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Commentary: JWST near-infrared detector degradation— finding the problem, fixing the problem, and moving forward
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1. Although the acronym H2RG refers only to the readout integrated circuit, here we follow convention in the astronomical community and use it to refer to the complete hybrid detector array.
2. J. P. Gardner, J. C. Mather, M. Clampin, R. Doyon, M. A. Greenhouse, H. B. Hammel, J. B. Hutchings, P. Jakobsen, S. J. Lilly, K. S. Long, J. I. Lunine, M. J. McCaughrean, M. Mountain, J. Nella, G. H. Rieke, M. J. Rieke, H.-W. Rix, E. P. Smith, G. Sonneborn, M. Stiavelli, H. S. Stockman, R. A. Windhorst, and G. S. Wright, “The James Webb Space Telescope,” Space Sci. Rev. 123, 485606 (Apr. 2006), arXiv:astro-ph/0606175.
3. Matthew A. Greenhouse, Vicki Balzano, Pamela Davila, Michael P. Drury, Jamie L. Dunn, Stuart D. Glazer, Ed Greville, Gregory Henegar, Eric L. Johnson, Ray Lundquist, John C. Mccloskey, Raymond G. Ohl, Robert A. Rashford, and Mark F. Voyton, “Status of the james webb space telescope integrated science instrument module system,” UV/Optical/IR Space Telescopes and Instruments: Innovative Technologies and Concepts V. Edited by Tsakalakos 8146, 262 (Sep 2011), http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011SPIE.8146E.262G&link_type=ABSTRACT.
4. The processed detector wafers are evaluated by current-voltage and optical testing of PECs. PECs are lithographically patterned adjacent to the main array(s) on the wafers, and are diced and wirebonded into leadless chip carrier (LCC) packages for cryogenic testing. They contain variable sized photodiodes, small test arrays, contact measurement structures, and other test devices used for evaluating the diode performance (dark current, quantum efficiency, contact resistance) and materials properties (diffusion length, carrier lifetime, etc.) for each processed wafer.
5. Markus Loose, James Beletic, James Garnett, and Min Xu, “High-performance focal plane arrays based on the hawaii-2rg/4g and the sidecar asic,” Focal Plane Arrays for Space Telescopes III. Edited by Grycewicz 6690, 10 (Sep 2007), http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007SPIE.6690E..10L&link_type=ABSTRACT.
6. Markus Loose, Mark C. Farris, James D. Garnett, Donald N. B. Hall, and Lester J. Kozlowski, “Hawaii-2rg: a 2k × 2k cmos multiplexer for low and high background astronomy applications,” IR Space Telescopes and Instruments. Edited by John C. Mather. Proceedings of the SPIE 4850, 867 (Mar 2003), http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003SPIE.4850..867L&link_type=ABSTRACT.
7. S. H. Moseley, Richard G. Arendt, D. J. Fixsen, Don Lindler, Markus Loose, and Bernard J. Rauscher, “Reducing the read noise of h2rg detector arrays: eliminating correlated noise with efficient use of reference signals,” High Energy 7742, 36 (Jul 2010), http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010SPIE.7742E..36M&link_type=ABSTRACT.
8. Bernard J. Rauscher, Richard G. Arendt, D. J. Fixen, Matthew Lander, Don Lindler, Markus Loose, S. H. Moseley, Donna V. Wilson, and Christos Xenophontos, “Reducing the read noise of hawaii-2rg based detector systems with improved reference sampling and subtraction (irs2),” Infrared Sensors 8155, 45 (Sep 2011), http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011SPIE.8155E..45R&link_type=ABSTRACT.
9. J. E. Jellison, “Gold-indium intermetallic compounds: Properties and growth rates,” NASA Document, 145 (1979), http://misspiggy.gsfc.nasa.gov/tva/meldoc/photonicsdocs/Jane.pdf&link_type=EJOURNAL.
10. G. W. Powell and J. D. Braun, “Diffusion in the gold-indium system,” Trans. AIME 230, 694699 (Jun 1964).
11. B. J. Rauscher, O. Fox, P. Ferruit, R. J. Hill, A. Waczynski, Y. Wen, W. Xia-Serafino, B. Mott, D. Alexander, C. K. Brambora, R. Derro, C. Engler, M. B. Garrison, T. Johnson, S. S. Manthripragada, J. M. Marsh, C. Marshall, R. J. Martineau, K. B. Shakoorzadeh, D. Wilson, W. D. Roher, M. Smith, C. Cabelli, J. Garnett, M. Loose, S. Wong-Anglin, M. Zandian, E. Cheng, T. Ellis, B. Howe, M. Jurado, G. Lee, J. Nieznanski, P. Wallis, J. York, M. W. Regan, D. N. B. Hall, K. W. Hodapp, T. Böker, G. De Marchi, P. Jakobsen, and P. Strada, “Detectors for the James Webb Space Telescope Near-Infrared Spectrograph. I. Readout Mode, Noise Model, and Calibration Considerations,” PASP 119, 768786 (Jul. 2007), arXiv:0706.2344.
12. Carl Stahle, et al., “JWST-RPT-017457: Executive summary: Root cause determination,” NASA Document, 110 (Apr 2011), http://www.jwst.nasa.gov/resources/017457.PDF&link_type=EJOURNAL.
13. Carl Stahle, et al., “JWST-RPT-017774: Executive summary 2d: Define tests to determine whether the existing detectors are qualified for flight,” NASA Document, 16 (Jul 2011), http://www.jwst.nasa.gov/resources/017774.PDF&link_type=EJOURNAL.
14. James Beletic, et al., Proc SPIE in press (2012).
15. D. Brent Mott, Augustyn Waczynski, Yiting Wen, Bernard J. Rauscher, Nicholas Boehm, Meng P. Chiao, Lantrinh Degumbia, Greg Delo, Roger Foltz, Emily Kan, David Alexander, Craig Cabelli, Brian Clemons, Joseph Connelly, Alex Dea, Rebecca Derro, Charles Engler, Ali Feizi, Ori Fox, Robert J. Hill, Thomas E. Johnson, Matthew Lander, Don J. Lindler, Markus Loose, Sridhar S. Manthripragada, Kevin Novo-Gradac, Wayne D. Roher, Robert Rosenberry, Kamdin Shakoorzadeh, Miles T. Smith, Donna Wilson, and Joseph Zino, “Characterization of the detector subsystem for the near-infrared spectrograph (nirspec) on the james webb space telescope,” High Energy 7021, 66 (Aug 2008), http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008SPIE.7021E..66M&link_type=ABSTRACT.


Image of FIG. 1.

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FIG. 1.

These dark images show the degradation versus time of several JWST 5 μm cutoff H2RGs. Each panel shows a dark image in inverse grayscale, where pixels with high currents show up as black. A dark image is a map of integrated charge under dark conditions. Parts prefixed with a “C” are NIRCam 5 μm H2RGs and parts prefixed with an “S” are NIRSpec 5 μm H2RGs. Panel (a) shows degradation in four NIRCam parts, (b) shows degradation in the NIRSpec “flight” parts, and (c) shows the degradation of a NIRSpec “flight spare”. Each dark image is taken with the detector enclosed in a completely blanked off dewar. The degradation manifests itself in the appearance of greater numbers of inoperable “warm” pixels.

Image of FIG. 2.

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FIG. 2.

(a) This figure shows a NIRSpec H2RG detector array. The H2RGs used by NIRCam and FGS/NIRISS differ only in the mechanical packaging. The photosensitive area measures about 36.72 × 36.72 mm2. The H2RG has 2040 × 2040 photosensitive HgCdTe pixels that are surrounded on all sides by a four pixel wide border of “reference pixels.” (b) Indium bump bonds are used to join the HgCdTe detector array to the silicon readout integrated circuit (ROIC).

Image of FIG. 3.

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FIG. 3.

Example of the increase in dark count rate for one pixel of a degraded detector. Here raw signal is measured in analog to digital converter units (ADU), and dark count rate is equal to the fitted slope. The blue data are for a good pixel and the red data are for the same pixel that has degraded with time.

Image of FIG. 4.

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FIG. 4.

(a) Pixel contact structure; (b) Scanning Electron Microscope (SEM) image of a non-degraded pixel in NIRCam detector C105; (c) SEM of degraded pixel in NIRCam detector C094.

Image of FIG. 5.

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FIG. 5.

(a) Inadequate barrier layer coverage; (b) Potential degradation mechanisms.

Image of FIG. 6.

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FIG. 6.

Degradation process in a pixel due to inadequate barrier layer.

Image of FIG. 7.

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FIG. 7.

This electrical circuit model of a degraded pixel accounts for the “RC”-like curvature of dark ramps (see Fig. 3). The red-highlighted components form in the HgCdTe immediately above the failed barrier layer. These cause the “RC”-like shape. This simple model does not attempt to explain the degradation in the photodiode that causes enhanced leakage current.

Image of FIG. 8.

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FIG. 8.

(a) SEM of a pixel corner in NIRCam detector C094; (b) X-ray elemental analysis (EDS) of the same area showing that Au and In have interdiffused to form an intermetallic compound (AuIn2) due to failure of the barrier layer.

Image of FIG. 9.

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FIG. 9.

(a) X-ray analysis (EDS) of red box area in SEM image demonstrates the presence of an In-Au intermetallic (AuIn2).

Image of FIG. 10.

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FIG. 10.

SEM and Backscatter Secondary Electron (BSE) image of detector pixel in C094.

Image of FIG. 11.

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FIG. 11.

Scanning Electron Microscopy images of pixels from a Process Evaluation Chip of NIRCam, NIRSpec, and FGS flight detectors. The presence of In-Au intermetallics from the breakdown of the barrier layer, as indicated by lighter shading in the In and Au layers, is present in all of the images.

Image of FIG. 12.

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FIG. 12.

The responsive quantum efficiency (RQE) of the new improved barrier layer detector arrays generally meets JWST requirements to within the ±10% zero point uncertainty for these measurements. This figure shows the RQE of two improved barrier layer H1RGs overlaid on NIRSpec requirements (Red) and the average of the four old-design NIRSpec “flight” and “flight spare” H2RGs (Gray). The H1RG prototypes use a NIRCam AR coating that is optimized for longer wavelengths than the NIRSpec coating that was used for the old design H2RGs. When this is taken into account, the performance of the improved barrier layer design is no worse than the old design.


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Table I.

Selected JWST NIRSpec Detector Requirements.

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Table II.

HgCdTe sensors in the JWST ISIM.

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Table III.

Key Physical Observations.

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Table IV.

Measured Performance of Prototype H1RGs.


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The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.


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Scitation: Commentary: JWST near-infrared detector degradation— finding the problem, fixing the problem, and moving forward