NAND Memory Read Timing for Shifted Threshold Voltage Distributions
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Solution Overview
Problem
In high-density memory devices, threshold voltage (Vt) distribution shift due to factors like programmed cells charge loss and noise leads to read failures, as pre-defined read levels fail to track the shifted distribution accurately.
Innovation Solution
A method for performing a read operation in NAND memory devices involves first sensing operations using a first read voltage and a set of develop times, followed by selecting an optimal read develop time based on these operations to accurately determine the threshold voltage of memory cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pre-defined read levels are used for reading memory cells, then the read operation is simple and fast, but read failures occur due to threshold voltage distribution shift
Solution Approach 1:
The patent performs preliminary sensing operations before the actual read operation to determine the optimal read develop time. This preliminary action characterizes the threshold voltage distribution and selects appropriate read parameters in advance, ensuring reliable reading even when threshold voltage shifts occur, without adding significant complexity to the main read operation.
Solution Approach 2:
The patent dynamically adjusts the read develop time based on the sensed threshold voltage distribution. Instead of using fixed pre-defined read levels, the system adapts the read parameters (specifically develop time) to match the actual state of the memory cells, thereby maintaining high read reliability under varying conditions.
2Measurement precision
If multiple sensing operations with different develop times are performed, then the threshold voltage distribution can be tracked accurately, but the read operation time increases
Solution Approach 1:
The patent performs a limited number of sensing operations with different develop times just enough to characterize the threshold voltage distribution and identify the optimal read parameter. This partial action approach achieves sufficient measurement precision without performing excessive sensing operations that would unnecessarily extend the read time.
Solution Approach 2:
The patent changes the develop time parameter in sensing operations to probe different regions of the threshold voltage distribution. By systematically varying this single parameter and analyzing the results, the system can accurately determine the optimal read develop time without needing to perform comprehensive measurements across all possible parameters.
Data Source
AI summary
A method for operating a memory device includes performing first sensing operations on memory cells of the memory device based on a first read voltage and a first set of develop times. The memory cells are coupled to a word line. The method further includes selecting a first read develop time from the first set of develop times based on results of the first sensing operations and performing a read operation on the memory cells based on the first read voltage and the first read develop time.