NAND Memory Read Circuits Using Elevated Word-Line Recovery
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Solution Overview
Problem
The reliability of NAND Flash memory devices is compromised due to charge migration between adjacent word lines during read operations, leading to read disturbance and poor data retention, especially as storage capacity increases.
Innovation Solution
Implementing a peripheral circuit that applies a recovery voltage greater than the supply voltage to word lines after read operations, either through a shortened or canceled recovery phase, to reduce charge migration and stabilize threshold voltage distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a full recovery phase is implemented to discharge word lines to supply voltage, then charge migration between adjacent word lines is reduced, but read operation duration increases
Solution Approach 1:
The patent applies a pass voltage to unselect word lines before the read operation to pre-charge them to a voltage level closer to the supply voltage. This preliminary action reduces the voltage differential that drives charge migration during the read operation, thereby improving data retention without requiring a prolonged recovery phase afterward.
Solution Approach 2:
The patent discharges unselect word lines to a recovery voltage that is greater than the supply voltage during the recovery phase. This excessive discharge action ensures that the word lines are brought to a voltage state that prevents charge migration more effectively than simply returning to supply voltage, while the overall recovery time is reduced through optimized voltage transitions.
2Productivity
If read voltage is applied to select word line to perform read operation, then data is read from memory cells, but charge migration occurs to adjacent unselect word lines causing read disturbance
Solution Approach 1:
The patent applies a pass voltage to unselect word lines simultaneously with or before applying the read voltage to the select word line. This preliminary anti-action creates a protective voltage state on adjacent unselect word lines that counteracts the electric field-induced charge migration, preventing read disturbance while allowing the read operation to proceed at full speed.
Solution Approach 2:
The patent changes the voltage parameter of unselect word lines from a low state to a pass voltage level during the read operation. This parameter change on adjacent word lines modifies the electric field distribution in the memory array, preventing charge migration from the select word line to unselect word lines while maintaining fast read operation timing.
3Loss of time
If recovery phase is shortened or canceled to improve read speed, then read operation duration is reduced, but charge migration between adjacent word lines increases
Solution Approach 1:
The patent performs the critical charge migration prevention action during the read operation itself by maintaining pass voltage on unselect word lines, rather than relying on a separate post-read recovery phase. This preliminary action during the active read period eliminates the need for a prolonged recovery phase, achieving both fast read speeds and reliable data retention.
Solution Approach 2:
The patent maintains the protective pass voltage on unselect word lines continuously throughout the read operation and into the recovery phase, rather than allowing it to drop immediately. This continuous protective action ensures that charge migration is prevented throughout the entire operation cycle, enabling shortened recovery times without sacrificing data retention reliability.
Data Source
AI summary
In certain aspects, a memory device includes an array of memory cells, word lines respectively coupled to rows of the memory cells, and a peripheral circuit coupled to the array of memory cells through the word lines. The peripheral circuit is configured to apply a read voltage to a select word line of the word lines, and discharge the select word line from a voltage greater than the read voltage to a first recovery voltage that is greater than a supply voltage. The discharging of the select word line stops when a voltage of the select word line reaches the first recovery voltage.