Ultra low-k dielectric mechanical property characterization

To meet electrical performance requirements, the industry is implementing ultra-low dielectric constant (ULK) materials in the back end of line interconnect structure. ULK dielectrics are inherently weak compared to traditional dielectrics and pose significant challenges to electronic packaging processes and reliability. Accurate mechanical properties are a prerequisite for upfront risk assessments associated with low-k integration using numerical simulations. In this paper, techniques used to characterize ULK dielectric elastic modulus and in-plane/out-of-plane coefficient of thermal expansion will be presented and the data for a candidate ULK dielectric will be summarized. Nanoindentation of ULK films on substrate was used to determine the plane strain modulus. In the direction normal to the film, the temperature gradient of the thermal expansion strain along the film thickness was measured by X-ray reflectivity. In the plane of the film, the temperature gradient of the biaxial thermal stress was obtained by the substrate curvature measurements. A method to deduce Poisson´s ratio of the thin ULK film is proposed using the data from the afore-mentioned characterization techniques.