Microstructure and mechanical properties of pulsed laser deposited boron nitride films

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.     

Growth of cubic boron nitride films by ibad and triode sputtering: development of intrinsic stress

Two methods are employed to evidenced the stress behavior in c-BN films. On the one hand, in depth stress profile of c-BN film, deposited by ion beam assisted evaporation, was performed by recording infrared spectra and substrate curvature after reactive ion etching (RIE) steps. It shows a peak of stress up to −17 GPa in the h-BN basal layer and a stress relaxation when the cubic phase appears. On the other hand, dynamic stress profiles of c-BN films deposited by a triode sputtering system, are obtained by recording infrared spectra and substrate curvature after various c-BN deposition times, with the same experimental conditions. Likewise, a peak of stress of −12 GPa is unmistakably observed in the h-BN basal layer followed by a stress release during c-BN nucleation, where an average value of −12 GPa is observed in the c-BN film volume. These results provide a support for the stress model proposed by McKenzie even if along with a minimum stress a high level of densification of the layer is needed.     

The effect of substrate surface roughness on the nucleation of cubic boron nitride films

Boron nitride (BN) films have been deposited on silicon (Si) substrates with a root-mean-square surface roughness between 0.2 and 170 nm using mass-selected ion beam deposition (MSIBD). Mechanical scratching by either diamond or alumina powders with different powder sizes was used for substrate pretreatment. The effect of substrate surface roughness on the subsequent growth of BN films at different ion energies (75–500 eV) was investigated by Fourier-transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and transmission electron microscopy (TEM). For rough substrates the surface morphology of the BN films changes from a granular structure at low ion energies to a flat and featureless surface at 500 eV ion energy. In the latter case grooves and valleys are first filled up with amorphous BN before turbostratic BN (t-BN) is formed. Eventually c-BN nucleates on the t-BN interfacial layer. The c-BN nucleation threshold energy of about 125 eV remains unchanged. Surface-like growth processes dominate at low ion energy, leading to the granular morphology. The observations are explained by ion impact on inclined micro-surfaces, leading to reduced projected ion ranges and enhanced sputtering and re-deposition into surface grooves and valleys.     

Microstructural characterization of diamond films deposited on c-BN crystals

The morphology and structure of diamond films, deposited on cubic boron nitride (c-BN) crystals by microwave-plasma-enhanced chemical vapor deposition, is studied by high-resolution scanning electron microscopy and micro-Raman spectroscopy. The c-BN crystals, with sizes of 200 to 350 μm and grown by a high-temperature/high-pressure technique, were embedded in a copper holder, and used as substrates in deposition runs of 15 min to 5 h. The nucleation centers for diamond appear as well-shaped cuboctahedral crystallites, having diameters of approximately 100 nm. With increasing deposition time the diamond crystallites grew larger, forming islands on the c-BN faces. In some cases, epitaxial growth was observed on the (111) c-BN faces where coalesced particles gave rise to very smooth regions. A number of diamond crystals with peculiar shapes are observed, such as a pseudo five-fold symmetry due to multiple twinning. Moreover, both randomly distributed carbon tubes, about 100 nm in diameter and 1 μm in length, and spherically shaped features are observed in samples prepared under the typical conditions of diamond deposition, this effect being ascribed to the influence of plasma-sputtered copper contamination. Quite unusual diamond crystals with a deep, pyramidal-shaped hole in the middle grew on the copper substrate between the c-BN crystals.     

Depth resolved stress investigations of c-BN thin films

Cubic boron nitride thin films were deposited by ion beam assisted deposition (IBAD) on (100)-oriented silicon cantilever structures prepared by standard micro-machining processes. This enables an accurate determination of the stress-induced bending of the beam by optical microscopy. Depth resolved characterisation of the coatings was achieved by subsequent back-etching and examination of the film stress and the IR data after each sputtering step. Cubic BN containing films exhibit a three layer sequence: non-cubic baselayer/transition h-BN→c-BN/c-BN toplayer. This layered sequence was verified by the evolution of the IR data as well as the stress distribution. These investigations confirm the existence of a transition region between the h-BN baselayer and the c-BN toplayer. Furthermore, the dependence of the depth-resolved c-BN stress σ_c-BN on the main deposition parameters was investigated. A stress reduction can be achieved by reducing the Ar/N₂ ratio and/or by increasing the ion energy. As this stress relief is correlated with an increase of the sp² bonded material within the c-BN toplayer, it can be concluded that stress relaxation occurs at the sp² bonded grain boundary material. Finally, the influence of the stress on the nucleation and the growth of c-BN containing films will be discussed.     

Study of stress and infra-red absorption line of BN films containing various fractions of cubic phase

This paper focuses on the stress dependence of infrared absorption lines of BN films synthesized by ion beam assisted deposition (IBAD) and containing various fractions of cubic phase. The compressive stress ranges from −3 to –11 GPa and is found to be conditioned by the content in the c-BN phase. This stress results in a characteristic shift towards high wavenumbers of the IR TO c-BN peak. A frequency shift rate of 3.0±0.5 cm⁻¹/GPa was derived, very close to the one measured for bulk c-BN under hydrostatic pressure. The effect of post-deposition annealing was analyzed. We found the compressive stress to be slightly modified by a post-deposition annealing at 800°C. The resulting relaxation effect is found to be most effective in the c-BN part of mixed films, and in the h-BN one in c-BN rich films (>80% of cubic phase). From the latter observation, the stress level retained initially in the sp²-bonded fraction should reach −10 GPa. This stress level suggests a mechanism involving the rhombohedral form of BN as a precursory phase for c-BN nucleation.     

Influence of d.c. substrate bias voltage on growth of cubic boron nitride films by radio frequency sputter

In this paper, the influence of substrate d.c. bias voltage on growth of cubic boron nitride (c-BN) films by radio frequency (RF) sputter is reported. Boron nitride films were deposited on p-type Si(100) wafers (8–15 Ωcm) which were biased by the d.c. voltage negatively with respect to ground. The sputtering target was hot pressed hexagonal boron nitride of 4 N purity. The sputtering gas was the mixture of nitrogen and argon. The boron nitride films were characterized with Fourier transform infrared (FTIR) spectra. At a RF power of 360 W and substrate d.c. bias voltage of −200 V, the films contained almost pure phase c-BN. It was shown that different substrate d.c. bias voltages resulted in different cubic phase contents in the c-BN films.     

Synthesis of adhesive c-BN films in pure nitrogen radio-frequency plasma

Cubic boron nitride (c-BN) thin films were prepared at 600 °C by radio-frequency (rf) plasma pulsed laser deposition. All c-BN films prepared in Ar-rich plasma have poor adhesion on Si(100) substrates, but those prepared in pure N₂ plasma can be maintained for more than 5 months without degradation. However, an increase of ion flux at an ion energy similar to that of pure N₂ plasma results in the peeling of c-BN films. Thus, application of pure N₂ plasma with suppressed ion flux can improve c-BN film adhesion. Under such conditions, an extended sp²-bonded interlayer is suspected, with the onset of the c-BN phase being delayed. Suppression of radiative damage in reduced nitrogen ion flux on both the c-BN and h-BN/t-BN phases are important for the adhesive of c-BN films.     

Effects of substrate bias voltage on adhesion of DC magnetron-sputtered copper films on E24 carbon steel: investigations by Auger electron spectroscopy

The adhesion strength of copper thin films on E24 carbon steel substrates was studied using the scratch test via the critical load. Coatings were deposited by a DC magnetron sputtering system. All substrates were mechanically polished; some of them were directly coated and others were ion-etched by argon ions prior to deposition process. The effects of substrate negative bias voltage during the film growth were investigated. Experimental results showed that the critical load depended on the bias voltage and that the higher bias voltage, the better adhesion. It was also observed that the deposition rate of deposited films gradually decreased with the increase of the substrate bias voltage. Furthermore, the working pressure during the substrate ion bombardment etching greatly affected the critical load. Scanning electron microscopy was used to observe the scratch tracks to accurately evaluate the critical load. Substrate surface profiles obtained by a mechanical profilometer showed that the critical load increased with the increase of the surface roughness. The analysis by Auger electron spectroscopy revealed that the interface, in case of an unbiased substrate, was relatively narrow and abrupt. However, in case of a bias voltage application, the interface was wider and more diffuse. These results suggest that the mechanisms involved in critical load enhancement are due firstly to the substrate surface roughness and the substrate temperature generated by the ion bombardment, secondly to the physical mixing in the interfacial domain and the densification of the deposited material created by the bias voltage.     

Low surface temperature synthesis and characterization of diamond thin films

Polycrystalline diamond films are deposited on p-type Si(100) and n-type SiC(6H) substrates at low surface deposition temperatures of 370–530 °C using a microwave plasma enhanced chemical vapor deposition (MPECVD) system. The surface temperature during deposition is monitored by an IR pyrometer capable of measuring temperature between 250 and 600 °C in a microwave environment. The lower deposition temperature is achieved by using an especially designed cooling stage. The influence of the deposition conditions on the growth rate and structure of the diamond film is investigated. A very high growth rate up to 1.3 μm/h on SiC substrate at 530 °C surface temperature is attributed to an optimized Ar-rich Ar/H₂/CH₄ gas composition, deposition pressure, and microwave power. The structure and microstructure of the films are characterized by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. A detailed stress analysis of the deposited diamond films of grain sizes between 2 and 7 μm showed a net tensile residual stress and predominantly sp³-bonded carbon in the deposited films.     

Transparent ultrananocrystalline diamond films on quartz substrate

Highly transparent ultrananocrystalline diamond (UNCD) films were deposited on quartz substrates using microwave plasma enhanced chemical vapor deposition (MPECVD) method. Low temperature growth of high quality transparent UNCD films was achieved by without heating the substrates prior to the deposition. Additionally, a new method to grow NCD and microcrystalline diamond (MCD) films on quartz substrates has been proposed. Field emission scanning electron microscopy (FESEM) and Raman spectroscopy were used to analyze the surface and structural properties of the films. The surface morphology of UNCD film shows very smooth surface characteristics. The transparent property studies of UNCD film on quartz substrate showed 90% transmittance in the near IR region. The transparent and dielectric properties of UNCD, NCD, and MCD films on quartz substrates were compared and reported.     

The influence of ion beam assisted deposition parameters on the properties of boron nitride thin films

We studied ion beam assisted deposition of cubic boron nitride thin films on silicon (100) and high speed steel. The boron nitride films were grown by the electron beam evaporation of pure boron (99.4%) and the simultaneous ion bombardment of a mixture of nitrogen and argon ions from a Kaufman ion source. At a constant boron evaporation rate, the ion energy, ion current density, substrate temperature and process gas mixture was varied. The thickness of the films was kept between 200 and 300 nm. Boron nitride films with >80% of the cubic phase (determined by Fourier transform infrared spectroscopy) were obtained with nitrogen/argon mixtures of 50/50 at ion energies of 450 eV and substrate temperatures of 400°C. The current density amounted to 0.45 mA cm⁻² at a nominal boron rate of 200 pm s⁻¹. Cubic boron nitride films were deposited on high speed steel by introducing a titanium interlayer for adhesion improvement.     

Evolution and properties of adherent diamond films with ultra high nucleation density deposited onto alumina

In this work, we report on adherent diamond films with thickness of up to 4.5 μm grown on polycrystalline alumina substrates. Prior to deposition, alumina substrates were ultrasonically abraded with mixed poly-disperse slurry that allows high nucleation density of values up to ∼5×10¹⁰ particles/cm². It was estimated that the minimal film thickness achieved for continuous films was ∼320 nm, obtained after a deposition time of 15 min with diamond particles density (DPD) of ∼4×10⁹ particles/cm². Continuous adherent diamond films with high DPD (∼10⁹ particles/cm²) were obtained also on sapphire surface after abrasion with mixed slurry and 15 min of deposition. However, after longer deposition time, diamond films peeled off from the substrates during cooling.The poor adhesion between the diamond and sapphire is attributed to the weak interface interaction between the film and the substrate and to difference in coefficient of thermal expansion. On the other hand, it is suggested that the reason for good adhesion between diamond film and alumina substrate is that high carbon diffusivity onto alumina grain boundaries allows strong touch-points at the grooves of alumina grains, and this prevents the delamination of diamond film. This adhesion mechanism, promoted by sub-micron diamond grain-size, is allowed by initial high nucleation density.The surface properties, phase composition and microstructure of the diamond films deposited onto alumina were examined by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high-resolution scanning electron microscopy (HR-SEM). The residual stress in the diamond films was evaluated by diamond Raman peak position and compared to a theoretical model with good agreement. Due to the sub-micron grain-size, the intrinsic tensile stress is high enough to partially compensate the thermal compressive stress, especially in diamond films with thickness lower than 1 μm.     

Influence of the substrate nature on the properties of nanocrystalline diamond films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been deposited by microwave plasma CVD from CH₄/N₂ mixtures on a variety of substrates such as polycrystalline diamond, cubic boron nitride, silicon, titanium nitride, and Ti–6Al–4V. The study aimed to investigate the influence of the chemical nature of the substrate, the surface roughness, and the pretreatment of the substrate on the nucleation, the bulk structure, and the mechanical and tribological properties of the NCD/a-C films. The present paper is especially devoted to the bulk structure of the films. By means of X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) it is shown that the bulk properties of the films are not affected by the properties of the substrate although these have a strong influence on the nucleation behaviour. XRD measurements show that – irrespective of the substrate used – the films contain diamond nanocrystallites of 3–5 nm diameter. From the Raman spectra it can be inferred that the crystallite/matrix ratio does not vary. The XPS measurements, finally, show that there are no great changes in the sp²/sp³ ratio of the matrix. These findings are discussed in view of possible growth mechanisms of NCD/a-C nanocomposite films.     

Effects of crystalline quality on the phase stability of cubic boron nitride thin films under medium-energy ion irradiation

To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) thin films, c-BN films with various crystalline qualities prepared by dual beam ion assisted deposition were irradiated at room temperature with 300 keV Ar⁺ ions over a large fluence range up to 2 × 10¹⁶ cm^− 2. Fourier transform infrared spectroscopy (FTIR) data were taken before and after each irradiation step. The results show that the c-BN films with high crystallinity are significantly more resistant against medium-energy bombardment than those of lower crystalline quality. However, even for pure c-BN films without any sp²-bonded BN, there is a mechanism present, which causes the transformation from pure c-BN to h-BN or to an amorphous BN phase. Additional high resolution transmission electron microscopy (HRTEM) results support the conclusion from the FTIR data. For c-BN films with thickness smaller than the projected range of the bombarding Ar ions, complete stress relaxation was found for ion fluences approaching 4 × 10¹⁵ cm^− 2. This relaxation is accompanied, however, by a significant increase of the width of c-BN FTIR TO-line. This observation points to a build-up of disorder and/or a decreasing average grain size due to the bombardment.     

Near-edge X-ray absorption fine structure spectroscopy of arcjet-deposited cubic boron nitride

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was employed to help determine the structure of boron nitride films grown by bias-enhanced chemical vapor deposition in a low-density supersonic arcjet flow. BN films containing 0.90% cubic boron nitride were analyzed by NEXAFS and compared with c-BN and h-BN reference spectra. The mainly cubic films have been shown previously to be nanocrystalline, which leads to the inability to obtain structural information from Raman scattering spectra. However, with NEXAFS, the nanocrystalline nature of the films does not strongly affect the structural interpretation. It is shown that films deposited with a bias of −75 V are primarily sp³ bonded. This high sp³ bonding character agrees with previous measrements based on infraredtransmission and reflectance spectroscopy, as well as X-ray photoelectron spectroscopy.     

Selective deposition of diamond films on insulators by selective seeding with a double-layer mask

Polycrystalline diamond films have been patterned on Si₃N₄/Si and SiO₂/Si substrates by selective seeding with a double-layer mask via hot-filament chemical vapor deposition. High quality in the patterned diamond films and high selectivity were obtained by the process. The diamond films deposited on the insulators at different CH₄/H₂ concentrations were studied by scanning electron microscopy and Raman spectroscopy. The process proved to be far less damaging to the substrates, and yet effective in developing patterns of diamond films on a large and different substrate.     

Hot filament chemical vapour deposition and wear resistance of diamond films on WC-Co substrates coated using PVD-arc deposition technique

Different Cr- and Ti-base films were deposited using PVD-arc deposition onto WC-Co substrates, and multilayered coatings were obtained from the superimposition of diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The behaviour of PVD-arc deposited CrN and CrC interlayers between diamond and WC-Co substrates was studied and compared to TiN, TiC, and Ti(C,N) interlayers. Tribological tests with alternative sliding motion were carried out to check the multilayer (PVD + diamond) film adhesion on WC-Co substrate. Multilayer films obtained using PVD arc, characterised by large surface droplets, demonstrated good wear resistance, while diamond deposited on smooth PVD TiN films was not adherent. Multilayered Ti(C,N) + diamond film samples generally showed poor wear resistance.Diamond adhesion on Cr-based PVD coatings deposited on WC-Co substrate was good. In particular, CrN interlayers improved diamond film properties and 6 μm-thick diamond films deposited on CrN showed excellent wear behaviour characterised by the absence of measurable wear volume after sling tests. Good diamond adhesion on Cr-based PVD films has been attributed to chromium carbide formation on PVD film surfaces during the CVD process.     

Tribological study of boron nitride films

Boron nitride (BN) films with different cubic and hexagonal phase compositions were deposited on silicon substrates via diamond interlayers by magnetron sputtering and electron cyclotron resonance microwave plasma chemical vapor deposition. The tribological behaviors of the BN films were investigated systematically using a ball-on-disc tribometer with silicon nitride as the counterpart. Comparison studies were also performed on sintered cubic and hexagonal BN compacts. The influence of phase compositions and surface roughness of BN coatings on their tribological characteristics was studied. The cubic BN (cBN) films showed excellent wear resistance against silicon nitride. The wear rate of the cBN films was estimated to be about 1.0 × 10^?7 mm³/N m by measuring the cross-sectional area of the wear track after the sliding test over a distance of 12 km.     

Selective etching of boron nitride phases

Cubic boron nitride (c-BN) films can be used as hard coatings and for electronic devices due to their outstanding material properties, but the gas phase deposition of c-BN is still a challenging task. Until now it has only been possible to achieve nanocrystalline c-BN layers via physical vapor deposition (PVD) methods with rather weak film qualities. Only a chemical vapor deposition (CVD) process for c-BN can produce high quality films with material properties similar to those of the product achieved by high pressure, high temperature processes (HPHT) conventional routes. Therefore it is essential to tune the individual steps in the CVD process (nucleation, growth and selective etching) in a similar manner to that for diamond CVD to enable continuous growth of c-BN.Since selective etching of hexagonal boron nitride (h-BN) and sp² phases is still a major problem, we investigated the interaction of h-BN and c-BN with different reactive gases — ammonia (NH₃), chlorine (Cl₂), hydrogen chloride (HCl) and boron trifluoride (BF₃) — regarding their etching behaviour and surface stabilisation properties. Etching ratios from ≈10:1 up to 450:1 were found in the temperature range 600–1300°C for the h-BN/c-BN system, clearly indicating a high selectivity due to kinetic effects.The reaction mechanisms will be discussed with respect to the kinetic differentiation of the degradation of c-BN and h-BN (selective etching). The morphological changes and the quality of the remaining BN phases was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared and Raman spectroscopy and these indicated a homogeneous decay of the individual phases. Since a homogeneous decay of c-BN resembles the reversed growth, the study of the interaction of both BN phases with reactive gases allowed us to collect more detailed information of the molecular mechanisms involved in the formation of the individual phases. These results will provide new routes for growing c-BN in a CVD process.