Design, characterization, and optimization of Al0.8Sc0.2N PMUTs for versatile sensing applications
Creators: name: Lyu, Haochen
Publisher: name: No Publisher Supplied
Year: 2025

description: Ultrasonic sensing technologies have seen rapid development across industries such as automotive, healthcare, and industrial automation, providing cost-effective and reliable solutions for distance measurement.Traditional ultrasonic sensors are primarily lead-based bulk piezoelectric ceramics, which typically have dimensions in the centimeter range. These sensors face several challenges, including complex fabrication processes, high energy consumption, large size, poor consistency between sensors, and acoustic impedance mismatch with the medium. Such limitations render them inadequate for meeting the current demands of intelligent, miniaturized, and integrated systems. In the past decade, the rapid development of MEMS technology has driven significant advancements in ultrasonic sensing technology. MEMS enables wafer-level manufacturing of ultrasonic sensors through semiconductor processes, significantly reducing manufacturing costs and energy consumption while miniaturizing the sensors to micro- and nanoscale dimensions. Currently, MEMS-based ultrasonic sensors are mainly categorized into capacitive (CMUT) and piezoelectric (PMUT) types. Among these, PMUTs have attracted considerable attention due to their more controllable fabrication process and their ability to operate without requiring a large DC bias voltage. This makes them particularly suitable for air-coupled applications such as distance detection, spatial positioning, and flow monitoring. The core thin-film materials used in PMUTs include PZT, zinc oxide, and aluminum nitride (AlN). While PZT offers a high piezoelectric coefficient, it is environmentally unfriendly due to its lead content. Additionally, PZT has a high dielectric constant, significant leakage current at high frequencies, and a high processing temperature, making it incompatible with CMOS technology and significantly increasing manufacturing costs. Zinc oxide, on the other hand, is simple to fabricate but suffers from low stiffness and poor resistance to high temperatures and corrosion, limiting its applicability under extreme conditions. AlN strikes a balance between manufacturing simplicity and material stability, being compatible with standard CMOS processes and offering excellent mechanical stability and resistance to high-temperature and corrosive environments. Furthermore, Sc doping significantly enhances the piezoelectric properties of AlN, making Sc-doped AlN the most commercially promising core piezoelectric thin-film material for PMUTs. Despite the widespread application of Sc-doped AlN-based PMUTs, several challenges remain, particularly in air-coupled applications: 1. The functional performance of Sc-doped AlN in high-temperature environments has not been extensively studied. While existing research indicates that Sc-doped AlN maintains high piezoelectric performance at elevated temperatures, critical parameters such as frequency drift, electromechanical coupling coefficient, and stress variations under extreme conditions remain unexplored. This lack of understanding hinders its reliable application in high-temperature environments. 2. Current application scenarios impose stringent frequency requirements on PMUTs. Low frequencies are preferred to reduce sound attenuation in the medium and extend the measurement range, while high frequencies are desired to shorten wavelengths and improve measurement accuracy. Existing approaches to achieving multi-frequency PMUTs typically involve controlling the amplitude and timing of array elements or integrating devices with different frequencies within the array. However, these methods compromise the advantages of PMUTs in terms of small size and low power consumption. 3. In near-field applications, PMUTs suffer from blind spots due to lingering vibrations after emitting sound waves. During this ringdown period, any received signals may be aliased. Additionally, the extended duration of the echo signals caused by this trailing vibration results in poor axial resolution. When two objects are close together, their reflected signals may overlap, further degrading resolution.end{enumerate} To address the issues above, this dissertation explores the design, characterization, and optimization of PMUTs for high-performance applications, with a focus on enhancing operational versatility, precision, and scalability. The research content can be divided into three main points: 1. Fabrication and Characterization of Scandium-Doped Aluminum Nitride Thin Films} This study begins with the deposition of 20% scandium-doped aluminum nitride ($ext{Al}_{0.8}ext{Sc}_{0.2}ext{N}$) films using physical vapor deposition (PVD) technology, followed by thin film etching. Stress and surface roughness uniformity of nine etched thin films were characterized to ensure the reliability of the fabrication parameters. Additionally, advanced material characterization techniques such as XRD, SEM, and EDS were employed to analyze the morphology and composition of the films. Finally, high-performance wafer-scale Al0.8Sc0.2N PMUT devices were successfully fabricated. 2. Performance Evaluation of PMUTs under High-Temperature Conditions The performance of PMUTs under high-temperature conditions was studied, including parameters such as resonant frequency, electromechanical coupling coefficient, and temperature stability. Finite element analysis (FEA) and experimental validations were conducted to investigate the effects of structural parameters, such as piezoelectric layer thickness, structural layer thickness, and back cavity diameter, as well as material variations like scandium doping, on the performance under extreme temperatures. The study also examined the reliability of PMUTs under thermal cycling conditions and characterized the frequency response at low temperatures. Results showed that when the operating temperature increased from room temperature to 200°C, PMUT devices with a back cavity diameter of 1000 µm and a top silicon thickness of 4 µm exhibited a temperature drift rate of 47.3%. The maximum electromechanical coupling coefficient improved by 68.6% at approximately 100°C compared to room temperature. Moreover, thermal cycling experiments revealed that the initial frequency shifted after temperature variations and could not recover over time, indicating the need for frequency calibration to ensure accurate measurements under extreme conditions. Low-temperature experiments further demonstrated that PMUTs maintained stable piezoelectric performance even at -30°C, confirming their reliability in extreme temperature environments. 3. Development and Characterization of a Mode-Switchable Multi-Frequency PMUT A novel mode-switchable multi-frequency PMUT (MS-MF-PMUT) was developed and characterized to achieve high-frequency consistency at the wafer scale, providing a mass-producible design for complex applications such as medical imaging and smart city technologies. This design addresses the challenges of low transmission sensitivity and high production complexity faced by current multi-frequency PMUTs. Simulations were first performed to obtain the frequency response characteristics of the device under different driving modes. Electrical characterizations were then conducted to analyze the frequency response, and dynamic vibration testing was used to study its resonant characteristics under different modes. Additionally, wafer-level frequency uniformity was investigated to assess its feasibility for mass production. Results showed that the device achieved maximum amplitudes of 350.2 nm/V and 218.6 nm/V in the two driving modes, respectively, and wafer-level frequency uniformity was maintained within ±8.14% and ±6.58% of the central frequency. These findings highlight its significant potential for large-scale production. 4. A novel driving method was proposed to improve axial resolution for near-range detection by employing forced vibration instead of the traditional resonant driving mode. Theoretical analysis was conducted to establish the relationship between the forced vibration driving frequency and the number of bursts. This relationship was verified through simulations and dynamic vibration testing. Acoustic tests were then performed to evaluate blind spots and axial resolution in near-range applications. Results indicated that, compared to traditional resonant driving, the proposed forced vibration method improved axial resolution by 440% and reduced blind spots by 37.5%. This demonstrates the effectiveness of the method in enhancing the near-range detection capabilities of PMUT sensors. The findings of this research contribute to the advancement of ultrasonic sensing technology, offering new insights into the design of PMUTs that are not only capable of multi-frequency and high-temperature operations but also optimized for both short-range and long-range applications. These innovations enhance the feasibility of PMUTs in diverse industries, including automotive, healthcare, and industrial sensing.