SRAM Gate Extensions With Unequal Depths for Margin Tuning
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Solution Overview
Problem
Existing strategies for integrating devices in a static random-access memory (SRAM) cell struggle to meet performance targets such as improved read margin and write margin.
Innovation Solution
A method of fabricating a semiconductor device involves forming a substrate with doped regions and isolation structures, followed by a series of etching processes to create divots of varying depths in the isolation structures. These divots allow for the formation of gate extensions of different depths, which can be used to tune the effective channel width of transistors in the SRAM cell.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional integration strategies are used in SRAM cells, then manufacturing simplicity is maintained, but performance targets such as read margin and write margin cannot be met
Solution Approach 1:
The gate structure is segmented into multiple portions with different extension depths into the isolation structure. Specifically, first gate portions extend to a first depth while second gate portions extend to a second depth, allowing independent optimization of transistor characteristics for different circuit functions (read vs. write operations) within the same SRAM cell
Solution Approach 2:
Different regions of the gate structure are given different properties through varying extension depths. The first gate portions with greater extension provide stronger control for transistors requiring higher drive strength, while second gate portions with lesser extension provide adequate control with smaller footprint for other transistors, optimizing local performance requirements
2Productivity
If feature size is reduced to increase integration density, then more components can be integrated, but performance targets such as read margin and write margin become harder to meet
Solution Approach 1:
Instead of solely relying on lateral dimension scaling, the invention exploits the vertical dimension by extending gates into the isolation structure at different depths. This third-dimensional approach allows performance optimization without further reducing the lateral feature sizes, thereby maintaining integration density while improving transistor control and performance margins
3Reliability
If gate extensions are used to improve transistor control, then read margin and write margin are improved, but manufacturing complexity increases due to multiple etching processes
Solution Approach 1:
The isolation structure is pre-formed with varying depths (first divots and second divots) before gate deposition. This preliminary preparation of the isolation structure with different depth regions allows subsequent conformal gate deposition to automatically create the desired multi-depth gate extensions, simplifying the overall manufacturing sequence by preparing the substrate in advance
Data Source
AI summary
A semiconductor structure includes a first isolation structure and a second isolation structure disposed in a substrate. The semiconductor structure includes a doped region interposed between the first isolation structure and the second isolation structure in the substrate. The semiconductor structure includes a gate structure disposed over the doped region. The semiconductor structure includes a first gate extension protruding from the gate structure into the first isolation structure, where the first gate extension has a first depth measured from a top surface of the substrate. The semiconductor structure further includes a second gate extension protruding from the gate structure into the second isolation structure, where the second gate extension has a second depth that is different from the first depth.