Characterizing the porosity in thin titanium films by laser-acoustics

Porosity may be harmful for the properties of thin films. Depositing dense films is often the aim of the technological development, although there are also applications requiring adjusting a special amount of pores in the film. In thin films, the pore dimension is in the sub-micrometer range so that detecting the pores is difficult. The laser-acoustic technique is shown to be able to indicate an enhanced porosity in thin films. It is based on surface acoustic waves and yields Youngʹs modulus of the film. The elastic modulus of the material is distinctly influenced by porosity. It reduces with increasing porosity and depends also on the shape of the pores. Test series of titanium films (approx. 2 μm thick) with different porosity were deposited on silicon. The laser-acoustic tests yielded elastic moduli for the films that were up to 40% lower compared to the value of the bulk material. Carefully prepared cross-sections of the films were investigated by scanning electron microscopy. The reducing Youngʹs modulus was found to correlate with increasing porosity. A pore volume fraction up to approximately 20% was estimated. A theory for Youngʹs modulus depending on porosity, also taking into account the pore shape, was employed to deduce information on the pore structure from the laser-acoustic results and the scanning electron micrographs. The laser-acoustic method was applied to an extended batch of films deposited with different technologies combining electron evaporation, DC-vacuum arc discharge and pulsed high current arc. It has proved a quick test method to evaluate the deposition of dense films.