Dynamic Read Voltage Adjustment for NAND Flash Memory
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Solution Overview
Problem
Non-volatile memory devices, such as NAND flash, face challenges in maintaining data integrity due to noise introduced by program disturb and inter-cell interference, leading to voltage level drops over time, which affects the accuracy of read operations as threshold voltage distributions shift and overlap, especially with increased program and erase cycles.
Innovation Solution
The method involves generating updated read voltages by estimating parameters using regularized least squares based on cell counts and properties of the memory device, such as program/erase cycles, to account for degradation and reduce variance, thereby improving the reliability of data retrieval across the lifespan of the memory device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional fixed read voltages are used, then initial read accuracy is maintained, but read errors increase over time due to voltage level drops from program disturb and inter-cell interference
Solution Approach 1:
The patent implements dynamic read voltage adjustment by transitioning from fixed initial read voltages to updated read voltages that adapt to cell degradation. The system continuously monitors cell voltage levels and adjusts read voltages accordingly, making the read operation dynamic rather than static to maintain accuracy throughout the memory device lifespan.
Solution Approach 2:
The patent changes the voltage parameter by calculating updated read voltages based on observed voltage level drops. The system modifies the read voltage values dynamically to compensate for degradation, directly applying parameter changes to maintain measurement precision despite aging effects.
2Reliability
If more read voltages are applied to account for degradation, then data retrieval robustness improves, but computational complexity and measurement time increase
Solution Approach 1:
The patent implements feedback by measuring actual cell voltage levels and using these measurements to update read voltage thresholds. The system creates a closed-loop control where read operations provide feedback about cell degradation, which is then used to adjust subsequent read voltages, improving robustness without requiring excessive complexity.
Solution Approach 2:
The patent performs preliminary measurements of cell voltage levels before final read operations. By conducting initial characterization measurements and using these to pre-calculate updated read voltages, the system prepares in advance to avoid complex real-time computations during actual data retrieval operations.
3Measurement precision
If multiple cell counts are obtained for parameter estimation, then accuracy of voltage threshold determination improves, but read operation time increases
Solution Approach 1:
The patent applies partial action by obtaining a sufficient but not excessive number of cell counts for parameter estimation. The system determines the minimum necessary measurements required to achieve accurate voltage threshold determination, avoiding unnecessary additional measurements that would waste time while ensuring sufficient data for reliable parameter estimation.
Data Source
AI summary
Disclosed are devices, systems and methods for improving the reliability of retrieving information from a memory device. An exemplary method includes obtaining a plurality of cell counts for each of a plurality of read voltages applied to the memory device, each of the plurality of cell counts representing a number of cells having a cell voltage value that is within a voltage band corresponding to the read voltage applied thereto, generating, based on the plurality of cell counts, the plurality of read voltages and a design parameter, a set of estimated parameters, the design parameter being based on one or more properties of the memory device, determining an updated read voltage based on the estimated set of parameters, and applying the updated read voltage to the memory device to retrieve the information from the memory device.