NAND Flash Read Voltage Correction Using Dual Read Sequences
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Solution Overview
Problem
Existing NAND flash memory systems face challenges in accurately determining the read voltage levels due to variations in threshold voltage distributions of memory cell transistors, leading to inefficiencies in data reading operations.
Innovation Solution
The memory system employs a dual-read sequence approach, where a first read sequence determines a correction value for read voltages based on the timing of current changes with word line voltage increases, followed by a second read sequence using the corrected voltages to enhance data reading accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If fixed read voltage levels are used in NAND flash memory, then the device structure remains simple and operation is easy, but data reading accuracy deteriorates due to threshold voltage variations
Solution Approach 1:
The patent applies preliminary action by performing a first read sequence before the actual data read operation to detect threshold voltage distribution characteristics. The controller uses this preliminary detection to determine correction values that are then applied in a second read sequence, ensuring accurate reading while maintaining relatively simple device structure.
Solution Approach 2:
The patent implements parameter changes by dynamically adjusting read voltage levels based on detected threshold voltage distributions. The controller modifies read voltage parameters in real-time according to the electrical characteristics of memory cells, transforming fixed voltage operation into adaptive voltage operation to improve reading accuracy.
2Measurement precision
If read voltage correction is performed to improve reading accuracy, then data reading precision improves, but operation time increases due to dual-read sequence
Solution Approach 1:
The patent applies periodic action by structuring the read operation into distinct phases: a first read sequence for detection and a second read sequence for actual data reading. This periodic structure allows the system to efficiently alternate between detection and data retrieval modes, minimizing overall time loss while maintaining high precision.
Solution Approach 2:
The system performs self-service by automatically detecting its own threshold voltage distribution characteristics and generating appropriate correction values without external intervention. The controller autonomously adjusts read voltages based on real-time electrical characteristics, eliminating the need for manual calibration and reducing operational overhead.
3Measurement precision
If threshold voltage distribution is detected to adjust read voltages, then reading accuracy improves, but detection and measurement difficulty increases
Solution Approach 1:
The patent uses an intermediary approach by introducing a detection mechanism that measures electrical characteristics (current or voltage) as an intermediate step. Instead of directly measuring threshold voltage distribution which is complex, the system detects easily measurable electrical parameters and uses these to infer threshold voltage characteristics, simplifying the detection process.
Solution Approach 2:
The patent replaces complex direct threshold voltage measurement with electrical characteristic detection. By substituting the measurement approach from direct threshold voltage sensing to electrical property measurement (current/voltage), the system achieves the same goal with simpler, more practical measurement techniques that are easier to implement in actual hardware.
Data Source
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AI summary
A memory device according to one embodiment includes strings, bit lines, word lines, a source line, and a controller. Each of the strings includes memory cells. The word lines are respectively coupled to the memory cells in each of the strings. The controller is configured to execute a first read operation including a first read sequence and a second read sequence. The controller is further configured to: in the first read sequence, increase voltages of the word lines at the same speed, and determine a correction value of each of a plurality of read voltages based on a timing at which an amount of current through the strings changes with an increase in voltages of the word lines; and in the second read sequence, execute a read operation using the read voltages to which the correction value is applied.