Wordline Bias Lookup Table for MLC Flash Memory
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Solution Overview
Problem
Multi-level cell (MLC) flash memory devices experience degradation in data reliability over their life cycle due to variations in threshold voltages across wordlines, leading to errors and reduced endurance, making them unsuitable for mission-critical applications without sophisticated algorithms and additional capacity measures.
Innovation Solution
A method is provided to determine and store bias values for wordlines based on program level distributions, which are used to offset shifts in threshold voltages, thereby reducing errors and improving the reliability and endurance of MLC flash memory by adjusting program and read operations dynamically.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multi-level cell flash memory is used to increase data capacity, then storage density is improved, but data reliability deteriorates due to threshold voltage variations across wordlines
Solution Approach 1:
The patent applies preliminary action by performing characterization operations during manufacturing to determine bias values for each wordline before the memory device is put into service. These bias values are stored in a lookup table and automatically applied during normal operation to compensate for threshold voltage shifts, preventing reliability degradation rather than correcting it after errors occur.
Solution Approach 2:
The patent changes the parameter of read voltage by dynamically adjusting it based on wordline-specific bias values stored in the lookup table. Instead of using fixed read voltages, the system selects appropriate voltage levels from the lookup table based on the current wordline being accessed, thereby compensating for manufacturing variations and threshold voltage shifts across different wordlines.
2Reliability
If sophisticated algorithms and wear-leveling are employed to mitigate MLC reliability issues, then data reliability is improved, but device complexity increases
Solution Approach 1:
The patent moves complexity from runtime operations to manufacturing time by pre-characterizing each wordline during manufacturing and storing bias values in a lookup table. This eliminates the need for complex runtime algorithms to compensate for wordline variations, as the compensation values are already determined and stored for immediate use during normal read operations.
Solution Approach 2:
The patent enables the memory device to self-correct for wordline variations by incorporating the bias values directly into the memory chip's lookup table. The device uses these pre-stored values automatically during operation without requiring external intervention or complex host-side algorithms, making the reliability improvement inherent to the device itself.
3Reliability
If read retry capabilities are implemented to handle errors, then data reliability is improved, but operational complexity and time overhead increase
Solution Approach 1:
The patent performs preliminary compensation by applying wordline-specific bias values before read operations occur. By pre-adjusting the read voltages based on stored bias values, the system prevents errors from occurring in the first place, eliminating the need for time-consuming read retry operations that would otherwise be required to correct errors after they occur.
Data Source
AI summary
Disclosed is an apparatus and method for determining memory cell bias information for use in memory operations. One or more memory die are selected from a group of memory die, and one or more memory blocks selected from the selected one or more memory die. A group of cells within the selected memory blocks are programmed and cycled. Bias values are generated based on comparing one or more program levels associated with respective wordlines with predetermined programming levels. The bias values are stored lookup table that is configured to be accessible at runtime by a memory controller for retrieval of the bias value during a memory operation.