Lower size limit of particle removal in liquid-assisted laser cleaning

Since the development of the liquid-assisted laser cleaning method known as “steam laser cleaning (SLC)”, its application in semiconductor manufacturing has been limited due to its wet nature and potential damage caused by electromagnetic-field enhancement. However, as the primary target size has now been shifted towards the sub-10-nm range, the potential of SLC needs to be reassessed in the unexplored domain. Because previous studies have been focused on particles over ∼100 nm, the process has hardly been optimized for smaller particles. Moreover, the chance of damage generation by large dielectric particles is small in the case of nanoscale particles. Consequently, this study aims to identify the smallest particle size that can be removed from a silicon surface for gold, polystyrene (PS), and alumina particles. After optimizing the process through numerical simulation and experiment, the damage threshold and the particle removal efficiency (PRE) are measured by varying the laser fluence and particle size. The lower size limit, as determined by a 90 % PRE criterion, is 3 nm for gold particles, 20 nm for PS particles, and 50 nm for alumina particles. Discussions are made on the mechanisms responsible for the relatively high cleaning performance observed for gold particles.