Diamond, because of its electrical and chemical properties, may be a suitable material for integrated sensing and signal processing. But methods to control chemical or biological modifications on diamond surfaces have not been established. Here, we show that nanocrystalline diamond thin-films covalently modified with DNA oligonucleotides provide an extremely stable, highly selective platform in subsequent surface hybridization processes. We used a photochemical modification scheme to chemically modify clean, H-terminated nanocrystalline diamond surfaces grown on silicon substrates, producing a homogeneous layer of amine groups that serve as sites for DNA attachment. After linking DNA to the amine groups, hybridization reactions with fluorescently tagged complementary and non-complementary oligonucleotides showed no detectable non-specific adsorption, with extremely good selectivity between matched and mismatched sequences. Comparison of DNA-modified ultra-nanocrystalline diamond films with other commonly used surfaces for biological modification, such as gold, silicon, glass and glassy carbon, showed that diamond is unique in its ability to achieve very high stability and sensitivity while also being compatible with microelectronics processing technologies. These results suggest that diamond thin-films may be a nearly ideal substrate for integration of microelectronics with biological modification and sensing.

The transport properties of diamond thin films are well known to be sensitive to the sp2/sp3-bonded carbon ratio, the presence of the grain boundaries and other defects, and to the presence of various impurities. In order to clarify the roles these factors play in the conduction mechanisms of nitrogen-doped ultrananocrystalline diamond (UNCD), Raman scattering, near edge x-ray absorption fine structure (NEXAFS), soft x-ray fluorescence (SXF), and secondary ion mass spectroscopy (SIMS) measurements were performed. Transmission electron microscopy analysis of nitrogen doped UNCD has previously indicated that the films are composed of crystalline diamond nano-grains with boundaries of amorphous carbon, and NEXAFS measurements reveal that the global amount of sp2-bonded carbon in these films increases slightly with nitrogen doping. The nitrogen content is quantified with high-resolution SIMS analysis, while NEXAFS and SXF indicates that the nitrogen exists primarily in tetrahedrally coordinated sites. These measurements indicate that the overall grain boundary volume of nitrogen-doped ultrananocrystalline diamond is increasing, while the grains themselves remain pure diamond. This supports our previously reported hypothesis that grain boundary conduction is the mechanism for the observed increase in conductivity in ultrananocrystalline diamond with nitrogen doping.

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Ultrananocrystalline diamond (UNCD) films were prepared by microwave plasma chemical vapor deposition using argon-rich Ar∕CH4 plasmas at substrate temperatures from ∼400 to 800°C. Different seeding processes were employed to enhance the initial nucleation density for UNCD growth to about 1011sites∕cm2. High-resolution transmission electron microscopy, near-edge x-ray absorption fine structure, visible and ultraviolet Raman spectroscopy, and scanning electron microscopy were used to study the bonding structure as a function of growth temperature. The results showed that the growth of UNCD films is much less dependent on substrate temperature than for hydrogen-based CH4∕H2 plasmas. UNCD with nearly the same nanoscale structure as those characteristic of high-temperature deposition can be grown at temperatures as low as 400°C with growth rates of about 0.2μm∕hr. The average grain size increased to about 8nm from 3 to 5nm that is characteristic of high-temperature growth, but the relative amounts of sp3 and sp2 bonding remained unchanged. These results suggest that the activation energy for UNCD growth is about 2–3Kcal∕mole compared with ∼28kcal∕mole for traditional growth chemistries, and that hydrogen plays an important role in the growth of UNCD films using hydrogen-poor plasmas.

Most MEMS devices are currently based on silicon because of the available surface machining technology. However, Si has poor mechanical and tribological properties which makes it difficult to produce high performance Si based MEMS devices that could work reliably, particularly in harsh environments; diamond, as a superhard material with high mechanical strength, exceptional chemical inertness, outstanding thermal stability and superior tribological performance, could be an ideal material for MEMS. A key challenge for diamond MEMS is the integration of diamond films with other materials. Conventional CVD thin film deposition methods produce diamond films with large grains, high internal stress, poor intergranular adhesion and very rough surfaces, and are consequently ill-suited for MEMS applications. Diamond-like films offer an alternative, but are deposited using physical vapour deposition methods unsuitable for conformal deposition on high aspect ratio features, and generally they do not exhibit the outstanding mechanical properties of diamond. We describe a new ultrananocrystalline diamond (UNCD) film technology based on a microwave plasma technique using argon plasma chemistries that produce UNCD films with morphological and mechanical properties that are ideally suited for producing reliable MEMS devices. We have developed lithographic techniques for the fabrication of UNCD MEMS components, including cantilevers and multilevel devices, acting as precursors to microbearings and gears, making UNCD a promising material for the development of high performance MEMS devices. We also review the mechanical, tribological, electronic transport, chemical and biocompatibility properties of UNCD, which make this an ideal material for reliable, long endurance MEMS device use.