Dummy Cell Segmentation for NAND Flash Charge Sharing
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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, particularly in self-boosting and local self-boosting schemes.
Innovation Solution
Incorporating dummy cells between memory cells in the string structure, which are selectively turned off during programming to prevent charge sharing and maintain the channel voltage, thereby preventing unwanted programming of program-inhibit cells.
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:
The string structure is segmented into multiple groups by inserting dummy cells between memory cell groups. This segmentation isolates the charge in each group's channel region, preventing charge sharing between groups while maintaining high integration density. The dummy cells act as barriers that divide the continuous channel into separate segments.
Solution Approach 2:
Dummy cells are inserted as intermediary elements between memory cell groups within the same string. These dummy cells serve as mediators that block the coupling and charge sharing between adjacent memory cell groups, thereby maintaining channel voltage stability without requiring additional strings or complex control circuitry.
2Productivity
If more memory cells are placed in each string, then integration density increases, but charge sharing phenomenon worsens reducing channel voltage
Solution Approach 1:
The long string with many memory cells is divided into shorter effective segments by inserting dummy cells. Each segment contains a manageable number of memory cells whose charge cannot share with other segments. This allows high integration density while controlling the charge sharing phenomenon through structural segmentation.
Solution Approach 2:
Dummy cells are extracted or inserted into the string structure at specific positions to remove the harmful coupling effect. By taking out the continuous channel connection and replacing it with dummy cell barriers, the charge sharing pathway is eliminated while preserving the high-density memory cell arrangement.
3Ease of operation
If self-boosting scheme is used for program-inhibit cells, then programming control is improved, but channel voltage drops due to charge sharing make programming difficult
Solution Approach 1:
The channel is segmented by dummy cells into isolated regions, allowing the self-boosting voltage to be maintained in program-inhibit cell groups without being drained by charge sharing with neighboring groups. This segmentation preserves the energy needed for programming control while enabling high-density integration.
Solution Approach 2:
Dummy cells act as intermediary barriers that prevent the leakage of boosted channel voltage from program-inhibit groups to adjacent groups. This mediation maintains the energy level required for programming control in each isolated group, solving the voltage drop problem caused by charge sharing.
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 use of dummy cells effectively minimizes charge sharing and maintains the channel voltage, ensuring reliable programming operations even in high integration density NAND flash memory devices by isolating the channel and preventing F-N tunneling in program-inhibit cells.
Implementation Method 1
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 non-volatile memory device includes a selection transistor coupled to a bit line. The device also includes a plurality of memory cells serially coupled to the selection transistor and at least one dummy cell located between the plurality of memory cells. The dummy cell is turned off during a programming operation of a memory cell located between the dummy cell and the selection transistor.