Effect of stress-induced phase transformation on nanomechanical properties of sputtered amorphous carbon films

Abstract

The nanomechanical properties of radio-frequency sputtered ultrathin carbon films measured by surface force microscopy were correlated to the carbon bonding structures analyzed by x-ray photoelectron and Auger electron spectroscopy. The films consisted of amorphous carbon (a-C) comprising both trigonal (sp2) and tetrahedral (sp3) carbon hybridizations. The sp3 carbon content in the a-C materials of films with nanohardness of 19–40 GPa was found to be in the range of 22%–28%. From variations of the binding energy of Ar 2p electrons and the sp3 carbon content with the film Ar content, a stress-induced phase transformation from sp2 to sp3 carbon was determined at compressive residual stress of about 14 GPa. Film hardening occurs due to material densification, which is controlled by the intensity of the energetic Ar+ bombardment and the flux ratio of incoming C atoms and Ar+ ions during film growth. The results of this study elucidate the underlying hardening mechanism in ultrathin sputter-deposited carbon films.