Observation of SiO2Nanoparticle Formation via UV Pulsed Laser Ablation in a Background Gas

SiO2 nanoporous films have recently gained significant popularity as low-k dielectric constant insulating films. In this study, we demonstrated the fabrication of a low-k SiO 2 nanoporous film consisting of SiO2 nanoparticles by pulsed laser deposition. However, to optimize the growth conditions of nanoporous films, it is important to understand the spatial and size distribution of nanoparticles produced by laser ablation. Here, we visualized laser-ablated SiO2 nanoparticles using a two-dimensional laser scattering imaging technique and investigated the influence of gas species and pressure on the spatial distribution of the nanoparticles in the gas phase. After the decay of the plasma emission within several microseconds, a spherical-shaped scattering image depicting the spatial distribution of SiO2 nanoparticles was obtained from a delay time of approximately 10 ms. In Ar gas, the sphericalshaped distribution of SiO 2 nanoparticles changed to an ellipsoidal shape with increasing pressure, whereas in O2 gas, it changed a mushroom-like shape. These results show that the gas species and pressure are dominant parameters affecting the spatial distribution of nanoparticle formation.