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Silicon Diffractive Optics for
Infrared Spectroscopy
Dan Jaffe
Why Infrared?
Why Spectroscopy?
Tools of the trade
How to make a better mousetrap out
of silicon
Infrared lets you see through dust into star forming
regions. It also gives you access to physical processes
you cannot study in the visible.
Diffraction gratings bend light like prisms, but through
a greater angle for a given wavelength separation.
This larger dispersion allows you to resolve more
closely spaced spectral features.
Grating Equation: ml=d(sina+sinb)
The grating equation gives allowable directions.
The blaze tells you how much power goes to which.
Key Science for High Resolution Near-IR
Spectroscopy:
Star Formation
Probe the
kinematics, densities, and
chemical properties of
unresolved disks.
Molecular lines at 3-5 mm
Pre-Main Sequence Stellar Astrophysics
•Rotational Properties (vsini)
•Magnetic Field Strength (Zeeman splitting)
•Accretion Rates (Brackett Line Profiles)
•Cluster Kinematics (radial velocities)
•Fundamental Parameters (Teff, log-g, abundances)
•Courtesy K. Covey (U. Washington/CfA)
Enable planet searches around low mass stars
and brown dwarfs. Sensitivity for cooler, low mass stars
is much better than in the visible.
Put color argument here
Determine the chemical evolution of the Galaxy.
Study isotopic ratios such as 18O/16O
The problem is that
everything
radiates in the
infrared and so
your entire
spectrograph must
be cooled to
almost absolute
zero.
Immersion Gratings are the Key to the New Spectrographs
An immersion grating is a grating in which grooves are immersed in a medium
with an index of refraction n.
ml  nG (sin a  sin b )
Rmax 
2nG L sin 
l
d b 2nG tan b

dl
l
An immersion grating gains you a factor of n2-n3 in
spectrograph volume.
Idea dates back to Fraunhofer (1822).
Reinvented in 1954 by Hulthén and Neuhaus
Patented in 1984 by Sica.
Never made to work because of production difficulties.
How precise does your grating have to be?
Assuming random errors in groove spacing, allowable wave front RMS
error is
2
  2 n

 
 exp   
2 RMS  
0
 
  l
 RMS   RMS / sin 
rms=25nm
We produce immersion gratings by a process of
photolithography and chemical micromachining.
Step 1. Purchase a boule of high purity material
Step 2. Cut the boule into disks
Material needs to be oriented to achieve various blaze angles
6.16o
54.7o
63.4o
Production
Step 3. Polish disks using chemical – mechanical polish (CMP).
Step 4. Deposit passivation layer.
The passivation layer will be patterned into an etching mask which
defines the grating period.
Production
Step 5. Deposit photoresist.
•Spin-on photoresist at 3500 rpm
Photoresist layer
Passivation layer
Silicon substrate
Production
Step 6. UV exposure through contact photolithography mask
(contact is a critical issue)
Step 7. Develop exposed photoresist
We have the image of the mask in photoresist:
Production
Step 8. Etch the passivation layer: Si3N4 is etched via reactive ion etching
(RIE).
+ + + + +
Substrate
Production
Step 9. Photoresist removal
Positive image of grating mask pattern in the passivation layer.
Production
Step 10. Anisotropic silicon etch in a KOH solution
Si + 2OH- + 2H2O  SiO2(OH)2-- + 2H2
Production
The exposed (111) crystal planes
are smooth on an atomic scale.
Production
Step 11. Remove the remaining
passivation layer: Remaining Si3N4 is
etched in concentrated phosphoric acid
at ~150oC.
Production
Step 12. Cut the disk into a prism
Production
Step 13: anti-reflective coating on the entrance face and reflective coating on
the groove surfaces
Si Immersion Grating
Production
Grating etched into
silicon puck
Puck cut into prism
and then polished
36 mm
elliptical C.A. corresponds to
a 24 mm circular beam
Flat entrance face
antireflection coated
Device completed by aluminizing the
grooves along the hypotenuse
Note: Only the bottom of
the coating matters
Evaluation
Combination of tests:
• efficiency measurements
• interferometric measurements
• point-spread function (PSF) measurements
• analysis of grating defects and aberrations (ghosts, scattered light)
Tests give us consistent results on grating performance and help us
analyze the sources of errors.
Evaluation
Sample 1D monochromatic spectra
Evaluation
Interferometric tests done at 632.8 nm.
Evaluation
Point spread function measurements also test
diffraction limited performance.
• Compare to optical performance of a flat mirror
• Analyze errors (ghosts) and compare to
periodic errors observed with interferometric tests
• Determine resolving power from FWHM
Demonstrated resolving power of up
to 75,000 with G1
Diffuse scattered light is caused by surface
microroughness and various macro defects.
Atomic force microscopy of a 5 mm
by 5 mm area of G2.
RMS roughness is 1.6 nm.
Current grooves are 80 mm wide
and 40 mm long.
If this were a 1m wide sidewalk, it would
be 0.5 km long. The bump in the
picture would be 60 mm.
Actual Flight Hardware for SOFIA
Planned Instrument for NASA IRTF
GMTNIRS
Planned instrument for the Giant Magellan Telescope