Development of low-k materials is critical for further improving the performance of integrated circuits. In this work, a novel type of fluorine-doped ultra-low-k porous SiCOH films has been produced. The chemical composition and bond configurations of the nanometer-scale porous films were determined using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. A low dielectric constant of 2.15 was achieved with an elastic modulus of 4.84GPa which is significantly larger than the minimum requirement of 4GPa for ultra-large-scale integrated circuit inter-layer dielectric applications. The cage/net Si-O-Si ratio method has been proposed to elucidate the influence of molecular structure on the mechanical properties of the film. The breaking and remaking of certain bonds under UV irradiation results in the formation of increased amounts of the net Si-O-Si molecular configuration which plays a major role in increasing the elastic modulus of the film. Furthermore, it was found that increase in elastic modulus is proportional to the duration of irradiation and an increase of 20% can be achieved after 4h of UV irradiation. The methodology and approach described in this paper can be further followed to study the mechanical properties of ultra-low k materials under various annealing and porosity conditions, and to use fluorine-doping and UV irradiation processes for optimum ultra-low k materials in application.