Spin-coated periodic mesoporous organosilica thin films - Towards a new generation of low-dielectric-constant materials

Periodic mesoporous organosilica (PMO) thin films have been produced using an evaporation-induced self-assembly (EISA) spin-coating procedure and a cationic surfactant template. The precursors are silsesquioxanes of the type (C2H5O)(3)Si-R-Si(OC2H5)(3) or R'-[Si(OC2H5)(3)](3) with R = methene (-CH2-) ethylene (-C2H2-), ethene (-C2H4-), 1,4-phenylene (C6H4), and R' = 1.3,5-phenylene (C6H3)- The surfactant is successfully removed by solvent extraction or calcination without any significant Si-C bond cleavage of the organic bridging groups R and R' within the channel walls. The materials have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), and Si-29 and C-13 magic-angle spinning (MAS) NMR spectroscopy. The d-spacing of the PMOs is found to be a function of R. Nanoindentation measurements reveal increased mechanical strength and stiffness for the PMOs with R = CH2 and C2H4 compared to silica. Films with different organic-group content have been prepared using mixtures of silsesquioxane and tetramethylorthosilicate (TMOS) precursors. The dielectric constant (k) is found to decrease with organic content, and values as low as 1.8 have been measured for films thermally treated to cause a self-hydrophobizing bridging-to-terminal transformation of the methene to methyl groups with concomitant loss of silanols. Increasing the organic content and thermal treatment also increases the resistance to moisture adsorption in 60 and 80%-relative-humidity (RH) environments. Methene PMO films treated at 500 degrees C are found to be practically unchanged after five days exposure to 80% RH. These low dielectric constants, plus the good thermal and mechanical stability and the hydrophobicity suggest the potential utility of these films as low-k layers in microelectronics.