Positronium time-of-flight measurements of porous low-k films

We demonstrate that positronium (Ps) time-of-flight (TOF) spectroscopy provides a powerful technique for probing the structure of porous low-dielectric constant (low-k) films. The emission of ortho-positronium (o-Ps) from the surface of a series of nano-porous silsesquioxane films was studied with a newly developed TOF spectrometer. Clear Ps emission peaks were observed from the two high porosity films (F38 and KI31), indicating their higher open porosity in comparison with other films. We also observed that the energy of o-Ps emission from F38 is much higher than that from KI31. It was concluded that the open pores in the former film are less tortuous, and o-Ps loses significantly less energy emerging from its surface than from that of latter film.