NAND Flash Verify Voltage Segmentation for Back Pattern Effect
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Solution Overview
Problem
The back pattern effect in NAND flash memory devices leads to errors during data reading due to changes in channel resistance and IV characteristics after programming, caused by uneven programming of memory cells, which affects the accuracy of threshold voltage determination.
Innovation Solution
Applying distinct voltages to unselected word lines that have been programmed versus those that have not been programmed, and coordinating these voltages with the voltage applied to the particular non-volatile storage element to accurately sense its condition during verify operations, thereby reducing errors from the back pattern effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single voltage is applied to all unselected word lines during verify operations, then the device complexity is reduced, but measurement precision deteriorates due to the back pattern effect causing artificial threshold voltage shifts
Solution Approach 1:
The patent segments the unselected word lines into two distinct groups: those that have been subjected to programming operations and those that have not. Each group receives a different voltage level during verify operations. This segmentation eliminates the back pattern effect by ensuring that programmed and unprogrammed word lines have comparable channel resistance, thereby maintaining accurate threshold voltage measurements without requiring complex additional circuitry.
Solution Approach 2:
The patent applies different voltage levels to different subsets of unselected word lines based on their programming status. Specifically, a first voltage is applied to unselected word lines that have been programmed, while a second voltage is applied to unselected word lines that have not been programmed. This local differentiation in voltage application compensates for the resistance changes caused by programming, eliminating the back pattern effect and improving measurement precision.
2Ease of manufacture
If programming is performed in sequence from source side to drain side, then the manufacturing process is simplified, but reliability deteriorates due to the back pattern effect causing reading errors
Solution Approach 1:
The patent applies the appropriate voltage to unselected word lines before the actual verify operation on the selected word line begins. By pre-establishing the correct voltage conditions on unselected word lines based on their programming status, the system eliminates the back pattern effect that would otherwise occur during the verify operation, thereby ensuring reading accuracy while maintaining the simple sequential programming approach.
Solution Approach 2:
The patent changes the voltage parameter applied to unselected word lines based on their programming history. Instead of using a uniform voltage, the system adjusts the voltage level to match the resistance characteristics of the specific subset of unselected word lines (programmed versus unprogrammed), thereby eliminating the back pattern effect and improving data reading reliability.
Data Source
AI summary
When performing a data sensing operation, including a verify operation during programming of non-volatile storage elements (or, in some cases, during a read operation after programming), a first voltage is used for unselected word lines that have been subjected to a programming operation and a second voltage is used for unselected word lines that have not been subjected to a programming operation. In some embodiments, the second voltage is lower than the first voltage.