Large-Scale Multi-Frequency Capacitive Micromachined Ultrasonic Transducer (CMUT) Arrays for Ultrasound Medical Imaging and Therapeutic Applications

• Author / Creator

  Maadi, Mohammad

• Ultrasonic transducers capable of operating over multiple frequency bands could have several interesting medical applications including multi-resolution multi-depth imaging for point-of-care ultrasound, imaging-therapy, super-harmonic contrast agent imaging, super-resolution imaging, image-guided drug delivery, and ultra-wideband ultrasound and photoacoustic imaging. However, multi-frequency arrays are difficult to realize using conventional piezoelectric transducer technology. Here, closely packed interlaced capacitive micromachined ultrasonic transducers (CMUTs) with different membrane sizes are designed to create multi-frequency arrays. CMUTs, compared to their piezoelectric counterparts, are a novel type of ultrasonic transducer that have wider bandwidth, sensitive receive performance, and offer natural integration with electronics. Besides, they do not suffer from self-heating problems which can apply many limitations for piezoelectric transducers. By applying a bias voltage, an electrostatic attraction force occurs between the top and bottom electrodes of the CMUT. The membrane is suspended over the gap until the bias voltage reaches the snap-down value which is named collapse voltage. The resonance frequency is determined by the size and thickness of the membrane. Thus, by interlacing membranes of different sizes, multi-frequency capabilities are realized.
  This thesis aims to address unmet needs for multi-frequency ultrasound arrays by comprehensive modeling, fabrication, and testing. Modeling developments include equivalent circuit models for large arrays of membranes, including the interaction of membranes. Previously, the theory for mutual acoustic interactions was developed for similar radiators, but not membranes of different sizes. Following development of analytical expressions for calculating the self- and mutual radiation impedance between dissimilar circular membranes, a fast and precise lumped equivalent circuit model for designing large-scale multi-frequency ultrasound realistic arrays was developed for both circular and square membranes.
  We also developed nanofabrication methods for novel multi-frequency CMUT arrays using a modified silicon-nitride sacrificial release process. We showed the feasibility of designed and fabricated multi-frequency arrays for several applications including multi-scale imaging, contrast agent imaging, etc. Aiming to further improve acoustic output, we then developed next-generation large-scale multi-gap multi-frequency CMUT arrays. In these arrays, low-frequency sub-elements have larger gap-sizes to permit a larger range of membrane motion for generating more acoustic power. Compared to low-frequency sub-elements, high-frequency cells are made with smaller gap-sizes to make them more sensitive to echoes coming back from the objects in receive mode. These arrays are designed to increase the acoustic power by a factor of 2-3 times. We additionally propose strategy to increase the acoustic power by applying electrical impedance matching networks to each CMUT element. By transmitting with low-frequencies and receiving nonlinear echoes from microbubble contrast agents with high-frequency sub-arrays, the proposed new technology may have applications for pre-clinical and clinical contrast imaging to permit background-free detection of contrast agents. Such background reduction compared to current methods could prove important for future targeted molecular imaging applications. Other applications of the arrays could include image-guidance with high-frequency sub-arrays and therapeutic modes with low-frequency sub-arrays. Such therapeutic modes could include high-intensity ultrasound based thermal or mechanical ablation, ultrasound-assisted drug release from acoustically-active carriers, and ultrasound-assisted permeabilization of tissues including the blood-brain barrier.

• Subjects / Keywords

  • Ultrasound Medical Imaging and Therapeutic Applications
  • Ultrasonic Transducers
  • Capacitive Micromachined Ultrasonic Transducer (CMUT)
  • MEMS
  • Multi-Frequency CMUT Arrays
  • Microfabrication

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-mddc-jt40

• License

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