A series of low- k films was exposed to UV-assisted curing (UV curing) with excimer lamps. The influence of the UV-curing wavelength, the UV-curing time, and the maximum pretreatment temperature were investigated. A mechanical, chemical, and optical characterization of this set of experiments is presented. It is revealed that the exposure to UV curing with 172 nm had sufficient energy to abundantly photodissociate Si-C H3 groups and to shrink the films. In turn, the elastic modulus was enhanced. As a side effect caused by the photodissociation of Si-CH groups, the content of Si-OH and Si-H moieties increased. Longer wavelengths (222 and 308 nm) showed less-drastic effects on low- k films because they do not provide sufficient energy to photodissociate Si-CH groups. This fundamental difference existing between different UV-curing wavelengths is evidenced by the optical characterization. Furthermore, this work also reveals the effect of pretreatment temperature on UV curing. Low pretreatment temperatures are required to keep enough photochemical reactivity and matrix mobility.