The Wide Field InfraRed Survey Telescope-Astrophysics Focused Telescope Asset (WFIRST-AFTA) mission is a 2.4-m class space telescope that will be used across a swath of astrophysical research domains. JPL will provide a high-contrast imaging coronagraph instrument—one of two major astronomical instruments. In order to achieve the low noise performance required to detect planets under extremely low flux conditions, the electron multiplying charge-coupled device (EMCCD) has been baselined for both of the coronagraph’s sensors—the imaging camera and integral field spectrograph. JPL has established an EMCCD test laboratory in order to advance EMCCD maturity to technology readiness level-6. This plan incorporates full sensor characterization, including read noise, dark current, and clock-induced charge. In addition, by considering the unique challenges of the WFIRST space environment, degradation to the sensor’s charge transfer efficiency will be assessed, as a result of damage from high-energy particles such as protons, electrons, and cosmic rays. Science-grade CCD201-20 EMCCDs have been irradiated to a proton fluence that reflects the projected WFIRST orbit. Performance degradation due to radiation displacement damage is reported, which is the first such study for a CCD201-20 that replicates the WFIRST conditions. In addition, techniques intended to identify and mitigate radiation-induced electron trapping, such as trap pumping, custom clocking, and thermal cycling, are discussed.
The WFIRST-AFTA (Wide Field InfraRed Survey Telescope-Astrophysics Focused Telescope Asset) is a NASA space observatory. It will host two major astronomical instruments: a wide-field imager (WFI) to search for dark energy and carry out wide field near infrared (NIR) surveys, and a coronagraph instrument (CGI) to image and spectrally characterize extrasolar planets. In this paper, we discuss the work that has been carried out at JPL in advancing Electron Multiplying CCD (EMCCD) technology to higher flight maturity, with the goal of reaching a NASA technology readiness level of 6 (TRL-6) by early-to-mid 2016. The EMCCD has been baselined for both the coronagraph's imager and integral field spectrograph (IFS) based on its sub-electron noise performance at extremely low flux levels - the regime where the AFTA CGI will operate. We present results from a study that fully characterizes the beginning of life performance of the EMCCD. We also discuss, and present initial results from, a recent radiation test campaign that was designed and carried out to mimic the conditions of the WFIRST-AFTA space environment in an L2 orbit, where we sought to assess the sensor's end of life performance, particularly degradation of its charge transfer efficiency, in addition to other parameters such as dark current, electron multiplication gain, clock induced charge and read noise.
The objectives of the Integrated Imaging Sensors (I2S) Program are rtwofold. The first is to develop and deliver a rifle sight containing a single aperture and optical path for receiving, combining, and viewing radiation from the separate infrared (IR) and visible bands in a single image simultaneously. The second is to develop a sensor array sensitive in the radiation band spanning approximately from 0.4 μm to 1.7 μm by "fusing" indium-gallium-arsenic material onto silicon charge coupled devices. The ability to coincidentally and simultaneously form images from these two separate radiation bands is expected to significantly improve the detection and identification of objects from the case where only one radiation band is employed. Additionally, extending the cutoff of the visible band from 0.9 μm to 1.7 μm is expected to enhance viewing in this band as there is more available light, and further lessons the exacting requirement of desigining nearly noise free detectors.
Adaptive optics applications require a camera and detector that are capable of very high frame rates, high sensitivity and low noise. The PixelVision Adapt III camera achieves over 85 percent quantum efficiency and 12 electrons read noise at 1500 fps using the Adapt III, back illuminated CCD, designed by PixelVision, Inc. and manufactured by Scientific Imaging Technologies, Inc. The CCD's imaging area consists of 80 X 80 36 micrometers square pixels and utilizes full frame transfer architecture with an additional 80 rows in the storage region. To achieve low noise at high speeds, every two columns are mixed together into one output amplifier. There are 40 output amplifiers on the CCD and 40 analog channels. Digital data is mixed to a series stream and is transmitted over fiber optic cables to a PCI bus data acquisition board. Windows95 software drivers allow continuous acquisition of data into system memory. A water- cooled housing eliminates turbulence created by forced air cooling.
This paper will describe in some detail tow new large area CCD image sensors designed specifically to be used either as a single imager or assembled in mosaics of CCDs. The devices have 2048 X 4096, 15 micrometers pixels; the difference being the orientation of the serial register. Performance data are presented on both front- and back-illuminated parts. In addition, a new production camera test system will also be described which is being used to screen test the Advanced Camera CCDs for the wide field and high resolution channels.
A new class of video rate imagers based on back-illuminated and thinned CCDs is available that shows promise to replace conventional image intensifiers for most military, industrial, and scientific applications. Thinned, back-illuminated CCDs (BCCDs) and electron-bombardment CCDs (EBCCDs) offer low light level performance superior to conventional image intensifier coupled CCD (ICCD) approaches. These new, high performance devices promise to expand the fields of science, provide high contrast, high resolution, low light level surveillance imaging, and make nighttime pilotage safer for military aviators. This paper presents experimental data which illustrates how responsivity, gain, and modulation transfer function (MTF) determine the low light imaging capability, the 'target of interest' signal to noise ratio (SNR) of each of these types of sensors. High SNR and MTF make BCCDs the imager of choice under moderately low light levels and EBCCDs the imager of choice under extremely low light level conditions.
In earlier work, a model of the back illuminated CCD was presented and used to predict optical quantum efficiency. In this work we expand on the model and find an analytical solution for the probability of collection of a carrier generated at a given depth. We apply our solution to find the theoretical quantum efficiencies for both electron bombardment and optical illumination and compare them to measurements taken on thinned, backside-enhanced, non- AR coated devices. A single set of parameters is found which shows a reasonable fit to both sets of data. Earlier models of electron-bombarded CCDs have failed to explain the measured nonzero gain at low energies, however our model shows nonzero gain at all energies.
Radiation hardness is critical for charge coupled devices used in the electron bombarded mode. Two types of damage in CCDs are caused by keV electron irradiation: a flatband voltage shift and an increase in interface state density. A flatband voltage shift is more catastrophic to device performance than an increase in interface state density, especially for MPP devices. The type of radiation damage a CCD is susceptible to depends on the process used to fabricate it. Results are presented which show that Tektronix CCDs fabricated with a straight silicon dioxide gate insulator exhibit an increase in interface state density but little if any flatband voltage shift.
This paper presents preliminary results on the performance of n-channel, backside-thinned charge-coupled devices (CCDs) as electron-bombarded-semiconductor (EBS) imagers for the detection of 1-10 keV electrons. The devices exhibit average EBS gains ranging from approximately 50 at 1 keV to 1600 at 10 keV. Device radiation tolerance has been investigated by exposing normally-clocked devices to 6 keV electron doses up to 0.01 Coulombs/cm2. Room temperature pre- and post-irradiation results are presented for these key device parameters: full well capacity, dark current, and charge transfer efficiency (CTE). At the maximum dose of 0.01 Coulombs/cm2, full well capacity decreases 9 from an initial value of 680,000 e-, and dark current increases from 2 to approximately 50 nA/cm2. There are no measurable changes in large signal CTE up to the maximum dose. Radiation damage at energies other than 6 keV is estimated by measurement of the x-ray generation efficiency of silicon as a function of electron energy. Device stability after temperature cycling has been studied by subjecting packaged devices to vacuum bakes of 24 hours at 300 degree(s)C. Full well, CTE, EBS gain, and output amplifier performance are unchanged after the extended temperature cycle, while dark current decreases slightly by 15. In summary, these initial results indicate that the CCD can function as both an efficient and robust electron imager.