An Overview of Detectors (with a digression on reference pixels)

STScI Calibration Workshop 1

An Overview of Detectors(with a digression on reference pixels)

Bernard J. RauscherNASA Goddard Space Flight Center

22 July 2010

JWST NIRSpec HAWAII-2RG


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


Common detector types for the visible through mid-IR

CCD

•

• 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, on-chip binning is possible

Near-IR array

• 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 arrayWFC3 CCDs

JWSTNIRSpec H2RG

JWST MIRI

Hybrid detector arrays

• • Intrinsic HgCdTe or InSb

photoconductor• NICMOS, IRAC, WFC3,

JWST NIR instruments

• • Extrinsic (intentionally doped)

Si:As photoconducor (other dopants are possible for longer wavelength response)

• IRAC and JWST/MIRI22 July 2010


Photon detection in semi-conductors• Photons are absorbed in the semi-conductor

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)

– No calibration issues –light is just not detected• For photon energies greater than about 1/3rd

of the bandgap (very blue light), multiple carrier creation becomes increasingly likely

– 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).

Rieke, G.H. 2010, Elixir School22 July 2010

“p-type” “n-type”


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.

22 July 2010


For WFC3 and JWST, the HgCdTe is graded to sweep charge (actually holes) to the depletion region

n-HgCdTe

p-HgCdTe(implant)

HgCdTe Buffer

Cap

Surface Passivation

CdZnTe substrate (removed)

In bump interconnect

H2RG ROIC

HAWAII-2RG pixel architecture. Photons enter from the bottom.

AR coating goes on at the dotted green line after the substrate has been removed

22 July 2010

n-HgCdTe

p-HgCdTe(implant)

HgCdTe Buffer

Cap

Surface Passivation

CdZnTe substrate (removed)

MBE Growth Direction

Detector Band Diagram

Valence bandholes

Conduction bandelectrons


What happens in a JWST NIR pixel?

22 July 2010

e-

photonHoles are collected in the depletion region where p-type HgCdTe meets n-type HgCdTe and electrical fields (arrows) are strong

e+

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

8

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)

Name Format (# of Pixel)

Pixel Pitch (mm) # of Outputs

H4RG-15 4096 × 4096 15 1, 4, 16, 32, 64

H4RG-10 4096 × 4096 10 1, 4, 16, 32, 64

H2RG 2048 × 2048 18 1, 4, 32

H1RG 1024 × 1024 18 1, 2, 16

Institutions, Observatories, and Programs Using HxRG Arrays

Wide-field Infrared Survey Explorer (WISE)Orbiting Carbon Observatory (OCO)Development Programs in Astronomy & Earth ScienceJames Webb Space Telescope (JWST) - NIRCam, NIRSpec, FGSJoint Dark Energy Mission (JDEM)Astronomy institutions and observatories: Calar Alto, Caltech, CFHT, ESO, 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. WisconsinSpace surveillance applicationsJoint Milli-Arcsecond Pathfinder Survey (J-MAPS)Development Programs in Astronomy

In Development, first on sky telescope test in 2011

• Source follower per detector (SFD) architecture is not unique to Teledyne. Raytheon has also used an SFD with their astronomical detector arrays

9

VresetReset

Voltage

Cell drainvoltage

DsubDetectorSubstrateVoltage

PhotovoltaicDetector

ReadSelect

ResetClock

IndiumBump

Source FollowerMOSFET gate

Photocharge integrates here

3-T ROIC Pixel CellColumn

BusDetector Pixel

ColumnSelect

BufferedOutput Disable

Output Bufferdrain voltage

Output

Horizontal Read Bus

Buffered OutputGreen: ClocksPurple: Bias voltages

HxRG Pixel in the ROIC


Some calibration “gotcha’s” and where they might originate in the sensor chip assemblies (SCA)

Open Pixels

Random Telegraph Noise (RTN)

Hot Pixels

Flux Dependent Linearity

Ghosts

Inter-Pixel Capacitance (IPC)

Electronic crosstalk

Fringing(only if substrate removal

was not complete)

Persistence and latent images

Inter-pixel sensitivity

variations (IPS)

Non-linear response(also electronics)

22 July 2010

Charge diffusion

Flatfield structure


An example of how understanding the device can aid understanding a calibration issue: reciprocity failure

• For NICMOS, strongest in the blue– Suggests surface trapping is important

• According to U. Michigan group, cooling helps– Suggests traps are shallow

22 July 2010

Courtesy Bob Hill


Some “gotcha’s” originate in the readout electronics

• 1/f noise (more on this later)– Particularly evident with SIDECAR ASIC in JWST ultra-

low-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

22 July 2010


Schematic of a MIR IBC Detector

Rieke, G.H. 2010, Elixir School22 July 2010


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!)

22 July 2010


How noise averages downwith multiple non-destructive reads

• 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

sread - Read noise per readn – Number of up-the-ramp groupsm – Number of frames per grouptf – Frame readout timetg – Group timef – Photonic current (includes dark current)


ADVANCED TOPIC: REFERENCE PIXELS

Richard G. Arendt1, Dale J. Fixsen1, Don Lindler1,Markus Loose2, Samuel H. Moseley1 & Bernard J. Rauscher1

1NASA Goddard Space Flight Center2Markury Scientific

22 July 2010

SPIE Telescopes & Instruments 17

Overview• 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 e- rms 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.

29 June 2010

• 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 will1. present a demonstration of the analysis and mitigation techniques, and2. describe how to improve the next generation of detectors and readout electronics

JWST NIRSpec H2RG sensor chip assembly (SCA)


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 noise2. 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

29 June 2010


Many references available for removing the extra noise

HAWAII-2RG Detector Array

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

Regular pixels (used as a reference) because they are vignetted and never see light

29 June 2010


AN EXAMPLE OF USING MORE AND BETTER REFERENCES

29 June 2010


Raw Test Data• Outputs 1-3 sample the

detector array, but single-ended (not differential which is the default)

• Output 4 samples the reference output

• For each frame, power spectra of the time-ordered 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


Fourier analysis of the raw dataPower Cross Power

29 June 2010

Cross power is a measure of the power that is correlated between the two data sets (e.g. real output vs. reference output)


Effect of different ways of using the references

• 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

– 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

29 June 2010

Traditional JWST Differential Differential w/ Freq. Dep. Gain Interleaved References

s = 13.5 e- s = 10.8 e- s = 9.6 e-


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

29 June 2010


The Noise Floor: Traditional vs. Optimal

Traditional CDS Optimal CDS

29 June 2010

The input data in both cases are a set of one hundred 88 frame up-the-ramp sampled darks

Ignore the right hand output. It is looking at the reference output, not photo-sensitive pixels

σCDS ~ 13.5 e- rms σCDS ~ 9.6 e- rms


Future prospects

29 June 2010

• 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 effective1. References need to be sampled above the 1/f “knee” frequency2. References need to be significantly quieter than the regular pixels that

they are intended to correct3. 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


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-the-ramp

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

29 June 2010


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

22 July 2010

Documents

An Overview of Detectors (with a digression on reference pixels)