UV Curing of Oxides

In the case of oxides, the exposure of a soft film, such as the as-deposited one, to a UV source is an alternative top-down process of inducing a densification of the inorganic matrix. This is a quite general effect whose extent depends on the chemical conditions of the synthesis, for instance, the degree of condensation and dimension of the oxide clusters in the sol. The pH of the sol is also very important because it also affects the extent of condensation of the film structure upon deposition. 

These condensation reactions work not only for silica alkoxides, but also for most of the metal alkoxide precursors. The photolysis of alcohols leads to bond 

The UV-induced photolysis of water gives atomic hydrogen and hydroxyl radicals and the highly reactive superoxide 2 [22], whereas photoinduced alcoholysis produces cleavage of the C—H, O—H, and C—C bonds, with the formation of different reactive species. Hydroxyl radicals are also formed in the presence of nitric acid as a catalyst 

The same condensation process is also activated upon UV irradiation of sol-gel films that could be done under different conditions, such as in vacuum, in controlled atmosphere, and in air. In general, it is observed, for instance, by in situ FTIR, that photoirradiation produces a decrease in the silanols and, in turn, an increase in the intensity of the sihca (or other oxides such as titania) absorption bands. The condensation of the films is also reflected by change in several properties, such as refractive index, thickness, hardness, and energy sur-fece. On the other hand, it has also been observed that the structural changes induced in the silica network are not limited to condensation, and even more intimate rearrangements of the silica tetrahedra from unstable folded linear Si02 structures to stable linear Si—O—Si bonds can be activated [23]. 

Direct photopatterning of sol-gel oxide films is also possible by chemical modification (chelation) of metal alkoxide precursors with an organic ligand lithography on the films is achieved by selective exposure of the films to UV light. The films respond as positive resists, and the unexposed areas are removed 

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