Pre-read Polarity Evaluation for Memory Read Margin
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Solution Overview
Problem
Memory devices face challenges in maintaining read margin and accuracy due to resistance drift in resistance variable memory cells, particularly when using random polarity reads, which can lead to uncorrectable errors and reduced retention and read disturb reliability.
Innovation Solution
A pre-read voltage evaluation is performed to determine sufficient read retry margin, allowing the controller to adjust the read retry polarity and potentially re-write drifted SET bits to maintain read margin, thereby avoiding frequent read retries and power consumption issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If random polarity reads are used to read memory cells, then read speed and simplicity are improved, but read margin and reliability deteriorate due to resistance drift causing uncorrectable errors
Solution Approach 1:
The patent applies preliminary action by performing a pre-read operation at a first polarity before the actual read operation. This pre-read evaluates the read margin by sensing current through the memory cell and comparing it to a threshold, allowing the system to determine whether to invert the polarity for the subsequent read operation. This advance evaluation prevents read margin failures caused by resistance drift while maintaining efficient random polarity reading.
2Measurement precision
If read retry operations are performed frequently to correct errors from resistance drift, then data accuracy is improved, but power consumption increases
Solution Approach 1:
The patent implements feedback by using the pre-read operation to evaluate read margin and provide information that controls whether a read retry with polarity inversion is necessary. The pre-read sensing current is compared to a threshold to generate feedback that determines the optimal polarity for the main read operation. This feedback mechanism ensures data accuracy by correcting polarity-related errors only when needed, thereby avoiding unnecessary read retries and reducing power consumption.
3Reliability
If higher read voltages are applied to compensate for resistance drift, then read margin is improved, but retention and read disturb reliability deteriorate
Solution Approach 1:
The patent applies dynamics by making the read voltage polarity adaptive rather than fixed. The system dynamically adjusts the polarity used for read operations based on real-time evaluation of read margin through pre-read operations. This dynamic adaptation allows the system to maintain adequate read margin by selecting the appropriate polarity without requiring consistently higher voltage levels, thereby reducing read disturb effects on retained data while ensuring reliable reads.
Data Source
AI summary
Systems, methods, and apparatus to evaluate read margin when reading memory cells in a memory device. In one approach, a controller of a memory device applies an initial read voltage of an initial polarity to memory cells. Errors from the read are used to determine whether read retry is needed. If so, a pre-read voltage of an opposite polarity is applied, and errors determined. Based on the errors from applying the pre-read voltage, a polarity is selected for the read retry voltage. The read retry voltage of the selected polarity is then applied to the memory cells.