A series of low-k films was exposed to UV-assisted curing ÍUV curingÍ 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-CH 3 groups and to shrink the films. In turn, the elastic modulus was enhanced. As a side effect caused by the photodissociation of Si-CH 3 groups, the content of Si-OH and Si-H moieties increased. Longer wavelengths Í222 and 308 nmÍ showed less-drastic effects on low-k films because they do not provide sufficient energy to photodissociate Si-CH 3 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.Nanoporous organosilicate glasses ÍOSGÍ have attracted considerable attention in technologies such as catalysis, 1 optical sensors, 2 lasers, 3 biotechnology, 4 separation, 5 and on-chip interconnects. 6-8 Film deposition techniques include plasma-enhanced chemical vapor deposition ÍPECVDÍ and spin coating of precursor suspensions prepared by the sol-gel process. 7 Both approaches exploit the use of sacrificial molecules, or porogens, to generate a pore network in the films. 7 The pore network is crucial in many of the applications but at the same time brings many challenges. An example is the application of OSG films as insulating media in on-chip interconnects of integrated circuits. 6-8 The pore network decreases the relative permittivity Ídielectric constant or k valueÍ of the material and in turn the interconnect signal delay. However, the pore network deteriorates the mechanical properties of the films, complicating their integration during aggressive processes such as chemical mechanical polishing.To enhance the poor mechanical properties, ultraviolet-assisted curing ÍUV curingÍ with broad-band lamps has been proposed. 9-19 It combines photochemical and thermal processes. The success of the UV curing is reported to be linked to enhanced matrix cross-linking, rearrangement of the bonding structure to stiffer configuration, or to densification of the matrix. The variety of the reported UV-curing effects suggests that the involved mechanisms must depend on the low-k material and on the UV lamp used.The first studies on UV curing of low-k films have been focused on the results or advantages of such processing. However, there is scarce fundamental research on which UV wavelengths and elemental processes are responsible for the material modification. To gain information on this issue, we have recently characterized the optical properties of PECVD low-k films in the energy region of 2-9 eV. Purged UV spectroscopic ellipsometry with different...