Abstract
The extreme physical and chemical properties of CVD diamond films have attracted many scientists and technologists to explore broad and often multidisciplinary applications. Nanocrystalline diamond (NCD) is a special form of CVD consisting of nanometer-sized diamond grains contributed by the high secondary nucleation rate on the growing surface in the argon-rich/hydrogen-poor ambient. The properties along with the microstructure of CVD diamond are thus modified by changing the gas-phase chemistry. Because of the small crystalline size compare to conventional microcrystalline diamond (MCD), NCD films exhibit smooth surfaces and are, therefore, of great value to many practical applications that require smooth diamond coatings such as SAW devices. The electrical properties of the diamond films also have drastic changes, for example going from an insulating to an electrically conducting material as a result of the network of conducted sp2 grain boundaries existed in NCD films. Further doping NCD with nitrogen will improve its electrical properties, such as conductivity and electron field emission. Control over the microstructure from micro-scale to nano-scale diamond grains therefore gives us the opportunity to exploit many of the unique properties of diamond and reach the full utilization of diamond as an engineering material. This work is organized in three sections: introduction, growth of NCD, and applications of NCD. The introduction includes revolution from carbon to CVD diamond (Chap. 1) showing the basic ideas from carbon atom, sp3 carbon bond, diamond structure to CVD process, and nanocrystalline diamond (Chap. 2) revealing in details the so-called NCD along with its growth mechanism, the latest techniques to deposit NCD films. The second section, growth of NCD, covers the study of co-deposition of MCD and NCD (Chap.3), and nucleation of NCD films (Chap.4). The former investigated the reported compositional mapping for MCD and NCD growth and demonstrated local high concentration of atomic hydrogen near the substrate contributed to the MCD growth in spite of the low/no hydrogen addition in a methane–argon mixture that was previously reported to grow only NCD. Nucleation, which is essential for NCD, will affect morphology, growth rate, adhesion of NCD films, and their applications. Various seeding process and the related effects are presented and discussed in Chap. 4. The final section is applications of NCD including SAW devices on NCD (Chap.5), electric field emission-doping of NCD (Chap.6), and advance applications of NCD (Chap. 7). Diamond has the hardest Young’s module showing the highest propagation speed among all materials. NCD is believed to have the same characteristic so that SAW devices on NCD can promote the operating frequency to meet the emerging demands for high-frequency communication. Chap. 5 describes introduction, theoretic model, and fabrication of SAW devices based on IDT/ZnO/NCD structure and demonstrates that NCD exhibits similar propagation speed as natural diamond. By means of doping, not only the structure but properties of NCD can be modified (Chap. 6). Nitrogen doping, which can be done by adding N2 to the hydrocarbon/argon mixtures, promotes the performance of NCD films in electrical properties, such as field emission and conductivity. Addition of hydrogen helps stabilize the plasma and contributes to the deposition of more phase-pure NCD. Their effects on NCD films will be reported in Chap. 6. The advance applications of NCD, described in Chap. 7, includes MEMS and biosensors. Diamond is a much better material compared to Si for MEMS because of its superior mechanical, chemical, thermal, electrical and tribological properties that make it possible to produce high performance diamond based MEMS devices that could work more reliably, especially in extreme environments. The fully biocompatible properties of diamond, on the other hand, make diamond, especially NCD, become an ideal material for biosensors.