Porous ultralow kappa dielectric film pores were sealed by 1-5 nm films of boron carbonitride, itself a dielectric. The BC0.9N0.07 films were deposited by chemical vapor deposition at 335 OC on etched and ashed blanket films and patterned films of porous methyl silsesquioxane (PMSQ). The penetration of Ta through the boron carbonitride film and into the porous low kappa substrate following exposure to TaF5 at 230 degrees C, or TaSix growth from TaF5 and Si2H6 at 230 degrees C, was used to test pore sealing. Tantalum profiles were evaluated using x-ray photoelectron spectroscopy depth profiling, back side secondary ion mass spectroscopy, and energy dispersive x-ray spectroscopy. A 3.9 nm, boron carbonitride film sealed PMSQ, which has an average pore diameter of 1.9 nm before etching. The diffuse nature of the BC0.9N0.07-PMSQ interface in electron energy loss profile maps suggests that some of the BC0.9N0.07 penetrates into the PMSQ until the pore openings are pinched off as the sealing film deposits. (C) 2007 American Vacuum Society.