Opto-acoustic technique to investigate interface of thin-film systems

Abstract

The adhesion strength of nano-scale thin-film systems is evaluated with an opto-acoustic technique. The thin-film specimens are oscillated with an acoustic transducer at a range of audible frequency and the resultant film-surface displacement is detected with an optical interferometer. For each film material, a pair of specimens is prepared; one is coated on a silicon substrate after the surface is treated with plasma bombardment, and the other is coated on an identical silicon substrate without a treatment. For comparison, a bare silicon specimen of the same dimension is tested. All coated specimen show greater film-surface displacement than the bare silicon specimen in the entire frequency range, confirming that the detected oscillation represents the differential displacement at the interface. In some cases, the specimen coated on the plasma treated substrate shows greater film-surface oscillation than the one coated on the untreated substrate. The frequency dependence of the oscillation indicates resonance-like behavior at a frequency orders-of-magnitude lower than the resonance estimated from the bonding strength.