The quality factor (Q) of inductors on silicon (Si) is limited by the series resistance of the metal at low frequency and by the substrate resistivity at high frequency. Oxide is generally used to isolate the useful signal of the inductor from the lossy Si substrate. However, stoichiometric silica (SiO2) is processed at high temperature which restricts the possibility of post-complementary metal-oxide semiconductor integration, and due to the large thermal mismatch with Si, thick oxide films introduce considerable stress. In this work, we apply electrochemically formed porous Si (PS) and intrinsic amorphous Si (i-a-Si:H) deposited at low temperature (250°C) in an approach asan isolation bilayer for planar inductors on Si for rf integrated circuits. An improvement of morethan 45% in Q was measured at a frequency of 6.4GHz for 1.6nH inductor incorporating PS andi-a-Si:H films. The experimental results presented indicate that these low-temperature materials are promising for the isolation of rf devices on low-resistivity Si.