Martin Angerer

Capacitive micromachined ultrasonic transducers (CMUTs) are a promising alternative to conventional piezoelectric transducers, offering superior design flexibility and broadband operational characteristics. However, their clinical and practical deployment is constrained by elevated driving voltages and limited acoustic power output, particularly when producing CMUTs based on polymers. This paper presents an end-to-end, measurement-driven experimental validation strategy for designing passive broadband impedance-matching networks that enhance transmitted acoustic power in immersed CMUT arrays while preserving bandwidth. Matching topologies are synthesized to operate near the theoretical Bode–Fano limit, and robustness to component tolerances is quantified through Monte Carlo yield analysis using realistic off-the-shelf component variations. The matching networks are then implemented and experimentally validated under representative unipolar pulse excitation, with far-field acoustic pressure characterized in both time and frequency domains and compared against numerical predictions. The results show that the optimized impedance matching increases transmit power by a factor of 2.1 at the cost of a 40% fractional-bandwidth reduction. These findings establish a directly applicable, validated framework for broadband impedance matching in polymer CMUT arrays and support its use as a cost-effective approach for ultrasound imaging and therapeutic systems.

This study investigates the effects of post-fabrication annealing on polymer-based capacitive micromachined ultrasonic transducers (polyCMUTs). These devices comprise microscopic diaphragms produced via photolithographic patterning of polymer layers. Critical point drying, required to release the diaphragms, can cause significant plastic deformation, thereby reducing electromechanical coupling. Post-fabrication annealing, carried out in incremental steps up to 190 °C, led to an effective increase in coupling by a factor of 5.4. Atomic Force Microscopy showed that the initial upward deflection of 162.7 nm decreased to 6.2 nm after annealing at 190 °C, while also improving surface uniformity. In parallel, the transducer’s resonance frequency rose from 2.33 MHz (unannealed) to 2.60 MHz, and the input impedance phase angle at resonance increased from −68.1° to −4.3°. Together, these changes indicate a significant improvement in resonator behavior and, consequently, device performance. Thus, post-fabrication annealing is an effective measure to achieve the designed performance while enhancing manufacturing yield, thereby increasing the applicability of polyCMUTs.

The ability of ultrasound imaging to deliver real-time visualization of tissue structures and surgical instruments can provide essential benefits in guiding medical interventions. In spinal cord injury research, small animal models are commonly used, but their size restricts the applicability of many standard ultrasound systems. Capacitive Micromachined Ultrasonic Transducers (CMUTs) offer advantages over traditional piezoelectric transducers, including a smaller form factor, high design flexibility, and improved acoustic performance. CMUT structures made of polymers (polyCMUTs) can be produced cost-effectively and quickly, while potentially offering flexible, biocompatible transducers for next-generation ultrasound systems. This study introduces the first polyCMUT probe designed for imaging spinal cords in rats. A compact 11 MHz, 64 channel probe with a 12.9 x 6.5 mm small tip was developed through a three-stage fabrication process, combining in-house manufactured polyCMUT arrays with electronics, integrated in a research imaging system. Performance evaluation included electrical impedance measurements, acoustic characterization, and in-vitro and ex-vivo imaging. Quality analysis validated the stability of the fabrication process, demonstrating high yield and minimal variability, with a standard deviation in resonance frequency of less than 1%. The probe successfully visualized key anatomical structures like the central canal as well as real-time imaging of needle insertion into tissue. However, distinguishing between gray and white matter remained challenging due to limitations in frequency and bandwidth. This study demonstrates the potential of the polyCMUT technology for developing tailored ultrasound solutions. Future work will focus on optimizing high-frequency performance and advancing toward in-vivo applications to provide meaningful tools in spinal cord injury research and therapeutic interventions.

This paper introduces a hybrid modeling approach to accurately predict the performance of polymer-based Capacitive Micromachined Ultrasonic Transducers (polyCMUTs) by coupling finite element analysis (FEA) with analytical methods. The coupled FEA and analytical (CFA) model integrates characteristics from a single-cell FEA into a multi-cell equivalent circuit. Acoustic cross-coupling between cells is considered using analytical methods, and the acoustic far-field is computed via the Rayleigh integral. We validated the model on rectangular designs with 11x11 cells and varying cell-to-cell pitches. CFA results showed in average less than 7% deviation from full FEA in terms of center frequency, fractional bandwidth, and peak sensitivity, while requiring less than 1% of the computation time. We also observed good agreements with measurements, with a deviation of 17% for the rectangular designs and less than 4% for a larger linear array element (428 cells) we recently produced. This makes the CFA model a powerful tool for fast design exploration and optimization of CMUTs.

The objective of this work was to investigate changes in the acoustic characteristics of micromachined transducers caused by acoustic cross-coupling between cells. We used hexagonal, polymer-based capacitive micromachined ultrasonic transducers (polyCMUTs) consisting of 127 cells connected in parallel. The distances between the cells were varied, while the cell dimensions and number of cells remained constant. The resulting changes in characteristics were evaluated in terms of peak frequency f_pk , fractional bandwidth FBW , peak transmit sensitivity S_pk and opening angle Φ _t . The study relies on results from an analytic multicell model (MCM) which considers cross-coupling effects between cells through a mutual acoustic impedance matrix. The results are compared with finite element (FE) analyses and measurements on fabricated prototypes. The manufacturing processes used to produce the polyCMUT prototypes are explained in detail. We found significant changes in all acoustic characteristics: as cell spacing increases, f_pk and Φ _t decrease, while S_pk gradually rises to about twice the initial value. The FBW varies due to the change in f_pk , peaking at small to intermediate cell-to-cell distances. While both modeling approaches cover the general effects, discrepancies in comparison to the measurements were identified. The FE model provided better fits than the analytic MCM, albeit at significantly higher computational costs. The effects on the acoustic characteristics were found strongest at lower frequencies and if many cells are in close proximity to each other. Hence, rotational symmetric or square transducers operating at lower frequencies are affected most. The results demonstrate that design approaches based on modeling single cells may lead to significant deviations from design goals. Both, analytic and FE models are suitable tools to estimate the effects of acoustic interactions and to predict the performance. This aids in meeting design requirements of micromachined ultrasound transducers consisting of multiple radiators.

A potential method for future breast cancer screening is 3D ultrasound computed tomography (USCT). The utilized image reconstruction algorithms require transducer characteristics fundamentally different from conventional transducer arrays, leading to the necessity of a custom design. This design has to provide random transducer positioning, isotropic sound emission as well as a large bandwidth and wide opening angle. In this paper, we present a new transducer array design to be utilized in a third generation 3D USCT system. Each system requires 128 cylindrical arrays, mounted into the shell of a hemispherical measurement vessel. Each new array contains a 0.6 mm thick disk with 18 single PZT fibers (0.46 mm diameter) embedded in a polymer matrix. Randomized positioning of the fibers is achieved with an arrange-and-fill process. The single-fiber disks are connected on both ends with a matching and backing disk using simple stacking and adhesives. This enables fast and scalable production. We characterized the acoustic field of 54 transducers with a hydrophone. Measurements in 2D showed isotropic acoustic fields. The mean bandwidth and opening angle are 131% and 42°, respectively (both -10 dB). The large bandwidth arises from two resonances within the utilized frequency range. Parameter studies using different models showed that the realized design is already close to the achievable optimum for the transducer technology used. Two 3D USCT systems were equipped with the new arrays. First images show promising results, with an increase in image contrast and a significant reduction of artifacts.

Ultrasound tomography for breast cancer imaging relies on high quantities of transducers. These should exhibit reliable performance, low variability and high production yield. This work aims for finding a suitable quality control method to identify manufacturing errors of ultrasound transducer arrays. Electromechanical impedance measurements were performed at five states during the array manufacturing process. Ten parameter were derived from each measurement at each state. The relative change of these parameters allows to evaluate their suitability for quality control. Suitable parameters must exhibit high sensitivity to changes in manufacturing, but low sensitivity to parasitic circuit elements. To quantify the separability, overlapping coefficients were calculated for each parameter between the current and the preceding manufacturing state. These coefficients reflect the intersecting area of two probability density functions which were calculated using a normal kernel function to avoid distributional assumptions. The bandwidth of the electrical input power was found the most suitable parameter for quality control. Identification of manufacturing defects is possible with a probability of 0.006 for false negative decisions. Additionally, it shows high overlapping coefficients when adding parasitic circuit elements. These results encouraged us to use the derived classifiers from easy-to-perform electromechanical impedance measurements for quality control of our ultrasonic transducer arrays.

A semi-automated packaging process for transducer array manufacturing is presented. As an major innovation, a transducer disc is integrated in a sandwich structure between a printed circuit board (PCB) and an acoustic matching layer. Each of the transducer discs contains 18 lead-zirconium-titanate (PZT) fibres embedded in epoxy. To interconnect the transducer array components, adhesive bonding and automatic pick-and-place processes were used. A pre-series was evaluated by measuring the electro-mechanical impedance (EMI) before and after the assembly. Statistical analysis showed consistent behaviour of the series resonance f s and the electro-mechanical coupling k eff before and after the packaging. This encouraged the manufacturing of 256 arrays. These arrays will now be integrated in an ultrasound computer tomography (USCT) system with 3D scanning aperture for breast cancer imaging. With this system, we intend to bridge the gap towards clinical use of full 3D USCT.

The requirements of our third generation 3D ultrasound computer tomography system (USCT) led to a transducer array design based on single-fibre piezocomposites. The used piezoelectric ceramic fibres exhibit a thickness to diameter ratio of 4/3. This ratio results in a proximity of the dominant axial and lateral vibration resonances. The standard Krimholtz-Leedom-Matthaei (KLM) model considers only one spatial dimension. This limits the validity of the modelled predictions for our application. By adding a coupled transmission line in series to the axial resonator, the standard model can be extended to take additional spatial dimensions into account. Using model parameters from literature resulted in a low model-to-measurement fit. To enhance the prediction accuracy, a brute-force optimisation over four model parameters was conducted. With the optimal parameter set, a fit of 90.3% could finally be achieved. The necessary model parameter changes were found reasonable for the piezocomposite design. With the presented model enhancement it is now possible to consider both dominant vibration modes of our transducers.