Field inversion in local oxidation of silicon regions of a CMOS integrated circuit is generally avoided thanks to a field implant. In this paper the feasibility of an implant-free isolation is studied. The parasitic charge in the interlayer dielectrics is attributed to the diffusion during nitride deposition of hydrogen which react with carbon in the underlying plasma oxide. Using a PECVD nitride as the upper layer of the inter-layer dielectrics allows to maintain the leakage current at an acceptable level.