The introduction of new dielectrics into silicon chip interconnection technology, to improve the electrical performance of ultra large-scale integration (ULSI), is marked by continuous revisions to meet the International Technology Roadmap for Semiconductors (ITRS) projection. Amorphous a-SiOC:H ($k$=2.9) deposited by plasma-enhanced chemical vapor deposition (PECVD) from linear precursors is now in scale up towards production. Using other precursors like cyclic siloxanes,$k$ value is reduced to 2.5 at least. The main way to reduce the dielectric constant is to introduce porosity into the film. In this work, a two-step porogen approach is followed to perform extreme low (EL) $κ$($k$<2.5) deposition. Firstly, a dual-phase thin film is deposited by PECVD using two advanced precursors: decamethylcyclopentasiloxane (D5) to create a a-SiOC:H matrix and a sacrificial organic precursor. Within the matrix, the organic precursor generates organic inclusions. In a second step, the organic phase is removed by a suitable curing to induce porosity. Various types of organic precursors are investigated and thin films are then characterized. Incorporation and removal of organic molecules from a-SiOC:H material are closely studied using infrared spectroscopy. Using cyclohexene oxide as organic precursor (porogen), κ value of the porous dielectric is measured at 2.2. Nitrogen adsorption isotherms measurements prove the cured material porosity. This work clearly demonstrates the ability of achieving an EL with a PECVD porogen approach using D5 as matrix precursor and new organic precursors as porogen.