NAND Flash Read Compensation for Back Pattern Effect
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Solution Overview
Problem
The back pattern effect in NAND flash memory systems leads to errors during data reading due to changes in channel resistance and threshold voltage shifts, as more memory cells are programmed, causing conductivity changes and altered current flow.
Innovation Solution
A method for reading data from non-volatile storage involves measuring resistance information for NAND strings, setting specific read parameters based on this data, and performing read operations with compensation for neighboring memory cells to account for condition changes and maintain accurate data retrieval.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple memory cells in a NAND string are programmed sequentially, then the memory cells can be programmed to their desired state, but the channel resistance increases and causes artificial threshold voltage shifts leading to read errors
Solution Approach 1:
The patent applies preliminary compensation actions during the programming process. Specifically, compensation voltages are applied to unselected memory cells before and during the programming of selected memory cells. This preliminary action prevents the channel resistance changes from causing threshold voltage shifts in unselected cells, thereby eliminating the back pattern effect before it can corrupt data.
Solution Approach 2:
The patent dynamically changes voltage parameters during programming operations. Different voltage levels are applied to selected versus unselected word lines, and these voltages are adjusted based on the programming state. By changing the voltage parameters applied to control gates of unselected memory cells, the patent maintains proper channel conductivity and prevents artificial threshold voltage shifts.
2Measurement precision
If verify operations are performed between programming pulses to determine target level, then programming can stop when target is reached, but channel conductivity changes cause current variations that affect verification accuracy
Solution Approach 1:
The patent changes the voltage parameters applied to unselected word lines during verify operations. By adjusting these voltage parameters, the patent maintains stable channel conductivity and consistent current flow conditions during verification, ensuring accurate threshold voltage measurements despite the programming state of memory cells.
3Productivity
If memory cells on adjacent word lines are programmed in sequence, then complete programming coverage is achieved, but threshold voltage shifts in unselected cells cause read errors
Solution Approach 1:
The patent applies preliminary compensation voltages to unselected memory cells on adjacent word lines before programming operations begin. This preliminary action ensures that when programming proceeds across multiple word lines, unselected cells are protected from threshold voltage shifts, maintaining data integrity while achieving complete programming coverage.
Solution Approach 2:
The patent employs feedback mechanisms where verify operations monitor the state of memory cells, and compensation voltages are adjusted based on the programming progress and measured threshold voltages. This feedback loop ensures that data integrity is maintained throughout the multi-word line programming process.
Data Source
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AI summary
When reading data from a non-volatile storage element that is part of a group of connected non-volatile storage elements, resistance information is measured for the group. One or more read parameters are set based on the measured resistance information. The read process is then performed using the one or more parameters.