Atomic layer deposition has become an indispensable tool in micro- and nanoelectromechanical systems fabrication due to its unique capabilities in depositing ultra-thin, conformal films with precise thickness control. The technique's self-limiting surface reactions enable uniform coatings even on high-aspect-ratio structures, addressing critical challenges in MEMS/NEMS manufacturing where three-dimensional geometries dominate device architectures. Unlike other deposition methods that struggle with shadowing effects, ALD provides consistent material coverage regardless of feature orientation or complexity, making it particularly valuable for creating functional layers in intricate MEMS/NEMS designs.

One of the most significant advantages in MEMS/NEMS applications lies in the exceptional control over mechanical stress in deposited films. Residual stress management proves crucial for device performance and reliability, as excessive stress can lead to deformation, delamination, or mechanical failure. Through careful selection of precursors, deposition temperatures, and process parameters, ALD enables tuning of intrinsic film stress from highly compressive to tensile states. For instance, aluminum oxide films can be engineered with near-zero stress when deposited at specific temperatures, while adjusting the oxygen source can modify the stress state by hundreds of megapascals. This level of control allows designers to compensate for stress-induced curvature in multilayer structures or create intentionally stressed films for actuation purposes.

The conformality of ALD coatings addresses multiple challenges in MEMS/NEMS fabrication simultaneously. Three-dimensional structures such as comb drives, cantilevers, and suspended membranes require uniform material properties throughout their entire surface area to ensure consistent electrical, mechanical, and thermal behavior. Traditional deposition methods often produce thickness variations that lead to performance inconsistencies, particularly in devices relying on precise capacitive or resonant behavior. ALD's ability to coat all surfaces equally eliminates these variations, enabling more predictable device operation. Furthermore, the technique's excellent step coverage proves invaluable for coating sidewalls and undercuts in released MEMS structures, where other methods would leave critical areas unprotected.

Material integration through ALD enables novel MEMS/NEMS functionalities that would be difficult or impossible to achieve with conventional fabrication approaches. The technique allows sequential deposition of different materials without breaking vacuum, creating sharp interfaces and preventing contamination. This capability facilitates the integration of dielectric, conductive, and piezoelectric materials within the same fabrication sequence. For example, alternating layers of aluminum oxide and titanium dioxide can create nanolaminates with tailored dielectric properties, while ruthenium or platinum ALD provides conductive paths in three-dimensional architectures. Such material combinations enable advanced MEMS/NEMS devices with integrated sensing, actuation, and passivation in a single fabrication process.

Surface engineering through ALD significantly enhances the reliability and lifetime of MEMS/NEMS devices. The technique can deposit pinhole-free barrier layers as thin as a few nanometers to protect sensitive components from environmental degradation. In devices containing moving parts, ALD coatings reduce stiction and wear through controlled surface chemistry and roughness. Alumina films have demonstrated particular effectiveness in reducing adhesion between contacting surfaces, while tungsten-based coatings improve wear resistance in sliding interfaces. These surface modifications occur without significantly altering device dimensions or mass, maintaining the delicate balance required for micro- and nanoscale mechanical systems.

The precision of ALD enables thickness-dependent effects that are exploited in advanced MEMS/NEMS designs. By controlling film growth at the atomic level, designers can create functional layers with thicknesses precisely tuned to device requirements. This proves especially valuable in optical MEMS, where interference effects demand nanometer-level thickness control, and in nanomechanical resonators, where film thickness directly influences resonant frequency. The technique's reproducibility ensures that these thickness-dependent properties remain consistent across wafers and between fabrication runs, a critical requirement for commercial MEMS/NEMS production.

Thermal management in MEMS/NEMS benefits substantially from ALD's capabilities. The technique can deposit ultra-thin, thermally conductive layers for heat spreading or insulating barriers for thermal isolation, depending on device requirements. Aluminum nitride films grown by ALD provide both thermal conductivity and electrical insulation, making them ideal for thermally sensitive MEMS/NEMS applications. Conversely, porous alumina layers can serve as effective thermal barriers in devices requiring temperature gradients. The conformal nature of these coatings ensures uniform thermal properties throughout three-dimensional device structures, preventing localized hot spots that could degrade performance or reliability.

Integration of ALD with existing MEMS/NEMS fabrication processes demonstrates remarkable compatibility. The technique operates at relatively low temperatures compared to many conventional deposition methods, allowing deposition on temperature-sensitive materials and structures. Post-processing steps such as annealing can further modify ALD film properties without damaging underlying device components. This compatibility extends to various substrate materials including silicon, silicon nitride, polymers, and even previously released MEMS structures. The ability to add functional ALD coatings at various stages of device fabrication provides designers with unprecedented flexibility in process flow optimization.

Challenges remain in optimizing ALD processes for specific MEMS/NEMS applications, particularly regarding throughput and material selection. While the technique offers unparalleled control over film properties, its cyclic nature can result in slower deposition rates compared to other methods. Process development continues to address this limitation through reactor design innovations and precursor chemistry improvements. Material options, though expanding rapidly, still face limitations in certain conductive and piezoelectric films where achieving desired crystallinity or stoichiometry proves challenging at lower deposition temperatures.

Future developments in ALD for MEMS/NEMS will likely focus on expanding material libraries and improving process efficiency while maintaining the technique's fundamental advantages. Emerging areas include area-selective ALD for patterned deposition without lithography steps, and the development of new precursor chemistries for specialized functional materials. As MEMS/NEMS devices continue shrinking in size and increasing in complexity, atomic layer deposition will play an increasingly vital role in enabling their fabrication and performance. The technique's unique combination of conformality, precision, and material versatility positions it as a cornerstone technology for next-generation micro- and nanoelectromechanical systems.