Shigeru Takahara

Photochemistry of Naphthalimide Photoacid Generators

The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.

Photoalignment Films of Polyesters with Photoreactive Main Chain

The photoreactive thin films of poly(oxyalkyleneoxy-1,4-phenylenediacryloyl), a PDA-n polyester (n=2–5), were prepared. Their photoreactions and the liquid crystal (LC) photoalignment by irradiation of linearly polarized UV light (LPL) were investigated. The PDA-n polyesters have photoreactive sites in the polymer main chain and not in the side chain which is different from poly(vinyl cinnamate) (PVCi), a well-known photoalignment polymer. These PDA-n thin films show different photoreactivities depending on the number of methylene units, n. Odd-numbered PDA-n polyesters with higher photosensitivity than even-numbered PDA-n polyesters show a dichroism due to irradiation with LPL which results in a homogeneous alignment of the nematic LC.

Mechanistic investigations of the photosensitized reactions of iron arene complexes

Abstract The decomposition of cyclopentadienyliron(II) cumene hexafluorophosphate ([CpFe(Cum)]PF 6 ) was investigated by singlet and triplet sensitization. [CpFe(Cum)]PF 6 quenched the excited state of various sensitizers, and decomposition of [CpFe(Cum)]PF 6 efficiently took place in CH 2 Cl 2 . The quenching rate constants of fluorescence or TT absorption of the sensitizers by [CpFe(Cum)]PF 6 were close to the diffusion rate constants. The rate constants were independent of the exo- or endo-thermicity of the photoinduced electron transfer process assumed to take place. However, they were dependent on the singlet and triplet excitation energies of [CpFe(Cum)]PF 6 and the sensitizers. Thus, the major quenching process would be the singlet and triplet energy transfer. In the case of using 3,3′-carbonyl- bis (7-diethylaminocoumarin) (KC-DA) as a visible-light-sensitizer, the decomposition of [CpFe(Cum)]PF 6 is inefficient, and a photochemically unstable intermediate was generated.

Performance of vinyl ether cross-linkers on resist for 193-nm lithography

A three-component positive working resist composed of an acrylate polymer, vinyl ether cross-linker, and photoacid generator was evaluated for vacuum UV lithography. The vinyl ether cross-linker using tricyclodecyl group of alicyclic compound has a high transparency. This resist system shows a relatively high sensitivity and has a good profiling by irradiation using 193 nm light from an ArF excimer laser. Moreover, we found that these cross-linking type resists have advantages with respect to its resist properties such as the acid diffusion and the dry etching resistance.

Novel dual‐mode photoresist based on decarboxylation by photogenerated base compound

The copolymers containing a carboxyl group were used as the two-component photopolymers in combination with dimethyl 4-(o-nitrophenyl)-2,6-dimethyl-1,4-dihydro-3,5-pyridinedicarboxylate (NMHP) as a photobase generator. Under irradiation by 365 nm light, NMHP is converted to dimethyl 4-(o-nitrosophenyl)-2,6-dimethyl-3,5-pyridinedicarboxylate (NMP). NMP acts as a base catalyst for the thermal decarboxylation of a carboxyl group in the polymer. The decarboxylation in the polymer provides a negative-working photoresist under the additional flood-exposure of 365 nm light before development. This process is called the photoassisted contrast (PAC) process. NMHP acts as a dissolution inhibitor for the aqueous base development of the polymers containing a carboxyl group but NMP promotes the dissolution of the polymers. This change in the solubility inhibition of NMHP offers a positive-working photoresist without the process of post-exposure baking (PEB) and the PAC process. Therefore, NMP, the photoproduct of NMHP, produces dual-tone-mode photoresists with the copolymers containing a carboxyl group.

Size Control of CdS Quantum-confined Ultrafine Particles by Photocatalytic Polymer Coating

CdS quantum-confined ultrafine particles (Q-ufp) were prepared in colloidal solutions in the presence of styrene monomer under photoirradiation. By means of transmission electron microscopy analysis, it has been experimentally verified for the first time that the prepared CdS Q-ufp were coated with polystyrene layer, and the particle size distribution was controlled by selecting the wavelength of the irradiating laser light: shorter wavelength light irradiation gave smaller CdS Q-ufp. A mechanism for particle size control through photocatalytic polymer coating is discussed to explain the experimental results; the particle size distribution or corresponding UV-vis absorption spectra were simulated using the proposed model. As a result, the oxidation potential of styrene monomer, relative to the highest occupied molecular orbital (HOMO) or the top of the valence band of CdS Q-ufp which depends on the particle size, is important for electron transfer necessary for initiating the polymer coating process.

Tetrahydropyranyl-protected poly(p -hydroxystyrene) containing vinyl ether compound for three-component photopolymers

Poly(p-hydroxystyrene)s (PHSs) partially protected by tetrahydropyranyl (THP), PPT-x, containing 2,2′-bis(4-(2-(vinyloxy)ethoxy)phenyl)propane as a crosslinking agent were used as three-component photopolymers in combination with diphenyliodonium 9,10-dimethoxyanthracene-2-sulfonate (DIAS) as a photoacid generator. The PPT-x with a higher THP-protecting ratio was deprotected at higher temperatures than the PPT-x with a lower THP-protecting ratio. The conversions of vinyl ether compound in the photopolymer films were not related to the THP-protecting ratios in PPT-x and increased with increasing baking time. Under different THP-protecting ratios and prebaking temperatures, the PPT-x showed varying lithographic behavior between a positive-working mode and a negative-working mode. These three-component photopolymer solutions of PPT-x have good preservation stability.

Water‐dispersible microgel modified with methacryloyl groups by ionic bonding on the surface and the photopolymer microgel

A novel water-dispersible reactive microgel, which had a diameter of 40–90 nm, was synthesized for photopolymer materials. The microgels have segments with substituted ammonium groups, to provide water solubility, in their polymer networked structure. It has unsaturated groups connected to the quaternary nitrogens by ionic bonding (I-type microgel). The I-type microgel was compared with one that has methacryloyl groups connected with the quaternary nitrogens of the microgel by covalent bonding (C-type microgel). The I-type microgels were able to separately control the modified amount of quaternary nitrogen and methacryloyl group. In the presence of 2,4-diethylthioxantone as a photoinitiator and pentaerthritol triacrylate as a crosslinker, the photopolymer containing the C-type or I-type microgels had sensitivity high enough for practical use. Not only the amount of the methacryloyl group of the microgel but the amount of the quaternary nitrogen affected the sensitivity and the rate of polymerization of the water-dispersible photopolymer containing the I-type microgels. Copyright