Nano-etched gratings for particle collimation and optical by hcj

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									  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

                           Solid-State Electronics Lab
                               University of Michigan
                  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!       Solid-State Electronics Lab
                                    University of Michigan
             The Solar Corona
  Solar
 Eclipse


                                        Composite
                                         Sensor
                                          Image




 The solar wind is a
 hail of charged and
   neutral particles
ejected from the Sun.
                            Solid-State Electronics Lab
                                University of Michigan
              The Solar Spectrum
                                    Lyman-alpha

                                                    103




                              1200-1270 Angstroms
Lyman-alpha




                              Solid-State Electronics Lab
                                  University of Michigan
   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

                                     Solid-State Electronics Lab
                                         University of Michigan
Transmission of Ly-alpha




               Target
              Features




                         Solid-State Electronics Lab
                             University of Michigan
     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.




                                   Solid-State Electronics Lab
                                       University of Michigan
    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
        1. Boron doping and EDP etch
        2. SOI wafer and dry etch                      Double-sided
                                                       membrane
    – Grating Etch                                     processing!!
        1. Femtosecond laser
        2. 2 micron lithography and electroplating
        3. Nanoimprint lithography and deep RIE      Solid-State Electronics Lab
                                                         University of Michigan
      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
                                    Solid-State Electronics Lab
                                        University of Michigan
        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
                                                            Solid-State Electronics Lab
                                                                 University of Michigan
            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
                               Solid-State Electronics Lab
                                   University of Michigan
              Nanoimprint and DRIE




 Sample pre-eched to
 500nm with 500nm oxide



• 15 min STS etch
• 12:1 aspect ratio
• <10nm scalloping
• 400nm features
                               Solid-State Electronics Lab
                                   University of Michigan
               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     Steps
               Chrome       1-4
               SiO2

               Si




                                   Step 5
                                            Solid-State Electronics Lab
                                                University of Michigan
                  Nanoimprinting (2)
     6. Polymer residual
        etch, chromium etch
     7. Dry-etch mask oxide

MRI8030
Chrome
SiO2

Si




                                  Solid-State Electronics Lab
                                       University of Michigan
                      Grating Etch (1)
  8. STS etch grating, stage 1




• 7 minute STS etch
• 8.5:1 aspect ratio
• slight roughness
• 150nm features
• 35nm mask undercut
• 0.18 mm/min etch
                                         Solid-State Electronics Lab
                                             University of Michigan
                Grating Etch (2)
9. Dry oxidize sample to narrow
                                            Concerns
   grating lines and create second-
                                       • Controllability of
   stage etch mask
10.STS etch to oxide layer etch-stop     oxidation must be
                                         within 10nm
                                       • Oxide stress
                                       • Aspect ratio of mask
                                         slowing or stopping
                                         second-stage etch
                                       • Stopping on buried
                                         oxide without
                                         widening lines
                                              Solid-State Electronics Lab
                                                  University of Michigan
   Gas Ratio Characterization




From standard Bosch (left) to first-run
modification (middle) to current process (right)
                                      Solid-State Electronics Lab
                                          University of Michigan
   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

                              Solid-State Electronics Lab
                                  University of Michigan
     Process Characterization
         Test              Scalloping      Profiles

Raise ratio of etch time   No effect       Widen
vs. passivation                            bottoms
Raise absolute gas         Minimal         Slow etch,
pressures                  effect          self-pass.
Raise SF6 vs. O2 ratio     More            Faster etch
during etch                scalloping
Raise absolute etch        More            Faster etch
time per cycle             scalloping

                                        Solid-State Electronics Lab
                                            University of Michigan
              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

                                                          Solid-State Electronics Lab
                                                              University of Michigan
          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


                                 Solid-State Electronics Lab
                                     University of Michigan
         Technique Summary
    Method                            Results
Femtosecond        Lines look good, but direct-write takes FAR too
                   long (on the order of months!) to make a decent
laser etch         -sized grating
Optical litho      Shorter lines look good, but lots of wet steps;
                   also, electroplating deemed not viable due to
w/electroplating   diffusion limitation within channels
Nanoimprint        Repeatable, large-area gratings with straight
                   sidewalls achieved; oxide mask etched too
with DRIE          quickly, though, so need better masking
                   technique
Double-step        Same as above, except that a second-stage
                   oxidation both narrows lines and serves as
DRIE               mask for a deeper etch; this should work!
                                                  Solid-State Electronics Lab
                                                      University of Michigan
Thank You!
Any questions?

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


                                               Solid-State Electronics Lab
                                                   University of Michigan
2 micron feature characterization
First try: 75 nm scallop         Second try: ~30 nm scallop




Third try: ~10 nm scallop   Fourth try: <8 nm scallop




                                                        Solid-State Electronics Lab
                                                            University of Michigan
    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




                                                          Solid-State Electronics Lab
                                                              University of Michigan

								
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