Air Gap Sidewall Spacer for Semiconductor Parasitic Capacitance
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Solution Overview
Problem
Semiconductor devices face challenges in reducing parasitic capacitance while maintaining high-speed operations at low voltage, which is exacerbated by the scaling down of semiconductor devices.
Innovation Solution
The formation of air gaps on the sidewalls of gate structures using multiple sacrificial spacers, with a narrow top portion and a capping layer to cap the air gap, reduces parasitic capacitance and increases the thickness of the air gap, thereby enhancing the operation rate and simplifying the fabrication process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If air gap is formed on sidewalls of gate structure, then parasitic capacitance is reduced and operation rate increases, but fabrication process complexity increases
Solution Approach 1:
The air gap formation process is segmented into multiple stages using different sacrificial spacers: a first sacrificial spacer forms the lower portion of the air gap, while a second sacrificial spacer forms the upper portion. This segmentation allows complex air gap structures to be built through simpler, sequential deposition and etching steps, resolving the contradiction between achieving parasitic capacitance reduction and managing fabrication complexity.
Solution Approach 2:
Sacrificial spacers are deposited preliminarily on the sidewalls of the gate structure before the actual air gap formation. These spacers serve as temporary structures that define the air gap geometry and are later removed to create the desired air gap. This preliminary action simplifies the overall process by pre-establishing the structural framework needed for parasitic capacitance reduction.
2Length of stationary object
If multiple sacrificial spacers are used to form air gap, then air gap thickness increases and parasitic capacitance reduces, but device structure becomes more complex
Solution Approach 1:
The first and second sacrificial spacers are nested sequentially on the gate structure sidewalls, with each spacer building upon the previous one. This nesting approach allows the air gap thickness to be increased in controlled increments through repeated deposition cycles, rather than requiring a single complex thick spacer structure, thus managing device structure complexity while achieving the desired air gap dimensions.
Solution Approach 2:
The air gap thickness is controlled by varying the thickness of sacrificial spacers in the vertical dimension, while the horizontal dimensions are managed by the gate structure geometry itself. This dimensional separation allows independent optimization of air gap thickness without proportionally increasing overall device complexity.
3Ease of manufacture
If air gap top portion is made narrow, then capping process becomes easier and fabrication difficulty reduces, but air gap volume decreases
Solution Approach 1:
The air gap structure employs local quality variation by having a narrower top portion and a wider bottom portion. The top portion is narrowed specifically at the capping region to facilitate easy capping material deposition, while the bottom portion maintains larger volume to preserve the parasitic capacitance reduction benefits. This localized geometric optimization resolves the contradiction between capping ease and air gap volume.
Solution Approach 2:
The air gap exhibits asymmetric geometry with different cross-sectional areas at the top versus the bottom. This asymmetry is deliberately designed so that the top portion (where capping is needed) is narrower, while the bottom portion (where capacitance effects are most significant) is wider, thereby simultaneously achieving ease of manufacture and sufficient air gap volume.
Data Source
AI summary
A method for fabricating a semiconductor device includes forming a gate structure over a substrate, forming a multi-layer sidewall spacer including a first sacrificial spacer which covers sidewalls of the gate structure and a second sacrificial spacer which is disposed on a sidewall of the first sacrificial spacer and recessed lower than an upper surface of the gate structure, forming an air gap having a narrower width top portion than a middle and a bottom portions, by removing the first and second sacrificial spacers, and forming a capping layer which caps the top portion of the air gap.