Probing Pore Characteristics in Low-K Thin Films Using Positronium Annihilation Lifetime Spectroscopy
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Probing Pore Characteristics in Low-K Thin Films Using Positronium Annihilation Lifetime Spectroscopy D. W. Gidley, W. E. Frieze, T. L. Dull1, J. N. Sun1, and A. F. Yee1 Department of Physics, University of Michigan, Ann Arbor, MI 48109 1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109 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 and organic-based films. The technique is sensitive to all pores (both interconnected and closed) in the size range from 0.3 nm to 300 nm, even in films buried under a diffusion barrier. PALS may be particularly useful in deducing the pore-size distribution in closed-pore systems where gas absorption methods are not available. 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 thin silica films that have been made porous by a variety of methods the pores are found to be interconnected and an average pore size is determined. In a mesoporous methyl-silsesquioxane film with nominally closed pores a pore size distribution has been determined. The sensitivity of PALS to metal overlayer interdiffusion is demonstrated. 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 There is currently a great deal of interest in introducing and characterizing nanometer-sized voids into thin silica and polymer films. Such porous films are being intensely pursued by the microelectronics industry as a strategy for reducing the dielectric constant of interlayer insulators in microelectronic devices. Unfortunately, there are relatively few techniques capable of probing the pore characteristics (average size, size distribution, and interconnectivity) in sub-micron films on thick substrates. This is particularly true if the voids are closed (not interconnected) so that gas absorption techniques are not available. Transmission electron microscope (TEM) images, for example, are inherently challenging to interpret in such amorphous insulators. Neutron scattering [1] and beam-based positronium annihilation lifetime spectroscopy (PALS) [2] have recently been used to determine an average pore size in silica films and beam-based Doppler broadening positron annihilation spectroscopy [3] has been used to probe open-volume in silsesquioxane films. In closed-pore systems PALS may be uniquely capable of deducing a pore-size distribution [4], even in films buried under
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