Technical Status of Near Infrared Detectors for SNAP

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Technical Statusof Near Infrared Detectors for
SNAP

• Gregory Tarlé
• University of Michigan
• July 2002

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Outline

• State of NIR Detectors
• Dark Current
• Quantum Efficiency
• Read Noise
• Intrapixel Variation Dithering
• Michigan NIR Detector Lab
• Summary

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NIR Detectors

• Fully half of the SNAP focal plane is now NIR
  sensitive.
• At present there are two main technologies for
  large megapixel arrays
• InSb under development by the ALADDIN
  (Advanced Large Area Detector Development in
  InSb) project.? ORION
• HgCdTe HAWAII (HgCdTe Array Wide Area Infrared
  Imager) multiplexer developed by Rockwell Science
  Center together with University of Hawaii.
• need for passive cooling ? single technology
  option (HgCdTe)

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State of HgCdTe NIR Detectors

• picture from RSC (progression)
• Here we discuss the state of the NIR detector
  technology (RSC) Hawaii-2, Hawaii-2 RG, InSb
  etc
• discuss performance
• WFC3 , NGST, noise, Idark, QE

• Hg(1-x)CdxTe ?c determined by x this
  determines operating temperature.
• Rockwell Science Center (RSC) is the principal,
  recognized source for Mercury Cadmium Telluride
  (HgCdTe) infrared focal plane arrays (FPAs).
• RSC has developed devices (NICMOS 256 x 256
  FPAs, WFC3 1024 x 1024) for the IR channel on
  HST. Also U Hawaii, ESO, Subaru, NGST

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HgCdTe Fabrication
Hybrid array has an array of HgCdTe detectors on
one layer and charge collection and addressing
circuitry (CMOS multiplexer) on another layer.
The two layers are connected with indium bump
bonds.
PACE Layer of CdTe is deposited on sapphire
substrate by chemical vapor deposition followed
by a layer of HgCdTe grown by liquid phase
epitaxy. MBE - Rockwell has developed a process
of fabricating detector layers in HgCdTe using
molecular beam epitaxy (MBE). This process
results in improved QE at short wavelength and
potential reduction in intrapixel variation.
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Hawaii RG Multiplexer

• NGST development
• (Hawaii-2RG)
• 0.25mm design rules
• 3-side close buttable
• 1, 4, or 32 outputs
• Bi-directional register to allow corner to
  center scanning
• Reference output R and internal reference
  columns/rows
• Guide window G output
• Fully selectable guide window within 2048 x 2048
  architecture
• Seamless guide mode/science mode readout
• Read noise 10 e- rms CDS ?N down to 3 e- rms
  w/multiple samples achieved with lc 5 mm
  material.

Four of these H1RG multiplexers are optically
stitched to produce a H2RG device
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SNAP NIR Detector Specifications
The Rockwell HgCdTe devices are our baseline
choice for the NIR system. WFC3 MBE material with
1.7 mm cutoff in the NGST 2k x 2k format (under
development) is a very good match for
SNAP. Performance Goals Read Noise 5
electrons (Fowler-4 read) Dark Current 0.1
e-/sec/pixel Quantum Efficiency gt 60
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NIR Detectors Dark Current
Dark current depends sensitively on the threshold
wavelength and the temperature.
SNAP requires dark currents below 0.1 e-/s/pix at
140?K
Measurements made at DCL (Goddard) on Hawaii-1R
with lc 1.7 mm at 150?K (courtesy of B. Hill).
Device Number
Achieved dark current meets SNAP specifications
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NIR Detectors Quantum Efficiency
Measurements made at Goddard-DCL on Hawaii-1R
(courtesy of B. Hill)
Early production lots have high QE but droops at
low wavelength.

• Rockwell seems to understand the MBE parameters
  that control QE.
• They have grown and tested new 1.7 mm material
• New devices have been tested at GSFC-DCL and have
  uniformly high QE (slightly higher dark current).

We are confident that the SNAP QE solution will
be inherited from WFC3 development.
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NIR Detectors Read Noise
SNAP specification of 5e- assumes 10e- for a
single CDS and that 4 reads at the beginning and
end of an exposure will reduce this by a factor
of ?4. Recent development RSC has confirmed
earlier GSFC-DCL measurements with 1.7 mm
material that showed large read noise for the
WFC3 devices (25 e- with a single CDS reduced to
15 e- with 4 CDS compared to 10e-/5e- obtained
with 5 mm material). The cause remains unclear.
According to RSC this is a property of the 1.7 mm
material (not the MUX) and is expected to
significantly improve for devices with larger
wavelength cutoff (1.9 - 2.0 mm) and consequently
larger dark current. A RD program involving
additional production runs is required to solve
this problem. An additional run has been
recommended for WFC3 and should be complete at
the start of SNAP RD. If WFC3 fails to solve
this problem we have two options 1) We undergo
additional production lots at RSC 2) We modify
our readout strategy.
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Intra-Pixel Variations

• SNAP under-samples by factor of 3 at 1mm (worst
  case).
• Sensitivity variations on the scale of a single
  pixel can significantly reduce photometric
  accuracy of under-sampled images.
• Accurate photometry ( 1 overall statistical,
  1 relative systematic) is current NIR
  performance target until SNAP science driven
  requirements have been passed down to the
  detector level.
• Intrapixel response for the MBE HgCdTe has not
  been measured yet.
• Intrapixel response may be fine as it is or it
  may not be. If not, Rockwell can work on
  improving the deposition process to sculpt the
  electric so as to more efficiently collect the
  charges near the pixel edges.
• Until we characterize the intrapixel response of
  an actual device, simulations remain our only
  tool to understand the impact of intrapixel
  variations on photometry.

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Intra-Pixel Variation and Photometry
PACE process produced devices have ? 0.8 but by
controlling growth process MBE HgCdTe devices
should have ? closer to unity. Simulations show
that dithering is required for a lt 0.98. Even
for ? 0.8 a 2x2 ½ pixel dither pattern can
reduce photometry error to negligible levels for
all wavelengths.
Fractional Measurement Error
Fractional Sensitive Area (?) ? RdA/A
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Michigan Infrared Detector Lab

• We have begun to set up HgCdTe evaluation
  facility at UofM
• First measurements will focus on intrapixel
  variation, read noise and dark current.
• Rockwell Science Center has delivered a H1RG
  multiplexer on a H2RG carrier.
• ARC has produced a dewar and a DAQ system.

• Setup has been customized for the H2RG. Initially
  we will use the H1RG multiplexer to debug the
  system and establish noise floor.
• The next step will be to acquire a science grade
  FPA incorporating all the experience of the WFC3
  and NGST developments to perform noise, QE and
  intrapixel variation studies.
• Eventually we will acquire additional science
  grade device(s) to verify that we can meet all
  SNAP NIR requirements.
• The facility will serve as a prototype for large
  scale HgCdTe characterization in Phase B.

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Ongoing HgCdTe Characterization Efforts

• Detector Characterization Lab (DCL) at Goddard
  Space Flight Center (PI Ed Cheng)
• The DCL currently supports the characterization
  of HgCdTe detectors for the Hubble Space
  Telescope Wide Field Camera 3 instrument.
• NASA has funded four laboratories to develop and
  assess the quality of NGST prototype detectors
• The University of Hawaii Laboratory (PI Donald
  Hall) will develop and characterize HgCdTe
  detectors manufactured by Rockwell Scientific.
• The University of Rochester Laboratory (PI
  William Forrest) will develop and characterize
  InSb detectors made by Raytheon Infrared
  Operations.
• The Independent Detector Testing Laboratory (PI
  Don Figer) at Space Telescope Science Institute
  and the Johns Hopkins University will
  characterize both HgCdTe and InSb detectors in a
  comparative hardware setup.
• The Laboratory at NASA Ames Research Center (PI
  Craig McCreight) is developing and characterizing
  SiAs mid-infrared detectors.

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Multiplexer
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Mapping FPA sub-pixel photometric response

• At ? 1.3 ?m, a diffraction-limited 5 ?m spot
  requires f lt 1.6
• Use precision molded aspherics, focal length
  2.75 mm, f 0.76
• Pinhole light source at 200 mm, vary
  longitudinal distance to compensate focal length
  variation with IR wavelength
• Move pinhole transversely to scan individual
  pixel 1 mm per pixel
• 4 ? 4 compound lens array or micro-lens array to
  test multiple sensor regions at one time
• Detailed design is underway and manufacture
  should be achieved early in FY03.

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NIR Detectors Summary

• RD Phase NIR detectors
• Obtain and characterize a science grade HgCdTe
  detector.
• Assume that problem with anomalous read noise
  will be solved by WFC3 and a solution compatible
  with SNAP requirements will be found by the start
  of SNAP RD. If not, we will need to find a
  solution. This may require additional HgCdTe
  production lot runs or a modified readout
  strategy.
• Noise reduction through multiple reads needs to
  be verified.
• Intrapixel variations will be studied.
• Demonstration that 2?2 dithering will produce
  adequate photometry precision.
• Perform detector modeling.
• Establish science driven requirements.
• Establish SNAP science grade specifications.
• Obtain and characterize a SNAP science grade
  HgCdTe detector (optional 2nd detector).
• Prepare for large scale FPA qualification.
• Demonstrate all SNAP NIR requirements can be met
  with a SNAP science grade device.
• Incorporate developments at RSC, WFC3 and NGST.