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Silicon Wafer Cleaning for EUV Reflectance Measurements by Cold, High-Pressure CO 2 Jet William Evans Brigham Young University
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Why EUV Optics? EUV Lithography Soft X-Ray Microscopes EUV Astronomy
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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.
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Hydrocarbon Buildups Lower Reflectance Reduced Reflectance with Hydrocarbon Thickness. Theoretical change in reflectance vs. grazing angle and organic thickness. (at λ=40.0 nm)
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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
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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.
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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.
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Results Sample # Exposure Time to CO2 (s) Thickness Before Opticlean ® (Å) Thickness w/ Opticlean ® Residue (Å) Thickness After CO2 Exposure (Å) 1025.9346.3246.67 2025.2747.9847.72 3526.8241.6940.68 4525.1942.7738.48 51024.8242.3942.00 61025.7546.9321.05
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Results (cont.)
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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.
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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.
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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
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