Flash Memory Dummy Cell Charge Sharing Mitigation
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Solution Overview
Problem
High integration density in NAND flash memory devices leads to increased coupling between memory cells, causing charge sharing and a decrease in channel voltage, which can result in undesirable programming operations, especially in highly integrated devices where local self-boosting schemes are ineffective due to high coupling ratios between word lines.
Innovation Solution
Incorporating a dummy cell in each string connected to a dummy word line, which is programmed to the highest state after an erase operation to block charge sharing and maintain channel voltage, thereby preventing unwanted programming of program-inhibit cells by minimizing charge sharing through the use of a dummy word line voltage that matches or is lower than unselected word lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If integration density of memory cells is increased, then device size is reduced, but coupling between memory cells increases causing charge sharing and channel voltage decrease
Solution Approach 1:
A dummy cell is introduced as an intermediary element between actual memory cells in a string. This dummy cell, when programmed to the highest state, acts as a barrier that prevents charge sharing between adjacent memory cells while maintaining the high integration density layout. The dummy cell serves as a mediator that isolates the electrical interaction between closely spaced memory cells.
Solution Approach 2:
The invention changes the state parameter of a specific cell (the dummy cell) to the highest programmed state, which creates a strong potential barrier. By controlling the threshold voltage of the dummy cell to be significantly different from other cells, it effectively blocks charge sharing paths without requiring physical separation of memory cells.
2Quantity of substance
If integration density is increased, then more memory cells fit in a string, but local self-boosting schemes become ineffective due to high coupling ratios between word lines
Solution Approach 1:
The dummy cell serves as an intermediary that decouples the electrical interaction between adjacent memory cells. By placing a programmed dummy cell between selected and unselected memory cells, the scheme prevents the propagation of boosted channel voltage to unselected cells, making the programming operation reliable even with high integration density.
3Quantity of substance
If memory cells are formed closer together, then integration density increases, but channel voltage drops due to charge sharing
Solution Approach 1:
The invention changes the threshold voltage parameter of the dummy cell to the highest possible value by programming it to the highest state. This parameter change creates a deep potential well that traps charge and prevents it from sharing with adjacent cells, thereby maintaining channel voltage at required levels despite close spacing.
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
The introduction of dummy cells effectively minimizes charge sharing and maintains channel voltage, ensuring reliable programming operations even in high integration density devices by isolating the channel voltage of program-inhibit cells, thus preventing unintended programming.
Implementation Method 1
an increase in integration density may also cause problems. For example, the more highly integrated the memory device is, the more closely the memory cells are formed. This reduction in space between the memory cells may lead to an increase in coupling between memory cells. Furthermore, as the integration degree of the flash memory device is increased, a string may include more memory cells. This increase in memory cells with less space between them may cause a charge sharing phenomenon that may reduce a boosted channel voltage
Implementation Method 2
NAND flash memory devices are programmed and erased using Fowler-Nordheim (F-N) tunneling. In F-N tunneling, electrons are injected/discharged from/to a channel region into/from a floating gate due to a large voltage difference between the channel region and a control gate of a cell transistor
Data Source
AI summary
A nonvolatile semiconductor memory device includes a string selection transistor coupled to a bit line. The device also includes a plurality of memory cells coupled in series to the string selection transistor, wherein at least one of the memory cells is configured to be in a programmed state during an erase procedure of the plurality of memory cells.