An Overview of Detectors
(with a digression on reference pixels)
JWST NIRSpec HAWAII-2RG
Bernard J. Rauscher
NASA Goddard Space Flight Center
22 July 2010
STScI Calibration Workshop
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Introduction
• After the diameter of the primary mirror, no
component affects the performance of an observatory
more than the detectors
• Detectors imprint a signature (e.g. dead pixels, hot
pixels, QE variations etc.) onto the data
• Calibrating out this signature is critical to getting the
most out of these observatories
• Need to understand how detectors function and why
specific signatures occur
• In this talk, I present an introduction to detectors with
an emphasis on JWST’s HAWAII-2RG (H2RG) sensor
chip assemblies (SCA)
22 July 2010
STScI Calibration Workshop
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Common detector types for the visible through mid-IR
Hybrid detector arrays
CCD
•
Photon collection separated from readout
–
–
•
Optimized detector layer collects charge
Optimized readout integrated circuit senses charge in place (it does not
move like in a CCD)
Multiple non-destructive reads typically used to beat down
read noise and integrate through cosmic ray hits
Mid-IR array
Near-IR array
WFC3 CCDs
•
•
Intrinsic Si photoconcuctor
•
Photons collected and charge
read out in same piece of
silicon
•
•
•
During readout, charge
physically moves from one
pixel to the next
Usual readout is correlated
double sampling
Because charge moves, onchip binning is possible
22 July 2010
JWST
NIRSpec H2RG
•
•
•
Intrinsic HgCdTe or InSb
photoconductor
NICMOS, IRAC, WFC3,
JWST NIR instruments
STScI Calibration Workshop
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•
•
JWST MIRI
Extrinsic (intentionally doped)
Si:As photoconducor (other
dopants are possible for
longer wavelength response)
IRAC and JWST/MIRI
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Photon detection in semi-conductors
•
•
Photons are absorbed in the semiconductor creating electron/hole pairs
For photon energies less than the
bandgap, the photo-conductor does not
respond to light unless it has been doped
(e.g. MIRI’s detectors)
–
•
For photon energies greater than about
1/3rd of the bandgap (very blue light),
multiple carrier creation becomes
increasingly likely
–
–
•
No calibration issues –light is just not
detected
“ptype”
“ntype”
Probable calibration issue for JWST. Both
NIRSpec and FGS use 5 micron cutoff
detectors at 600 nm
RQE > DQE!
To see how MIRI’s Si:As detectors work,
compare the diagram of crystal structure
(above) with the band gap diagrams
(below). To free an electron in intrinsic
material (1) requires a certain energy
indicated by the band gap. It takes less
energy to free charge carriers from
impurities (2) and (3).
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Rieke, G.H. 2010, Elixir School
How a JWST near-IR array works
Readout
Integrated Circuit
(ROIC)
Simplified structure of a hybrid IR array detector. In
real arrays, there is often an epoxy backfill
between the indium bumps.
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For WFC3 and JWST, the HgCdTe is graded to sweep charge
(actually holes) to the depletion region
Detector Band Diagram
p-HgCdTe
(implant)
H2RG ROIC
Surface
Passivation
In bump
interconnect
Cap
Surface
Passivation
HgCdTe
Buffer
Cap
n-HgCdTe
HgCdTe Buffer
CdZnTe
substrate
(removed)
Conduction band
electrons
p-HgCdTe
(implant)
n-HgCdTe
MBE Growth Direction
CdZnTe substrate
(removed)
HAWAII-2RG pixel architecture. Photons
enter from the bottom.
AR coating goes on at the dotted
green line after the substrate has
been removed
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What happens in a JWST NIR pixel?
Outside the depletion region, E fields are weaker
and charge can diffuse between pixels
QE depends on wavelength. Blue light is
absorbed near the surface and red light
is absorbed deep in the detector
If an anomaly is strongest in the blue, it
might be a surface effect. If it is
strongest in the red, it might affect
deeper detector layers
e-
e+
photon
22 July 2010
STScI Calibration Workshop
Holes are collected in
the depletion region
where p-type HgCdTe
meets n-type HgCdTe
and electrical fields
(arrows) are strong
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JWST’s H2RGs are part of the Teledyne HxRG family
H: HAWAII: HgCdTe Astronomical Wide Area Infrared Imager
x: Number of 1024 (or 1K) pixel blocks in x and y-dimensions
R: Reference pixels
G: Guide window capability
 Substrate-removed HgCdTe for simultaneous visible &
infrared observation
 Hybrid Visible Silicon Imager; Si-PIN (HyViSI)
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Pixel Pitch
# of Outputs Institutions, Observatories, and Programs Using HxRG Arrays
(mm)
Name
Format
(# of Pixel)
H1RG
1024 × 1024
18
H2RG
2048 × 2048
18
H4RG-10
4096 × 4096
10
H4RG-15
4096 × 4096
15
Wide-field Infrared Survey Explorer (WISE)
Orbiting Carbon Observatory (OCO)
Development Programs in Astronomy & Earth Science
James Webb Space Telescope (JWST) - NIRCam, NIRSpec, FGS
Joint Dark Energy Mission (JDEM)
Astronomy institutions and observatories: Calar Alto, Caltech, CFHT, ESO,
1, 4, 32 ESTEC, Gemini, GSFC, IRTF, ISRO, IUCAA, JHU-APL, Keck, LBNL, LMU,
MIT, MPIA, MPS, OCIW, Penn State, RIT, SALT, SAO, Subaru, TATA, U.
Arizona, UCLA, UC Berkeley, U. Hawaii, U. Rochester, U. Toronto, U.
Wisconsin
Space surveillance applications
1, 4, 16, 32, Joint Milli-Arcsecond Pathfinder Survey (J-MAPS)
64
Development Programs in Astronomy
1, 2, 16
1, 4, 16, 32,
In Development, first on sky telescope test in 2011
64
HxRG Pixel in the ROIC
Green: Clocks
Purple: Bias voltages
Output
Buffered Output
Buffered
Output Disable
Horizontal Read Bus
Output Buffer
drain voltage
Dsub
Reset
Clock
Detector
Substrate
Voltage
Photovoltaic
Detector
Column
Select
Vreset
Reset
Voltage
Read
Select
Indium
Bump
Source Follower
MOSFET gate
Photocharge integrates here
Detector Pixel
•
Cell drain
voltage
3-T ROIC Pixel Cell
Column
Bus
Source follower per detector (SFD) architecture is not unique to Teledyne.
Raytheon has also used an SFD with their astronomical detector arrays
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Some calibration “gotcha’s” and where they might originate in the
sensor chip assemblies (SCA)
Electronic crosstalk
Random Telegraph
Noise (RTN)
Ghosts
Open Pixels
Inter-Pixel
Capacitance (IPC)
Hot Pixels
Charge diffusion
Persistence and
latent images
Flatfield structure
Inter-pixel
sensitivity
variations (IPS)
Flux Dependent
Linearity
Fringing
(only if substrate removal
was not complete)
Non-linear response
(also electronics)
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An example of how understanding the device can aid
understanding a calibration issue: reciprocity failure
Courtesy Bob Hill
• For NICMOS, strongest in the blue
– Suggests surface trapping is important
• According to U. Michigan group, cooling helps
– Suggests traps are shallow
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Some “gotcha’s” originate in the readout electronics
• 1/f noise (more on this later)
– Particularly evident with SIDECAR ASIC in JWST ultralow-power & temperature operation
– Also seen in ground based controllers
• Bars & bands
– Happen when one part of the system pulls down the
biases for another
• Tails
– Caused by settling time issues in the readout
electronics and harnesses
• Pedestal drifts
– Caused by unstable biases
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Schematic of a MIR IBC Detector
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Rieke, G.H. 2010, Elixir School
Readout
• For CCDs, charge is moved to the
output, sensed, and discarded
– Nevertheless, noise performance
of CCDs is outstanding. JWST’s
NIR and MIR arrays do not yet
match them
• For NIR and MIR arrays, charge is
sensed in place by the ROIC
– Can use multiple non-destructive
reads to average down noise and
integrate through cosmic ray hits!
– Achieving CCD like noise
performance with JWST’s NIR
arrays will require new readout
approaches (yes, we are working
on this!)
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How noise averages down
with multiple non-destructive reads
sread - Read noise per read
n – Number of up-the-ramp groups
m – Number of frames per group
tf – Frame readout time
tg – Group time
f – Photonic current (includes dark current)
•
•
Model does not include 1/f noise, will under estimate the noise of JWST’s
SIDECAR based detector systems somewhat
This differs slightly from what is shown in Rauscher et al., PASP, 119, 768 (due to a
transcription error while finalizing the manuscript)
– Error caught by Massimo Robberto of STScI (Thanks!)
– Massimo presents an independent derivation that expands this result somewhat in an internal
STScI memo (please speak to Massimo for details)
22 July 2010
STScI Calibration Workshop
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Richard G. Arendt1, Dale J. Fixsen1, Don Lindler1,
Markus Loose2, Samuel H. Moseley1 & Bernard J. Rauscher1
1NASA
Goddard Space Flight Center
2Markury Scientific
ADVANCED TOPIC: REFERENCE
PIXELS
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STScI Calibration Workshop
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Overview
•
•
•
•
•
•
•
1.
2.
Performance of 2kx2k Teledyne HAWAII-H2RG detectors and SIDECAR
ASICs is key to the success of the JWST mission
Broadband imaging is generally background limited. With QE ~ 80%,
only incremental improvement still possible
Spectroscopy & narrow band imaging are generally detector noise
limited –large improvements still possible even with NIRSpec’s 6 erms total noise requirement
We have begun a program to analyze the noise characteristics of the
NIRSpec detector subsystem, studying the correlations among the
detector outputs and with the reference output, as well as the
temporal correlations in a given detector section.
JWST NIRSpec H2RG sensor chip
assembly (SCA)
Using the measured characteristics of the noise correlations, we can determine the optimal coefficients for
the removal of correlated noise as a function of frequency. By using all available reference sources, and by
adding more frequent references, we can potentially reduce the noise by a factor of two
We find that there is a frequency dependent gain and a frequency dependent correlation between the
regular pixels and the reference pixels and the reference output
In this talk we will
present a demonstration of the analysis and mitigation techniques, and
describe how to improve the next generation of detectors and readout electronics
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SPIE Telescopes & Instruments
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Principal Components Analysis
•
•
•
•
Principal components analysis (PCA) puts noise studies on a firm quantitative
foundation
Computed the covariance matrix of vertical and horizontal cuts across the detector
array, as well as in ~ 64 x 64 pixel regions
Computed the eigensystem of the covariance matrix and sorted the eigenvectors by
descending eigenvalue
Major noise components of Flight NIRSpec detector subsystem are
1. 1/f noise
2. Alternating column pattern noise
•
•
These components are highly correlated with available references and can be
removed using standard techniques
Almost all of the correlation is temporal –there is little difference between pixels
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Many references available for removing the extra noise
4 rows of reference pixels along
the “bottom” and “top” edges of
each detector array
4 columns of reference pixels
along the “left” and “right” edges
of each detector array
A separate reference output that
is always available for all pixels
HAWAII-2RG Detector Array
Regular pixels (used as a
reference) because they are
vignetted and never see light
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AN EXAMPLE OF USING MORE AND
BETTER REFERENCES
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Raw Test Data
• Outputs 1-3 sample the
detector array, but singleended (not differential
which is the default)
• Output 4 samples the
reference output
• For each frame, power
spectra of the timeordered data are
calculated for each
output. Results averaged
over 88 frames of a single
ramp.
29 June 2010
Op #1
Op #2
Op #3
Op #4
(REFOUT)
Appearance of raw single-ended data. The
horizontal banding indicates the presence
of highly correlated 1/f noise
SPIE Telescopes & Instruments
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Fourier analysis of the raw data
Power
Cross Power
Cross power is a measure of the power that is correlated between the two data sets
(e.g. real output vs. reference output)
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Effect of different ways of using the references
Traditional JWST Differential
s = 13.5 e•
•
s = 10.8 e-
Interleaved References
s = 9.6 e-
Traditional JWST differential feeds the H2RG’s reference output to the SIDECAR ASIC’s
differential inputs
Differential w/ Frequency dependent gain digitizes everything in single ended mode. A
frequency dependent weighting is applied to the H2RG’s reference output before it is
subtracted
–
–
•
Differential w/ Freq. Dep. Gain
This weighting account for gain difference at low freuqncy
And lower degree of correlation at high frequency
Interleaved references jump out to read 8 blanked off pixels every 128 pixels. Includes
corrections for 1/f and alternating columns
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SPIE Telescopes & Instruments
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Power at the Nyquist Frequency
•
•
Expanded view of power near and at the Nyquist frequency for one of the detector outputs
Symbols show results before and after optimal use of reference pixels and outputs
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SPIE Telescopes & Instruments
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The Noise Floor: Traditional vs. Optimal
The input data in both cases are a set of one hundred 88 frame up-the-ramp sampled
darks
Traditional CDS
σCDS ~ 13.5 e- rms
Optimal CDS
σCDS ~ 9.6 e- rms
Ignore the right hand output. It is looking at the reference output, not photo-sensitive
pixels
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SPIE Telescopes & Instruments
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Future prospects
• This work highlights the importance of sampling low-noise references
frequently and weighting the references by frequency
• In current generation H2RG detector arrays, reference pixels in rows and
columns are: (1) too far away and (2) too noisy to suppress 1/f noise
• In current generation SIDECAR ASICs, there is no good way to weight the
H2RG’s reference output by frequency in the differential input
• To be most effective
1. References need to be sampled above the 1/f “knee” frequency
2. References need to be significantly quieter than the regular pixels that
they are intended to correct
3. Reference corrections need to take into account possible frequency
dependent weighting between the reference signal and the signal that is
being corrected
• These goals can be met by many different ROIC and readout electronics
designs
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To sum up: More & Better References
• Noise of JWST’s NIR detectors is much better than we
thought!
• SIDECARs are injecting correlated noise
– Can be removed by using more and better references
– Almost all the correlation is temporal rather than spatial
– Must work in Fourier domain; reference corrections must be
frequency weighted
• Flight NIRSpec DS has total noise ~ 6 e- rms for 88 up-theramp samples (EXPTIME ~ 900 s)
• The techniques describe here should drop that to about 3.5
e- rms without changing the hardware
• Work is ongoing to demonstrate these improvements in
practice
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Summary
• In this short talk, I’ve tried to present an overview of
common astronomical detectors for the visible through
mid-IR
– Emphasis on JWST’s HAWAII-2RG near-IR arrays
• Briefly discussed some of the anomalies that are expected,
and where they originate in hybrid near-IR arrays
– Others will no doubt discuss many of these further at this
conference
• Discussed how using more references more effectively can
significantly improve the performance of JWST’s detector
limited instruments
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