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