April D. Jewell

Recent developments and results of new ultraviolet reflective mirror coatings

Astronomical observations in the Lyman–ultraviolet (91 – 122 nm) are limited in part by the performance of reflective coatings. Currently, the best reflective mirror options for the UV wavelength range of 90 -122 nm are LiF+Al (R ~ 60% from 102 – 200 nm) and SiC (R ~ 30 % from 90 – 200 nm). Higher reflectivity coatings in the 90 – 122 nm range will improve sensitivity and allow for more complex instrumentation. We are working to develop, laboratory test and eventually space test new reflective UV coatings (R > 70% from 90 – 115 nm) that also preserve high-reflectivity performance (R > 80% from 115 – 800 nm) throughout the longer-wavelength vacuum ultraviolet and visible spectral bands. We present a progress report on our work with new protective thin film deposition techniques of metal fluorides (MgF2 and AlF3) on high intrinsic broadband reflective metal (aluminum) surfaces. We present first test results from both traditional and atomic layer deposition processes. In this paper, we discuss the current status of the deposition process, coating substrates, reflectivity measurements for optical through far-ultraviolet wavelengths as well as environmental storage sensitivities.

Wide band antireflection coatings deposited by atomic layer deposition

Here we describe the development of optical coatings for silicon-based detectors for astronomy, planetary and terrestrial applications. We have used atomic layer deposition (ALD) to develop broadband (i.e. 320-1000 nm) antireflection (AR) coatings on silicon substrates with the ultimate goal of incorporating these AR coatings with existing detector technologies. Materials characterization was used to study film and interface quality of these coatings. We are able to achieve precision growth of single and multilayer films to significantly reduce reflection losses for this region of spectrum and provide tailored, repeatable performance targeted for specific applications.

Charge-coupled devices detectors with high quantum efficiency at UV wavelengths

Abstract. We report on multilayer high efficiency antireflection coating (ARC) design and development for use at UV wavelengths on CCDs and other Si-based detectors. We have previously demonstrated a set of single-layer coatings, which achieve >50% quantum efficiency (QE) in four bands from 130 to 300 nm. We now present multilayer coating designs that significantly outperform our previous work between 195 and 215 nm. Using up to 11 layers, we present several model designs to reach QE above 80%. We also demonstrate the successful performance of 5 and 11 layer ARCs on silicon and fused silica substrates. Finally, we present a five-layer coating deposited onto a thinned, delta-doped CCD and demonstrate external QE greater than 60% between 202 and 208 nm, with a peak of 67.6% at 206 nm.

Superlattice-doped silicon detectors: progress and prospects

In this paper we review the physics and performance of silicon detectors passivated with wafer-scale molecular beam epitaxy (MBE) and atomic layer deposition (ALD). MBE growth of a two-dimensional (2D) doping superlattice on backside-illuminated (BSI) detectors provides nearly perfect protection from interface traps, even at trap densities in excess of 1014 cm-2. Superlattice-doped, BSI CMOS imaging detectors show no measurable degradation of quantum efficiency or dark current from long-term exposure to pulsed DUV lasers. Wafer-scale superlattice-doping has been used to passivate CMOS and CCD imaging arrays, fully-depleted CCDs and photodiodes, and large-area avalanche photodiodes. Superlattice-doped CCDs with ALD-grown antireflection coatings achieved world record quantum efficiency at deep and far ultraviolet wavelengths (100-300nm). Recently we have demonstrated solar-blind, superlattice doped avalanche photodiodes using integrated metal-dielectric coatings to achieve selective detection of ultraviolet light in the 200-250 nm spectral range with high out-of-band rejection.

Detector performance for the FIREBall-2 UV experiment

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 charge coupled 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.

Current progress in the characterization of atomic layer deposited AlF3 for future astronomical ultraviolet mirror coatings

Reflective aluminum (Al) mirrors for astronomical telescopes are traditionally protected by a transmissive overcoat. The optical, mechanical and chemical properties of this overcoat material strongly affect the spectral reflective properties and durability of the mirror system. We are developing atomic layer deposited metal fluorides and assessing their applicability for future astronomical space missions in the ultraviolet and visible wavelengths. We are currently performing depositions on silicon wafers to serve as a basis for the metal-fluoride on Al depositions. In this paper we present reflectance, surface roughness, environmental storage and polarization sensitivity results of thin layers of AlF3 on silicon. Atomic layer deposited coatings of AlF3 grown at 100 and 200 °C yield good optical characteristics deduced from reflectance measurements from 90 – 800 nm and spectroscopic ellipsometry measurements from 200 – 800 nm, which are consistent with calculations from optical constants derived by our group and from the literature. Atomic force microscopy (AFM) measurements demonstrate a 15% increase in surface roughness for a ~25 nm film with respect to a silicon reference. Temporary storage in a gN2 box minimally affects the UV reflectance of ~30 nm of AlF3 on Si. Overall, these coatings have proven to be versatile and optically stable in the early phases of development.

High efficiency CCD detectors at UV wavelengths

The Faint Intergalactic Redshifted Emission Balloon (FIREBall) is a NASA/CNES balloon-borne ultraviolet multi-object spectrograph designed to observe the diffuse gas around galaxies (the circumgalactic medium) via line emission redshifted to ~205 nm. FIREBall uses a ultraviolet-optimized delta doped e2v CCD201 with a custom designed high efficiency five layer anti-reflection coating. This combination achieves very high quantum efficiency (QE) and photon-counting capability, a first for a CCD detector in this wavelength range. We also present new work on red blocking mirror coatings to reduce red leak.

Characterizing environmental effects on visible and UV reflectance of ALD-coated optics

Numerous atomic and molecular transitions that provide important diagnostics for astrophysical research exist in the Lyman-ultraviolet (LUV; 91.2 - 121.6 nm) and far-ultraviolet (FUV; 121.6 - 200 nm) bandpasses. Future astronomy and planetary science missions require the development of mirror coatings with improved reflectance between 90 - 200 nm which maintain optical performance in visible and IR wavelengths (320 - 2000 nm). Towards this end, we have developed an atomic layer deposition (ALD) process for optical coatings to enhance the efficiency of future space observatories. We measured the reflectance from 115-826 nm of sample optics, consisting of silicon wafers coated with lithium fluoride films deposited via ALD. We also measured the reflectance of sample optics stored in various environments, and characterized the effect of storage environment on visible and UV optical performance over week-long time scales. Minimal change in optical performance was observed for wavelengths between 200 and 800 nm, regardless of storage environment.

Atomic Layer Deposited (ALD) coatings for future astronomical telescopes: recent developments

Atomic Layer Deposition (ALD) can create conformal, near stoichiometric and pinhole free transmissive metal fluoride coatings to protect reflective aluminum films. Spectral performance of astronomical mirror coatings strongly affect the science capabilities of astronomical satellite missions. We are utilizing ALD to create a transmissive overcoat to protect aluminum film mirrors from oxidation with the goal of achieving high reflectance (> 80%) from the UV (~100 nm) to the IR (~2,000 nm). This paper summarizes the recent developments of ALD aluminum fluoride (AlF3) coatings on Al. Reflectance measurements of aluminum mirrors protected by ALD AlF3 and future applications are discussed. These measurements demonstrate that Al + ALD AlF3, even with an interfacial oxide layer of a few nanometers, can provide higher reflectance than Al protected by traditional physical vapor deposited MgF2 without an oxide layer, below ~115 nm.

Ultrathin protective coatings by atomic layer engineering for far ultraviolet aluminum mirrors

Conventional aluminum-coated mirrors operating at far ultraviolet wavelengths (90–200 nm) utilize protective overcoats of metal fluoride thin films deposited by physical vapor deposition. The use of atomic layer deposition (ALD) holds promise in improving spatial reflectance uniformity and reducing the required thickness of the protective layers. Achieving a stable, pinhole-free, ultrathin (<3 nm) overcoat would allow protected Al mirrors to approach the ideal Al intrinsic reflectivity in the challenging, but spectrally-rich, 90–115 nm range. However, combining ALD methods with high performance evaporated Al layers has technical challenges associated with the formation of undesirable interfacial oxide. To overcome this issue, we demonstrate the use of thermal atomic layer etching (ALE) methods to remove this oxide prior to ALD encapsulation. This paper describes our continuing work to optimize new ALD processes for the metal fluoride materials of MgF2, AlF3 and LiF. We also describe new work on low temperature (<200 °C) ALE methods utilizing a fluorination-volatilization approach that has been incorporated into our mirror development efforts. The scalability of this overall approach and the environmental stability of ALD/ALE Al mirrors is discussed in the context of possible future astrophysics applications such as the NASA LUVOIR and HabEx mission concepts. The use of this combined ALE/ALD method may also enable a fabrication platform in space that can renew or reconfigure protective overcoats on Al mirrors on-orbit, as an alternative to other space-based metal coating methods considered previously.