Porous low-k thin films




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  Plasma Treatment

Characterization of the nanoporous structure of low-k films is not trivial to the semiconductor industry, and few techniques are applicable.  It can be even more difficult to detect any structural changes (which could be very subtle) after integration processing.  We will demonstrate the capability of PALS for exploring structural change of interconnected porous low-k films after plasma treatment in integration processes.  


A mesoporous MSQ film with the thickness of 280 nm is used for systematic studies.  The pores are highly interconnected and open to the surface with an average pore size of 2.5 nm in diameter.  Several blanket porous MSQ specimens were exposed to a pretreatement of nitrogen-based plasma for various lengths of time.  One other film was treated with the oxygen plasma ashing process.  

  Figure 1.  Ps vacuum intensity in the plasma pretrated (PPT) and oxygen ashed films.  An unprocessed film is examined for comparison.

PALS tests were performed on these “plasma pretreated” (PPT) films and the O2 ashed sample.  Fitted Ps intensities in vacuum are plotted in Figure 1.  All the treated films have Ps intensity consistent with simply backscattered Ps in the vacuum, indicating that 

  • The interconnected pores in the MSQ film are sealed off at least on the surface after the plasma treatment process.  

  • A densified, non-porous  layer is formed at the surface 

  • The densified layer performs effectively as a Ps diffusion barrier, preventing the escape of Ps into vacuum.

Figure 2.  "Total" Ps formed in mesopores (including Ps diffusing into vacuum), after correcting for Ps backscattering into vacuum.  Numbers on the right side of the plot are film thickness determined using SEM.  


Figure 2 shows the systematic comparison of the total intensity of Ps formed in the mesoporosity.  
  • The amount of mesopores has dramatically decreased in all the processed films.
  • The average pore size of the remaining porosity, is found to be unchanged, suggesting that after the PPT processing, the surface of the porous low-k film is densified without affecting the porous structure underneath.  
  • Ps film intensity decreases as the plasma pretreatment time increases, which means the densified layer is thickened with longer exposure to the plasma.   

Compared to the non-porous film, porous low-k is much more susceptible to plasma damage.  The connected pore structure appears to facilitate film damage by providing the reactive species from the plasma easy diffusion paths into the film.  The mechanism of forming a dense skin layer after these integration processes is not fully understood.  The possibility of achieving controllable, thin, dense, skin layers as a passive diffusion barrier or diffusion barrier platform is proposed for future studies.


  •   J. N. Sun, D.W. Gidley, Y. F. Hu, W. E. Frieze and E. T. Ryan, Applied Physics Letter, 81,8, 1447 (2002)