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Title: Nucleation and growth of cubic boron nitride thin films
Other Titles: Li fang dan hua peng bo mo de xing he yu sheng zhang yan jiu
Authors: Chan, Chit-yiu (陳喆垚)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Boron nitride
Thin films
Notes: CityU Call Number: TA455.N5 C425 2005
Includes bibliographical references (leaves 147-154)
Thesis (Ph.D.)--City University of Hong Kong, 2005
xvii, 193 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: Cubic boron nitride (cBN) is a synthetic and an intrinsically super-hard material with the second highest hardness and thermal conductivity next to diamond. Cubic BN with its tetrahedral sp3 structure is isostructural and isoelectronic to diamond. Diamond is far the most extreme material but cBN surpasses diamond in some properties. Unlike diamond, cBN is chemically inert to molten ferrous materials and resistant to oxidation up to 1200 °C at atmospheric conditions. These properties make cBN more attractive than diamond in many mechanical and tribological applications. Cubic BN has the widest bandgap (6.2 ± 0.2 eV) among III-V semiconductors, high electron mobility and hole mobility. In contrast to diamond it maintains high resistivity even at extreme temperatures. It can be doped for both p- and n-type conductivities and is piezoelectric. Therefore cBN is a potential candidate for construction of high-temperature, high power and high-speed electronic devices operating in harsh environment. Although cBN films are very attractive they have not been implemented into practical applications because of many accoutered problems like poor quality, low phase purity, high compressive stress (up to 20 GPa), poor adhesion to the substrates, small deposited area and limited film thickness (≤200 nm). These cBN properties suggest that the development of novel technologies for synthesis of high-quality, large-area and thick cBN films is very challenging. This thesis presents a viable route towards the practical applications of cBN coatings through better understanding of the nucleation and growth behavior of the deposited cBN thin films. The aforementioned limitations have been overcome by the introduction of fluorine chemistry mediating by a complex He-Ar-N2-BF3-H2 plasma induced in an electron cyclotron resonance (ECR) system. This deposition method enables the preparation of thick cBN films (>1 μm) with low internal stress and over large areas of silicon (Si) substrates. Fourier transform infrared (FTIR) spectroscopic examination shows that the films are composed of >80 % cBN phase. Detail analysis of BN structures employing high resolution transmission electron microscopy (HRTEM) and transmission electron energy loss spectroscopy (EELS) revealed that a pure cBN layer is formed on top of an initial graphitic BN layer which accounts for the insignificant hexagonal BN (hBN) signal in FTIR spectra. The characteristic transverse optical (TO) and longitudinal optical (LO) phonons modes of cBN, generally appearing in the spectra of cBN crystals synthesized by high pressure high temperature (HPHT) methods, were found in our cBN films which are the indicatives of large cBN crystallites. It was found that the ion bombardment, gas composition, substrate temperature and growth time significantly affect the phase purity and crystallinity of cBN films. However, the growth of cBN still follows the typical pattern with a graphitic precursor, amorphous/turbostratic BN (aBN/tBN) at the Si substrate interface. This work however illustrates that aBN/tBN are not necessary precursor layers for cBN growth. Using diamond substrates can eliminate aBN/tBN layer and cBN can directly be deposited on diamond at suitable deposition parameters. The distinctive features of cBN films grown on diamond in reference to those on Si substrates were also investigated. The critical experimental conditions inducing nucleation and maintaining the growth of cBN were revealed based on the detail HRTEM studies of interfacial cBN−diamond structures. The equivalent atomic configurations in cBN and diamond, and the alignment of their crystallographic planes suggest ‘perfect’ epitaxy in the HRTEM view-field. This work thus shows that diamond substrates play vital role at synthesis of high quality cBN films. Regarding to the successful deposition of cBN using low energetic ion bombardment (−20 V) and fluorine chemistry, a surface chemical vapor deposition (CVD) growth process of cBN was developed. A new CVD surface growth mechanism, initiated by the introduction of fluorine chemistry, is also outlined herein. The proposed mechanism is based on the combination of HRTEM structural studies of the utmost layers of the cBN films, surface analysis of the film surface composition after exposure to different plasma environments and an optical emission spectroscopic examination of plasma with variable composition particularly fluorinated species. The surface roughness is another parameter controlling cBN nucleation and growth which was investigated at synthesis of cBN films employing a mass-selected ion beam deposition (MSIBD) which takes advantage in independent control of deposition parameters. Different surface roughness (0.2 to 170 nm) was obtained by scratching Si surface with proper grain sizes of diamond or alumina powders. The analysis of grown BN films show the interfacial tBN thickness increases, cBN nucleation threshold shift towards the higher ion energy with the increase in substrate roughness, and the orientation relationship of tBN with Si becomes more random. The differences in cBN volume fraction are however less obvious at higher ion energy (500 eV). The resulting featureless film morphology at high ion energy is probably associated with the preferred subsurface growth process. On the other hand, low ion energy leads to a surface-like growth process which predominantly yields granular morphology. The fundamental understanding of the deposition parameters has led to the development of technology providing cBN films that are thick enough for reliable evaluation of mechanical properties. The measured hardness strongly depends on the crystallinity and crystal size/grain boundaries of the cBN films. The consistent and repeatable hardness of 70 GPa and elastic modulus of 800 GPa were measured on smoothed cBN films. The magnitudes of hardness and elastic modulus measured here are the highest ever obtained from the measurement of cBN films, and they are comparable to the values reported for cBN crystals synthesized by HPHT methods. The works proves that the properties, adherence, film stability and overall thickness of cBN films are considerably affected by the chemical nature of substrates. Growing very thick cBN films (>3 μm) can easily be demonstrated on polycrystalline diamond by ECR-PECVD at very low bias voltage (–30 V) with assistance of fluorine chemistry. Cubic BN films also grow on polycrystalline diamond in epitaxial relationship which is important for development of device quality cBN and engineering of novel electronic devices. The works indicates that growing large single crystal cBN films might be feasible at particle energies approaching the values of thermal energies using plasma enhanced chemical vapor deposition (PECVD). Evidently cBN still nucleates at effective bias of about –12 V. The cBN films grown by PECVD do not show top thin non-cubic layers which contrast the films prepared by PVD. The surface free of non-cubic BN phases and absence of interfacial precursor aBN/tBN layers enables to grow extremely thin films, which allows constructing cBN-diamond (cBND) super-lattices. Absence of non-cubic phases on cBN surface also suggests that existing models cannot elucidate the cBN formation at PECVD since the particle energy is well below atomic displacement energy. Growing cBN by PECVD is more likely surface process. Advantages of ECR-plasma coupled with fluorine chemistry are demonstrated to be capable of preparing high quality, low stress cBN films. The ECR technique in combination with a universal diamond buffer layer provides cBN–diamond bilayer composite with exciting mechanical properties particularly suitable for machining ferrous metals. Therefore cBND was deposited on cemented WC cutting inserts at 800 °C. The cutting test revealed that cBND adhesion and properties are already good enough for some industrial applications though cutting tools failed. The failure is however in the WC region whereas the separation of cBN and diamond from substrate is not observed. Thus the research work presented is important milestone towards the first industrial exploitation of cBN films.
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