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Updated: Jun 28, 2026
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Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding
Published on: January 9, 2014
Evaluation of wafer bonded CMUTs with rectangular membranes featuring high fill factor.
Serena H Wong1, Mario Kupnik, Xuefeng Zhuang
1Edward L Ginzton Laboratory, Stanford University, Stanford, CA, USA. hwong@stanford.edu
Summary
Optimizing capacitive micromachined ultrasonic transducer (CMUT) fill factor is key for performance. Reducing cell spacing can cause issues, but wider support posts and aspect ratio adjustments improve reliability and operational modes.
Area of Science:
- Microelectromechanical Systems (MEMS)
- Acoustic Transduction
- Ultrasonic Transducer Technology
Background:
- Capacitive micromachined ultrasonic transducers (CMUTs) performance is enhanced by increasing fill factor.
- Fill factor in rectangular CMUTs is determined by cell-to-cell spacing and aspect ratio.
Purpose of the Study:
- Investigate the impact of cell-to-cell spacing and aspect ratio on CMUT performance.
- Analyze the nonuniformity of collapse voltage and the presence of higher-order modes in CMUTs.
- Determine optimal design parameters for improved CMUT operation.
Main Methods:
- Utilized white light interferometry to measure cell deflection nonuniformity.
- Employed a 2-D finite element model (FEM) to simulate and analyze CMUT behavior.
- Conducted impedance and interferometer measurements to study modal frequencies and operational modes.
Main Results:
- Reduced cell spacing amplified collapse voltage nonuniformity to 18.4% due to support post bending.
- Increasing support post width to 1.67 times membrane thickness mitigated post bending.
- Higher aspect ratios caused the (1,3) modal frequency to approach the (1,1) modal frequency.
- Rectangular CMUTs operated in higher-order modes in immersion under CW excitation, reducing output pressure.
Conclusions:
- CMUT design parameters significantly influence collapse voltage uniformity and operational modes.
- Wider support posts are crucial for mitigating collapse voltage nonuniformity caused by reduced cell spacing.
- Rectangular CMUTs are unsuitable for continuous wave (CW) operation in immersion due to higher-order mode excitation.
