IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 40 (2007) 5266–5274 doi:10.1088/0022-3727/40/17/038 Structural evolution of nanoporous silica thin films studied by positron annihilation spectroscopy and Fourier transform infrared spectroscopy N Patel 1 , S Mariazzi 1 , L Toniutti 1 , R Checchetto 1 , A Miotello 1 , S Dir` e 2 and R S Brusa 1 1 Dipartimento di Fisica, Universit` a di Trento, Via Sommarive 14, I-38050 Povo (TN), Italy 2 Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Universit` a di Trento, Via Mesiano 77, I-38050 Povo (TN), Italy E-mail: checchet@science.unitn.it, brusa@science.unitn.it and patel@science.unitn.it Received 28 May 2007, in final form 10 July 2007 Published 16 August 2007 Online at stacks.iop.org/JPhysD/40/5266 Abstract Three series of silica thin films with thicknesses in the 300 nm range were deposited by spin coating on Si substrates using different compositions of the sol precursors. Film samples were thermally treated in static air at temperatures ranging from 300 to 900 ◦ C. The effect of sol precursors and thermal treatment temperature on the film porosity was analysed by Fourier transform infrared (FTIR) spectroscopy, depth profiling with positron annihilation spectroscopy (DP-PAS) and the analysis of the capacitance–voltage (C–V ) characteristic. The maximum of the total porosity was found to occur at a temperature of 600 ◦ C when removal of porogen and OH groups was completed. Film densification due to the collapsing of the pores was observed after drying at 900 ◦ C. DP-PAS provides evidence that the increase in the total porosity is related to a progressive increase in the pore size. The increase in the pore size never gives rise to the onset of connected porosity. In the silica film samples prepared using a low acidity sol precursor, the pore size is always lower than 1 nm. By increasing the acid catalyst ratio in the sol, larger pores are formed. Pores with size larger than 2.3 nm can be obtained by adding porogen to the sol. In each series of silica film samples the shift of the antisymmetric Si–O–Si transversal optical (TO 3 ) mode upon thermal treatment correlates with a change of the pore size as evidenced by DP-PAS analysis. The pore microstructure of the three series of silica films is different at all the examined treatment temperatures and depends on the composition of the precursor sol. 1. Introduction Membranes are thin permeable layers that separate two regions of a chemical system and regulate the transport of molecules from and to their different parts. The preferential transport of a particular kind of molecule depends on: (i) the solubility value and the diffusion mechanism of that molecule in the membrane material as in organic perm-selective membranes or (ii) the size of the molecule as in microporous membranes [1]. Microporous membranes have pores with sizes in the sub- nanometre range and separate the component of a gas mixture by a molecular sieving effect. When the pore sizes reach 0022-3727/07/175266+09$30.00 © 2007 IOP Publishing Ltd Printed in the UK 5266