Silicon Wafer Cleaning for EUV Reflectance Measurements by Cold, High-Pressure CO 2 Jet William Evans Brigham Young University

Why EUV Optics?  EUV Lithography  Soft X-Ray Microscopes  EUV Astronomy

Hydrocarbons and “Stuff”  Thin films and silicon wafers naturally build up a layer of contaminants.  The contaminants interfere with measurements of the optical properties of the mirror.

Hydrocarbon Buildups Lower Reflectance Reduced Reflectance with Hydrocarbon Thickness. Theoretical change in reflectance vs. grazing angle and organic thickness. (at λ=40.0 nm)

Cleaning Methods Many different cleaning methods have been used to clean silicon wafers for thin film deposition.  Opticlean ®  Problem: 20 A Residue  Oxygen Plasma Etch  Problem: Not Local and Can Oxidize Non-Protected Surfaces  High Intensity UV Light in Air  Problem: Oxidizes Metals, even Gold  Cold, High-Pressure CO 2 Jet

Cold, Pressurized CO 2  We used a CO 2 Snow Cleaning Solenoid Unit pressurized CO 2 gun by Applied Surface Technologies.  The unit uses freezing, pressurized CO 2 to blow particles and contaminates off of a surface.  We tested this unit on silicon wafers with a native oxide layer.

Testing Procedure  We tested the CO 2 gun cleaning system by the following procedure.  Measured the apparent oxide layer.  Applied Opticlean ®.  Again measured the apparent oxide.  Cleaned the samples with CO 2.  Again measured the apparent oxide thickness.

Results Sample # Exposure Time to CO2 (s) Thickness Before Opticlean ® (Å) Thickness w/ Opticlean ® Residue (Å) Thickness After CO2 Exposure (Å)

Results (cont.)

Why the Discrepancy?  The two 10-s samples showed some obvious differences in the effectiveness of the CO 2 gun.  This is probably due to the CO 2 gun.  CO 2 gun cleans macroscopically as compared the others, which are atomistic.  Also, the non-uniform removal of contaminates could have been influenced by cleaning geometry. The sample and gun were hand-held, so the distance and angle varied from cleaning to cleaning.

Results, Conclusions, and Directions for Further Study  In conclusion, the CO 2 jet appears promising, because it was able to remove the residue in one case.  Specifically the cold, high pressure CO 2 unit was effective in removing Opticlean ® from silicon wafers when used for a sufficient amount of time at a proper distance.  Further tests are needed to specify how to use this cleaning procedure for a variety of substrates.

Acknowledgements  We would especially like to thank  Dr. David Allred  Dr. Lindford  Richard Sandberg  Andrew Jaquier  Chris Verhaaren  Amy Baker  Kristin Bestor  The BYU Thin Film Optics Research Group