Decohesion of Thin Films from Ceramic Substrates
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FISHER, and A.G. EVANS
Center For Advanced Materials, Lawrence Berkeley Laboratory University of California, Berkeley, CA 94720 ABSTRACT Decohesion of thin films from ceramic or semiconductor substrates is strongly influenced by internal stresses in films and stress concentrations from edges or flaws as well as by interfacial fracture energy. Residual stresses can cause spontaneous delamination, splitting and curling of films under tension or delamination, buckling and spalling of films under residual compression, even with good interfacial bonding. Delamination behavior is considered using simple fracture mechanics models, supplemented with preliminary measurements of interfacial fracture energies. Formation conditions largely control internal stresses in films; whereas fracture energies are dictated by interfacial chemistry and mechanical factors such as plasticity. INTRODUCTION Failure of systems having ceramic-metal interfaces is usually brittle or semi-brittle and can be treated using fracture mechanics. This permits quantitative evaluation of behavior and correlation of chemical and mechanical aspects such as sensitivity to cleanliness, processing, or residual stresses. From a fracture mechanics perspective, factors controlling strength are categorized under: stress state, fracture toughness, and inititating flaw population. The stress state depends strongly upon internal stresses and stress concentrations from edges and from elastic modulus mismatch as well as applied forces. The fracture energy depends upon crack location; it reflects mechanical or microstructural aspects such as plasticity or crack deflection as well as interfacial chemical and structural factors controlling crack tip behavior. Flaw populations are difficult to characterize and impart a statistical aspect to strength; important sources include incomplete interfacial bonding, residual pores, microcracks around reaction products or inclusions, and surface damage from contact or machining. Inhomogeneous plasticity can cause stress concentrations as well as initiate cracks. This paper addresses the role of several of these factors in thin film delamination. Conventional strength or pull tests (e.g. the "Scotch tape test") are hard to interpret and often poorly reproducible. Thus, simpler limiting situations are emphasized for which film delamination is less sensitive to flaw size, and other techniques are applied. BACKGROUND Crack extension is driven by the strain energy release rate which is: G = (-dW/dA + dU/dA)
where W and U are, respectively the external free energy and internal strain energy; and A, the crack area. This quantity can be compared to thermodynamic or chemical energies. It is also related to crack stress concentration factors although the relationship is complicated when elastic moduli differ across the crack plane, an important issue with combined tensile and shear loading [1]. @Present address, Materials Group, Cal. Santa Barbara, CA 93106
Dept of Mechanical Engineering, Univ of
Mat. Res. Soc. Symp. Proc. Vol. 54. " 198
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