Abstract
The elastic property of the film-substrate interface of thin-film systems is characterized with an opto-acoustic method. The thin-film specimens are oscillated with an acoustic transducer at audible frequencies, and the resultant harmonic response of the film surface is analyzed with optical interferometry. Polystyrene, Ti, Ti-Au and Ti-Pt films coated on the same silicon substrate are tested. 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. Experiments indicate that both the surface-treated and untreated specimens of all film materials have resonance in the audible frequency range tested. The elastic constant of the interface corresponding to the observed resonance is found to be orders of magnitude lower than that of the film or substrate material. Observations of these resonance-like behaviors and the associated stiffness of the interface are discussed.
Highlights
Thin-film coating is used in a wide variety of applications ranging from micro-electro-mechanical systems (MEMS) to surface treatment of mechanical systems for wear-resistance enhancement.The recent trend indicates that reduced film thickness in a number of applications leads to better performance
A thin-film system specimen was oscillated from the rear surface of the substrate in a range of audible frequencies, and the harmonic response of the film surface displacement was measured with a Doppler vibrometer and a Michelson interferometer
The choice of the acoustic frequency allowed us to oscillate the film and substrate as rigid bodies, so that the harmonic response represented the dynamics of the interface
Summary
Thin-film coating is used in a wide variety of applications ranging from micro-electro-mechanical systems (MEMS) to surface treatment of mechanical systems for wear-resistance enhancement. It is important to evaluate the adhesion strength using probing frequencies within a frequency range that thin-film systems are subject to and to examine if the interface exhibits resonance-like behaviors. Considering the above issues, we have devised an opto-acoustic method to characterize the film-substrate interface in a range of audible frequencies. By sweeping the driving frequency of the transducer, it is possible to explore for resonance-like behavior of the interface Using this method, we studied the effect of a pre-coating surface treatment. We describe the opto-acoustic method, present the experimental study on the effect of the surface treatment for the silicon-based thin-films and discuss the results of the experiments focusing on the resonance behaviors observed in both the untreated and treated specimens. A possible elastic mechanism behind the resonance-like behavior observed in the audible frequency is discussed
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