Influence of polymer porogens on the porosity and mechanical properties of spin coated Ultra Low k dielectrics

Structural properties deduced from Fourier Transform Infrared spectroscopy, ellipsometric porosimetry (EP), X-ray reflectivity and grazing incidence small-angle X-ray scattering (GISAXS) are correlated with dielectric and mechanical properties of Ultra Low k layers prepared by spin coating. Different sacrificial polymers were used as porogens and different porogen loadings were tested. After crosslinking and porogen removal by thermal treatment, the GISAXS images indicate a fairly isotropic distribution, with a slight in-plane correlation of the pores. Dielectrics having the same dielectric constant close to 2.2, have almost the same pore volume fraction ~ 40%, but the azimuthally averaged GISAXS profiles display porogen-dependent shapes, leading to different sizes and inter-pore correlations. For a given porogen, the increase of the volume fraction from ~ 10% to ~ 50% causes k to decrease from 2.7 to 1.9, with a slight increase in the pore size. Thermal curing, supplemented or not by UV illumination, hardly changes the porous structure, as observed by EP and GISAXS. Finally, the influence of the pore size on the elastic properties appears to be weak in the range studied (2.5–10 nm), while changes in the mechanical properties are governed mainly by the pore volume fraction and skeleton cross-linking.