Transcript Recent Performance Improvements, Calibration Techniques and Mitigation Strategies for Large-format HgCdTe Arrays G.

Recent Performance Improvements,
Calibration Techniques and Mitigation
Strategies for Large-format HgCdTe Arrays
G. Finger, R. Dorn, S. Eschbaumer, D. Ives, L. Mehrgan, M.
Meyer, J. Stegmeier
Introduction
Hawaii-2RG close to prefect wrt basic parameters
noise, QE, darkcurrent
 comparison of 4 methods to determine conversion gain
 persistence of HgCdTe Hawaii-2RG arrays
 mitigation strategy to reduce persistence
 method to measure persistence in darkness
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Noise comparison H2RG #119 / H2RG #184
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H2RG #119 (X-Shooter)
25.3 erms on IR active pixels
7.7 erms on reference pixels
H2RG #184 (KMOS)
6,9 erms on IR active pixels
5.8 erms on reference pixels
bond pad contact resistance
improved
 noise reduced
from 25.3 to 6.9 erms
Readout noise < 10 erms for DCS
on 5 new science arrays
(KMOS and SPHERE)
Noise of KMOS arrays with Fowler sampling
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Reduce noise with multiple
nodestructive sampling
Noise 2.2 erms for
32 Fowler pairs
Noise map of crossdispersed spectrum
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slit open / warm instrument shutter closed
integration time = 600s ( 903 nondestructive readouts)
limited by shot noise of photon background which is dominated
by scattered light of K-band (5E-2 e/s/pixel)
K order 11
J order 26
Dark current of lc =2.5 mm HgCdTe
1
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2
Crossdispersed echelle
spectrum with slit closed
Cut levels :
0 - 5E-3 e/s/pixel
at T=81K , Vbias=1V
Dark current of lc =2.5 mm HgCdTe
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dark current
outside optical field:
4.2 E-4 e/s/pixel
dark current in J
1.3 E-3 e/s/pixel
T=81K, VBIAS = 1V
Dark current versus temperature
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Quantum efficiency high
over the entire sensitive
range of the array
Measurement at optical
wavelengths pending
IPC with single pixel reset
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uniformly illuminate array
with high flux
integration time 1 s
Use guide mode of
Hawaii-2RG mux
guide window size 1x1
Reset single pixel before
readout
integration time < 500ms
IPC with single pixel reset
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uniformly illuminate array
with high flux
integration time 1 s
Use guide mode of
Hawaii-2RG mux
guide window size 1x1
Reset single pixel before
readout
integration time < 500ms
Observe
capacitive coupling
on next neighbors
recent improvements of IPC
H2RG #184
H2RG #226
•improvements in multiplexer layout
resulted in reduction of
coupling coefficient a:
a#184=1.7%
a#226=1.4%
Conversion gain and single pixel reset
• Ori Fox method of classical propagation of errors:
used also by Teledyne

Assuming a
true variance
Cij is covariance
between pixels i and j
Conversion gain and single pixel reset
• Ori Fox method of classical propagation of errors:
used also by Teledyne
a
a
a
a
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Assuming a
true variance
Cij is covariance
between pixels i and j
• a=0.17 : correction factor 1+8a +52 a2 =1.13
Conversion gain and single pixel reset
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variance versus signal:
2.26 e/ADU
single pixel reset IPC
correction
1.96 e/ADU
Conversion gain from integrated autocorreolation
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variance versus signal:
2.26 e/ADU
single pixel reset IPC
correction
1.96 e/ADU
integrated autocorrelation
versus signal:
2.04 e/ADU
Conversion gain by capacitance comparison method
Relais
Reset
SFD
Vreset
Cext
C0
V
Dsub
Detector
(Vext,1  Vext, 2 )Cext 
dc level drift
on external capacitor
slope a = C0/Cext
npixel
V
i 1
n ,i
C0
sum of signal
of all pixels
Conversion gain by capacitance comparison method
Reset
Relais
SFD
Vreset
Cext
Ccryo
C0
V
Dsub
Detector
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Ccryo difficult to estimate without risk
for detector:
Ccryo includes capacitances
of cable, preamplifier board
and wirebond ceramics
Ceramic capacitors on HAWAII-2RG
wirebond ceramics show strong
temperature dependence:
T=296 K C=1mF
T=77 K C=276 nF
Conversion gain by capacitance comparison method
Reset
Relais
SFD
Vreset
Cext
Ccryo
C0
Dsub
Detector
a0 measured with Cext removed
a
C0  Cext
a
1
a0
C0=32.8 fF
Ccryo  Cext
a
a0  a
Ccryo=394 nF
C0/e=205e/mV (in our setup: 1.89e/ADU)
V
comparison of methods to obtain conversion gain
method
conversion gain
C
[e/ADU]
[e/mV]
[fF]
variance versus signal
2.26
245
39.2
integrated autocorrelation (Moore et al)
2.04
221
35.4
single pixel reset (Fox et al)
1.96
212
34.0
capacitance comparison
1.89
205
32.8
Remarks:
•first three methods are stochastic (rely on noise measurement)
•single pixel reset measures coupling coefficient
but assumes only coupling to next neighbors
•capacitance comparison is direct and robust method
taking into account coupling to all pixels
taking into account cable capacitance and
cold ceramic capacitors at detector
Persistence: X-Shooter as test bench
•with slit closed:
instrumental background:
4.2E-4 e/s/pixel
•ideal for persistence tests
Persistence: lamp on
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DIT=1.65s slit open ThAr lamp
Persistence: slit closed
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first dark exposure with DIT=128s after 2048s exposure with open slit
Persistence versus stimulus
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Persistence of first
2 min. dark exposure
is ~6.3e-4 of stimulus
Persistence at different wavelengths
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Persistence almost
the same at
l=1.07 mm
and l=2.2 mm
Persistence model of Roger Smith
traps populated when exposed to mobile
electrons and holes
pn -junction
n
p
charge trapped when location of trap
becomes undepleted and
is released in next dark exposure
Persistence model of Roger Smith
traps populated when exposed to mobile
electrons and holes
pn -junction
n
p
charge trapped when location of trap
becomes undepleted and
is released in next dark exposure
Mitigation of persistence: global reset detrapping
traps populated when exposed to mobile
electrons and holes
pn -junction
n
p
charge trapped when location of trap
becomes undepleted and
is released in next dark exposure
keep global reset switch closed after science exposure
allow de-trapping of charge
Mitigation of persistence: global reset detrapping
•Slit open
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Mitigation of persistence: global reset detrapping
•First 2 minute dark exposure
without global reset de-trapping
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Mitigation of persistence: global reset detrapping
•First 2 minute dark exposure
with global reset de-trapping
•Keep reset switch of all pixels
permanently closed with global reset
for 128 s at the end of bright exposure
to force depletion width to stay
wide avoiding population of traps
• de-trapping time is 128 s
• close slit and return to normal
operating mode taking dark exposures
•Persistence in first dark exposure
reduced by factor of 9
30
Mitigation of persistence: global reset detrapping
•First 2 minute dark exposure
with global reset always closed during bright exposure
• if reset closed before switching on
bright source and kept closed until
slit closed again persistence is zero
•global reset is an electronic shutter
which protects detector from
persistence while exposed to bright
illumination
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Mitigation of persistence: global reset detrapping
• In first 2 minute dark
without global reset de-trapping
exposure intensity of
persistence is reduced by
a factor of 9 with
global reset de-trapping
•Duration of detrapping
128 s
with global reset de-trapping
reset always closed during bright exposure
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Method to measure persistence in darkness
l
l
l
hypothesis:
persistence is generated by the change of the voltage across pn
junction of pixel diode
instead of using light to shrink depletion region
reduce bias voltage in selected area of array
using the window mode of the Hawaii-2RG multiplexer
and the global rest
outside window normal operation of the array
Persistence electrical /optical
•Generated with bias change
in selected area using global reset
•Generated with light source
Persistence electrical /optical
• red diamonds:
persistence generated with
light source on /off
•black triangles:
in selected area using global reset
persistence generated with
bias low / high
• decay with similar time constants
Persistence measured in darkness
•Measure persistence
of all 3 KMOS detectors in one go
uniformity, cosmetics, dark current. readout noise, persistence
•GL scientific mosaic mount
•128 channel cryo-preamps , flex boards
and vacuum connectors
Persistence measured in darkness
•Mosaic test facility:
no window
no optics
detector covered by black plate
flux < 1E-3 e/s/pixel
Persistence measured in darkness
•integration time 120 sec
•operating temperature 66K
•generated with bias change in darkness
on selected area using global reset
Persistence versus time
•persistence lasts for > 1500s
•persistence is
device dependent
•array # 184 is better
Persistence versus temperature
•persistence is
device dependent
•persistence of devices
#211 and #212 has a
maximum at T=66K
•persistence of device #184
does not have this
temperature maximum
Persistence versus detrapping time
•peristence is decreases with
increasing detrapping time
Persistence versus duration of illumination
•persistence increases
when detector is exposed to
the bright source
for a longer time
Persistence versus signal intensity
•persistence increases with
increasing stimulus
•bias = DSUB – VRESET
VRESET=0.5V
increasing signal
( brighter light source)
Global reset de-trapping: on sky test
• after global reset detrapping
vertical stripes
in first difference images
of two 1200s exposures
• intensity of stripes in
first difference
~ 0.03 e/s/pixel
Global reset de-trapping: on sky test
• profile of vertical
stripes
in difference images
of two 1200s exposures
• intensity of stripes in
first difference
~ 0.03 e/s/pixel
Global reset de-trapping: on sky test
• vertical
stripes
in first difference images
of two 1200s exposures
• intensity of stripes in
first difference
~ 0.03 e/s/pixel
• stripes located at
start of fast shift register
Global reset de-trapping: on sky test
•
keep clocks running during
global reset detrapping
• no vertical stripes
in first difference images
of two 600s exposures
Global reset de-trapping: on sky test
• profile of vertical
stripes
in difference images
of two 1200s exposures
• intensity of stripes in
first difference
~ 0.03 e/s/pixel
•intensity of stripes in
second difference
~ negligible
•to be further investigated
Global reset de-trapping: on sky test
• automatic flexure compensation: line intensity 45000 e/s/pixel
Global reset de-trapping: on sky test
• automatic flexure compensation: line intensity 45000 e/s/pixel
•First 1500 s dark exposure : no persistence !
Conclusion
readout noise improved by a factor of 2 on new H2RG
(6.9 erms single DCS)
 shot noise limited operation achieved in cross dispersed spectrometer in J
 PTF corrected by single pixel reset factor yields conversion gain
which agrees with capacitance comparison method within 3%
 persistence model confirmed by experiment:
traps at edge of valence and conduction bands
persistence is a consequence of changing bias voltage
no persistence is expected with CTIA since bias voltage constant
 new method to measure persistence in darkness
 global reset detrapping successfully tested on sky without residuals
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HAWK-I first light
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Tarantula Nebula
THE END
TLI: Threshold Limited Integration
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Multiple nondestructive
readouts scheme
Set saturation level
If signal exceeds
saturation level, readout
not used to calculate slope
of integration ramp
Extrapolate signal to DIT
In effect different
integration times for pixels
exceeding saturation level
TLI: Threshold Limited Integration
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Multiple nondestructive
readouts scheme
Set saturation level
If signal exceeds
saturation level, readout
not used to calculate slope
of integration ramp
Extrapolate signal to DIT
In effect different
integration times for pixels
exceeding saturation level
TLI: Threshold Limited Integration
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Multiple nondestructive
readouts scheme
Set saturation level
If signal exceeds
saturation level, readout
not used to calculate slope
of integration ramp
Extrapolate signal to DIT
In effect different
integration times for pixels
exceeding saturation level
Signal can be >> 107 e
Gain of
> 2 orders of magnitude
in dynamic range
TLI: Threshold Limited Integration
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with TLI
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without TLI
Saturation level
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Multiple nondestructive
readouts scheme
Set saturation level
If signal exceeds
saturation level, readout
not used to calculate slope
of integration ramp
Extrapolate signal to DIT
In effect different
integration times for pixels
exceeding saturation level
Full well 1E5 electrons
With TLI it is possible to
integrate > 1E7 electrons
Trapping model
Mobile
electrons
Depleted
-
-+
- - -
-
-
Trapped
electrons
- - -
Trapped
holes
-
-
N
P
+ + +
++
+
+ +
+ + +
++
+
+ +
dark idle
high flux signal
reset
next dark exp.
(large reverse bias)
(low bias)
(large reverse bias)
(small bias reduction)
Mobile
holes
++
+
+ +
All traps have released their charge
in depletion region
R.Smith, SPIE 7021-22, Marseille 2008-06-24
As signal accumulates
the depletion width is
reduced. Traps newly
exposed to charge can
capture some mobile
carriers.
At “reset” the wider
depletion region is
restored, but trapped
charge stays behind.
++
+
+ +
The released charge reduces
the bias voltage. persistence
Dark current versus temperature
generationrecombination
limited above 110K
Idark exp(-Teff/T)
Idark exp(-Egap/(1.7 KT))
 surface leakage and
tunneling below 100K
1.7E-3 e/s/pixel
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