Anisotropic Elastic Properties of Low-k Dielectric Materials
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Anisotropic Elastic Properties of Low-k Dielectric Materials A.A. Maznev1, A. Mazurenko1, G. Alper1, C.J.L. Moore1, M. Gostein1, Michelle T. Schulberg2, Raashina Humayun2, Archita Sengupta2, Jia-Ning Sun2 1 Philips Advanced Metrology Systems, Natick MA 01760 2 Novellus Systems, Inc., San Jose, CA 95134 ABSTRACT A non-contact optical technique based on laser-generated surface acoustic waves (SAWs) was used to characterize elastic properties of two types of thin (150-1100 nm) low-k films: more traditional non-porous organosilicate glass PECVD films (k=3.0) and novel mesoporous silica films fabricated in supercritical CO2 (k=2.2). The acoustic response of the non-porous samples is well described by a model of an elastically isotropic material with two elastic constants, Young’s modulus and Poisson’s ratio. Both parameters can be determined by analyzing SAW dispersion curves. However, the isotropic model fails to describe the SAW dispersion in the mesoporous samples. Modifying the model to allow a difference between in-plane and out-of plane properties (i.e., a transversely isotropic material) results in good agreement between the measurements and the model. The in-plane compressional modulus is found to be 2-3 times larger than the out-of plane modulus, possibly due to the anisotropic shape of the pores. Elastic anisotropy should therefore be taken into account in modeling mechanical behavior of low-k materials.
INTRODUCTION While low-k dielectrics are being pursued by the semiconductor industry for their electrical characteristics, their mechanical properties such as hardness and elastic modulus are equally critical to provide structural integrity to the interconnect structures [1]. Techniques currently accepted in the industry for measuring mechanical properties (e.g. nanoindentation) are based on the assumption that film properties are isotropic. In thin film deposition, however, the substrate presents an asymmetric condition that may lead to structural anisotropy resulting from onedimensional shrinkage, preferential alignment of polymer chains or ordering or orientation of the pores. Consequently, elastic properties of thin film materials may be different for the directions parallel and perpendicular to the plane of the substrate. Nanoindentation is not adequate for characterization of such anisotropic materials. Methods based on surface acoustic waves (SAWs), on the other hand, have proven capable of determining anisotropic elastic properties of thin films [2]. In this work, we used a non-contact technique utilizing laser-generated SAWs to measure the elastic properties of two classes of low-k films: PECVD organosilicate glass (k~3.0) and a novel mesoporous silica material (k~2.2). We will show that in the former case the measurements are well described by the isotropic model of the elastic properties of the low-k material. In the latter case, the material is found to exhibit strong elastic anisotropy, with the inplane compressional modulus 2-3 times larger than the out-of-plane modulus. We will discuss t
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