Filtration of airborne nanoparticles down to single-digit nanometer range

The rapid development of nanotechnology brings new challenges to aerosol filtration, which plays a critical role in controlling air quality and protecting workers’ health. Localized high concentration nanomaterials may be emitted in certain processes in the semiconductor industry. Examples include silica and alumina nanoparticles in chemical-mechanical polishing processes, potential use of hafnium oxide nanoparticles in advanced immersion lithography, and carbon nanotubes (CNTs) that may be used in self-assembly or advanced packaging processes. There were serious concerns whether the conventional filters could be efficient against exceedingly small particles. Diffusion and interception are important filtration mechanisms for nanoparticles. Diffusion loss is stronger for small particles therefore the filtration efficiency increases with decreasing particle size in the nanometer range. However, nanoparticles may bounce though filters due to their high thermal energy, which reduces the filtration efficiency. Recent studies indicate the filtration efficiency increases as the particle size decreases down to 2 – 3 nm. There exists possibility of thermal rebound for smaller particles or at elevated temperatures.