Quantum Efficiency of AR-coated
MBE Devices
HfO2 (optimized for
~330 nm)
MBE processed
Device thickness=45µm
T=20°C
HfO2/SiO2 (broadband,
low fringing)
Temperature Dependence of Quantum
Efficiency Near Band Edge
Si Bandstructure: Indirect
Ga-As Bandstructure: Direct
Back-Illumination Process for Enhanced UV
Performance
Rim-thinned silicon wafer
Ultra-high-vacuum MBE system
Deep Depletion CCDs 1.
Electric potential
The electric field structure in a CCD defines to a large degree its Quantum Efficiency (QE). Consider
first a thick frontside illuminated CCD, which has a poor QE.
Cross section through a thick frontside illuminated CCD
In this region the electric potential gradient
is fairly low i.e. the electric field is low.
Potential along this line
shown in graph above.
Any photo-electrons created in the region of low electric field stand a much higher chance of
recombination and loss. There is only a weak external field to sweep apart the photo-electron
and the hole it leaves behind.
Deep Depletion CCDs 2.
Electric potential
In a thinned CCD , the field free region is simply etched away.
Cross section through a thinned CCD
There is now a high electric field throughout the
full depth of the CCD.
This volume is
etched away
during manufacture
Problem : Thinned CCDs may have good blue
response but they become transparent
at longer wavelengths; the red response
suffers.
Red photons can now pass
right through the CCD.
Photo-electrons created anywhere throughout the depth of the device will now be detected. Thinning
is normally essential with backside illuminated CCDs if good blue response is required. Most blue
photo-electrons are created within a few nanometers of the surface and if this region is field free,
there will be no blue response.
Deep Depletion CCDs 3.
Electric potential
Ideally we require all the benefits of a thinned CCD plus an improved red response. The solution is to use a
CCD with an intermediate thickness of about 40mm constructed from Hi-Resistivity silicon. The increased
thickness makes the device opaque to red photons. The use of Hi-Resistivity silicon means that there are no field
free regions despite the greater thickness.
Cross section through a Deep Depletion CCD
Problem :
Hi resistivity silicon contains much lower
impurity levels than normal. Very few wafer
fabrication factories commonly use this
material and deep depletion CCDs have to
be designed and made to order.
Red photons are now absorbed in
the thicker bulk of the device.
There is now a high electric field throughout the full depth of the CCD. CCDs manufactured in this way
are known as Deep depletion CCDs. The name implies that the region of high electric field, also known as
the ‘depletion zone’ extends deeply into the device.
Deep Depletion CCDs 4.
The graph below shows the improved QE response available from a deep depletion CCD.
The black curve represents a normal thinned backside illuminated CCD. The Red curve is actual data from
a deep depletion chip manufactured by MIT Lincoln Labs. This latter chip is still under development.The blue
curve suggests what QE improvements could eventually be realised in the blue end of the spectrum once
the process has been perfected.
Deep Depletion CCDs 5.
Another problem commonly encountered with thinned CCDs is ‘fringing’. The is greatly reduced
in deep depletion CCDs. Fringing is caused by multiple reflections inside the CCD. At longer
wavelengths, where thinned chips start to become transparent, light can penetrate through and be
reflected from the rear surface. It then interferes with light entering for the first time. This can give
rise to constructive and destructive interference and a series of fringes where there are minor
differences in the chip thickness.
The image below shows some fringes from an EEV42-80 thinned CCD
For spectroscopic applications, fringing can render some thinned CCDs unusable, even those
that have quite respectable QEs in the red. Thicker deep depletion CCDs , which have a much
lower degree of internal reflection and much lower fringing are preferred by astronomers
for spectroscopy.
LBNL 2k x 2k Quantum Efficiency
Quantum Efficiency of state-of-the-art CCDs
Quantum Efficiency (%)
100
LBNL
90
MIT/LL high rho
80
Marconi
70
60
50
40
30
20
10
0
300
400
500
600
700
800
900
1000
1100
From “An assessment of the optical detector systems of the W.M. Keck Observatory,”
Wavelength
J. Beletic, R. Stover, K Taylor, 19 January
2001. (nm)
2 layer anti-reflection coating: ~ 600A ITO, ~1000A SiO2
Fully-depleted pin diode radiation detector
Photons:
Near IR – Visible: 1 electron hole pair/photon
UV/x ray/g ray: E(eV)/3.6 electron hole pairs/photon
To Amplifier
VSUB
~ 80 electron hole
pairs/mm for minimum
ionizing particles
(High Energy Physics)
Slope r/esi = qND/esi
Over depleted
LBNL 2k x 4k (100mm wafer)
1478 x 4784
10.5 mm
2k x 4k
15 mm
1294 x 4186
12 mm
Measurements at Lick Observatory
Fully-depleted, back-illuminated 1024 x 512 (15mm)2 CCD fabricated at
Dalsa Semi
30 minute dark (3 e-/pixel-hr at –150C)
500nm flat field
All at 80V Vsub (overdepleted)
400nm flat field
Visible vs Near-IR imaging
LBNL 2k x 2k results
Image: 200 x 200 15 mm LBNL CCD in Lick Nickel 1m.
Spectrum: 800 x 1980 15 mm LBNL CCD in NOAO KPNO spectrograph.
Instrument at NOAO KPNO 2nd semester 2001 (http://www.noao.edu)
Correlated Double Sampler (CDS) 1.
The video waveform output by a CCD is at a fairly low level : every photo-electron in a pixel
charge packet will produce a few micro-volts of signal. Additionally, the waveform is
complex and precise timing is required to make sure that the correct parts are amplified and
measured.
The CCD video waveform , as introduced in Activity 1, is shown below for the period of
one pixel measurement
Vout
t
Reset feedthrough
Reference level
Charge dump
Signal level
The video processor must measure , without introducing any additional noise, the Reference level
and the Signal level. The first is then subtracted from the second to yield the output signal voltage
proportional to the number of photo-electrons in the pixel under measurement. The best way to
perform this processing is to use a ‘Correlated Double Sampler’ or CDS.
Correlated Double Sampler (CDS) 2.
The CDS design is shown schematically below. The CDS processes the video waveform and outputs
a digital number proportional to the size of the charge packet contained in the pixel being read. There
should only be a short cable length between CCD and CDS to minimise noise.The CDS minimises the
read noise of the CCD by eliminating ‘reset noise’. The CDS contains a high speed analogue processor
containing computer controlled switches. Its output feeds into an Analogue to Digital Converter (ADC).
R RD OD
Reset switch
CCD On-chip Amplifier
.
Inverting Amplifier
-1
OS
ADC
Input Switch
Polarity Switch
Computer Bus
Pre-Amplifier
Integrator
Correlated Double Sampler (CDS) 3.
The CDS starts work once the pixel charge packet is in the CCD summing well and the CCD reset
pulse has just finished. At point t0 the CCD wave-form is still affected by the reset pulse and so
the CDS remains disconnected from the CCD to prevent this disturbing the video processor.
t0
t0
Output wave-form of CCD
Output voltage of CDS
-1
Correlated Double Sampler (CDS) 4.
Between t1 and t2 the CDS is connected and the ‘Reference ‘ part of the waveform is sampled.
Simultaneously the integrator reset switch is opened and the output starts to ramp down linearly.
t1 t2
t1
Reference
window
-1
t2
Correlated Double Sampler (CDS) 5.
Between t2 and t3 the ‘charge dump’ occurs in the CCD. The CCD output steps negatively by an amount
proportional to the charge contained in the pixel. During this time the CDS is disconnected.
t2t3
t1
-1
t2 t3
Correlated Double Sampler (CDS) 6.
Between t3 and t4 the CDS is reconnected and the ‘signal’ part of the wave-form is sampled. The input to
the integrator is also ‘polarity switched’ so that the CDS output starts to ramp-up linearly. The width of the
signal and sample windows must be the same. For Scientific CCDs this can be anything between 1 and 20
microseconds. Longer widths generally give lower noise but of course increase the read-out time.
t3 t4
t1
Signal
window
-1
t2 t3
t4
Correlated Double Sampler (CDS) 7.
The CDS is then once again disconnected and its output digitised by the ADC. This number , typically a
16 bit number (with a value between 0 and 65535) is then stored in the computer memory. The CDS
then starts the whole process again on the next pixel. The integrator output is first zeroed by closing
the reset switch. To process each pixel can take between a fraction of a microsecond for a
TV rate CCD and several tens of microseconds for a low noise scientific CCD.
t2 t3
t4
Voltage to be
digitised
The type of CDS is called a ‘dual slope integrator’.
A simpler type of CDS known as a ‘clamp and sample’
only samples the waveform once for each pixel.
It works well at higher pixel rates but is noisier
than the dual slope integrator at lower pixel rates.
t1
-1
ADC
Pixel Size and Binning 6.
The first row is transferred into the serial register
Pixel Size and Binning 5.
Stage 1 :Vertical Binning
This is done by summing the charge in consecutive rows .The summing is done in the serial register. In the
case of 2 x 2 binning, two image rows will be clocked consecutively into the serial register prior to the serial
register being read out. We now go back to the conveyor belt analogy of a CCD. In the following animation
we see the bottom two image rows being binned.
Charge packets
Pixel Size and Binning 7.
The serial register is kept stationary ready for the next row to be transferred.
Pixel Size and Binning 8.
The second row is now transferred into the serial register.
Pixel Size and Binning 9.
Each pixel in the serial register now contains the charge from two pixels in the image area. It
is thus important that the serial register pixels have a higher charge capacity. This is achieved
by giving them a larger physical size.
Pixel Size and Binning 10.
Stage 2 :Horizontal Binning
This is done by combining charge from consecutive pixels in the serial register on a special electrode
positioned between serial register and the readout amplifier called the Summing Well (SW).
The animation below shows the last two pixels in the serial register being binned :
SW
1
2
3
Output
Node
Pixel Size and Binning 11.
Charge is clocked horizontally with the SW held at a positive potential.
SW
1
2
3
Output
Node
Pixel Size and Binning 12.
SW
1
2
3
Output
Node
Pixel Size and Binning 13.
SW
1
2
3
Output
Node
Pixel Size and Binning 14.
The charge from the first pixel is now stored on the summing well.
SW
1
2
3
Output
Node
Pixel Size and Binning 15.
The serial register continues clocking.
SW
1
2
3
Output
Node
Pixel Size and Binning 16.
SW
1
2
3
Output
Node
Pixel Size and Binning 17.
The SW potential is set slightly higher than the serial register electrodes.
SW
1
2
3
Output
Node
Pixel Size and Binning 18.
SW
1
2
3
Output
Node
Pixel Size and Binning 19.
The charge from the second pixel is now transferred onto the SW. The binning is now complete
and the combined charge packet can now be dumped onto the output node (by pulsing the voltage
on SW low for a microsecond) for measurement.
Horizontal binning can also be done directly onto the output node if a SW is not present but this can
increase the read noise.
SW
1
2
3
Output
Node
Pixel Size and Binning 20.
Finally the charge is dumped onto the output node for measurement
SW
1
2
3
Output
Node
Bloomin
g
columns
Saturated
stars
Anti-blooming CCD can eliminate this
effect:
Blooming
No blooming
One solution: Anti-blooming
CCDs
Anti-blooming CCDs
have additional gates to
bleed off the overflow
due to saturation
The problem is
these gates
cover 30% of the
pixel. This
results in
reduced
sensitivity,
Residual Images
If the intensity is too high this will leave a residual
image. Left is a normal CCD image. Right is a bias
frame showing residual charge in the CCD. This can
effect photometry
Solution: several dark frames readout or shift image
Fringing
CCDs especially back illuminated ones are bonded
to a glass plate
SiO2
10 mm
Glue 1 mm
Glass
When the glass is illuminated by monochromatic light
it creates a fringe pattern. Fringing can also occur
without a glass plate due to the thickness of the CCD
 (Å)
6600
6760
6920
7080
7280
7460
7650
7850
8100
8400
Depending on the CCD fringing becomes important
for wavelengths greater than about 6500 Å
Signal-to-Noise
Ratio
Readout Noise
0
1
3
10
Readout noise in electrons
Intensity
High readout noise CCDs (older ones) could
seriously affect your Signal-to-Noise ratios of
Basic CCD reductions
• Subtract the Bias level. The bias level is an
artificial constant added in the electronics to ensure
that there are no negative pixels
• Divide by a Flat lamp to ensure that there are no
pixel to pixel variations
• Optional: Removal of cosmic rays. These are high
energy particles from space that create „hot pixels“
on your detector. Also can be caused by natural
radiactive decay on the earth.
Bias
Overscan region
Pixel
Most CCDs have an overscan region, a portion of the
chip that is not exposed so as to record the bias level.
The prefered way is to record a separate bias (a dark
with 0 sec exposure) frame and fit a surface to this.
This is then subtracted from every frame as the first
Flat Field Division
Raw Frame
Flat Field
Raw divided by Flat
Every CCD has different pixel-to-pixel sensitivity, defects,
dust particles, etc that not only make the image look bad, but
if the sensitivity of pixels change with time can influence your
results. Every observation must be divided by a flat field after
bias subtraction. The flat field is an observation of a white
lamp. For imaging one must take either sky flats, or dome
flats (an illuminated white screen or dome observed with the
telescope). For spectral observations „internal“ lamps (i.e.
Biases, Flat Fields and Dark Frames 4.
If there is significant dark current present, the various calibration and science frames
are combined by the following series of subtractions and divisions :
Science Frame
Dark Frame
Science
-Dark
Output Image
Science -Dark
Flat Field Image
Flat-Bias
Flat
-Bias
Bias Image
Dark Frames and Flat Fields 5.
In the absence of dark current, the process is slightly simpler :
Science Frame
Bias Image
Science
-Bias
Output Image
Science -Bias
Flat-Bias
Flat Field Image
Flat
-Bias
Noise Calibration
Definitions:
N_ad - Noise in A/D converter units
N_e - Noise in electrons
S_ad - Signal in A/D converter units
S_e - Signal in electrons
g
- Gain factor (electrons/adu)
S_e = g × S_ad
N_e = g × N_ad
g²×(N_ad)² = (g × N_ad)² = (N_e)² = S_e = g × S_ad
g = S_ad / (N_ad)²
Principle of Aperture Photometry
Star
Aperture
Sky Annulus
Signal in aperture: Star + aperture_area x sky_average
Signal in Annulus: annulus_area x sky_average
Signal of Star:
aperture_signal – aperture_area x sky_average
V-band sky brightness variations
Near-Infrared Detector Arrays
- The State of the Art Klaus W. Hodapp
Institute for Astronomy
University of Hawaii
Historic Milestones
• 1800: Infrared radiation discovered (Herschel with his
thermometers)
• 1960s and 70s: Single detectors (PbS, InSb …)
• 1980s: First infrared arrays (322, 5862, 642, 1282)
• 1990: NICMOS-3 (2.5mm PACE-1 HgCdTe)
• 1991: SBRC 2562 (InSb)
• 1994: HAWAII-1 (2.5mm PACE-1 HgCdTe)
• 1995: Aladdin (InSb)
• 2000: HAWAII-2 (2.5mm PACE-1 HgCdTe)
• 2002: HAWAII-1RG (5.0μm MBE HgCdTe)
• 2002: HAWAII-2RG (5.0μm MBE HgCdTe)
• 2002: RIO 2K×2K NGST InSb
• 2009: HAWAII-4RG NSF grant (last week)
Some of the Material is from :
An Introduction to Infrared Detectors
Dick Joyce (NOAO)
NEWFIRM 4K x 4K array; Mike Merrill
19 July 2010
NOAO Gemini Data Workshop
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Now that you know all about CCDs…..
•
•
•
•
•
Introduction to the infrared
Physics of infrared detectors
Detector architecture
Detector operation
Observing with infrared detectors
– Forget what you know about CCDs….
– Imaging and spectroscopy examples
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Define infrared by detectors/atmosphere
Eric Becklin, SOFIA
vis
•
•
•
•
near-ir
mid-ir
far-ir
“visible”: 0.3 – 1.0 μm; CCDs
Near-IR: 1.0 – 5.2 μm; InSb, H2O absorption
Mid-IR : 8 – 25 μm; Si:As, H2O absorption
Far-IR: 25 – 1000 μm; airborne, space
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CCD, IR: physics is the same
• Silicon is type IV element
• Electrons shared covalently in
crystalline material
– Acts as insulator
– But electrons can be excited to conduction
band with relatively small energy (1.0 eV =
1.24 μm), depending on temperature
• Internal photoelectric effect
• Collect electrons, read out
C. Kittel, Intro. to Solid State Physics
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Extrinsic Photoconductor
• Silicon is type IV element
• Add small amount of type V (As)
• Similar to H atom within Si crystal
– Extra electron bound to As nucleus
– Very small energy required for excitation
(48 meV = 26 μm)
• Sensitive through mid-IR
C. Kittel, Intro. to Solid State Physics
19 July 2010
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Intermetallic Photoconductor
• Make Si-like compound
– III-V (InSb, GaAs)
– II-VI (HgxCd1-xTe)
• Semiconductors like Si, but with
different energy gap for
photoexcitation
– HgCdTe 0.48 eV = 2.55 μm
– InSb
0.23 eV = 5.4 μm
But, can excite electrons by other means……
19 July 2010
NOAO Gemini Data Workshop
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Wavelengths of
High Performance Detector
Materials
Si PIN
InGaAs
SWIR HgCdTe
MWIR HgCdTe
InSb
LWIR
HgCdTe
Si:As IBC
Approximate detector temperatures for dark currents << 1 e-/sec
Materials for Infrared Detectors
Collection of High-Performance
CMOS Detectors
InSb 2K x 2K,
25 µm pixels
3D stacked CMOS
wafer sandbox
HgCdTe 2K x 2K, 18 µm pixels
HgCdTe 2K x 2K,
20 µm pixels
Monolithic CMOS 4K
x 4K, 5 µm pixels
HgCdTe 4K x 4K mosaic,
18 µm pixels
Hawaii-2RG Heritage
All Successfully Developed on 1st Design Pass
1987
1990
1994
1994
-2
-1
16,384 pixels
70,000 FETs
CDS: <50e-
65,536 pixels
250,000 FETs
CDS: <30e-
65,536 pixels
250,000 FETs
CDS: <20e-
4.2 million pixels
>13 million FETs
Expect CDS <10e-
1.05 million pixels
>3.4 million FETs
CDS: <10e-
Exploiting Many Lessons
Learned to Minimize Development Risk
And Enable Next Generation Performance
2000
1998
-1R
CDS: <TBD e-
Transition to 0.25µm CMOS
With Full Wafer Stitching and
Low-Power System-on-Chip ASIC
Infrared Arrays
•Diode Array
•Multiplexer
•Readout Electronics
Electric Field in a CCD 1.
Electric potential
The n-type layer contains an excess of electrons that diffuse into the p-layer. The p-layer contains an
excess of holes that diffuse into the n-layer. This structure is identical to that of a diode junction.
The diffusion creates a charge imbalance and induces an internal electric field. The electric potential
reaches a maximum just inside the n-layer, and it is here that any photo-generated electrons will collect.
All science CCDs have this junction structure, known as a ‘Buried Channel’. It has the advantage of
keeping the photo-electrons confined away from the surface of the CCD where they could become trapped.
It also reduces the amount of thermally generated noise (dark current).
p
n
Potential along this line shown
in graph above.
Cross section through the thickness of the CCD
NIR Photodiode Array Technologies
Problems:
•Substrate availability
•Thermal expansion match to Si
•Lattice match to detector material
•LPE HgCdTe on Sapphire (PACE-1): Rockwell, CdTe buffer
•MBE HgCdTe on CdZnTe: Rockwell, thin or substrate removed, AR coated
•InSb (Raytheon): Bulk material, p-on-n, thinned, AR coated
•LPE HgCdTe on CdZnTe: Raytheon, thick
•MBE HgCdTe on Si: Raytheon, ZnTe and CdTe buffer, thick, thin in future
HAWAII-1
Rockwell Science Center
• 10241024 2.5mm HgCdTe detector
array
• 4 Quadrant architecture
• 4 Output amplifiers
• 18.5 mm pixels
• LPE HgCdTe on sapphire (PACE-1)
• Use of external JFETs possible
• Available for purchase
HAWAII-1 Focal Plane Array
Open Shutter
Close Shutter
Reset
Reset
Diode Bias Voltage
0.5 V
kTC Noise
Reset-Read Sampling
0V
Time
Readout
Recharge Noise in Capacitors
Energy stored in a capacitor: E = ½ Q²/C
Noise Energy must be: E_n = ½kT
Noise Charge: ½ (Q_n)²/C = ½kT
(Q_n)² = kTC
Q_n = √ kTC
Example:
Capacitance: 50 fF, T=37 K
k = 1.38 e-23 J/K
Q_n = √ kTC
Q_n = 5 e-18 C
With q_e = 1.6 e-19 C
Q_n = 32 electrons rms
Open Shutter
Close Shutter
kTC noise
Reset
Reset
Readout
CDS Signal
Diode Bias Voltage
0.5 V
Double Correlated Sampling
0V
Time
Readout
Open Shutter
Close Shutter
kTC noise
Reset
Reset
Readout
MCS Signal
Diode Bias Voltage
0.5 V
Fowler (multi) Sampling
0V
Time
Readout
Open Shutter
Close Shutter
kTC noise
Reset
MCS Signal
Reset
Diode Bias Voltage
0.5 V
Up-the-Ramp Sampling
0V
Time
So, here’s what we have to deal with..
• Raw K-band image of field shows
stars, but also substantial sky
signal
• Sky signal intensity varies over
field
– Large-scale variations
• Illumination
• Quantum efficiency variations
– Small-scale variations
• Pixel-to-pixel variations
•
•
•
Array defects
High dark current pixels (mavericks)
These can be corrected by appropriate
calibration images
– Dark frames (bias)
– Flatfield images
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When we try this (CCD style)…
• Obtain science images
• Obtain calibration images
– Dark frames at same integration time
– Flatfield images of uniform target
•
•
•
•
•
Subtract dark frame from science images
Divide dark-subtracted images by flatfield
 Image of science field with uniform sky
level
Subtract (constant) sky level from image
But, here is what we get…..
– Better, but still see substantial sky
variations
Small flatfield errors on sky still larger than faint science targets
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Since the sky is the problem…
• Subtract out the sky ( or as much as
possible) before the flatfield
correction
• Obtain two images of field, move
telescope between
• Subtract two images
– Eliminate almost all sky signal
– Subtracts out dark current, maverick
pixels
•
•
Divide by flatfield image
Result has almost no sky structure
Subtracting sky minimizes effects of flatfield errors
(but noise increased by 1.4)
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Typical sequence for IR imaging
• Multiple observations of science field with small telescope motions in
between (dithering)
– Sky background limits integration time
– Moving sources samples sky on all pixels
– Moving sources avoids effects of bad/noisy pixels
•
Combine observations using median filtering algorithm
– Effectively removes stars from result sky image
– Averaging reduces noise in sky image
•
•
Subtract sky frame from each science frame  sky subtracted images
Divide sky subtracted images by flatfield image
– Dome flat using lights on – lights off to subtract background
– Sky flat using sky image – dark image using same integration time
– Twilight flats – short time interval in IR
•
Shift and combine flatfielded images
– Rejection algorithm (or median) can be used to eliminate bad pixels from final image
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NOAO Gemini Data Workshop
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Here’s what it looks like….
Sky frame
Median
Subtract sky,
divide each by
Flatfield
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NOAO Gemini Data Workshop
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Shift and combine images
• NGC 7790, Ks filter
• 3 x 3 grid
• 50 arcsec dither offset
Bad pixels eliminated
From combined image
Higher noise in corners
than in center (fewer
combined images)
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NOAO Gemini Data Workshop
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This works fine in sparse fields, but what
about crowded fields, extended targets?
• In addition to dithered observations of science field (still necessary for
sampling good pixels), it is necessary to obtain dithered observations
of a nearby sparse field to generate a sky image.
• Requires additional observing overhead, but this is the only way to
obtain proper sky subtraction
“And if you try to cheat, and don’t take the proper number of
sky frames, then you get what you deserve”
--Marcia Rieke
19 July 2010
NOAO Gemini Data Workshop
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An example: M42
Raw image in narrowband H2 filter
Off-source sky frame
Sky-subtracted, flatfielded image
19 July 2010
NOAO Gemini Data Workshop
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Mid-infrared strategy
• Sky background at 10 μm is 103 – 104 greater than in K band
– Detector wells saturate in very short time (< 50 ms)
– Very small temporal variations in sky >> astronomical source intensities
•
•
Read array out very rapidly (20 ms), coadd images
Sample sky at high rate (~ 3 Hz) by chopping secondary mirror (15 arcsec)
– Synchronize with detector readout, build up “target” and “sky” images
– But tilting of secondary mirror introduces its own offset signal
•
Remove offset by nodding telescope (30 s) by amplitude of chop motion
– Relative phase of target changed by 180° with respect to chop cycle
– Relative phase of offset signal unchanged
– Subtraction adds signal from target, subtracts offset
•
http://www.gemini.edu/sciops/instruments/t-recs/imaging
……
chop
19 July 2010
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HAWAII-1
•
•
•
•
•
•
Quantum efficiency (50% - 60%)
Dark current 0.01 e-/s (65K)
Read noise about 10 - 15 e- rms CDS
Residual image effect
Some multiplexer glow
Fringing
3600 s
128 samp
T= 65K
Internal
FETs
External
JFETs
optimized
Fringing in PACE-1 material
1997
1998
Residual Images in PACE-1 HAWAII-1 Arrays
Aladdin
Raytheon Center for Infrared Excellence
•
•
•
•
•
•
•
10241024 InSb detector array
4 Quadrant architecture
32 Output amplifiers
27 mm pixels
Thinned, AR coated InSb
Three generations of multiplexers
“Foundry Run” distribution mode
Aladdin
•
•
•
•
•
•
Quantum efficiency high (80% - 90%)
Dark current 0.2 - 1.0 e-/s
Read noise about 40 e- rms CDS
Charge capacity 200,000 eResidual image effect
No amplifier glow
Aladdin frame taken with SPEX (J. Rayner)
NIRI Aladdin Image of AFGL2591
HAWAII-2
Rockwell Science Center
• 20482048 2.5mm HgCdTe detector
array
• 4 Quadrant architecture
• 32 Output amplifiers
• 3 Output modes available
• 18.0 mm pixels
• Use of external JFETs possible
• Reference signal channel
underway for ProCam-2
• Also migrating to
0.13µm on newest
programs to boost
performance via Cu and
low-k interlayer
dielectrics
10
DRAM CMOS
RSC FPA
Minimum Feature (µm)
•
•
Continuing to Aggressively Use
5 Designs in 0.25µm
CMOS
3.3/1.8V 0.18µm CMOS
1
After Isaac (1999)
0.1
1965
1970
1975
1980 1985 1990 1995
Year of Introduction
2000
2005
HAWAII-2: Photolithographically Abut 4 CMOS
Reticles to Produce Each 20482 ROIC
Twelve 20482 ROICs per 8” Wafer
Submicron Stepper Options
Canon 16mm x 14 mm
GCA 20mm x 20 mm
ASML 22mm x 27.4 mm
Reticle-Stitching: 2048x2048 ROIC
20482 Readout Provides Low Read Noise for Visible and MWIR
HAWAII-2 Reference Signal
New Developments
• Multiplexers:
• Detector Materials:
•
•
•
•
•
•
•
•
•
•
•
HAWAII-1R
HAWAII-1RG
HAWAII-2RG
Abuttable 2K2K
RIO developments
MBE HgCdTe on CdZnTe
MBE HgCdTe on Si
Cutoff wavelength
Thinning
Substrate removal
AR coating
RSC Approach
HAWAII - 2RG
HgCdTe2 Astronomy Wide Area Infrared Imager
with 2k Resolution, Reference pixels and Guide Mode
• HgCdTe detector
– substrate removed to achieve 0.6 µm sensitivity
• Specifically designed multiplexer
– highly flexible reset and readout options
– optimized for low power and low glow operation
– three-side close buttable
• Two-chip imaging system: MUX + ASIC
– convenient operation with small number of
clocks/signals
– lower power, less noise
3-D Barrier to Prevent Glow from Reaching
the Detector
HgCdTe
Detector
p-type
n+
Indium
Interconnect
Low-Noise CMOS Multiplexer
Overglass
Metal 3
Analog Capacitor
Metal 2
Metal 1
Poly 1
CMOS (LOCOS)
Prototype 2×2 Mosaic for NGST
Ground-Based Camera Projects
2K*2K IR Arrays
•IfA ULB
•UKIRT WFC
•CFHT WIRCAM
•Gemini GSAOI
•ESO VISTA
•Keck KIRMOS
Block Diagram
I/O Pads & output buffers
serial
interface
clock
buffers
decoders for horizontal start and stop address
fast guide shift register + logic
fast normal shift register + logic
5 MHz column buffers
glow and crosstalk shield
decoders for vertical start and stop address
Slow guide shift register + logic
Slow normal shift register + logic
glow and crosstalk shield
4 rows and columns containing reference pixels
Additional row of reference pixels
for diagnostic purposes
2048 x 2048 pixel array
(2040 x 2040 sensitive pixels)
4 rows and columns containing reference pixels
• All pads located
on one side
(top)
• Approx. 110
doubled I/O
pads (probing
and bonding)
• Three-side
close buttable
• 18 µm pixels
• Total
dimensions:
39 x 40.5 mm²
FPA Housings in NIRCam
LW FPAs
and
Housings
Module A
Module B
SW FPAs
and
Housings
*OBA Struts and Brackets not shown
Spectroscopy uses similar strategy
1.89
1.42 1.13 0.94
• Example: GNIRS spectrum
– R ~ 2000, cross-dispersed
– 0.8 – 2.5 μm in five orders
• Strong, wavelength-dependent sky
– OH emission lines 0.8 – 2.3 μm
– Thermal continuum 2.0 + μm
– Atmospheric absorption > 2.3 μm shows up
as emission in thermal
• Need to subtract out sky
2.55
1.91 1.53 1.27 1.09
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Subtract sky by dithering along slit
• First 900s exposure
• Move QSO 4 arcsec along
slit, expose
• Subtract
• Eliminates most of sky
lines
– OH emission time variable
– Very small (.02 pixel)
instrument flexure
– Remove residual sky using
software
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Summary
• Infrared arrays utilize same physics as CCDs
• Architecture is different from CCDs
– Hybrid construction: separate detector and readout
– Unit cell: row/column addressing – no charge transfer
– Nondestructive readout – double; multiple correlated sampling
•
Low temperature operation
– Minimize detector dark current (bad electrons)
– Minimize thermal radiation from instrument (bad photons)
•
More bad photons – sky is limiting factor in infrared
– Imaging: sky >> astronomical signals
– Spectroscopy: sky bright, emission lines
– Strategy: dithering to eliminate sky contribution
Review article: George Rieke 2007, Ann. Rev. Astr. Ap. 45, 77.
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