SPIE 2012 Amsterdam Talk: Silicon Immersion Gratings

This deck is my invited talk at the SPIE Astronomical Telescopes and Instrumentation conference in Amsterdam. The talk describes the immersion grating for IGRINS, and all the process steps and metrology that went into it.

Transcript

1. Paper # 8450-99
   Near infrared metrology
   of high performance
   silicon immersion gratings
   Michael Gully-Santiago
   The University of Texas at Austin, USA
   8:30 AM Friday, July 6, 2012
   Modern Technologies in Space- and Ground-
   based Telescopes and Instrumentation II
   SPIE Astronomical Telescopes and
   Instrumentation 2012
   

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2. Past
   •  Diffraction gratings
   •  Immersion grating history
   •  Grating production at UT Austin
   •  Fabrication and process
   Present
   •  IGRINS Immersion grating:
   CA1a
   •  Metrology:
   –  interferometry
   –  spectral purity
   –  efficiency
   Future
   •  Direct writing: Electron beam
   lithography
   -0.15 0.00 0.15 -0.15 0.00 0.15 -0.05 0.00 0.05
   Marsh,  Mar,  &  Jaffe  2007  
   

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3. Normal blazed reflection grating
   Diffraction Gratings & Applications Loewen and Popov 1997
   β  
   α  
   Air  
   n≈1.0  
   Metal  
   €
   mλ = σ(sinα + sinβ)
   σ  
   gra0ng  
   normal  
   λ  
   

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4. Immersion grating
   Diffraction Gratings & Applications Loewen and Popov 1997
   β`  
   α`  
   Si  
   n≈3.4  
   Metal   σ  
   gra0ng  
   normal  
   λ/n
   €
   m λ
   n
   = σ(sin ʹ′
   α + sin ʹ′
   β )
   

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5. Spectral resolution in immersion
   •  R = λ/δλ
   
   •  R = m N
   where
   m=diffraction order
   N=number of illuminated
   grooves
   •  increase the operational
   diffraction order
   •  increases the spectral
   resolution for a given size
   grating SPIE  
   

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6. Immersion gratings are not new
   Fraunhofer 1787 – 1826
   first noted the effect of increased spectral
   resolution by immersing gratings in
   liquids of high refractive index
   Huelthén and Neuhaus, Nature, 1954
   

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7. Heritage Si gratings from UTexas group
   Grisms:
   SOFIA FORCAST
   SPIE: Deen et al. 2008
   JWST NIRCAM
   SPIE: Jaffe et al 2008,
   Gully-Santiago et al.
   2010
   Immersion gratings:
   Marsh, Mar, & Jaffe 2007
   

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8. IGRINS,  the  Immersion  Gra0ng  Infrared  Spectrometer    
   University of Texas at Austin & Korea Astronomy and Space Science Institute (KASI)
   R=λ/Δλ=40,000  
   observes  all  of  the  H  (1.45-­‐1.90  µm)  and  K  (2.00-­‐2.45  µm)  band  
    atmospheric  windows  in  a  single  exposure  
   winter  2013  commissioning  
   2.7  m  Harlan  J.  Smith  Telescope  
   

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9. IGRINS uses an immersion echelle grating
   

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10. Silicon crystal properties
    Monolithic  crystalline  Si  ingot    
    Scanning  electron  micrographs  
    (SEM)  of  Si  crystal  planes:    
    Top:  conven0on  Si  wafer  
    BoTom:  Bias  cut  Si  wafer  
    

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11. Wang  et  al.  2010  
    Processing
    

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12. Wang  et  al.  2010  
    Processing
    

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13. Wang  et  al.  2010  
    Processing
    

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14. Wang  et  al.  2010  
    Processing
    

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15. Wang  et  al.  2010  
    Processing
    

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16. CA1
    We cut two immersion gratings
    from this parent substrate.
    30
    5
    - .1434°
    THETA
    ETCHED SURFACE
    1
    3
    27
    PARENT SUBSTRATE AS PROVIDED
    TO VENDOR
    ETCHED
    PROFILE LAYOUT
    53.5
    Wang  et  al.  2010  
    90.2
    30
    5
    - .1434°
    THETA
    90.34°
    103
    71.66°
    B
    C
    D
    8 7 6 5 4 3 2
    NOTES:
    1. VENDOR WILL GRIND FACE 2 0.1434 DEGREES (THETA) CW TO FACE 1
    2. VENDOR WILL SAW PIECE A FROM PIECE B . CUT WILL FALL WITHIN 5mm KERF
    SHOWN IN SKETCH
    3. FACE 3 SHOULD THEN BE GROUND PARALLEL TO FACE 2
    4. FACE 4 SHOULD THEN BE GROUND AT 71.66 and 89.68 DEGREE COMPOUND ANGLE
    TO ETCHED GRATING SURFACE (SEE SHEET 1)
    NOTES CONT'D:
    5. PROCESS REPEATS FOR PIECE B
    6. PIECE B TO BE CUT AT SAME SETUP AS A
    7. CONTACT DR. WEISONG WANG AT 512-471-0886 OR DR. DAN JAFFE AT 51
    WITH ANY QUESTIONS
    ETCHED SURFACE
    1
    3
    2
    A
    B
    DIRECTION OF ROTATION
    27
    93
    ETCHED SURFACE
    4
    53.5
    100
    

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17. 25 mm
    CA1a
    The IGRINS immersion grating
    13
    CE
    1
    B 71.66°±.025°
    A
    WHERE INDICATED
    4
    CORNER EFFECT DUE TO SHAPE OF PARENT MATERIAL
    FOR PROCESS USING 1200 GRIT WHEEL
    .25mm BEVEL EXCEPT
    6. CONTACT DR. WEISONG WANG AT 512-471-0886 OR DR. DAN JAFFE AT 512-471-3425 WITH ANY QUESTIONS
    5
    NOTE:
    TNESS 1/4 WAVE AT 632.8nm OVER
    RT OF 90%
    OCESS
    D
    C
    6 5 4 3 2 1
    BEAM PROJECTED
    ON GRATING SURFACE
    30.6
    30
    90.2
    4
    BEAM PROJECTED
    ON ENTRANCE SURFACE
    ETCHED LINES PERPENDICULAR
    TO DATUM A
    TO PROFILE ON SHEET 2
    ) ANGLE SHOWN
    ONLY TO ORIENT VIEWER
    90.34°
    (
    90.2
    D
    C
    B
    3 2 1
    BEAM PROJECTED
    ON GRATING SURFACE
    OF PARENT MATERIAL
    30.429
    29
    2.5
    88.95
    30.5
    84.502
    DETAIL B
    SCALE 4 : 1
    30.429
    30.482
    ACE
    Unrealised  Prism  shape   Actual  shape:  
    Leaves  on  unused  Si  material  
    Polished  and  AR  coated  entrance  face  
    

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18. Michael    Gully-­‐SanGago  
    

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19. Aluminum  
    Silicon  
    n=3.4   Gra0ng  normal  
    β ~ α =  δ = 71.56°
    
    δ
    
    

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20. Gra0ng  
    normal  
    δ
    
    αʼ#
    βʼ
    
    σ
    
    m =
    2nσ sinδ
    λ
    =119th order
    €
    R =
    λ
    δλ
    = mN
    €
    =119 × 2890
    = 345,000
    

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21. Metrology:
    – Surface roughness
    – Optical Interferometry
    – Spectral purity
    – Efficiency in immersion
    

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22. 2.4 Å 16.6Å
    24.2 Å
    Measured  surface  roughness  of  a  10  mm  
    thick  immersion  gra0ng  prototype  
    The  wet  etching  process  introduces  
    surface  roughness  on  the  groove  facets  
    S0ll,  the  roughness  is  much  less  than  a  
    wavelength,  and  has  negligible  impact  on  
    the  overall  efficiency  
    Surface Roughness
    

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23. 632 nm in air is comparable
    to ~2.2 µm in immersion
    25 mm beam is projected
    over ~80 mm across the
    hypotenuse of the R3
    echelle
    Marsh,  Mar,  and  Jaffe  2007  
    Optical interferometry
    

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24. View Slide

25. View Slide

26. λ  =  632.8  nm  interferometry  of  the  CA1a  entrance  face  
    Post  AR  coa0ng    
    Peak  to  valley:  0.06  waves  
    25  mm  aperture  
    

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27. λ   =  632.8  nm  interferometry  of  the  CA1a  R3  facets    
    Post  aluminiza0on  
    Peak  to  valley:  0.17  waves  
    25  mm  aperture  
    

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28. -20 -10 0 10 20
    x (FWHMs)
    10-6
    10-5
    10-4
    10-3
    10-2
    10-1
    100
    Normalized Flux
    -20 -10 0 10 20
    x (FWHMs)
    -20
    -10
    0
    10
    20
    y (FWHMs)
    Spectral purity
    PSF at l = 632.8 nm from 25 mm beam interferogram
    

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29. -20 -10 0 10 20
    x (!632
    /D
    25
    )
    -20
    -10
    0
    10
    20
    y (!632
    /D
    25
    )
    -20 -10 0 10 20
    x (!632
    /D
    25
    )
    -20
    -10
    0
    10
    20
    y (!632
    /D
    25
    )
    -20 -10 0 10 20
    x (!632
    /D
    25
    )
    -20
    -10
    0
    10
    20
    y (!632
    /D
    25
    )
    The  x-­‐  and  y-­‐  scales  are  angles  in  units  of  λ/D  for  λ=632.8  nm  and  D=25  mm.      
    All  figures  have  the  same  color  scale:  0.003  -­‐  1.0    
    λ=632.8  nm    
    D=25  mm  
    λ=543  nm    
    D=20  mm  
    λ=632.8  nm    
    D=20  mm  
    Measured  PSFs  
    Zygo  Synthe0c  PSF  
    Spectral purity
    

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30. -20 -10 0 10 20
    x (!632
    /D
    25
    )
    10-6
    10-5
    10-4
    10-3
    10-2
    10-1
    100
    Normalized Flux
    Model Measured
    λ (nm)   Ig
    /Il
    Ig
    /Il
    Meas./Model
    Green 543.5 0.0012 0.0016 1.30
    Zygo (red) 632.8 0.0009 0.0009 1.00
    Meas. (red) 632.8 0.0009 0.0011 1.22
    IR 1523 0.0018 <0.0120 <6.52
    Marsh,  Mar,  &  Jaffe  2007  
    Amplitude:  A  =  3.2  nm  
    Period:  P=1.6  mm  
    -50 0 50
    x (!632
    /D
    25
    )
    10-6
    10-5
    10-4
    10-3
    10-2
    10-1
    100
    Normalized Flux
    Ghost amplitude
    

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31. Custom robotic grating efficiency test apparatus
    *1.45 < λ (µm) < 2.3
    *δλ ~ 1.0 nm
    *D ~ 10 mm
    *Al ref. mirror
    *unpolarized
    

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32. *Efficiency relative to Al mirror
    *Slightly different incidence angles, α, for H- and K- band measurements (red/blue lines)
    *Dotted line is 75%, range is 68-80% over both bands
    *Gray background is atmospheric transmission over Kitt Peak (Kinkle et al. 2003)
    *T ~ 295 K
    

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33. *Black dots show the observed blaze peaks.
    …as refractive index changes the wavelengths that were once on-blaze are now off-
    blaze.
    Temperature-­‐dependent  refracGve  index  of  silicon…  
    Frey,  Leviton,  &  Madison  2006  
    

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34. Metrology Summary
    •  Facet surface roughness is ~16 Å
    –  contributes negligibly to efficiency loss
    •  Peak to valley surface error is 0.17 waves
    –  Diffraction limited in the wavelength range of
    interest
    •  Spectral purity meets specification
    –  0.16% ghosts at 543 nm (1.9 µm in immersion)
    are below 0.25% specification
    •  In immersion efficiency is typically 70-75% on
    blaze over 1.5-2.3 µm
    –  Exceeds IGRINS specification
    

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35. Future work preview:
    better and bigger gratings
    •  Better:
    –  reduce repetitive error amplitude
    –  reduce large scale surface error
    •  Bigger:
    –  Currently limited to 100 mm diameter substrate
    –  Next generation instruments (GMTNIRS?) will
    require 150 mm, or up to 200 mm boules
    Contact lithography will not work for larger
    gratings
    

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36. Direct writing experiments:
    electron beam lithography (JPL) & UV nanoruler
    (MIT)
    •  NASA JPL Microdevices Lab (MDL)
    – JEOL 9300-FS
    •  Non-flat surfaces
    •  Large Computer generated holograms
    •  Grayscale lithography
    Historical challenge has been periodic errors
    introduced from e-beam field stitching
    errors
    

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37. -0.30 0.00 0.30 -0.30 0.00 0.30 -0.10 0.00 0.10
    -0.30 0.00 0.30 -0.30 0.00 0.30 -0.10 0.00 0.10
    E-beam direct writing initial results
    First  prototype  immersion  
    gra0ng  
    Second  immersion  
    gra0ng,  improved  wri0ng  
    strategy  
    10  mm  thick  R3  clones  of  the  IGRINS  surface,  directly  wriSen  with  the  JEOL  9300FS  
    25  mm  beam  interferograms  on  the  same  color  scale,  in  waves  of  632  nm  surface  deviaGon  
    Measured   55  term  Zernike  fit   Residual  
    

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38. Future gratings
    GMTNIRS  
    Giant  Magellan  Telescope  Near-­‐IR  
    spectrograph  
    

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39. Other future projects
    1) Bonded Grisms
    2) Cryogenic VPH gratings for the Near-IR
    3) Polarization properties of Immersion
    gratings and VPHs
    H Band polarized efficiency
    1500 1600 1700 1800 1900
    h (nm)
    0.0
    0.2
    0.4
    0.6
    0.8
    1.0
    Efficiency
    Hyeonju  Jeong,  
    KASI/UT,  2011  
    M.  Gully-­‐SanGago  et  al.,  in  progress  
    

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40. Thank you
    NASA Graduate Student Researchers Program
    NSF ATI Grant AST-0705064
    NASA APRA Grant NNX10AC68G
    UT Austin
    Dan Jaffe
    Weisong Wang
    Cindy Brooks
    Casey Deen (now at MPIA Heidelberg)
    JPL MDL
    Dan Wilson
    Rich Muller
    

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41. EXTRAS
    

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42. Immersion grating
    Diffraction Gratings & Applications Loewen and Popov 1997
    Si  
    n≈3.4  
    Metal   σ  
    gra0ng  
    normal  
    λ/n
    €
    m λ
    n
    = σ(sin ʹ′
    α + sin ʹ′
    β )
    

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43. Immersion grating
    Diffraction Gratings & Applications Loewen and Popov 1997
    Si  
    n≈3.4  
    Metal   σ  
    gra0ng  
    normal  
    λ/n
    €
    m λ
    n
    = σ(sin ʹ′
    α + sin ʹ′
    β )
    

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44. Immersion grating
    Diffraction Gratings & Applications Loewen and Popov 1997
    Si  
    n≈3.4  
    λ/n
    €
    m λ
    n
    = σ(sin ʹ′
    α + sin ʹ′
    β )
    λ  
    Air  
    n≈1.0  
    

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45. In  e-­‐beam  there  is  a  grid  of  spectral  and  spa0al  dimension  s0tching  ghosts.  
    We  have  reduced  spectral  ghosts  to  negligible  levels.  
    With  no  aTempt  at  correc0on,  the  spa0al  ghosts  are  at  a  level  of  Ig
    /IL
     =  5  x  10-­‐4  
    0.00 0.15 -0.05 0.00 0.05
    Residual  ager  the  first  55  Zernike  polynomials  are  removed  
    

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46. Theory: CA1, !="=71.57, n
    Si
    =n(#), T=295K
    -30 -20 -10 0 10 20 30
    $ (deg)
    0.0
    0.2
    0.4
    0.6
    0.8
    1.0
    1.2
    Relative flux
    # = 1500.0 nm
    # = 1510.0 nm
    118
    119
    120
    121
    122
    Theory: CA1, !="=71.57, n
    Si
    =n(#), T=295K
    -30 -20 -10 0 10 20 30
    $ (deg)
    0.0
    0.2
    0.4
    0.6
    0.8
    1.0
    1.2
    Relative flux
    # = 2300.0 nm
    # = 2325.0 nm
    76
    77
    78
    79
    Predicted  efficiency  spectra  in  immersion,  normalized  to  the  blaze  peak.  
    The  diffracGon  and  refracGon  angles    depend  sensiGvely  on  the  refracGve    
    index  of  Silicon.  
    

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47. IR PSF measurement in immersion at 1523 nm
    *Not a diffraction-limited measurement
    *CA1a is consistent with a reference mirror
    -5 0 5
    x (!1523
    /D
    25
    )
    0.0
    0.2
    0.4
    0.6
    0.8
    1.0
    Normalized Flux
    Airy
    Mirror
    CA1a
    

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48. View Slide

49. View Slide

50. Vacuum  
    Silicon  
    n=3.4  
    Incident  beam  
    Diffracted  beams  
    

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51. Vacuum  
    Silicon  
    n=3.4  
    Incident  beam  
    Diffracted  beams  
    Gra0ng  normal  
    

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52. Vacuum  
    Silicon  
    n=3.4   Gra0ng  normal  
    β ~ α =  δ = 71.56°
    
    

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53. Vacuum  
    Silicon  
    n=3.4   Gra0ng  normal  
    β ~ α =  δ = 71.56°
    
    

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54. Gra0ng  
    normal  
    δ
    
    αʼ#
    βʼ
    
    σ
    
    mλ
    n
    = σ(sin ʹ′
    α + sin ʹ′
    β )
    ʹ′
    α ≈ ʹ′
    β ≈ δ
    LiSrow  autocollimaGon:  
    GraGng  equaGon:  
    m =
    2nσ sinδ
    λ
    =
    2 × 3.44 × 27.36 × 0.95
    1.5
    =119th order
    €
    R =
    λ
    δλ
    = mN
    N =
    W
    σ
    =
    25mm
    27.36µm
    = 914 grooves
    =119 × 914
    =109,000
    cf.  Loewen  &  Popov  
    

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55. Orien0ng,  cujng,    
    polishing,  nitriding  
    2006-­‐  2008  
    Lithography  
    Nov  2009  
    July  2011  
    AR  Coa0ng/  
    aluminiza0on  
    June  2011  
    Cujng  prisms  
    Feb  2010  
    Op0cal  evalua0on  
    IR  evalua0on  
    Sept  2011  
    Timeline  of  gra0ng  produc0on  and  evalua0on  
    

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56. Periodic  errors  originate  in  the  UV  
    photolithography  process,  specifically  
    the  transla0on  stage  motor  drive  
    Since  CA1,  we  have  reduced  the  
    periodic  errors  aTributable  to  the  
    motor  drive  to  negligible  levels    
    

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57. *The angle q is relative to the optical axis
    *The blaze envelope angular width goes as Δθ = λ/w
    *The Free Spectral Range is the available bandwidth in a given order, it goes as Δλ = λ/
    m
    On  the  next  slide  I  illustrate  how  diffracted  angles  depend  on  refrac0ve  index  
    

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58. Wang  et  al.  2010  
    Processing
    

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59. View Slide

60. View Slide

61. View Slide