Radiation environment performance of JWST prototype FPAs M. E. McKelvey*a, K. A. Ennicoa, R. R. Johnsona, P. W. Marshalld, R. E. McMurraya, Jr., C. R. McCreighta, J. C. Pickelc, R. A. Reedb aNASA Ames Research Center, Moffett Field, CA 94035-1000 bNASA Goddard Space Flight Center, Greenbelt, MD 20771 cPickel R&T, Inc., Bonsall, CA 92003 dConsultant to GSFC ABSTRACT As the logical extension of the 20-year mission of the Hubble Space Telescope, NASA plans to launch the James Webb Space Telescope (JWST, formerly NGST) near the end of this decade. As Hubble's scientific and technological successor, equipped with a 6-meter-class deployable mirror, JWST will allow observations of the very early universe and initial formation of galaxies at levels not achievable today. JWST's unprecedented sensitivity cannot be utilized without a new class of IR focal plane arrays whose performance matches that of the telescope. In particular, JWST focal planes must be able to withstand the ionizing-particle radiation environment expected for its Lagrange-point (L2) orbit and ten-year mission lifetime goal. To help determine their suitability for JWST, NASA is evaluating prototype megapixel-class readouts and hybrid detector arrays under proton bombardment to simulate the anticipated JWST lifetime radiation dose. This report describes the results of early tests on devices from two manufacturers using photovoltaic (HgCdTe or InSb) candidate near-infrared detector structures. Results to date have shown encouraging performance, along with some areas of continuing concern. 1.INTRODUCTION We describe here characterization tests performed on two prototype candidate infrared detector focal plane arrays (FPAs) for JWST's near-infrared (NIR) instruments, including the Near-Infrared Camera (NIRCAM) Near-Infrared Spectrometer (NIRSpec), and Fine-Guidance Sensor. Rockwell Science Center's H-1RG and Raytheon Vision System's SB291 are megapixel-class building blocks designed to be representative of the larger structures proposed for JWST. Both devices are optimized around JWST NIR requirements (Table 1), with design operating temperatures between 30K and 37K. Orbiting the second Lagrange point 1.5 million km from earth, JWST will be exposed to a potentially damaging cosmic ray environment. Depending upon the level of shielding provided to the detector arrays in the final design, a lifetime dose of 5krad(Si) to 50krad(Si) is anticipated. The SB291 (and its 2048X2048 counterpart, the SB304) draws on Raytheon's successful 'ALADDIN' and 'Orion' readout architectures1, applying refinements tailored for the near-IR application. The SB291 is essentially a 'quadrant' of the four-output SB304, and the readout is mated to an indium antimonide (InSb) detector substrate at a pixel pitch of 25µm. InSb detector material requires a 30K operating temperature for the SB291, compared to 37K for the H-1RG. The H-1RG (and the H-2RG, its 2048X2048 version) is a hybrid device comprising a sophisticated software- configurable silicon readout circuit and a mercury-cadmium-telluride (HgCdTe) detector array optimized for the JWST NIR spectral range. The readout includes a serial port to allow user selection of various operating modes, including the number of parallel outputs and output stage amplifier configuration. Several clocking and reset modes are available, including 'guide mode' operation wherein a selectable subarray may be dedicated to a guide star and read out independently. Pixel read rates up to 5MHz are available. *[email protected]; tel. (650)604-3196; fax (650)604-1094 Other groups2,3 have characterized a wide range of performance metrics for these devices. Our laboratory test effort has been tightly focused on evaluating the performance degradation to be expected for JWST lifetime radiation dose levels. We have placed particular emphasis on dark current activation, a crucial determinant in meeting JWST's science goals. In Section 2, we highlight the development program for these devices. Section 3 describes the devices under test in further detail. Section 4 describes the test program and equipment, and Section 5 summarizes the test results. Table 1. Relevant JWST NIR Detector Requirements.4 Parameter Requirement Goal Wavelength Range (µm) 0.6 - 5 0.6 - 5 Operating Temp. (K) 30 - 37 Highest possible FPA Format Multiple 20482 SCAs Multiple 20482 SCAs QE (%) >70 - 80 > 90 - 95 Dark Current (e-/s) Fits noise budget <0.01 Read Noise (e-) <9 (Fowler 8) <2.5 (Fowler 8) Well Capacity (e-) >60,000 >1E5 Power (mW/Mpixel) ~1 0.1 2.DEVELOPMENT PROGRAM To understand the performance limitations imposed by the space radiation environment on the candidate near-IR JWST focal plane technologies, a program of testing and analysis was set up in conjunction with the Radiation Effects Working Group at the NASA Goddard Space Flight Center. The tested devices were produced through parallel NASA NIR focal plane development contracts at Rockwell Scientific (5-µm cutoff HgCdTe) and at Raytheon Vision Systems (InSb), which ran from 1998 - 2003. Both contractors worked to demonstrate that JWST performance requirements could be met, and that the more ambitious performance goals could be approached. Among the performance challenges were achieving extremely low total noise (<10 e-) over extended (≥1000 s) integration times, near-vanishing dark current levels, and high-performance anti-reflection coatings to cover the 0.6 to 5µm range. The vendors developed both 1024 x 1024 and 2048 x 2048 element detector arrays and the associated two-dimensional multiplexers. Because in each case the architecture, processing steps, and detector geometries of the 10242 and 20482 devices were identical, the smaller-format devices were selected for testing convenience. Our lab was tasked with characterizing pre- and post-irradiation effects on the bare 10242 multiplexers (Rockwell H-1RG and Raytheon SB-291), and also the full hybrid arrays in five separate cyclotron tests. Testing was conducted at the vendor-selected optimum temperature within the allowable JWST range. 3.DESCRIPTION OF DEVICES 3.1. Raytheon SB291 ROIC and InSb SCA The 1024 x 1024-element SB291 readout multiplexer is a Source-Follower-per-Detector (SFD) design with three transistors per unit cell, formed in an epitaxially-grown layer on a Si substrate. Its architecture is similar to that of the proven CRC-206 'ALADDIN III',5 but with a contiguous megapixel rather than independently-read quadrants. The SB291 uses a 25 µm pixel, and input/output (I/O) pads on the SB291 are confined to one side so that the device is three- side buttable if used as the building block for a larger focal plane array. Four pixels in adjacent columns are addressed simultaneously with each valid pixel clock, yielding a column-interleaved output. A column of reference unit cells (identical to the other unit cells but with no detector attached) is added at either side of the readout, creating a 1032x1024 addressable area. The reference pixels serve to provide a 'black level' that may be used to subtract offset drifts in long-integration measurements. The device is a very close cousin to the SB270 'Orion' ROIC6. To minimize power dissipation and possible glow while maintaining acceptable pixel SFD amplifier bandwidth, only two columns of unit cells are powered at a given time. Unit cell current is applied just before a given unit cell is addressed in order to provide sufficient drive capability to slew the output bus during the address period. At any one time this 'slew' current source is applied to the unit cells being addressed and to the unit cells one address ahead. Thus only eight columns of pixels see the source-follower current at a given time. The design goal for readout rate was 600 kpixels/s total through four outputs, with a minimum unit cell address time of 6.7 µs. Both row-by-row and global reset modes are provided. There is no dedicated reset bias supplied to the unit cells. Instead, the reset switch shorts the integrating node to the drain of the integrating FET so detectors are reset to the drain potential plus an offset determined by charge redistribution associated with the opening of the reset and enable switches in the mux. Fowler sampling7 was used in our tests to optimize read noise performance. A number of lot splits were produced for this effort, with varied parameters including doping and thickness of the epitaxial layer. In the end, the vendor was asked to select the lot split to be evaluated under proton bombardment, and one ROIC from that lot split was selected for radiation testing. The ROIC was tested separately from the SCA to allow mux- and detector-related effects to be distinguished. After candidates were evaluated in our laboratory a particular hybrid device was selected for proton bombardment to a dose of 5krad(Si). All the devices produced were engineering-grade prototypes, and as such had a number of cosmetic and functional defects. Of the SCAs whose ROIC substrate came from the same lot split as that selected for the bare ROIC test, the part deemed least likely to produce excess dark current from these defects was selected for the proton test. 3.2. Rockwell H-1RG ROIC and HgCdTe SCA The HAWAII-1RG readout multiplexer is also an SFD design, with an addressable area of 1024x1024 including a four- pixel-wide border of reference pixels, which are mixed in with the data stream by default. Each of two parallel outputs reads out a contiguous 512 columns (including reference pixels). Global reset mode is the default, and the H-1RG does incorporate a dedicated line to carry the reset potential to all unit cells. Input and output lines are confined to one edge of the device to make it three-side buttable as part of a larger focal plane array. The readout contains a sophisticated array of built-in functions controlled via a set of registers that can be set 'on the fly'.