Crystallography and structural evolution of cubic boron nitride films during bias sputter deposition

Abstract

The mechanism and the crystallography of the growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrates by radio-frequency bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy. Particular attention has been paid to the atomic structure of graphitic (sp 2-bonded) BN and grain boundaries in the c-BN films. The c-BN films grow in the sequence of amorphous boron nitride (a-BN), turbostratic boron nitride (t-BN) and c-BN layers, similar to previous results. The sp 2-bonded BN material consists of small regions, 1–5 nm thick, forming in a layered manner parallel to the substrate surface. Each region consists of parallel lamellae in both the hexagonal and rhombohedral (h-BN and r-BN, respectively) configurations. Three orientation relationships between h-BN and r-BN phases were determined: OR-1: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (0001) h-BN‖(0001) r-BN; OR-2: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 1 ̄ ) r-BN ; OR-3: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 2) r-BN . The r-BN crystallites within the sp 2-bonded BN grow preferentially in such a way that the (01 1 ̄ 2) r-BN ‖(01 1 ̄ 1 ̄ ) r−BN plane is almost parallel to the grain boundary. Twinning about both the basal planes and the {01 1 ̄ 1} r-BN planes is common within r-BN phase. The c-BN crystallites adjacent to the graphitic BN layer are highly twinned, the {111} c-BN twin planes being parallel to the basal planes of the sp 2-bonded BN. The c-BN phase nucleates on the oriented graphitic BN layer in a semicoherent manner and obeys specific orientation relationships with the hexagonal and rhombohedral phases, namely: OR-I: [ 2 ̄ 110] r-BN ‖[110] c-BN OR-II: [ 2 ̄ 110] r-BN ‖[ 1 ̄ 1 ̄ 0] c-BN (01 1 ̄ 1 ̄ ) r-BN ‖( 1 ̄ 11) c-BN ( interface plane) (01 1 ̄ 2) r-BN ‖(1 1 ̄ 1 ̄ ) c-BN ( interface plane) (0001) r-BN ≈‖(1 1 ̄ 1) c-BN (0001) r-BN ≈‖(1 1 ̄ 1) c-BN They are related by a rotation of 180° about their common axis, [0001] r-BN ‖[1 1 ̄ 1] c-BN , [2 1 ̄ 1 ̄ 0] h-BN ‖[110] c-BN (01 1 ̄ 2) h-BN ‖(001) c-BN (0001) h-BN ‖(1 1 ̄ 1) c-BN In the mainly c-BN region of the films, twins are present about more than one of the sets of {111} c-BN planes. The intrinsic microstructure of the c-BN films and the crystallography between the cubic and graphitic BN phases show that the structure of cubic boron nitride is directly related to the structure of the precursor phases. The atomic structure of an interface depends on the orientation relationship between adjacent c-BN grains and the boundary inclination. The grain boundaries consist of twin boundaries when two adjacent grains are oriented close to the [110] c-BN zone axis and the boundary plane is parallel to the {111} c-BN close-packed planes of both grains. However, a thin layer, 1–2 nm, of sp 2-bonded BN forms between the c-BN grains when the boundary plane inclines a few degrees from the {111} c-BN planes of the adjacent grains.