Christian Bretthauer

Acoustic microelectromechanical sensors and actuators like microphones or microspeakers often use cantilever-based structures. The fluidic resistance due to the air slit between the cantilevers plays a crucial role for acoustic key parameters like the low-frequency roll-off or the sound pressure level of such device. The opening and closing behavior of such air slits becomes complex when the displacement profile of the cantilevers is considered, leading to nonuniform slit openings, slit widths and Knudsen number-based corrections for the airflow resistance along the cantilever arc length. To extract the fluidic resistances of the air slits, the application of finite element methods (FEMs) is necessary. However, such methods require time-consuming simulations, which make the design of acoustic microelectromechanical system devices cumbersome. Using a physics-based analytic model can mitigate the computational efforts by extracting fluidic resistances utilizing microfluidic equations. The here presented model uses various geometric cantilever parameters in combination with discretizing the fluidic slit resistance along the arc length of the cantilever to calculate the fluidic resistance for each displacement. The model considers rarefied gases for small air slits of 500 nm as well as static prebending induced by e.g. residual mechanical stress in fabrication processes. A four-flap triangular cantilever structure is used to verify the derived physics-based analytic model with FEM simulations. The proposed model shows a good agreemend with the conducted FEM simulations over a large range of geometric parameters. In future work, it can be implemented in circuit simulators allowing a lumped element representation of the microphone or microspeaker.

Strain gauges are regarded as the gold standard in various force sensing applications, including minimally invasive surgical devices, due to their high sensitivity and ability for miniaturization at millimeter scale. However, further reduction in size and improvement concerning the limit of detection are desirable to enable seamless integration into highly constrained environments, such as needle tips, without compromising performance. Capacitive micromachined ultrasonic transducers (CMUTs), though not previously applied to direct force measurement, offer significant potential as high-sensitivity alternatives when their resonant characteristics are effectively harnessed. In this work, we present a piston-structured CMUT with a thinned back electrode, configured as a force-sensing element mounted on a deflecting cantilever. The system is evaluated using both numerical simulation and experimental analysis. The numerical simulation is verified analytically and validated by measuring the deflection of the top plate. As a proof of concept, the performance of the CMUT is characterized via impedance-based frequency tracking, avoiding full circuit integration. We demonstrate the feasibility of CMUTs as force-sensitive resonators, achieving sensitivities of 0.80 Hz/mN under tensile loading and 0.76 Hz/mN under compression, with a linearity error of 2.69% and an initial response threshold of 20.5 mN. These results establish the functionality of CMUTs as force-sensing elements and lay the groundwork for further optimization toward a new class of highly integrable force sensors.

This paper introduces a novel system designed to enhance the robustness of airborne ultrasound technology against in-band interference. By integrating MEMS-based transceivers, the system achieves ultrasonic pulse-echo operation up to 100 kHz while maintaining low power consumption. A dedicated hardware readout block effectively addresses the signal distortion and encodes the transmission intervals in the received signal. The experimental results demonstrate the key role of hardware-defined reference points for the start of the frame for accurate echo detection. This optimization ensures superior performance compared to the reference system, especially in the presence of in-band interference. The system’s multi-channel readout feature shows potential for applications like 3D object detection and tracking, even in noise-heavy environments.

In this work, novel airborne capacitive micromachined ultrasonic transducers (CMUTs) based on a dual-backplate (DBP) technology are presented. In contrast to conventional CMUTs, these transducers use a three-electrode-based capacitive system, where the membrane is placed between two highly-perforated counter electrodes, enabling enlarged displacement amplitudes in electrostatic actuation and wide and tunable bandwidth due to a ventilated air cavity. Fabricated DBP-CMUT prototypes therefore show exceptionally high receive and transmit sensitivities of -34.5 dB(V/Pa) and 259 nm/V, respectively, in their 84 kHz resonance. The viscous dissipation introduced by ventilating the cavity results in a wide factional bandwidth (FBW) of 29%. An applicability of the developed CMUT for airborne ranging is demonstrated in pulse-echo based ranging measurements, where the distance of a sound reflecting metal plate can be clearly detected by a single CMUT operated in a transceiver mode.

Capacitive micromachined ultrasonic transducers (CMUTs) are the next-gen ultrasound transducers for diverse applications. CMUTs provide high bandwidth and sensitivity and have excellent integrability with CMOS processes. Additionally, our CMUT design enables mobile and battery driven devices with a small form factor and requiring voltages as low as < 30 V. Here, we present a novel coupling approach and demonstrate two sensor applications: fill level sensing and touch sensing. With a package containing a stack of silicone and epoxy, we achieve excellent coupling into different materials such as mold plastics and aluminum via direct or intermediate acoustic impedance matching. This enables an universal solution for coupling into almost any kind of material. Fill level sensing is demonstrated through a ca. 4 mm-thick aluminum containment wall with levels from 2.3 mm to 600 mm. Furthermore, a touch sensor is realized via detecting a reduced echo amplitude due to the presence of a finger. Measurements through a commercial decor panel and a ca. 4 mm-thick aluminum plate show a maximum signal difference of 6.5 % and 19.2 % in the echo, respectively. Additionally, we investigate several interference and coupling phenomena in the layered system for the touch scenario. By that, we enable a wide field of applications, where CMUTs are coupled to hard materials such as plastics and metals, as found in touch sensors in e.g. consumer electronics, as well as liquid (flow / fill level) monitoring in chemistry, pharmaceutical and food industry. Furthermore, structural health monitoring for smart industry and non-destructive testing are feasible with our approach as well.

Capacitive micromachined ultrasonic transducers (CMUTs) represent an accepted technology for ultrasonic transducers, while high bias voltage requirements and limited output pressure still need to be addressed. In this paper, we present a design for ultra-low-voltage operation with enhanced output pressure. Low voltages allow for good integrability and mobile applications, whereas higher output pressures improve the penetration depth and signal-to-noise ratio. The CMUT introduced has an ultra-thin gap (120 nm), small plate thickness (800 nm), and is supported by a non-flexural piston, stiffening the topside for improved average displacement, and thus higher output pressure. Three designs for low MHz operation are simulated and fabricated for comparison: bare plate, plate with small piston (34% plate coverage), and big piston (57%). The impact of the piston on the plate mechanics in terms of resonance and pull-in voltage are simulated with finite element method (FEM). Simulations are in good agreement with laser Doppler vibrometer and LCR-meter measurements. Further, the sound pressure output is characterized in immersion with a hydrophone. Pull-in voltages range from only 7.4 V to 25.0 V. Measurements in immersion with a pulse at 80% of the pull-in voltage present surface output pressures from 44.7 kPa to 502.1 kPa at 3.3 MHz to 4.2 MHz with a fractional bandwidth of up to 135%. This leads to an improvement in transmit sensitivity in pulsed (non-harmonic) driving from 7.8 kPa/V up to 24.8 kPa/V.

Fan-out wafer-level packaging (FOWLP) is an interesting platform for Microelectromechanical systems (MEMS) sensor packaging. Employing FOWLP for MEMS sensor packaging has some unique challenges, while some originate merely from the fabrication of redistribution layers (RDL). For instance, it is crucial to protect the delicate structures and fragile membranes during RDL formation. Thus, additive manufacturing (AM) for RDL formation seems to be an auspicious approach, as those challenges are conquered by principle. In this study, by exploiting the benefits of AM, RDLs for fan-out packaging of capacitive micromachined ultrasound transducers (CMUT) were realized via drop-on-demand inkjet printing technology. The long-term reliability of the printed tracks was assessed via temperature cycling tests. The effects of multilayering and implementation of an insulating ramp on the reliability of the conductive tracks were identified. Packaging-induced stresses on CMUT dies were further investigated via laser-Doppler vibrometry (LDV) measurements and the corresponding resonance frequency shift. Conclusively, the bottlenecks of the inkjet-printed RDLs for FOWLP were discussed in detail.