The changes in microstructure of a specially prepared boron nitride (BN) film as a function of film depth were studied by high resolution transmission electron microscopy (HRTEM) and other materials analysis tools. These changes were then correlated to the changes in processing parameters during film growth. The analyzed film was fabricated by the four-step ion-assisted deposition procedure known to be effective in film-stress engineering for the formation and retention of a thick cubic BN (cBN) layer with a three-step buffer-layer deposition. In this deposition, the energy of the ions assisting cBN formation was increased stepwise from 200 to 280, and then to 360 eV [S.F. Wong, C. W. Ong, G.K.H. Pang, K.Z. Baba-Kishi, W. M. Lau, J. Vac. Sci. Technol. A 22 (2004) 676]. The nominal thickness of the cBN layer was 650 nm and that for each of the three buffer layers was about 160 nm. Both the HRTEM and electron diffraction results confirmed that the top cBN layer, with a thickness of 643 nm, consisted of cBN grains with a preferred orientation of their c-axis along the film growth direction. In comparison, the three-step buffer layer deposition yielded complex and intriguing microstructures. In the first buffer layer adjacent to the substrate, grains containing sp(2) planes with a preferred orientation of their basal planes parallel to the film growth direction were the main constituents. The increase of ion energy from 200 to 280 eV for the formation of the second buffer layer first led to an enrichment of the concentration of these sp(2) grains with the preferred orientation. Then, bending of some of the sp(2) planes into curved microstructures was evident. The microstructure became very complex and displayed multiple phases including some amorphous structures. The presence of a cBN-like phase was indeed detected by electron energy loss spectroscopy. This complex microstructure persisted until it was replaced by the cBN structure, without abrupt change when the ion energy was increased from 280 to 360 eV for the deposition of the third buffer layer. It is proposed that small grains with cBN-like sp(3) bonding configurations are present in the 2nd and 3rd buffer layers, probably with crystalline domains less than 5 nm and thus difficult for direct detection even by HRTEM. A sufficient accumulation of these cBN nuclei transformed the buffer layer to the cBN layer. (c) 2005 Elsevier B.V. All rights reserved.