Characterizing Porosity in Nanoporous Thin Films Using Positronium Annihilation Lifetime Spectroscopy
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Characterizing Porosity in Nanoporous Thin Films Using Positronium Annihilation Lifetime Spectroscopy J.N. Sun, D. W. Gidley, Y.F. Hu, W.E. Frieze and S. Yang1 Department of Physics, University of Michigan, Ann Arbor, MI 48109 1
Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974
ABSTRACT Depth profiled positronium annihilation lifetime spectroscopy (PALS) has been used to probe the pore characteristics (size, distribution, and interconnectivity) in thin, porous films, including silica, organic and hybrid films. PALS has good sensitivity to and resolution of all pores (both interconnected and closed) in the size range from 0.3 nm to 30 nm, even in films buried under a diffusion barrier. In this technique a focussed beam of several keV positrons forms positronium (Ps, the electron-positron bound state) with a depth distribution that depends on the selected positron beam energy. Ps inherently localizes in the pores where its natural (vacuum) annihilation lifetime of 142 ns is reduced by collisions with the pore surfaces. The collisionally reduced Ps lifetime is correlated with pore size and is the key feature in transforming a Ps lifetime distribution into a pore size distribution. In hybrid films made porous by a degradable porogen PALS readily detects a percolation threshold with increasing porosity that represents the transition from closed pores to interconnected pores. PALS is a non-destructive, depth profiling technique with the only requirement that positrons can be implanted into the porous film where Ps can form. INTRODUCTION In recent years, there have been extensive efforts to develop low-dielectric constant materials in order to reduce the resistance-capacitance delay in integrated circuits (IC) [1]. Organosilicate – silsesquioxane (SSQ), an inorganic/organic hybrid [2,3], has been intensively investigated as an on-chip, low-dielectric constant material. One reason for this is that it is difficult to obtain good mechanical and thermal stability using pure materials, organic or inorganic. The dielectric constants of dense SSQ’s are usually less than 3 due to the reduced crosslinking density and lower material density when compared to conventional silica (k≈4). The resistance to thermal decomposition is generally better than organic films due to the increased network stability. As the feature size of IC’s shrink below 0.10 µm, nanoporosity is being incorporated into SSQ materials to produce ultra low-k films (k
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