Mechanical Characterization of Multilayer Thin Film Stacks Containing Porous Silica Using Nanoindentation and the Finite
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Mechanical Characterization of Multilayer Thin Film Stacks Containing Porous Silica Using Nanoindentation and the Finite Element Method Ke Li a, Subrahmanya Mudhivarthi b,c, Sunil Saigal a, and Ashok Kumar b,c a Department of Civil Engineering, b Department of Mechanical Engineering, and c Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, FL 33613, U.S.A. ABSTRACT Novel metal/dielectric material combinations are becoming increasingly important for reducing the resistance-capacitance (RC) interconnection delay within integrated circuits (ICs) as the device dimensions shrink to the sub-micron scale. Copper (Cu) is the material of choice for metal interconnects and SiO2 (with a dielectric constant k = ~ 3.9) has been used as an interlevel dielectric material in the industry. To meet the demands of the international road map for semiconductors, materials with a significantly lower dielectric constant are needed. In this study, the effects of porosity and layer thicknesses on the mechanical properties of a multilayer thin film (Cu, Ta and SiO2)-substrate (Si) system are examined using nanoindentation and finite element (FE) simulations. A micromechanics model is first developed to predict the stress-strain relation of the porous silica based on the homogenization method for composite materials. An FE model is then generated and validated to perform a parametric study on nanoindentation of the Cu/Ta/SiO2/Si system aiming to predict the mechanical properties of the multilayer film stack. INTRODUCTION A number of new organic and inorganic materials are being investigated for their applicability as interlayer dielectric materials [1]. One way to obtain dielectric materials with a low dielectric constant (k) is to introduce air voids into silica (SiO2). However, the introduction of porosity deteriorates the mechanical properties of the dielectric material and thus the mechanical reliability of the entire IC device when undergoing chemical-mechanical planarization processes. Thus, in-depth understanding of the mechanical behavior of this class of low-k materials (i.e., porous silica) and the overall mechanical performance of thin film stacks containing porous silica layers is essential for the reliable application of these dielectric materials. An extensively used technique for characterizing the mechanical behavior of thin films is nanoindentation, whereby properties such as hardness and elastic modulus can be obtained [2]. The applicability of nanoindentation to porous low-k materials, nevertheless, needs further justifications for lack of understanding of pore crushing involved in the indentation process [1]. In this work, a parametric study is performed to investigate the mechanical performance of thin film stacks containing Cu, tantalum (Ta) and SiO2 (solid or porous) as the interconnect layer, the barrier layer, and the dielectric layer, respectively. Porous silica produced by Iskandar et al. [3] is taken as the porous dielectric material. A micromechanics model is first dev
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