Nondestructive dynamic evaluation of thin NiTi film adhesion

Thin layer materials may be considered as composites of the substrate and deposited film. The degree of stress transfer between the two components, i.e. the adhesion, will influence the dynamical response of the thin layer composite characterized by its resonance frequency. A semi-empirical adhesion parameter γ was defined which varies between 1, indicating perfect adhesion, and 0, indicating no adhesion. Resonance measurements were carried out on the Ni₅₀Ti₅₀/SiO₂/Si system, employing a high-resolution mechanical vibration technique. To confirm that the parameter γ truly reflects the quality of the adhesion, NiTi films of different thicknesses were sputter-deposited on a Si substrate and 1 - γ, reflecting the degree of deviation from pure elastic coupling through the interface, was measured. It was found that NiTi/Si yields γ values of about 0.8, reflecting partial adhesion. Annealing improves the adhesion of NiTi films on Si. This improvement of the adhesion was followed in situ.