The structures and properties of hydrogen silsesquioxane (HSQ) films produced by thermal curing

Abstract

The structures and properties of hydrogen silsesquioxane (HSQ) films produced by curing were studied in the temperature range of 240–340 °C for various curing times. The experimental results show that the transformation of the cage structure to the network structure is the major reaction in the studied temperature range. The cage–network transformation can be explained by two-stage zero order kinetics. The rate constant of the first stage is 10–35 times more than that of the second stage. The activation energy and frequency factor of the cage–network transformation are 64.63 kJ mol⁻¹ and 2.76 × 10⁴ s⁻¹ for the first stage while those of the second stage are 38.59 kJ mol⁻¹ and 4.11 s⁻¹, respectively. The difference is probably because the network structure of the second stage limits the structural transformation and results in a small frequency factor. The porosity of the cured HSQ films increases rapidly with curing time in the first 10 min and then slowly for the remaining time. The FE-SEM (Field Emission-Scanning Electron Miscroscope) result suggests the formation of nano-pores in the cured film. The evolution of porosity is probably due to the outgassing of the reaction side-product (SiH₄), the trapped solvent (4-methylpentan-2-one) or the cage/network transformation. The last two factors contribute significantly as shown by the refractive index results of the cured films. The increasing film thickness with increasing curing time and temperature indicates the evolution of porosity in the HSQ film.