CHARACTERIZATION OF TRANSPARENT AND OPAQUE THIN FILMS USING LASER PICOSECOND ULTRASONICS

Abstract This paper describes new applications of laser picosecond ultrasonics to the nondestructive testing of transparent and opaque thin films of thickness in the range 20 nm to 10 μm. The optical pump-probe technique is used to excite ultrasonic stress pulses with acoustic wavelength of order 20 nm and detect time-dependent reflectance variations. Samples of practical importance such as silicon oxide anticorrosion coatings or thin film chromium electrodes are investigated. Transparent silicon oxide films on stainless steel substrates give reflectance variations characterized by beating oscillations superimposed on step-like changes. These effects are modelled as a sum of an interference contribution from light reflected by the moving stress pulse in the transparent film and a contribution from the ultrafast stressinduced film surface displacement of order 10-3 nm, detected for the first time. We show how both the longitudinal sound velocity and the film thickness can be derived, provided tha its refractive index is known. Results for echo detection in opaque films are also presented. The resolution for relative reflectance variations of order 10-7 allowed five echoes from an aluminium-sapphire interface to be resolved. An echo from the Cr-Cr interface of a double layer electrode of chromium from an amorphous silicon image sensor was also detected. This indicates the presence of a carbon polymer layer with thickness of atomic order left over after reactive ion etching.