The effect of porogen loading on the stiffness and fracture energy of brittle organosilicates
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Ting Y. Tsui Chemical Engineering Department, University of Waterloo, Nanotechnology Institute, Waterloo, Ontario N2L 3G1, Canada
Joost J. Vlassaka) School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (Received 4 April 2008; accepted 16 September 2008)
Integrating porous low-permittivity dielectrics into Cu metallization is one of the strategies to reduce power consumption, signal propagation delays, and crosstalk between interconnects for the next generation of integrated circuits. The porosity and pore structure of these low-k dielectric materials, however, also affect other important material properties in addition to the dielectric constant. In this paper, we investigate the impact of porogen loading on the stiffness and cohesive fracture energy of a series of porous organosilicate glass (OSG) thin films using nanoindentation and the double-cantilever beam (DCB) technique. The OSG films were deposited by plasma-enhanced chemical vapor deposition (PECVD) and had a porosity in the range of 745%. We show that the degree of porogen loading during the deposition process changes both the network structure and the porosity of the dielectric, and we resolve the contributions of both effects to the stiffness and fracture energy of the films. The experimental results for stiffness are compared with micromechanical models and finite element calculations. It is demonstrated that the stiffness of the OSG films depends sensitively on their porosity and that considerable improvements in stiffness may be obtained through further optimization of the pore microstructure. The cohesive fracture energy of the films decreases linearly with increasing porosity, consistent with a simple planar through-pore fracture mechanism.
I. INTRODUCTION
Nanoporous OSG coatings with relative dielectric constants smaller than 2.5 are considered for use as an intra-metal dielectric in future generations of advanced integrated circuits. Their low dielectric constant is achieved by introducing a substantial fraction of nanometer-sized pores into a hybrid organic-inorganic matrix.1–3 Subtractive porosity levels of up to 45% can be reached by changing the loading of porogen (pore generator) during the film deposition process. While introducing porosity may be beneficial for the dielectric properties of the coatings, porosity does have a negative impact on the mechanical properties of the coatings, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0005 J. Mater. Res., Vol. 24, No. 1, Jan 2009
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which are critical to the integrity of the dielectrics during integrated circuit fabrication.1–3 Nemat-Nasser and Hori reviewed a large body of theoretical work on modeling the effect of porosity on the bulk elastic response of porous materials.4 Micromechanics models,5–7 which combine the exact Eshelby inclusion solution8 and various assumptions on approximating the deformation interaction between pores, c
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