A first PECVD process incorporating a silicon oxide precursor alone and then with an organo-silicon precursor with increasing flow while the flow of the silicon oxide precursor is reduced to zero provides a graded carbon adhesion layer whereby the content of C increases with layer thickness and a second PECVD process incorporating an organo-silicon precursor including an organic porogen provides a multiphase ultra-low k dielectric. The multiphase ultra-low k PECVD process uses high frequency radio frequency power just above plasma initiation in a PECVD chamber. An energy post treatment is also provided. A porous SiCOH dielectric material having a k less than 2.7 and a modulus of elasticity greater than 7 GPa is formed.