Toughness and Contact Behavior of Conventional and Low- k Dielectric Thin Films
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Toughness and Contact Behavior of Conventional and Low-k Dielectric Thin Films Robert F. Cook, Dylan J. Morris and Jeremy Thurn1 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, U.S.A. 1 Advanced Mechanical Technology, Seagate Technology Bloomington, MN, 55435, U.S.A. ABSTRACT A comprehensive indentation fracture mechanics framework is established that allows the fracture properties of thin films to be determined. The framework is composed of four stress-intensity factors characterizing the stress fields arising from (i) elastic contact, (ii) wedging, (iii) residual elastic-plastic mismatch and (iv) pre-existing film stress. The amplitudes of the stress-intensity factors depend on the deformation properties of the film and vary throughout the indentation cycle. The toughness values of a PVD alumina, for which (iii) and (iv) are dominant, and a low-k film, for which (i), (ii) and (iv) are dominant, are evaluated. INTRODUCTION Dielectric films are used in a wide variety of advanced technologies: microelectronic, photonic, magnetic storage and micro-electromechanical systems (MEMS). In all cases, the dielectric performs a (frequently primary) role as a structural element, in addition to providing appropriate insulating, optical or permeability characteristics. As most dielectrics are brittle, knowledge of film toughness (controlling all aspects of film fracture) and sharp contact behavior (the dominant mode of crack initiation) is then of crucial importance for advanced device design. Indentation methods using sharp probes have been used for a considerable time to characterize materials at small scales—with the attendant material economy and ease of specimen preparation and testing—and are thus an obvious choice for mechanical characterization of dielectric films. Such methods have been used to determine plastic properties (for example, by correlating the mean supported contact stress, the hardness, with yield stress in metals [1]), fracture properties (by relating indentation crack lengths to toughness in glasses, ceramics and semiconductors [2, 3]) and elastic properties (by relating indentation contact stiffness to modulus in a wide range of materials [4]). This paper describes a variety of controlled-flaw, indentation-based techniques used to determine the toughness of two sets of dielectric films. The first are physical vapor deposited (PVD) aluminum oxide films of order 10 µm thick and use macroscopic methods, including conventional indentation and optical observation of indentation-induced film and interface cracks, to estimate film toughness and interfacial fracture resistance. The second are plasma-enhanced chemical vapor deposited (PECVD) low-k films of order 1 µm thick and use ultra-microscopic depth-sensing indentation (DSI) methods, "nanoindentation," and low voltage scanning electron microscopy (SEM) of cracks, to estimate film toughness. New models are presented that describe the variation of crack length with indentation load and permit fi
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