A class of ultra low dielectric constant polymethylsilsesquioxane (PMSQ) films with T-8(Me4NO)(8) polyhedral oligomeric silsesquioxanes (T-8 POSS) as double-effective porogen was studied. Through the Me4NO- groups of T-8 POSS attacking the Si-O-Si chains of PMSQ, the POSS will be connected to the PMSQ crosslink system. POSS has the cage structure, which acted as the closed pores (<= 1.5 nm). On the other hand, the Me4NO- groups served as the sacrificial template. When they decomposed after annealing, the open pores were then left in the films. The introduction of T-8 POSS can greatly decrease the dielectric constant of PMSQ and effectively improve its mechanical strength owing to T-8(Me4NO)(8) interconnected with PMSQ. These continuous and smooth films were prepared by spin-coating with thickness in the range of 60-200 nm. The dielectric constant of the films could be controlled by adjusting the proportion of porogen. These films showed good mechanical strength and ultra low dielectric constant. In particular, a POSS/PMSQ film with ultra low dielectric constant of 1.6 and modulus of 7 GPa had been prepared on silicon wafer by spin-coating. (C) 2010 Elsevier B.V. All rights reserved.