Internal matrix structure of low-κ mesoporous silica and its relation to mechanical properties

Poor mechanical and thermal properties are the chief problems in implementing mesoporous low-kappa materials in future integrated circuits. Much work has been done on measuring these physical properties of some candidate low-kappa materials; however, there exists no work in the literature tying the internal matrix properties of mesoporous dielectrics to their bulk mechanical properties. Mesoporous silica films were deposited by us over a wide range of porosities, pore size distribution, structure, interconnectivity and chemistry using a binary solvent sol-gel process and by sintering. A detailed pore size and structure characterization was done by using ellipsometric porosimetry (EP) followed by the measurement of the elastic modulus of these films. Both films consisted of mainly two types of pore geometries which were tied to the size of the pores. Sintered films show a narrower pore size distribution with an increased population of micropores in comparison with as-deposited films. We have correlated the elastic properties to the internal matrix structure of the silica pore network and shown that they depend strongly on the film porosity and the pore size distribution in the film. We propose that sintering helps in improving the mechanical strength by modifying the pore size distribution to incorporate a higher fraction of cylindrical micropores in the film. The contribution of the internal solid matrix was minimal as it was found to be perfect in our as-deposited films and our sintering conditions did not modify the backbone structure significantly. Lastly, finite-element simulations for two different pore geometries are shown and support the argument that cylindrical pores show much better mechanical properties than tubular pores for the same porosity. (C) 2004 Elsevier B.V. All rights reserved.