Detector performance for the FIREBall-2 UV experiment

Abstract

We present an overview of the detector for the upcoming Faint Intergalactic Red-shifted Emission Balloon (FIREBall-2) experiment, with a particular focus on the development of device-integrated optical coatings and detector quantum efficiency (QE). FIREBall-2 is designed to measure emission from the strong resonance lines of HI, OVI, and CIV, all red-shifted to 195-225 nm window; its detector is a delta-doped electron multiplying chargecoupled device (EM-CCD). Delta-doped arrays, invented at JPL, achieve 100% internal QE from the UV through the visible. External losses due to reflection (~70% in some UV regions) can be mitigated with antireflection coatings (ARCs). Using atomic layer deposition (ALD), thin-film optical filters are incorporated with existing detector technologies. ALD offers nanometer-scale control over film thickness and interface quality, allowing for precision growth of multilayer films. Several AR coatings, including single and multi-layer designs, were tested for FIREBall-2. QE measurements match modeled transmittance behavior remarkably well, showing improved performance in the target wavelength range. Also under development are ALD coatings to enhance QE for a variety of spectral regions throughout the UV (90-320 nm) and visible (320-1000 nm) range both for space-based imaging and spectroscopy as well as for ground-based telescopes.Keywords: Ultraviolet, EMCCD, AR coating, delta doping, FIREBall, atomic layer deposition FIREBALL-2The Faint Intergalactic Red Shifted Emission Balloon (FIREBall-2) is a balloon-borne UV spectrograph funded jointly by NASA and CNES. Briefly, FIREBall-2 is designed to observe emission from the circumgalactic medium (CGM), the diffuse gas around galaxies. The primary targets include line emission from HI (Lyman-α, 121.6 nm) at a redshift of z=0.7; OVI (103.3 nm) at z=1.0; and CIV (154.9 nm) at z=0.3. The instrument is optimized for narrowband observations spanning the stratospheric window (200-210 nm) centered at 205 nm. FIREBall-2 is a follow on to FIREBall-1, which was launched on two separate occasions in 2007 and 2009. [1][2][3][4] FIREBall-1 was a technical and engineering success, but elucidated the need for lower detection limits and FIREBall-2, resulting in changes to the spectrograph design and the detector. The focus of this manuscript is on the FIREBall-2 detector, specifically development of Electron Multiplying CCDs (EMCCDs) with unprecedented quantum efficiency (QE) within the FIREBall-2 observation band. Details of the spectrograph design and detector noise performance are provided elsewhere, including within these Proceedings. FIREBALL-2 DETECTORThe FIREBall-2 detector development is a collaborative effort shared between the Jet Propulsion Laboratory (JPL), the California Institute of Technology, and Columbia University. The detector is based on e2v's CCD201-20, an electron multiplying charge-coupled device (EMCCD). EMCCDs are conventional CCDs in which one of the register phases is replaced with a high voltage (40-50 V) gain register that multip...

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

“…Here we show two examples of results obtained from back illumination and passivation with 2D growth combined with custom coatings in the FUV and NUV for EMCCDs. Results of this process as manifested in QE are shown in Figure 9 [ 34 , 37 , 40 ]. Figure 9 a shows the QE results of coated delta doped arrays for an imaging spectrometer that covers the challenging part of the spectrum from 100–300 nm.…”

“…The first time JPL passivated a CCD using MBE, the passivation layer comprised 5 nm of silicon doped at 3 × 10 20 B/cm −3 , i.e ., 3-D doping, plus a sacrificial 1-nm un-doped silicon cap layer to form the surface oxide [ 29 ]. In all subsequent devices, JPL has used 2D doping for surface passivation [ 17 , 18 , 28 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]. During MBE growth, a thin layer of un-doped silicon is grown on the substrate to form an atomically clean, uniform silicon surface.…”

“…This chemical-reaction-driven process allows for the precise control of stoichiometry, thickness, and uniformity, while producing highly dense and pinhole free films with sharp and well-defined interfaces. These characteristics make ALD ideal for preparing optical coatings based on single and multilayer films, and we have employed this technique to develop highly effective AR coatings in the challenging far UV spectral range [ 34 , 37 , 39 , 40 ].…”

Delta-doped electron-multiplying CCDs for FIREBall-2

“…However, based on our recent flight results, and calibration data in the field, it is possible to switch to a five-layer AR coating without major loss in QE in our target bandpass. 32 7 FIREBall-2 as a Pathfinder for Future UV Telescopes…”

High-efficiency UV/optical/NIR detectors for large aperture telescopes and UV explorer missions: development of and field observations with delta-doped arrays

“…While multilayer AR coatings provide high QE with a narrow peak, variations in film thicknesses will result in a peak shift. 55 In the case of FIREBall-2, which is naturally constrained to a very narrow bandpass we consider AR coatings with five or fewer layers with a broader bandpass that encompasses the stratospheric window. Test coatings were prepared on test samples and functional test devices; test samples were evaluated in terms of their optical performance and related properties (e.g., surface and interface roughness) prior to moving to test devices.…”