Large Format Focal Plane Arrays

Rockwell Scientific Company (RSC) produces a wide variety of Sensor Chip Arrays (SCAs) for demanding astronomical imaging and spectroscopy applications. These devices achieve high quantum efficiency, low readout

noise and extremely low dark current. Presently, RSC array formats range from 256x256 (PICNIC) to 2Kx2K (HAWAII-2RG). The HAWAII-2RG [1] is the fifth generation RSC astronomy Readout Integrated Circuit (ROIC) and contains many desirable features including:

• reference pixels (row and column plus reference output)

• independent readout of a guide window of adjustable size

• complete glow suppression from output buffer and clock drivers

• radiation single event upset error-detection and correction

• Schmidt triggering of the clock signals

• low power output amplifiers

• three-side close buttability

• three reset options (frame, line and pixel)

• full ROIC programmability via 3-line digital interface

• 100-500 kHz "slow" output mode

As shown in Fig. 1, the ROIC can be hybridized to both infrared and visible RSC detector technologies. Typical infrared astronomy applications require spectral response out to 1.7 ^m (NIR), 2.5 ^m (SWIR) and 5 ^m (MWIR), however RSC has also fabricated a Long-Wave Infrared (LWIR) 10 ^m cutoff 512x512 SCA using the HAWAII-1RG as the ROIC. RSC has two technologies for fabricating infrared detector material:

• PACE-1 Liquid Phase Epitaxy (LPE) for SWIR,

• Molecular Beam Epitaxy (MBE) for NIR through LWIR applications.

PACE-1 HgCdTe is grown on sapphire substrate and MBE HgCdTe is grown on lattice matched CdZnTe substrate. While the cost of sapphire substrates is lower than CdZnTe, the CdZnTe crystal structure is better matched to HgCdTe and produces higher quality detectors. Consequently PACE is relatively more economical while MBE provides higher performance; the user can choose which IR detector material is most appropriate for the application.

For visible spectral sensitivity, RSC has two detector technologies:

• substrate-removed MBE HgCdTe (discussed in Section 1.1),

• Si PIN detector - named HyViSI™ for Hybrid Visible Silicon Imager (discussed in Section 1.2).

While substrate removed MBE HgCdTe provides continuous visible through infrared spectral coverage, HyViSI offers lower cost and higher operating temperature at visible wavelengths.

Focal Plane Array
Figure 1. The RSC astronomy array family, an industry leader for 20 years.

1.1 Mosaic Focal Plane Arrays (FPAs)

Mosaic FPAs using several RSC SCAs are now in use at ground-based observatories, and are being designed and assembled for space telescopes. These arrays are enabling astronomers to continue to push the boundaries of astronomical science. Figure 2 shows an image taken at the UKIRT by the WFCAM imager, which employs a mosaic of four HAWAII-2 SWIR SCAs. While this paper was written, Canada-France-Hawaii Telescope was commissioning a mosaic of four HAWAII-2RG SCAs in its WIRCAM instrument (see Fig. 3). The University of Hawaii has an operating 4K*4K HAWAII-2RG mosaic and the Gemini and European Southern observatories are also commissioning mosaics of four HAWAII-2RG SWIR SCAs.

Figure 2. Image taken by a mosaic of four HAWAII-2 SCAs in the WFCAM instrument on the UKIRT telescope (image courtesy of JAC).
Figure 3. Packaged mosaic of HAWAII-2RG SCAs for the WIRCAM instrument on the CFHT telescope (image courtesy of GL Scientific and CFHT)

The development of high performance detectors with the feature-rich HAWAII-2RG ROIC convinced the JWST team at NASA to use RSC's substrate-removed SWIR and MWIR SCAs in the Near Infrared Camera (NIRCam) and Near Infrared Spectrometer (NIRSpec), both instruments include mosaics of HAWAII-2RG SCAs [2].

1.2 Silicon PIN HyViSI TM Visible Sensors

In addition to the infrared arrays, RSC also produces SCAs based on silicon technology that is sensitive to the visible spectrum. RSC's HyViSI™ utilizes Si PIN detectors mated to the several RSC ROICs to attain low noise images in the visible range [3].

Figure 4 shows the quantum efficiency of a HyViSI SCA. The quantum efficiency is greater than 80% for the 500-900 nm range, and achieves greater than 50% QE in the deep red, between 900 and 1000 nm, due to the thickness of the silicon detector layer. The detector material has a bias voltage so that it is fully depleted which minimizes charge diffusion - point spread functions of less than 7.5 |m FWHM have been demonstrated. These arrays consistently show high operability (>99.99%), single CDS read noise of <13 electrons at 100 kHz clock rate, dark current of less than 0.001 electrons/pixel/sec at 140 K, and response non-uniformity less than 4%.

Large Vnir Focal Plane Arrays

200 300 400 500 600 TOO 800 900 1000 1100

200 300 400 500 600 TOO 800 900 1000 1100

W.iveleivith, nm

Figure 4. Quantum efficiency of Rockwell Scientific's HyViSI Si PIN arrays. The QE at each wavelength is a function of the AR coating.

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