Nano-hybridization techniques based on the pyrolytic reactions of organo-soluble metallic precursors dissolved in poly(amic acid)s followed by spontaneous precipitation of metal/inorganic nano-particles in solid polyimide (PI) films is facile and effective for functionalization of PI optical and electronic materials. The organic/inorganinc PI nanohybrid materials, which were recently developed by the authors, having a variety of functionalities such as a) high refractive indices, b) low refractive indices, c) controlled thermo-optical property and its anisotropy, d) high polarizing property, and e) high thermal conductivity are reviewed with future prospects on their advanced opto-electronic applications.
Novel polyimide (PI)/epoxy hybrid material for single-step photo-patterning of optical waveguides was prepared by
blending of a semi-aromatic fluorinated poly(amic acid silylester) (PASE), a cycloaliphatic epoxy compound, and a
photo-acid generator. A large refractive index change (δn, > 0.01), which is sufficient for multi-mode optical waveguides,
was obtained between the polymer films prepared with and without UV irradiation. The refractive index change was
generated by cationic reaction between the silylated carbonyl ester groups of PASE and epoxy rings, which was initiated
by UV irradiation and promoted by successive thermal curing. The difference in molecular structures, which results in
the refractive index changes, were characterized by FT-IR measurements, and it was clarified that the films with and
without UV irradiation showed PASE and PI structures, respectively. These films exhibited high thermal stability higher
than 230°C, which are desirable for waveguide fabrication for optical inter-connects and lightwave circuits. Using this
hybrid material, channel-type optical waveguides were successfully fabricated by the single-step photo-patterning
procedure without development by aqueous or organic solvents, which is more facile and economical for waveguide
mass-fabrication.
The light emitting property of zinc oxide (ZnO) nanoparticles, which were spontaneously precipitated and dispersed in fluorinated polyimide films has been studied because this hybrid material exhibits interesting properties and could be widely applied in optoelectronics and photonics. Polyimides (PI) provide high thermal and chemical stability and outstanding electrical properties. In addition, ZnO presents excellent optical properties due to its wide band gap (3.37 eV) at room temperature and large exciton bonding energy (60 meV), which can be used in light-emitting diodes, transparent electrodes, and piezoelectric devices. By adding small amount of Zinc compounds (5 mol %), either Zinc hexafluoroacetylacetonate dihydrate or Zinc nitrate hexahydrate, to precursor solutions of polyimides followed by thermal curing at 350~390°C, the green light emission at ca. 520 nm of polyimides has been significantly enhanced by 10 to 15 times respectively.
When zinc concentration excesses the saturation level, light emission decreases and emission peak was shifted to higher wavelengths due to the aggregation of ZnO. This can be explained by the quantum confinement mechanism and the interaction between the oxygen of ZnO and PI functional groups. Zinc compound precursors and curing temperature and atmosphere affect the light emitting behavior which will be discussed in detail.
A new ArF matrix based on poly(vinylsulfonamide) has been developed. Sulfonamides, whose pKa values are comparable to those of phenols, can be used as acidic functional groups in the design of chemically amplified resist (CAR) and aqueous base developable resist. Various poly(N-alkyl vinylsulfonamide)s were prepared (R=H, CH2CF3, Pr, 1-adanmantyl), and showed high transparency at round 193 nm region. Depending on the substituents, the dissolution rates of films in a 2.38 wt% aqueous tetraethylammonium hydroxide solution were varied from 500 to 0.0035 nm/s. The tetrahydropyranyl (THP) protected poly(vinylsulfonamide) was prepared and the deprotection of THP was easily occurred when the photoresist containing a photoacid generator was exposed to UV light, followed by post-exposure baking.
Thermo-optic coefficients dn/dT for TE- and TM-polarized lights (dnTE/dT and dnTM/dT, respectively) were measured for aromatic polyimide (PI) films formed on Si Substrates. The values of dn/dT for the average refractive index (dnav/dT) is in a range of -52 to -87 ppm/K, and amorphous PI films with large nav have large |dnav/dT|, which is well accounted by the temperature derivative of the Lorentz-Lorenz equation. On the other hand, the absolute values of dnTE/dT were significantly larger than those for dnTM/dT even for the PI films with very small polarization dependence in refractive index (Δn; in-plane/out-of-plane birefringence). This result cannot be explained by the temperature derivative of the Vuks equation, which represents that the large polarization dependence in dn/dT is caused by the large Δn. Polarization dependence in dn/dT for the PI films with small Δn is caused by the temperature dependence in large stress birefringence. Based on these results, large thermal expansion and small stress birefringence (small residual stress or small polarization anisotropy) is essential for achieving large |dnav/dT| and small polarization dependence in dn/dT. To demonstrate this assumption, dn/dT measurements were carried out for PI films derived from a fluorine-containing dianhydride and aliphatic diamines, which exhibit loose chain packing and small polarizability anisotropy. As a result, these PI films exhibit small polarization dependence in dn/dT, and relatively large dnav/dT compared to the aromatic PI films with almost the same nav.
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