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Pran Mukherjee, Thomas H. Zurbuchen, L. Jay Guo, and Fred A. Herrero
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
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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!
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Composite Sensor Image
The Solar Corona Solar Eclipse Composite Sensor Image The solar wind is a hail of charged and neutral particles ejected from the Sun.
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The Solar Spectrum Lyman-alpha 103 Angstroms Lyman-alpha
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Primary Application: UV filter
Requirements: block energetic photons, particularly Lyman-alpha UV at nanometers high geometric transparency to allow atoms through high aspect ratio to collimate atoms self-supported
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Transmission of Ly-alpha
Target Features
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Transmission of Particles
Modeled solid sheet of 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.
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Applications and Technologies
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!!
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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
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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
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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
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Nanoimprint and DRIE 15 min STS etch 12:1 aspect ratio
Sample pre-eched to 500nm with 500nm oxide 15 min STS etch 12:1 aspect ratio <10nm scalloping 400nm features
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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 Mold with 50% duty cycle MRI8030 SiO2 Si Chrome Steps 1-4 Step 5
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Nanoimprinting (2) Polymer residual etch, chromium etch
Dry-etch mask oxide MRI8030 Chrome SiO2 Si
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Grating Etch (1) 7 minute STS etch 8.5:1 aspect ratio slight roughness
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
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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
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Gas Ratio Characterization
From standard Bosch (left) to first-run modification (middle) to current process (right)
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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
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Process Characterization
Test Scalloping Profiles Raise ratio of etch time vs. passivation No effect Widen bottoms Raise absolute gas pressures Minimal effect Slow etch, self-pass. Raise SF6 vs. O2 ratio during etch More scalloping Faster etch Raise absolute etch time per cycle
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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
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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
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Technique Summary Method Results Femtosecond laser etch
Lines look good, but direct-write takes FAR too long (on the order of months!) to make a decent-sized grating Optical litho w/electroplating Shorter lines look good, but lots of wet steps; also, electroplating deemed not viable due to diffusion limitation within channels Nanoimprint with DRIE Repeatable, large-area gratings with straight sidewalls achieved; oxide mask etched too quickly, though, so need better masking technique Double-step DRIE Same as above, except that a second-stage oxidation both narrows lines and serves as mask for a deeper etch; this should work!
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Thank You! Any questions?
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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
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2 micron feature characterization
First try: 75 nm scallop Second try: ~30 nm scallop Third try: ~10 nm scallop Fourth try: <8 nm scallop
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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
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