Interfacial mechanical testing of atomic layer deposited TiO2 and Al2O3 on a silicon substrate by the use of embedded SiO2 microspheres

Abstract

In this paper the authors present a next generation measurement system for interfacial mechanical testing of especially atomic layer deposited (ALD) thin films. SiO₂ microspheres were embedded in 100 and 300 nm thick ALD TiO₂ and Al₂O₃, deposited at 110 °C, 200 °C and 300 °C on a silicon substrate. The embedded microspheres were detached using a fully programmable semi-automatic microrobotic assembly station employed to carry out the lateral pushing and detaching force F (μN) measurement. The area of interfacial fracture A (μm²) was measured using scanning electron microscopy and digital image analysis to calculate critical stress of interfacial fracture σ (MPa). Work W (J) and energy release rate G (J m⁻²) of interfacial fracture were also calculated from the measurement results. Interfacial fracture from the film-substrate interface occurred only for TiO₂ deposited at 200 °C which had a crystalline structure with the biggest grain size, signifying that for all of the other samples, film adhesion was excellent, and significantly better than film cohesion. Quantitatively this means that thin film interfacial adhesion to the substrate was also higher than the values of the critical stresses and the measured energy release rates. Interfacial toughness seems to be related to film thickness and crystallinity in the case of TiO₂, but with Al₂O₃ the interfacial toughness seems to increase with the deposition temperature. The method presented in this paper is generic, and can be applied for the evaluation of interfacial mechanical properties, such as adhesion, between any various film-substrate-sphere system of choice.