Physical Vapor Deposition (PVD) is a critical technology for the fabrication of Microelectromechanical Systems (MEMS) and Nanoelectromechanical Systems (NEMS). It enables the deposition of thin films with precise control over thickness, composition, and stress, which are essential for the performance and reliability of microscale and nanoscale devices. Among the key materials deposited via PVD for MEMS/NEMS applications are silicon oxides (SiOx), silicon nitrides (SiNx), and metals, each playing a vital role in the functionality of cantilevers, membranes, and actuators.

One of the primary challenges in MEMS/NEMS fabrication is managing residual stress in thin films. Excessive tensile or compressive stress can lead to device deformation, delamination, or failure. PVD techniques such as sputtering and evaporation allow for fine-tuning of stress through process parameters like power, pressure, and substrate temperature. For instance, SiNx films deposited via reactive sputtering can exhibit stress ranging from highly compressive to tensile, depending on the nitrogen-to-silicon ratio and deposition conditions. Adjusting these parameters enables the engineering of stress-neutral SiNx layers, which are crucial for suspended structures like cantilevers and membranes. Similarly, SiOx films deposited by PVD can be tailored for low stress, making them suitable for optical MEMS and interferometric applications where flatness is critical.

Step coverage is another critical consideration in PVD for MEMS/NEMS. Unlike Chemical Vapor Deposition (CVD), which offers excellent conformality, PVD typically results in directional deposition, leading to uneven coverage on high-aspect-ratio features. However, techniques such as collimated or ionized sputtering improve step coverage by enhancing the directionality of deposited species. This is particularly important for devices like RF switches and inertial sensors, where uniform metal coatings on sidewalls are necessary for electrical conductivity and mechanical integrity. For example, gold or aluminum layers deposited via PVD must exhibit sufficient step coverage to ensure reliable electrical contacts in RF MEMS switches.

Etch selectivity is a key factor in patterning PVD-deposited films for MEMS/NEMS. The choice of material and deposition method influences the etch rates in wet or dry etching processes. SiNx films, for instance, are often used as etch stops due to their high resistance to hydrofluoric acid (HF), which readily etches SiOx. This selectivity is exploited in the fabrication of membranes and released structures, where sacrificial oxide layers are removed without attacking the structural nitride. Similarly, metal layers such as chromium or titanium serve as adhesion promoters and etch masks, with their etch rates carefully controlled to avoid undercut or over-etching during device patterning.

Applications of PVD in MEMS/NEMS span a wide range of devices. In sensors, stress-controlled SiNx membranes are employed in pressure sensors and microphones, where mechanical stability and sensitivity are paramount. For bioMEMS, PVD-deposited gold or platinum layers provide biocompatible electrodes for neural probes and biosensors. RF MEMS switches benefit from low-stress metal films that ensure reliable actuation and minimal deformation over millions of cycles. Additionally, PVD is instrumental in fabricating piezoelectric actuators, where precise thickness control of materials like aluminum nitride (AlN) or zinc oxide (ZnO) is necessary for optimal electromechanical coupling.

The versatility of PVD extends to advanced MEMS/NEMS applications such as optical filters and tunable capacitors. In these devices, alternating layers of SiOx and SiNx or metals form distributed Bragg reflectors or variable capacitors, with each layer’s optical or electrical properties finely tuned via deposition parameters. The ability to deposit multilayered structures with minimal stress and high uniformity makes PVD indispensable for these precision applications.

Despite its advantages, PVD faces challenges in MEMS/NEMS fabrication, particularly in achieving conformal coatings on complex 3D structures. Techniques like atomic layer deposition (ALD) often complement PVD in such cases, though PVD remains the preferred method for its high deposition rates and material versatility. Furthermore, the integration of PVD with other processes, such as deep reactive ion etching (DRIE), requires careful optimization to avoid compatibility issues like film peeling or contamination.

In summary, PVD is a cornerstone technology for MEMS/NEMS fabrication, offering unparalleled control over film stress, composition, and thickness. Its applications in sensors, RF switches, and bioMEMS highlight its critical role in enabling high-performance microscale and nanoscale devices. Continued advancements in PVD techniques, including improved step coverage and stress engineering, will further expand its utility in emerging MEMS/NEMS technologies.