Semiconductor Storage Device DLA Read Operation for Threshold Voltage Shift Correction
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Solution Overview
Problem
NAND-type flash memory devices face challenges in preventing read errors due to variations in threshold voltages of memory cells, which can shift due to coupling between adjacent word lines during write operations, leading to decreased reliability and performance.
Innovation Solution
The semiconductor storage device employs a direct look-ahead (DLA) read operation that involves multiple read processes with varying discharge times and voltages to correct for threshold voltage shifts, ensuring accurate data retrieval by selectively using read results from different states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple read processes with different discharge times are performed to correct threshold voltage shifts, then read accuracy is improved, but read operation time increases
Solution Approach 1:
The patent performs a first read operation before the second read operation to obtain preliminary information about the threshold voltage state. Based on the result of the first read, the controller decides whether a second read with different discharge time is necessary, thereby preparing in advance and avoiding unnecessary additional read operations
Solution Approach 2:
The patent implements a multi-stage read approach where a first read with one discharge time is performed, and a second read with a different discharge time is conditionally performed only when needed. This partial action approach ensures high read accuracy by using multiple discharge times when necessary, while avoiding the time penalty of always performing both reads
2Measurement precision
If the discharge time of the sense node is extended to detect threshold voltage shifts, then detection precision is improved, but operation speed decreases
Solution Approach 1:
The patent dynamically adjusts the discharge time of the sense node based on the read stage and detected conditions. The first read uses one discharge time setting while the second read uses a different discharge time setting, allowing the system to optimize between speed and precision dynamically rather than using a fixed discharge time
Solution Approach 2:
The patent employs periodic read operations with alternating discharge time characteristics. By performing sequential reads with different discharge times and selecting results based on threshold voltage shift detection, the system achieves both high detection precision and maintains operational speed through structured periodic measurement cycles
Data Source
AI summary
A semiconductor storage device includes first and second memory cells, first and second word lines connected to the first and second memory cells, respectively, a bit line connected to the first and second memory cells, and a sense amplifier including a sense node. During a first read, a controller applies a first read voltage to the second word line and determines a read result. During a second read, the controller discharges the sense node for a first time period while applying a second read voltage to the first word line to determine a first read result, and discharges the sense node for a second time period while applying the second read voltage to determine a second read result. The controller determines read data based on the first read result, the second read result, and the read result of the second memory cell.