Positronium time of flight measurements of an open-pored spin-on low-k mesoporous film

摘要

Depth-profiled positronium time of flight (Ps-TOF) was measured by implanting slow positron pulses at variable energies into an open-pored spin-on low-k mesoporous film with bimodal pore size distribution arising from zeolite micropores and interparticle mesopores. We estimate the energy of the positronium (Ps) that diffuses out of the target into the vacuum to investigate the slowing down of Ps by collisions with the walls of the micropores (0.5 nm in diameter) and mesopores (4 nm in diameter). The obtained Ps-TOF spectra showed that the temperature of the emitted Ps depends on the positron implantation depth, i.e., the number of collisions with the walls. The data indicate that the pore tortuosity is low in the present sample. However, the slowing down rate seems to increase when the Ps temperature is high. This is probably because when the Ps temperature is high, the Ps energy is higher than the Ps confinement energy for micropores (similar to 3 eV), and the apparent tortuosity increases because there are millions of micropores. The Ps formed in this film has to travel a long distance to escape from the same depth into the vacuum until it slows down below the confinement energy of the zeolite micropores.