Papers

ICMCTF1999 Session D1: Synthesis, Characterization and Applications of Cubic Boron Nitride and Carbon Nitride Materials

Wednesday, April 14, 1999 1:30 PM in Room Forum/Senate/Committee

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		D1-1 Quantum Mechanical Methods Applied to the Study of the Structure and Formation of Boron Nitride Films D.R. McKenzie, C.M. Goringe (University of Sydney, Australia); D.G. McCulloch (RMIT University, Australia) The success of the Tight Binding and Density Functional Theory approaches to the quantum mechanics of many electron systems has enabled powerful molecular dynamics computer codes such as that of the Parrinello gropup of the Max Planck Institute, Stuttgart, Germany, to be developed which are capable of realistic simulations of the formation of structure in solids. Because of the complexity of the bonding in carbon and boron nitride with sp1, sp2 and sp3 hybridisations possible, these sophisticated quantum approaches, also called ab initio approaches, are essential to give realistic structures. Carbon has been extensively studied with ab initio methods, with good results for both the structure and the electronic properties. A review of this work will be given showing that not only are the structural predictions correct, but that a phase diagram for amorphous structures can be predicted. This phase diagram is applicable to structures quenched from the melt and agrees well with observations for films made by condensing carbon ion plasma from a cathodic arc source. In particular, the transition from an sp2 rich phase to an sp3 rich phase is predicted to occur at approximately the same pressure as deduced from experimental measurements of stress and sp3 fraction and from approximate theory developed from equilibrium thermodynamics. The BN system provides a good opportunity to extend the ab initio simulations to a binary alloy. The issues we wish to address are: the fraction of tetrahedral environments as opposed to hexagonal environments in the structure, the degree of chemical order, that is, the fraction of unlike (B-N) bonds as opposed to like (N-N,B-B) bonds, the presence of preferred orientation in non-hydrostatic stress fields and the amorphisation energy. The amorphisation energy we define as the energy difference between the amorphous and crystalline structures of comparable density. Because of fundamental considerations of computer time restrictions, the ab initio methods are not capable of reproducing phenomena such as crystallisation which rely on thermal diffusion, and so can only describe amorphous systems. The BN networks were produced by molecular dynamics simulation beginning with a molten sample of 64 atoms. Two different densities were studied, covering the range corresponding to the hexagonal and cubic BN crystalline forms. The structures were analysed using radial distribution functions, ring statistics and a chemical order parameter. As a comparison, a network of aluminium nitride, AlN was also formed and analysed. AlN forms a crystal structure closely related to the wurtzite form of BN, which has the same density as the cubic form of BN. We found that BN at the c-BN density prefers a crystalline structure over an amorphous structure, more so than does AlN. This explains the universal observation of the crystalline form of c-BN rather than the amorphous form in experimental synthesis work, whereas there have been reports of the amorphous form of AlN. We found that AlN is more chemically ordered than BN at both densities. This reflects the energy difference between like and unlike bonds. We found that the amorphous network at the c-BN density did not have a dominance of tetrahedral environments, so that the existence of the BN equivalent of tetrahedral amorphous carbon, t a-C, the highly tetrahedral amorphous carbon network, does not appear likely. Finally we found that strong preferred orientation develops in a two dimensional carbon analogue system when an anisotropic stress field is applied. The nature of the orientation agrees with considerations based on equilibrium thermodynamics.
2:10 PM		D1-3 Enhanced Cubic Phase Formation in rf-PECVD Boron Nitride Films J. Vilcarromero, M.N.P. Carreno, I. Pereyra (Escola Politécnica da USP, Universidade de São Paulo, Brazil) PECVD stoichiometric Boron Nitride has been mainly prepared using gases precursors, as Borazine, Diborane, BCl₄, NH₃, and N₂, with a flow ratio among the N and Boron containing gases close to 1 resulting in predominant h-BN phase in the films. In this work, we show that the c-BN phase is enhanced, even at low temperatures, by a very low B/N ratio in highly H₂ diluted gas mixtures. The hydrogen dilution also inhibits the NH_n bonds and the amorphous phase in the samples. In addition, we observed that c-BN phase increases when silicon is introduced as p-dopant. The thin films were grown in a capacitived coupled rf-PECVD using a B₂H₆, N₂ and H₂ atmosphere at substrate temperatures below 350 ⁰C. The B₂H₆/N₂ flow ratio was varied from 1.6 x 10^-2 to 6.7 x 10^-4 in H₂ dilution from 40 to 60 % of flow rate. The B and N content was determined by nuclear reaction analyses and Rutherford Backscattering Spectroscopy. The structural properties were studied using Infrared Spectroscopy, Raman Scattering and X-Ray Diffraction experiments. The thin films present up to 50 % of c-BN content, deduced from the infrared measurements. The c-BN vibration mode, located around 1057 cm^-1, is an indication of low stressed films. The mechanical properties, as stress and hardness, were also studied and related with the structural properties. Supported by brazilian agencies: Fapesp and CNPq
2:30 PM		D1-4 Synthesis and Characterization of C-BNFilms Prepared by Ion Beam Assisted Deposition and Triode Sputtering M. Ben el Mekki (ATIPIC, Spain); M.A. Djouadi, V. Mortet, E. Guiot (Institution ENSAM, France); G. Nouet (Institution LERMAT, Caen, France); N. Mestres (Institution ICMAB, Spain) Cubic boron nitride films deposited on c-Si substrate by ion beam assisted deposition (IBAD) and triode sputtering (TS) are studied as a function of the material composition and the microstructure and crystallinity of the films. The methods of characterization used in this study are: X-ray photoelectron, infrared and Raman, spectroscopy. High resolution cross sectional TEM is used to confirm the optical results. In the case of films prepared by IBAD, the working temperature is about 250ºC and the ion energy is 400 - 500 eV. The films are prepared from different gas mixture ratio (nitrogen + argon ions to thermal boron atoms ratio), the boron is supplied by evaporation of elemental boron. Concerning the films prepared by TS, the study is performed over two series of samples: The first one consists of a series of samples deposited at constant parameters under variation of substrate bias voltage in the range 0 - 50 V. The second series is prepared at constant parameters, changing the deposition time from 5 to 50 min, the boron is supplied by a pulverization of boron target and the nitrogen and argon are supplied by introducing a constant flux of 40% of argon and 60% of nitrogen. The particle-size dependence of frequencies and damping of optical phonons is studied for all samples from analyses of Raman scattering and infrared spectra. A very important difference between the particle-size of IBAD samples and TS samples is observed. A progressing chemical etching by phosphoric acid at 80ºC and infrared characterization announce a sequence of sp² and sp³ layers in the case of samples prepared by TS while in the case of samples prepared by IBAD we have a mixture of sp² and sp³ phases with high content of sp³ structure. All analyses are in full agreements with TEM results.
2:50 PM		D1-5 A Study of Initially Grown BN Films on Silicon by Direct Negative Boron Ion beam. E.-S. Byon, K.-H. Lee, S.-R. Lee (Korea Institute of Machinery and Materials, KOREA); J.-H. Yoon (Changwon National University, KOREA); S.I. Kim (SKION Corporation) The early growth stage of c-BN films on silicon substrate has been investigated using boron ion beam. The specific range of incident ion current and ion energy values is known as a key parameter for synthesis of c-BN films. More energetic bombardment is required to initially form than to subsequently grow c-BN. In conventional ion-assisted deposition techniques, the energy of incident ion beams cannot be precisely controlled because of the neutral particles involved and the Gaussian energy distribution of the ions. This paper reports on an experimental investigation for the initial growth behavior of c-BN films on Si(100) substrate using purely negative boron ion beam and conventional nitrogen ion beam sources. The negative boron ion beam is ideal to obtain a well defined ion energy and a current and consequently a deposition rate. Prior to deposition, the substrates in ground potential were sputter-cleaned either by a negative boron ion beam or a positive nitrogen beam. The precisely controlled negative boron ions with an energy of 50-300 eV were irradiated to grow BN layers. The activated nitrogen beam was also applied simultaneously during the growth stage. The effect of boron ion energy and flux of both the ions and the activated particles on the initially grown layer was parametrically investigated to see how the transition from non-cubic to cubic phase in a growing BN film takes place. The films were characterized by FTIR, AFM, HRTEM, AES and nano-indentor. The detailed significances will be presented.
3:30 PM		D1-7 The Mechanism of c-BN Deposition in Hydrogen Plasmas I. Konyashin, J. Bill, F. Aldinger (Max Planck Institute for Metals Research, Germany) The mechanism of the formation of sp³-hybridized boron nitride during PACVD in a hydrogen plasma has been experimentally studied using samples of amorphous, hexagonal and cubic boron nitride, and B-, N- and H-containing precursors. The formation of cubic boron nitride is found to be possible as a result of selective etching of poorly crystallized sp²- hybridized boron nitride from the surface of the growing boron nitride film. The formation of sp³ bonds of c-BN is thought to be possible due to the impact of hydrogen atoms or ions on the surface of the growing film, and sp³ dangling bonds of c-BN can be stabilized as a result of their passivation by hydrogen atoms. The deposition of cubic boron nitride appears to proceed through the formation of metastable BNH_x species in the gas phase as a result of hydrogen abstraction from the molecule of B-N-H precursors in the hydrogen plasma. Thus, PACVD of cubic boron nitride in hydrogen plasmas appears to occur in much the same way as diamond chemical vapor deposition from hydrocarbon - hydrogen gas mixtures.
3:50 PM		D1-8 The Use of Pulsed Laser Ablated Boron Nitride Interlayers for Improving the Adhesion of Cubic Boron Nitride Films G. Reisse, S. Weissmantel (University of Applied Sciences Mittweida, Germany) Boron nitride films were prepared by pulsed laser ablation from a boron nitride target using a KrF-excimer laser, where the growing films were deposited in nitrogen atmosphere or bombarded by a nitrogen/argon ion beam. Films deposited at high growth rates or weak ion bombardment are hexagonal with amorphous to turbostratic microstructure (l-BN) and show high adhesive strength to silicon and stainless steel substrates. By using them as intermediate layers, the adhesion of pure cubic boron nitride films (c-BN) can significantly be improved. l-BN films and l-BN/h-BN/c-BN layer systems have been investigated by in-situ ellipsometry, infrared spectroscopy and cross-section and plan-view high-resolution transmission electron microscopy including diffraction. The mechanical properties, i.e. stress, hardness and adhesion, of those films and layer systems are presented. l-BN films deposited at high laser energy densities have compressive stresses as high as 11.5 GPa. Films deposited at low laser energy densities have stresses in the range of 4.7 to 1.3 GPa and a Vickers hardness in the range of 18.6 to 7.5 GPa depending on substrate temperature and ion bombardment. The compressive stresses of 400 nm thick adherent c-BN films were estimated to be 4.5 GPa.
4:10 PM		D1-9 Synthesis of Carbon Nitrides Thin Films by Pulsed Laser Deposition in a Remote Nitrogen Plasma C. Jama, O. Dessaux, P. Goudmand (University of Science and Technology of Lille, France); J.M. Soro, D. Rats, J. von Stebut (Ecole des Mines de Nancy, France) Carbon nitride is one of the new carbon materials which shows very interesting properties. However substrate pre-treatment processes are required to obtain good adhesion of carbon nitride thin films. The improvement of the adhesion properties by using different pre-treatment processes prior to carbon nitride deposition are presented. In the present study such films were prepared in a remote nitrogen plasma excited by a microwave generator. Infra-red Pulsed Laser Deposition (PLD) technique has been used to vaporise a graphite target. This reactive PVD process was adopted to produce thick coatings in the range of 1µmon different substrates. A very wide spanned investigation implying assessment of general coating properties is presented : - surface chemical composition, - microstructure, texture and morphology. These general properties are correlated with the coating mechanical properties. We adopted the following micro-mechanical testing techniques : - depth sensing indentation in the ultra-micro range for hardness and elasticity, - single pass conventional microscratch-testing (in the mN load range) for scratch hardness and dynamic brittleness as well as general failure mechanisms as related to local structural defects, - multipass low cycle friction fatigue scratch operation at sub-critical loads for assessment of fatigue cracking and delamination damage production. The relevant adhesion characteristics along with mechanical properties of the deposited films will be discussed in detail.
4:30 PM		D1-10 Crystalline SiCN: A Hard Material Rivals to Cubic BN L.C. Chen, P.D. Kichambare (National Taiwan University, Taiwan, R.O.C.); K.H. Chen (Institute of Atomic & Molecular Sciences, Academia Sinica, Taiwan, R.O.C.); T.R. Lu (National Chiao-Tung University, Taiwan, R.O. China); C.T. Kuo (National Chiao-Tung University, Taiwan, R.O.C.) Covalently bonded crystalline carbon nitride (c-CN), preferably in the ß-C₃N₄ structure that is isomorphic to ß-Si₃N₄, has long been predicted to have a high bulk modulus superior to that of diamond. However, growth of binary c-CN with crystal sizes large enough to enable measurement of its properties has not been achieved so far. Interestingly, exceptional hardness and yield strength have been reported for some CN films even with a very limited degree of crystallinity. Hardness of CN films up to 20 GPa has been reported. In this article, large (several tens of microns), well-faceted ternary SiCN crystals have been grown by microwave plasma-enhanced chemical vapor deposition. A semiconductor grade gas mixtures containing H₂, CH₄, N₂ and SiH₄ with various ratio as precursors were used for deposition of these crystalline films. The ternary compound (C; Si)_xN_y exhibits a hexagonal structure and consists of a network wherein the Si and C are substitutional elements. While the N content of the compound is about 35-40 at. %, the extent of Si substitution varies and can be as low as 10 at. %. Mechanical properties of the SiCN compounds have been studied by nano-indentation. From the nanoindentation studies, we estimate the hardness of the SiCN crystals to be around 30 GPa, well above most reported values for CN films.
4:50 PM		D1-11 Deposition of Carbon Nitride Films by Ionised Magnetron Sputtering Y. Kusano, Z.H. Barber, J.E. Evetts, I.M. Hutchings (University of Cambridge, United Kingdom) Carbon nitride films were deposited on to silicon and sodium chloride substrates by an ionised magnetron sputtering system under various conditions. The influence of inductively coupled rf plasma generated between a dc magnetron sputtering target and the substrates, and dc bias voltages applied at the substrates on deposition rates and chemical structures of the films was investigated. The films were characterised by Fourier transform infrared spectroscopy (FTIR) and Rutherford back-scattering (RBS). When pure nitrogen gas was used for the sputtering gas with substrate bias voltages between -100 and +100 volts, the deposition rate increased with the presence of the inductive plasma, while the chemical structures of the films were unchanged. However, the deposition rates of films deposited in an argon/nitrogen mixture increased drastically with the presence of the inductive plasma only when the substrates were positively biased. The films deposited in an argon/nitrogen mixture with the inductive plasma under positive bias showed higher contents of sp and sp² hybridised carbon bonded with nitrogen, than those without the inductive plasma and/or under negative bias voltages on the substrate. The results can be interpreted in terms of selective etching of the films by argon ions in the plasma.