Deposited Boron Nitride Research Articles

Attempts of boron nitride deposition on different substrates under the influence of the electric field present in the reaction zone

Attempts were undertaken to obtain deposits of hexagonal boron nitride under the influence of either the electric field or current of various frequency flowing through the substrate. It was demonstrated that magnetic field as well as electrical potential difference created in the reaction zone both influence the way BN layers are formed. The layers grow with the formation of fine crystallites in the center of the substrate with their size increasing while moving from the center of the substrate to its edges. Distinct correlation between the electric fields line density and the sizes of the crystallites was observed. The distribution of the crystallites depends on current frequency used. It was found that there is an optimal current frequency for which the substrate is covered with the homogeneous layer of BN. The adhesion of this deposit depends strongly on the kind of the substrate used.

In situ ellipsometry growth characterization of dual ion beam deposited boron nitride thin films

Pure hexagonal h, as well as mixed-phase cubic/hexagonal c/h boron nitride (BN) thin films were deposited onto [001] silicon substrates using the dual ion beam deposition technique. The BN thin films were grown under UHV conditions at different substrate temperatures and ion beam bombarding parameters. Thin-film growth was monitored using in situ spectroscopic ellipsometry at 44 wavelengths between 420 and 761 nm. The in situ ellipsometric Ψ and Δ data were compared with two-layer growth model calculations for the mixed-phase c/h BN, and with one-layer growth model calculations for pure h-BN growth. In situ data provide information on the optical properties of deposited h-BN and c/h-BN material, film thickness, and BN growth rates. A virtual interface approach is employed for the optical properties of the silicon substrate. The growth and nucleation of c-BN observed here confirms the cylindrical thermal spike model. The results for composition and thickness of the BN films were compared to those obtained from ex situ infrared transmission measurements and high-resolution transmission electron microscopy investigations.

Cubic boron nitride deposition on silicon substrates at room temperature by KrF excimer laser ablation of h-BN

A hexagonal-BN target was ablated by high-fluence (6 and 12 J/cm2) KrF excimer laser pulses in low-pressure (5 Pa) N2 atmosphere, without any aid of ion bombardment and heating of the Si substrate. Investigations of the deposited films show that the cubic-BN phase was deposited. The film deposited at 6 J/cm2 appears to contain a higher cubic phase content with respect to the one deposited at 12 J/cm2.

Reactions in the initial stage of the CVD of BN — a quantum chemical investigation

For a survey over the initial steps in the chemical vapor deposition of boron nitride, we have investigated the mechanism and the energetic course of the gas phase reaction of boron trichloride with ammonia at the level of second-order Moller-Plesset perturbation theory. Furthermore, we have studied dimerization and trimerization of intermediate products. The results affirm previous views that the formation of triaminoborane consists of three sequential substitutions in which a chlorine atom is exchanged for an amino group. While the overall reaction is found to be exothermic, the last substitution reaction is endothermic. In addition we have been able to show that each substitution is a two-step process: an unhindered formation of an ammonia adduct and the subsequent elimination of hydrogen chloride which proceeds through a transition state. Oligomerization and adduct formation are shown to be concurrent processes. We find that the dimerizations are also two-step reactions. At first, a van der Waals complex is formed. The following proper dimerization requires the surmounting of a barrier. From the reaction profile and the relative stabilities of oligomers and adducts we conclude that the polymerization is favored at the stage of BCl(NH2)2.

The mechanism of cubic boron nitride deposition in hydrogen plasmas

Deposition of cubic boron nitride in hydrogen plasma is thought to proceed as a result of selective etching of sp2-hybridized boron nitride from the surface of the growing boron nitride film, the impact of hydrogen atoms or ions leading to formation of c-BN sp3 bonds on the surface and formation of metastable BNHx species in the gas phase. The mechanism of nucleation and growth of c-BN crystals in hydrogen plasmas is proposed.

Microstructure and mechanical properties of pulsed laser deposited boron nitride films

Abstract 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 without or at weak ion bombardment (such films will be called l-BN in this paper) 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 and hardness, of these films and layer systems are presented. l-BN films deposited at higher laser energy densities have compressive stresses as high as 11.5 GPa. Films deposited at lower 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.

Plasma assisted chemical vapour deposition of boron nitride coatings from using BCl 3–N 2–H 2–Ar gas mixture

Boron Nitride coatings have been deposited by plasma-assisted chemical vapour deposition (PACVD) from BCl 3/N 2/H 2/Ar gas mixtures in a hot wall capacitively coupled radio-frequency (13.56 MHz) reactor. The nature of active species in the plasma during deposition was determined by Optical Emission Spectroscopy (OES) and Mass Spectrometry (MS). The plasma characterisation was performed as follows: first, an Ar/H 2 plasma was studied in order to understand the influence of molecular hydrogen in the discharge mixture. Then the two precursors N 2 and BCl 3 were added and the new gas mixture studied. Finally the deposition plasma was investigated. These characterisations were correlated to the microstructure and c-BN concentrations determined by Scanning Electron Microscopy (SEM) and Fourier Transformed Infrared Spectroscopy (FTIR). The study demonstrates the major role of atomic hydrogen on the possible mechanisms leading to BN deposition: —the introduction of hydrogen in Ar/N 2 controls the nature of the NH x (from N to NH 3) species in the gas phase. These results are correlated to the relative amount of NH groups in the films, —by a modification of the excitation state of the plasma (n e, T e) the introduction of H 2 can increase the dissociation rate of the boron precursor BCl 3 and, reacting with chlorine, leads to the formation of HCl. This corresponds to an increase in the growth rate of the coatings. Finally, BN samples containing 5% of cubic phase were treated by Ar, Ar/H 2 and Ar/Cl 2 plasmas. These post treatments demonstrated that ion assisted preferential etching of h-BN by H or Cl atoms could be used to obtain large concentrations of c-BN coatings and possibly offer a new route for deposition of low stress cubic boron nitride.

Field emission properties of BN coated Si tips by pulsed ArF laser deposition

Thin films of boron nitride (BN) were deposited by pulsed ArF laser ablation of a pyrolitic BN target in an N2 ambient with and without substrate bias and also in a nitrogen plasma. The structural property studies with infrared and reflection high-energy electron diffraction show that the films possess cubic BN and hexagonal or amorphous phase of BN. Deposition of cubic BN is enhanced by application of substrate bias whereas crystallinity is degraded by the introduction of nitrogen plasma. The field emission of electrons was tested for BN coated Si tips, and it was found that the field emission of electrons enhanced when the cubic BN was coated.

Internal friction and torsional creep behavior of chemically vapor deposited boron nitride

Dense hexagonal BN processed via chemical vapor deposition (CVD) was tested with respect to damping, shear modulus, and torsional creep rate up to temperatures as high as ≈2300 °C. The microstructural characteristics of the material both before and after creep testing were studied by high-resolution electron microscopy (HREM). The CVD process yields a homogeneous nanosized microstructure with no other secondary phase detectable. Damping experiments revealed no plastic relaxation during testing up to ≈2000 °C, which is consistent with the fact that also no creep deformation could be detected below such a high temperature. Small porosity and an increased amorphization process were noted by HREM inspection after stress exposure at ≈2300 °C. These phenomena may be responsible for both the enhanced damping capacity and the creep rate of the material which, in the range of the present testing conditions, seems to follow the simple viscoelastic behavior of a Maxwell solid.

Flow-Tube Investigation of the High-Temperature Reaction between BCl3 and NH3

The reaction between BCl3 and NH3 to produce dichloroboramine (Cl2BNH2) and HCl was probed using a flow-tube reactor interfaced to a molecular-beam mass-sampling system. A composite method that couples the detailed simulation of the fluid mechanics and chemical reactions occurring in the flow tube to an algorithm for nonlinear least-squares optimization was used to extract the kinetic parameters from the experiment. The rate constant for this gas-phase reaction is k (cm3 mol-1 s-1) = 4.21 × 1011 exp[−(8350 cal mol-1)/(RT)] with an estimated uncertainty of 10−15% over the temperature range 670−920 K. The results indicate that surface reactions involving BCl3 and NH3 occur in the flow tube, but their contributions to the observed chemical behavior of the system can be quantified and therefore do not affect the kinetic measurements reported here. In addition, the low-energy barrier to this gas-phase reaction has important implications for the chemical vapor deposition of boron nitride from BCl3 and NH3.

Cathodic Arc Evaporation - A Versatile Tool for Thin Film Deposition

Cathodic arc evaporation as a deposition technique (cathodic arc deposition CAD) is rapidly developing, emerging different novel variants. Three new fields of science and technology of CAD are reviewed: i) an analytical model of reactive CAD of metal cathodes, ii) the deposition of amorphous CN x films by filtered carbon arc deposition combined with a nitrogen ion source and iii) the first realization of a CAD process with a hot boron cathode which among others is suitable for cubic boron nitride deposition.

Laser ablation and deposition of boron nitride in a vacuum and in the presence of N 2 and NH 3

Hexagonal boron nitride pellets have been evaporated by a doubled Nd:YAG laser, and the removed material has been deposited on to Si (100) substrates. The ablation has been performed either in a vacuum or in the presence of nitrogen and ammonia at different pressures. The laser fluence and substrate temperature have been varied to optimise the deposition conditions. The best films, from a morphological and compositional point of view, were deposited in a nitrogen atmosphere at room temperature.

Modelling of boron nitride: Atomic scale simulations on thin film growth

Molecular-dynamics simulations on ion-beam deposition of boron nitride are presented. A realistic Tersoff-like potential energy functional for boron nitride, which was specially fitted to ab initio-data, has been used. The impact of energetic boron and nitrogen atoms on a c-BN target is simulated with energies ranging from 10 to 600 eV. The structural analysis of the grown films shows that a loose, dominantly sp2-bonded structure arises at high ion flux. In no case the formation of a sp3-bonded phase is observed, but the obtained films partially reveal textured basal planes as found in experiment. Two different growth regimes are identified for ion energies above and below 100 eV.

On the role of ion bombardment in cubic boron nitride deposition

Abstract Thin film deposition of cubic boron nitride (c-BN) is nowadays investigated in two different ways; the first is ion assisted deposition, the second a chemical approach without significant ion assistance. The latter is less well investigated, and up to now its suitability for c-BN deposition has not been proved. On the other hand, for ion assisted deposition universal parameter ranges have been found, and first models focusing on special aspects of the ion bombardment (subplantation, stress, sputtering) exist. The sputter model which was developed by our group is slightly modified, assuming the modification with the highest growth velocity to be favoured. With this formulation, all macroscopic parameter dependencies (ion to atom ratio, ion energy, ion mass, angle of ion incidence and substrate temperature) are well described. In this contribution, the microscopic processes during deposition will be discussed in light of recent experiments. The nucleation of c-BN yielding different structures (an amorphous layer followed by textured h-BN and finally nanocrystalline c-BN) and parameter dependencies is also included. The role of ion bombardment in general is twofold: on the one hand, it is up to now inevitably necessary to grow the c-BN phase at all temperatures; on the other hand, it causes problems of which high stress and, as a consequence, poor adhesion are the most significant. Adhesion failure occurs, if the forces at the interface exceed the atomic adhesion forces. Therefore, besides the reduction of film stress, improvement of the interface is worthy of investigation. Modelling of the latter is complicated by the complex nucleation sequence mentioned above. Some conventional means to improve adhesion (adhesion layers, stress reduction by annealing or high energy ion implantation) exist. Nevertheless, in view of applications, improvement of adhesion by a suitable choice of deposition parameters is highly desirable. In a simple model, stress is related to the defect concentration within the material which is governed by a balance between defect production due to ion bombardment, and thermal as well as ion assisted recombination processes. The model predicts the decrease of the stress with increasing ion energy in agreement with experimental data. The ion to atom ratio is of minor importance, whereas increasing the angle of ion incidence and the substrate temperature are predicted to decrease stress. Furthermore, approaches to change the nucleation behaviour during deposition are outlined. These are expected to improve the interface characteristics in view of the adhesion problem.

A thermodynamic approach to chemical vapor deposition of boron nitride thin films from borazine

Thermodynamic analysis of the chemical vapor deposition (CVD) of boron nitride has been performed for the BNH system using the method of the minimization of Gibbs' free energy. The calculations were carried out for two gas mixtures with borazine: (1) B 3N 3H 6 + N 2 and (2) B 3N 3H 6 + NH 3 for total pressures P tot of 1 and 10 −2 Torr, temperature range 800–2300 K and for a variety of initial gas mixture compositions. Theree different modifications of boron nitride (hexagonal h-BN, cubic c-BN and wurtzite w-BN) were taken into consideration. It was obtained that c-BN is formed at temperatures up to 1804 K, h-BN modification is most stable above this temperature, w-BN is a metastable phase at all possible variations of process conditions. Mixture borazine with nitrogen is more promising to deposit h-BN at low pressure.

Deposition of boron nitride on graphite at 1300 K

A method for the deposition of BN onto graphite and other substrates is described. Boron trichloride (BCl3) and ammonia (NH3) diluted with Ar were used as reacting gases. The deposition process was carried out at 1300 K as well as lower temperatures in an open system at pressures of 1 atm. The consequences of the introduction of hydrogen to the system were considered. It was demonstrated that the replacement of argon with hydrogen increases the efficiency of the process as well as the theoretical rate of BN deposition. However, the acceleration of the deposition seems to be unprofitable, because the resulting supersaturation leads to the formation of an amorphous phase. The modification of the experimental conditions were proposed.

Porous Vycor Membranes Modified by Chemical Vapor Deposition of Boron Nitride for Gas Separation

This study focuses on the characterization of porous Vycor membranes modified by chemical vapor deposition of boron nitride (B‐N‐C‐H) for gas separation. The B‐N‐C‐H films were deposited on mesoporous Vycor tubes using triethylamine borane complex and ammonia as precursors. The effects of deposition temperature and reactant flow geometry on permselectivity of membranes with respect to various permeant gases were investigated. High selectivities (up to 50,000) were achieved between small molecules (He, ) and large molecules (, Ar, ). The measured activation energies for the He and permeability are 9.5 kcal/mol and 12 kcal/mol, respectively. The membranes synthesized at lower temperatures and lower ammonia flow rates showed good mechanical and chemical stability.

Preparation of boron nitride films by multi-source plasma CVD method

Boron nitride films were deposited from BF 3 and NH 3 gases using the multi-source plasma CVD technique. This new technique controls to activate each source gas. Each kind of source gas was introduced separately through the plasma generator. After excitation of each gas, these gases were introduced to the front of the substrate. On the other hand, widely spread sheet-like plasma generated above the substrate by dc arc-plasma gun under magnetic field also excited the gas in front of the substrate, and boron nitride film was deposited on it. Also rf power was applied between the grounded chamber and the substrate electrode during the deposition. We have investigated the effect of substrate self-bias, to accelerate ions toward the growing surface, on the structure of the boron nitride films. Deposited boron nitride films were characterized by infrared (IR) absorption spectroscopy. The results of IR absorption spectroscopy indicated the structure of the deposited boron nitride films became hexagonal at lower substrate bias. As the self-bias increased, the shape of the spectra was changed and the broad absorption band near the position of cubic boron nitride formation appeared.

Properties of pulsed laser deposited boron nitride films

Pulsed laser ablation using an excimer laser of 248 nm wavelength was applied to prepare boron nitride films, where the ablation from boron nitride as well as elemental boron targets was studied. The growing films were bombarded by a nitrogen or a nitrogen/argon ion beam to obtain stoichiometric films and to investigate ion induced modifications of structure and properties. Depending on deposition parameters the refractive index of the boron nitride films in the visible wavelength range varies between 2.5 and 1.7 and the optical energy band gap between 2.0 and 4.5 eV. Most films were found to be completely sp 2-bonded and amorphous to nanocrystalline with a turbostratic microstructure. Only boron nitride films deposited from the boron target and bombarded with nitrogen/argon ions show also an absorption peak in the infrared spectrum that indicates the presence of the cubic phase.

Deposition of Boron Nitride Films from BB'B"-Trichloroborazine

The chemical vapor deposition of boron nitride from BB'B''-trichloroborazine was investigated in a microbalance equipment with a hot wall reactor. It was possible to monitor the evaporating and depositing masses simultaneously, so that the kinetics of the CVD-process could be measured in detail. The evaporation rate was studied in the temperature range between 285 and 330 K. The activation energy of the evaporation was determined to be 65 ± 2 kJ/mole. The deposition process was investigated at temperatures between 973 and 1233 K in an argon atmosphere at total pressures between 250 and 2000 Pa. The deposition of boron nitride on silicon could be described by a Langmuir reaction type. A gas phase decomposition of BB'B''-trichloroborazine was observed, so that the concentration at the substrate was dependent on the residence time of the precursor in the hot zone of the reactor. The deposited layers were colorless and amorphous and had the correct 1/1 stoichiometry between boron and nitrogen (WDX-Analysis). The layers contained less than 1 % of chlorine. SiC-Tyranno-fiber fabrics were coated homogeneously with BN-layers of 200-300 nm thickness. A tensile strength decrease of approximately 15 % was observed for the coated fibers. Oxidation experiments showed that the layers were oxidized at temperatures above 873 K in an Ar/O 2 -atmosphere leading to a brittle behavior of the fibers.