Carbon overcoat: Structure and bonding of Z-DOL

Abstract

Thin carbon films (∼10 nm) sputter-deposited on silicon wafers from a graphite target were examined by ESR (electron spin resonance) spectroscopy, and atomic force microscopy. The study revealed that such carbon films are best described as a stack of closely packed spheroidal granules with diameters of several nanometers, with the actual dimension depending on the sputtering condition. The atomic bonding scheme within each individual granule would be either the sp2 type (graphitic) or the sp3 type (diamond like). The granules of the sp3 type host the dangling bonds responsible for the ESR signal. Each spheroid of the sp3 type thus comprises a mantle and a core; the mantle layer is formed when freshly prepared film is exposed to ambient atmosphere and labile molecules (e.g., oxygen or water) diffuse into the spheroid and react with dangling bonds. The mantle layer thus formed becomes a protective shield for the dangling bonds remaining in the core. The density of dangling bonds within the core may be as high as 1 per 100 carbon atoms. When sputter-deposited films were left in an intimate contact with molecules with hydroxyl groups (e.g., water or Z-DOL), an irreversible reaction occurred involving dangling bonds. The reaction is ascribed to a hydrogen atom transfer from the hydroxyl group to a dangling bond in the core and is postulated to be the mechanism of the Z-DOL bonding. The atomic force microscopy examination revealed surface features that were consistent with the stacked spheroid model. The spheroid diameter inferred from the 〈roughness〉rms measurement is in good accord with the dimension estimated from the ESR study.