Microstructure of <111>‐Textured Cubic Boron Nitride Film Deposited under Oxygen‐Containing Atmosphere

The cross‐sectional and planar microstructures of a cubic boron nitride (cBN) thin film with a <111>‐preferential orientation are observed using transmission electron microscopy. The cBN films are deposited by unbalanced magnetron sputtering under the condition that oxygen is added to a 20 sccm Ar–N₂(25%) gas mixture. Thecross‐sectional view of the cBN film deposited with the addition of 0.4 sccm of oxygen shows a dual‐phase structure: turbostratic boron nitride (tBN) layers are filled between cBN columns, and the planar view shows that cBN crystals were surrounded by a tBN matrix. The films deposited under less than 0.4 sccm of oxygen addition show a single‐phase structure with no tBN layers between the cBN columns. The difference between dual‐ and single‐phase structures is that they have preferred <111> and <220> orientations. This texture variation is interpreted as being due to the low residual stress of the dual‐phase‐structured cBN film and the low surface energy of the cBN (111) plane. The residual stress of the dual‐phase structure is significantly lower than that of the single‐phase structure. This is attributed to the compressive residual stress relieved by the tBN layers formed between the cBN columns.