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Recent Advances

in Cubic Boron Nitride Deposition

W.J. Zhang, I. Bello, Y. Lifshitz, and S.T. Lee Abstract This article reviews recent progress in the deposition of thick, adherent, cubic boron nitride (c-BN) films. Most of the previous work applied ion-assisted physical vapor deposition methods to deposit c-BN films. The ion impact was successful in nucleating c-BN crystallites, but it resulted in a very small crystallite size and introduced stress, which caused the delamination of films thicker than 100 nm. Recent efforts to reduce the stress and obtain thicker films are described. The limited success of these attempts motivated us to explore chemical vapor deposition methods based on fluorine chemistry. We review this work, detailing the success of depositing thick (20 m), stress-free, adherent films with a larger crystallite size and significantly better crystalline quality. Keywords: chemical vapor deposition (CVD), superhard coating materials, thin films.

Introduction Boron nitride (BN), like carbon, may be formed in three main crystalline forms: cubic (c-BN), wurtzite (w-BN), and hexagonal (h-BN); these are analogous to cubic diamond, hexagonal diamond, and graphite, respectively. The first two are sp 3bonded; the latter is sp 2-bonded. Cubic BN is the most attractive BN allotrope, due to a unique set of properties that make it second only to—and sometimes superior to—cubic diamond: high hardness, high thermal conductivity, wide bandgap, chemical inertness, and high thermal stability. It is better than diamond for mechanical applications related to ferrous materials because diamond reacts to form soot in high-temperature ferrous applications.1 Cubic BN deposition is typically accomplished using ion-assisted methods by which the film grows in sequence from an amorphous material (a-BN), to turbostratic BN (t-BN) with the basal planes perpendicular to the surface, and finally to c-BN with the (111) planes parallel to the t-BN basal planes. All these deposition methods generate energetic species of tens to hundreds of electronvolts that produce c-BN films but at the same time induce significant compressive stress leading to delamination of the films, thus limiting the achievable thickness of the film to
100 nm. Mirkarimi et al.2 reviewed c-BN deposi-

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tion efforts up to 1996. More recent investigations have focused on different ways to reduce the stress created in depositing thick and adherent c-BN films. This article summarizes these attempts and emphasizes the success of novel chemical routes using fluorine chemistry, analogous to the chemical vapor deposition (CVD) of diamond.

Physical Vapor Deposition Routes for c-BN Synthesis and Resulting Film Properties Cubic BN films have been prepared by a variety of ion-assisted physical vapor deposition (PVD) methods, including magnetron sputtering, ion-assisted deposition, biased plasma deposition, and ion-beam deposition.2–4 Bombardment by energetic species (tens to hundreds of electronvolts) coupled with exposure to boron and nitrogen species plays a ke