Testing ultra-thin films by laser-acoustics

Evaluating ultra-thin films of a few nanometers thickness is still a challenge for mechanical testing. The capability of the laser-acoustic technique was investigated to measure the Young's modulus of diamond-like carbon films with thickness down to 5 nm. The elastic modulus has proved to characterize an important mechanical property of hard films which are used for wear protection. It can non-destructively be determined by the laser-acoustic technique that is based on measuring the dispersion of surface acoustic waves. This acoustic wave mode is very sensitive to thin films which was previously shown for diamond-like carbon films with thickness down to 60 nm. Diamond-like carbon films with thickness from 5 to 30 nm were deposited by the pulsed arc technique on pure silicon and on silicon coated with a metal layer of 80 nm thickness. The demand for the accuracy of the measuring instruments and for the measuring conditions, such as bandwidth and sample dimension, is discussed for testing such as ultra-thin films.