Optical characterization of sputtered aluminum nitride thin films – correlating refractive index with degree of c-axis orientation

Aluminum Nitride (AlN) is a well-known compound piezoelectric material with high Complementary Metal-Oxide Semiconductor process compatibility. Previous results show that the piezoelectric coefficient correlates with the c-axis orientation of AlN. In this work, the optical properties of reactively sputtered AlN thin-films have been investigated to find a relation with the structural properties of the film (i.e. c-axis orientation) using spectroscopic ellipsometry. The results show that for almost the same film thickness, highly c-axis orientated films have higher refractive indices compared to films with low c-axis orientation or amorphous layers. A relationship between (002) peak intensity measured with x-ray diffractometry and refractive index is shown. At 546 nm wavelength, the refractive index decreased from 2.15 for films with high (002) peak intensity, to 2.11 for films with about half the peak intensity, and further down to 1.90 for films without preferred orientation. Additionally, with the same sputtering conditions, the variation of the film thickness seems to have no significant effect on the refractive index. The results of AlN thin-film mass density obtained from x-ray reflectivity measurements are consistent with refractive index measurements. The mass density of AlN thin-films with high c-axis orientation that resulted in a higher refractive index is 2.99 g/cm−3, compared to 3.23 g/cm−3 for epitaxial grown AlN layers. This value decreased to 2.82 g/cm−3 for films with poor c-axis orientation.