Resistance of siloxane-based, low-dielectric-constant (low-k) dielectrics against heat and moisture stress is clarified. The organo-silica-glass (OSG) and the silicon-oxycarbide are shown to be the most reliable: the k-values are stable even after a heating test at 650 degreesC and a pressure cooker treatment for 100 h. This stability is high enough to ensure the low-k property throughout fabricating multilevel interconnects and long-term reliability after the fabrication. This is shown to be due to the stability of Si-CH3 bonds and Si-CHn-Si bonds incorporated in the OSG and the silicon-oxycarbide. The stability of the OSG in real low-k interlevel dielectric structure was also demonstrated using four-level interconnect test devices. The low-k property still remains even after the reliability tests, showing that the low-k interlayer dielectric structure is sufficiently resistant to heat and moisture stresses.