title: Ultra-Low-Voltage CMUTs and Innovative Acoustic Coupling for Touch and Fill Level Sensing
Creators: name: Merbeler, Fabian
Publisher: name: UNSPECIFIED
Year: 2024

description: Ultrasound is a well-established sensing method in various industries and applications like medical imaging, underwater sonars, non-destructive testing, parking sensors in cars, flow metering, and many more. 
Compared to radar and optical approaches, ultrasound systems are more cost-effective, less complex, and can penetrate virtually any kind of material.
The last three decades showed the rise of micromachined ultrasonic transducers (MUT), utilizing processes of the semiconductor industry. 

One type of MUTs is capacitive micromachined ultrasonic transducers (CMUTs), which offer many advantages over piezoelectric MUTs (PMUT). Therefore, CMUTs are the central device for the research conducted in this thesis. 

Based on the existing literature on CMUTs, the research question addresses three areas to utilize the full potential of CMUTs:
1. A more efficient CMUT with reduced bias voltages as well as optimized output pressures. 
2. A dedicated acoustic coupling approach is necessary to introduce CMUTs into real applications. There is little research in this area, but it is essential for commercialization. 
3. Novel applications need to be demonstrated to prove the feasibility and advantages of CMUTs beyond medical imaging.

To achieve these goals, two key CMUT features are introduced: a relatively small vacuum gap and a piston stiffening on the top side of the movable plate. The designs are implemented in Infineon's sacrificial release technology. Two CMUT generations are characterized for resonance, pull-in voltage, and acoustic output on a newly-developed hydrophone setup. Due to the piston stiffening, an increase of TX sensitivity of 7.8 kPa/V to 24.8 kPa/V is found for the same plate diameter. However, the pull-in voltage of 7.4 V without piston and 25V with piston is low compared to 30 V up to over 100 V in recent literature.

Acoustic coupling is realized with a silicone film and a low-shrinking epoxy inside a pre-molded package, allowing for efficient clamp-on application to many acoustically hard surfaces. The design, material choice, processing, and acoustic impedance matching are discussed. An universal impedance matching scheme for plastics and metals is introduced, further incorporating wide-band width capabilities.

Last, two novel use cases are demonstrated: through-wall fill level sensing and touch sensing through a surface, both with ultrasound pulses. Both are conducted through metal and additionally through a plastics-metal decor panel for touch detection. The system response concerning package echoes is analyzed and allows for time-of-flight fill level measurement through ~4 mm aluminum from a water height of 2.3 mm up to &gt;610mm, limited by the setup solely. Pulse-echo touch experiments show a maximum signal difference of the surface echoes between touch and no-touch of 19.2 % for aluminum, and manifold frequency-dependent standing wave phenomena in the decor panel.

In conclusion, the research presented with this thesis advances the application of CMUTs and their design for low-voltage operation, together with an universal and wide-band acoustic coupling concept.
Further research is advised for an extended study of acoustic coupling materials, and the investigation of package design with thin-film acoustics, e.g. to suppress package echoes and improve reproducibility in sample building. Moreover, the CMUT design space may be elaborated further, e.g. for wider pistons combined with smaller gaps. Ultimately, more refined signal processing, on-the-edge processing, and ASIC-integration should be investigated.