Low density materials, such as hydrogen silsesquioxane (HSQ), can offer lower dielectric constants. With HSQ, a low value of K can be achieved if the density of Si-H bonding is maintained at a high level and the formation of -OH bonds and absorption or creation of water in the film is minimized. In this work, we have studied the use of hydrogen plasma to improve the quality of HSQ. In addition, the effects of N-2 and O-2 plasma post-treatments on HSQ are investigated. The leakage current of HSQ decreases as the H-2 plasma treatment time is increased. However, the leakage current of HSQ increases as the N-2 and O-2 plasma treatment time is increased. A model is proposed to explain the role of hydrogen in HSQ. The hydrogen passivates the surface of porous HSQ. The H-2 plasma treatment provides additional hydrogen passivation of the HSQ. On the other hand, the N-2 and O-2 plasma treatments reduce the hydrogen passivation of HSQ. As a result, both the leakage current and dielectric constant increase as the N-2 and O-2 plasma treatment time is increased. Finally, HSQ with H-2 plasma treatment is used as the intermetal dielectric in a multilevel interconnection structure. Consistent with our model, the via resistance decreases with increasing H-2 plasma treatment time.