Variable-Energy Positron Annihilation as Highly Sensitive Nanoporosimetry for Porous Thin Films

Abstract

Positron annihilation was applied to measuring critical pore sizes in various materials. In recent years, positron annihilation with a variable-energy positron beam has emerged as a powerful tool for the investigation of porous thin films synthesized as low-k dielectrics, high performance gas sensor materials, and so on. This paper is a brief overview of recent progress in nanopore characterization of thin films by means of positron annihilation with a description of several important issues relevant to positron annihilation.

Investigation on the variation in the fine structure of plasma-polymerized composite membrane by positron annihilation spectroscopy

“…The so-called pick-off annihilation with electrons in molecules was used to calculate the mean free-volume radius R (Å to nm) based on an established semi-empirical correlation equation, which is based on a spherical-cavity model. Detailed descriptions of slow-positron beam and data analysis can be found elsewhere [13][14][15]. The oPs intensity (I 3 ) can give us some information on the properties of materials, but it is strongly influenced by various factors-freevolume fraction, pore sizes, and chemical nature of the environment in which chemically active groups can inhibit positronium (Ps) formation through electron capture.…”

Positron Annihilation as a Method to Characterize Porous Materials

“…We note that PAS has been used for 30-40 years to sensitively detect monovacancies and small vacancy clusters in metals and semiconductors (2), whereas Ps annihilation spectroscopy has been developed over a similar period to probe subnanometer, intermolecular voids in polymers (3). The extension of Ps annihilation spectroscopy into materials with engineered nanoporosity is a natural outgrowth of the recent surge in interest in nanotechnology coupled with advances in positron beam technology that allow controlled implantation of positrons into technologically important thin films, especially low-k dielectric films in the microelectronics industry (4)(5)(6).…”

“…high-momentum core electron annihilation and/or more Ps formation, both leading to a higher S-parameter. The S-parameter can also be sensitive to the chemistry of the film and increases in environments in which Ps quenching is large, for example, in films in which paramagnetic centers have been introduced (6) or in which there are free radicals present. However, whereas DBS can explore relative changes of the S-and W-parameters as a function of film porosity, without sufficient calibration DBS has difficulty in determining the relative contributions of all the effects that can influence the pair momentum.…”

Positron annihilation study of silica films templated by a cationic surfactant

“…1, S for the second region, solely attributed to the as-deposited films, increases gradually with increasing W CTAB from 0 to 15 wt%. The enhanced S with increasing W CTAB may be due to the chemical composition [16,17] Figure 2 depicts the relationship between S and W obtained for the CTAB-silica composite films, corresponding to the respective highest value of S . It is known that a linear relationship between the S and W parameters is observable not only when the annihilation site of positrons gradually changes with constant formation probability of positronium for a given system [16,17], but also when positrons annihilate at the same kind of pores with different concentrations [18,19].…”