Thermal stress analysis and characterization of thermo-mechanical properties of thin films on an elastic substrate.
Date of Issue2011
School of Mechanical and Aerospace Engineering
Stress and thermomechanical behavior of thin films deposited atop a substrate upon thermal loading/unloading have been investigated extensively in the past decades, which was partially stimulated by the development of micro- and nanotechnology. However, most of the previous effort in analysis was focused on thin films in their linear elastic deformation range. On the other hand, either very dedicated equipments or complicated procedures were required for the determination of their properties. This dissertation presents a systematic and detailed study of the thermally induced stress in multilayer thin films within both the elastic and elastic-plastic deformation ranges and several approaches to determine the thermomechanical properties of thin films. The elastic analysis based on the linear strain assumption results in the closed-form solutions and approximations (for very thin films). The condition that the film stress is of the same sign is identified for bilayer cases. Subsequently, the investigation is extended into the elastic-plastic deformed films in bilayer structures. Closed-form solutions of the maximum, average and minimum film stresses and curvatures are obtained for plastically deformed films. The difference among the maximum stress, average stress and Stoney stress in films is investigated systematically. In addition, the result of a case study reveals that the yield start point may be estimated as a linear function of temperature in the elastic-plastic deformation range. A simple approach to determine the values of five thermomechanical properties of thin films, namely, the Young’s modulus, coefficient of thermal expansion, yield start stress, strain hardening modulus and Poisson’s ratio, is proposed. The approach is based on the conventional curvature test on bilayer structures upon temperature variation, which can be easily implemented using many conventional techniques. Some simple and generic approaches for characterization of thin films with nonlinear stress versus strain relationship and/or temperature dependent material properties are proposed. In the case of very thin films, analytical solutions are obtained. Approaches and solutions developed here are applied to investigate the moduli of metallic films. The film thickness effect on the modulus of Ag films is observed. The approach developed here is also utilized to identify the critical role of a compressive stress in thin TiO2 layer atop NiTiCu film in the reversible trench phenomenon.
DRNTU::Engineering::Materials::Microelectronics and semiconductor materials::Thin films