Gregory Delo

A comparison of charge transfer efficiency measurement techniques on proton damaged n-channel CCDs for the Hubble Space Telescope Wide-Field Camera 3

We examine proton-damaged charge-coupled devices (CCDs) and compare the charge transfer efficiency (CTE) degradation using extended pixel edge response, first pixel response, and /sup 55/Fe X-ray measurements. CTEs measured on Marconi and Fairchild imaging sensors CCDs degrade similarly at all signal levels, though some of the Fairchild CCDs had a supplementary buried channel.

Hot pixel annealing behavior in CCDs irradiated at -84/spl deg/C

A Hubble space telescope wide field camera 3 e2v CCD was irradiated while operating at -84/spl deg/C and the dark current studied as a function of temperature while the charge coupled device was warmed to a sequence of temperatures up to a maximum of +30/spl deg/C. The device was then cooled back down to -84/spl deg/ and remeasured. Hot pixel populations were tracked during the warm up and cool down. Hot pixel annealing began below -40/spl deg/C and the anneal process was largely completed by the time the detector reached +20/spl deg/C. There was no apparent sharp annealing temperature. Although a large fraction of the hot pixels fell below the threshold to be counted as a hot pixel, they nevertheless sustained a higher leakage rate than the remaining population. The mechanism for hot pixel annealing is not presently understood. Room temperature irradiations do not adequately characterize the hot pixel distributions for cooled applications.

Hot pixel behavior in WFC3 CCD detectors irradiated under operational conditions

A Hubble Space Telescope Wide Field Camera 3 (WFC3) CCD detector was tested for radiation effects while operating at -83°C. The detector has a format of 2048 x 2048 pixels with a 15 μm square pixel size, a supplemental buried channel, an MPP implant, and is back side illuminated. Detector response was tested for total radiation fluences ranging from 1x103 to 2.5x109 of 63.3 MeV protons/cm2 and for a range of beam intensities. Radiation damage was investigated and the annealing of damage was tested by warming up to +30°C. The introduction rate of hot pixels and their statistics, hot pixel annealing as a function of temperature and time, and radiation changes to the mean value of dark current were investigated. Results are compared with the experiences of other HST instruments.

A study of hot pixel annealing in the Hubble Space Telescope Wide Field Camera 3 CCDs

A Hubble Space Telescope Wide Field Camera 3 (WFC3) CCD detector was tested for radiation effects while operating at -83C. The goal of the experiment was to evaluate the introduction and annealing rates of hot pixels and to assess the dynamics of that process. The device was irradiated while cold and warmed to +30°C for a 4 hour soak, then cooled back down to -83°C. Hot pixel populations were tracked during warm up and cool down. The results showed that the hot pixels begin to anneal around -40°C and the anneal process was largely completed before the detector reached +30°C. It was also found that, although a large fraction of the hot pixels dropped below the threshold, they remained warmer than the remaining population.

The Wide-Field Camera 3 detectors

The Wide-field Camera 3 (WFC3) is a fourth-generation instrument planned for installation in Hubble Space Telescope (HST). Designed as a panchromatic camera, WFC3s UVIS and IR channels will complement the other instruments onboard HST and enhance the observatorys scientific performance. UVIS images are obtained via two 4096×2051 pixel e2v CCDs while the IR images are taken with a 1024×1024 pixel HgCdTe focal plane array from Teledyne Imaging Sensors. Based upon characterization tests performed at NASA/GSFC, the final flight detectors have been chosen and installed in the instrument. This paper summarizes the performance characteristics of the WFC3 flight detectors based upon component and instrument-level testing in ambient and thermal vacuum environments.

Mechanisms and Temperature Dependence of Single Event Latchup Observed in a CMOS Readout Integrated Circuit From 16–300 K

Heavy ion-induced single event latchup (SEL) is characterized in a commercially available CMOS readout integrated circuit operating at cryogenic temperatures. SEL observed at 24 K and below is believed to be possible when free carriers produced by an ion strike initiate an exponential increase in the free carrier density via shallow-level impact ionization (SLII). This results in a large current increase that proceeds to a sustained latched state, even though the classic condition for parasitic bipolar gain product is not met since it is much less than unity. The LET threshold for SEL is significantly lower at 20 K as compared to 300 K although the saturated cross section is 2-3 times higher at 300 K. The temperature dependence of the SEL cross section is characterized from 16-300 K. SEL behavior attributed to the classical cross-coupled parasitic bipolar model is observed from ~135-300 K, and the reduction in the SEL cross section is remarkably modest as the temperature is lowered from room temperature to ~200 K. Temperature dependent electrical latchup characterization of a 130 nm pnpn test structure also indicates a change in the latchup behavior at ~50 K consistent with the SLII mechanism.

Wide Field Camera 3 CCD Quantum Efficiency Hysteresis: Characterization and Mitigation

In ground testing of the Hubble Space Telescope Wide Field Camera 3 (HST/WFC3), the CCDs of its UV/visible channel exhibited an unanticipated quantum efficiency hysteresis (QEH) behavior. The QEH first manifested itself as an occasionally observed contrast in response across the format of the CCDs, with an amplitude of typically 0.1-0.2% or less at the nominal -83°C operating temperature, but with contrasts of up to 3-5% observed at warmer temperatures. The behavior has been replicated in the laboratory using flight spare detectors and has been found to be related to an initial response deficiency of ~5% amplitude when the CCDs are cooled with no illumination. A visible light flat-field (540nm) with a several times full-well signal level is found to pin the detector response at both optical (600nm) and near-UV (230nm) wavelengths, suppressing the QEH behavior. We have characterized the timescale for the detectors to become unpinned (days for significant response loss at -83°C and have developed a protocol to stabilize the response in flight by flashing the WFC3 CCDs with the instruments internal calibration system.

Laboratory and sky testing results for the TIS H4RG-10 4k x 4k 10-micron visible CMOS-hybrid detector

We present both laboratory and telescope testing results describing the performance of the H4RG-10 CMOS-Hybrid detector. The H4RG-10 is the largest visible hybrid array currently in existence and shows great potential for use in future space missions. We report read noise, dark current, pixel connectivity, persistence, and inter-pixel capacitance measurements for the temperature range 110-240 K. We report on quantitative astrometric and qualitative photometric performance of the instrument based on observations made at USNOs Flagstaff Station observatory and establish an upper limit to the astrometric performance of the detector. We discuss additional testing and future work associated with improving detector performance.

Radiation effects in WFC3 IR detectors

A Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) flight-like IR detector was tested for radiation hardness by exposing it to high energy protons while operating at the nominal flight temperature of 150 K. The detector is a 1.7 μm cutoff HgCdTe detector with a CdZnTe substrate. The device is hybridized to a silicon multiplexer. The detector response was tested for gradually increasing fluence from less than 1x103 to a total of 5x109 63 MeV protons/cm2. Dark current changes were evaluated after each step. An increase in dark current and new hot pixels were observed after large steps of irradiation. The increased dark current was observed to partially anneal at 190K and fully anneal at room temperature. Radiation effects, hot pixel distribution, and results of annealing at different temperatures are presented here.

Selection of the infrared detectors for Wide Field Camera 3 on the Hubble Space Telescope

Wide Field Camera 3 is a fourth generation instrument for the Hubble Space Telescope (HST), to be installed during the next HST Servicing Mission 4. For its infrared channel Rockwell Scientific Company has developed a new type of HgCdTe 1Kx1K detector, called WFC3-1R, with cutoff wavelength at 1.7μm and 150K operating temperature. The WFC3-IR detectors are based on HgCdTe MBE grown on a CdZnTe substrate and use a new type of multiplexer, the Hawaii-1R MUX. Two flight detectors, a prime and a spare, have been recently selected on the basis of the measures performed at NASA Goddard Research Center - Detector Characterization Laboratory. These parts show quantum efficiency higher than 80% at λ=1.6μm and greater than 40% at λ>1.1μm, readout noise of ~25 e- rms with double correlated sampling, and mean dark current of ~0.04 e/s/pix at 150K. We show that the IR channel of WFC3, equipped with one of these flight detectors, beats the instrument requirements in all configurations and promises to have a discovery efficiency significantly higher than NICMOS. In particular, a two-band wide-area, deep survey made with WFC3 exceeds the discovery efficiency of NICMOS before and after the installation of NCS by a factor of 15 and 10, respectively.