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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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Presentation on theme: "Silicon Wafer Cleaning for EUV Reflectance Measurements by Cold, High-Pressure CO 2 Jet William Evans Brigham Young University."— Presentation transcript:

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

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

3 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.

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

5 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

6 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.

7 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.

8 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

9 Results (cont.)

10 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.

11 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.

12 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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