Advances in High Sound Pressure Piezoelectric Micromachined Ultrasonic Transducers

Academic Background

Ultrasonic transducers are widely used in object detection, non-destructive testing (NDT), biomedical imaging, and therapeutic treatments. Compared to traditional bulk ultrasonic transducers, piezoelectric micromachined ultrasonic transducers (PMUTs) offer advantages such as small size, low power consumption, and wide bandwidth, making them suitable for applications in consumer electronics and the Internet of Things (IoT), including ranging, gesture recognition, fingerprint sensing, and 3D imaging. However, these small sensors have relatively low output pressure, which limits their signal transmission capabilities in various applications. For example, the state-of-the-art aluminum nitride (AlN)-based PMUT arrays have only achieved a transmission distance of 4 meters. To expand the use of PMUTs in applications such as mid-air haptics, loudspeakers, and acoustic tweezers, the main challenge lies in achieving a high output sound pressure level (SPL).

The transmission characteristics of PMUTs are primarily defined by the mechanical structural design and the active piezoelectric material, leading to the need for new materials to improve performance. Although AlN is the most commonly used piezoelectric material, its piezoelectric coefficient is relatively low (e31;f ≈ -1 C/m²). By adjusting material composition, such as incorporating 36% scandium (Sc) into AlN (ScAlN) films, the piezoelectric coefficient can be increased to -2.3 C/m². However, lead zirconate titanate (PZT), which can produce relatively high output pressure, suffers from low receiving sensitivity due to its high dielectric constant, and the presence of lead limits its use in certain applications. Therefore, the search for lead-free piezoelectric materials to further enhance PMUT performance has become a research focus.

Source of the Paper

This paper, authored by Fan Xia, Yande Peng, Wei Yue, Mingze Luo, Megan Teng, Chun-Ming Chen, Sedat Pala, Xiaoyang Yu, Yuanzheng Ma, Megha Acharya, Ryuichi Arakawa, Lane W. Martin, and Liwei Lin, was published in the journal Microsystems & Nanoengineering in 2024. The paper details the design, fabrication, and applications of high sound pressure PMUTs based on sputtered potassium sodium niobate (KNN) films in haptic feedback, loudspeakers, and rangefinders.

Research Process

Design and Fabrication

The transduction process of PMUTs involves three energy domains: electrical, mechanical, and acoustic. PMUTs convert electrical excitation signals into acoustic waves through electromechanical-acoustic coupling. The PMUTs designed in this paper feature a circular unimorph diaphragm structure, consisting of a 2-μm-thick KNN film as the active piezoelectric layer and a 5-μm-thick silicon device layer as the elastic layer. The dual-electrode geometry enhances vibration displacement and output pressure through differential driving. Simulation results show that the fundamental flexural mode of the PMUT exhibits a stress reversal point at 67% of the radius under clamped boundary conditions, and the differential drive configuration maximizes the use of the entire piezoelectric diaphragm to increase output.

The fabrication process begins with the deposition of a 25-nm-thick zinc oxide (ZnO) adhesion layer and a 200-nm-thick platinum (Pt) bottom electrode layer on a 6-inch silicon-on-insulator (SOI) wafer, followed by the deposition of a 2-μm-thick KNN film via radio frequency (RF) magnetron sputtering at 500°C. Subsequently, a 10-nm-thick ruthenium oxide (RuO2) layer and a 100-nm-thick Pt layer are deposited and patterned as the inner circular and outer ring top electrodes. Via openings to access the bottom electrode are created by wet etching the KNN film, and the backside silicon cavity is defined by a silicon deep reactive-ion etching (DRIE) process.

Characterization and Testing

The crystal structure of the KNN film and the mechanical properties of the PMUTs were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD results show that the KNN film has good crystallinity, and SEM images reveal the multilayer diaphragm structure and the thickness of each layer. Electrical characterization tests show that as the diaphragm radius increases, the resonant frequency of the PMUT decreases from 241 kHz to 21.2 kHz. Mechanical vibration tests indicate that under a differential driving scheme, the center displacement of the PMUT reaches 1.23 μm under a 200 mVp-p excitation, corresponding to a displacement sensitivity of 12.3 μm/V.

Acoustic performance tests show that the PMUT achieves a sound pressure level (SPL) of 133 dB at a 1 cm axial distance and 111.6 dB at 10 cm, with a transmission sensitivity 5-10 times higher than that of AlN-based PMUTs. Nonlinear behavior studies reveal that the vibration frequency of the PMUT drifts with increasing displacement, consistent with the Duffing nonlinear model.

Main Results

Haptic Feedback Application

A 15×15 KNN PMUT array generates a focal pressure of 2900 Pa at a distance of 15 mm under a 12 Vp-p driving voltage, corresponding to an SPL of 160.3 dB. This represents the highest output pressure achieved by an airborne PMUT array as a haptic actuator. Through a pulse-width modulation (PWM) scheme, the PMUT array can provide non-contact haptic stimulation on human palms, with instant haptic feedback achieved in 90% of volunteer tests.

Loudspeaker Application

A single PMUT element with a resonant frequency of 22.8 kHz can generate an SPL of 105 dB at a 3 cm axial distance. Using an amplitude modulation (AM) scheme, the PMUT can produce uniform acoustic output within the audible frequency range of 20 Hz to 20 kHz, with an SPL of approximately 85 dB. Although the structure has not been optimized for speaker applications, the successful generation of audible sound demonstrates the strong output capability of the KNN PMUT.

Rangefinder Application

Pulse-echo measurements using a single PMUT element demonstrate good transceiving ability, with objects detectable up to 2.82 meters away. The pulse-echo measurement results show that the echo amplitude is 0.32 mV at a distance of 1.5 meters, with a time-of-flight (TOF) of 8.6 ms. By optimizing array design and acoustic packaging, the detection range of the PMUT can be further extended.

Conclusion

This study demonstrates high sound pressure PMUTs based on sputtered KNN films, with the KNN film exhibiting good crystal quality in the 001 orientation and a high piezoelectric coefficient. A single KNN PMUT with a resonant frequency of 106.3 kHz under 4 Vp-p achieves a large vibration amplitude of up to 3.74 μm/V and an SPL of 132.3 dB at a 1 cm axial distance and 111.6 dB at 10 cm, with a transmission sensitivity 5-10 times higher than that of state-of-the-art AlN-based PMUTs. Through structural design, packaging optimization, and customized electronics, the performance of PMUTs can be further improved.

In applications such as haptic feedback, loudspeakers, and rangefinders, KNN PMUTs have demonstrated their advantages of high sound pressure and low driving voltage. In the future, these PMUTs are expected to find widespread use in fields such as acoustic cooling, portable ultrasound imaging, cardiovascular monitoring, non-destructive testing, flowmeters, underwater imaging, and acoustic tweezers.

Research Highlights

1. High Sound Pressure Output: KNN PMUTs achieve high sound pressure output under low driving voltages, 5-10 times higher than AlN-based PMUTs.
2. Multifunctional Applications: Demonstrates strong performance in haptic feedback, loudspeakers, and rangefinders.
3. Lead-Free Material: KNN, as a lead-free piezoelectric material, has a high piezoelectric coefficient and low dielectric constant, making it suitable for various applications.
4. Innovative Design: The differential drive and dual-electrode geometry maximize the use of the piezoelectric diaphragm, enhancing output performance.

This study provides new insights into the design and application of high sound pressure PMUTs, showcasing their great potential in various fields.