selection.
3Measurement precision
If multiple search voltages are applied to count on cells at different time points, then the determination of evaluation period becomes accurate, but the read operation complexity increases
Solution Approach 1:
The patent segments the read operation into distinct phases: applying search voltages at different time points, counting on cells at each phase, and determining the evaluation period based on these counts. This segmentation allows for precise evaluation period determination while organizing the complex procedure into manageable, systematic steps that can be efficiently executed.
Data Source
AI summary
A memory device includes memory cells connected to a plurality of word lines. The memory device also includes a read operation performer configured to perform a read operation of applying an equalizing voltage to the plurality of word lines and applying a read voltage to a selected word line. The memory device further includes a fail cell counter configured to count a number of on cells among selected memory cells connected to the selected word line at each of time points. The memory device additionally includes a read operation controller configured to control the read operation performer to determine a length of an evaluation period based on a result of comparing the number of on cells at each of the time points, and configured to sense a voltage of bit lines respectively connected to the selected memory cells after the evaluation period elapses from the time.