Revealing hidden pore structure in nanoporous thin films using positronium annihilation lifetime spectroscopy

The highly inhomogeneous pore morphology of a plasma-enhanced-chemical-vapor-deposited ultralow-k dielectric film (k=2.2) has been revealed using depth-profiled positronium annihilation lifetime spectroscopy (PALS) combined with progressive etch back of the film surface. The film is found to have a dense surface layer, an intermediate layer of 1.8 nm diameter mesopores, and a deep region of similar to 3 nm diameter mesopores. After successively etching of the sealing layer and the isolated 1.8 nm pore region, PALS reveals that the underlying large pores are highly interconnected. This inhomogeneous pore structure is proposed to account for observed difficulties in film integration. (C) 2005 American Institute of Physics.