Multi-Level Cell Programming Shadow-Sequence for NAND Flash Coupling
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Solution Overview
Problem
As integration density of NAND flash memory arrays increases, adverse coupling effects during programming of multi-level cells (MLC) become significant, particularly word line coupling, which affects the programming of adjacent cells.
Innovation Solution
A method of programming MLC in a non-volatile memory device that involves shadow-programming followed by main-programming, where MLC are grouped and programmed in a specific sequence to reduce or prevent bit-line and word line coupling by adjusting threshold voltage distributions, using techniques like incremental pulse programming.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional programming methods are used for MLC in high-density NAND flash memory arrays, then programming speed is maintained, but adverse coupling effects (word line coupling and bit-line coupling) significantly affect adjacent MLC
Solution Approach 1:
The programming operation is segmented into multiple phases: pre-programming phase where MLC are programmed to intermediate states, and main programming phase where final states are achieved. This segmentation allows coupling effects to be managed across different programming stages, improving accuracy while maintaining speed.
Solution Approach 2:
The patent applies preliminary action by performing pre-programming of MLC to intermediate states before the main programming operation. This preliminary programming establishes a baseline state that reduces the impact of coupling effects during the subsequent main programming phase, thereby improving programming accuracy.
2Productivity
If integration density of NAND flash memory arrays is increased, then device size and power consumption are reduced, but coupling effects during programming become more significant
Solution Approach 1:
The patent applies local quality by differentiating programming operations for MLC based on their positional relationships. MLC are categorized into first MLC (with first neighboring MLC) and second MLC (with second neighboring MLC), and different programming sequences and methods are applied to each group to manage coupling effects locally.
Solution Approach 2:
The MLC array is segmented into different groups based on their coupling relationships. The programming operation is divided into sequential steps that first handle pre-programming of specific groups, then proceed to main programming. This segmentation allows the system to maintain high integration density while managing the increased coupling effects through structured programming sequences.
3Ease of operation
If word line coupling is addressed through conventional programming, then programming operation is simple, but threshold voltage distributions of adjacent MLC are adversely affected
Solution Approach 1:
The programming operation is segmented into pre-programming and main programming phases. During pre-programming, MLC are programmed to intermediate states with controlled threshold voltage changes. During main programming, final states are achieved with reduced coupling effects. This segmentation maintains operational simplicity while improving threshold voltage distribution precision.
Solution Approach 2:
The patent applies preliminary action by performing pre-programming to intermediate states before main programming. This preliminary step establishes a foundation that reduces the impact of word line coupling on threshold voltage distributions during the subsequent main programming phase, thereby improving manufacturing precision without significantly complicating the overall operation.
4Reliability
If programming sequence is optimized to reduce coupling, then programming accuracy is improved, but programming time increases
Solution Approach 1:
The programming operation is segmented into pre-programming and main programming phases, with specific sequences for first MLC and second MLC. By organizing the programming sequence this way, the patent achieves improved accuracy through better coupling management while minimizing time overhead through efficient phase transitions and parallel processing capabilities.
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 effectively reduces or prevents adverse coupling between adjacent MLC, improving the accuracy and efficiency of programming by minimizing the impact of threshold voltage changes during most significant bit (MSB) and least significant bit (LSB) programming.
Implementation Method 1
The channel of the string selection transistor is boosted to a voltage equal to the power voltage less a threshold voltage of the string selection transistor. As a result, the string selection transistors coupled to the non-selected cells are turned OFF. The selected cells are programmed by Fowler-Nordheim tunneling between the floating gates and respective channels.
Data Source
AI summary
A method of programming a multi-level cells (MLC) commonly coupled to a word line in a non-volatile memory device includes shadow-programming first MLC to a first shadow state, shadow-programming second MLC to a second shadow state less than the first shadow state, and then main-programming the first MLC from the first shadow state to a first final state and main-programming the second MLC from the second shadow state to the second final state less than the first final state.