A high durable and low-cost mold was fabricated with hot-embossing a sol-gel derived organically modified silicate film using a commercially available grating as a mother mold. Close pattern transfer from the mother mold to the film (daughter mold) was confirmed. The thermooptic coefficient of the daughter mold was measured and found to be unchanged during 9 times heating and cooling cycle. This result implies that the mold has no residual stress and so leads to fine patterning in embossing process. We hot-embossed polymethyl methacrylate (PMMA) films not with the expensive mother mold, but with the daughter mold. More than thirty PMMA grating replicas could be successively fabricated using one daughter mold without cleaning and treating the surface. This is due to the hydrophobicity of the daughter mold. After heating at 250°C for 120 hours, the water contact angle was approximately above 90°, and so the daughter mold did not lose its hydrophobicity. Because of the hydrophobicity, no significant problem that the daughter mold stuck to the PMMA film was observed for the successive embossing. It was confirmed with AFM observation that there was no difference in pattern dimension between the daughter mold and PMMA replicas. Hence, the daughter mold is high durable. Moreover, we measured the transmitted diffraction light power of the fabricated PMMA gratings, and found that there was no difference in the diffraction efficiency of the gratings. Therefore, the daughter mold is suitable for low-cost hot embossing of polymer materials.
Thermally stable grating was fabricated with hot-embossing with the application of sol-gel derived silsesquioxane film. Planar films consisted of phenylsilsesquioxane and methylsilsesquioxane showed thermal stability with respect to the low birefringence, less than 1×10-3, and no intrinsic absorption from 400 to 1700nm for the prolonged heating at 200°C for 10hours. The refractive index of the film was controllable from 1.49 to 1.56 at 632.8nm wavelength with increasing a molar ratio of the phenyl contents from 0.3 to 1.0, where the index controllability was ±0.0005. A short embossing process, less than 15 minutes, was demonstrated. In-situ mode-line measurement for the embossed grating showed change in refractive index under heating and cooling cycle was linear and reversible. A high thermooptic coefficient (dn/dT) of the films from room temperature to 150°C, which was approximately -2×10-4 despite of its composition, was stable after subsequent nine thermal cycles. The measurement at the temperature up to 300°C showed streak propagation in the film and the prolonged heating at 150°C for 700 hours indicated no change in refractive index. This material is considered to be suitable for optical device application, since it showed the combined advantages of the both organic and inorganic materials such as embossing capability and high TO coefficient with thermal stability.