Airgap Isolation for Bulk Substrate RF Switches
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Solution Overview
Problem
Integrating RF switches into bulk substrates is challenging due to the need to drop large voltages across junctions and the presence of large depletion layers, which add substantial area to the layout, unlike silicon on insulator (SOI) substrates.
Innovation Solution
A method involving the formation of an amorphous layer and an airgap in a bulk substrate, creating a completely isolated transistor with an underlying airgap, eliminating the need for junctions between the transistor and the substrate by using shallow trench isolation and deep trench structures to form an airgap underneath the transistor channel.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If RF switches are integrated into bulk substrates using conventional methods, then the device can be manufactured in standard bulk process, but large depletion layers add substantial area to the layout
Solution Approach 1:
The invention extracts the substrate material directly beneath the transistor channel by forming deep trenches and removing the silicon, replacing it with an airgap. This removes the source of large depletion layers while maintaining bulk process compatibility, thereby reducing layout area without sacrificing manufacturability
Solution Approach 2:
The invention changes the physical parameter of the substrate region under the transistor from solid silicon to airgap (vacuum). This parameter change eliminates the need for large depletion layers in high resistivity substrates, reducing the required layout area while maintaining electrical isolation
2Ease of manufacture
If RF switches are integrated into bulk substrates, then the device can be manufactured in standard bulk process, but large voltages must be dropped across junctions
Solution Approach 1:
By removing the substrate silicon beneath the transistor and replacing it with an airgap, the invention eliminates the pn junction between the well and substrate. This extraction removes the harmful voltage drop across junctions while maintaining compatibility with standard bulk manufacturing processes
Solution Approach 2:
The airgap acts as an intermediary between the transistor and the bulk substrate, providing electrical isolation without requiring a pn junction. This mediator eliminates the voltage drop issue while allowing the device to be manufactured in standard bulk processes
3Object-generated harmful factors
If airgap is formed under the transistor channel, then voltage drop across junctions is prevented, but the device complexity increases
Solution Approach 1:
The invention segments the substrate by forming deep trenches that isolate the region beneath the transistor channel. This segmentation creates discrete airgap regions that can be precisely controlled, achieving voltage isolation while managing structural complexity through systematic division of the substrate
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 fully isolates the FET, preventing voltage drop across well-to-substrate junctions and reducing layout area, allowing for efficient integration of RF switches into standard bulk silicon processing without disturbing adjacent elements.
Implementation Method 1
an amorphous layer under an active region of a substrate
Implementation Method 2
fully isolates the FET, preventing voltage drop across well-to-substrate junctions
Data Source
AI summary
A field effect transistor (FET) with an underlying airgap and methods of manufacture are disclosed. The method includes forming an amorphous layer at a predetermined depth of a substrate. The method further includes forming an airgap in the substrate under the amorphous layer. The method further includes forming a completely isolated transistor in an active region of the substrate, above the amorphous layer and the airgap.