Double Trench Isolation for Void-Free NAND Flash Memory
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Solution Overview
Problem
Conventional methods face challenges in forming isolation structures with high aspect ratios in NAND flash memory arrays, leading to voids in the filling material, which results in poor isolation between NAND strings and increased power consumption.
Innovation Solution
A double trench process is employed, where a first trench is formed and selectively etched to expose the bottom surface, followed by the creation of a second trench that extends deeper, allowing selective oxidation to fill the second trench while keeping the first trench sides unoxidized, enabling void-free filling with a deposited dielectric material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If a single trench structure is used with high aspect ratio, then isolation depth is improved, but voids form in the filling material
Solution Approach 1:
The isolation trench is divided into two segments: a first trench with lower aspect ratio that can be filled without voids, and a second trench extending deeper from the first trench. This segmentation allows each trench to have optimized dimensions for its filling process, resolving the contradiction between depth and filling quality.
Solution Approach 2:
The first trench is formed and filled with dielectric material before forming the second trench. This preliminary action ensures that the filling material is deposited in a controlled environment with manageable aspect ratio, preventing void formation while still achieving the required isolation depth through the subsequent second trench.
2Length of stationary object
If conventional deposition is used for high aspect ratio trenches, then isolation depth is achieved, but voids are formed in the filling material
Solution Approach 1:
The isolation structure is segmented into two trenches with different depths and filling methods. The first trench is filled by deposition with controlled aspect ratio to avoid voids, while the second trench extends the isolation depth. This segmentation resolves the contradiction between achieving sufficient isolation depth and maintaining filling reliability without voids.
Solution Approach 2:
The aspect ratio parameter is changed by dividing the total trench depth into two segments. The first trench has a lower aspect ratio suitable for void-free deposition, while the second trench provides additional depth. This parameter change allows the system to achieve both sufficient isolation depth and reliable void-free filling.
3Manufacturing precision
If a double trench structure is formed, then void-free filling is achieved, but process complexity increases
Solution Approach 1:
The isolation trench formation is segmented into two distinct trenches formed by separate etching and filling processes. While this increases process steps, each segment can be optimized independently for void-free filling, resolving the contradiction between manufacturing precision and process complexity by allowing targeted process optimization.
Solution Approach 2:
Different regions of the isolation structure are given different qualities: the first trench has optimized dimensions for deposition filling, while the second trench provides additional depth. This local quality differentiation allows each region to be processed optimally, achieving void-free filling overall despite the increased process complexity.
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 allows for the formation of isolation trenches without voids, even at aspect ratios beyond conventional limits, enhancing isolation between NAND strings and reducing power consumption.
Implementation Method 1
The covering layers may then be removed from bottom surfaces of the first trenches to expose underlying silicon (e.g. by anisotropic etching) while leaving sidewalls covered
Implementation Method 2
The bottom surfaces may then be selectively etched by using an etch chemistry that is selective to silicon thereby forming second trenches
Implementation Method 3
Oxidation is then used to selectively fill the second trenches (where silicon is exposed) while maintaining the first trenches (where silicon is covered) substantially unaffected
Implementation Method 4
first trenches filled by deposition of suitable material (e.g. polysiliazane)
Data Source
AI summary
Isolation is provided by forming a first trench, depositing a cover layer on the bottom and sidewalls of the first trench, selectively removing the cover layer from the bottom and forming a second trench extending from the bottom of the first trench. The second trench is then substantially filled by thermal oxide formed by oxidation and the first trench is subsequently filled with a deposited dielectric.