Independent Depth-Controlled Shallow Trench Isolation for Semiconductor Memory
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Solution Overview
Problem
Conventional shallow trench isolation structures in semiconductor memory arrays suffer from line shortening effects and optical interference, leading to oval-shaped active area patterns that reduce process window and electric performance due to the use of a single lithographic and dry etching process with a single photomask, resulting in uniform trench depth.
Innovation Solution
The implementation of a method to create rhomboid-shaped active areas with independent depth-controlled shallow trench isolation structures by using multiple hard masks and etch processes to form trenches with varying depths, allowing for deeper trenches along one direction and shallower trenches along another, improving electric isolation and reducing coupling effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single lithographic process and single dry etching process are used to form shallow trench isolation, then the manufacturing process is simple, but the active area pattern becomes oval-shaped with reduced surface area due to line shortening effect and optical interference
Solution Approach 1:
The patent divides the single lithographic and etching process into multiple sequential steps: first forming initial trenches with a first photomask and first etching process, then forming additional trenches with a second photomask and second etching process. This segmentation allows each step to contribute to the final rhomboid pattern, avoiding the line shortening and optical interference effects that plague single-step processes while maintaining manufacturing feasibility.
Solution Approach 2:
The patent introduces a temporal dimension by using multiple lithographic and etching steps performed at different stages, rather than attempting to achieve the complex rhomboid pattern in a single planar step. The first etching creates preliminary trenches, and the second etching adds additional trenches to complete the rhomboid shape, effectively using process sequencing to overcome optical limitations.
2Device complexity
If a single photomask is used to define the active area pattern, then the manufacturing process is straightforward, but the transferred pattern has smaller surface area than the original mask pattern due to line shortening effect
Solution Approach 1:
The patent uses two separate photomasks instead of one: the first photomask defines the initial trench patterns, and the second photomask defines the additional trench patterns that complete the rhomboid active areas. This segmentation of the patterning function allows each mask to be optimized for its specific purpose, avoiding the line shortening effect that would occur if a single mask attempted to directly define the complete complex pattern.
Solution Approach 2:
The first photomask and first etching process perform a preliminary action by creating initial trenches that define part of the active area boundaries. This preliminary structuring enables the second photomask to work with a pre-defined framework, allowing the final pattern to achieve the desired surface area without suffering from the line shortening that would occur in a single-step direct patterning approach.
3Ease of manufacture
If uniform trench depth is used in shallow trench isolation, then the manufacturing process is simple, but the electric isolation between adjacent word lines is insufficient and coupling effects increase
Solution Approach 1:
The patent implements non-uniform trench depths by using selective etching: the first etching process creates trenches of a certain depth, and the second etching process creates additional trenches of different depths in specific locations. This local differentiation of trench depth allows enhanced electric isolation where needed (deeper trenches between word lines) while maintaining simpler shallower trenches in other areas, thus improving reliability without requiring complete uniformity throughout.
Solution Approach 2:
The patent changes the etch depth parameter between different etching steps. The first dry etching process etches to a first depth, and the second dry etching process etches to a second depth that is greater than the first depth in specific regions. This parameter change in trench depth is achieved through selective masking and sequential etching, allowing optimization of electric isolation performance by having deeper trenches where word line isolation is critical.
Data Source
AI summary
A method of manufacturing a semiconductor structure. A patterned first hard mask is formed on a substrate. The patterned first hard mask includes first trench patterns extending along a first direction. A second hard mask is then formed on the patterned first hard mask. A patterned photoresist layer is formed on the second hard mask. The patterned photoresist layer includes second trench patterns extending along a second direction. The second trench patterns intersect first trench patterns. Using the patterned photoresist layer as an etch mask, a first etch process is performed to transfer the second trench patterns into the patterned first hard mask and the second hard mask. Subsequently, using the patterned first hard mask as an etch mask, a second etch process is performed to transfer the first trench patterns and the second trench patterns into the substrate.