Elsevier

Thin Solid Films

Volume 515, Issue 3, 23 November 2006, Pages 1058-1062
Thin Solid Films

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

https://doi.org/10.1016/j.tsf.2006.07.069Get rights and content

Abstract

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.

Introduction

Boron nitride exists in its equilibrium state either as the sp2-bonded hexagonal (h-BN) or sp3-bonded cubic (c-BN) modification, depending on temperature and pressure conditions. Intensive studies have been addressed recently to thin film synthesis of cubic boron nitride by physical vapor deposition (PVD) and plasma-assisted chemical vapor deposition (PA-CVD) techniques [1], [2], [3], [4], [5], [6], inspired by the well-known outstanding properties of the bulk material, such as high hardness, chemical inertness against ferrous metals and oxides [7] and excellent thermal conductivity, electrical resistivity, and optical transparency in the visible- and infrared-range. These make c-BN a promising candidate for a wide range of applications, as protective coatings for cutting tools [8], functional material for high-temperature, high-power optical and electronic devices [9]. As a wide band-gap semiconductor, c-BN can be either like diamond p-type doped or n-type doped [10], [11], [12].

Cubic boron nitride have been successfully grown by ion-assisted PVD and PA-CVD methods, in which an adequate ion impact is considered to be indispensable for the formation [13]. This, however, results in lattice defects, binding deformation, and hence large levels of biaxial stress causing film delamination restricting maximum film thickness to a few 100 nm only. Despite the impressive progress in the c-BN thin film technology and immense effort put into the development of new synthesis concepts the stress/adhesion problem still prevents a successful industrial application. However, several groups, recently, made important progress in overcoming this limitation, allowing the deposition of c-BN films with thickness above 2 μm [14], [15], [16], [17], [12]. Still, most of these approaches are not compatible with industrial needs [18]. In order to be able to deposit thick, adherent c-BN films a combination of stress reduction after nucleation and an adhesion enhancing layer concept has to be developed. Recently, a new concept for stress reduction and adhesion enhancement has been published, featuring a 2 μm thick, well-adherent boron nitride with an overall c-BN content of 79% and a hardness of 60 GPa in the c-BN top-layer was achieved by a layered coating concept and oxygen addition [19], [20]. In this work a new advanced layered coating concept is presented where the nucleation of the cubic phase was first prevented by the given conditions before stepwise initiation of the c-BN nucleation avoiding an abrupt change of the phase composition and properties. The emphasis of this work is lying on the detailed investigation of the BN:O coating in respect to the morphology, the constitution and nanoscaled bonding state of the grown layered c-BN:O coating.

Section snippets

Film deposition

In the present work, boron nitride films were deposited by unbalanced radio-frequency (r.f.) magnetron sputtering (13.56 MHz) using a hot-pressed hexagonal boron nitride target of 99.999% in purity, 75 mm in diameter and 3 mm in thickness at a substrate temperature of TS = 400 °C. The r.f. power applied to the target was 500 W. The distance between the target and the substrates was 120 mm. The base pressure before deposition was lower than 5 × 10 4 Pa. During the deposition the argon flow rate was

Results and discussions

The concept of the four-layered, in total 3 μm thick oxygen containing boron nitride films is illustrated in Fig. 1. The Ar gas flow was kept constant during the whole deposition process at 45 sccm, as described in Section 2.1. The coating consists of a two-step ≈ 800 nm-adhesion layer, followed by a ≈ 200 nm thick c-BN nucleation layer, and a 2 μm c-BN:O top layer.

The morphology of the cross section as well as the surface of the coating is illustrated by the SEM image in Fig. 2, showing a dense

Conclusions

The growth of thick c-BN coatings was carried out by reactive r.f. magnetron sputtering from a h-BN target in an Ar/N2/O2 atmosphere with variable N2 and O2 gas flow conditions. The deposition resulted in the synthesis of low-stress, adherent 3 μm thick boron nitride film with good mechanical properties and overall c-BN content of 65%. In the first step of the adhesion layer the nucleation of hexagonal boron nitride was gradually realized by increasing the nitrogen content achieving the

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