The reduction of the dielectric constant of interlayer insulating films in ultra large scale integrated circuits is an important factor in improving resistance-capacitance delay. A "porous diamond" film, composed of nanoscale diamond particles, is proposed as a material with a low dielectric constant, and is prepared by spin coating a 5% diamond colloidal solution mixed by 4 nm diamond nanoparticles in purified water. High-resolution scanning electron microscope observations show that the film contains nanoscale pores. The mechanical strength and adhesion of the porous diamond films were improved by forming the chemical bonds between diamond nanoparticles and also between the nanoparticles and the substrate. The chemical bonds were created by introducing a bi-functional silane coupler such as hexachlorodisiloxan molecules followed by annealing at 300 degreesC. A low dielectric constant of 1.63, estimated from the refractive index, was obtained by using high purity diamond nanoparticles after the reinforcing process by hexachlorodisiloxan. The porous diamond films are expected to have a higher thermal stability and a superior mechanical strength compared with other porous low dielectric constant materials. (C) 2003 American Institute of Physics.