Thickness and sound velocity measurement in thin transparent films with laser picosecond acoustics

A new thin-film optical testing method is described for both thickness and sound velocity of transparent films on opaque substrates with laser picosecond acoustics using the optical pump-probe technique. The theory of excitation and detection of ultrasonic stress pulses for this geometry is presented in detail together with experimental results for sputtered thin films of silica of thickness 200 nm–2 μm on amorphous germanium substrates. Reflectance variations, measured as a function of pump-probe delay time, are characterized by echoes and beating oscillations superimposed on periodic steplike changes. These effects are modeled as a sum of an echo contribution from the stress-induced modulation of the substrate reflectance, an interference contribution from the light reflected by the moving stress pulse in the transparent film, a contribution from the modulation of the light on transmission through this stress pulse, and a contribution from the stress-induced ultrafast vibrations of the film interfaces of order 10−3 nm. The latter contribution arises from thin-film interference effects that represent a novel detection mechanism for surface vibrations in the picosecond regime. Sound velocity and thickness are derived from the data to an accuracy of a few percent, and the photoelastic constant of the transparent film is determined.