Nonvolatile Memory Read Method Voltage Adjustment
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Solution Overview
Problem
Flash memory devices experience read failures due to charge leakage, which causes threshold voltage distributions of programmed cells to shift, leading to incorrect data readouts, particularly in Multi-Level Cell (MLC) flash memories.
Innovation Solution
A read method involving a first and second select read voltage is applied to the selected word line, with the second voltage being lower than the first, and if a read failure occurs, a program operation is performed to cure the failure, ensuring accurate data retrieval.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high select read voltage is applied to read memory cells, then the read operation can detect programmed cells, but charge leakage causes threshold voltage shift leading to read failures
Solution Approach 1:
The patent divides the read operation into multiple stages with different select read voltages. A first read operation uses a higher select read voltage to detect cells with significant charge leakage, while a second read operation uses a lower select read voltage to accurately read cells without leakage. This segmentation allows the system to handle different cell states appropriately, resolving the contradiction between detection capability and read reliability.
Solution Approach 2:
The patent dynamically adjusts the select read voltage based on the detected cell state. When charge leakage is detected in the first read operation, the system switches to a lower select read voltage for the second read operation. This dynamic voltage adjustment optimizes both the detection of programmed cells and the prevention of read failures, resolving the contradiction between measurement precision and reliability.
2Reliability
If the threshold voltage distribution of OFF cells shifts into the ON cell distribution due to charge leakage, then data stored as '0' may be read as '1', but maintaining a sufficient read margin requires lower read voltages
Solution Approach 1:
The patent performs a preliminary read operation at a higher select read voltage to detect cells that have experienced charge leakage before performing the actual read operation. This preliminary detection allows the system to identify cells whose threshold voltage distribution has shifted, enabling subsequent corrective actions such as reprogramming or using appropriate read voltages to maintain data integrity and sufficient read margin.
Data Source
AI summary
Disclosed is a read method of a non-volatile memory device which includes performing a first read operation in which a first read voltage is applied to a selected word line. If a read fail arises at the first read operation, a second read operation is performed in which a second read voltage lower than the first read voltage is applied to the selected word line. If no read fail arises at the second read operation, the read fail generated at the first read operation is cured by performing a program operation.