Porous low-k thin films




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 Pore percolation

PALS is capable of distinguishing between open and closed pore structure based on the intensity of the Ps signal in vacuum.  

  • In interconnected open pores, the highly mobile Ps atoms readily escape into vacuum 

  • From closed pore films only backscattered Ps in vacuum can be observed.  

This will allow us to determine the evolution of pore structures and the percolation threshold, i.e., the closed-to-open pore transition in porous films.  

Figure 1. Ps intensities

A series of porous MSSQ films from Lucent, with different weight percentage of porogen (range from 0 ~ 50%) are used to illustrate PALS sensitivity to percolation threshold.  The intensities of the Ps annihilating in the film and in vacuum are plotted in Figure 1.

The in-film intensity results from Ps annihilating in the closed porosity (Io-Ps, including Ps in both mesopores and micropores) while the vacuum intensity (Ivacuum) consists of the systematically backscattered Ps (~ 2 % at 5.0 keV) and the escaped Ps from open porosity.  

When the porogen loading increases to 25 wt%

  • Ps annihilation fraction in the film sudden drops 

  • Ps signal in vacuum sharply increases.  

Obviously, some Ps formed in the film has managed to diffuse into vacuum as the porogen loading goes above 25 wt%.  Therefore, this loading is believed to be the nominal pore percolation threshold for Ps (more precisely, the threshold is in the range of 20-25%).  At this threshold, the mesopores start to merge with or connect to the surrounding pores and, as a result, open paths to the free surface can be formed.

Figure 2.  Pore sizes and dielectric constants.

To determine the pore size, a capping layer (as Ps diffusion barrier) is required for pore size determination in those films above percolation threshold whereas below the percolation threshold, there is no need for a capping layer, since all the pores generated in the films are closed and isolated. 

  • Micropores around 1 nm on average are intrinsic to the matrix MSSQ material and are present in all of the film.  

  • Mesopores become distinguishable in films beyond 15 wt% porogen and bigger in size as the porogen loading increases.  

  • The dielectric constant, k, decreases as a result of higher porosity in these films. 


  • PALS is sensitive to the percolation threshold, which can be easily identified by the escaped Ps signal in vacuum.


  • J. N. Sun, D. W. Gidley, Y. F. Hu, W. E. Frieze and S. Yang, Characterizing Porosity in Nanoporous Thin Films Using Positronium Annihilation Lifetime Spectroscopy, Materials Research Society Symposium Proceeding (2002)