Adhesion Thin Ductile Films Using Stressed Overlayers and Nanoindentation
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Adhesion Thin Ductile Films Using Stressed Overlayers and Nanoindentation M.J. Cordill, N.R. Moody1, D.F. Bahr School of Mechanical and Materials Engineering Washington State University, Pullman, WA 1 Sandia National Laboratories, Livermore, CA ABSTRACT Differently stressed films of tungsten on silicon dioxide have been studied to determine the interfacial fracture toughness and the Mode I fracture energy release rate of tungsten on glass. Tungsten films with a low compressive stress (less than 1GPa) had nanoindentation tests performed on them to induce buckling. Using mechanics based models and the dimensions of the buckles the fracture energy release rate and the phase angle of loading (Ψ) were calculated to be between 3.8 and 13 J/m2. By varying the residual stress in the film it was possible to examine regions of pure shear (Mode II) interfacial fracture as well as mixed mode interfacial fracture toughness of this system. A similar tungsten film was then used as stressed overlayer on sputtered Pt films on silicon dioxide to determine the fracture energy release rate. Nanoindentation was required to induce buckling, as the overlayer alone did not cause spontaneous buckling. The stressed overlayer method and nanoindentation were used to determine the interfacial toughness of the Pt/silica system to be 1.4 J/m2. INTRODUCTION Adhesion of thin films onto substrates is of great importance for both the microelectronics industry and the development of MEMS devices (microelectro mechanical systems). The adhesion of these films can be determined using a variety of methods, including scratch testing and nanoindentation. Scratch testing drags a tip across a surface to induce delamination in the form of spalling or buckles [1, 2]. Nanoindentation [2-4] uses a nanoindenter to induce delamination at the interface of the film in the form of blisters. Another method used to determine the adhesive properties of thin films is the stressed overlayer method [2, 5], which uses highly stressed films deposited on top of the film system of interest. The stress in the overlayer can delaminate the underlying films in the form of straight and telephone cord buckles, provided that there is enough strain energy in the overlayer. The stressed overlayer method is easy to implement and an array of thin film systems can and have been tested [6-9]. This technique uses mechanics based models to calculate the interfacial fracture toughness of the film system using dimensions of the buckles or blisters produced. If the stressed overlayer does not have enough stress to delaminate the underlying films, nanoindentation can be used in conjunction with the stressed overlayer to induce delamination. The coupling of these two techniques is useful in the fact that the same adhesion mechanics can be used and interfaces with high interfacial fracture toughness values can be tested. When nanoindentation is used to induce delamination the fracture energy release rate is calculated using the dimensions of the indentation blister. The indentation blisters
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