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Kinetic Inductance Detectors – a New Imaging Technology for Observations In and From Space

Final Report Summary - SPACEKIDS (Kinetic Inductance Detectors – a New Imaging Technology for Observations In and From Space)

Executive Summary:
Widely used detector technologies for far-infrared (FIR) to millimetre wavelengths, such as semiconductor and transition-edge superconducting bolometers, have significant fabrication and operational difficulties, and can lead to a high degree of system complexity for the large arrays demanded by the next generation of astronomical and Earth observing missions such as SPICA, CoRE+ and Millimetron and FIRI. The Kinetic Inductance Detector (KID) is a relatively new superconducting device which can provide photon noise limited sensitivity over the entire FIR-mm range. The SPACEKIDS project has developed and evaluated large arrays of KIDs, and has demonstrated their suitability for future space-based observations at far-infrared to millimetre wavelengths.

Satellite-based observations in this wavelength range are very important to astronomers who want to investigate the origins of stars, galaxies, and indeed the Universe. And observations of the Earth from space in this wavelength range will provide climate scientists with crucial data with which to constrain their predictions of climate change. Additionally, meteorologists will benefit from improved global maps of atmospheric temperature, humidity and precipitation.

KIDs are simple both to fabricate and to integrate, and can be read out with a high multiplexing ratio, dramatically reducing the complexity of cryogenic interconnections, cabling and electronics. The simplification at the cryogenic level is very attractive and large arrays can enable a dramatic increase in mapping speed for broad-band imaging, as well as new applications in spectropolarimetry and hyperspectral imaging. SPACEKIDS drew together some of the leading European institutes with expertise in FIR detector technology. The main aims were: (i) design of KID arrays with improved multiplexing ratio and uniformity; (ii) optimisation of performance in the presence of cosmic rays; (iii) optimisation of sensitivity and optical coupling; (iv) demonstration of wide-band electronics coupled to large format KID arrays.

A variety of detector materials and geometries have been explored to optimise the KID detectors for specific applications. Tests have shown that aluminium films can achieve the ultimate required sensitivity over the entire far-infrared to millimetre wavelength range. Tests on other materials continue.

A detailed study of the coupling of electromagnetic radiation to different implementations of KID detectors (antenna-coupled and lumped-element) has been carried out, and the issue of cross-coupling in an array has been addressed.

Operation of detectors in the harsh environment of space is a challenge. KID performance in the presence of a cosmic ray flux, representative of a space environment, has been optimised through design, modelling, fabrication and testing.

The development of readout electronics for large arrays of detectors has formed a large part of this work. A suitable readout system has been designed and demonstrated and works to specification.

A number of mission outlines for astronomy and Earth observation have been identified for which KIDs can be a competitive detector choice, and the detector requirements for each mission idea have been derived. Future mission concepts have been proposed that will optimally exploit the unique advantages afforded by large arrays of kinetic inductance detectors.

Large-format arrays of detectors have been built and demonstrated. These arrays have been tested in dedicated systems that proved the performance required for such future satellite missions. Two such arrays were developed and tested – one for a low photon background (astronomy) case, and another for a high photon background (Earth observation) case.

The ultimate achievement of the project has been to prove the performance of large arrays of KIDs, and to demonstrate their suitability and advantages to future satellite missions.

Project Context and Objectives:
The purpose of this project was to bring together the leading European groups to develop KID arrays with specifications tailored towards the most relevant future space missions. These developments include:
1. Reaching photon noise limited sensitivity and at least 80% optical coupling over the full bandwidth of the instrument
2. Reducing the susceptibility to cosmic rays to such a level that the observing speed is reduced at most by 20% due to cosmic ray hits in space compared to a negligible cosmic ray hit rate
3. Improving the number of pixels per unit readout bandwidth by reducing microwave cross talk to a level that at least 1000 KIDs can be read-out in a 1 GHz bandwidth with a cross talk level below 5%

The ultimate objective of the development programme was to demonstrate the technology advancements listed above in the production and testing of two large (kilo-pixel class) focal plane arrays. One of these was optimised for ultra-low photon background (astrophysics) applications with high Q factor, and one was optimised for high photon background (Earth observation) applications (low Q).
On top of this, we developed a bespoke 2 GHz bandwidth readout electronics system, capable of reading out low Q and high Q KIDs.

Project Results:
At the conclusion of its three year programme, we are pleased to report that the SPACEKIDS project has achieved all of its major objectives, with some significant enhancements, and with the schedule of deliverables largely followed as planned.

We have conducted a review of future potential mission concepts for astrophysics (low photon background) and Earth observation (high photon background) applications. From these concepts, we extracted the relevant specifications for the large demonstration arrays, and prioritised the tests to be carried out. We then assessed some of these mission concepts in more detail, producing more detailed design concepts that would be enabled or enhanced by the unique capabilities of KIDs.

For astrophysics applications, we detailed two mission concepts; a far-infrared grating spectrometer coupled to a large-aperture cooled telescope, and a CMB polarimetry mission. Both of these concepts are well aligned with proposals in development for the ESA M5 mission call.

For Earth observation applications, we have examined a mission concept that will have a significant impact in meteorology and climatology applications. The instrument will produce temperature and humidity profiles with much better accuracy and resolution compared to the instruments that are currently under development for MetOp-SG. And the retrieval of ice cloud parameters will be far superior to the capabilities of the ICI instrument, also under development for MetOp-SG. This will have a large impact on constraining essential climate variables for global climate models. The concept has been analysed and presented in significant detail, and a development roadmap has been produced.

Through modelling and experimental verification, we have demonstrated that KID arrays can be produced with negligible cross-coupling between pixels. We identified an issue with TiN KIDs, in that their sensitivity is currently lower than model predictions. But it was found that aluminium LEKID (Lumped Element KID) detectors can achieve the photon-noise limit. Sensitivities needed for low-background instruments were measured with antenna-coupled KIDs, with photon-limited performance down to background powers of 100 AW.

Cosmic ray susceptibility has been minimised by the addition of a layer of Ti to rear side of the detector chips to reduce phonon propagation in the array substrate. With this design feature, only nearest-neighbour pixels see ionising particle hits.

Very significant progress has been made in the design of LEKID arrays, with high-fidelity models produced. The SPACEKIDS consortium can now manufacture 200-nm wide Al LEKID absorbers from 15 nm thick Al films, which is a world-leading capability.

For antenna-coupled KIDS, a novel dual-band leaky-wave lens-coupled antenna has been developed, and a fabrication and integration scheme for silicon lens array developed. We have shown the successful fabrication and operation of a 1.4 – 2.8 THz single polarisation leaky-wave antenna-coupled MKID. The beam pattern and frequency response match very well the model calculations and the device is background limited down to 0.05 fW incident radiation power. The limiting sensitivity corresponds to an NEP of 2∙10-19 W/√Hz.
Additionally we have made test chips of twin slot antenna-coupled devices at 350 and 850 GHz. Both of these devices have frequency response and beam pattern in excellent agreement with the simulations. The limiting sensitivity was only measured for the 850 GHz device, given by a NEP = 4∙10-19 W/√Hz. These two device geometries were used for the main demonstrators of the project in kilo-pixel arrays.

We have designed and tested a kpixel LEKID front-illuminated array for WP5. We also successfully fabricated a LEKID operating at 1-2 THz using 200 nm wide lines of 15 nm thick aluminium. We have shown that the device works and responds to THz illumination. But more detailed tests and new devices are needed for further testing. This work will continue in CAB and SRON after the end of the project using our own funding sources.

The development of readout electronics for large KID arrays was essentially completed during years 1 and 2. Manufacturing of the final readout electronics hardware was completed at the beginning of year 3. We have verified that the readout system achieves full compliance with the requirements. This readout development was significantly expanded with respect to the originally proposed work, in that two systems were developed. A 2-4 GHz system was provided for the high-background LEKID detector option, and a 4-8 GHz system for the antenna-coupled KIDs.

High- and low-background large format arrays have been produced, and fully characterised in dedicated test facilities. Initial testing of the arrays indicated a stray-light issue, which was resolved by the addition of a stray-light absorbing layer. This led to a three-month delay in the final test programme, which was accommodated by no-cost extension to the project, granted by the REA. All tests on the large format arrays are complete, and we are pleased to report that all priority-1 tests were successfully carried out both demonstrator systems, meeting the requirements in all respects, and the goals in most cases. It is important to note that there was no need to de-scope the project due to issues faced as part of this workpackage and that in our continuing post-SPACEKIDS work we will, by the end of 2016, fully test two alternative designs not chosen at the detector down-select meeting These arrays are currently in hand and will undergo the same test programme as the baseline designs adding to the overall output of the SPACEKIDS project.

The second SPACEKIDS international workshop was held at the European Space Technology Centre (ESTEC), Netherlands, on 10th March 2016. This workshop had an attendance of 58 people from a diverse array of sectors, including representation from the European Space Agency, UK Met Office, UK Centre for Earth Observation Instrumentation, European academia and research institutes, and European industry. The focus of this workshop was the presentation of the key achievements of the SPACEKIDS project, and the presentation of the instrument and mission concepts to the potential end-user community.

To date, 13 refereed and seven conference papers been published (or accepted for publication) based on SPACEKIDs work. The refereed papers include journals such as Nature and Physical Review B. A full list may be found in the WP7 report. The SPACEKIDS project has been mentioned in many other talks and presentations, including invited talks in China, Russia and the US. A full list is being compiled.

A separately-funded study has been completed successfully for the UK Space Agency and the Centre for Earth Observation & Space Technology (CEOI-ST). This was an instrument and mission concept study for a new KID-based instrument for climatology and meteorology applications, work that was directly enabled by the preliminary work carried out in the context of the SPACEKIDS Earth-observation concept study. The work was funded for a period of 12 months, and involved researchers from Cardiff University, UK Met Office and the University of Hamburg. The outputs from this work also fed in to the SPACEKIDS Earth observation instrument concept, and thus significantly enhanced this aspect of the project.

Also as a result of SPACEKIDS developments, Cardiff University was awarded £40,000 by the UK Science and Technologies Facilities Council (STFC) as an impact acceleration grant for advancing potential commercial applications for KID technologies. This study involved investigating TiN films for potentially higher temperature operation, such that systems such as a THz camera (under development with SPACEKIDS partner QMCI) could be implemented without the need for a 3He cooler.
Potential Impact:
The technical progress made by SPACEKIDS will enable new scientific research in astronomy and Earth observation through the maturation of a novel detector technology suitable for future space-borne observations with advanced imaging and spectroscopic instruments. The technology addresses regions of the electromagnetic spectrum in which high performance space-ready detector technology currently does not exist, and significantly reduces system integration complexity and cost. SPACEKIDS developments have ensured a European strategic lead and reduce the dependence on key technologies and capabilities. SPACEKIDS technology will satisfy the requirements of a wide range of future space-borne astronomical and Earth-observing missions with array format sensors suitable for the wavelength range of approximately 30 to 3,000 μm. SPACEKIDS will have direct impact on the capability of future space missions.

Scientific areas in astronomy that may benefit from SPACEKIDS include direct imaging and spectroscopy of exoplanets, FIR spectroscopic and Interferometric study of galaxy evolution, star formation and planetary system development, cosmic microwave background imaging and polarimetry. Potential Earth-observing applications include ice cloud remote sensing, high resolution fast imaging humidity and precipitation monitors, and atmospheric chemistry monitoring (ozone, bromine, chlorine, etc.).
The knowledge and experience gained with KID technology will also enable further innovative developments in detector technology, for example extending the technology to cover shorter wavelengths (e.g. MIR-optical-X-ray) and generating new ideas for future space science and earth observation missions. The SPACEKIDS team has actively publicised the potential of this technology to the scientific community, promoting opportunities for new research alliances and strengthening existing collaborations with international space partners.

By having developed detector arrays for both low and high background illumination conditions we anticipate significant applications not only in space-borne projects but also in ground-based scientific and industrial applications. The wider scientific community anticipates broad research-community and commercial exploitation of this region of the electromagnetic spectrum but currently regards the absence of advanced sensor technologies as one of the fundamental issues holding back this exploitation. The knowledge and expertise created by this programme in design, fabrication and testing of sensor arrays in Europe will therefore allow rapid development of new instruments and capabilities for new applications as they arise.
The industrial partners in this consortium have identified the commercial potential of the technology in areas as diverse as security, non-destructive testing, process control and analysis and healthcare.
As well as the economic benefits that tend to arise from the development of new technology in the form of new devices, techniques and applications, SPACEKIDS has trained early-career researchers through giving them an opportunity to work on advanced technologies. With the team’s strong record in public outreach, it has also contributed to the inspiration of young people to follow technical and scientific careers, thus contributing to the maintenance of a highly skilled European workforce. Most of the great advances in astronomy in recent decades have revolutionised our understanding of the nature and history of the universe. This has been driven by technical innovations comparable to those that SPACEKIDS was designed to bring about. The general public has a great interest and fascination with these developments, showing that new technology and new science are of great cultural significance. In addition, the new Earth observation capabilities that SPACEKIDS has opened up will make a contribution to improving our understanding of the atmosphere and climate of our planet. This is of paramount importance to the future of mankind.

List of Websites:

Prof. Matt Griffin - Coordinator -
Dr. Pete Hargrave - Technical manager -