Ultrananocrystalline and Nanocrystalline Diamond Thin Films for MEMS/NEMS Applications

Anirudha V. Sumant, Orlando Auciello, Robert W. Carpick, Sudarsan Srinivasan, and James E. Butler of smooth diamond films with uniform thickness and micro/nanostructure over large area substrates (≥150 mm diameter wafer), as recently demonstrated.1 Growth of thin (0.1 to 5 micron thick) diamond films on nondiamond substrates, typically conventional electronic substrates such as Si, SiC, and AlN, is performed as the initial step in the fabrication of diamond-based MEMS/NEMS. Diamond films are grown by chemical vapor deposition (CVD) using hot filament–activated or plasma-activated processes.2–4 Growth of diamond by CVD on nondiamond substrates requires a surface pretreatment, or “seeding,” to enhance the nucleation of diamond grains, often by seeding/coating the substrate with small diamond nanoparticles.4 Conventional CVD film deposition methods employ microwave plasma-enhanced CVD or hot filament CVD with hydrogen-rich chemistry [H2 (balance)/CH4 (0.1 to 4%)].2–4 Polyor microcrystalline diamond films, with thicknesses between 0.1–5 μm,5 are produced when the initial nucleation density is low (<1010/cm2). Microcrystalline diamond (MCD) films exhibit a columnar grain structure whose large grains (1–5 μm) coarsen with thickness. Correspondingly, they exhibit a rough, highly faceted morphology whose root mean square (RMS) roughness is typically ~10% of the film thickness. On the other hand, when very high nucleation densities are achieved (>1012/cm2), relatively smooth, high-quality nanocrystalline diamond (NCD) films with thicknesses from 0.03–5 microns4,6 can be grown using 0.1 to 1% CH4 in H2 at temperatures of about 900°C. When the CH4 content is increased to about 4%, smoother, fine-grained NCD films are grown, but with a higher percentage of nondiamond carbon (i.e., sp2 bonds and noncrystalline configurations). Once the diamond nuclei are present from the seeding process, growth is mostly homoepitaxial (including twinning and defect formation) on the seeds, with some nondiamond carbon incorporated in the grain boundaries. In this diamond CVD growth process, the principal carbon growth species are the CH3 radicals.8–10 Atomic hydrogen is critical to drive the hydrogen abstraction reactions that (1) prepare the CH3 adsorption site by removing a hydrogen atom from the hydrogen-terminated diamond surface and (2) abstract the hydrogen atoms from the adsorbed CH3 and nearby surface sites, thereby permitting the carbon atom to insert into a diamond lattice site. The atomic hydrogen also preferentially etches the undesirable graphitic or amorphous carbon phases that might grow simul taneously with the diamond phase. Atomic hydrogen also etches Abstract There has been a tireless quest by the designers of microand nanoelectro mechanical systems (MEMS/NEMS) to find a suitable material alternative to conventional silicon. This is needed to develop robust, reliable, and long-endurance MEMS/NEMS with capabilities for working under demanding conditions, including harsh environments, high stresses, or with contacting and sliding surfaces. Diamond is one of the most promising candidates for this because of its superior physical, chemical, and tribomechanical properties. Ultrananocrystalline diamond (UNCD) and nanocrystalline diamond (NCD) thin films, the two most studied forms of diamond films in the last decade, have distinct growth processes and nanostructures but complementary properties. This article reviews the fundamental and applied science performed to understand key aspects of UNCD and NCD films, including the nucleation and growth, tribomechanical properties, electronic properties, and applied studies on integration with piezoelectric materials and CMOS technology. Several emerging diamond-based MEMS/NEMS applications, including high-frequency resonators, radio frequency MEMS and photonic switches, and the first commercial diamond MEMS product—monolithic diamond atomic force microscopy probes—are discussed.