Oleg Zabeida, Artem Shelemin, Pedro Avila, Jolanta Klemberg-Sapieha, Ludvik Martinu, Polytechnique Montréal, Montréal, Québec, Canada
Polyhedral oligomeric silsesquioxanes (POSS) are 3-D cage molecules with specific structural and chemical properties. These hybrid organic-inorganic molecules possess a rigid 8‑silicon (Si8O12) core to which different organic groups are attached. Their molecular symmetry, rigidity and multifunctionality make POSS-derived materials good candidates for high-performance microelectronic, aerospace, and biomedical applications. POSS is often used as an additive to the polymer matrix to improve its mechanical durability, decrease dielectric loss or reduce water uptake.
In this work we present an ion beam assisted thermal evaporation (IBA-THE) technique developed to deposit thin POSS films for ophthalmic applications. The interaction between the ion beam plasma and the evaporated POSS molecules leads to their partial dissociation and the creation of highly interlinked transparent films in which the cage structure is partially preserved. The addition of oxygen to the ion source working gas leads to rapid etching of hydrocarbon groups evidenced by the appearance of CO and CO2 species in the gas phase. As a result, less organic, more SiO2-like films are deposited. Following a detailed analysis of the effect of experimental parameters on gas-phase processes using residual gas analysis and optical emission spectroscopy, we propose a simplified model of the IBA-THE process.
The mechanical and optical properties of POSS films can be adjusted by varying the of ion beam current, gas composition and evaporation rate. In the case when octa-silane POSS was used as a precursor, the refractive index, n, of films deposited with Ar-only ion beam assistance varied from 1.42 to 1.79. An addition of oxygen resulted in films with n values from 1.46 to 1.55. These films were also more transparent with k values as low as 5x10-4. The optimized hybrid POSS films demonstrated a good adhesion to the glass and plastic substrates, Young’s modulus of 20-40 GPa, and an elastic recovery of 50-60%. The incorporation of POSS films in anti-reflective stacks deposited on plastic substrates resulted in their improved durability and environmental stability.