2.2.1

Visible – SWIR - MWIR range

Detection in the extended SWIR wavelengths is one of the first wishes of Sofradir. Indeed, since early 2000, Sofradir developed various detectors in order to answer customers’ needs for imagery or hyperspectral applications. The first applications were mainly addressed by matrix sensitive arrays: Neptune (500x256 30μm pitch), Saturn (1000x256 30μm pitch), Mars (320x256 30μm pitch) and Scorpio (640x512 15μm pitch).

As examples, a Neptune detector with an adapted cut-off wavelength was launched in December 2014 onboard HAYABUSA-2 probe of the Japanese Space Agency (JAXA) that aims to study an asteroid “1999 JU3” after a 3-years space journey.

More recently, three detectors have been launched in March 2016 in the scope of ESA EXOMARS TGO mission. Therefore, three new SOFRADIR detectors flew to Mars, bringing onboard two MARS SW-MW ([2.2;4.3μm]) detectors and one SCORPIO SW-MW ([2.6;4.2μm]) detector (see Figure 2-3). These detectors are respectively integrated in NOMAD (Nadir and Occultation for Mars Discovery) and ACS (Atmospheric Chemistry Suite) instruments.

Figure 2-2:

Sofradir skills areas

Figure 2-3:

MARS MW IDDCA integrated in NOMAD instrument

Another Neptune SW-MW detector was implemented into the MicrOmega IR microscope developed by IAS (Institut d’Astrophysique Spatiale at Orsay, France) with the support of CNES (Centre National d’Etudes Spatiales, the French space agency). This detector is expected to be launched in the next ESA EXOMARS mission, aiming to analyze Mars ground surface composition.

For METOP-SG mission, another program named 3MI instrument integrates a NEPTUNE FPA. This program has been started at SOFRADIR mid-2015 and the selected detector design is derived from a NEPTUNE FPA with an adjusted cut-off wavelength (2.3μm at 185K) coupled with a SATURN package in its passive cooling version (see Figure 2-4).

Figure 2-4:

SATURN SWIR – Passive cooling

Indian space agency (ISRO) has chosen Sofradir for the CHANDRAYAAN-2 mission. A derivative of NEPTUNE detector in its active cooling version (with a RICOR K508 cryocooler) will be delivered. This program started at the end of 2015 and consists in using a NEPTUNE detector ROIC coupled with a SWIR MWIR detection circuit. The packaging integrates a specific filter located between the FPA and the window in order to address the four requested bands. The first flight model delivery is expected beginning of 2019.

Apart from these “standard” sensitive arrays, SOFRADIR has developed other specific detectors for space applications in order to fit with customer needs in SWIR range: a linear detector (Sentinel-2), and two large size detectors (NGP and ALFA). Sentinel-2 and NGP detectors have been selected for first missions, as briefly presented hereafter, while ALFA detector is currently under development, and aims to address in a first time astronomy and science applications. This detector as well as its design and definition is more detailed in chapter 3 of this paper.

SOFRADIR was selected in 2008 by AIRBUS Defense & Space and ESA for the development of infrared detectors for the Sentinel-2 mission (part of the Copernicus program). For this mission, linear arrays detectors have been designed and screened to operate for the in-orbit lifetime of Sentinel-2 satellites (over 7 years). They include three SWIR linear arrays of 1298 pixels at 15 micron pitch, incorporated in two different MCT detection circuits which are hybridized on the same readout circuit (see Figure 2-5 for retina and associated package). A significant flight models production phase have been performed for this program leading to the delivery of 27 flight models in total for the completion of the first phase (satellites A/B) in early 2014.

Figure 2-5:

Sentinel-2 Focal Plane Array, Detector Package and image of French Riviera with Sentinel-2A satellite in IR waveband using SOFRADIR detectors (courtesy of ESA)

Then in 2015, the second phase (satellites C/D) has been started, with the objective to deliver 24 supplementary detectors. Delivery of these models is scheduled over 2017 and 2018.

Regarding NGP detector, it has been selected by Airbus DS and ESA for Sentinel-5 mission, also part of Copernicus program, which aims to do air-quality mapping. After a precursor phase (TROPOMI) launched in December 2017 with a Saturn SWIR detector onboard, the next phase will be done with NGP SWIR models (1024x1024 15μm pitch) proposed in a passive cooling version (see Figure 2-6). First flight models are aimed to be delivered end of 2018/beginning of 2019.

Figure 2-6:

NGP sensor in Sentinel-5 detector package

The same detector is also used for Microcarb mission, which aims to monitor Carbon quantity in the atmosphere. For this application, the NGP detector is currently being adapted to VISIR configuration in order to fit with the mission needs.

Table 1 gives an overview of the different SWIR detectors developed by SOFRADIR, from visible to MWIR wavelengths, and their status (some programs have been completed by Sofradir but have not been launched yet).

Table 1:

Main programs using SWIR detectors

2.2.2

VLWIR range

VLWIR MARS detector (see Figure 2-7) development for space has been started in 2014 with SAC ISRO in the frame of GISAT program. This detector is composed of a FPA of 320x256 with a 30μm pitch sensitive in the VLWIR band, with a cut-off wavelength of 14.9μm and an operating temperature at 50K. The spectral range is defined by a six bands strip filter from 7.1μm to 13.5μm. The retina and the filter are mounted in a sealed package closed by a germanium window, which is optimized for transmission in the waveband from 7μm to 13.5μm. For this program, six flights models are expected to be delivered in 2020.

Figure 2-7:

MARS detector

For the SGLI instrument (on board GCOM-C), SOFRADIR has been selected by NEC TOSHIBA Corporation and the Japanese space agency (JAXA) for the development and production of the long wave infrared detector sensitive up to 13 μm at an operating temperature of 55 K. This detector is sensitive in two definite wavebands, with a central wavelength of 10.8μm for the first one and 12μm for the other. The package and the retina are presented in Figure 2-8. The program has been completed successfully in 2013 with the delivery of all flight models, meeting both customer requirements and the schedule of the program. This detector is now successfully used in space on board the GCOM-C satellite since end of 2017.

Figure 2-8:

SGLI detector

Table 2:

Main programs using VLWIR detectors

2.2.3

Custom products

Sofradir was selected in 2011 by Thales Alenia Space and ESA (European Space Agency) to develop and produce the infrared detectors for the European future meteorological program MTG (Meteosat Third Generation). For this program, Sofradir is developing the detectors for two different systems:

For the FCI satellite, four types of detectors (see Figure 2-9), covering wavebands from 1.3 μm up to 14 μm shared in 11 spectral channels, are developed for use around 60K.

Figure 2-9

FCI NIR/IR detection assemblies

Each detector has the same overall definition. It is comprised of a retina (one ROIC on which one or two MCT arrays are hybridized), a sealed package with spectral filters and an entrance window associated to a space cryogenic flex cable with a space connector. The retinas of the different FCI detectors are linear arrays of pixels with pitches varying between 15 and 25 μm.

Five flight models of each type of detectors (meaning 20 flight models in total) have to be manufactured and delivered in the frame of this program between mid-2018 and end of 2019.

Regarding the IRS satellite, two types of detectors (see Figure 2-10) covering infrared waveband from 3.3 μm up to 14.7 μm are developed for use around 55K.

Figure 2-10:

IRS detection assembly

Each detector has the same overall definition. It is comprised of a large size retina with a format 160x160 and 90 μm pitch, a non-sealed package and a space cryogenic flex cable with a space connector. Three flight models of each type of detectors (meaning 6 flight models in total) have to be manufactured and delivered in the frame of this program between mid-2018 and end of 2019.

More recently, in 2015, Sofradir has been selected by the German Space Agency to develop and to manufacture the METimage infrared detector ot be integrated in the instrument by Airbus Defense and Space Germany in the frame of METOP-SG mission.

For this development, several detectors are expected; all with the same packaging design (see Figure 2-11): one covering both SWIR and MWIR wavebands, with cut-off wavelength is around 5.5μm, one covering LWIR waveband, with cut-off wavelength is around 12.5μm, and one covering VLWIR waveband, with cut-off wavelength is around 14μm. All these detectors have to operate with a FPA temperature of 60K. SMWIR FPA will be composed by an equivalent of 180x153 sub-pixels array with 30μm pitch, whereas LVWIR FPA will be composed by two detection circuits “LWIR” and “VLWIR” respectively equivalent to 108x153 sub-pixels array and 36x153 sub-pixels array with 30μm pitch.

Figure 2-11:

METimage detector

Four flight models of each type of detectors (meaning 8 flight models in total) have to be manufactured and delivered in the frame of this program between mid-2019 and mid-2020.

Table 3:

Main programs with custom detectors

3.2.1

ROIC architecture

The ALFA ROIC is composed by a matrix of 2048 by 2048 pixels with 15μm pitch. Therefore its overall size is close to 30x30 mm². In order to achieve such dimension, the stitching technique has been implemented in foundry. The validation of this technique is very important for future ROIC developments, as from now there is no more limitation on ROIC size.

ALFA ROIC architecture is given in Figure 3-1. In green these are the analog parts, while in blue these are the digital ones. The data are provided through 32 outputs, with an additional reference output which is linked to a pixel not connected to photodiode: this pixel is maintained under the reference bias level.

Figure 3-1:

ALFA ROIC architecture

3.2.2

ROIC operating modes

ALFA ROIC has four operating modes. The default one, called Science mode, allows the user to get images of the full matrix with its best performances (low noise, low power consumption, weak glow generation) with a readout frequency up to 100kHz. The data are available with 1, or 4 or 32 outputs. In order to operate the detector faster, a Fast mode is available, with a readout frequency up to 6MHz, giving a full frame reading time of 24ms with 32 outputs. The Window mode allows defining and reading up to 3 windows with adjustable sizes, as well as the Tracking mode which is an interlaced readout of the full field and the windows. Main characteristics of each mode are given in Table 5.

Table 5:

Operating modes main characteristics

Thanks to SFD input stage, the ROIC readout mode is a non-destructive readout, and there are several types of reset, all available in both full frame modes and windowing modes. Indeed, the reset can be done on each pixel individually, or line by line, or also on the full frame/window.

Figure 3-2:

ROIC reset types

3.2.3

ROIC expected performances

The table below summarizes the performances expected for ALFA ROIC:

Table 6:

ALFA ROIC performances

As one can see, the simulation results show that ALFA ROIC fits with ESA requirements, hence it is well adapted to science applications.