Positron beam analysis of structurally ordered porosity in mesoporous silica thin films

Positron beam techniques have been employed to characterise low-k dielectric silica based films, which have two or three dimensional structures of nanometre size pores. Pore fractions vary from 5 to 50%. Positrons implanted in the layer slow down and eventually annihilate with the electrons from the layer. However, in pores of the dielectric films positronium (Ps) is formed before annihilation takes place. The two states of Ps (para-positronium (p-Ps) and ortho-positronium (o-Ps)) are formed with rather different life times of 125 ps and 142 ns, respectively. The behaviour of Ps in the porous material can be described as hot particles with 1 eV initial kinetic energy, which lose their energy by frequent collisions with the atoms of the pore walls. When the pores are interconnected or separated by thin walls allowing permeation of the Ps some of the Ps will effuse from the film into the vacuum. The 2D-ACAR technique enables one to monitor the velocity distribution of the annihilating p-Ps and shows an increasing fraction of surface emitted p-Ps when positrons are implanted closer to the film surface. Measurements of Doppler broadening of 2-gamma annihilation and detection of 3-gamma events (o-Ps) give insight into the frequency of other annihilation events. Combining the results, a complete picture can be obtained of the interactions and transport of the Ps particles in the material. By modelling the Ps behaviour information is obtained on structural parameters of the porous material. A transport model based on multi energy group diffusion of particles describes the results well.