**Figure 7.**

*(a−c) Examples of different morphologies of undoped polycrystalline CVD diamond films grown using different C:H ratios in the source gas: (a) faceted microcrystalline diamond typically grown under standard deposition conditions, with <2% CH4 in H2, (b) smoother nanocrystalline diamond grown as before, but with >4% CH4 in H2, and (c) "cauliflower" nanocrystalline diamond grown with >4% CH4 in H2. Reproduced with permission from ref. [31]. Copyright 2020 American Chemical Society.*

carbon-containing groups and H atoms by injecting energy. The plasma density of MPCVD is higher than that of HFCVD so that high-purity diamond can be prepared. Due to the limitation of microwave wavelength and microwave resonator, a large-area uniform film cannot be obtained. Therefore, MPCVD is not suitable for industrial production. In order to further improve the quality and growth rate of diamond, the improvement and innovation of microwave cavity have become the focus of research. The stability of microwave frequency and the singleness of microwave energy will still be the direction that needs to be overcome in the future. With the development of GaN devices, the third-generation semiconductor solid-state microwave source

*Growth of Diamond Thin Film and Creation of NV Centers DOI: http://dx.doi.org/10.5772/intechopen.108159*

will possibly replace the magnetron and become an inevitable choice for the modularization and miniaturization of high-power and high-stability microwave sources. In addition, the addition of multi-beam energy coupling excitation plasma such as radio frequency, laser, and other energy will also open a new platform for MPCVD equipment.

Despite the polycrystalline diamond grown by CVD techniques, the growth of ultrathin diamond film has also attracted extensive attention. In the past years, theoretical calculations based on density functional studies have predicted the chemically induced transformation of multilayer graphene into a diamond-like film [40, 41]. Recently, in experiment, the fluorination of graphene sheets in Bernalstacked bilayer graphene generated by CVD over a single-crystal CuNi(111) surface can cause the creation of interlayer carbon–carbon bonds, resulting in a fluorinated diamond monolayer [42].
