SAPHIRA (320x256/24μm) is the first of a family of HgCdTe APD infrared sensors for NIR/SWIR sensing in low flux conditions for scientific applications. The paper will present the status of the latest developments of detectors suitable for a range of ground and space applications including wavefront sensing, low flux imaging, Lidar and high speed optical communications. The next generation SAPHIRA (512x512/24μm) array has an architecture specific to pyramid wavefront sensors, supported by a consortium comprising European Southern Observatory, Max Planck Institute, NRC Herzberg Institute and Potsdam University. A 1kx1k/15μm, 3-side buttable sensor called Ike Pono after the Hawaiian for ‘far-vision’ is aimed at extreme, low-flux imaging, supported by NASA through the University of Hawaii, Institute of Astronomy. A large area device suitable for LIDAR and supported by NASA also demonstrates a performance suitable for high speed optical communication applications. Most recently the development of a 2k x 2k/ 15μm device for scientific imaging has started with the support of ESA.
Linear-mode avalanche photodiode arrays, LmAPDs, based on bandgap engineered HgCdTe, grown by Metal Organic Vapour Phase Epitaxy, MOVPE, can produce virtually noise-free infrared sensors. These are required for applications in big science, security and biochemistry. A custom device called SAPHIRA (320x256/24μm) has been designed specifically for LmAPDs. SAPHIRA has been deployed as a wavefront sensor for adaptive optic systems in nine major telescopes and notably five devices control the four 8.2 metre telescopes of the VLT interferometer. These demanding applications have driven frames rates up to 200 kframes/s and avalanche gains to x600 achieving read noise as low as 0.26 e- rms and enabling single photon counting. The detector is baselined for time-domain astronomy vital for exoplanet spectroscopy and understanding the physics of active stellar objects. The three 30 metre class telescopes currently under construction and the three candidate space telescopes, HabEx, Luvoir and EZE will depend on noise-free infrared detectors at very low dark current. Work at the University of Hawaii and European Southern Observatory has demonstrated dark currents in the 4-10 electrons/hour range and with avalanche gain offers the prospect of higher science return from these instruments. A 1kx1k/15μm 3-side buttable array is currently in development to service extreme low background applications especially spectroscopy. A 512x512/24μm SAPHIRA array with 64 parallel video outputs is in development for pyramid wavefront sensing on the European Extremely Large telescope, ELT, mirror co-phasing and rapid time-domain astronomy.
With gain-normalized dark currents reduced below 10 e-/s/pixel, HgCdTe Avalanche Photo-Diode arrays operated in linear mode are now widely accepted as the sensors of choice for adaptive optics wavefront sensing of natural guide stars in the wavelength range 1 to 2.5 um. At lower temperatures the intrinsic dark currents in Leonardo’s SAPHIRA 320 x 256 @ 24um pitch arrays have recently been shown to be four orders of magnitude lower, approaching 0.001 e-/s/pixel. This has opened the way to megapixel class APD arrays offering major advantages for a wide range of low background astronomical observations.
We report the status of development of a 1k x 1k @ 15um pixel Read-Out Integrated Circuit (ROIC) for a three-side close-buttable, 16 output, MOVPE HgCdTe array with reference pixels and reference outputs. The design has been optimized for minimum ROIC glow following a detailed investigation of glow sources in the SAPHIRA ME1001 ROIC. This involved precisely overlaying J-band glow images on layout drawings from EDA software to identify all the sources of glow.
As of the time of submission of this abstract, the Architecture definition is in its third iteration and the program is on track to produce ROICs around mid-2018. In parallel, Mark 20 SAPHIRAs optimized for very low background operation with the goal of delaying the onset of tunnel-tunnel current to bias voltages producing useful avalanche gain are being characterized along with modified SAPHIRAs with 15um pixels.