Structure and elastic properties of amorphous silicon carbon nitride films

Amorphous SiC_xN_y films with various compositions were deposited by ion-beam sputtering. The bonding characteristics, Young's modulus, and density of the films were investigated using x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, surface acoustic wave spectroscopy, x-ray reflection, and molecular-dynamics (MD) simulations. It was observed that the Young's modulus decreases from 260±20 to 85±12 GPa while the density decreases from 3.45±0.2 to 2.3±0.3 g/cm³ as the carbon content of the films increases from 0% to 68%. FTIR and XPS spectra indicate an increasing proportion of double and triple bonds with higher carbon content of the films. These experimental results were compared with the Young's moduli and the infrared spectra obtained from density-functional-based MD simulations. Also for these model calculations the Young's modulus drops from 237±54 to 109±25 GPa as the carbon content increases from 0% to 69%. It can be seen that the formation of C=C, C=N, C≡C, and C≡N bonds, together with the occurrence of terminating nitrogen atoms for the films with higher carbon content, are responsible for the degradation of the sp³ network, and therefore for a lower Young's modulus and density.