Toyonobu Yoshida

Vapour phase deposition of cubic boron nitride

Abstract This paper reviews the “state-of-the-art” of the vapour phase deposition of cubic boron nitride (cBN) in terms of the nucleation and growth processes on ion bombardment, and provides an insight into the solution of the inherent disadvantages of ion-assisted processing for cBN deposition. To this end, current experimental and theoretical topics related to the vapour phase deposition of cBN are examined, and the initial growth stage is discussed in conjunction with recent findings derived from novel sputtering and plasma chemical vapour deposition. Finally, a qualitative growth scheme of cBN is proposed based on the concept of dynamic compressive strain, and research needs for the future development of cBN thin films are identified.

INITIAL STAGE OF CUBIC BORON NITRIDE FILM GROWTH FROM VAPOR PHASE

Low pressure inductively coupled plasma enhanced chemical vapor deposition (ICP–CVD) made it possible to deposit stoichiometric boron nitride (BN) films with thickness gradients in lateral direction, which clearly demonstrate the progresses of the cubic BN (c‐BN) growth. Under optimal deposition conditions, single‐phase c‐BN grows on the initially deposited sp2‐bonded BN layer, which had a critical sp2‐BN‐thickness of around 30–100 nm, depending on the substrate potential. It was also found that there exists a very close correlation between the c‐BN formation and Ar incorporation, which was lower than the detection limit in the sp2‐bonded BN layer, while approximately 0.4 at.u2009% Ar was detected in the c‐BN layer. Furthermore, drastic change of the strained state in the film and the surface morphology upon the transition of the growth phase from sp2‐bonded BN to c‐BN was confirmed. As a whole, the comprehensive characterization of the initial progress of the c‐BN formation suggested that a highly compressed...

Growth process of cubic boron nitride films in bias sputter deposition

Abstract Intending to study phase evolution of cubic boron nitride (cBN) films during r.f. bias sputter deposition, surfaces of the cBN films in different growth stages were examined systematically. Argon content at the film surface increased during the deposition of initial sp2-bonded layer growth, and saturated at about 1.5 at. %, and remained constant after the formation and growth of cubic phase, accompanied with an increase of compressive film stress up to 5 GPa. This increase of stress is confirmed to be the result of averaging film stress over the double-layered cBN film, i.e. over the low-stressed initial layer and high-stressed cubic layer. Atomic force microscope observations in the tapping mode revealed self-affine fractal nature and kinetic roughening of the film surface during the growth, and root mean square roughness changed from 0.3 nm of an initial layer, to 0.9 nm accompanied with peculiar surface smoothing at the initial stage of cBN formation. Moreover, the evolution of the surface roughness was clearly characterized by the change of modes in each growth process, i.e. an initial layer growth stage, a transition stage, and a cubic layer growth stage.

Threshold sheath potential for the nucleation and growth of cubic boron nitride by inductively coupled plasma enhanced chemical-vapor deposition

The nucleation and growth conditions of c-BN films were studied in low-pressure inductively coupled plasma enhanced chemical-vapor deposition. The threshold sheath potentials needed for the nucleation and the growth of c-BN were sustained by two step Vsheath deposition. The threshold sheath potential for the growth Vsheathg was found to be significantly lower than the threshold value required for nucleation Vsheathn. Under our experimental condition, the values of Vsheathg and Vsheathn were found to be 45 and 65 V.

Microstructure and nanomechanical properties of cubic boron nitride films prepared by bias sputter deposition

Abstract Nanoindentations, using a triangular based cube-corner shaped diamond tip, were tried to study the mechanical properties of cubic boron nitride (cBN) films. Since the cBN film has a layered structure consisting of an initial sp2-bonded BN (amorphous BN and turbostratic BN) layer and a sp3-bonded BN (cBN) layer, which was previously studied by cross-sectional transmission electron microscopy (TEM), cBN films in different growth stages were prepared for the measurements. Hardness and elastic modulus were evaluated as a function of the penetration depth from the obtained loading-unloading curves using a method described by Oliver et al. The maximum hardness and elastic modulus of a 110-nm thick cBN film (including an approximately 50-nm thick initial layer) were approximately 10 and 3 times, respectively, larger than the values evaluated for a silicon wafer substrate. The dependence of the evaluated hardness and the elastic modulus on film thickness and penetration depth can be explained in terms of the layered structure of the cBN film, i.e. soft initial sp2-bonded BN layer with low elastic modulus, and hard cBN layer with high elastic modulus.

Mass and energy measurements of the species responsible for cBN growth in rf bias sputter conditions

Mass and energy measurements of ions and neutrals impinging on a substrate surface were performed during radio frequency (rf) bias sputter deposition of cubic boron nitride (cBN) thin films in a pure Ar discharge. The sampling system was rf driven to measure the correct energy of ions impinging to the rf driven substrate. The ion energy distributions showed asymmetric bimodal shapes and the energy spreads varied with the masses of ions and the negative substrate bias voltage (Vs). Ar+ was the most dominant ion, and the average energy and energy spread changed with Vs from 90 to 310 eV and from 40 to 140 eV, respectively. The flux ratio of Ar+ to B (Φi/ΦB) increased from 1.0 to 2.3 with a decrease in the target power input. cBN could be deposited above a threshold of the total momentum transfer per depositing boron atoms; 138u2009(eVu2009amu)1/2, which is comparable to the values reported in the ion beam assisted depositions. Therefore, argon ions with an energy of 135 to 380 eV are considered to be equally contri...

THE ROLE OF RADICALS AND CLUSTERS IN THERMAL PLASMA FLASH EVAPORATION PROCESSING

The prominent features of plasma flash evaporation may be caused by cluster deposition under high net flux of radicals. In order to clarify such features quantitatively, the atomic oxygen flux (Γo) and cluster size (d) in 200 Torr RF thermal Ar-O2 plasma were measured on the substrate. It was found that Γo higher than 1019 atoms/cm2·s can be easily acheived in this processing and this value corresponds to frozen atomic oxygen fraction 7 %. The cluster size of YBCO was estimated to be about 0.3 to 10 nm. 0.8 nm cluster deposition under Γo higher than 6 × 1018 atoms/cm2·s made it possible to deposit fine epitaxial YBCO films with Tc = 90K on SrTiO3 (100) with extremely high deposition rate around 2.2 μm/min.

Effects of phase regulation on ion energy distribution in RF bias sputtering

Ion energy distributions (IEDs) at a radio frequency (RF) driven substrate electrode were measured in an rf (13.56 MHz) bias sputtering system with a phase shifting unit for regulating the two rf voltages applied to the target (upper) and the substrate (lower) electrodes. Without the regulation, the IED was of a centrally peaked shape, not of a bimodal shape which is a characteristic of ions accelerated across an RF sheath, while the IED with the regulation showed a strongly asymmetric bimodal saddle shape. Moreover, it was found that the energy spread and asymmetry of IEDs and the ion current varied with the phase angle difference, and were correlated with the measured sheath thickness variation. These results were discussed with a simple collisionless oscillating sheath model.