NAND String Middle-Out Programming for Residual Electron Reduction
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Solution Overview
Problem
NAND flash memory systems face challenges in reducing residual electrons after sensing operations, which can lead to program disturb and unintentional programming due to trapped electrons in the polysilicon channel, affecting data retention and programming accuracy.
Innovation Solution
Implementing a middle-out programming sequence where memory cell transistors in the middle of the NAND string are programmed and verified before the ends, and using control circuitry to bias the polysilicon channel to remove residual electrons by charging it to a voltage greater than ground, thereby reducing electron trapping and enhancing channel boosting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional programming sequences are used, then programming speed is maintained, but residual electrons cause program disturb and unintentional programming
Solution Approach 1:
The patent segments the programming sequence into distinct phases: a first programming sequence that programs memory cells from one end of the NAND string toward the middle, and a second programming sequence that programs from the other end toward the middle. This segmentation allows residual electrons to be managed in controlled stages, preventing program disturb while maintaining overall programming efficiency.
Solution Approach 2:
The patent applies preliminary actions by performing verify operations before complete programming is finished. The verify operation checks whether memory cells have reached their final programmed states, allowing the system to stop programming early for cells that are already correctly programmed, thus preventing unnecessary electron injection that could cause program disturb.
2Reliability
If middle-out programming sequence is used, then residual electrons are reduced and programming accuracy improves, but programming complexity increases
Solution Approach 1:
The programming operation is divided into two sequential segments: first programming memory cells from one end toward the middle, then programming from the other end toward the middle. This segmentation simplifies the management of residual electrons by handling them in two controlled passes rather than attempting to manage all cells simultaneously, reducing overall system complexity.
Solution Approach 2:
The patent incorporates verify operations that provide feedback on the programming status of memory cells. These verify operations detect whether cells have reached their final programmed states, allowing the programming sequence to adapt and stop early when appropriate, thereby simplifying control while improving reliability.
3Measurement precision
If verify operations are performed frequently, then programming accuracy is ensured, but operation time increases
Solution Approach 1:
The verify operation is performed as a preliminary check before completing the full programming sequence. By verifying early whether memory cells have already reached their final programmed states, the system can avoid unnecessary additional programming cycles, thereby reducing total operation time while maintaining programming accuracy.
Solution Approach 2:
The patent applies partial action by performing verify operations only on necessary portions of the memory block rather than exhaustive verification of all cells. This selective verification approach ensures programming accuracy for critical cells while minimizing the time penalty associated with comprehensive verification.
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 reduces residual electrons in the NAND string, preventing program disturb and improving programming accuracy and data retention by ensuring that memory cells reach their final states without interference.
Implementation Method 1
control circuitry to bias the polysilicon channel to remove residual electrons by charging it to a voltage greater than ground
Data Source
AI summary
Systems and methods for reducing residual electrons within a NAND string subsequent to performing a sensing operation using the NAND string or during the sensing operation. A middle-out programming sequence may be performed in which memory cell transistors in the middle of the NAND string are programmed and program verified prior to programming and verifying other memory cell transistors towards the drain-side end of the NAND string and/or the source-side end of the NAND string. In one example, for a NAND string with 32 memory cell transistors corresponding with word lines WL0 through WL31 from the source-side end of the NAND string to the drain-side end of the NAND string, the memory cell transistor corresponding with word line WL16 may be programmed and program verified prior to programming the memory cell transistors corresponding with word lines WL15 and WL17.