Infrared detection is widely used in astrophysics and plays a key role in several space missions aiming for example at scanning the sky to discover new objects (coolest stars, dust-obscured galaxies, exo-planets …) or studying the evolution of the universe, where light is redshifted in the infrared range. In many cases the space telescope involves an HgCdTe infrared detector operating at low frame rate over long integration time. Due to the very low input signal, dark current and readout noise are essential figures that must be minimized to get the best detector sensitivity. This kind of application also requires very large focal plane array (FPA) often relying on a butting arrangement of large detectors. The trend is to increase the single detector format from 1Kx1K to 2Kx2K and 4Kx4K. For very large formats, material quality and detector process may affect the production yield and the global infrared FPA cost. As a result the detector format could result from a trade-off taking into account producibility.
The Laboratoire Electronique et Traitement de l’Information (LETI) of the Commissariat à l’Energie Atomique (CEA, Grenoble, France) has been involved in the development of infrared detectors based on HgCdTe (MCT) material for over 30 years, mainly for defence and security programs [1]. Once the building blocks are developed at LETI (MCT material process, diode technology, hybridization, …), the industrialization is performed at SOFRADIR (also in Grenoble, France) which also has its own R&D program [2].
In past years, LETI also developed infrared detectors for space astrophysics in the mid infrared range – the long wave detector of the ISOCAM camera onboard ISO – as well as in the far infrared range – the bolometer arrays of the Herschel/PACS photometer unit –, both instruments which were under the responsibility of the Astrophysics department of CEA (IRFU/SAp, Saclay, France).
Nowadays, the infrared detectors used in space and ground based astronomical instruments all come from vendors in the US. For programmatic reasons – increase the number of available vendors, decrease the cost, mitigate possible export regulations, …– as well as political ones – spend european money in Europe –, the European Space Agency (ESA) defined two roadmaps (one in the NIR-SWIR range, one in the MWIR-LWIR range) that will eventually allow for the procurement of infrared detectors for space astrophysics within Europe.
The French Space Agency (CNES) also started the same sort of roadmaps, as part of its contribution to the different space missions which involve delivery of instruments by French laboratories. It is important to note that some of the developments foreseen in these roadmaps also apply to Earth Observations.
One of the main goal of the ESA and CNES roadmaps is to reduce the level of dark current in MCT devices at all wavelengths. The objective is to use the detectors at the highest temperature where the noise induced by the dark current stays compatible with the photon noise, as the detector operating temperature has a very strong impact at system level. A consequence of reaching low levels of dark current is the need for very low noise readout circuits.
CEA and SOFRADIR are involved in a number of activities that have already started in this framework. CEA/LETI does the development of the photo-voltaic (PV) layers – MCT material growth, diode technologies–, as well as some electro-optical characterisation at wafer, diode and hybrid component levels, and CEA/IRFU/SAp does all the electro-optical characterisation involving very low flux measurements (mostly dark current measurements). Depending of the program, SOFRADIR can also participate in the development of the hybrid components, for instance the very low noise readout circuits (ROIC) can be developed either at SOFRADIR or at CEA/LETI.
Depending of the component specifications, the MCT epitaxy can be either liquid phase (LPE, which is the standard at SOFRADIR for production purposes) or molecular beam (MBE), the diode technology can be n/p (standard at LETI and SOFRADIR) or p/n (under development for several years now) [3], and the input stage of the ROIC can be Source Follower per Detector (SFD for very low flux low noise programs) or Capacitive Trans Impedance Amplifier (CTIA for intermediate flux programs) [4].
This paper will present the different developments and results obtained so far in the two NIR-SWIR and MWIR-LWIR spectral ranges, as well as the perspectives for the near future. CEA/LETI is also involved in the development of MCT Avalanche Photo Diodes (APD) that will be discussed in other papers [5,6].
CEA and SOFRADIR have been manufacturing and characterizing near infrared detectors in the frame of ESA's near infrared large format sensor array roadmap to develop a 2Kx2K large format low flux low noise device for space applications such as astrophysics. These detectors use HgCdTe as the absorbing material and p/n diode technology. The technological developments (photovoltaic technology, readout circuit, ...) are shared between CEA/LETI and SOFRADIR, both in Grenoble, while most of the performances are evaluated at CEA/IRFU in Saclay where a dedicated test facility has been developed, in particular to measure very low dark currents. The paper will present the current status of these developments at the end of ESA's NIRLFSA phase 2. The performances of the latest batch of devices meet or are very close to all the requirements (quantum efficiency, dark current, cross talk, readout noise, ...) even though a glow induced by the ROIC prevents the accurate measurement of the dark current. The current devices are fairly small, 640x512 15μm pixels, and the next phase of activity will target the development of a full size 2Kx2K detector. From the design and development, to the manufacturing and finally the testing, that type of detector requests a high level of mastering. An appropriate manufacturing and process chain compatible with such a size is needed at industrial level and results obtained with CEA technology coupled with Sofradir industrial experience and work on large dimension detector allow French actors to be confident to address this type of future missions.
Ultra High Speed (UHS) imaging is at the forefront of the imaging technology for some years now. These image sensors
are used to shoot high speed phenomenon that require about hundred images at Mega frame-per-seconds such as
detonics, plasma forming, laser ablation… At such speed the data read-out is a bottleneck and CMOS and CCD image
sensors store a limited number of frames (burst) on-chip before a slow read-out. Moreover in recent years 3D integration
has made significant progresses in term of interconnection density. It appears as a key technology for the future of UHS
imaging as it allows a highly parallel integration, shorter interconnects and an increase of the fill factor. In the past we
proposed an idea of 3D integrated burst image sensor with on-chip A/D conversion that overcome the state of the art in
term of frame-per-burst. This sensor is made of 3 stacked layers respectively performing the signal conditioning, the A/D
conversion and the burst storage. We present here different solutions to implement the analogue front-end of the first
layer. We will describe three circuits for three purposes (high frame rate, power efficiency and sensitivity). To support
our point, we provide simulation results. All these front-ends perform global shutter acquisition.
F. Simoens, J. Meilhan, S. Gidon, G. Lasfargues, J. Lalanne Dera, J. L. Ouvrier-Buffet, S. Pocas, W. Rabaud, F. Guellec, B. Dupont, S. Martin, A. C. Simon
CEA-Leti has developed a monolithic large focal plane array bolometric technology optimized for 2D real-time imaging
in the terahertz range. Each pixel consists in a silicon microbolometer coupled to specific antennas and a resonant
quarter-wavelength cavity. First prototypes of imaging arrays have been designed and manufactured for optimized
sensing in the 1-3.5THz range where THz quantum cascade lasers are delivering high optical power. NEP in the order of
1 pW/sqrt(Hz) has been assessed at 2.5 THz.
This paper reports the steps of this development, starting from the pixel level, to an array associated monolithically to its
CMOS ROIC and finally a stand-alone camera. For each step, modeling, technological prototyping and experimental
characterizations are presented.
In this paper, we present the design of the MWIR channels of EChO. Two channels cover the 5-11 micron spectral
range. The choice of the boundaries of each channel is a trade-off driven by the science goals (spectral features of key
molecules) and several parameters such as the common optics design, the dichroic plates design, the optical materials
characteristics, the detector cut-off wavelength. We also will emphasize the role of the detectors choice that drives the
thermal and mechanical designs and the cooling strategy.
Through collaboration between III-V Lab and CEA-Leti, a 640 x 512 InGaAs image sensor with 15 μm pixel pitch has
been developed. Based on a thinned substrate, the photodiode array detects the light from the visible to the near infrared
wavelength (0.4 to 1.7 μm) with a dark current lower than 18 fA per pixel at room temperature. The readout IC (ROIC)
design in a standard CMOS 0.18 μm technology is presented. The pixel circuit is based on a capacitive transimpedance
amplifier (CTIA) stage with two selectable charge-to-voltage conversion gains. The input stage has been optimized for
low noise performance in the high gain mode. In this mode, the charge-to-voltage conversion factor is 17.6 μV/electron
and the full well capacity is above 105 x 103 electrons. The integration time can be set up to the frame period thanks to a
rolling shutter approach. The frame rate can be up to 120 fps or 60 fps if the Correlated Double Sampling (CDS)
capability of the circuit is enabled. The readout noise measured in CDS with short exposure time is around 30 electrons
for a dynamic range of 71 dB in high-gain mode and 108 electrons and 79 dB in low-gain mode.
KEYWORDS: Avalanche photodetectors, Mercury cadmium telluride, Readout integrated circuits, Mid-IR, Sensors, Short wave infrared radiation, Single photon, Electrons, Photodetectors, Signal to noise ratio
Proportional photon detection has been demonstrated using linear mode HgCdTe avalanche photodiodes (APDs)
hybridized on a specially designed read-out integrated circuit (ROIC). The ROIC was designed to detect photons at a
moderate bandwidth (10 MHz) with a low noise of 10 electrons per characteristics time of the ROIC and to be
compatible with large area-small pixel focal plane array (FPA) applications. Proportional photon counting was
demonstrated by reproducing the Poisson statics for average photon number states ranging between m=0.8 to 8 photons,
at low to moderate avalanche gains M=40-200, using both mid-wave infrared (MWIR) and (short-wave infrared) SWIR
HgCdTe APDs. The probability distribution function of the gain was estimated from the analysis of the amplitude of
detected residual thermal photons in the MWIR APDs. The corresponding probability distribution functions was
characterized by a low excess noise factor F and high asymmetry which favours a high photon detection efficiency
(PDE), even at high threshold values. An internal PDE of 90 % was estimated at a threshold level of 40 % of the average
signal for a single photon. The dark count rate (DCR) was limited by residual thermal photons in the MWIR APD to
about 1 MHz. A geometrical and spectral filtering of this contribution is important to achieve the ultimate performance
with MWIR detectors. In this case, the DCR was estimated by interpolation to about 8 kHz. The SWIR HgCdTe APD
device had a lower residual photon flux (60 kHz), but was found to be limited by tunnelling dark current noise at high
gains at a rate of 100 kHz.
Three-dimensional (3-D) flash light detection and ranging (LADAR) imaging is based on time of flight (TOF) measurement of a single laser pulse. The laser pulse coming back from the observed object will be detected only if the number of photons received by each pixel generates a signal greater than the pixel noise. In order to extract this weak photonic signal from the noise we use the high gain and low excess noise of the HgCdTe avalanche photodiode (APD) arrays developed at CEA/LETI. The sensor consists of a 30-μm pitch APD detector array hybridized to a 320×256 pixels ROIC for passive and active imaging. In passive mode the focal plane array behaves like a thermal imager and we measured 30 mK of noise-equivalent temperature difference. In active imaging mode, each pixel sensed the time of flight and the intensity two-dimensional (2-D) of a single laser pulse. Laboratory tests show a range noise of 11 cm for 4300 photoelectrons per pixel and detection limit under 100 photoelectrons. The sensor was also used during a field trial to record 2-D and 3-D real-time videos. The quality of the images obtained demonstrates the maturity of HgCdTe-APD-array technology.
Hybrid InGaAs focal plane arrays are very interesting for night vision because they can benefit from the nightglow
emission in the Short Wave Infrared band. Through a collaboration between III-V Lab and CEA-Léti, a 640x512 InGaAs
image sensor with 15μm pixel pitch has been developed.
The good crystalline quality of the InGaAs detectors opens the door to low dark current (around 20nA/cm2 at room
temperature and -0.1V bias) as required for low light level imaging. In addition, the InP substrate can be removed to
extend the detection range towards the visible spectrum.
A custom readout IC (ROIC) has been designed in a standard CMOS 0.18μm technology. The pixel circuit is based on a
capacitive transimpedance amplifier (CTIA) with two selectable charge-to-voltage conversion gains. Relying on a
thorough noise analysis, this input stage has been optimized to deliver low-noise performance in high-gain mode with a
reasonable concession on dynamic range. The exposure time can be maximized up to the frame period thanks to a rolling
shutter approach. The frame rate can be up to 120fps or 60fps if the Correlated Double Sampling (CDS) capability of the
circuit is enabled.
The first results show that the CDS is effective at removing the very low frequency noise present on the reference
voltage in our test setup. In this way, the measured total dark noise is around 90 electrons in high-gain mode for 8.3ms
exposure time. It is mainly dominated by the dark shot noise for a detector temperature settling around 30°C when not
cooled. The readout noise measured with shorter exposure time is around 30 electrons for a dynamic range of 71dB in
high-gain mode and 108 electrons for 79dB in low-gain mode.
Spectral signatures of solid materials in the THz range can provide spectroscopic information for chemical identification.
Previously we have demonstrated the absorption coefficient extraction by scanned imaging of QCL THz beams
attenuated through explosive samples. The detection was achieved by a unique pixel addressed within an uncooled
antenna-coupled microbolometer 160x120 array specifically designed for the 1-5 THz range. This detector technology
developed at CEA-LETI relies on amorphous silicon bolometer know-how and aims at opening the way to real-time
video rate, with potential low cost.
We report complementary tests of imaging in reflection configuration and the first tests of a second prototype where
320x240 bolometers are monolithically processed above a CMOS read-out circuit.
CEA-Leti has developed a dual mode infrared array detector for passive (thermal) or active 2D and 3D imaging. Very
high sensitivity in 3D mode of operation is achieved by using an HgCdTe avalanche photodiode array with linear gain.
The 30 μm pitch detector array is hybridized to a 320x256 pixels Readout Integrated Circuit (ROIC). In passive mode,
the 3.6x106 e- well capacity and the low noise of the ROIC allow to reach photon noise limited NETD. For active
imaging mode, each pixel measures the time of arrival (3D) and the intensity (2D) of one laser pulse. A sensor based on
a detector array with a cut off wavelength of 4.6μm at 80K was fabricated and tested. This paper describes the pixel
architecture and presents ranging performances obtained in laboratory conditions. The first 2D and 3D active videos
obtained during a field trial of our focal plane array are presented.
We report our latest development of HgCdTe electron avalanche photodiode (e-APD) with Cd compositions between 0.3
to 0.41; exponentially increasing gain, synonym of exclusive electron multiplication, was observed in all the devices up
to M>600, associated with low noise factors F=1.2; a record high gain of M=7000 was measured in e-APDs with λc=4.6μm at 80K, which shows on the stability of the junction; the equivalent input dark current decreases with increasing band-gap and a record low value of 2 aA, was obtained in a λc=2.9 μm e-APD at M=24 and spectral response measurements have shown that the gain and quantum efficiency is conserved down to the UV. Dedicated ROICs have
been designed for passive and active laser assisted imaging. A passive imaging ROIC for low flux application have been
designed for a full frame readout speed of 1.5 kfps and an equivalent input noise lower than 2 electrons. Two active
imaging ROICs have already been validated with e-APD arrays. Dual mode passive and active 2D (range gated) e-APDs
FPAs have been made using with cut-off wavelengths ranging from 2.9μm to 5.3μm at T=80 K. On the best devices, the
operability in gain and noise exceeds 99.6% and relative gain dispersion lower than 10 %, independently of the
wavelength at gains M=10-100. First characterizations of multi-mode 3D/2D FPA have shown on a range resolution
below 15 cm (1 ns).
KEYWORDS: Staring arrays, 3D image processing, Pulsed laser operation, LIDAR, Avalanche photodetectors, Ranging, 3D metrology, Readout integrated circuits, Mercury cadmium telluride, Signal to noise ratio
CEA-Leti has developed a 320x256 FPA for 3D flash LADAR active imaging. The readout IC (ROIC) performs time-of-flight
(TOF) measurement in addition to 2D intensity imaging with a single emitted laser pulse. The FPA consist of a
ROIC hybridized to a 30 μm pitch HgCdTe avalanche photodiode (APD) array. The illuminator used for testing this FPA
is a 1.57 μm laser producing 8 ns pulses with a maximum energy of 8 mJ per pulse. This paper describes the readout IC
pixel architecture and presents ranging performances obtained in laboratory conditions. The first 2D and 3D active
images obtained during the first field trial of our prototype LADAR system are presented.
KEYWORDS: Signal to noise ratio, Long wavelength infrared, Staring arrays, Prototyping, Readout integrated circuits, Electrons, High dynamic range imaging, Photodiodes, Mercury cadmium telluride, Sensors
CEA-Leti MINATEC has been involved in infrared focal plane array (IRFPA) development since many years, with performing HgCdTe in-house process from SWIR to LWIR and more recently in focusing its work on new ROIC architectures. The trend is to integrate advanced functions into the CMOS design for the purpose of applications
demanding a breakthrough in Noise Equivalent Temperature Difference (NETD) performances (reaching the mK in LWIR band) or a high dynamic range (HDR) with high-gain APDs. In this paper, we present a mid-TV format focal plane array (FPA) operating in LWIR with 25μm pixel pitch, including a new readout IC (ROIC) architecture based on
pixel-level charge packets counting. The ROIC has been designed in a standard 0.18μm 6-metal CMOS process, LWIR n/p HgCdTe detectors were fabricated with CEA-Leti in-house process. The FPA operates at 50Hz frame rate in a snapshot integrate-while-read (IWR) mode, allowing a large integration time. While classical pixel architectures are limited by the charge well capacity, this architecture exhibits a large well capacity (near 3Ge-) and the 15-bit pixel level ADC preserves an excellent signal-to-noise ratio (SNR) at full well. These characteristics are essential for LWIR FPAs
as broad intra-scene dynamic range imaging requires high sensitivity. The main design challenges for this digital pixel
array (SNR, power consumption and layout density) are discussed. The electro-optical results demonstrating a peak NETD value of 2mK and images taken with the FPA are presented. They validate both the pixel-level ADC concept and its circuit implementation. A previously unreleased SNR of 90dB is achieved.
CEA-Leti has developed a new 320x256 hybrid focal plane array (FPA) for flash LADAR imaging. The detector array
consists of 30μm pixel pitch MWIR HgCdTe avalanche photodiodes operating at 80K and the readout integrated circuit
(ROIC) is fabricated on a standard 0.18μm CMOS process. The custom ROIC can operate as a passive thermal imager or
a flash LADAR imager. In this second mode, each pixel will provide the time of flight measurement (3D) and the
returned intensity (2D) of one laser pulse. For the first laboratory trials the e-APD photodiode array performances were
measured in passive mode and the same FPA was then tested in one shot LADAR mode. This paper describes the
readout IC pixel architecture and reports the first electro-optical test results in both passive and active modes. This new
prototype takes advantage of the latest developments of the partnership between Sofradir and CEA-Leti.
CEA Leti has recently developed a new readout IC (ROIC) with pixel-level ADC for cooled infrared focal plane arrays
(FPAs). It operates at 50Hz frame rate in a snapshot Integrate-While-Read (IWR) mode. It targets applications that
provide a large amount of integrated charge thanks to a long integration time. The pixel-level analog-to-digital
conversion is based on charge packets counting. This technique offers a large well capacity that paves the way for a
breakthrough in NETD performances. The 15 bits ADC resolution preserves the excellent detector SNR at full well
(3Ge-). These characteristics are essential for LWIR FPAs as broad intra-scene dynamic range imaging requires high
sensitivity. The ROIC, featuring a 320x256 array with 25μm pixel pitch, has been designed in a standard 0.18μm CMOS
technology. The main design challenges for this digital pixel array (SNR, power consumption and layout density) are
discussed. The IC has been hybridized to a LWIR detector fabricated using our in-house HgCdTe process. The first
electro-optical test results of the detector dewar assembly are presented. They validate both the pixel-level ADC concept
and its circuit implementation. Finally, the benefit of this LWIR FPA in terms of NETD performance is demonstrated.
We report the latest developments of MW HgCdTe electron initiated avalanche photo-diodes (e-APDs) focal plane
arrays (FPAs) at CEA-LETI. The MW e-APD FPAs are developed in view of ultra-sensitive high dynamic range
passive starring arrays, active 2D/3D and dual-mode passive-active imaging, which is why both the passive imaging
performance and the gain characteristics of the APDs are of interest. A passive mode responsivity operability of 99.9%
was measured in LPE and MBE e-APDs FPAs associated with an average NETD=12mK, demonstrating that dual mode
passive-active imaging can be achieved with LETI e-APDs without degradation in the passive imaging performance. The
gain and sensitivity performances were measured in test arrays and using a low voltage technology (3.3V) CTIA test
pixel designed for 3D active imaging. The CTIA and test arrays measurements yielded comparable results in terms of
bias gain dependence (M=100 at Vb=-7V), low excess noise factor (=1.2) and low equivalent input current
(Ieq_in<1pA). These results validated the low voltage CTIA approach for integrating the current from a HgCdTe e-APD
under high bias. The test array measurements demonstrated a relative dispersion below 2% in both MBE and LPE e-
APDs for gains higher than M>100, associated with an operability of 99%. The operability at Ieq_in<1pA at M=30 was 95%. A record low value of Ieq_in=1fA was estimated in the MBE e-APDs at M=100, indicating the potential for using the MW e-APDs for very low flux applications. The high potential of the MW e-APDS for active imaging was
demonstrated by impulse response measurements which yielded a typical rise time lower than 100ps and diffusion
limited fall time of 900ps to 5ns, depending on the pixel pitch. This potential was confirmed by the demonstration of a
2ns time of flight (TOF) resolution in the CTIA e-APD 3D pixel. The combined photon and dark current induced
equivalent back ground noise at f/8 with a cold band pass filter at λ=1.55μm was 2 electrons rms for an integration time
of 50ns.
CEA Leti has demonstrated the good performances of its MWIR HgCdTe avalanche photodiode arrays. Gains above 20
at a moderate bias voltage of 5V have typically been measured with an excess noise factor of only 1.2. The next
generation of infrared focal plane arrays will take advantage of these characteristics to address new applications, reduce
system complexity and enhance performances. One of the main opportunities offered by avalanche photodiode detectors
concerns long range active imaging. This paper reports the development of two novel pixel architectures for 3D active
imaging based on flash LADAR technology. Both pixels have been designed in a standard 0.35μm CMOS process and
perform time-of-flight measurement in addition to 2D intensity imaging with a single emitted laser pulse. The analog
input circuits have been optimized to allow fast pulse detection while providing robustness to process variability. A
small readout IC demonstrator has been fabricated and coupled to a 10x10 avalanche photodiode array at 40μm pixel
pitch. The first test results in lab conditions show good
electro-optical performances with a ranging resolution around 30cm (2ns).
KEYWORDS: Modulators, Digital filtering, Analog electronics, Readout integrated circuits, Amplifiers, Staring arrays, Signal to noise ratio, Indium nitride, Digital electronics, Sensors
Designing a digital IR focal plane array (IRFPA) requires fulfilling very stringent requirements in terms of power consumption, silicon area and speed. Among the various ADC architectures like successive approximation, ramp or over-sampled converters, the best choice strongly depends on the application. We believe that sigma-delta converters, in spite of their quite high power consumption, are a promising solution for high-performance and medium size FPA, e.g. 320x240.
This paper presents the design of a second-order incremental sigma-delta ADC dedicated to cooled (77K) IRFPA applications. System-level simulations used to define the modulator parameters and specify its analog building blocks are presented. Circuit design of the switched-capacitor modulator and the digital decimation filter is described. The column ADC including the filter has been implemented in a standard 0.35μm CMOS process on the basis of a 25μm pitch and lead to a total length of 3200μm.
Test chips including a single ADC have been manufactured end of 2006. The first measurement results, at 77K, are presented along with perspectives and future developments. They demonstrate the following performance: 81dB Signal-to-Noise Ratio (SNR), 13 bits Effective Number Of Bits (ENOB) and 270μW power consumption at 17kSamples/s rate.
CEA/LETI has been working for several years on the development of HgCdTe-based infrared dual band detectors [3]. Since 2001 CEA/LETI is also involved in a large program for the demonstration of dual band QWIP FPAs presenting large format and small pitch. This study is carried out with the QWIP team of THALES Research and Technology (TRT) in charge of QWIP design, MBE growth and GaAs processing for the detector side. As part of this program TRT investigated different quantum structures and pixel architectures for the realization of two-band FPAs for MWIR/LWIR and LWIR/LWIR applications. At the end of this study a choice of the most appropriate architecture was done. On its side, CEA/LETI designed readout circuits optimized for the selected dual-band QWIP. TRT delivered QWIP arrays and CEA/LETI proceeded to the assembly, integration and electro-optical characterization. The aim of this paper is to describe the architecture of these dual-band demonstrators and to present the first results concerning their electro-optical performances measured at 70K and 65K.
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