S. Ulrich

Phase separation in magnetron sputtered superhard BCN thin films

Abstract The similar crystalline structure of diamond and cubic boron nitride suggests the synthesis of superhard thin films containing boron, carbon and nitrogen. BCN thin films have been prepared by reactive r.f.-magnetron sputtering of a hexagonal boron nitride target in an argon/acetylene atmosphere of a constant pressure of 2 × 10−2 mbar. A deposition temperature of 600 °C leads to nearly hydrogen-free BCN films. At a flow ratio of 0.05% C2H2/Ar, the composition of the deposited BCN films is close to B5CN3 as a result of energy dispersive X-ray (EDX) and elastic recoil detection (ERD) measurements. By applying a r.f.-substrate bias, the ion energy has been varied at a constant ratio of ions to film-forming particles of 4. The BCN films show a maximum in stress at an ion energy of 110 eV due to knock-on subplantation of argon ions and a phase separation into carbon, boron and cubic boron nitride regions as a result of X-ray diffraction investigations. Auger electron- and infra-red spectroscopy.

Phase stability and stress relaxation effects of cubic boron nitride thin films under 350 keV ion irradiation

Abstract To investigate the effect of radiation damage on the stability and the compressive stress of cubic boron nitride (c-BN) films, samples either prepared by r.f. magnetron sputtering or ion beam-assisted sputter deposition (IBAD) were irradiated at room temperature with 350 keV inert ions (Kr + ). FTIR spectra taken before and after each irradiation step clearly demonstrate that c-BN is stable under this medium energy bombardment. Furthermore, additional AES measurements show that the average film stoichiometry is not affected by the ion fluences used in the present experiments. While the observed broadening of the different lines in the IR spectra with increasing ion fluences points to a build-up of disorder and/or a decreasing average grain size due to the bombardment, the additionally found significant shift of the line related to the c-BN TO-mode towards smaller wave numbers, indicates a strong relief of the compressive stress present in the as-prepared films. This irradiation-induced stress relief could be independently confirmed by profilometer measurements yielding post-bombardment values of 5.1 GPa as compared to 20.5 GPa before irradiation.

Subplantation effect in magnetron sputtered superhard boron carbide thin films

Abstract Superhard amorphous boron carbide films with a film thickness of about 2 μm have been prepared by r.f.-magnetron sputtering of a boron carbide target in a pure argon discharge at a gas pressure of 1.6 × 10−3 mbar. The flux ratio of the argon ions to boron and carbon atoms has been kept constant at 3.5, while the energy of the argon ions is varied by applying a d.c.-substrate bias. The effect of argon ion implantation measured by Rutherford back scattering is discussed. When the argon ion energy is increased, the mechanical properties show extreme values at an argon ion energy of 74 eV, which can be explained quantitalively by knock-on subplantation. Stress up to 6.7 GPa and a micro-hardness up to 72 GPa are obtained. The hardness enhancement is correlated with the increase of stress. The influence of preferential sputtering of boron or carbon from the deposited B4C film can be neglected.

Low-temperature formation of β-silicon carbide

Abstract Silicon carbide thin films were deposited by unbalanced radio frequency (RF) (13.56 MHz) magnetron sputtering from a silicon carbide target in a pure argon discharge. Deposition parameters were 80 W RF target power, 1.6 × 10 −3 mbar argon pressure and 6 cm target substrate distance. As determined with energy and mass analysis, the flux of the film-forming particles ( Φ Si + Φ C ≈ 0.6 × 10 15 cm −2 s −1 ) consists of mainly neutral silicon and carbon atoms with typical energies of a few electronvolts. The flux of the plating particles ( Φ Ar + ≈ 1.5 × 10 15 cm −2 s −1 ) is composed of argon ions with a well-defined energy of 24 eV. The low film growth rate, in combination with a high flux ratio of plating argon ions to film-forming particles Φ Ar +/( Φ Si + Φ C ) of about 2.5, shifts the temperature of the phase transition from amorphous silicon carbide to nanocrystalline silicon carbide from normally 1000 °C in the case of plasma-enhanced chemical vapour deposition (PECVD) or chemical vapour deposition (CVD) deposition techniques down to 420 °C. The films were characterized by measurements of the mechanical properties, e.g. hardness and stress, as well as X-ray diffraction (XRD) and Auger electron spectroscopy (AES). Due to the crystallization at 420 °C the stress of the films is reduced from 6.3 GPa (at 60 °C and for the amorphous phase) to approximately 0.8 GPa. The hardness is between 53 and 37 GPa. At higher plating energies (> 85 eV) the ion-plating-induced densification is diminished by preferential sputtering of silicon and consequently stoichiometry and hardness are negatively affected.

Knock-on subplantation-induced formation of nanocrystalline c-BN with r.f. magnetron sputtering and r.f. argon ion plating

Abstract Boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering from a hexagonal boron nitride (h-BN) target with r.f. argon ion plating at an argon gas pressure of 8 × 10 −4 mbar. The content of cubic boron nitride (c-BN) is very high, as determined by infrared absorption spectroscopy (IR, 84% c-BN), factor analysis of Auger electron spectroscopy (AES, approximately 100% sp 3 ) and X-ray reflectivity (approximately 100% sp 3 ). Furthermore, the films were characterized by stress measurements and atomic force microscopy (AFM). The film-forming particles (fluxΦ n ) are mainly sputtered neutral boron and nitrogen atoms, and the plating particles (fluxΦ i ) are argon ions. The current density is about 2.25 mA cm −2 , as determined from energy and mass analysis. The energy dependence due to r.f. substrate bias shows a maximum in the c-BN content at 137 eV at an arrival ratioΦ i /Φ n = 13 where AFM investigations show a minimum of the area roughness of 0.2 nm. Increasing the arrival ratio (to 20 and 66), the optimal c-BN formation is shifted to lower energies (87 eV and 62 eV respectively) which is in agreement with the subplantation model. Stress reduction experiments, such as UV-assisted deposition and post-annealing, are discussed.

Variation of carbon concentration, ion energy, and ion current density of magnetron-sputtered boron carbonitride films

Abstract Diamond, cubic boron nitride and also ternary materials consisting of boron, carbon and nitrogen exhibit an extraordinary combination of extreme mechanical and physical properties due to their bonding characteristics and crystal structure. This results in a high application potential in protective and functional layers. Taking into account the special properties of these phases, the compositions inside the B=C=N concentration triangle are of particular interest, as novel superhard phases are expected to occur. In this work, boron carbonitride films of variable compositions were produced by means of reactive radio frequency (r.f.) magnetron sputtering in combination with ion bombardment. Examination up to now has revealed carbon concentrations between 16 and 27 at.% and a boron/nitrogen ratio varying between 1.08 and 1.26. For a carbon concentration of 12 at.%, the ion energy was varied between 125 and 300 eV at a constant ratio of ions to film-forming particles, Φ ion / Φ BCN . Peak analyses of the differentiated Auger spectra gave no indication of any generation of B=C bonds. The contents of the h-BN phase and c-BN phase as well as the threshold conditions and optimum conditions for c-BN formation were investigated for 8 energy variations at 18 values of Φ ion / Φ BCN between 0.1 and 0.5. IR spectroscopy showed that the maximum fraction of sp 3 -hybrid BN bonds reached approximately 61% at an ion energy of 200 eV and an ion current density of 0.5 mA cm −2 . The film properties can be strongly influenced by the flux ratio of ions to film-forming particles, Φ ion / Φ BCN , and by the ion energy. Generally, the effect on the film properties by increasing the flux ratio Φ ion / Φ BCN or by decreasing the ion energy is often the same. This statement is discussed theoretically.

Strain relaxation of boron nitride thin films on silicon

Exploiting the high brilliance of synchrotron radiation, we performed surface-sensitive and depth-resolved x-ray scattering experiments on thin films of boron nitride grown on Si(001) substrates. In-plane strains of different structural phases, namely turbostratic and cubic, grain sizes and textures were determined. Annealing the films up to temperatures of 1000 °C leads to large strain relaxation of about 70%, while the grain size stays constant at 80 A.

General stress reduction mechanisms for the deposition of cubic boron nitride thin films

Abstract Intense ion bombardment has been necessary so far to produce cubic boron nitride thin films independent of the deposition technique used. Unfortunately, residual stress is one of the consequences of ion–surface interactions, leading to low adhesion and limited film thickness. In this study, boron nitride thin films were deposited by r.f.magnetron sputtering of a hexagonal boron nitride target combined with r.f. argon ion bombardment. The films were characterized by X-ray reflectivity analysis, X-ray Auger electron and infra-red spectroscopy as well as stress analysis. Stress reduction mechanisms are discussed: (1) deposition at higher substrate temperatures, (2) post-annealing, (3) post-ion implantation, (4) addition of a third alloying component, (5) multilayer concept, and (6) optimization of the deposition parameters.

Interface investigations by infrared spectroscopy and X-ray reflectivity measurements of cubic boron nitride thin films

Abstract Cubic boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering of a hexagonal boron nitride target in a pure argon discharge in combination with r.f. argon ion bombardment. At a flux ratio of the argon ions relative to the film forming boron and nitrogen atoms of 20, and an argon ion energy of 87 eV, the total content of sp 3 -bonds of a 300 nm film amounts to 92.8%, according to infrared analysis. This film has a hexagonal interface between the silicon substrate and the cubic top layer which is estimated to be 15.6 nm on the basis of infrared spectroscopy as well as X-ray reflectivity. From the infrared investigations of the film thickness dependence it can be concluded that 72% of the sp 2 -bonds in the 300 nm c-BN film are formed in the hexagonal interface region, and only 18% between the cubic crystallites in the top layer. Consequently, the content of the cubic phase in the top layer amounts to 98.6%, which is in agreement with the density of 3.502 g/cm 3 as determined by X-ray reflectivity measurements.