Highlights

Nucleation of Ultrananocrystalline Diamond Films on Si

Scientific Achievement

Ultrananocrystalline diamond (UNCD) films are of interest because of their exceptional mechanical, tribological, electrochemical, and biocompatible properties.  TEM has been used to characterize the microstructure and chemical bonding in UNCD films grown using a low-pressure heat-assisted bias-enhanced nucleation and growth (BEN-BEG) process.  The films have nanoscale surface roughness, high growth rate, uniform nano-sized crystalline diamond grains and a very low content of sp² bonded carbon atoms, which lead to them exhibiting promising tribological properties.  We have used energy-filtered TEM (EFTEM) imaging and HREM to carry out direct mapping of the chemical bonding and atomic arrangement at the interface between the UNCD and the substrate.

Elemental maps and chemical bonding maps of carbon and silicon have been extracted from series of EFTEM images of the UNCD/Si interfaces.  In order to prevent damage, experimental parameters were established for minimum exposure time and maximum signal-to-noise ratio.  Several background subtraction techniques were tested that enabled us to obtain maps of the sp² and sp³ bonding of carbon adjacent to the interface with the Si substrate.  The near-edge fine structure of the Si-L transition enabled Si, SiC and SiO₂ to be distinguished, and within the limits of measurement sensitivity the formation of SiO₂ could be excluded at the interface.  Preferential regions of sp³ bonding close to the interface were identified as nucleation sites for the formation of UNCD.  Based on our TEM data we were able to propose a growth mechanism for the UNCD films on the bias-pretreated substrate that included the following steps: (1) silicon etching and the formation of silicon (111) oriented triangular facets produced by ion bombardment during the initial hydrogen plasma cleaning process; (2) formation of amorphous carbon and textured graphite in the valleys of the Si substrate and formation of the SiC phase at the peaks of the Si substrate; (3) preferential nucleation of sp³-bonded diamond clusters on the Si-C phase; (4) UNCD film growth from the sp³-bonded regions nucleated at the peaks of the Si surface triangular profile.

Significance

We have demonstrated that cross-sectional HREM in conjunction with chemical bonding mapping are powerful tools for studying the interfacial chemistry of UNCD films on a Si substrate, and can also be applied to studies of other film growth.  Increased knowledge of the atomic structure and interfacial chemistry at the UNCD/Si interfaces has allowed us to understand the growth mechanisms of the BEN-BEG UNCD film and thus to synthesize UNCD films with desired mechanical and tribological properties for MEMS/NEMS devices.  This work has been published in Appl. Phys. Lett. 92, 133113 (2008).  The experimental procedures and algorithms established during this study will be used to extend elemental and chemical mapping from two to three dimensions.

Performers

X. Y. Zhong, B. Kabius, J. Hiller, Y. C. Chen, O. Auciello (Argonne-MSD)