Thin-film transformations and volatile products in the formation of nanoporous low-k polymethylsilsesquioxane-based dielectric

The thermal transformation of spin-cast thin films to produce nanoporous low-k dielectric layers has been investigated using polymethysilsesquioxane (PMSSQ) for the low-k matrix and polymethylmethacrylate-co-dimethylaminoethylacrylate (PMMA-co-DMAEMA) as the porogen which is volatilized to leave nanopores in the matrix. Surface analysis methods, including time of flight-secondary ion mass spectrometry and x-ray photoelectron spectroscopy, and thin film analysis by small-angle neutron scattering revealed the kinetics of matrix crosslinking, while thermal desorption mass spectrometry showed the evolution of gaseous reaction products from porogen and matrix during the complex chemical transformations which occur with thermal cycling from 100 degrees C to 450 degrees C. Matrix crosslinking occurs primarily at lower temperatures (100-225 degrees C), while porogen diffusion and decomposition begins somewhat above 200 degrees C, leading to phase separation which creates the final nanoporous structure. Since matrix and porogen reaction kinetics have some overlap, relative kinetics can be important: e.g., matrix crosslinking proceeds more rapidly for PMSSQ precursors with high Si-OH content cf. low SiOH content, with implications for the morphology of porogen-derived nanostructure. As surface species transform (matrix crosslinking) and disappear (porogen volatilization), their complements are seen in the gas phase as reaction and decomposition products. Porogen decomposition is ligand selective, in that the N-containing ligand of DMAEMA is volatilized at considerably lower temperatures (similar to 200 degrees C) than that (similar to 400 degrees C) for the remaining species (the PMMA ligand and the common backbone for both PMMA and DMAEMA). (c) 2005 American Vacuum Society.