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Infrared image sensor developments supported by the European Space Agency
Accepted Manuscript Infrared Image Sensor developments supported by the European Space Agency K. Minoglou, N. Nelms, A. Ciapponi, H. Weber, S. Wittig, B. Leone, P.E. Crouzet PII: DOI: Reference:
S1350-4495(18)30427-4 https://doi.org/10.1016/j.infrared.2018.12.010 INFPHY 2799
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
9 June 2018 7 December 2018 8 December 2018
Please cite this article as: K. Minoglou, N. Nelms, A. Ciapponi, H. Weber, S. Wittig, B. Leone, P.E. Crouzet, Infrared Image Sensor developments supported by the European Space Agency, Infrared Physics & Technology (2018), doi: https://doi.org/10.1016/j.infrared.2018.12.010
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Infrared Image Sensor developments supported by the European Space Agency K. Minoglou*, N. Nelms, A. Ciapponi, H. Weber, S. Wittig, B. Leone, P.E. Crouzet ESA-ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands Phone: +31 71 5653797 Email: [email protected] Keywords: ESA, infrared detectors, space missions, earth observation, astronomy
ABSTRACT The European Space Agency (ESA) has an on-going interest in the development of new detectors and the continuous optimization of available detectors across the waveband for space instrumentation. This paper presents the status of current infrared image sensor development activities supported by the Agency.
1
INTRODUCTION
Detectors form a cornerstone in the measurement capabilities of space missions, sensing radiation from infrared to X-rays and beyond, and consequently European Space Agency (ESA) is always concerned to have the highest possible detector performance available to instrument developers. As previously reported [1], in addition to on-going developments focusing on visible Charge Coupled Devices (CCDs) and Complementary Metal Oxide Semiconductor (CMOS) image sensors, high-performance infrared detectors are identified as key components for several upcoming ESA missions in both astronomy and earth observation applications. In support of these demands, the Agency is undertaking developments in the Near Infrared/Short Wave Infrared (NIR/SWIR), Mid Wave Infrared (MWIR) and Long Wave Infrared/Very Long Wave Infrared (LWIR/VLWIR) wavebands aiming at ensuring the availability of suitable detection systems for the already planned and future missions. This paper is presenting the status of infrared detector developments by the Agency. It is not an exhaustive list of on-going activities, as this is out of the scope of such a review. The objective is to focus on the link between the identified space needs, the strategic technology Roadmaps and the related implementation and Research and Development (R&D) developments.
2
SPACE DETECTOR REQUIREMENTS
Detectors are one of the basic elements in astronomical space instrumentation. Typically, telescopes are based on focal planes with one or more detectors, operating in a staring or stepand-stare mode. Linear 1D detectors but also, most commonly, 2D array configurations are used, that require cooled and stable operation. One could wonder why there is a need for continuous detector developments when it comes to space instruments and why it is not possible to use always devices off-the-shelf. The answer is rather straightforward if we consider the fact that detectors performance can be the key limiting parameter for space instrumentation. The desired needs of the science community and the end users, along with
the state-of-the-art, are driving to a large extend the new instrument requirements. On the other hand, the functionality and performance of optical instruments is determined to a very high degree by the available detectors. Unfortunately, most of the times, those two are not in agreement. In both the visible and infrared wavebands, there are a number of common instrument-level, performance drivers. Improvements in related parameters are in turn the drivers behind the numerous detector development activities described in the following sections. Those performance drivers, shown in Figure 1, are summarized below.
Figure 1 Common performance drivers for visible and IR detectors
Larger overall array size – Higher pixel count – Big pixels This detector design parameter is linked to the demand for increased spatial and spectral resolution while maintaining or improving the dynamic range. For space applications, a large pixel is effectively in the 10-20 um range, typically square for imaging applications but can be rectangular when optimized for some instrument configurations. The pixel size specification impacts the maximum signal levels. To be noted here that the size of the infrared detector arrays is smaller compared to the visible ones due to the smaller size of the available substrate materials and the relevant process and technology limitations. Lower read-noise The driver for low read-noise translates to an increased dynamic range, achievable within existing charge handling capabilities. This impacts both image and spectral feature discrimination. Spectral response – material and Antireflective coating (ARC) properties This design parameter relates to the improvements in both narrowband and broadband response for different spectral applications. It requires developments in both semiconductor
material properties (e.g. epitaxial thickness, fine tuning of alloys and growth uniformity for infrared detectors) and post-processing (e.g. anti-reflection coatings). Higher quantum efficiency It is directly related to the maximization of signal to noise ratio. Lower dark current Reduction in dark current has two major impacts at instrument level – better performance (increased dynamic range) at lower temperature or equivalent performance at higher operating temperature. This is a very critical performance driver, especially for infrared detectors were cryo cooling is needed very often. Imaging performance – cross-talk, Mean Transfer Function (MTF) MTF and cross-talk are both closely linked to several other detector design parameters and need to be carefully considered in relation to the type of detector being developed (e.g. CCD, CMOS Image Sensors). Quantum Efficiency (QE) is a good example of parameter interaction with MTF. Reliability – radiation tolerance Exposure to high-energy radiation is a hazard that all detectors launched in to space experience. Radiation hardening techniques are typically included in custom detector developments to provide increased resistance to single event effects and both ionizing and non-ionizing radiation. Operating temperature – dark current again Lower temperatures typically require more spacecraft resources and can have significant impacts upon instrument design. Improved performance at higher operating temperatures is a consistent aim. As mentioned before, this is of very high importance for instruments using infrared detectors. Availability Achieving the best detector performance is very important but to be useful the detector needs to be available. This is a critical aspect, especially considering the fact that space applications alone are rarely able to support commercial viability. Cost Detector cost continues to become more and more relevant due to a number of factors including focal plane sizes, competing technologies and overall economic scrutiny. Cost can also be a factor affecting availability.
3
SPACE MISSIONS NEEDS FOR INFRARED DETECTORS
In this section we try to identify which mission will (or may) benefit from the development of infrared image sensors. Also, the relevant needs that shall be satisfied in order to meet mission needs and to prepare for possible future missions.
3.1
Earth Observation Missions
Infrared detectors have applications in many areas of future Earth Observation ESA missions. Those detectors are needed in e.g. Carbon measuring missions, atmospheric limb sounders, Thermal InfraRed (TIR) instruments and future Sentinel missions, based on the analyses of the Copernicus Space Component evolution. ESA has supported most of the related User Requirements Workshops organized by the European Commission (EC). Further Earth Observation activities will be coordinated with any future engagement in analyses (e.g. gap analyses) of user requirements for the evolution of Copernicus by the Commission and adapted to answer in the best possible way to any identified priorities. For the CO2 monitoring mission, which is considered a high priority mission, a task force has defined the user requirements for a high resolution imaging spectroscopy capability with fast global revisit for emission monitoring [2]. Detectors with a cut-off of 2.2um will be needed. The precise thermal infrared sensing capability over land and coastal zones and water bodies has been discussed at the EC Copernicus Agriculture and Forestry Applications User Requirements Workshop 1. The report2 of the workshop calls for multi-spectral (three bands min. covering from 0.4um to 2.5um, including TIR) observations, daily revisit and spatial resolution of 50m or better, with radiometric performance suited to estimate soil temperature and crop water use (1k or better). On-going activities in ESA are also studying the trade-offs to identify and compare costeffective solutions based on non-cooled and cryo-cooled sensors. Persisting High-Resolution Optical Imaging from geostationary orbit has been studied in the context of Geo-Oculus. This is a sensor with about 17 bands from 412nm to 12um with high spatial resolution, an instantaneous field of view up to 300km by 300km. This study was followed by GEO-HR, which has less bands but an improved spatial resolution for an Instantaneous Field of View (IFOV) of 100km wide. Based on these mission concept ideas, IR sensor would be needed in the SWIR with increased format and reduced dark signal capabilities. The last requirement would be linked to a reduction of the cut-off wavelength to 2.1um. For future mission concepts with better spatial and temporal resolution, the detector pitch needs to be further reduced (below 15um) and the read-out speed needs to be faster. For the Geo-HR concept large arrays need to be generated by butting detectors in the focal plane. For future Light Detection And Ranging (Lidar) missions avalanche photodiode detectors at 1.6um or 2um need to be available for low flux Lidar signal detection.
3.2
Science Missions
The critical requirements for Science missions can in general be stated as high quantum efficiency and very low noise (dark current and read out noise), properties driven by the inherent low light levels of the applications. For the moment, the Science missions under development make use of existing US technology, in most cases Mercury Cadmium Telluride (MCT) arrays from Teledyne (USA). On the European side, as it will be presented in the next chapters, Sofradir (France) is currently working on the development of a 2k² SWIR detector to compete with the H2RG array from Teledyne. 1 2
http://www.copernicus.eu/events/agriculture-and-forestry-applications-user-requirements-workshop http://copernicus.eu/agri-forestry-workshop
A future ESA Science mission requiring high performance detection technology in the NIR, SWIR and MWIR concerns the characterisation via remote sensing of the atmospheres of Exoplanets. A potential mission concept (ARIEL) envisages four photometric channels in the VNIR range (0.5-1.95μm) and an IR spectrometer covering 1.95-7.8μm. The baseline focal plane technology is MCT. More details for this development are presented in section 4.2.1. Table 1 shows a summary of the technology development needs regarding the main future Mission families. Table 1 Needs for infrared detectors to cover future Science Missions Mission / Mission Type
Technology Development Need
time frame
Priority
Exoplanet characterization as e.g. Darwin, ECHO, Ariel, UVOIR
Large Low Noise IR detector arrays (1024x1024 or larger, Noise << 1e-/s for short wavelengths, <10e-/s for long wavelengths)) covering (0.4)0.8-2.5µm, 2.510/12µm and 10/12-16µm, operating above 35K Large IR detector arrays (1024x1024) from 0.42.5/5µm, operating above 90/100K
2021
high
2021
Large Low Noise IR detector arrays (1024x1024 ) covering (0.4)0.8-2.5µm, operating between 10K and 40K
2021
high (same technology as above) Medium/ high
Cryogenic FEE to read out above detectors at Cryo, limiting harness needs and reduce sensitivity to electromagnetic compatibility (EMC), micro vibrations etc Future survey missions will require large Arrays in the Visible/Near Infrared (VIS/NIR), buildup of multiple detector arrays. To simplify the design of the Infrared Focal Plane Array (FPA), large, buttable arrays in the VIS/NIR are required. Curved arrays would simplify the overall optical architecture and are considered very useful. Time Delay Integration (TDI) readout of NIR detectors would enable full sky surveys a la GAIA to be extended to cover also stars currently hidden in dust
2021
high
2025
medium
IR detectors for Planetary missions e.g Castaway, Castalia, IR Fine Guidance Sensors operating at Cryogenic temperatures and in the Vis/NIR e.g. SPICA, ECHO, Ariel Cryogenic Front End Electronics (FEE)
Large survey missions (post GAIA, post Euclid)
4
INFRARED DETECTOR DEVELOPMENTS
The Agency policy is to develop and maintain its strategic approach to detector developments using both technology-push and technology-pull. For example, a new detector development can be linked to a specific mission and therefore, its requirements are based on the mission needs. On the other hand, a detector development can be initiated to increase the Technology Readiness Level (TRL) of a specific technology and show that basic performances can be achieved. In this case, the detector developments are harmonized to cover a large part of the electromagnetic spectrum and to maintain a stability and coherency in the European detector development policy. As presented in the previous sections, high-performance infrared detectors are identified as key components for several upcoming ESA missions in both Earth Observation and Science applications. In support of these demands, ESA is undertaking numerous developments in the NIR/SWIR, MWIR and LWIR/VLWIR wavebands. In particular, demand for detector arrays operating in the NIR and SWIR wavebands (from 1.0 to 3.0 um) is very high. In response, ESA initiated a long-term programme aimed at making large format infrared detector arrays
with cut-offs at 2.1um and 2.5um available within Europe. Currently, regarding infrared detectors, 2 roadmaps have been defined and cover: NIR Large Format Sensor Array (ALFA-N) Technology Development Plan: Program aimed at developing a 2K x 2K, very low noise, very low dark current, MCT array with the supporting Application-Specific Integrated Circuit (ASIC). Low Dark Current 2D MWIR to VLWIR MCT Detectors Technology Development Plan: Coordination and synergy of detector development activities at ESA aiming towards the next generation of MCT 2D MWIR to LWIR detectors. The ALFA-NIR and ALFA-Controller roadmap is shown in Figure 2. Currently, the ALFA-N phase 1 and 2 are finished and phase 3 is on-going. For the ALFA-Controller, the phase 1 is finished and the phase 2 is about to be kicked-off. The MWIR/LWIR detector development roadmap is shown in Figure 3. The programme comprises a number of activities, the status and results of which are presented in the following sections.
Figure 2 ALFA-NIR and ALFA-Controller roadmap funded through ESA Technology Research Programme (TRP) and Core-Technology Programme (CTP).
Figure 3 Coordinated development activities related to MWIR/LWIR/VLWIR MCT detector developments for both Earth Observation and Science missions
4.1 4.1.1
NIR/SWIR Detectors
THE ALFA PROGRAM
4.1.1.1 Detector developments Phase 1 and phase 2 of the detector development are now complete, as is the first stage of the ASIC development programme. The Phase 2 detector development was run as two parallel activities with Leonardo (UK) and CEA-LETI (France). Leonardo developed a 1280 x 1032 MCT hybrid array, with a 15 um pixel pitch and sourcefollower per detector (SFD) architecture. The detector has a cut-off wavelength of 2.1 um and very low dark currents were measured (<1 e-/p/s at 120 K) but the measured quantum efficiency is of order only 25%. However, recent developments at Leonardo have resulted in the demonstration of much higher QE (~70% at 2 um and at 85K) using an MCT avalanche photodiodes (APD) structure and a 24 um pitch Readout Integrated Circuit (ROIC). CEA-LETI developed a 640 x 480, also with 15 um pitch array with SFD pixel architecture, a charge-handling capacity of 80 ke- and a read-noise of 10 e-rms. Very low dark currents have been measured, 1 e-/p/s at a temperature of 100 K. The detector tested [3] at the end of phase 2 showed very promising results (see Figure 4). The best detector, the CEA/LETI (FR) Liquid Phase Epitaxy (LPE) device (640*512 pixels, 15µm pitch), has been proton and then gamma irradiated. The results after the proton radiation are presented in [4] and show an increase of only 24% of the dark current after irradiation up to a fluence of 2.27e11p+ .cm-2.
Figure 4 Quantum efficiency measured on detectors as deliverable from the ALFA-N Phase 2 roadmap
The program is at the moment on phase 3 leaded by Sofradir (France) and has the goal to design, manufacture and characterize a large format 2048 x 2048 MCT array of 15x15um pixel pitch, cut-off wavelength: 2.1 um, cut-on wavelength: 0.8 um,. The target/design specs are: QE >70% with ARC, Charge Handling Capacity (CHC) is 60 ke-, read-noise <18 e-rms (single Correlated Double Sampling (CDS)), dark current: 0.1 e-/p/s @ 100K, No. outputs 32 (read-out through 1,4 or 32) and possibility for non-destructive readout. The detailed design review has been completed successfully and the ROIC is under manufacture with the foundry. It is expected to be delivered in July 2018 with first ROIC level results available in the autumn.
4.1.1.2 Control ASIC developments Two control ASIC developments were performed with Integrated Detector electronics AS (IDEAS) (Norway) and Caeleste (Belgium) and successfully completed in 2017 and 2015 respectively. The control ASIC developed by IDEAS includes a Serial Peripheral Interface (SPI) programmable clock sequencer with 40 waveform outputs and 8 digital inputs. It also features, 16 programmable analog outputs, 4 Low drop-Out regulators (LDOs) supplying power to external units and 5 x 12 bit video inputs operating at 3MHz. Data from 8 digital inputs may also be captured to generate an Interrupt Request (IRQ) for a master-controller. The ASIC is designed in IDEAS own 350nm radiation-hard library. Standalone testing is completed and the functionality is validated to 77K. The control ASIC (NIRCA I) developed by Caeleste includes a programmable clock sequencer with 32 clock outputs, 16 x 10-bit programmable bias voltages, 4 x 16-bit video channels operating at up to 200 kHz and a SpaceWire interface. It is designed using the radiation-hard DARE-180 library. The stand-alone testing has been completed, with full functionality down to 77 K demonstrated, although an issue with the layout of the Analog-toDigital Converters (ADCs) has been identified resulting in an effective performance of 12bits.
As indicated in Figure 2, follow-on activities are on-going to optimise the developments described in this section. Two separate activities of developing a control ASIC are on-going. The first one is targeting Earth Observation application and the second one is oriented toward Science application and in particular the ALFA detector. The Earth Observation targeted activity (NIRCA mkII) is contracted to IDEAS and has the goal of developing an ASIC (see Figure 5) capable of providing clocks and bias voltages to operate the detector as well as providing video processing and digitisation (16 channels, 14 bit, up to 12 Msps), leading to a two-chip detection system (ASIC + detector). The NIRCA mkII is designed to be interface-compatible as a minimum with the Sofradir NGP detectors and the Leonardo ME950. The critical design review is planned for November 2018. In parallel, there is a Science oriented activity aim to further develop a cryogenic, control and digitization ASIC predominantly for optimized large area NIR/SWIR detector hybrid. The phase 1 of the activity has been completed with a prototype ASIC developed by Caeleste (Belgium) that has been tested [5] using a dedicated test set up at ESTEC. For the second phase, the activity is contracted to again to Caeleste (Belgium) and kicked-off in June 2018. The completion of the activity is scheduled for end of 2019. Finally, although the Control ASIC developments are discussed under the context of NIR/SWIR detector developments, it should be pointed out that they are not limited to use with these detectors and their capabilities are applicable to many CMOS ROIC hybrids.
Figure 5 Schematic layout of the control ASIC
4.1.2
SYNERGY WITH EU
In the frame of a synergy between the ESA program ALFA and EU/REA program called ASTEROID (ASTronomy EuROpean Infrared Detection) [6], an infrared SWIR detector is under development. ASTEROID will allow the development of technologies to prepare the future industrialization (large volume) of 2K² ALFA detectors and large infrared FPA in general for other applications. The target is a 2k² FPA with 15 µm pitch, with specific parameters which are listed in the table below.
Table 2 ASTEROID detector specifications
Parameter Format Pixel Pitch ROIC Input Stage Operating Wavelength Quantum Efficiency Operating Temperature Dark Current (at 100K) Linear Well Capacity (non-linearity ≤3%) Cross talk (inter pixel capacitance) Cross talk (other contributions) Readout noise (single CDS) Readout speed
4.1.3
Value 2048 x 2048 15um SFD (Source Follower per Detector) 0.4/0.8 – 2.0/2.5 um ≥70% ≥100K ≤0.1e-/p/s ≥60ke≤2% ≤3% ≤18e- rms ≥100kHz
OPTIMIZATION OF LONG, MODULAR LINEAR InGaAs IMAGERS
This activity aims to develop the next generation of long-linear InGaAs arrays with improved noise performance for commercial applications. This activity was granted to Xenics (Belgium). The objective is to design and fabricate a long, modular linear 1x2048 pixel InGaAs array, with standard SWIR InGaAs of 1.7μm cut-off. The ROIC is a custom stitched design by Xenics and fabricated at OnSemi 0.18um technology. The output configuration is 8 or 16 analog video channels at 60 MHz. The heritage on this activity is the previous successful development for Proba-V sensor with 3000 pixels, 25μm pitch and wire bonded [7]. For PROBA-V the main challenge was to manufacture a long SWIR focal plane of more than 3000 effective pixels, sensitive in the spectral range up to 1.6μm. Long focal plane arrays were achieved by mechanical butting of three linear photodiode arrays. The device operates at room temperature, and the detector package was accommodated to the payload opto-mechanical constraints. However, the above described type of SWIR detector design, based on wire bonding technology, is showing its limits in PROBA-V. The first limit was observed in terms of Signal to Noise Ratio (SNR) due to high FPA electrical noise, introduced by the parasitic detector capacitance and specifically the bonding pad capacitance. Another limit is situated in opto-mechanical performances. The co-planarity of the mechanically butted arrays limits the payload depth of focus tolerances or the use of faster optics. When going to smaller pitches the present detector technology limits the FPA MTF and hence the payload MTF. To reduce the SWIR detector noise figures the current GSTP activity with Xenics was initiated by ESA The purpose of this GSTP was to design a dedicated ROIC and to demonstrate a commercial “flip-chip” linear detector array with a 12.5μm pitch. The intention of this activity is twofold: (a) Demonstrate a significant noise reduction with respect to wire bonded devices (a factor 5 is expected for the highest sensitivities; for the lower sensitivities, the noise performance is gradually dominated by the feed-back capacitor) and hence much better radiometric properties for low signal levels in the SWIR region up to 1.6μm. (b) Demonstrate the modularity of the design so that arrays of various length (from 512 pixels to 2048 pixels) could be built in a straightforward manner.
Currently a demonstrator with one 2K linear InGaAs chip wire bonded on a PCB is successfully fabricated. An appropriate custom ceramic package has been developed (see Figure 6). The noise optimization of the camera hardware/software and full characterization is on-going.
Figure 6 Left: Picture of one assembled FPA. Right: Custom ceramic package
4.1.4
EVALUATION OF MCT APD DETECTORS FOR FUTURE SPACE APPLICATIONS
Many astronomy missions are investigating very low-flux sources, requiring long integration times (e.g. several hundreds of seconds) which places severe constraints on both detector operation and performance, in particular very low noise contribution from the detector readout. In the majority of photodiode based detectors, the read noise performance is effectively limited by the first stage of amplification, whether it is in-pixel or external to the detector. This situation can be overcome through the use of APDs, where an internal amplification of the signal occurs though carrier impact ionization under the influence of a high electric field. The concept for APDs dates back to the 1980s [8] and the first APD arrays were produced in the early 2000s [9]. The solid state mechanisms for avalanche gain in MCT have been well described and three recommended papers are [10], [11] and [12]. In recent years, MCT APD detectors have been shown to have almost single-carrier ionization (electron-initiated or eAPD) leading to excess noise factors of close to 121, unlike that in Si or InGaAs APDs. The main focus of application for APD arrays in astronomy is currently in the area of Adaptive Optics but it is clear that there are a number of other possibilities. In light of this, the Agency is investigating the application of MCT ADP arrays for future space missions through a contract initiated in 2016 with Leonardo (UK). The investigations were focused on the effects of radiation on the performance of MCT APDs. The selected device was a ‘Saphira’ from Leonardo, previously designed & developed for wavefront sensors and interferometry applications in astronomical telescopes, with 320 x 256 x 24µm pixels, baseline dark current ~0.03 e-/p/s and avalanche gain > 500. The activity’s work plan included Total Ionizing Dose (TID) and proton irradiation campaigns @ 100K and @ 293K. Due to the nature of the effects under investigation, great effort was spent on the definition of
the test plan and the development of a specific test bench able to accommodate cryo cooled and light induced tests under irradiation.
Figure 7 Developed test bench for the radiation tests of APD detectors
Analysis of results is on-going and first indication is that radiation did not have a different effect on APDs than on standard MCTs. However, due to technical difficulties at proton test house, not all the objectives of this activity were met. Currently there are discussions on a possible follow-up.
4.2 4.2.1
MWIR Detectors
DETECTOR DEVELOPMENT FOR ARIEL MISSION
This development is targeting the Ariel mission [13] requirement for the AIRS instrument. CEA/Leti (France) are developing an MCT hybrid detector sensitive in the MWIR waveband from 2 to 8 um. This activity focuses on the development of the photosensitive material and the optimization of the detector electro-optical performance (dark current and QE) particularly at low temperature (around 50 K). For this reason, a standard and well understood Capacitive Transimpedance Amplifier (CTIA) ROIC has been selected. Two batches of detectors will be manufactured, with batch#2 taking advantages of lessons learned from batch#1. The material is p-type for the two batches, while several pixel architecture (metal layer overlap, intra and interpixel passivations) are investigated per device and per wafer. The kick-off meeting took place in May 2017. The detailed design review is passed and batch#1 is now manufactured and being characterised at circuit and diode level at 80 K. The detailed characterisation at detector level at a lower temperature will take place from November 2018 on. The completion of the activity is foreseen end of 2019.
4.2.2
DEVELOPMENT OF LOW DARK CURRENT MWIR/LWIR DETECTORS
This activity is on-going with 2 parallel contracts granted. The first one is led by AIM (Germany) and the other by Sofradir (France) both aiming to develop an MCT hybrid detector with low dark current and low noise ROIC operating at low temperature. The study is performance oriented and does not focus on any specific MCT technology or any specific ROIC architecture. For both activities, the key requirements are: cut-off wavelength of 12.5 um at 40K, dark current density < 2.5e-11A.cm-2 (<0.08e-/p/s) at 40K and detection efficiency ≥ 60%. The title of the activity indicates a development in both the MWIR and the LWIR. However, it was decided at the early stages of the requirement definition to focus only on the 12.5 um cutoff. Sofradir manufactured the detectors and started the characterizations of the chosen ROIC. In order to reach the 40K operating temperature, the setup has been updated and the characterization of the hybrid will start in the coming months. Results of 35% decrease of dark current in the diffusion regime and a demonstration of p-on-n technology with a long cutoff wavelength in the infrared range are presented in [14]. In the AIM activity, also a design of a new ROIC with 4 readout segments has been developed: 7T-CTIA, 5T-CTIA non radiation tolerant, 5T-CTIA radiation tolerant, source follower. The setup is now ready for the characterization of the final hybrid devices (p-on-n and n-on-p MCT materials).
4.2.3
VANADIUM DIOXIDE HIGH-RESOLUTION UNCOOLED BOLOMETER ARRAY
This activity targets to demonstrate the feasibility of a large-format uncooled microbolometer array detectors in 8-12 µm spectral range approaching the performance of cooled instruments for future low-cost, low-mass, remote sensing instruments. The activity was granted to Xenics (Belgium). The developed bolometer (see Figure 8) features a vanadium oxide µbolometer detector technology with 1024×1024 array size, 17μm pixel pitch, using a custom ROIC. Currently, a large-format 2D detector array and associated ROIC were designed and manufactured. The active test pixels were found to be compliant to the requirements, and provided for a thermal time constant of 6.73 ms and a responsivity of 912 kV/W. The 1/f noise coefficient of active and reference pixels were measured to be of the order of 1.10-12. However, initial tests revealed issue with ROIC design, which required a redesign of the ROIC. The re-design is on-going. The end of project is scheduled in October 2018 due to additional technical delays.
Figure 8 Left: Two-level microbolometer pixel geometry. Right: Microbolometer pixel with double length, 90 degree supporting legs
4.3
LWIR/VLWIR Detectors
Longer wavelength measurements require detectors with a longer cut-off wavelength and narrower bandgap, and consequently require much colder operating temperatures to reduce thermally generated dark current to acceptable levels. This has a number of impacts at both detector and spacecraft level e.g. ROICs must be able to operate at cryogenic temperatures (as low as 20 K in some cases) and the spacecraft has to be able to supply the necessary resources for cooling the detector and maintaining a stable background environment. An alternative or complementary approach is to reduce the dark current as much as possible through engineering of the MCT layer such that the detector can be operated at a higher temperature while still achieving the required performance. In an effort to improve both the performance and the knowledge of detector capabilities, ESA has initiated a series of coordinated activities, the latest results from which are presented below.
4.3.1
LOW DARK CURRENT 2D MCT DETECTOR DEVELOPMENT
Two parallel activities targeting “Low dark current 2D MCT detector development” (11.5 um and 14.5 um cut-off) with Leonardo (UK) and AIM (Germany) have been completed recently. Leonardo investigated several design/process options aimed at reducing dark currents further than that exhibited by their standard process. At AIM, custom developments of both n-on-p and p-on-n planar MCT diode arrays using LPE have demonstrated a significant reduction in dark current below Rule 07 [15], Those results show also an improve compared to previous AIM LWIR/VLWIR technology (Figure 9). Up to date, the LWIR and VLWIR detectors of AIM are the state-of the art with respect to the lowest dark current achieved [16]. Also, some very promising results have been acquired during a continuation of the contract over the originally planned end date, when the diodes were tested in higher temperatures, opening the way to allow higher temperature operation within acceptable low levels of the dark current. Following on from this parallel development, ESA has now initiated further development activities of array technology with 12.5 um cut-off wavelength for operation down to 40 K or lower. Those are the activities mentioned in the section 4.2.2
Figure 9 Quantum efficiency (top) and dark current (bottom) measurements for LWIR diodes for AIM p on-n planar technology, showing flat QE response and dark current improvements
4.3.2
LOW DARK CURRENT VLWIR T2SL INFRA RED DETECTORS
ESA is also interested in alternative infrared detector technologies to MCT and recognizes that significant progress has been made using III-V materials. Since proposed in 1980s [17][19], the InAs/(In,Ga)Sb Type II super-lattice structure (T2SL) has gained a lot of interest for the infrared detection applications. For example, there is a potential lower cost of quantum well detector technologies such as Quantum Well Infrared Photodetector (QWIPs), Quantum Dot Infrared Photodetectors (QDIPs), and T2SL photodetectors compared to MCTs and InSb. However, the relative performance of these technologies has not been sufficiently high to allow them to be widely used except from very limited applications. Studies on performance parameter sensitivity analysis to determine the relative importance of various FPA performance parameters to the overall performance of a long range imaging system are very useful to provide comparisons of performance for various detector technologies from the perspective of end-to-end system performance [20]. In parallel to those studies, the relevant technology needs to be further improved. Performance of MWIR and LWIR T2SL detectors has not achieved its theoretically predicted limit and to fully realize the T2SL potential methods of suppression of various dark current components have to be developed [21].
Following a study funded by ESA and led by University of Cardiff in the UK, T2SL detectors have been identified as a potential alternative for future LWIR and VLWIR applications. As a result, ESA has initiated an activity with IRNova (Sweden), to investigate the capabilities of T2SL arrays in a direct comparison with the performance achieved using MCT technology (cut-off wavelengths 11.5 um and 14.5 um). The activity includes the design, manufacturing and testing of high performance, low dark current VLWIR detector array using a Commercial off-the-shelf (COTS) ROIC (FLIR Systems) of 320×256 array size, 30μm pixel pitch and 384×288 array size, 25μm pixel pitch. For the T2SL the options investigated are InAs/GaSb:p-on-n, InAs/GaSb:n-on-p and InAs/InAsSb:p-on-n. Currently, the results of the first run of material growth have been reviewed and the second run is on-going. Process development and detector fabrication/evaluation is in progress. In order to use the optimum of the various studied options and to reach the best performance, the need to use a different ROIC was identified. The activity is on-going with significant indications that the T2SL detectors can be a strong competitor to MCT with high QE, low dark current and superior uniformity to MCT. Further results as well as investigations of T2SL structures for the SWIR wavebands can be found in [22]-[25].
5
DISCUSSION
Infrared detectors are one of the key enabling technologies for a wide range of space missions in both the Earth Observation and Scientific domains, ranging from high-resolution multispectral imagers to “low cost” infrared systems for Earth Remote Sensing. In order to realize the desired status of technical capabilities and industrial strength, the developments covering the infrared detector technology must address the following key areas: Constant effort is needed on maintaining and expanding “mainstream” infrared (e.g. MCT / InGaAs) capability. e.g. pursue increase in format for IR detectors with impact on substrate size, small pixel pitch hybridisation and ROIC development (including stitching). Explore and develop promising (alternate / additional) technologies e.g. III-V compounds, Type-II super-lattice structures, uncooled thermal detectors, APDs, p-on-n MCT structures, materials and architectures targeting lower dark-current and higher operating temperatures. For ESA, an additional key objective is the European non-dependence and the further performance enhancement in European detector capabilities It should be noted here that the Space market is not possible to sustain the needed capability at foundry level. Therefore synergies with other sectors must be exploited e.g. development of Infrared detectors has been until relatively recently supported mainly by defence applications but is now being pursued in the commercial sector, encouraging research outside of the defence sector.
6
CONCLUSIONS
Infrared detectors play a key role in the design and implementation of space instruments, both for earth observation and for astronomy applications. The European Space Agency is aiming on providing to the scientific community and to the end users detectors of high performance, meeting the demanding needs of space missions. Additional to the optimization of the performance, their uninterrupted availability is of paramount importance. Maintaining and
expanding detector manufacturing capabilities and foundries in Europe is a key objective of the Agency. Using both technology-push and technology-pull ESA is maintaining its strategic approach to detector developments, through both internal and external consultation, resulting in the initiation of targeted detector development activities across the waveband.
7
REFERENCES
[1] N. Nelms et al., “The status of European Space Agency supported detector developments”, Proc. SPIE 9915, High Energy, Optical, and Infrared Detectors for Astronomy VII, 991502 (27 July 2016), doi: 10.1117/12.2232517 [2] Pinty B., G. Janssens-Maenhout, M. Dowell, H. Zunker, T. Brunhes, P. Ciais, D. Dee, H. Denier van der Gon, H. Dolman, M. Drinkwater, R. Engelen, M. Heiman, K. Holmlund, R. Husband, A. Kentarchos, Y. Meijer, P. Palmer and M. Scholze (2017): An Operational Anthropogenic CO2 Emissions Monitoring & Verification Support capacity - Baseline Requirements, Model Components and Functional Architecture, doi: 10.2760/08644, European Commission Joint Research Centre, EUR 28736 EN [3] P-E. Crouzet et al., “First characterization of the NIR European Large Format Array detectors at ESTEC", SPIE 9639 (2015) [4] D. Gooding et al. “Large format array NIR detectors for future ESA astronomy missions: characterization and comparison”. Proceedings Volume 9915, High Energy, Optical, and Infrared Detectors for Astronomy VII; 99151G (2016) https://doi.org/10.1117/12.2231179 [5] F. Lemmel et al. “Large format array controller (aLFA-C): tests and characterisation at ESA”. Proceedings Volume 9915, High Energy, Optical, and Infrared Detectors for Astronomy VII; 99151N (2016) [6] https://cordis.europa.eu/project/rcn/210172_en.html [7] J. Bentell et. al, “3000 pixel linear InGaAs sensor for the Proba-V satellite”, Proc. SPIE 7862, Earth Observing Missions and Sensors: Development, Implementation, and Characterization, 786206 (4 November 2010); doi: 10.1117/12.869647 [8] C. T. Elliott et al., J.Vac.Sci.Technol.A (USA), 8 , 1251, (1990) [9] J. D. Beck et al., Proc. SPIE, 4454, 188, (2001) [10] J. Rothman et al., Electron Mater, 40, No 8, 1757, (2011) [11] J. D. Beck et al., Advanced photon counting techniques, SPIE Conference Series, 8033, (2011) [12] M. A. Kinch et al., Chapter 21, “HgCdTe Electron Avalanche Photodiodes”, Mercury Cadmium Telluride Growth, Properties and Applications, published by Wiley, (2011) [13] https://ariel-spacemission.eu/ [14] N. Péré-Laperne et al., "Low dark current p-on-n technology for space applications," Proc. SPIE 10404, Infrared Sensors, Devices, and Applications VII, 104040G (30 August 2017); doi:10.1117/12.2275359 [15] W.E. Tennant, "MBE HgCdTe Technology: A Very General Solution to IR Detection, Described by ''Rule 07'', a Very Convenient Heuristic", JEM, 37(9), 1406, (2008) [16] S. Hanna et al., “Low dark current LWIR and VLWIR HgCdTe focal plane arrays at AIM”, Proc. SPIE Remote Sensing, 1000-25, (2016) [17] G. A. Sai-Halasz, et al., “A new semiconductor superlattice,” Applied Physics Letters, vol. 30, no. 12, pp. 651–653, (1977) [18] L. Esaki, “InAs-GaSb superlattices-synthesized semiconductors and semimetals,” Journal of Crystal Growth, vol. 52, no. 1, pp.227–240, (1981) [19] D. L. Smith et al, “Proposal for strained type II superlattice infrared detectors,” Journal of Applied Physics, vol. 62, no. 6, pp. 2545–2548, (1987) [20] L.Terence et al., “Relative performance analysis of IR FPA technologies from the perspective of system level performance”, Infrared Physics & Technology, Volume 84, (2017), Pages 7-20, ISSN 1350-4495, https://doi.org/10.1016/j.infrared.2017.03.007. [21] E. A. Plis, “InAs/GaSb Type-II Superlattice Detectors”, Advances in Electronics, Volume 2014, Article ID 246769, 12 pages, http://dx.doi.org/10.1155/2014/246769 [22] L. Höglund et al, “Very long wavelength type-II InAs/GaSb superlattice infrared detectors”, Proc. SPIE 10624, Infrared Technology and Applications XLIV, 1062401 (April 17, 2018) https://doi.org/10.1117/12.2292977 [23] L. Höglund et al., “Advantages of T2SL: results from production and new development at IRnova”, Proceedings Volume 9819, Infrared Technology and Applications XLII; 98190Z (2016)
[24] L. Höglund et al,. “Manufacturability of type-II InAs/GaSb superlattice detectors for infrared imaging”, Infrared Physics & Technology, Volume 84, (2017), Pages 28-32, ISSN 1350-4495, https://doi.org/10.1016/j.infrared.2017.03.002. [25] Y. Uliel et al., “InGaAs/GaAsSb Type-II superlattice based photodiodes for short wave infrared detection”, Infrared Physics & Technology, Volume 84, (2017), Pages 63-71, ISSN 1350-4495, https://doi.org/10.1016/j.infrared.2017.02.003.
Highlights
Space instrumentation requires continuous detector developments High-performance infrared detectors are key components for upcoming space missions On-going activities aiming to improve “mainstream” infrared technologies Additional activities targeting promising (alternate / additional) technologies
S1350-4495(18)30427-4 https://doi.org/10.1016/j.infrared.2018.12.010 INFPHY 2799
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
9 June 2018 7 December 2018 8 December 2018
Please cite this article as: K. Minoglou, N. Nelms, A. Ciapponi, H. Weber, S. Wittig, B. Leone, P.E. Crouzet, Infrared Image Sensor developments supported by the European Space Agency, Infrared Physics & Technology (2018), doi: https://doi.org/10.1016/j.infrared.2018.12.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Infrared Image Sensor developments supported by the European Space Agency K. Minoglou*, N. Nelms, A. Ciapponi, H. Weber, S. Wittig, B. Leone, P.E. Crouzet ESA-ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands Phone: +31 71 5653797 Email: [email protected] Keywords: ESA, infrared detectors, space missions, earth observation, astronomy
ABSTRACT The European Space Agency (ESA) has an on-going interest in the development of new detectors and the continuous optimization of available detectors across the waveband for space instrumentation. This paper presents the status of current infrared image sensor development activities supported by the Agency.
1
INTRODUCTION
Detectors form a cornerstone in the measurement capabilities of space missions, sensing radiation from infrared to X-rays and beyond, and consequently European Space Agency (ESA) is always concerned to have the highest possible detector performance available to instrument developers. As previously reported [1], in addition to on-going developments focusing on visible Charge Coupled Devices (CCDs) and Complementary Metal Oxide Semiconductor (CMOS) image sensors, high-performance infrared detectors are identified as key components for several upcoming ESA missions in both astronomy and earth observation applications. In support of these demands, the Agency is undertaking developments in the Near Infrared/Short Wave Infrared (NIR/SWIR), Mid Wave Infrared (MWIR) and Long Wave Infrared/Very Long Wave Infrared (LWIR/VLWIR) wavebands aiming at ensuring the availability of suitable detection systems for the already planned and future missions. This paper is presenting the status of infrared detector developments by the Agency. It is not an exhaustive list of on-going activities, as this is out of the scope of such a review. The objective is to focus on the link between the identified space needs, the strategic technology Roadmaps and the related implementation and Research and Development (R&D) developments.
2
SPACE DETECTOR REQUIREMENTS
Detectors are one of the basic elements in astronomical space instrumentation. Typically, telescopes are based on focal planes with one or more detectors, operating in a staring or stepand-stare mode. Linear 1D detectors but also, most commonly, 2D array configurations are used, that require cooled and stable operation. One could wonder why there is a need for continuous detector developments when it comes to space instruments and why it is not possible to use always devices off-the-shelf. The answer is rather straightforward if we consider the fact that detectors performance can be the key limiting parameter for space instrumentation. The desired needs of the science community and the end users, along with
the state-of-the-art, are driving to a large extend the new instrument requirements. On the other hand, the functionality and performance of optical instruments is determined to a very high degree by the available detectors. Unfortunately, most of the times, those two are not in agreement. In both the visible and infrared wavebands, there are a number of common instrument-level, performance drivers. Improvements in related parameters are in turn the drivers behind the numerous detector development activities described in the following sections. Those performance drivers, shown in Figure 1, are summarized below.
Figure 1 Common performance drivers for visible and IR detectors
Larger overall array size – Higher pixel count – Big pixels This detector design parameter is linked to the demand for increased spatial and spectral resolution while maintaining or improving the dynamic range. For space applications, a large pixel is effectively in the 10-20 um range, typically square for imaging applications but can be rectangular when optimized for some instrument configurations. The pixel size specification impacts the maximum signal levels. To be noted here that the size of the infrared detector arrays is smaller compared to the visible ones due to the smaller size of the available substrate materials and the relevant process and technology limitations. Lower read-noise The driver for low read-noise translates to an increased dynamic range, achievable within existing charge handling capabilities. This impacts both image and spectral feature discrimination. Spectral response – material and Antireflective coating (ARC) properties This design parameter relates to the improvements in both narrowband and broadband response for different spectral applications. It requires developments in both semiconductor
material properties (e.g. epitaxial thickness, fine tuning of alloys and growth uniformity for infrared detectors) and post-processing (e.g. anti-reflection coatings). Higher quantum efficiency It is directly related to the maximization of signal to noise ratio. Lower dark current Reduction in dark current has two major impacts at instrument level – better performance (increased dynamic range) at lower temperature or equivalent performance at higher operating temperature. This is a very critical performance driver, especially for infrared detectors were cryo cooling is needed very often. Imaging performance – cross-talk, Mean Transfer Function (MTF) MTF and cross-talk are both closely linked to several other detector design parameters and need to be carefully considered in relation to the type of detector being developed (e.g. CCD, CMOS Image Sensors). Quantum Efficiency (QE) is a good example of parameter interaction with MTF. Reliability – radiation tolerance Exposure to high-energy radiation is a hazard that all detectors launched in to space experience. Radiation hardening techniques are typically included in custom detector developments to provide increased resistance to single event effects and both ionizing and non-ionizing radiation. Operating temperature – dark current again Lower temperatures typically require more spacecraft resources and can have significant impacts upon instrument design. Improved performance at higher operating temperatures is a consistent aim. As mentioned before, this is of very high importance for instruments using infrared detectors. Availability Achieving the best detector performance is very important but to be useful the detector needs to be available. This is a critical aspect, especially considering the fact that space applications alone are rarely able to support commercial viability. Cost Detector cost continues to become more and more relevant due to a number of factors including focal plane sizes, competing technologies and overall economic scrutiny. Cost can also be a factor affecting availability.
3
SPACE MISSIONS NEEDS FOR INFRARED DETECTORS
In this section we try to identify which mission will (or may) benefit from the development of infrared image sensors. Also, the relevant needs that shall be satisfied in order to meet mission needs and to prepare for possible future missions.
3.1
Earth Observation Missions
Infrared detectors have applications in many areas of future Earth Observation ESA missions. Those detectors are needed in e.g. Carbon measuring missions, atmospheric limb sounders, Thermal InfraRed (TIR) instruments and future Sentinel missions, based on the analyses of the Copernicus Space Component evolution. ESA has supported most of the related User Requirements Workshops organized by the European Commission (EC). Further Earth Observation activities will be coordinated with any future engagement in analyses (e.g. gap analyses) of user requirements for the evolution of Copernicus by the Commission and adapted to answer in the best possible way to any identified priorities. For the CO2 monitoring mission, which is considered a high priority mission, a task force has defined the user requirements for a high resolution imaging spectroscopy capability with fast global revisit for emission monitoring [2]. Detectors with a cut-off of 2.2um will be needed. The precise thermal infrared sensing capability over land and coastal zones and water bodies has been discussed at the EC Copernicus Agriculture and Forestry Applications User Requirements Workshop 1. The report2 of the workshop calls for multi-spectral (three bands min. covering from 0.4um to 2.5um, including TIR) observations, daily revisit and spatial resolution of 50m or better, with radiometric performance suited to estimate soil temperature and crop water use (1k or better). On-going activities in ESA are also studying the trade-offs to identify and compare costeffective solutions based on non-cooled and cryo-cooled sensors. Persisting High-Resolution Optical Imaging from geostationary orbit has been studied in the context of Geo-Oculus. This is a sensor with about 17 bands from 412nm to 12um with high spatial resolution, an instantaneous field of view up to 300km by 300km. This study was followed by GEO-HR, which has less bands but an improved spatial resolution for an Instantaneous Field of View (IFOV) of 100km wide. Based on these mission concept ideas, IR sensor would be needed in the SWIR with increased format and reduced dark signal capabilities. The last requirement would be linked to a reduction of the cut-off wavelength to 2.1um. For future mission concepts with better spatial and temporal resolution, the detector pitch needs to be further reduced (below 15um) and the read-out speed needs to be faster. For the Geo-HR concept large arrays need to be generated by butting detectors in the focal plane. For future Light Detection And Ranging (Lidar) missions avalanche photodiode detectors at 1.6um or 2um need to be available for low flux Lidar signal detection.
3.2
Science Missions
The critical requirements for Science missions can in general be stated as high quantum efficiency and very low noise (dark current and read out noise), properties driven by the inherent low light levels of the applications. For the moment, the Science missions under development make use of existing US technology, in most cases Mercury Cadmium Telluride (MCT) arrays from Teledyne (USA). On the European side, as it will be presented in the next chapters, Sofradir (France) is currently working on the development of a 2k² SWIR detector to compete with the H2RG array from Teledyne. 1 2
http://www.copernicus.eu/events/agriculture-and-forestry-applications-user-requirements-workshop http://copernicus.eu/agri-forestry-workshop
A future ESA Science mission requiring high performance detection technology in the NIR, SWIR and MWIR concerns the characterisation via remote sensing of the atmospheres of Exoplanets. A potential mission concept (ARIEL) envisages four photometric channels in the VNIR range (0.5-1.95μm) and an IR spectrometer covering 1.95-7.8μm. The baseline focal plane technology is MCT. More details for this development are presented in section 4.2.1. Table 1 shows a summary of the technology development needs regarding the main future Mission families. Table 1 Needs for infrared detectors to cover future Science Missions Mission / Mission Type
Technology Development Need
time frame
Priority
Exoplanet characterization as e.g. Darwin, ECHO, Ariel, UVOIR
Large Low Noise IR detector arrays (1024x1024 or larger, Noise << 1e-/s for short wavelengths, <10e-/s for long wavelengths)) covering (0.4)0.8-2.5µm, 2.510/12µm and 10/12-16µm, operating above 35K Large IR detector arrays (1024x1024) from 0.42.5/5µm, operating above 90/100K
2021
high
2021
Large Low Noise IR detector arrays (1024x1024 ) covering (0.4)0.8-2.5µm, operating between 10K and 40K
2021
high (same technology as above) Medium/ high
Cryogenic FEE to read out above detectors at Cryo, limiting harness needs and reduce sensitivity to electromagnetic compatibility (EMC), micro vibrations etc Future survey missions will require large Arrays in the Visible/Near Infrared (VIS/NIR), buildup of multiple detector arrays. To simplify the design of the Infrared Focal Plane Array (FPA), large, buttable arrays in the VIS/NIR are required. Curved arrays would simplify the overall optical architecture and are considered very useful. Time Delay Integration (TDI) readout of NIR detectors would enable full sky surveys a la GAIA to be extended to cover also stars currently hidden in dust
2021
high
2025
medium
IR detectors for Planetary missions e.g Castaway, Castalia, IR Fine Guidance Sensors operating at Cryogenic temperatures and in the Vis/NIR e.g. SPICA, ECHO, Ariel Cryogenic Front End Electronics (FEE)
Large survey missions (post GAIA, post Euclid)
4
INFRARED DETECTOR DEVELOPMENTS
The Agency policy is to develop and maintain its strategic approach to detector developments using both technology-push and technology-pull. For example, a new detector development can be linked to a specific mission and therefore, its requirements are based on the mission needs. On the other hand, a detector development can be initiated to increase the Technology Readiness Level (TRL) of a specific technology and show that basic performances can be achieved. In this case, the detector developments are harmonized to cover a large part of the electromagnetic spectrum and to maintain a stability and coherency in the European detector development policy. As presented in the previous sections, high-performance infrared detectors are identified as key components for several upcoming ESA missions in both Earth Observation and Science applications. In support of these demands, ESA is undertaking numerous developments in the NIR/SWIR, MWIR and LWIR/VLWIR wavebands. In particular, demand for detector arrays operating in the NIR and SWIR wavebands (from 1.0 to 3.0 um) is very high. In response, ESA initiated a long-term programme aimed at making large format infrared detector arrays
with cut-offs at 2.1um and 2.5um available within Europe. Currently, regarding infrared detectors, 2 roadmaps have been defined and cover: NIR Large Format Sensor Array (ALFA-N) Technology Development Plan: Program aimed at developing a 2K x 2K, very low noise, very low dark current, MCT array with the supporting Application-Specific Integrated Circuit (ASIC). Low Dark Current 2D MWIR to VLWIR MCT Detectors Technology Development Plan: Coordination and synergy of detector development activities at ESA aiming towards the next generation of MCT 2D MWIR to LWIR detectors. The ALFA-NIR and ALFA-Controller roadmap is shown in Figure 2. Currently, the ALFA-N phase 1 and 2 are finished and phase 3 is on-going. For the ALFA-Controller, the phase 1 is finished and the phase 2 is about to be kicked-off. The MWIR/LWIR detector development roadmap is shown in Figure 3. The programme comprises a number of activities, the status and results of which are presented in the following sections.
Figure 2 ALFA-NIR and ALFA-Controller roadmap funded through ESA Technology Research Programme (TRP) and Core-Technology Programme (CTP).
Figure 3 Coordinated development activities related to MWIR/LWIR/VLWIR MCT detector developments for both Earth Observation and Science missions
4.1 4.1.1
NIR/SWIR Detectors
THE ALFA PROGRAM
4.1.1.1 Detector developments Phase 1 and phase 2 of the detector development are now complete, as is the first stage of the ASIC development programme. The Phase 2 detector development was run as two parallel activities with Leonardo (UK) and CEA-LETI (France). Leonardo developed a 1280 x 1032 MCT hybrid array, with a 15 um pixel pitch and sourcefollower per detector (SFD) architecture. The detector has a cut-off wavelength of 2.1 um and very low dark currents were measured (<1 e-/p/s at 120 K) but the measured quantum efficiency is of order only 25%. However, recent developments at Leonardo have resulted in the demonstration of much higher QE (~70% at 2 um and at 85K) using an MCT avalanche photodiodes (APD) structure and a 24 um pitch Readout Integrated Circuit (ROIC). CEA-LETI developed a 640 x 480, also with 15 um pitch array with SFD pixel architecture, a charge-handling capacity of 80 ke- and a read-noise of 10 e-rms. Very low dark currents have been measured, 1 e-/p/s at a temperature of 100 K. The detector tested [3] at the end of phase 2 showed very promising results (see Figure 4). The best detector, the CEA/LETI (FR) Liquid Phase Epitaxy (LPE) device (640*512 pixels, 15µm pitch), has been proton and then gamma irradiated. The results after the proton radiation are presented in [4] and show an increase of only 24% of the dark current after irradiation up to a fluence of 2.27e11p+ .cm-2.
Figure 4 Quantum efficiency measured on detectors as deliverable from the ALFA-N Phase 2 roadmap
The program is at the moment on phase 3 leaded by Sofradir (France) and has the goal to design, manufacture and characterize a large format 2048 x 2048 MCT array of 15x15um pixel pitch, cut-off wavelength: 2.1 um, cut-on wavelength: 0.8 um,. The target/design specs are: QE >70% with ARC, Charge Handling Capacity (CHC) is 60 ke-, read-noise <18 e-rms (single Correlated Double Sampling (CDS)), dark current: 0.1 e-/p/s @ 100K, No. outputs 32 (read-out through 1,4 or 32) and possibility for non-destructive readout. The detailed design review has been completed successfully and the ROIC is under manufacture with the foundry. It is expected to be delivered in July 2018 with first ROIC level results available in the autumn.
4.1.1.2 Control ASIC developments Two control ASIC developments were performed with Integrated Detector electronics AS (IDEAS) (Norway) and Caeleste (Belgium) and successfully completed in 2017 and 2015 respectively. The control ASIC developed by IDEAS includes a Serial Peripheral Interface (SPI) programmable clock sequencer with 40 waveform outputs and 8 digital inputs. It also features, 16 programmable analog outputs, 4 Low drop-Out regulators (LDOs) supplying power to external units and 5 x 12 bit video inputs operating at 3MHz. Data from 8 digital inputs may also be captured to generate an Interrupt Request (IRQ) for a master-controller. The ASIC is designed in IDEAS own 350nm radiation-hard library. Standalone testing is completed and the functionality is validated to 77K. The control ASIC (NIRCA I) developed by Caeleste includes a programmable clock sequencer with 32 clock outputs, 16 x 10-bit programmable bias voltages, 4 x 16-bit video channels operating at up to 200 kHz and a SpaceWire interface. It is designed using the radiation-hard DARE-180 library. The stand-alone testing has been completed, with full functionality down to 77 K demonstrated, although an issue with the layout of the Analog-toDigital Converters (ADCs) has been identified resulting in an effective performance of 12bits.
As indicated in Figure 2, follow-on activities are on-going to optimise the developments described in this section. Two separate activities of developing a control ASIC are on-going. The first one is targeting Earth Observation application and the second one is oriented toward Science application and in particular the ALFA detector. The Earth Observation targeted activity (NIRCA mkII) is contracted to IDEAS and has the goal of developing an ASIC (see Figure 5) capable of providing clocks and bias voltages to operate the detector as well as providing video processing and digitisation (16 channels, 14 bit, up to 12 Msps), leading to a two-chip detection system (ASIC + detector). The NIRCA mkII is designed to be interface-compatible as a minimum with the Sofradir NGP detectors and the Leonardo ME950. The critical design review is planned for November 2018. In parallel, there is a Science oriented activity aim to further develop a cryogenic, control and digitization ASIC predominantly for optimized large area NIR/SWIR detector hybrid. The phase 1 of the activity has been completed with a prototype ASIC developed by Caeleste (Belgium) that has been tested [5] using a dedicated test set up at ESTEC. For the second phase, the activity is contracted to again to Caeleste (Belgium) and kicked-off in June 2018. The completion of the activity is scheduled for end of 2019. Finally, although the Control ASIC developments are discussed under the context of NIR/SWIR detector developments, it should be pointed out that they are not limited to use with these detectors and their capabilities are applicable to many CMOS ROIC hybrids.
Figure 5 Schematic layout of the control ASIC
4.1.2
SYNERGY WITH EU
In the frame of a synergy between the ESA program ALFA and EU/REA program called ASTEROID (ASTronomy EuROpean Infrared Detection) [6], an infrared SWIR detector is under development. ASTEROID will allow the development of technologies to prepare the future industrialization (large volume) of 2K² ALFA detectors and large infrared FPA in general for other applications. The target is a 2k² FPA with 15 µm pitch, with specific parameters which are listed in the table below.
Table 2 ASTEROID detector specifications
Parameter Format Pixel Pitch ROIC Input Stage Operating Wavelength Quantum Efficiency Operating Temperature Dark Current (at 100K) Linear Well Capacity (non-linearity ≤3%) Cross talk (inter pixel capacitance) Cross talk (other contributions) Readout noise (single CDS) Readout speed
4.1.3
Value 2048 x 2048 15um SFD (Source Follower per Detector) 0.4/0.8 – 2.0/2.5 um ≥70% ≥100K ≤0.1e-/p/s ≥60ke≤2% ≤3% ≤18e- rms ≥100kHz
OPTIMIZATION OF LONG, MODULAR LINEAR InGaAs IMAGERS
This activity aims to develop the next generation of long-linear InGaAs arrays with improved noise performance for commercial applications. This activity was granted to Xenics (Belgium). The objective is to design and fabricate a long, modular linear 1x2048 pixel InGaAs array, with standard SWIR InGaAs of 1.7μm cut-off. The ROIC is a custom stitched design by Xenics and fabricated at OnSemi 0.18um technology. The output configuration is 8 or 16 analog video channels at 60 MHz. The heritage on this activity is the previous successful development for Proba-V sensor with 3000 pixels, 25μm pitch and wire bonded [7]. For PROBA-V the main challenge was to manufacture a long SWIR focal plane of more than 3000 effective pixels, sensitive in the spectral range up to 1.6μm. Long focal plane arrays were achieved by mechanical butting of three linear photodiode arrays. The device operates at room temperature, and the detector package was accommodated to the payload opto-mechanical constraints. However, the above described type of SWIR detector design, based on wire bonding technology, is showing its limits in PROBA-V. The first limit was observed in terms of Signal to Noise Ratio (SNR) due to high FPA electrical noise, introduced by the parasitic detector capacitance and specifically the bonding pad capacitance. Another limit is situated in opto-mechanical performances. The co-planarity of the mechanically butted arrays limits the payload depth of focus tolerances or the use of faster optics. When going to smaller pitches the present detector technology limits the FPA MTF and hence the payload MTF. To reduce the SWIR detector noise figures the current GSTP activity with Xenics was initiated by ESA The purpose of this GSTP was to design a dedicated ROIC and to demonstrate a commercial “flip-chip” linear detector array with a 12.5μm pitch. The intention of this activity is twofold: (a) Demonstrate a significant noise reduction with respect to wire bonded devices (a factor 5 is expected for the highest sensitivities; for the lower sensitivities, the noise performance is gradually dominated by the feed-back capacitor) and hence much better radiometric properties for low signal levels in the SWIR region up to 1.6μm. (b) Demonstrate the modularity of the design so that arrays of various length (from 512 pixels to 2048 pixels) could be built in a straightforward manner.
Currently a demonstrator with one 2K linear InGaAs chip wire bonded on a PCB is successfully fabricated. An appropriate custom ceramic package has been developed (see Figure 6). The noise optimization of the camera hardware/software and full characterization is on-going.
Figure 6 Left: Picture of one assembled FPA. Right: Custom ceramic package
4.1.4
EVALUATION OF MCT APD DETECTORS FOR FUTURE SPACE APPLICATIONS
Many astronomy missions are investigating very low-flux sources, requiring long integration times (e.g. several hundreds of seconds) which places severe constraints on both detector operation and performance, in particular very low noise contribution from the detector readout. In the majority of photodiode based detectors, the read noise performance is effectively limited by the first stage of amplification, whether it is in-pixel or external to the detector. This situation can be overcome through the use of APDs, where an internal amplification of the signal occurs though carrier impact ionization under the influence of a high electric field. The concept for APDs dates back to the 1980s [8] and the first APD arrays were produced in the early 2000s [9]. The solid state mechanisms for avalanche gain in MCT have been well described and three recommended papers are [10], [11] and [12]. In recent years, MCT APD detectors have been shown to have almost single-carrier ionization (electron-initiated or eAPD) leading to excess noise factors of close to 121, unlike that in Si or InGaAs APDs. The main focus of application for APD arrays in astronomy is currently in the area of Adaptive Optics but it is clear that there are a number of other possibilities. In light of this, the Agency is investigating the application of MCT ADP arrays for future space missions through a contract initiated in 2016 with Leonardo (UK). The investigations were focused on the effects of radiation on the performance of MCT APDs. The selected device was a ‘Saphira’ from Leonardo, previously designed & developed for wavefront sensors and interferometry applications in astronomical telescopes, with 320 x 256 x 24µm pixels, baseline dark current ~0.03 e-/p/s and avalanche gain > 500. The activity’s work plan included Total Ionizing Dose (TID) and proton irradiation campaigns @ 100K and @ 293K. Due to the nature of the effects under investigation, great effort was spent on the definition of
the test plan and the development of a specific test bench able to accommodate cryo cooled and light induced tests under irradiation.
Figure 7 Developed test bench for the radiation tests of APD detectors
Analysis of results is on-going and first indication is that radiation did not have a different effect on APDs than on standard MCTs. However, due to technical difficulties at proton test house, not all the objectives of this activity were met. Currently there are discussions on a possible follow-up.
4.2 4.2.1
MWIR Detectors
DETECTOR DEVELOPMENT FOR ARIEL MISSION
This development is targeting the Ariel mission [13] requirement for the AIRS instrument. CEA/Leti (France) are developing an MCT hybrid detector sensitive in the MWIR waveband from 2 to 8 um. This activity focuses on the development of the photosensitive material and the optimization of the detector electro-optical performance (dark current and QE) particularly at low temperature (around 50 K). For this reason, a standard and well understood Capacitive Transimpedance Amplifier (CTIA) ROIC has been selected. Two batches of detectors will be manufactured, with batch#2 taking advantages of lessons learned from batch#1. The material is p-type for the two batches, while several pixel architecture (metal layer overlap, intra and interpixel passivations) are investigated per device and per wafer. The kick-off meeting took place in May 2017. The detailed design review is passed and batch#1 is now manufactured and being characterised at circuit and diode level at 80 K. The detailed characterisation at detector level at a lower temperature will take place from November 2018 on. The completion of the activity is foreseen end of 2019.
4.2.2
DEVELOPMENT OF LOW DARK CURRENT MWIR/LWIR DETECTORS
This activity is on-going with 2 parallel contracts granted. The first one is led by AIM (Germany) and the other by Sofradir (France) both aiming to develop an MCT hybrid detector with low dark current and low noise ROIC operating at low temperature. The study is performance oriented and does not focus on any specific MCT technology or any specific ROIC architecture. For both activities, the key requirements are: cut-off wavelength of 12.5 um at 40K, dark current density < 2.5e-11A.cm-2 (<0.08e-/p/s) at 40K and detection efficiency ≥ 60%. The title of the activity indicates a development in both the MWIR and the LWIR. However, it was decided at the early stages of the requirement definition to focus only on the 12.5 um cutoff. Sofradir manufactured the detectors and started the characterizations of the chosen ROIC. In order to reach the 40K operating temperature, the setup has been updated and the characterization of the hybrid will start in the coming months. Results of 35% decrease of dark current in the diffusion regime and a demonstration of p-on-n technology with a long cutoff wavelength in the infrared range are presented in [14]. In the AIM activity, also a design of a new ROIC with 4 readout segments has been developed: 7T-CTIA, 5T-CTIA non radiation tolerant, 5T-CTIA radiation tolerant, source follower. The setup is now ready for the characterization of the final hybrid devices (p-on-n and n-on-p MCT materials).
4.2.3
VANADIUM DIOXIDE HIGH-RESOLUTION UNCOOLED BOLOMETER ARRAY
This activity targets to demonstrate the feasibility of a large-format uncooled microbolometer array detectors in 8-12 µm spectral range approaching the performance of cooled instruments for future low-cost, low-mass, remote sensing instruments. The activity was granted to Xenics (Belgium). The developed bolometer (see Figure 8) features a vanadium oxide µbolometer detector technology with 1024×1024 array size, 17μm pixel pitch, using a custom ROIC. Currently, a large-format 2D detector array and associated ROIC were designed and manufactured. The active test pixels were found to be compliant to the requirements, and provided for a thermal time constant of 6.73 ms and a responsivity of 912 kV/W. The 1/f noise coefficient of active and reference pixels were measured to be of the order of 1.10-12. However, initial tests revealed issue with ROIC design, which required a redesign of the ROIC. The re-design is on-going. The end of project is scheduled in October 2018 due to additional technical delays.
Figure 8 Left: Two-level microbolometer pixel geometry. Right: Microbolometer pixel with double length, 90 degree supporting legs
4.3
LWIR/VLWIR Detectors
Longer wavelength measurements require detectors with a longer cut-off wavelength and narrower bandgap, and consequently require much colder operating temperatures to reduce thermally generated dark current to acceptable levels. This has a number of impacts at both detector and spacecraft level e.g. ROICs must be able to operate at cryogenic temperatures (as low as 20 K in some cases) and the spacecraft has to be able to supply the necessary resources for cooling the detector and maintaining a stable background environment. An alternative or complementary approach is to reduce the dark current as much as possible through engineering of the MCT layer such that the detector can be operated at a higher temperature while still achieving the required performance. In an effort to improve both the performance and the knowledge of detector capabilities, ESA has initiated a series of coordinated activities, the latest results from which are presented below.
4.3.1
LOW DARK CURRENT 2D MCT DETECTOR DEVELOPMENT
Two parallel activities targeting “Low dark current 2D MCT detector development” (11.5 um and 14.5 um cut-off) with Leonardo (UK) and AIM (Germany) have been completed recently. Leonardo investigated several design/process options aimed at reducing dark currents further than that exhibited by their standard process. At AIM, custom developments of both n-on-p and p-on-n planar MCT diode arrays using LPE have demonstrated a significant reduction in dark current below Rule 07 [15], Those results show also an improve compared to previous AIM LWIR/VLWIR technology (Figure 9). Up to date, the LWIR and VLWIR detectors of AIM are the state-of the art with respect to the lowest dark current achieved [16]. Also, some very promising results have been acquired during a continuation of the contract over the originally planned end date, when the diodes were tested in higher temperatures, opening the way to allow higher temperature operation within acceptable low levels of the dark current. Following on from this parallel development, ESA has now initiated further development activities of array technology with 12.5 um cut-off wavelength for operation down to 40 K or lower. Those are the activities mentioned in the section 4.2.2
Figure 9 Quantum efficiency (top) and dark current (bottom) measurements for LWIR diodes for AIM p on-n planar technology, showing flat QE response and dark current improvements
4.3.2
LOW DARK CURRENT VLWIR T2SL INFRA RED DETECTORS
ESA is also interested in alternative infrared detector technologies to MCT and recognizes that significant progress has been made using III-V materials. Since proposed in 1980s [17][19], the InAs/(In,Ga)Sb Type II super-lattice structure (T2SL) has gained a lot of interest for the infrared detection applications. For example, there is a potential lower cost of quantum well detector technologies such as Quantum Well Infrared Photodetector (QWIPs), Quantum Dot Infrared Photodetectors (QDIPs), and T2SL photodetectors compared to MCTs and InSb. However, the relative performance of these technologies has not been sufficiently high to allow them to be widely used except from very limited applications. Studies on performance parameter sensitivity analysis to determine the relative importance of various FPA performance parameters to the overall performance of a long range imaging system are very useful to provide comparisons of performance for various detector technologies from the perspective of end-to-end system performance [20]. In parallel to those studies, the relevant technology needs to be further improved. Performance of MWIR and LWIR T2SL detectors has not achieved its theoretically predicted limit and to fully realize the T2SL potential methods of suppression of various dark current components have to be developed [21].
Following a study funded by ESA and led by University of Cardiff in the UK, T2SL detectors have been identified as a potential alternative for future LWIR and VLWIR applications. As a result, ESA has initiated an activity with IRNova (Sweden), to investigate the capabilities of T2SL arrays in a direct comparison with the performance achieved using MCT technology (cut-off wavelengths 11.5 um and 14.5 um). The activity includes the design, manufacturing and testing of high performance, low dark current VLWIR detector array using a Commercial off-the-shelf (COTS) ROIC (FLIR Systems) of 320×256 array size, 30μm pixel pitch and 384×288 array size, 25μm pixel pitch. For the T2SL the options investigated are InAs/GaSb:p-on-n, InAs/GaSb:n-on-p and InAs/InAsSb:p-on-n. Currently, the results of the first run of material growth have been reviewed and the second run is on-going. Process development and detector fabrication/evaluation is in progress. In order to use the optimum of the various studied options and to reach the best performance, the need to use a different ROIC was identified. The activity is on-going with significant indications that the T2SL detectors can be a strong competitor to MCT with high QE, low dark current and superior uniformity to MCT. Further results as well as investigations of T2SL structures for the SWIR wavebands can be found in [22]-[25].
5
DISCUSSION
Infrared detectors are one of the key enabling technologies for a wide range of space missions in both the Earth Observation and Scientific domains, ranging from high-resolution multispectral imagers to “low cost” infrared systems for Earth Remote Sensing. In order to realize the desired status of technical capabilities and industrial strength, the developments covering the infrared detector technology must address the following key areas: Constant effort is needed on maintaining and expanding “mainstream” infrared (e.g. MCT / InGaAs) capability. e.g. pursue increase in format for IR detectors with impact on substrate size, small pixel pitch hybridisation and ROIC development (including stitching). Explore and develop promising (alternate / additional) technologies e.g. III-V compounds, Type-II super-lattice structures, uncooled thermal detectors, APDs, p-on-n MCT structures, materials and architectures targeting lower dark-current and higher operating temperatures. For ESA, an additional key objective is the European non-dependence and the further performance enhancement in European detector capabilities It should be noted here that the Space market is not possible to sustain the needed capability at foundry level. Therefore synergies with other sectors must be exploited e.g. development of Infrared detectors has been until relatively recently supported mainly by defence applications but is now being pursued in the commercial sector, encouraging research outside of the defence sector.
6
CONCLUSIONS
Infrared detectors play a key role in the design and implementation of space instruments, both for earth observation and for astronomy applications. The European Space Agency is aiming on providing to the scientific community and to the end users detectors of high performance, meeting the demanding needs of space missions. Additional to the optimization of the performance, their uninterrupted availability is of paramount importance. Maintaining and
expanding detector manufacturing capabilities and foundries in Europe is a key objective of the Agency. Using both technology-push and technology-pull ESA is maintaining its strategic approach to detector developments, through both internal and external consultation, resulting in the initiation of targeted detector development activities across the waveband.
7
REFERENCES
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Highlights
Space instrumentation requires continuous detector developments High-performance infrared detectors are key components for upcoming space missions On-going activities aiming to improve “mainstream” infrared technologies Additional activities targeting promising (alternate / additional) technologies