Plasma-Assisted Cleaning by Electrostatics (PACE)

As feature sizes shrink, particle contamination on EUV masks used in the fabrication process of semiconductor chips is an increasingly difficult problem that leads to lower wafer throughput and higher costs of chip production. Current practices to remove particulates off of masks include using a sulfuric acid bath, ultrasonic cleansing, and rinsing in de-ionized water. However, nanometer-scale etching occurs through this cleaning process in addition to the presence of residual contamination due to the chemicals used, which leads to feature devolution. Currently, pellicles are used to protect the reticle with the pellicle being transparent to 193 nm light; however with current EUV technology being developed for 13.5 nm light, the pellicle is no longer transparent at this wavelength and thus cannot be used. Other mask-cleaning processes such as laser-induce plasma cleaning (LIP) run the risk of substrate damage due to potentially destructive methods. Plasma-assisted electrostatic cleaning involves using a potential drop in a plasma sheath electric field and charge imbalance between the particle and the substrate to propel the nanoparticles off the surface. Through applying a positive bias to the substrate and using a weak local plasma to charge the particles, the contamination is removed from the surface. As the particle size decreases the amount of time to charge the particle is longer thus leading to a longer removal period. However, as long as the particle is in the plasma sheath region, there is no theoretical limit on the size of the particle removed.