Radiation Physics and Chemistry 68 (2003) 345–349 Characterizing porosity in nanoporous thin films using positronium annihilation lifetime spectroscopy J.N. Sun, Y.F. Hu, W.E. Frieze, D.W. Gidley* Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA Abstract Depth profiled positronium annihilation lifetime spectroscopy (PALS) is an extremely useful probe of the pore characteristics of nanoporous thin films in general and low-dielectric constant ðkÞ thin films in particular. PALS is sensitive to all pores (both closed and open) in the size range from 0.3 to B300 nm and to the closed-to-open pore transition. Deduced pore sizes have been extensively compared with other techniques in an ongoing round-robin with other laboratories. The application of PALS in issues related to Cu/low-k film microchip integration will be demonstrated. r 2003 Elsevier Ltd. All rights reserved. Keywords: PALS; Porous films; Low-k dielectric films 1. Introduction Porosity control is an important aspect of the emerging field of nanoengineering. Introducing engi- neered porosity into microelectronic materials such as low-k dielectric films, into biotechnology films such as bio-membranes and polymeric selective permeation filters, and into chemical systems such as catalytic films is attracting intense research interest. As a specific example of the broadly applicable positronium annihila- tion lifetime spectroscopy (PALS) technique we focus herein on the extensive efforts to develop low-k materials in order to reduce the resistance–capacitance delay in integrated circuits (IC). According to the 1999 International Technology Roadmaps of Semiconduc- tors, incorporating nanoporosity is required to produce ultra low-k films ðko2:2Þ as the feature size of IC’s shrink below 0.10 mm. Understanding the pore structure, such as pore size, distribution, and pore interconnectiv- ity in porous low-k films is crucial since these parameters are fundamentally important in determining electrical and mechanical properties. Unfortunately, there are relatively few techniques capable of probing nanometer- sized pore characteristics in sub-micron films on thick substrates. Beam-based PALS (Gidley et al., 1999, 2000a; Petkov et al., 2000) has recently been demon- strated to be powerful in determining interconnected and/or closed porous structure in low-k films and beam- based Doppler broadening positron annihilation spec- troscopy (Petkov et al., 1999) has been used to probe void volume and pore interconnectivity. In this paper we will review the methodology of PALS in exploring the pore characteristics (size, distribution, and interconnec- tivity) in thin (sub-micron), porous low-k films. A model calibration that relates positronium annihilation lifetime with pore size has been developed and a recent round- robin comparison with other pore characterization techniques in various low-k films will be presented. PALS has also been used to investigate diffusion barrier integrity and the structural changes of porous low-k films during their integration with Cu. 2. PALS experimental technique In using PALS with thin films, an electrostatically focused beam of several keV positrons generated in a high vacuum system is implanted into the sample. The ARTICLE IN PRESS *Corresponding author. Fax: +1-734-764-5153. E-mail address: gidley@umich.edu (D.W. Gidley). 0969-806X/03/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00182-8