(574a) Aspects of a Novel Method for the Pore Size Analysis of Thin Silica Films Based on Krypton Adsorption at Liquid Argon Temperature (87.3k)

Thin mesoporous (silica) films have important applications in many fields (e.g, sensors and low-k dielectrics). Detailed knowledge of the pore size and pore volume is crucial in order to optimize the application of such thin films for instance as low k-materials in microelectronic applications. However, the pore size analysis of such films is difficult, also because convenient methods such as nitrogen and argon adsorption at 77.4 K and 87.3 K [1,2] cannot be easily applied for the pore size analysis of thin films. Such films are only a couple of hundred nanometers thick, and the application of nitrogen (77.4 K)and argon adsorption (87.3K) is (because of the high saturation pressure of 760 Torr) not sensitive enough to detect the small pressure changes due to adsorption on thin films (deposited for instance on a Si wafer). Krypton adsorption provides in principle an alternative because Krypton has very low saturation pressures at liquid nitrogen (77.4 K) and liquid argon temperature (87.3K), i.e. ca. 1.6 Torr and ca. 13 Torr, respectively. Until now however, krypton adsorption has not been routinely used for pore size analysis at these temperatures because the sorption and phase behavior of krypton far below its triple point temperature (115.8 K) is still under investigation. Methods for pore size analysis based on the Kelvin equation (e.g., BJH approach) cannot be applied here, and the study of krypton adsorption by approaches based on statistical mechanics (such as Non-Local Density Functional Theory (NLDFT) and molecular simulation) is still under development [3]. Hence, in order to address these problems we have performed systematic krypton adsorption experiments on selected zeolites and mesoporous silica molecular sieves (e.g., MCM-41, MCM-48 [4], SBA-15). This allowed us to systematically study the effect of confinement on the sorption and phase behavior of krypton below the triple point temperature, and to develop a procedure applicable for the pore size analysis of thin mesoporous films by krypton adsorption at 87.3 K. This novel method (which is "NLDFT traceable") allows one to determine the pore size distribution of thin films in the pore diameter range from ca. 0.7 nm to 10 nm. We demonstrate in our presentation the applicability of this novel method for the pore size analysis of thin silica films (thickness: ca. 200-500 nm) [5], synthesized according to ref. [6].

References

[1] M. Thommes, In: Nanoporous Materials Science and Engineering (edited by Max Lu and X. S. Zhao), World Scientific, 2004, Chapter 11, p. 317- 364 [2] S. Lowell, J. Shields, M.A. Thomas, M. Thommes, Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, Kluwer Academic Publisher, Dordrecht, Boston, London, 2004. [3] F. Hung, B. Coasne, K. Gubbins, F. Siperstein, M. Thommes, M. Sliwinska-Barkowiak, Studies in Surface Science and Catalysis, in press (2006) [4] M. Thommes, R. Köhn, M. Fröba, J. Phys. Chem. B 104 (2000), 7932 ; [5] M. Thommes, N. Nishiyama, S. Tanaka , Studies in Surface Science and Catalysis, submitted (2006) [6] Y Oku, N. Nishiyama, S. Tanaka , K. Ueyama, N. Hata , T. Kikkawa , Mat. Res. Soc Symp. Proc 716 (2002), 587