Design and evaluation of CMUTs for medical ultrasound imaging with a focus on non-invasive blood pressure waveform monitoring

Abstract

CMUT, MEMS ultrasound, quality factor, damping, hybrid fabrication, screen printing, non invasive blood pressure, ultrasonic sensors. This thesis investigates micromachined ultrasonic transducers (MUTs) for non-invasive blood pressure (BP) waveform monitoring, with a focus on capacitive micromachined ultrasonic transducers (CMUTs). The work combines: (i) a critical review of ultrasonic BP-waveform sensing; (ii) experimental analysis of CMUT topology, quality factor, and damping; and (iii) the development of a hybrid CMUT fabrication process that integrates printed conductors with surface-micromachined cavities and membranes. First, four CMUT topologies fabricated in a PolyMUMPs process are studied, combining spring versus straight arms with rocker versus fixed anchoring. Laser Doppler vibrometer in air and vacuum is used to extract resonance frequency and quality factor as a function of dc bias and ac drive. A pull-in-normalized common operating point is introduced, and paired air/vacuum data are analyzed using an additive 1/Q model to separate fluidic and intrinsic losses. By comparing the four permutations, rocker anchors are identified as the dominant intrinsic loss source, suitable for low Q factor operations where high axial resolution is required whereas straight fixed rims provide a high-Q, suitable for efficient transmit/receive operation. Second, the thesis reviews ultrasonic devices for continuous non-invasive BP waveform monitoring, examining bulk piezoelectric, PMUT, and CMUT approaches. The analysis highlights that most reported systems still rely on bulk probes or PMUTs, and that CMUT-based solutions—particularly on flexible or low-cost substrates—remain underexplored. Finally, a hybrid CMUT-like structure is demonstrated on glass by combining a sputtered aluminum bottom electrode, a LOR sacrificial layer patterned with AZ 5214E, an SU-8 membrane with etch holes, and a fully cured screen-printed silver top electrode. Temperature and chemical compatibility are mapped, and a 2 × 2 array is fabricated. Profilometry and vibrometry confirm a printed silver thickness of approximately 9.2 μm, two clear mechanical resonances at 1.01 MHz and 1.42 MHz with quality factors of approximately Q1 ≈ 6.6 and Q2 ≈ 235, and a collapse voltage around 20 V. The results show that CMUT-like devices can be realized with a reduced tool set, providing a practical platform for future low-cost and potentially flexible ultrasonic BP-waveform sensors with reduced fabrication duration.

Date	23 Jan 2026
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Ricardo Izquierdo (Supervisor), Paul Vahé Cicek (Co-supervisor) & Alexandre Robichaud (Co-supervisor)