High contrast imaging and characterization of faint exoplanets require a coronagraph instrument to efficiently suppress the host star light to 10-9 level contrast over a broad spectral bandwidth. The NASA WFIRST mission plan includes a coronagraph instrument to demonstrate the technology needed to image and characterize exoplanets. Lyot coronagraph masks designed to serve at the focal plane followed by a Lyot stop will be key elements in the WFIRST coronagraph and in future advanced missions such as LUVOIR (Bolcar (2019) and HabEx (Morgan 2019, Martin 2019)). Shaped pupil masks designed to work in reflective geometry are also employed in the WFIRST Coronagraph. High-contrast performance reaching much better than 10-9 contrast requires very tight design, fabrication tolerances, and material properties to meet a wide range of specifications, including precise shapes, micron-scale island features, ultra-low reflectivity regions, uniformity, wavefront quality, etc. In this paper, we present all the critical analytical and measured properties of materials and designs in relation to the results from our coronagraph testbeds.
NASA’s WFIRST-AFTA mission concept includes the first high-contrast stellar coronagraph in space. This coronagraph will be capable of directly imaging and spectrally characterizing giant exoplanets similar to Neptune and Jupiter, and possibly even super-Earths, around nearby stars. In this paper we present the plan for maturing coronagraph technology to TRL5 in 2014-2016, and the results achieved in the first 6 months of the technology development work. The specific areas that are discussed include coronagraph testbed demonstrations in static and simulated dynamic environment, design and fabrication of occulting masks and apodizers used for starlight suppression, low-order wavefront sensing and control subsystem, deformable mirrors, ultra-low-noise spectrograph detector, and data post-processing.
Long Wavelength infrared photodetectors based on Type-II superlattices from the 6.1Å system hold great promise for a wide variety of applications. However, as these materials are fabricated into focal plane arrays for real world applications, the small pixel sizes that are required can result in unacceptably high dark current due to a significant contribution of surface-induced leakage. These surface currents could be substantially reduced or even eliminated by the application of an appropriate passivation material. But, while a considerable amount of effort has gone into developing passivation processes and materials for these detectors (e.g. PECVD SiO2, polyimides, etc.), there is no one widely adopted standard technique in use today. Atomic layer deposition has the possibility of being an excellent method for depositing passivation because of the wide variety of materials that are readily available via ALD and the ability to conformally coat arbitrary topographies that may be found in the patterning of LWIR FPAs. In this work, fundamental materials characterization results and electrical test data will be presented for two wide band gap, high-K dielectrics (Titanium Oxide and Hafnium Oxide) looking at their nucleation and growth behavior on substrates of relevant III-V materials such as GaSb and InAs using ellispometry, XPS, and XRD. These results will be compared to more conventional passivation strategies to highlight the unique features of the ALD technique.
We describe recent progress in the development of anti-reflection coatings for use at UV wavelengths on CCDs
and other Si-based detectors. We have previously demonstrated a set of coatings which are able to achieve
greater than 50% QE in 4 bands from 130nm to greater than 300nm. We now present new refinements of these
AR-coatings which will improve performance in a narrower bandpass by 50% over previous work. Successful test
films have been made to optimize transmission at 190nm, reaching 80% potential transmission.
The technology of Atomic Layer Deposition (ALD) holds promise to enable a future strategic mission that can address both ultraviolet (UV) astrophysics and optical exoplanet science with a shared telescope. The technology path to a shared telescope requires the development of a mirror coating with high reflectance from 100 nm to 1000 nm, and low polarization effects (i.e., s-p phase shifts that can vary with angle of incidence across a primary and secondary mirror) in the optical range. Currently, UV coatings have low reflectance, and conventional optical coatings have poor polarization properties for high-contrast coronagraph applications. In this paper we attempt to take a first step toward solving both problems simultaneously by using ALD, taking advantage of the fact that ALD can potentially produce mirror coatings with denser layers than conventional coatings (hence better reflectance, durability, and water resistance). In addition, ALD can potentially produce coatings with new composite materials (hence better control of polarization). We report here the results of our initial experiments with mirror coatings using ALD.
Current FUV instrumentation is seriously compromised by poor reflectivity. The best existing coatings for the 90 – 115 nm range are SiC (30% reflectivity across the band) and LiF/Aluminum (60% reflectivity from 100 nm to 115 nm). An improved coating therefore would enable the production of vastly more sensitive instruments in the 90 – 200 nm range. An additional goal in the development of an alternate FUV coating is to overcome the well-documented hygroscopic behaviors of LiF coatings, which currently impose handling concerns that in turn drive cost and schedule. The coatings we will develop in this effort must also function well through the conventional silicon-based detector bandpass (200 nm to 1100 nm). By ensuring that these new coatings are usable at many wavelengths, we will make it possible to incorporate ultraviolet instruments into future large missions without compromising the science capability of other instruments or increasing cost and risk due to handling issues. We present new results of the coating process and discuss our new ALD processes.
Current FUV instrumentation is seriously compromised by poor reflectivity. The best existing coatings for the 90 - 115
nm range are SiC (30% reflectivity across the band) and LiF/Aluminum (60% reflectivity from 100 nm to 115 nm). An
improved coating therefore would enable the production of vastly more sensitive instruments in the 90 - 200 nm range.
An additional goal in the development of an alternate FUV coating is to overcome the well-documented hygroscopic
behaviors of LiF coatings, which currently impose handling concerns that in turn drive cost and schedule. The coatings
we will develop in this effort must also function well through the conventional silicon-based detector bandpass (200 nm
to 1100 nm). By ensuring that these new coatings are usable at many wavelengths, we will make it possible to
incorporate ultraviolet instruments into future large missions without compromising the science capability of other
instruments or increasing cost and risk due to handling issues.
In this paper, we report the latest results on our development of delta-doped, thinned, back-illuminated CMOS imaging
arrays. As with charge-coupled devices, thinning and back-illumination are essential to the development of high
performance CMOS imaging arrays. Problems with back surface passivation have emerged as critical to the prospects
for incorporating CMOS imaging arrays into high performance scientific instruments, just as they did for CCDs over
twenty years ago. In the early 1990's, JPL developed delta-doped CCDs, in which low temperature molecular beam
epitaxy was used to form an ideal passivation layer on the silicon back surface. Comprising only a few nanometers of
highly-doped epitaxial silicon, delta-doping achieves the stability and uniformity that are essential for high performance
imaging and spectroscopy. Delta-doped CCDs were shown to have high, stable, and uniform quantum efficiency across
the entire spectral range from the extreme ultraviolet through the near infrared. JPL has recently bump-bonded thinned,
delta-doped CMOS imaging arrays to a CMOS readout, and demonstrated imaging. Delta-doped CMOS devices exhibit
the high quantum efficiency that has become the standard for scientific-grade CCDs. Together with new circuit designs
for low-noise readout currently under development, delta-doping expands the potential scientific applications of CMOS
imaging arrays, and brings within reach important new capabilities, such as fast, high-sensitivity imaging with parallel
readout and real-time signal processing. It remains to demonstrate manufacturability of delta-doped CMOS imaging
arrays. To that end, JPL has acquired a new silicon MBE and ancillary equipment for delta-doping wafers up to 200mm
in diameter, and is now developing processes for high-throughput, high yield delta-doping of fully-processed wafers
with CCD and CMOS imaging devices.