recent performance improvements, calibration techniques and mitigation strategies for large-format...

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

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

Introduction

Noise comparison H2RG #119 / H2RG #184

H2RG #119 (X-Shooter)25.3 erms on IR active pixels7.7 erms on reference pixels

H2RG #184 (KMOS)6,9 erms on IR active pixels5.8 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

Reduce noise with multiple nodestructive sampling

Noise 2.2 erms for 32 Fowler pairs

Noise map of crossdispersed spectrum 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)

J order 26

K order 11

Dark current of c =2.5 m HgCdTe

Crossdispersed echelle spectrum with slit closed

Cut levels :

0 - 5E-3 e/s/pixelat T=81K , Vbias=1V

1

2

dark current outside optical field:4.2 E-4 e/s/pixel

dark current in J1.3 E-3 e/s/pixel

T=81K, VBIAS = 1V

Dark current of c =2.5 m HgCdTe

Dark current versus temperature

Quantum efficiency high over the entire sensitive range of the array

Measurement at optical wavelengths pending

IPC with single pixel reset

uniformly illuminate arraywith high flux integration time 1 s

Use guide mode of Hawaii-2RG muxguide window size 1x1

Reset single pixel before readoutintegration time < 500s

uniformly illuminate arraywith high flux integration time 1 s

Use guide mode of Hawaii-2RG muxguide window size 1x1

Reset single pixel before readoutintegration time < 500s

Observe capacitive coupling on next neighbors

IPC with single pixel reset

recent improvements of IPC

•improvements in multiplexer layout resulted in reduction of coupling coefficient :

#184=1.7% #226=1.4%

H2RG #184

H2RG #226

Conversion gain and single pixel reset

Assuming a

• Ori Fox method of classical propagation of errors: used also by Teledyne

Cij is covariancebetween pixels i and j

true variance


Assuming a

• Ori Fox method of classical propagation of errors: used also by Teledyne

Cij is covariancebetween pixels i and j

• =0.17 : correction factor 1+8 =1.13

true variance


variance versus signal:

2.26 e/ADU single pixel reset IPC

correction1.96 e/ADU

Conversion gain from integrated autocorreolation

variance versus signal:

2.26 e/ADU single pixel reset IPC

correction1.96 e/ADU

integrated autocorrelation versus signal: 2.04 e/ADU

Conversion gain by capacitance comparison method

Vreset

Dsub

CextC0

Reset SFD

V

Relais

Detector

01

,2,1, )( CVCVVnpixel

iinextextext

dc level drifton external capacitor

sum of signal of all pixels

slope = C0/Cext


Vreset

Dsub

CextC0

Reset SFD

V

Relais

Detector

Ccryo

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=1FT=77 K C=276 nF


0

0

0

1extcryoext CCCC

C0=32.8 fF Ccryo=394 nF

C0/e=205e/mV (in our setup: 1.89e/ADU)

Vreset

Dsub

CextC0

Reset SFD

V

Relais

Detector

Ccryo

0 measured with Cext removed

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

DIT=1.65s slit open ThAr lamp

Persistence: slit closed

first dark exposure with DIT=128s after 2048s exposure with open slit

Persistence versus stimulus

Persistence of first 2 min. dark exposure is ~6.3e-4 of stimulus

Persistence at different wavelengths

Persistence almost the same at

=1.07 m

and =2.2 m

Persistence model of Roger Smith

p npn -junction

charge trapped when location of trapbecomes undepleted and is released in next dark exposure

traps populated when exposed to mobileelectrons and holes

p npn -junction



Persistence model of Roger Smith

Mitigation of persistence: global reset detrapping

p npn -junction



keep global reset switch closed after science exposure allow de-trapping of charge

28

•Slit open


29

•First 2 minute dark exposure without global reset de-trapping


30

•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

•First 2 minute dark exposure with global reset de-trapping


31

• 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

•First 2 minute dark exposure with global reset always closed during bright exposure


32

• In first 2 minute dark exposure intensity of persistence is reduced by a factor of 9 with global reset de-trapping•Duration of detrapping 128 s

without global reset de-trapping

with global reset de-trapping

reset always closed during bright exposure


Method to measure persistence in darkness

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 gouniformity, cosmetics, dark current. readout noise, persistence

•GL scientific mosaic mount •128 channel cryo-preamps , flex boardsand vacuum connectors


•Mosaic test facility:no windowno opticsdetector covered by black plateflux < 1E-3 e/s/pixel


•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


• profile of vertical stripes in difference images of two 1200s exposures• intensity of stripes in first difference ~ 0.03 e/s/pixel


• 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


• keep clocks running during global reset detrapping• no vertical stripes in first difference images of two 600s exposures


• 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


• automatic flexure compensation: line intensity 45000 e/s/pixel


• automatic flexure compensation: line intensity 45000 e/s/pixel•First 1500 s dark exposure : no persistence !

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

Conclusion

HAWK-I first light

Tarantula Nebula

THE END

TLI: Threshold Limited Integration

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



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



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

with TLI

without TLI

Saturation level

Trapping model

P

N

-- - -

-

++++

+

next dark exp.

(small bias reduction)

The released charge reduces the bias voltage. persistence

- - -

+ + +

-- - -

-

++++

+

high flux signal

(low bias)

As signal accumulates the depletion width is reduced. Traps newly exposed to charge can capture some mobile carriers.

Trapped holes

Trapped electrons

- - -

+ + +

++++

+

-- - -

-

reset

(large reverse bias)

At “reset” the wider depletion region is restored, but trapped charge stays behind.

Depleted

Mobile electrons

Mobile holes

dark idle

(large reverse bias)

All traps have released their charge in depletion region

++++

+

-- - -

-

-+

R.Smith, SPIE 7021-22, Marseille 2008-06-24

Dark current versus temperature

generation-recombination 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

recent performance improvements, calibration techniques and mitigation strategies for large-format...

Documents

single pixel resetassuming

single pixel resetvariance

eadusingle pixel

eadu conversion gain

readout integration

high flux integration

c0cextconversion gain

j true varianceconversion