3D NAND Staggered Wordline Grounding for Read Window Stability
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Solution Overview
Problem
The read window budget (RWB) degradation in 3D NAND flash memory devices is exacerbated by threshold voltage (Vt) shift and cell-to-cell variation due to temperature changes and random telegraph noise, which is amplified by charge traps and grain boundaries in vertical channels, limiting performance and scaling.
Innovation Solution
Implementing a staggered or sequential grounding method for wordlines in 3D NAND arrays, where wordlines are transitioned to ground in a controlled sequence after a read operation, providing a grounding path for charges to discharge, reducing trapped charges at grain boundaries and minimizing temperature sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional simultaneous grounding of all wordlines is used, then the read operation is simple and fast, but charge traps and grain boundaries in vertical channels cause Vt distribution spreading and RWB degradation
Solution Approach 1:
The patent divides the wordline grounding operation into sequential segments rather than simultaneous grounding. Wordlines are grounded in stages, with intermediate wordlines grounded between top and bottom wordlines, allowing charges to discharge progressively through the vertical channels and preventing Vt distribution spreading caused by charge traps and grain boundaries.
Solution Approach 2:
The patent applies preliminary grounding of intermediate wordlines before completely discharging the vertical channels. By grounding intermediate wordlines first, the system creates discharge paths that allow charges to be removed from grain boundaries and charge traps before the final grounding, thereby reducing RWB degradation from temperature changes and random telegraph noise.
2Reliability
If smaller program steps are used to compensate for RWB loss, then RWB degradation is reduced, but programming time increases
Solution Approach 1:
The patent maintains continuous and efficient programming operation by implementing staggered wordline grounding that allows full-programming steps to be used. The sequential grounding mechanism continuously manages charge discharge during reading, eliminating the need to reduce program step size, thereby maintaining both programming speed and RWB performance without time penalty.
3Reliability
If operating temperature range is limited to reduce RWB loss, then RWB degradation is reduced, but use cases and deployment are significantly limited
Solution Approach 1:
The patent replaces the thermal management approach (limiting temperature range) with an electrical control mechanism (staggered wordline grounding). By using sequential grounding to actively manage charge discharge and Vt distribution, the system eliminates the need to restrict operating temperatures, thereby maintaining RWB performance across the full temperature range without sacrificing adaptability.
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 improves read noise and reduces RWB degradation, allowing for continued scaling and performance enhancement in 3D NAND arrays by effectively managing charge discharge and temperature effects.
Implementation Method 1
transitioning a selected wordline of the multiple wordlines from the read bias voltage to ground; delaying a transitioning of the other wordlines from the read bias voltage to ground
Data Source
AI summary
At the end of or after a reading operation in a 3D (three dimensional) NAND array, the wordlines of the 3D NAND array can be transitioned to ground in a staggered manner. The 3D NAND array includes a 3D stack with multiple wordlines vertically stacked, including a bottom-most wordline, a top-most wordline, and middle wordlines between the bottom-most wordline and the top-most wordline. A controller that controls the reading can set the multiple wordlines to a high voltage at the end or after the reading operation and then transition a selected wordline of the multiple wordlines from the high voltage to ground prior to transitioning the other wordlines to ground. Thus, the controller will transition the other wordlines from the high voltage to ground after a delay.