Adaptive Read Voltage Controller for NAND Flash Memory
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Solution Overview
Problem
Current memory systems face challenges in accurately reading multi-bit data from NAND flash memory due to variations in threshold voltage distributions, leading to errors in data retrieval, especially when threshold voltage distributions shift or overlap, which affects the reading accuracy and reliability.
Innovation Solution
The implementation of an M-point Vth tracking operation, where the controller adjusts the reading voltages based on detected valley positions between different states of memory cell transistors, allowing for shift reading to minimize error bits by estimating optimal reading voltages for each page, thereby improving reading accuracy across multiple pages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple reading voltages are used to read multi-bit data from NAND flash memory, then reading capacity increases, but reading accuracy decreases due to threshold voltage distribution variations
Solution Approach 1:
The patent implements dynamic voltage adjustment by detecting valley positions in threshold voltage distributions and adapting reading voltages accordingly. The controller performs Vth tracking operations to determine optimal reading voltages for each page, allowing the system to dynamically adjust voltages based on actual memory state rather than using fixed voltages, thereby maintaining high reading capacity while improving accuracy.
Solution Approach 2:
The patent changes the voltage parameter adaptively by estimating optimal reading voltages for each page based on detected valley positions. Instead of using predetermined fixed voltages, the system calculates and applies adjusted voltages (e.g., first reading voltage, second reading voltage, third reading voltage) that are optimized for the current threshold voltage distribution, resolving the contradiction between capacity and accuracy.
2Device complexity
If fixed reading voltages are used for all pages, then device complexity is reduced, but reading accuracy deteriorates when threshold voltage distributions shift
Solution Approach 1:
The patent implements feedback mechanisms through Vth tracking operations where the controller detects valley positions in threshold voltage distributions and uses this information to adjust reading voltages. The system continuously monitors and adapts based on actual memory conditions, improving reliability without requiring overly complex manual intervention by automating the feedback loop.
Solution Approach 2:
The patent performs preliminary Vth tracking operations to detect valley positions and estimate optimal reading voltages before actual data reading occurs. By preparing and determining the appropriate voltages in advance for each page, the system ensures high reading reliability without increasing operational complexity during the actual data retrieval process.
3Ease of operation
If traditional reading operations are used, then operation simplicity is maintained, but error rate increases due to overlapping threshold voltage distributions
Solution Approach 1:
The patent segments the reading process into multiple stages: first performing Vth tracking operations to detect valley positions, then using these detections to determine page-specific reading voltages. This segmentation allows the complex error reduction functionality to be separated from the simple data reading operation, maintaining ease of use while significantly reducing error rates caused by overlapping threshold voltage distributions.
Data Source
AI summary
According to an embodiment, a semiconductor memory, on receiving a first command, applies a voltage within a first range and a voltage within a second range to a word line and reads a first bit from a memory cell, and, on receiving a second command, applies a voltage within a third range to the word line and reads a second bit from the memory cell. The controller issues the first command a plurality of times and changes the voltages to be applied to the word line within the first range and the second range in accordance with the plurality of first commands, specifies a first and second voltage within the first and the second range, respectively, and estimates a third voltage within the third range. The voltage applied to read the second bit is the estimated third voltage.