Word Line Group Reference Voltage Tuning for NAND Data Retention
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Solution Overview
Problem
Existing memory devices, particularly non-volatile memory devices like NAND, face challenges in maintaining accurate data storage over time due to degradation in the ability of memory cells to retain charges, with current block-level error avoidance schemes lacking sufficient granularity.
Innovation Solution
Implementing a system that adjusts reference voltages on a group of word lines basis, considering the age of stored data, to compensate for the memory cells' charge retention capabilities, combining block-level and word line group-level adjustments for improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If block-level reference voltage adjustment is used, then error avoidance capability is improved, but granularity of adjustment is insufficient
Solution Approach 1:
The patent segments the memory block into multiple word line groups (e.g., first word line group, second word line group, etc.), allowing independent reference voltage adjustment for each group. This segmentation enables finer granularity control compared to traditional block-level adjustment, where all word lines in a block shared the same reference voltage. Each word line group can now be individually optimized based on its specific charge retention characteristics.
Solution Approach 2:
The patent applies local quality by allowing different reference voltage adjustments for different word line groups within the same block. Specifically, the first reference voltage adjustment value is applied to the first word line group while a second reference voltage adjustment value is applied to the second word line group. This enables localized optimization of read accuracy for each word line group based on its individual performance characteristics.
2Measurement precision
If reference voltages are adjusted based on data age, then read accuracy for old data is improved, but system complexity increases
Solution Approach 1:
The patent implements preliminary action by determining reference voltage adjustment values in advance based on data age categories. The system pre-establishes multiple reference voltage adjustment values corresponding to different data age ranges (e.g., recent data, intermediate-age data, old data). When a read operation is requested, the system simply selects the appropriate pre-determined adjustment value based on the data's age category, avoiding complex real-time calculations during the read operation.
Solution Approach 2:
The patent changes the reference voltage parameter dynamically based on data age. The system determines a data age category (first category, second category, third category, etc.) and selects corresponding reference voltage adjustment values from a set of pre-determined values. This parameter change approach allows the system to adapt to degradation in charge retention over time while maintaining a relatively simple implementation through discrete categories rather than continuous adjustment.
3Adaptability or versatility
If multiple reference voltage adjustment values are maintained, then adaptability to different data ages is improved, but memory resource usage increases
Solution Approach 1:
The patent manages multiple reference voltage adjustment values as discrete parameters associated with different data age categories rather than storing extensive calibration data. The system maintains a limited set of adjustment values (first reference voltage adjustment value, second reference voltage adjustment value, third reference voltage adjustment value, etc.), each corresponding to a specific data age category. This parameter-based approach provides good adaptability while consuming minimal memory resources compared to storing full voltage lookup tables or calibration profiles.
Data Source
AI summary
Methods, systems, and devices for reference voltage adjustment for word line groups are described. In some examples, one or more components of a memory system may determine a duration that data has been stored to one or more memory cells. Based on the duration, a voltage value of one or more reference voltages may be adjusted accordingly. For example, a voltage value of one or more reference voltages may be adjusted based on the duration. Moreover, the reference voltage values may be adjusted differently in response to the memory cells having stored data for a relatively longer duration, as opposed to memory cells that have stored data for a relatively shorter duration. The adjusted reference voltages may be used during a subsequent read operation. The voltage value of the one or more reference voltages may be adjusted on a word-line group by word-line group basis.