NAND Flash Memory Select Gate Segmentation
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Solution Overview
Problem
Existing memory arrays face complexity and real estate challenges due to the need for 'dummy' word lines to protect threshold voltages of programmable source and drain select gates during erase operations in NAND flash memory devices, leading to inefficiencies and increased complexity.
Innovation Solution
Implementing a memory array configuration with non-programmable source select gates and programmable drain select gates, where the programmable drain select gates are configured similarly to memory cells, allowing for adjusted threshold voltages that are not erased during operations, thereby eliminating the need for dummy word lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If programmable source and drain select gates are used to address threshold voltage variability, then select gate uniformity is improved, but dummy word lines are required to protect programmed threshold voltages during erase operations, increasing device complexity and real estate requirements
Solution Approach 1:
The select gate functionality is segmented into two distinct components: a non-programmable source select gate and a programmable drain select gate. This segmentation allows the drain select gate to be optimized for threshold voltage uniformity through programming while the source select gate maintains simplicity, eliminating the need for dummy word lines to protect source select gate thresholds during erase operations.
Solution Approach 2:
Different quality characteristics are applied to different parts of the select gate system. The drain select gate has programmable threshold voltages with high uniformity for precise control, while the source select gate has fixed threshold voltages that do not require protection during erase operations. This local differentiation resolves the contradiction by applying programming only where necessary.
2Adaptability or versatility
If programmable select gates are used, then threshold voltage adjustment capability is improved, but the need for dummy word lines increases real estate requirements
Solution Approach 1:
The select gate system is divided such that only the drain select gate requires programming capability and associated dummy word lines, while the source select gate remains non-programmable and does not require dummy word lines. This reduces the overall real estate requirement compared to having programmable capability throughout the entire select gate system.
Solution Approach 2:
Instead of making both source and drain select gates programmable (which would maximize adaptability but also maximize real estate requirements), the invention inverts the approach by making only the drain select gate programmable. This selective application of programming capability achieves sufficient adaptability while minimizing real estate consumption.
3Reliability
If both source and drain select gates are made programmable, then threshold voltage control is improved, but the memory array requires dummy word lines that increase complexity and reduce productivity
Solution Approach 1:
High threshold voltage control reliability is applied locally only to the drain select gate where programming is needed for uniformity, while the source select gate uses fixed threshold voltages that do not require dummy word lines. This local quality differentiation maintains sufficient reliability for operation while improving productivity by eliminating unnecessary dummy word lines.
Solution Approach 2:
The dummy word line requirement is extracted and removed from the source select gate portion of the system. By making the source select gate non-programmable, the invention eliminates the need for dummy word lines to protect source select gate thresholds during erase operations, thereby improving memory array productivity and efficiency.
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 configuration reduces complexity and real estate requirements by maintaining programmed threshold voltages of select gates, enhancing switching capability and uniformity, and allowing for higher control gate voltages without reliability issues, resulting in improved performance and efficiency.
Implementation Method 1
Changes in threshold voltage of the cells, through programming (which is sometimes referred to as writing) of charge storage nodes (e.g., floating gates or charge traps) or other physical phenomena
Implementation Method 2
The source and drain select gates may be field-effect transistors having a fixed threshold voltage
Data Source
AI summary
Memories and their memory arrays are disclosed. One such memory array has a string of series-coupled memory cells, a non-programmable select gate coupled in series to a first end of the string of series-coupled memory cells, and a programmable select gate coupled in series to a second end of the string of series-coupled memory cells.