Mechanism of nucleation and growth of cubic boron nitride thin films

The mechanism and the crystallography of the nucleation and growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrate by RF bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Both methods provide experimental information showing no sp²-bonded BN layer formation in the subsurface region of c-BN phase. This is clear evidence for layer-by-layer homoepitaxial growth of cubic boron nitride without graphitic monolayers in the near-surface region of the film. The turbostratic boron nitride (t-BN) consists of thin sub-layers, 0.5–2 nm thick, growing in such a way that a sub-layer normal is almost parallel to the growth direction. t-BN also comprises a large volume fraction of the grain boundaries with high interface energies. The present result and the finding by Shtansky et al. [Acta Mater. 48, 3745 (2000)], who showed that an individual sub-layer consists of parallel lamellae in both the hexagonal +h-BN) and rhombohedral (r-BN) configurations, demonstrate that high intrinsic stress in the films is due to the complex structure of sp²-bonded BN. The crystallography of c-BN films indicates heteroepitaxial nucleation of cubic phase on the graphitic BN structural precursor. The present results are consistent with stress-induced c-BN formation.     

Mechanism of nucleation and growth of cubic boron nitride thin films

The mechanism and the crystallography of the nucleation and growth of cubic boron nitride (c-BN) films deposited on 100-oriented silicon substrate by RF bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Both methods provide experimental information showing no sp²-bonded BN layer formation in the subsurface region of c-BN phase. This is clear evidence for layer-by-layer homoepitaxial growth of cubic boron nitride without graphitic monolayers in the near-surface region of the film. The turbostratic boron nitride (t-BN) consists of thin sub-layers, 0.5–2 nm thick, growing in such a way that a sub-layer normal is almost parallel to the growth direction. t-BN also comprises a large volume fraction of the grain boundaries with high interface energies. The present result and the finding by Shtansky et al. [Acta Mater. 48, 3745 (2000)], who showed that an individual sub-layer consists of parallel lamellae in both the hexagonal (h-BN) and rhombohedral (r-BN) configurations, demonstrate that high intrinsic stress in the films is due to the complex structure of sp²-bonded BN. The crystallography of c-BN films indicates heteroepitaxial nucleation of cubic phase on the graphitic BN structural precursor. The present results are consistent with stress-induced c-BN formation.     

Enhanced deposition of cubic boron nitride films on roughened silicon and tungsten carbide-cobalt surfaces

We report the influence of substrate surface roughness on cubic boron nitride (cBN) film deposition under low-energy ion bombardment in an inductively coupled plasma. Silicon and cemented tungsten carbide-cobalt (WC-Co) surfaces are roughened by low-energy ion-assisted etching in a hydrogen plasma, followed by deposition in a fluorine-containing plasma. Infrared absorption coefficients are measured to be 22,000 cm⁻¹ and 17,000 cm⁻¹ for sp²-bonded BN and cBN phases, respectively, for our films. For the silicon substrates, the film growth rate and the cBN content in the film increase with increasing the surface roughness, while the amount of sp²BN phase in the film shows only a small increase. A larger surface roughness of the substrate results in a smaller contact angle of water, indicating that a higher surface free energy of the substrate contributes to enhancing growth of the cBN film. For the WC-Co substrates, the film growth rate and the cBN content in the film increase similarly by roughening the surface.     

New approach in depositing thick, layered cubic boron nitride coatings by oxygen addition—structural and compositional analysis

Cubic boron nitride (c-BN) can be produced by PVD and PA-CVD techniques by intensive ion bombardment leading to highly stressed films limiting its use in industrial applications. Various attempts have been undertaken to reduce the compressive stress of c-BN thin films. A significant reduction in compressive stress and a substantially improved adhesion was achieved by a new coating concept consisting of a two-step adhesion-promoting base layer, a compositional-graded nucleation layer obtained by a stepwise decrease of the oxygen content in the Ar/N₂/O₂ atmosphere and a low-stressed c-BN:O top layer with controlled oxygen addition. The four-layer c-BN:O film with a thickness of 3 μm was deposited by unbalanced radio frequency magnetron sputtering of a hot-pressed hexagonal boron nitride target on silicon substrates. The adhesion layer was deposited in a mixed Ar/O₂ atmosphere of 0.26 Pa with a stepwise increased nitrogen gas flow and a subsequent increase of the ion energy by increasing the substrate bias from 0 to − 250 V. The c-BN nucleation was gradually initiated by decreasing the O₂ gas flow. The present study was focused on the investigation of the morphology, the microstructure on the nanoscale, and the bonding structure using scanning electron microscopy (SEM), Fourier-Transmission infra-red spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) employing analytical scanning transmission electron microscopy (ASTEM). The HRTEM images revealed a four-layer coating consisting of a gradual nucleation of t-BN, on which a gradual nucleation of c-BN was achieved by decreasing the oxygen gas flow.     

Cubic phase content and structure of BN films from an X-ray absorption study

X-ray absorption near-edge structure (XANES) was used to study the cubic boron nitride (c-BN) content in the BN films deposited on various substrates by different physical vapor deposition or plasma-enhanced chemical vapor deposition methods. By fitting the XANES curves of thin-film samples using standard spectra of pure c-BN and sp(2)-bonded BN in the films with suitable weight factors, the c-BN contents at the film's surface region and across the film's thickness have been determined quantitatively. The results agree well with the previous transmission electron microscopic observations. The method is proved to be independent of the optical properties of thin film and provides a possibility to evaluate the cubic content of BN films accurately.     

Epitaxy of cubic boron nitride on (001)-oriented diamond

Cubic boron nitride (c-BN), although offering a number of highly attractive properties comparable to diamond, like hardness, chemical inertness and a large electronic bandgap, up to now has not found the attention it deserves. This mostly has to do with preparational problems, with easy chemical routes not available and, instead, the necessity to apply ion-bombardment-assisted methods. Hence, most of the c-BN samples prepared as thin films have been nanocrystalline, making the prospect of using this material for high-temperature electronic applications an illusion. Although heteroepitaxial nucleation of c-BN on diamond substrates has been demonstrated using the high-pressure-high-temperature technique, none of the low-pressure methods ever succeeded in the epitaxial growth of c-BN on any substrate. Here, we demonstrate that heteroepitaxial c-BN films can be prepared at 900 degrees C on highly (001)-oriented diamond films, formed by chemical vapour deposition, using ion-beam-assisted deposition as a low-pressure technique. The orientation relationship was found to be c-BN(001)[100]||diamond(001)[100]. High-resolution transmission electron microscopy additionally proved that epitaxy can be achieved without an intermediate hexagonal BN layer that is commonly observed on various substrates.     

Deposition of B4C/BCN/c-BN multilayered thin films by r.f. magnetron sputtering

Thin films of cubic boron nitride (c-BN) and B₄C/BCN/c-BN multilayers, were deposited by r.f. (13.56 MHz) multi-target magnetron sputtering from high-purity (99.99%) h-BN and a (99.5%) B₄C targets, in an Ar (90%)/N₂ (10%) gas mixture. Films were deposited onto silicon substrates with (100) orientations at 300 °C, with r.f. power density near 7 W/cm². In order to obtain the highest fraction of the c-BN phase, an r.f. substrate bias voltage between − 100 and − 300 V was applied during the initial nucleation process and − 50 to − 100 V during the film growth. Additionally, B₄C and BCN films were deposited and analyzed individually. For their deposition, we varied the bias voltage of the B₄C films between − 50 and − 250 V, and for the BCN coatings, the nitrogen gas flow from 3% to 12%. A 300-nm-thick TiN buffer layer was first deposited to improve the adhesion of all samples. X-ray diffraction patterns revealed the presence of c-BN (111) and h-BN phases. FTIR spectroscopy measurements indicate the presence of a peak at 780 cm^− 1 referred to as “out-of-plane” h-BN vibration mode; another peak at 1100 cm^− 1 corresponds to the c-BN TO mode and the “in-plane” vibration mode of the h-BN at 1400 cm^− 1. BN films deposited at 300 °C at a pressure of 4.0 Pa and under − 150 V of nucleation r.f. bias, applied for 35 min, presented the highest c-BN fraction, near 85%. By using 32 layers, it was possible to deposit a 4.6-μm-thick c-BN film with adequate mechanical properties and good adhesion to the substrate.     

Nanocrystalline diamond growth on different substrates

Nanocomposite films consisting of diamond nanoparticles of 3-5 nm diameter embedded in an amorphous carbon matrix have been deposited by means of microwave plasma chemical vapour deposition (MWCVD) from CH₄/N₂ gas mixtures. Si wafers, Si coated with TiN, polycrystalline diamond (PCD) and cubic boron nitride films, and Ti-6Al-4V alloy have been used as substrates. Some of the substrates have been pretreated ultrasonically with diamond powder in order to enhance the nucleation density n_nuc. It turned out that n_nuc depends critically on the chemical nature of the substrate, its smoothness and the pretreatment applied. No differences to the nucleation behaviour of CVD PCD films were observed. On the other hand, the growth process seems to be not affected by the substrate material. The crystallinity (studied by X-ray diffraction) and the bonding environment (investigated by Raman spectroscopy) show no significant differences for the various substrates. The mechanical and tribological properties, finally, reflect again the influence of the substrate material: on TiN, a lower hardness was measured as compared to Si, PCD and c-BN, whereas the adhesion of c-BN/nanocrystalline diamond (NCD) system was determined by that of the c-BN film on the underlying Si substrate.     

Physics of epitaxy and c-BN films optimized growth

The cubic zinc-blende phase of boron nitride (c-BN) affords a plethora of potential applications based on material hardness, low chemical reactivity, high thermal conductivity and also on its very large band gap. A modern growth experiments are currently carried out to elaborate monocrystalline c-BN films with a strong film-substrate adherence and up to now several substrates have been used. However, although the film concentration of c-BN is increasing, the growth optimization is still a question under debate. Optimum conditions must be found in energetic growth techniques (IBAD) as well as substrate conditions (temperature, polarization). In this field basic physics is at the very heart of any strategy which aims to elaborate high quality c-BN films. This involves the physics of heteroepitaxy, an aspect of the growth process which consists in taking advantage of complementary freedom degrees as substrate choice and introduction of buffer layers to optimize heterointerfaces. Although several substrates have been tested an optimized choice still remain to be found. On the other hand, the strategy of buffer layers to improve film quality has been less investigated. In this communication, we give the state of art of this field and we discuss the problem of the optimization of c-BN film quality and we demonstrate that the variation of c-BN content in BN films grown on different substrates can be understood on the basis of elasticity theory.     

Preparation of turbostratic and cubic boron-nitride films by electron-cyclotron-resonance,plasma-assisted,chemical vapour deposition

Boron-nitride films were prepared on single-crystal silicon substrates by electron-cyclotron-resonance (ECR) plasma-assisted, chemical vapour deposition (CVD), using B₂H₆ and nitrogen as the source gases, without intentional heating of the substrate. Turbostratic boron-nitride (t-BN) films were obtained at a maximum deposition rate of 1.8 nms^–1. A cubic boron nitride (c-BN) phase formed in the t-BN films after application of a radio-frequency (r.f.) bias to the substrates at a voltage of 156–172 V and at a maximum deposition rate of 0.08 nms^–1.     

Molecular orbital calculations of hydrogen storage in carbon and boron nitride clusters

Hydrogen gas storage ability in carbon and boron nitride (BN) clusters was investigated by molecular orbital calculations. From single point energy calculations, H₂ molecules would enter from hexagonal rings of C₆₀ and B₃₆N₃₆ clusters and octagonal rings of B₂₄N₂₄ cluster because of lower energy barrier. Chemisorption calculation of hydrogen for BN clusters showed that hydrogen bonding with nitrogen atoms was more stable than that with boron atoms. Stability of H₂ molecules in BN clusters seems to be higher than that of carbon clusters.     

Thick c-BN coatings - Preparation, properties and application tests

Due to the outstanding properties of cubic boron nitride (c-BN) - c-BN is the second hardest of all known materials, has a high wear resistance and a high thermal stability - this material is very promising for a broad range of applications, especially for cutting tools, both as bulk and as a coating material. The state-of-the-art is the use of sintered cutting inserts with c-BN grains. Such c-BN grains are synthesized in an expensive high-pressure-high-temperature process.The requirements for cutting tools continuously increase in production engineering and this leads to a strong demand for new super hard tool coatings. Cubic boron nitride coatings could be an attractive solution. Unfortunately, the preparation of thick c-BN coatings, on the μm scale, is difficult, due to some serious drawbacks and has been successful only in the last years for a few research groups worldwide.PVD processes allow the preparation of c-BN films thicker than 2 μm on silicon and 1 μm c-BN top layers on pre-coated cemented carbide cutting inserts. Measurements of mechanical properties like hardness and Young's modulus reveal that the properties of the c-BN coatings, with hardness of about 60 GPa, are nearly identical to those of c-BN bulk material.Results of systematic turning and milling tests of different coatings in combination with a c-BN top-layer on cemented carbide cutting inserts will be presented in detail. The new results confirm the high potential of c-BN coatings on cutting tools.     

Thin films of boron nitride grown by CVD

For the first time, thin films of boron nitride were deposited by chemical vapour deposition on to polished silicon and other metal substrates using the inorganic compound H₃BNH₃ (aminodiborane) and ammonia as carrier gas. The substrate temperature was varied from 400 to 600°C. The films were chemically inert and adherent to the substrates. The FTIR spectrum of the film showed B-N-B absorption at 800 cm⁻¹, B-N stretching at 1056 cm⁻¹, and also a weak absorption at 1340cm⁻¹ corresponding to B-N-B bending vibration. Deposited films also exhibited X-ray diffraction pattern with interplanar spacing with (002) plane of hexagonal boron nitride.     

HFPECVD法沉积BN薄膜及其生长机理的研究

本文介绍了用HFPECVD(hot filament plasma enhanced chemical vapor deposition)法制备BN薄膜.通过红外吸收谱和x射线衍射图谱分析确定,射频功率和反应气体(N2)气流量显著影响薄膜中立方相BN(c-BN)的相对含量.当射频功率小于200W时,薄膜中立方相的相对含量随它的增加而增大;而当射频功率大于200W时,则随它增加而减小.当N2气流量增加时,薄膜生长速率增加,但立方相的相对含量却减少.最后通过对不同沉积时间样品的红外吸收谱的分析对BN薄膜的生长机理进行了探讨.     

立方氮化硼薄膜制备与性质研究新进展

立方氮化硼(c-BN)具有优异的物理和化学性质, 在力学、光学和电子学等方面有着广泛的应用前景. 自上世纪80年代开始, 低压沉积c-BN薄膜的研究迅速发展, 到90年代中期达到高潮, 随后进展缓慢, c-BN薄膜研究转入低潮. 近年来, c-BN薄膜研究在几方面取得了突破, 如获得与衬底粘附良好、厚度超过1μm的c-BN厚膜; 成功实现了c-BN单晶薄膜的异质外延生长; 此外, 在c-BN薄膜力学性质和过渡层微结构研究方面也取得了进展. 本文主要评述最近几年c-BN薄膜研究在以上几方面取得的最新进展.     

Desolvation-Triggered Versatile Transfer-Printing of Pure BN Films with Thermal–Optical Dual Functionality

Although hexagonal boron nitride (BN) nanostructures have recently received significant attention due to their unique physical and chemical properties, their applications have been limited by a lack of processability and poor film quality. In this study, a versatile method to transfer-print high-quality BN films composed of densely stacked BN nanosheets based on a desolvation-induced adhesion switching (DIAS) mechanism is developed. It is shown that edge functionalization of BN sheets and rational selection of membrane surface energy combined with systematic control of solvation and desolvation status enable extensive tunability of interfacial interactions at BN–BN, BN–membrane, and BN–substrate boundaries. Therefore, without incorporating any additives in the BN film and applying any surface treatment on target substrates, DIAS achieves a near 100% transfer yield of pure BN films on diverse substrates, including substrates containing significant surface irregularities. The printed BNs demonstrate high optical transparency (>90%) and excellent thermal conductivity (>167 W m⁻¹ K⁻¹) for few-micrometer-thick films due to their dense and well-ordered microstructures. In addition to outstanding heat dissipation capability, substantial optical enhancement effects are confirmed for light-emitting, photoluminescent, and photovoltaic devices, demonstrating their remarkable promise for next-generation optoelectronic device platforms.     

Mass spectrometric study of low-pressure inductively coupled plasma for chemical vapor deposition of cubic boron nitride films

Chemical species in plasma are crucial for understanding the mechanism of cubic boron nitride film vapor phase deposition and controlling the film structure. In this study, the plasma condition for cubic boron nitride deposition by low-pressure inductively coupled plasma-enhanced chemical vapor deposition using B₂H₆, N₂, and Ar as reactant gases has been diagnosed by a quadrupole mass spectrometer with an ion energy analyzer. The ionization potentials of B_XH_Y (X=1–2, Y=0–6) decomposed from B₂H₆ have been measured to be between 11.6 and 18.9 eV. B₂H₆ was totally ionized to B⁺ together with small amounts of BH⁺, BH₂⁺ and B₂H_Y⁺ in plasma above the 2 kW input power. N₂ was only partially ionized, and the degree of ionization increased with increasing Ar partial pressure. Neutral species under the present plasma environment were N₂, Ar and He, but N and H were not detected even by appearance mass spectrometry. Our results demonstrate that the main sources for cubic boron nitride formation are ions produced in plasma. The interaction between N₂ and the growth surface suppresses the cubic boron nitride formation by enhancing the tBN growth, and this surface interference can be reduced by introducing Ar into the system.     

r.f.PCVD法制备c-BN膜的研究

采用 Ｆ Ｔ Ｉ Ｒ、 Ｔ Ｅ Ｍ、 Ｓ Ｅ Ｍ 等技术, 对在渗硼层表面经ｒ.ｆ. Ｐ Ｃ Ｖ Ｄ 沉积的 Ｂ Ｎ 膜进行了研究试验证明, 与采用 Ｎ_２ 气和 Ｈ_２ 气相比, 以 Ａｒ ＋ １０ｖｏｌ％ Ｈ_２ 作为载气, 所获得的膜层ｃ-Ｂ Ｎ 含量最高, 膜厚最大, 可达４.６μｍ , 且膜基结合良好而以 Ｎ_２ 气或 Ｈ_２ 气为载气时, 前者会导致膜基结合力大大下降, 后者会引起沉积速度明显降低结果表明, 对于 Ｐ Ｃ Ｖ Ｄ 过程, 控制ｃ- Ｂ Ｎ 形成的主要因素是离子轰击能量的转移, 而不是氢的选择溅射过程试验获得的膜层由ａ- Ｂ Ｎ 和ｃ- Ｂ Ｎ 组成, ｃ- Ｂ Ｎ的尺寸为２０ ～４０ｎｍ     

Plasma assisted chemical vapour deposition of boron nitride coatings from using BCl3–N2–H2–Ar gas mixture

Boron Nitride coatings have been deposited by plasma-assisted chemical vapour deposition (PACVD) from BCl₃/N₂/H₂/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/H₂ plasma was studied in order to understand the influence of molecular hydrogen in the discharge mixture. Then the two precursors N₂ and BCl₃ 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/N₂ controls the nature of the NH_x (from N to NH₃) 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 H₂ can increase the dissociation rate of the boron precursor BCl₃ 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/H₂ and Ar/Cl₂ 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.     

Parameter spaces for the nucleation and the subsequent growth of cubic boron nitride films

The data existing in the literature about the deposition of cubic boron nitride thin films were reviewed critically in order to establish the parameter spaces of c-BN nucleation and growth. The ion energy E_i, the flux ratio F (=incoming ions/incoming boron atoms), the ion mass m_i, (or the ratio Ar/N₂, respectively), and the substrate temperature T_s, had already been identified as the decisive parameters which are, however, interdependent. Earlier data collections on c-BN deposition had shown that, irrespective of the deposition technique used, a well-defined c-BN region exists in the F/E_i parameter space, in which the deposition of c-BN is possible. Similar regions exist in the F/m_i and F/T_s parameter spaces. The present collection extends these older diagrams considerably, especially to the low energy region. From this extention it can be concluded that the momentum transfer concepts proposed in the literature fail to explain the data. Furthermore, the older collections were considered valid for nucleation and growth likewise. However, in recent years data have been published showing that the boundaries of the c-BN regions are different for nucleation and growth. After successful nucleation, subsequent growth can occur either at reduced ion bombardment (either energy or flux ratio or ion mass) and also at reduced temperatures. The existing data for this parameter reduction have been collected in this paper. It will be shown that the growth depends in a similar way as the nucleation on the (interdependent) ion bombardment parameters but no longer on temperature. This means that the nucleation and growth of c-BN are based on different, although in both cases ion-induced, mechanisms.