Plasma-Assisted Cleaning by Electrostatics (PACE)

The need for a non-contact contamination removal technique has been exhibited by various companies. While an EUV compatible pellicle is being researched, contamination will become an ongoing problem to overcome. Some techniques that are being considered for contamination removal include the use of Shockwaves which are potentially damaging, as well as rolling contamination off of a surface. Depending on feature size, rolling or moving of contamination horizontally across a surface is limited as there is a strong possibility that the contamination will get trapped in between features. Plasma- Assisted Cleaning by Electrostatics (PACE) is a non-contact removal method that utilizes charge imbalances to remove particles perpendicular to the surface. A positive bias is applied to the top of the sample for conducting samples or to the substrate behind an insulating sample. This positive bias draws in net electrons from the plasma to the entire surface. The contamination charges negatively and the positive bias is removed and switched with a negative bias. The combination of substrate/particle charge imbalance as well as electric field effects from the plasma sheath provide for the removal mechanism. Surface damage has been avoided by staying below the sputtering threshold for the surface materials of the samples. Recent tests on 2.5 nm ruthenium on Si/Quartz using the PACE technique coupled with Atomic Force Microscopy data has shown no roughening of the surface and approximately 90% removal efficiency of contamination. In addition, Auger Electron Spectroscopy scans show no removal of the 2.5 nm ruthenium capping layer. Removal results for 30 nm to 220 nm PSL particles as well as select other contamination materials on samples comparable with EUV masks in addition to damage assessments will be presented.