Growth of adhesive cubic phase boron nitride films without argon ion bombardment

Abstract

Previously, in situ bombardment of massive ions (Ar⁺, Kr⁺, etc.) was considered to be necessary for the formation of c-BN films. Because of the accumulated stress, bombardment of massive ions has led to the formation of c-BN films with poor adhesion. Here we show that c-BN films can be grown without involving bombardment of massive ions. This is achieved by using plasma-assisted pulsed-laser deposition (PLD) in pure N₂ RF plasma. Furthermore, we show that c-BN films can be grown in a vacuum (∼10^− 5 mbar during growth) by PLD without auxiliary ion source. We show that these are possible at a reduced deposition rate. Energetic growth species initiated by PLD and the pure N₂ plasma is sufficient to form adhesive c-BN films at moderate deposition rate as long as the energy transfer rate per growth species is sufficient.

Introduction

Cubic phase boron nitride (c-BN) films have many desirable properties such as extreme hardness comparable to diamond, high thermal conductivity, wide band gap (~6 eV) and higher resistance to oxidation and ferrous metals than diamond [1]. In the past two decades, it was reported that ion bombardment was necessary for growing c-BN films, especially with massive ions of Ar or Kr [1], [2], [3], [4], [5], [6]. However, poor adhesion was generally observed which hindered the applications of c-BN films. Implantation of Ar or Kr ions into the deposited films is expected to cause structural damage and stress, especially at high ion energy conditions [7]. This is one of the reasons for the poor adhesion.

In addition, c-BN films are found to delaminate in a unique manner. In general, c-BN films were grown with a microstructure consisting of sp² and sp³-bonded phases [1], [2], [3]. The c-BN phase is not directly grown on the substrate but on top of a BN interlayer. This interlayer consists of a sp² bonded turbostratic (t-BN) layer and an amorphous BN (a-BN) layer next to the substrate. A significant difference in mechanical properties and internal stress occurs between this interlayer and the c-BN film on top [8]. This is also one of the reasons for poor adhesion of c-BN films where mechanical failure occurred near the boundary of the sp²/sp³ phases.

Previously, we have reported the successful growth of the c-BN films in pure N₂ plasma by RF plasma assisted pulsed laser deposition (PLD) with a novel laser light source (5th harmonic generation of Nd:YAG lasers, wavelength λ ∼ 213 nm, pulse duration ∼3 ns) [7], [9]. We found that c-BN films grown in N₂ plasma are more adhesive than those grown with the addition of Ar ions [7]. In this work, the growth of c-BN films in pure N₂ plasma by the same technique was attempted, however, with a commonly accessible laser—the 4th harmonic generation of Nd : YAG laser (λ ∼266 nm, pulse duration ∼6 ns). As will be discussed, this is theoretically more challenging. In addition, we found that energetic BN growth species generated by laser ablation in a vacuum is sufficient for the growth of c-BN films without auxiliary ion bombardment from other ion sources.