Contoured Drain-Select Isolation in 3D NAND for Threshold Stability
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Solution Overview
Problem
Three-dimensional NAND memory devices experience a downward drift in threshold voltage due to damage of the outer blocking dielectric during the formation of drain-select-level isolation structures, leading to electron de-trapping and reduced reliability.
Innovation Solution
The formation of contoured drain-select-level isolation structures that laterally protrude into memory opening fill structures while avoiding lateral protrusions between neighboring pairs, ensuring the outer blocking dielectric is completely removed in ungated regions, thereby reducing electron de-trapping and stabilizing the threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional drain-select-level isolation structures are formed, then device fabrication is simplified, but the outer blocking dielectric is damaged causing threshold voltage drift and reduced reliability
Solution Approach 1:
The isolation structure employs a contoured, curved profile instead of a straight vertical shape. The structure laterally protrudes into memory openings at different heights, creating a meandering path that avoids damaging the outer blocking dielectric while maintaining effective isolation between adjacent memory openings. This curved geometry resolves the contradiction by protecting the dielectric (improving reliability) while still achieving isolation functionality.
Solution Approach 2:
The isolation structure extends in multiple spatial dimensions with lateral protrusions at different vertical levels. By adding vertical dimensionality to the isolation approach, the structure can selectively enter memory openings at different heights without requiring complex lateral positioning, thus maintaining ease of manufacture while improving reliability through precise dielectric avoidance.
2Manufacturing precision
If the isolation structure laterally protrudes into memory openings, then isolation effectiveness is improved, but risk of damaging the outer blocking dielectric increases
Solution Approach 1:
The contoured profile with lateral protrusions at different heights allows the isolation structure to precisely navigate around the outer blocking dielectric. The curved path enables the structure to laterally protrude into memory openings for effective isolation while following a trajectory that avoids the dielectric layer, thus achieving high isolation precision without causing dielectric damage.
Solution Approach 2:
The isolation structure is designed with pre-determined lateral protrusions at specific heights that are formed before final dielectric processing. This preliminary configuration ensures that the structure automatically avoids the outer blocking dielectric during subsequent processing steps, achieving precise isolation without causing damage.
3Ease of manufacture
If straight vertical isolation structures are used, then fabrication process is simpler, but threshold voltage stability deteriorates due to electron de-trapping
Solution Approach 1:
The contoured isolation structure with its curved, meandering profile provides superior threshold voltage stability compared to straight vertical structures. The curved geometry prevents direct contact with and damage to the outer blocking dielectric, eliminating electron de-trapping pathways while maintaining fabrication simplicity through conformal deposition processes that naturally follow the curved profile.
Data Source
AI summary
A three-dimensional memory device includes an alternating stack of insulating layers and electrically conductive layers memory openings vertically extending through the alternating stack, memory opening fill structures located in the memory openings, and a drain-select-level isolation structure vertically extending through a subset of layers within the alternating stack and laterally extending generally along a first horizontal direction with lateral undulations along a second horizontal direction such that a first electrically conductive layer within the alternating stack is divided into a set of at least two first drain-select-level electrode strips. The drain-select-level isolation structure laterally protrudes into each memory opening fill structure in a first row and a second row and does not laterally protrude into the first drain-select-level electrode strips within gaps between neighboring pairs of memory opening fill structures.