Extended Lower Source/Drain Structure for Buried Stacked FET Contacts
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Solution Overview
Problem
Forming contacts to the lower source/drain (S/D) in stacked field-effect transistors (FETs) is challenging due to its burial underneath the top S/D, making existing contact formation methods ineffective.
Innovation Solution
A lower S/D is epitaxially grown over an active channel stack and a dummy stack, with the dummy stack subsequently removed, leaving a dummy stub, enabling a double-wide lower S/D to electrically connect to a lower S/D contact metalized over the stub.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If a stacked FET structure is used to achieve 3D monolithic integration and area reduction, then device density and area efficiency are improved, but forming contacts to the lower source/drain becomes increasingly difficult
Solution Approach 1:
A dummy stack is formed adjacent to the active channel stack before growing the lower source/drain. The dummy stack serves as a placeholder that enables the lower source/drain to be grown in a laterally extended configuration. After the lower source/drain is formed, the dummy stack is removed, leaving a dummy stub that maintains the extended structure. This preliminary action of adding the dummy stack allows contact formation to proceed effectively.
Solution Approach 2:
The dummy stack acts as an intermediary structure during the fabrication process. It provides the necessary space and structural support for growing the lower source/drain in an extended configuration that would not be possible with the active channel stack alone. The dummy stack mediates between the need for compact 3D integration and the need for accessible lower source/drain contacts.
2Device complexity
If the lower source/drain is grown only over the active channel stack, then device structure is simplified, but the lower source/drain cannot be effectively contacted due to burial underneath the top S/D
Solution Approach 1:
The lower source/drain is grown to extend laterally in a direction perpendicular to the active channel stack, utilizing the lateral dimension provided by the adjacent dummy stack. This lateral extension creates an accessible surface for contact formation, transforming the lower source/drain from a completely buried structure to one that is partially exposed through the lateral extension over the dummy stub.
3Reliability
If a laterally extended lower source/drain is grown over both active channel stack and dummy stack, then effective electrical connection to lower S/D is enabled, but additional fabrication steps and structure are required
Solution Approach 1:
The dummy stack is temporarily added to enable the formation of the laterally extended lower source/drain, then removed after serving its purpose. The removal leaves a dummy stub that maintains the lateral extension of the lower source/drain. This approach allows the dummy stack to be discarded after fulfilling its intermediary function, recovering the space while preserving the beneficial extended structure for contact formation.
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
Facilitates effective electrical connection to the lower S/D, allowing signal conveyance to the active lower FET, thereby overcoming the challenge of reaching buried contacts in stacked FET devices.
Implementation Method 1
growing a lower source/drain (S/D) across the active channel stack and the dummy stack
Data Source
AI summary
Embodiments herein include semiconductor structures with an active channel stack having an upper field-effect transistor (FET) and a lower FET vertically stacked below the upper FET The semiconductor structure may also include a dummy stub adjacent to the active channel stack, a lower source/drain (S/D) connected to the active channel stack and laterally extended over the dummy stub, and an upper S/D connected to the active channel stack above the lower S/D.