NAND Flash Memory Cell Voltage Control for Threshold Distribution
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Solution Overview
Problem
In NAND type nonvolatile semiconductor memory devices, the sequential writing of data from memory cells disposed on a source line side leads to variations in threshold voltages, resulting in a broadened threshold voltage distribution, which affects data storage reliability.
Innovation Solution
A nonvolatile semiconductor memory device configuration where the control unit applies different control voltages to word lines connected to memory cells, with cells closer to the source line receiving a lower voltage than those closer to the bit line, ensuring uniform electron storage and reducing threshold voltage variations during write and erase operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If sequential programming is performed from source line side memory cells, then data write operation can be completed, but threshold voltage distribution broadens
Solution Approach 1:
The cell string is divided into multiple memory cell groups, each group being programmed separately. This segmentation allows independent control of programming conditions for each group, preventing threshold voltage distribution broadening while maintaining write operation completeness.
Solution Approach 2:
Different programming conditions are applied to different memory cell groups based on their specific characteristics. The control unit adjusts programming parameters locally for each group to compensate for position-dependent variations, ensuring uniform threshold voltage distribution across the entire cell string.
2Device complexity
If uniform programming conditions are applied to all memory cells, then programming process is simple, but threshold voltage variations increase
Solution Approach 1:
The programming process dynamically adjusts conditions based on memory cell group characteristics. The control unit determines appropriate programming conditions for each group based on their positions and characteristics, creating a adaptive programming strategy that maintains simplicity while ensuring uniformity.
Solution Approach 2:
The control unit monitors and evaluates the state of memory cell groups during programming and adjusts conditions accordingly. This feedback mechanism ensures that programming conditions are optimized for each group, preventing threshold voltage variations while maintaining process manageability.
3Manufacturing precision
If different control voltages are applied to word lines based on memory cell position, then threshold voltage distribution is narrowed, but control complexity increases
Solution Approach 1:
The word lines are segmented into different control voltage regions corresponding to different memory cell groups. Each region receives appropriately adjusted control voltages, allowing precise threshold voltage control while organizing complexity through structured segmentation.
Solution Approach 2:
Control voltage parameters are changed according to memory cell group characteristics and positions. The control unit adjusts voltage parameters locally for each group, optimizing threshold voltage distribution while managing complexity through systematic parameter adjustment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach helps maintain a narrower threshold voltage distribution, ensuring consistent data storage and retrieval by adjusting the electric field strength across memory cells based on their position, thereby enhancing data integrity and storage efficiency.
Implementation Method 1
adjusting the electric field strength across memory cells based on their position
Data Source
AI summary
A nonvolatile semiconductor memory device according to an embodiment comprises a control unit, during a data erase, applying at least to a word line connected to a memory cell disposed most to a source line side a lower control voltage than that applied to a word line connected to a memory cell disposed most to a bit line side, of a plurality of word lines connected to at least a plurality of memory cells mutually written with data of an identical number of bits in a cell string.