We describe recent advances in backside passivation of large-format charge-coupled devices (CCDs) fabricated on 200- mm diameter wafers. These CCDs utilize direct oxide bonding and molecular-beam epitaxial (MBE) growth to enable high quantum efficiency in the ultraviolet (UV) and soft X-ray bands. In particular, the development of low-temperature MBE growth techniques and oxide bonding processes, which can withstand MBE processing, are described. Several highperformance large-format CCD designs were successfully back-illuminated using the presented process and excellent quantum efficiency (QE) and dark current are measured on these devices. Reflection-limited QE is measured from 200 nm to 800 nm, and dark current of less than 1e- /pixel/sec is measured at 40°C for a 9.5 μm pixel.
We report our progress toward optimizing backside-illuminated silicon P-type intrinsic N-type complementary metal oxide semiconductor devices developed by Teledyne Imaging Sensors (TIS) for far-ultraviolet (UV) planetary science applications. This project was motivated by initial measurements at Southwest Research Institute of the far-UV responsivity of backside-illuminated silicon PIN photodiode test structures, which revealed a promising QE in the 100 to 200 nm range. Our effort to advance the capabilities of thinned silicon wafers capitalizes on recent innovations in molecular beam epitaxy (MBE) doping processes. Key achievements to date include the following: (1) representative silicon test wafers were fabricated by TIS, and set up for MBE processing at MIT Lincoln Laboratory; (2) preliminary far-UV detector QE simulation runs were completed to aid MBE layer design; (3) detector fabrication was completed through the pre-MBE step; and (4) initial testing of the MBE doping process was performed on monitoring wafers, with detailed quality assessments.
We report our progress toward optimizing backside-illuminated silicon PIN CMOS devices developed by Teledyne Imaging Sensors (TIS) for far-UV planetary science applications. This project was motivated by initial measurements at Southwest Research Institute (SwRI) of the far-UV responsivity of backside-illuminated silicon PIN photodiode test structures described in Bai et al., SPIE, 2008, which revealed a promising QE in the 100-200 nm range as reported in Davis et al., SPIE, 2012. Our effort to advance the capabilities of thinned silicon wafers capitalizes on recent innovations in molecular beam epitaxy (MBE) doping processes. Key achievements to date include: 1) Representative silicon test wafers were fabricated by TIS, and set up for MBE processing at MIT Lincoln Laboratory (LL); 2) Preliminary far-UV detector QE simulation runs were completed to aid MBE layer design; 3) Detector fabrication was completed through the pre-MBE step; and 4) Initial testing of the MBE doping process was performed on monitoring wafers, with detailed quality assessments. Early results suggest that potential challenges in optimizing the UV-sensitivity of silicon PIN type CMOS devices, compared with similar UV enhancement methods established for CCDs, have been mitigated through our newly developed methods. We will discuss the potential advantages of our approach and briefly describe future development steps.
The TMT first light Adaptive Optics (AO) facility consists of the Narrow Field Infra-Red AO System (NFIRAOS) and the associated Laser Guide Star Facility (LGSF). NFIRAOS is a 60 × 60 laser guide star (LGS) multi-conjugate AO (MCAO) system, which provides uniform, diffraction-limited performance in the J, H, and K bands over 17-30 arc sec diameter fields with 50 per cent sky coverage at the galactic pole, as required to support the TMT science cases. NFIRAOS includes two deformable mirrors, six laser guide star wavefront sensors, and three low-order, infrared, natural guide star wavefront sensors within each client instrument. The first light LGSF system includes six sodium lasers required to generate the NFIRAOS laser guide stars. In this paper, we will provide an update on the progress in designing, modeling and validating the TMT first light AO systems and their components over the last two years. This will include pre-final design and prototyping activities for NFIRAOS, preliminary design and prototyping activities for the LGSF, design and prototyping for the deformable mirrors, fabrication and tests for the visible detectors, benchmarking and comparison of different algorithms and processing architecture for the Real Time Controller (RTC) and development and tests of prototype candidate lasers. Comprehensive and detailed AO modeling is continuing to support the design and development of the first light AO facility. Main modeling topics studied during the last two years include further studies in the area of wavefront error budget, sky coverage, high precision astrometry for the galactic center and other observations, high contrast imaging with NFIRAOS and its first light instruments, Point Spread Function (PSF) reconstruction for LGS MCAO, LGS photon return and sophisticated low order mode temporal filtering.
We provide an update on the development of the first light adaptive optics systems for the Thirty Meter Telescope
(TMT) over the past two years. The first light AO facility for TMT consists of the Narrow Field Infra-Red AO
System (NFIRAOS) and the associated Laser Guide Star Facility (LGSF). This order 60 × 60 laser guide star
(LGS) multi-conjugate AO (MCAO) architecture will provide uniform, diffraction-limited performance in the
J, H, and K bands over 17-30 arc sec diameter fields with 50 per cent sky coverage at the galactic pole, as
is required to support TMT science cases. Both NFIRAOS and the LGSF have successfully completed design
reviews during the last twelve months. We also report on recent progress in AO component prototyping, control
algorithm development, and system performance analysis.
This paper summarizes progress of a project to develop and advance the maturity of photon-counting detectors for
NASA exoplanet missions. The project, funded by NASA ROSES TDEM program, uses a 256×256 pixel silicon Geigermode
avalanche photodiode (GM-APD) array, bump-bonded to a silicon readout circuit. Each pixel independently
registers the arrival of a photon and can be reset and ready for another photon within 100 ns. The pixel has built-in
circuitry for counting photo-generated events. The readout circuit is multiplexed to read out the photon arrival events.
The signal chain is inherently digital, allowing for noiseless transmission over long distances. The detector always
operates in photon counting mode and is thus not susceptible to excess noise factor that afflicts other technologies. The
architecture should be able to operate with shot-noise-limited performance up to extremely high flux levels,
>106 photons/second/pixel, and deliver maximum signal-to-noise ratios on the order of thousands for higher fluxes. Its
performance is expected to be maintained at a high level throughout mission lifetime in the presence of the expected
Dark current for back-illuminated (BI) charge-coupled-device (CCD) imagers at Lincoln Laboratory has historically been higher than for front-illuminated (FI) detectors. This is presumably due to high concentrations of unpassivated dangling bonds at or near the thinned back surface caused by wafer thinning, inadequate passivation and low quality native oxide growth. The high dark current has meant that the CCDs must be substantially cooled to be comparable to FI devices. The dark current comprises three components: frontside surface-state, bulk, and back surface. We have developed a backside passivation process that significantly reduces the dark current of BI CCDs. The BI imagers are passivated using molecular beam epitaxy (MBE) to grow a thin heavily boron-doped layer, followed by an annealing step in hydrogen. The frontside surface state component can be suppressed using surface inversion, where clock dithering reduces the frontside dark current below the bulk. This work uses surface inversion, clock dithering and comparison between FI and BI imagers as tools to determine the dark current from each of the components. MBE passivated devices, when used with clock dithering, have dark current reduced by a factor of one hundred relative to ion-implant/laser annealed devices, with measured values as low as 10-14 pA/cm2 at 20°C.
The CCD detectors in the X-ray Imaging Spectrometers (XIS) aboard Suzaku have been equipped with a precision
charge injection capability. The purposes of this capability are to measure and reduce the detector degradation
caused by charged particle radiation encountered on-orbit. Here we report the first results from routine operation
of the XIS charge injection function. After 12 months' exposure of the XIS to the on-orbit charged particle
environment, charge injection already provided measurable improvements in detector performance: the observed
width of the 5.9 keV line from the onboard calibration source was reduced from 205 eV to less than 145 eV.
The rate of degradation is also significantly smaller with charge injection, so its benefit will increase as the
mission progresses. Measured at 5.9 keV, the radiation-induced rate of gain degradation is reduced by a factor
of 4.3 ± 0.1 in the front-illuminated sensors when injecting charge greater than 6 keV equivalent per pixel. The
corresponding rate of degradation in spectral resolution is reduced by a factor 6.5 ± 0.3. Injection of a smaller
quantity of injected charge in the back-illuminated XIS sensor produces commensurately smaller improvement
factors. Excellent uniformity of the injected charge pattern is essential to the effectiveness of charge injection in
The Extreme-Ultraviolet Variability Experiment (EVE) is a component of NASA's Solar Dynamics Observatory (SDO)
satellite, aimed at measuring the solar extreme ultraviolet (EUV) irradiance with high spectral resolution, temporal
cadence, accuracy, and precision. The required high EUV quantum efficiency (QE), coupled with the radiation dose to
be experienced by the detectors during the five year mission (~1 Mrad), posed a serious challenge to the charge-coupled
device (CCD) detectors. MIT Lincoln Laboratory developed the 2048 × 1024 pixel CCDs and integrated them into the
detector system. The devices were back-side thinned and then back surface passivated using a thin, heavily boron-doped
silicon layer grown by molecular beam epitaxy (MBE) at less than 450°C. Radiation-hardness testing was performed
using the Brookhaven National Laboratory's National Synchrotron Light Source (BNL/NSLS). The MBE-passivated
devices were compared against devices with back surfaces passivated with a silver charge chemisorption process and an
ion-implant/furnace anneal process. The MBE devices provided both the highest QE at the required (-100°C) operating
temperatures, and superior radiation hardness, exceeding the goals for the project. Several flight-ready devices were
delivered with the detector system for integration with the satellite.
We have developed X-ray CCD sensors for the Astro-E2 X-ray Imaging
Spectrometer. Here we describe the performance benefits obtained from two innovations implemented in the CCD detectors developed for this instrument. First, we discuss the improved radiation tolerance afforded by a novel charge-injection structure. Second, we demonstrate for the first time the potential of a previously-developed chemisorption charging backside treatment process to produce back-illuminated X-ray sensors with excellent soft X-ray spectral resolution as well as improved quantum efficiency. We
describe the changes in X-ray event detection algorithms required to obtain this improved performance, and briefly compare the performance of XIS sensors to that of back-illuminated detectors currently operating on-orbit.
We have performed precise measurements of x-ray absorption constants for all the thin films comprising CCD gate structure, namely, phosphorous doped polysilicon, silicon dioxide, and silicon nitride. X-ray absorption of these films shows large oscillations around the corresponding absorption edges: nitrogen K, oxygen K, silicon L and K. As a result, quantum efficiency of a CCD in the soft x-ray range deviates significantly from the generally assumed simple model predictions. In order to cover the range of energies from 60 eV to 3000 eV transmission measurements were performed at several synchrotron beamlines at ALS, PTB BESSY, SRC. A model of the CCD response with near edge x-ray absorption structure taken into account predicts a very complicated shape of the energy dependence of the quantum efficiency around silicon and oxygen absorption edges. Experimental measurements of CCD quantum efficiency relative to a calibrated detector were performed at BESSY for both frontside illuminated and backside illuminated CCDs for energies around the oxygen absorption edge. Experimental results were found to be in a good agreement with our model.
As part of our program to select and calibrate flight- quality, x-ray CCD detectors for the AXAF CCD imaging spectrometer, we have developed efficient detector screening methods. Our screening protocol, which measures device-level performance parameters (including noise, dark current and charge transfer efficiency) as well as x-ray spectral resolution in the 0.3 - 6 keV band, allows us quickly to identify which of the greater than 30 flight candidate detectors warrant the expenditure of severely limited time available for calibration. The performance criteria used to rank devices are discussed, and the details of the measurement methods are presented. Summary results of the screening measurements are presented for a large sample of devices, and detailed data on selected devices are described. We find that the performance variations among the sample of flight devices to be relatively small but significant.
NOAA has commissioned a solar x-ray imager to be built for use on the GOES spacecraft. The mission of the SXI is to provide soft x-ray imagery of the Sun. The current instrument design employs a microchannel plate detector stack to convert the incident x-rays to an electrically detectable signal. In this paper, we discuss the SXI performance improvements possible by replacing the detector with a back-illuminated, x-ray sensitive CCD fabricated using technology developed at MIT/LL. In addition to a description of the x-ray sensitive CCD, we discuss possible improvements in data quality, reduction in instrument mass and power requirements, and simplified instrument handling.
focusing on work on large device formats, improvements in quantum efficiency, and reduction of CCD degradation in the natural space-radiation environment. Research was based on a 420 x 420-pixel frame-transfer device and a new 1024 x 1024-pixel device. To obtain high quantum efficiency from the visible into the UV, a technology for making back-illuminated versions of these devices is being developed. Quantum efficiencies greater than 80 percent in the 500-800 nm band have been obtained with a SiO antireflection coating. Particular attention is given to the problem of charge-transfer inefficiency degradation caused by energetic protons in space-based systems. It is shown that CCDs can be significantly hardened to radiation effects by a combination of special buried channel potential profiles and operation at temperatures around 150 K, where the trap sites created by the protons have emission times much longer than the clock periods.
An analytical model has been developed for predicting the spectral response of thinned, p+-doped back-illuminated charge-coupled device (CCD) imagers. The device is divided into two regions: a thin, uniformly doped p+ layer used to passivate the illuminated back surface from external electrical effects, and a p- region that extends from the p+ region across the approximately 10-micrometers thickness of the device to the potential well in the buried channel. The one-dimensional steady-state continuity equation for low-injection conditions has been solved analytically for the surface p+ region, which is characterized by electron diffusion length and coefficients appropriate for the doping level and a surface recombination velocity Sn that represents the loss of photoelectrons at the surface. All photoelectrons generated in the p- region are assumed to be collected in the buried channel because of the long diffusion length and the presence of a field sweeping the carriers into the CCD channel. The effect of multiple internal reflections on photoabsorption at long wavelengths is included. The quantum efficiency of this device is calculated as a function of the depth and recombination velocity of the p+ surface layer, using Sn as the only independent fitting parameter, and matches experimental results well over the wavelength range from 360 to 1100 nm.