Influence of crystal properties on the absorption IR spectra of polycrystalline AlN thin films

Abstract

In this paper Fourier transform infrared (FTIR) transmission spectroscopy is presented as a technique suitable for the routine morphological study of polycrystalline AlN thin films. We have compared FTIR spectra and X-ray diffraction (XRD) patterns of wurtzite polycrystalline AlN films deposited on Si(001) by RF reactive sputtering of an Al target in an N 2/Ar mixture. The preferred orientation and crystal quality of the films were investigated by XRD. The FTIR spectra showed absorption bands due to vibrational modes of AlN bonds, in particular, the A1(TO) mode at 612 cm −1 and the E1(TO) mode at 672 cm −1. The ratio of integrated areas of A1(TO) and E1(TO) absorption bands is related to the preferred orientation of the films, which ranges from films with purely (00.2)-oriented grains to films with grains tilted by approximately 60° to the surface normal. Energy shifts of the IR absorption modes were observed in relation to biaxial residual stress present in the films. As residual stress varied from tensile to compressive, E1(TO) peak energy increased, while A1(TO) peak energy decreased. It is concluded that FTIR transmission spectroscopy has the potential to be used as a routine characterization technique for in-line process control of polycrystalline AlN thin films.