Aluminum nitride (AlN) has emerged as a critical material for microelectromechanical systems (MEMS) due to its exceptional piezoelectric properties, high thermal conductivity, and compatibility with semiconductor fabrication processes. Unlike other piezoelectric materials such as lead zirconate titanate (PZT), AlN is lead-free, making it environmentally favorable and suitable for applications where regulatory restrictions apply. Its wide bandgap and chemical stability further enhance its reliability in harsh environments. This article focuses on the use of AlN in MEMS for piezoelectric actuation and sensing, particularly in resonant sensors, energy harvesters, and inertial devices.

One of the most significant advantages of AlN in MEMS is its strong piezoelectric response along the c-axis of its wurtzite crystal structure. The piezoelectric coefficients d33 and d31 for AlN are approximately 5.1 pm/V and -2.1 pm/V, respectively. While these values are lower than those of PZT, AlN compensates with low dielectric losses, high acoustic velocity, and excellent temperature stability. These characteristics make it ideal for high-frequency resonant devices where minimal energy dissipation and precise frequency control are required. Additionally, AlN films can be deposited using sputtering techniques with high uniformity and low stress, enabling integration with CMOS processes.

Resonant sensors leveraging AlN’s piezoelectric properties are widely used in mass sensing, pressure sensing, and gas detection. In these devices, an AlN thin film is typically patterned into a suspended membrane or cantilever that vibrates at a resonant frequency when electrically excited. The resonant frequency shifts in response to external stimuli such as added mass or pressure changes, allowing for highly sensitive detection. For example, AlN-based resonant mass sensors have demonstrated mass resolution in the picogram range, making them suitable for biochemical sensing applications. The high quality factor (Q-factor) of AlN resonators, often exceeding 10,000 in vacuum, ensures sharp resonance peaks and improved signal-to-noise ratios. Furthermore, AlN’s temperature stability reduces frequency drift, enhancing long-term reliability.

Energy harvesting is another key application where AlN’s piezoelectric properties are exploited to convert ambient mechanical vibrations into electrical energy. AlN-based energy harvesters are particularly attractive for powering wireless sensor nodes and low-power electronics in environments where battery replacement is impractical. The design typically involves a cantilever or diaphragm structure with an AlN layer sandwiched between two electrodes. When subjected to vibrations, the strain-induced piezoelectric potential generates an alternating voltage across the electrodes. Research has shown that optimized AlN harvesters can achieve power densities in the range of tens to hundreds of microwatts per square centimeter under realistic vibration conditions. The absence of lead in AlN also makes these devices more sustainable compared to PZT-based harvesters.

Inertial devices such as accelerometers and gyroscopes benefit from AlN’s ability to provide both actuation and sensing functions. In piezoelectric accelerometers, an AlN layer detects acceleration-induced strain, generating a proportional voltage output. These devices offer advantages over capacitive accelerometers in terms of lower power consumption and simpler readout circuitry. AlN-based gyroscopes utilize the Coriolis effect, where a primary resonant mode is excited piezoelectrically, and the orthogonal secondary mode induced by rotation is detected via the same AlN layer. The high stiffness and low mechanical damping of AlN contribute to improved sensitivity and bandwidth in inertial sensors. Recent advancements have demonstrated monolithic integration of AlN inertial sensors with CMOS circuitry, enabling compact and low-cost solutions for navigation and motion tracking applications.

The fabrication of AlN MEMS devices involves several critical steps to ensure optimal piezoelectric performance. Sputter deposition is the most common method for growing AlN thin films, with process parameters such as substrate temperature, nitrogen-to-argon ratio, and DC bias significantly influencing film quality. A highly c-axis-oriented film is essential for maximizing piezoelectric response, which is typically verified using X-ray diffraction (XRD) measurements. Post-deposition annealing can further improve crystallinity and reduce defects. Patterning of AlN is achieved through dry etching techniques such as reactive ion etching (RIE) with chlorine-based chemistries, which provide high selectivity and anisotropic profiles. Device integration often requires careful stress management to avoid warping or delamination of thin-film stacks.

Despite its advantages, AlN faces challenges in MEMS applications. The relatively low piezoelectric coefficients compared to PZT limit the energy conversion efficiency in some scenarios. Researchers are exploring doping strategies, such as scandium doping, to enhance AlN’s piezoelectric properties without compromising its other beneficial characteristics. Scandium-doped AlN (ScAlN) has shown a significant increase in d33, with values reaching up to 20 pm/V at high Sc concentrations. However, doping introduces complexities in deposition control and may affect film stress and thermal stability. Another challenge is the need for high-quality electrodes that ensure low contact resistance and minimal energy loss. Molybdenum and platinum are commonly used due to their compatibility with AlN and low interfacial resistance.

Future developments in AlN MEMS are likely to focus on improving material properties through advanced doping techniques and heterostructure engineering. The integration of AlN with 2D materials or other wide-bandgap semiconductors could enable novel device functionalities. Additionally, the rise of AI-driven design optimization and additive manufacturing techniques may accelerate the development of next-generation AlN MEMS devices with tailored performance metrics. Applications in emerging fields such as wearable health monitors, structural health monitoring, and IoT sensors will further drive demand for high-performance AlN-based solutions.

In summary, aluminum nitride stands out as a versatile material for MEMS applications requiring piezoelectric actuation and sensing. Its combination of favorable electromechanical properties, environmental compatibility, and CMOS integration potential makes it a preferred choice for resonant sensors, energy harvesters, and inertial devices. Ongoing research aims to overcome current limitations and expand the scope of AlN-based MEMS technologies, ensuring their continued relevance in an increasingly interconnected and automated world.