Capacitive micromachined ultrasonic transducers using commercial multi-user MUMPs process: capability and limitations

Area of Science:

• Microelectromechanical Systems (MEMS)
• Ultrasound Transducer Technology
• Materials Science

Background:

• Capacitive micromachined ultrasonic transducers (cMUTs) are crucial for various imaging applications.
• Customized fabrication processes for cMUTs are often expensive and inaccessible.
• The Multi-User MEMS Processes (MUMPs) offers a potentially low-cost alternative for cMUT fabrication.

Purpose of the Study:

• To construct cMUTs utilizing the MUMPs process.
• To analyze the capabilities and limitations of MUMPs for cMUT fabrication compared to custom processes.
• To propose design considerations to overcome identified limitations.

Main Methods:

• Fabrication of cMUTs using the standard MUMPs process.
• 1D modeling and Finite Element Analysis (FEA) for performance prediction.
• Experimental characterization of fabricated cMUT devices.

Main Results:

• MUMPs process imposes limitations on material and gap thicknesses, affecting independent control of collapse voltage and center frequency.
• Achieved center frequencies up to 9 MHz with collapse voltages under 200 V, suitable for medical and NDE imaging.
• Identified and addressed issues with series resistance and polysilicon electrode shorting, proposing a dielectric layer solution.
• Demonstrated a novel beam construction yielding a 6% bandwidth pressure pulse in air.

Conclusions:

• MUMPs process presents unique limitations for cMUT fabrication compared to custom processes.
• These limitations can be mitigated through appropriate design strategies.
• MUMPs provides an accessible and cost-effective platform for academic and industrial cMUT research and development.