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U3.8.1

Fracture patterns in thin films and multilayers Alex A. Volinsky University of South Florida, Department of Mechanical Engineering, Tampa FL 33620 USA [email protected] Dirk C. Meyer, Tilmann Leisegang, Peter Paufler Institut für Strukturphysik, Fachrichtung Physik der Technischen Universität, D-01062 Dresden, Germany ABSTRACT While there are many stress relief mechanisms observed in thin films, excessive residual and externally applied stresses cause film fracture. In the case of tensile stress a network of through-thickness cracks forms in the film. In the case of compressive stress thin film buckling is observed in the form of blisters. Thin film delamination is an inseparable phenomenon of buckling. The buckling delamination blisters can be either circular, straight, or form periodic buckling patterns commonly known as telephone cord delamination morphology. While excessive biaxial residual stress is the key for causing thin film fracture, either in tension, or compression, it is the influence of the external stress that can control the final fracture pattern. In this paper we consider phone cord buckling delamination observed in compressed W/Si and TiWN/GaAs thin film systems, as well as spiral and sinusoidal though-thickness cracks observed in Mo/Si multilayers under 3-point high-temperature bending in tension. INTRODUCTION Thin films can support high levels of residual stress (up to several GPa), which are typically higher in compression compared to tension. Regardless of the residual stress sign, it causes substrate bending, and the resulting curvature can be used to calculate stress level in a thin film using Stoney’s equation [1]. At higher levels of residual stress, or when in addition, external stress is applied, thin film fracture can occur. In case of residual tensile stress throughthickness cracking, film delamination, or even substrate cracking is observed [2]. Thin films buckle, delaminate and spall from the substrate when in compression. In a general, simplified form the strain energy release rate, G, in a stressed film, regardless of the stress sign is: G=Z

σ 2f h Ef

(1),

where σf is the stress in the film, h is the film thickness, Ef is the modulus of elasticity, and Z is a dimensionless cracking parameter. More accurately, the energy release rate averaged over the front of advancing isolated crack is: G = g (α , β )

π (1 − ν 2 )σ 2f h 2E f

(2),

where g(α,β ) is a function of the Dundurs parameters α and β, and can be found in [3-4]. Film fracture or delamination is observed when the strain energy release rate exceeds the film (Gf) or the interfacial (ΓI) toughness, respectively (G>Gf, or G>ΓI). One can avoid these types of failures

U3.8.2

by either reducing the film thickness, or the stress. Practically, the film thickness is easier to control. For a given stress level, there is a certain critical film thickness, at which failures are observed. This paper describes repeatable periodic fracture patterns observed in thin films and multilayers with tensile and compressive residu