Porous inorganic thin films from bridged silsesquioxane sol–gel precursors

• Porous inorganic films are synthesized from phenyl-bridged silsesquioxanes.
• An inorganic network results at curing temperatures > 500 °C with 35% pore volume.
• The refractive index decreases to 1.244 at 600 nm for curing temperature of 700 °C.
• Inorganic films present pores of 2 nm average diameter in close packing structure.
• Inorganic films from bridged silsesquioxanes have small uniformly-arranged pores.

A sol–gel process was exploited to produce porous inorganic thin films from phenyl-bridged silsesquioxanes. The evolution of both structural and optical properties of the starting hybrid sol–gel films were monitored, during synthesis and successive thermal curing steps, by Fourier transform infrared (FT-IR) spectroscopy, differential thermal analysis (DTA), thermogravimetric analysis (TGA) and spectroscopic ellipsometry (SE). Involved chemical species, structural and chemical modifications were identified when thermal treatments at increasing temperatures in the range of 60–800 °C were applied to the hybrid films. The progressive formation of a crosslinked silica network, template elimination and film densification were observed, resulting in completely inorganic porous thin films of low refractive index. The distinctiveness of this system directly comes from the extremely controlled and uniform dispersion of the porogen at a molecular level, which is intrinsic to the bridged silsesquioxane precursor choice. A quantitative porosity analysis was performed by environmental ellipsometric porosimetry (EEP), studying inorganic film optical and mechanical properties under different relative humidity conditions. A transmission electron microscopy (TEM) in-situ characterization of porosity size and distribution confirms the presence of a spatially structured organization of pores of a few nanometers in diameter.