Photosensitive polyimide/silica hybrid optical materials: Synthesis, properties, and patterning

In conventional ionic salt photosensitive polyimides, large volume shrinkage during imidization would be occurred due to eliminating pendant photosensitive moieties, such as 2-methyl acrylic acid 2-dimethylamino-ethyl ester (MDAE). In this study, the volume shrinkage of photosensitive poly(4,4′-(hexafluoroisopropylidenediphthalic anhydride)-co-oxydianiline) (6FDA-ODA)/MDAE was largely reduced by photocrosslinking MDAE with a coupling agent and the silica domain in the hybrid materials. The used coupling agents were 3-methacryloxypropyl trimethoxysilane (MPTMS) or (4-vinylphenethyl)trimethoxysilane (VPTMS). The coupling agent and the silica domain are designed primarily for reducing the volume shrinkage and enhancing the thermal properties, respectively. The retention of MDAE in the prepared hybrid films is supported by X-ray photoelectron spectroscopy (XPS) and thickness variation during curing process. The silica domain in the hybrid materials from TEM analysis was in the range of 10-50 nm, which was formed by the coupling agent and tetramethoxysilane. The silica domain significantly enhanced the thermal properties of the prepared hybrid films in comparison with parent fluorinated polyimide, including the glass transition temperature and coefficient of thermal expansion. The prepared hybrid materials also exhibited reduced refractive index and optical loss by increasing the silica. The SEM diagram suggested the prepared photosensitive hybrid materials could obtain lithographical patterns with a good resolution. These results indicate that the newly prepared photosensitive polyimide/silica hybrid materials may have potential applications for optical devices.