Porous low-k thin films




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 Pore Characteristics

As the feature size in integrated circuits shrinks below 0.10 mm, an ultra low-k material (k<2.2) is required.  The dominant approach is to lower material density by incorporating meso- and/or micropores into a material network. 

  • Mesoporous typically describes materials having pores 2-50nm in diameter  

  • Microporous is used to describe materials containing pores less than 2nm in diameter

There are a number of reliability concerns about porous low-k films, such as the mechanical strength, dielectric strength, and processing contamination.  In order to address these concerns, we must understand the pore structure, since parameters such as pore size, distribution, and pore interconnectivity in porous low-k films.  Unfortunately, the characterization of nanopore structure in low-k films is difficult because the films are typically less than 1 mm thick and must be characterized as prepared on a thick substrate. 


Beam-based positronium annihilation lifetime spectroscopy (PALS), a unique, non-destructive technique to measure void volume in thin films, has recently appeared to be a promising tool to probe the porous structure in low-k materials.  The particular strength of PALS is that it is a non-destructive technique, sensitive to both open and closed pores in a wide range (especially small pores) even if they are hidden beneath a covering layer.  We demonstrate herein the methodologies of using PALS to study Open porosity, Closed porosity and the transition from closed to open pores, Pore percolation in low-k films.