Multiple Select Gates with Distinct Threshold Voltages for NAND Flash
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Solution Overview
Problem
Current memory technologies face challenges in increasing memory density while minimizing disturb effects and program disturb during programming operations in NAND flash memory arrays, particularly due to capacitive coupling between select lines, which can lead to data corruption.
Innovation Solution
The implementation of multiple select gates connected in series between strings of series-connected memory cells and a data line, with threshold voltages adjusted to mutually exclusive ranges, allows for selective connection and isolation of NAND strings to the data line, reducing capacitive coupling and enhancing operational reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If memory cell spacing is reduced to increase memory density, then memory density is improved, but capacitive coupling between select lines increases causing program disturb
Solution Approach 1:
The patent divides the select gate functionality into multiple separate select gates (first select gate and second select gate) connected in series between the NAND string and data line. This segmentation allows independent control of each select gate, enabling precise selection while reducing unwanted capacitive coupling effects on adjacent lines.
Solution Approach 2:
The patent applies different threshold voltage characteristics to different select gates. The first select gate has a first threshold voltage and the second select gate has a second threshold voltage, creating local quality differences that enable selective activation based on voltage levels, thereby mitigating program disturb in high-density configurations.
2Reliability
If multiple select gates with different threshold voltages are connected in series, then program disturb is reduced, but device complexity increases
Solution Approach 1:
The multiple select gates with different threshold voltages serve universal selection functionality across different voltage ranges. This multi-functionality allows a single select gate structure to handle both low-voltage and high-voltage selection requirements, reducing the need for entirely separate selection circuits and thereby managing complexity.
Solution Approach 2:
The patent utilizes parameter changes in threshold voltage to differentiate select gate behavior. By adjusting the threshold voltage parameter of each select gate, the system achieves reliable program disturb mitigation through voltage-level differentiation, which simplifies control logic compared to more complex isolation mechanisms.
Data Source
AI summary
Apparatus having a plurality of strings of series-connected memory cells, and methods of their operation, where each of the strings of series-connected memory cells is selectively connected to the same data line through a respective plurality of select gates connected in series between that string and the data line. One select gate of each of the pluralities of select gates has a threshold voltage within a first range of threshold voltages, and each remaining select gate of each of the pluralities of select gates has a threshold voltage within a second range of threshold voltages mutually exclusive from the first range of threshold voltages. Each of the select gates having a threshold voltage within the first range of threshold voltages has its control gate isolated from any of the other select gates having a threshold voltage within the first range of threshold voltages.