PECVD deposited porous SiCOH with ultralow dielectric constant has been successfully integrated as the insulator in advanced interconnects to decrease the RC delay. The effects of NH3 plasma treatment and the effectiveness of the dielectric repair on molecular structures at the surface and buried interface of a pSiCOH film deposited on top of a SiCNH film on a Si wafer were fully characterized using sum frequency generation vibrational spectroscopy (SFG), supplemented by X-ray photoelectron spectroscopy. After exposure to NH3 plasma for 18 s, about 40% of the methyl groups were removed from the pSiCOH surface, and the average orientation of surface methyl groups tilted more towards the surface. The repair method used here effectively repaired the molecular structures at the pSiCOH surface but did not totally recover the entire plasma-damaged layer. Additionally, simulated SFG spectra with various average orientations of methyl groups at the SiCNH/pSiCOH buried interface were compared with the experimental SFG spectra collected using three different laser input angles to determine the molecular structural information at the SiCNH/pSiCOH buried interface after NH3 plasma treatment and repair. The molecular structures including the coverage and the average orientation of methyl groups at the buried interface were found to be unchanged by NH3 plasma treatment and repair.