## 1 Introduction  
Sensors are at the heart of modern scientific and technological progress, driving advancements in information technology beyond computation and communication, and enabling \"perception\" and \"intelligence\". The increasing demand for intelligent sensors is reflected in the proliferation of smart applications, including autonomous vehicles, smart homes, and smart cities [1, 2].  
Micro-Electromechanical System (MEMS) technology plays a key role in enabling intelligent sensors by integrating sensing and actuation at the microscale through semiconductor micro- and nanofabrication processes. MEMS sensors achieve conversion between various physical quantities, mechanical vibrations, and electrical signals through structural design. Their small size, low power consumption, high reliability, and ease of integration make MEMS sensors particularly advantageous for applications requiring miniaturization and intelligent functionality [3].  
Among piezoelectric materials used for MEMS sensors, aluminum nitride (AlN) and scandium-doped AlN (AlScN) have attracted considerable attention due to their compatibility with CMOS processes. AlN-based sensors have found rapid industrial development in RF MEMS [4; 5], optical MEMS [6; 7], and acoustic MEMS applications [8; 9]. Acoustic MEMSs, specifically micromachined ultrasonic transducers (MUTs), have become a major area of focus due to their potential for smart and portable applications [10; 11].  
Traditional bulk ceramic ultrasonic transducers, primarily made from sintered PZT-based piezoelectric ceramics, have significant drawbacks. Their relatively large size, high power consumption, and manufacturing inconsistencies make them unsuitable for modern smart applications [12; 13; 14]. Compared to these traditional transducers, MEMS-based ultrasonic sensors, such as AlN piezoelectric micromachined ultrasonic transducers (PMUTs), offer substantial advantages, including smaller size, lower power consumption, and improved integrability with CMOS technology. These benefits make PMUTs particularly suitable for portable devices, medical ultrasound imaging, and gesture recognition applications [15; 16; 17; 18; 19].  
A major advantage of AlN-based MEMS sensors is their ability to operate under high-temperature conditions. Traditional PZT-based piezoelectric sensors exhibit reduced piezoelectric performance at elevated temperatures, due to thermal effects that affect electric dipole alignment [20]. In contrast, AlN, with its stable wurtzite crystal structure and lack of a Curie temperature, maintains good piezoelectric performance even at high temperatures, making it suitable for use in harsh environments [21].  
Despite the advantages of AlN-based PMUTs, research on their performance under high-temperature environments remains limited [22]. Although AlN has demonstrated good high-temperature stability as a piezoelectric material, a systematic study on the performance of AlN PMUTs, particularly concerning resonant frequency, electromechanical coupling, and stress variations under high-temperature conditions, is lacking. This gap in the literature presents a significant opportunity for understanding the reliability and optimization of PMUTs in harsh environments.  
In this work, we address this gap by investigating the high-temperature performance of AlN PMUTs fabricated using standard SOI processes. This study aims to evaluate the changes in performance parameters such as resonance frequency and electromechanical coupling under elevated temperatures, thereby contributing to the understanding of PMUT reliability and optimization for applications in harsh environments.