There is no standard of nanopore size calibration for sub-micron thin films. Recently, a systematic round-robin of identical porous low-k thin films from different manufacturers was performed to compare the results of beam-PALS, specular X-ray reflectivity (SXR)/ small angle neutron scattering (SANS), and traditional N2 porosimetry (BET).
The low-k films studied are made from a variety of materials, such as silica, silsesquioxane and polymers from different processing methods. In Figure 1 we present the pore size comparison on eleven low-k films. All these films are detected to have interconnected, open pores. In the pore diameter range from 2 to 15nm, reasonable agreement in open pore size measurement is observed. For a better illustration, the data are replotted in Figure 2, focusing on comparing PALS with other techniques directly.
Figure 2. Round-robin comparison on PALS technique with other methods
It appears that PALS results agree better with EP and BET methods while SANS/SXR methods generally present relatively high values. We definitely do not have enough data to draw definitive conclusions. In this figure, PALS results on pore size are obtained by RTE model assuming two-dimensional infinitely-long square channels. The RTE model has been shown to be a highly successful and predictive model.
These isolated comparisons suggest a successful agreement on average pore size has been obtained for open pores. The sorption porosimetry EP and the traditional gas absorption methods have the capability of detecting pore-size distribution in open pore structure, while PALS and SANS/SXR have the advantages of probing both open and closed pores.