Deep UV-Blocking Particle Filter Using High Aspect Ratio Si Nanogratings with Smooth Sidewalls

1. Deep UV-Blocking Particle Filter Using High Aspect Ratio Si Nanogratings with Smooth Sidewalls Pran Mukherjee,Thomas H. Zurbuchen, L. Jay Guo, and Fred A. Herrero

2. Outline • Introduction to Application • Summary of Fabrication Techniques • Review of NIL/DRIE Technique • Conclusion From standard Bosch (left) to first-run modification (middle) to current process (right) Sneak Peak!

3. The Solar Corona Solar Eclipse Composite Sensor Image The solar wind is a hail of charged and neutral particles ejected from the Sun.

4. Lyman-alpha The Solar Spectrum Lyman-alpha 103 1200-1270 Angstroms

5. Primary Application: UV filter • Requirements: • block energetic photons, particularly Lyman-alpha UV at 121.6 nanometers • high geometric transparency to allow atoms through • high aspect ratio to collimate atoms • self-supported

6. Transmission of Ly-alpha Target Features

7. Transmission of Particles • Modeled solid sheet of 15000 angstroms Si • Grating should be ~30-50% open area, so double penetration depths • Atoms <15-20 keV/nucleon stopped; solar wind ranges from 1-2 keV/nucleon.

8. Applications and Technologies • Applications • Deep UV photon filters transparent to particles • Collimators for particle detectors • Polarizers • High aspect ratio molds • Broad-spectrum pushbroom sensor • Technologies • Thin silicon membrane • Boron doping and EDP etch • SOI wafer and dry etch • Grating Etch • Femtosecond laser • 2 micron lithography and electroplating • Nanoimprint lithography and deep RIE Double-sided membrane processing!!

9. Fabrication Techniques • Three techniques attempted • Femtosecond laser etch • Optical lithography • Nanoimprint lithography with DRIE • Constraints • Grating etch time • Grating line width • Aspect ratio • Sidewall straightness

10. Femtosecond Laser Etch Joglekar, et al., Proc. Natl. Acad. Sci. 101(16), 5856–5861, 2004 • Laser energy profile allows submicron etching at material damage threshold • Lower energy makes smaller holes, but need more shots in both length and depth dimensions to make trenches • 1kHz laser would take 12 years to etch 2cm square grating! • 10MHz and faster lasers becoming available

11. Optical lithography • Ion implant etch stop • 2-micron frontside features of varied lengths • Combination DRIE/EDP backside etch • Sputter and electroplate Au • 30 micron long lines look perfect • Lines longer than 60 microns have stiction problems; these are 420 microns long

12. Nanoimprint and DRIE Sample pre-eched to 500nm with 500nm oxide • 15 min STS etch • 12:1 aspect ratio • <10nm scalloping • 400nm features

13. Mold with 50% duty cycle MRI8030 Chrome SiO2 Si Nanoimprinting (1) • Make or buy SOI wafer • Grow 200 nm mask oxide • Evaporate 10nm chromium • Deposit thermal polymer (MRI 8030) • Nano-imprint grating into polymer Steps 1-4 Step 5

14. Nanoimprinting (2) • Polymer residual etch, chromium etch • Dry-etch mask oxide MRI8030 Chrome SiO2 Si

15. Grating Etch (1) • STS etch grating, stage 1 • 7 minute STS etch • 8.5:1 aspect ratio • slight roughness • 150nm features • 35nm mask undercut • 0.18 m/min etch

16. Grating Etch (2) • Concerns • Controllability of oxidation must be within 10nm • Oxide stress • Aspect ratio of mask slowing or stopping second-stage etch • Stopping on buried oxide without widening lines • Dry oxidize sample to narrow grating lines and create second-stage etch mask • STS etch to oxide layer etch-stop

17. Gas Ratio Characterization From standard Bosch (left) to first-run modification (middle) to current process (right)

18. Oxide Etch Characterization • 200nm oxide masks rapidly etched away • C4F8 passivation layer etches oxide, platen bias enhances effect • Oxygen content reduces Si etch rate by ~20x, but not oxide etch rate • Reducing platen bias reduces oxide-etch rate

19. Process Characterization

20. Process Comparison • Bosch Process • Etch Step: 160 sccm SF6, 16 sccm O2, 12 seconds, 20W platen, 800W coil • Passivate: 85 sccm C4F8, 8 seconds, 0W platen, 800W coil • 20/15 mT base pressure for etch/passivation (set by valve angle) • Our Process • Etch Step: 20 sccm SF6, 75 sccm O2, 9 seconds, 150W platen, 550W coil • Passivate: 100 sccm C4F8, 3 sccm SF6, 12 seconds, 0W platen, 500W coil • 0.7 mT base pressure • Biggest differences • Absolute gas pressures • Percentage of oxygen in etch step • Ratio of etch/passivation times • Etch slows from 2-5 microns per minute to 0.2 microns per minute

21. Future Concerns • Fix undercutting in 100nm process • Double-etch process where oxidation of primary 100nm feature narrows lines and creates second-stage 50nm mask • Create crosshatched mold to avoid stiction • Back-etch • Plug pinholes in final grating

22. Technique Summary

23. Thank You!Any questions?

24. Nanoimprint and DRIE Process Mold with 50% duty cycle • Make or buy SOI wafer • Grow 200 nm mask oxide • Evaporate 10nm chromium • Deposit thermal polymer (MRI 8030) • Nano-imprint grating into polymer • Polymer residual etch, chromium etch • Remove polymer • Dry-etch mask oxide • STS etch grating, stage 1 • Dry oxidize sample to narrow grating lines and create second-stage etch mask • STS etch to oxide layer etch-stop MRI8030 Steps 1-4 Chrome SiO2 Si Step 5 Step 10 Step 6 Steps 7-8 Step 9 Step 11

25. 2 micron feature characterization First try: 75 nm scallop Second try: ~30 nm scallop Third try: ~10 nm scallop Fourth try: <8 nm scallop

26. 350 nanometer characterization • Required significant tweaking to reduce oxide mask etching, but we achieved ~7.5:1 aspect ratio gratings with ~7 nm scalloping and 350nm features