Understanding adhesion failure in low-k dielectric stack during Chemical-Mechanical Polishing
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Understanding adhesion failure in low-k dielectric stack during Chemical-Mechanical Polishing F.Iacopi1,2, D.Degryse1,3, I.Vos1, M.Patz4 and K.Maex1,2 1
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium; 2 E.E. Dept., Katholieke Universiteit Leuven, Belgium; 3 Mech.Eng.Dept., Universiteit Gent, Belgium; 3 JSR Corporation, Tsukuba, Japan
ABSTRACT Low-k dielectrics are currently starting to replace SiO2 in advanced on-chip interconnects. Being low surface energy materials, a major concern for the integration of such dielectrics in Cu damascene processes is given by adhesion failure at the interfaces of the low-k films with cap or liner layers during Chemical Mechanical Polishing (CMP). A cross-comparison between interface fracture energies as measured through four-point bending, CMP experiments and stress fields computed by Finite Element Modeling, shows that it is not possible to predict failures by setting a ‘universal’ threshold on adhesion strength. In particular, we report on the critical role of CMP –induced shear stress on the onset of interface debonding, and the way the shear load is transmitted to the critical interfaces. The top-down load transmission depends on the position of the weak interfaces, and on the way the intermediate films in the stack can confine such load through their thickness and mechanical (elastic and plastic) properties. INTRODUCTION Mechanical failure of low-k –based dielectric stacks upon Chemical-Mechanical Polishing (CMP, [1]) is one major challenge for advanced interconnects manufacturability. Low-k dielectric films show invariably poor mechanical properties. The elastic modulus of mesoporous dielectrics can be as low as 2 GPa [2], and low-k films are typically low surface energy films showing low adhesion strength to other layers [3]. Multilevel metallization for advanced Cu/low-k interconnects requires repeated CMP processes, therefore ensuring mechanical stability of low-k –based stacks during manufacturing is extremely challenging. Extensive studies have been dedicated to the extraction of the key parameters and corresponding boundaries governing failure mechanisms upon CMP. Nevertheless, the achievement of a more detailed understanding has been limited by the large number of parameters determining both the effective CMP load and the mechanical failure mechanisms of low-k –based interconnects. In a previous work we demonstrated a good correlation between interfacial adhesion strength of the low-k films to other layers in the stack and the extent of mechanical failure (at macro- and micro-scale) during CMP [4]. On the other hand, we pointed out that requirements on interfacial adhesion strength differ significantly depending on the position of the considered ‘weak’ interface (i.e. interface where a low-k film is involved). The present study aims to a more detailed understanding of such requirements by supporting experiments with Finite Elements (FE) simulations. The focus lies on the properties of the interconnect structures and their effect on mechanical stability for a fix
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