In this chapter, application of optical interferometry for the characterization of thin-film adhesion to the substrate is discussed. The thin-film system is configured as one of the end mirrors of a Michelson interferometer and oscillated with an acoustic transducer from the substrate side. The oscillation causes sinusoidal displacement of the film surface around the initial (neutral) position, and the interferometer detects its amplitude as the relative phase difference behind the beam splitter. When the driving frequency of this oscillation is tuned to a range where the film-substrate interface is dominantly oscillated, the elasticity of the interface can be analyzed from the oscillation amplitude. The principle of this method is straightforward but in reality, fluctuation of the initial phase (the relative phase corresponding to the initial film position) compromises the signal. A technique known as the carrier fringe method along with spatial frequency domain analysis is employed to reduce the noise associated with the initial phase fluctuation. The possibility of the present method to analyze the so-called blister effect on thin-film adhesion is discussed.
Part of the book: Optical Interferometry