Is stress necessary to stabilise sp3 bonding in diamond-like carbon?

The role of compressive stress in producing sp(3) bonding in diamond-like carbon is of interest both technologically and scientifically. Stress limits the maximum thickness of adherent films, and it is desired to produce much thicker films for protective coatings and for making micro-electromechanical systems. Stress is important theoretically, because it is often linked to the deposition process. A strong correlation between macroscopic stress and sp(3) fraction in diamond-like carbons has been noted, particularly for tetrahedral amorphous carbon (ta-C). However, a survey of data shows that a given stress produces films with sp(3) contents between 20 and 85%, while for a given sp(3) content, stresses between 2 and 19 GPa have been found. We propose that the main cause of stress is ion bombardment. and that a low energy of only 20 eV/ion is needed to produce films with an sp(3) content over 70%. We discuss the various models linking stress and the sp(3) fraction in ta-C. The role of densification vs. compressive stress in stabilising sp(3) bonding is also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.