Microstructure and nanomechanical and optical properties of single- and multi-layer carbon films synthesized by radio frequency sputtering

Submicrometer-thick hard carbon films were deposited on smooth Si(1 0 0) substrates by radio frequency sputtering. To avoid the development of high residual stresses during film growth under energetic ion bombardment, films were synthesized by either sputtering alternating ultrathin layers of soft and hard amorphous carbon (a-C) or by compounding a-C films with small amounts of nitrogen during deposition. Hardness and elastic modulus measurements were obtained from indentation experiments performed with a surface force microscope and a nanoindenter using the slope of the unloading curve and the continuous stiffness method. The thickness and hardness of a multi-layer film consisting of five bilayers of soft/hard ultrathin a-C layers were found to be equal to ∼115 nm and ∼28 GPa, respectively, and the corresponding values of a single-layer nitrogenated amorphous carbon (a-CNx) film (deposited under conditions similar to those of the hard ultrathin a-C layers) were found to be equal to ∼222 nm and ∼20 GPa. The microstructure and interface roughness of the films were examined by X-ray diffraction (XRD) and X-ray reflectivity (XRR), respectively, and the optical properties by spectroscopic ellipsometry (SE). XRD results showed that the multi-layer film is amorphous, and that some crystalline phases are present in the single-layer a-CNx film. XRR studies revealed that the density of the soft and hard a-C layers and the relatively thick a-CNx film is equal to 2.2, 2.5 and 2.39 g/cm3, respectively. According to the SE results, the multi-layer a-C film is dense and rich in sp2 (∼70 at.%) carbon bonds, whereas the single-layer a-CNx film exhibits high optical absorption at low photon energy.