CMUT Fabrication via Local Oxidation for Precise Gap Control
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Solution Overview
Problem
Existing CMUT fabrication methods face challenges such as stiction, stress sensitivity, and difficulty in controlling membrane thickness and gap height, leading to issues with parasitic capacitance and breakdown voltage.
Innovation Solution
The method involves depositing an oxidation-blocking layer on a silicon substrate, patterning it to form post and cavity regions, and then thermally oxidizing to grow oxide posts, allowing for precise control of gap and post thickness through fusion bonding, while maintaining low surface roughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If sacrificial release process is used with silicon nitride membrane layer, then CMUT structure can be formed, but stiction problems occur and membrane stress is sensitive to deposition conditions
Solution Approach 1:
The patent extracts and eliminates the sacrificial polysilicon layer through small channels after the membrane is formed, rather than using it as a permanent structural element. This extraction approach allows the membrane to be released from the sacrificial layer without experiencing stiction problems, while maintaining the benefits of the silicon nitride membrane deposition process.
Solution Approach 2:
The patent uses the sacrificial polysilicon layer as a temporary intermediary structure that facilitates membrane formation and deposition, then removes it to achieve the final structure. This intermediary approach allows controlled deposition conditions during formation while eliminating the stiction issue in the final released structure.
2Ease of manufacture
If successive deposition and etching steps are used to set membrane thickness, then membrane can be formed, but difficulty in controlling membrane thickness occurs
Solution Approach 1:
The patent performs preliminary membrane formation through deposition on the sacrificial polysilicon layer, establishing the basic membrane structure. The final membrane thickness is then precisely controlled through a subsequent etching step that removes material to achieve the exact desired thickness, separating the formation and thickness control functions into distinct stages.
3Ease of manufacture
If non-uniform nitride deposition is used to seal cavity, then cavity sealing can be achieved, but gap height or thickness control becomes difficult
Solution Approach 1:
The patent segments the cavity sealing and gap height control functions into separate process steps. The cavity is sealed through deposition of silicon nitride layers, while the gap height is independently controlled through subsequent selective etching of the oxide layer, allowing each parameter to be optimized without compromising the other.
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 enables CMUTs with low parasitic capacitance, high breakdown voltage, and cost-effective, reproducible fabrication without the need for chemical-mechanical polishing, providing independent control over critical dimensions.
Implementation Method 1
depositing an oxidation-blocking layer, such as a silicon-nitride layer, onto a substrate of an oxidation-enable material
Implementation Method 2
The substrate is then thermally oxidized, such as through a LOCOS process, to grow one or more oxide posts from the post region
Implementation Method 3
A membrane layer is then bonded to the post, preferably through fusion bonding
Data Source
AI summary
The current invention provides methods of fabricating a capacitive micromachined ultrasonic transducer (CMUT) that includes oxidizing a substrate to form an oxide layer on a surface of the substrate having an oxidation-enabling material, depositing and patterning an oxidation-blocking layer to form a post region and a cavity region on the substrate surface and remove the oxidation-blocking layer and oxide layer at the post region. The invention further includes thermally oxidizing the substrate to grow one or more oxide posts from the post region, where the post defines the vertical critical dimension of the device, and bonding a membrane layer onto the post to form a membrane of the device. A maximum allowed second oxidation thickness t2 can be determined, that is partially based on a desired step height and a device size, and a first oxidation thickness t1 can be determined that is partially based on the determined thickness t2.