Methods and apparatus for preparing a porous low-k dielectric material on a substrate are provided. The methods optionally involve the use of ultraviolet radiation to react with and remove porogen from a porogen containing precursor film leaving a porous dielectric matrix and further exposing the dielectric matrix to ultraviolet radiation to increase the mechanical strength of the dielectric matrix. Some methods involve activating a gas to create reactive gas species that can clean a reaction chamber. One disclosed apparatus includes an array of multiple ultraviolet sources that can be controlled such that different wavelengths of light can be used to irradiate a sample at a time.