Silica aerogel can capture flying particles in EUV tools

A flying particle in a vacuum tool necessarily has a higher velocity than the critical velocity determined by the particle material and the surface material of chamber walls or tool structures. A flying particle loses kinetic energy at every collision with a surface and the velocity is reduced by about 10% a collision. When the velocity of the particle is below the critical velocity, the particle is captured by the surface at the next collision. We must catch the particles before they reach the reticle surface not protected by pellicle. It is a well-known fact that silica aerogels can capture stardust in space. Typical size of the stardust is a few tens of microns and the estimated velocity is a few tens of kilometers per second. On the other hand, typical size of particles in EUV tools is about 100 nm and the typical velocity is about a few hundreds of meters per second. The ratio of kinetic energy of the two is about 1:300,000,000,000. Can silica aerogels capture such low energy particles in the same way as stardust without bouncing the particles or generating debris? The question motivated us to evaluate the properties of silica aerogels in a vacuum; we evaluated the outgassing rate and the particle trapping performance of several aerogels with different densities, 0.012 - 0.19 g/cm(3). We found that some aerogels captured more than 90% incident particles and the outgassing rate is sufficiently low for EUV tools.