Nanoporous structure of methyl-silsesquioxane films using monoenergetic positron beams

The size and depth distributions of pores in silica-based intermetal-dielec tric materials were studied using monoenergetic positron beams. Doppler bro adening spectra of the annihilation radiation and lifetime spectra of posit rons were measured for methyl-silsesquioxane (MSSQ) spin-on-glass films. Th e size distribution of pores in the MSSQ films fabricated with 8% porogen l oad was found to be bimodal, with the major peaks located at 3 and 8 nm(3). Increasing the porogen load from 8% to 40% caused the smaller pores (3 nm( 3)) to disappear and 30-nm(3) ones to appear; these pores were considered t o be interconnected, and this structure makes it possible for positronium ( Ps) atoms to find paths towards the surface and to escape into vacuum. The 8%-porogen MSSQ films had low porosity near the Si substrate. From measurem ents of the temperature dependence of the self-annihilation rate of ortho-P s, we discuss the relationship between o-Ps emission into vacuum and the po re structure. (C) 2001 American Institute of Physics.