Examination of Film Thickness Dependence on Acoustic Impedance of Gold and Chromium Thin Films by Scanning Acoustic Microscopy

Thickness induced changes in acoustic impedance of gold (Au) and chromium (Cr) thin films are studied with scanning acoustic microscopy (SAM). Thin films are produced by thermal evaporation technique on BK7 glass substrates with varying thicknesses between 40 nm to 200 nm. In acoustic impedance (AI) mode, the microscope generates two-dimensional acoustic impedance maps of the thin films and micrometer resolution helps determining the surface defects on these films. On the other hand, acoustic impedance value is found to increase as thickness increases for both Au and Cr thin films indicating increased elasticity, therefore, hardness. The mean and standard deviation values of acoustic impedance of Cr thin films were found as 1.901 ± 0.050 MRayl for 40 nm, 1.905 ± 0.045 MRayl for 80 nm, 1.943 ± 0.049 MRayl for 120 nm, 1.964 ± 0.049 MRayl for 160 nm and 1.987 ± 0.052 MRayl for 200 nm. The mean and standard deviation values of acoustic impedance of Au thin films were found as 1.725 ± 0.026 MRayl for 80 nm and 1.954 ± 0.047 MRayl for 200 nm. This success achieved by SAM, demonstrates its potential in monitoring thin film surfaces even with very small thicknesses.