Elastic modulus. A suitable quantity for characterization of thin films

Young's modulus is often underestimated in its relevance for thin film characterisation. It represents the stiffness of the material that correlates with the atomic structure. The microstructure of thin films may distinctly deviate from that of the bulk materials and may depend on the special deposition conditions. For instance Young's modulus of refractory carbide and nitride films depends on the stoichiometry. For carbon films the modulus can vary in the wide range from < 100 GPa up to about 1000 GPa. Boron nitride films have similar properties. Their Young's modulus may be in the range between 35 GPa and 600 GPa. Furthermore, for hard coatings the elastic modulus often correlates with the hardness that is difficult to measure reliably for submicrometer thick films. This suggests Young's modulus to be an excellent indicator for testing, classifying, and optimising thin films. A laser-acoustic measuring technique was developed to measure Young's modulus of thin films with thickness do wn to 100 nm. It uses short laser pulses for generating wide-band ultrasonic surface waves. The measurement is performed on plane surface with a dimension of minimum 5 mm x 10 mm. Special specimen preparation is not required. Results obtained for diamond-like carbon and boron nitride films are presented, which show the film modulus to depend on deposition conditions as ion energy, angle of incidence of the plasma plume, substrate temperature, or atom-ion ratio. This reveals the method to be very suitable for optimising the deposition technology. The subplantation model is applied to explain the variation of the properties of the diamond-like carbon films.