Recent advances in miniaturization of IR imaging technology have led to a growing market for mini thermal-imaging
sensors. In that respect, Sofradir development on smaller pixel pitch has made much more compact products available to
the users. When this competitive advantage is mixed with smaller coolers, made possible by HOT technology, we
achieved valuable reductions in the size, weight and power of the overall package. At the same time, we are moving
towards a global offer based on digital interfaces that provides our customers simplifications at the IR system design
process while freeing up more space. This paper discusses recent developments on hot and small pixel pitch technologies
as well as efforts made on compact packaging solution developed by SOFRADIR in collaboration with CEA-LETI.
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).
New applications require high sensitivity infrared (IR) sensors in order to detect very low incident fluxes. Laser
gated imaging has, in particular, additional specific needs. IR sensors for this type of application are synchronized
with eye-safe lasers, and have to detect a weak signal backscattered from the target on the order of 10 photons per
pulse. They also have to be able to operate with a very short integration time, typically one hundred nanoseconds,
to gate the backscattered signal around the target. In partnership with Sofradir, CEA/LETI (France) has developed
high quality HgCdTe avalanche photodiodes satisfying these requirements. In parallel, specific studies have been
carried out at the Read-Out Circuit level to develop optimized architectures. Thanks to these advances, a new
Integrated Dewar Detector Cooler Assembly has been developed. This new product is the first step in a road-map
to address low flux infrared sensors in the next few years.
The optimization of HgCdTe avalanche-photo-diodes (e-APD) and focal plane arrays (FPA) are reported. The gain
performances was measured in planar homo-junction APDs with Cd compositions between xCd=0.3 to 0.41.
Exponentially increasing gain, synonym of exclusive electron multiplication, was observed in all the devices up to
M>100. The high gain at high xCd opens the path to thermo-electric cooled active imaging at T=200K. This perspective is
corroborated by the demonstration of high gains and low excess noise factor F=1.2 in extrinsically doped MW-APDs,
which enables reduced dark-current at high temperatures. The equivalent input dark current (Ieq_in) decreased from 200
fA to 1 fA, when the cut-off wavelength was reduced from λ
c=5.2 μm to 4.1 μm at M=100 and T=80 K. This shows that
sensitivity can be optimized by increasing xCd, at the cost of increased reverse bias. A new horizontal-gain-well (HGW)
hetero-structure was processed to optimize the sensitivity at high gain and low bias. The first HGW-APDs had gains
comparables with MW e-APDs and 50 times lower dark-current at T=200 K. They did also display surprisingly high
quantum efficiency in the MWIR range, ηpeak=30%, which enables thermal imaging at high operating temperature. The
high performance of MW-APDs was confirmed by the characterizations of a first 320x256 30μm pitch APD-FPA,
yielding a 99.8 % operability, low gain dispersion (<10%) and low noise equivalent photons (NEPh=3 at tint=1 μs) for
gains up to M=70. The maturity of the DEFIR HgCdTe e-APD-FPA technology was highlighted by the first
demonstration of passive amplified imaging.
The CMOS silicon focal plan array technologies hybridized with infrared detectors materials allow to cover a wide
range of applications in the field of space, airborne and
grounded-based imaging. Regarding other industries which are
also using embedded systems, the requirements of such sensor assembly can be seen as very similar; high reliability, low
weight, low power, radiation hardness for space applications and cost reduction. Comparing to CCDs technology,
excepted the fact that CMOS fabrication uses standard commercial semiconductor foundry, the interest of this
technology used in cooled IR sensors is its capability to operate in a wide range of temperature from 300K to cryogenic
with a high density of integration and keeping at the same time good performances in term of frequency, noise and power
The CMOS technology roadmap predict aggressive scaling down of device size, transistor threshold voltage, oxide and
metal thicknesses to meet the growing demands for higher levels of integration and performance.
At the same time infrared detectors manufacturing process is developing IR materials with a tunable cut-off wavelength
capable to cover bandwidths from visible to 20μm. The requirements of third generation IR detectors are driving to
scaling down the pixel pitch, to develop IR materials with high uniformity on larger formats, to develop Avalanche
Photo Diodes (APD) and dual band technologies.
These needs in IR detectors technologies developments associated to CMOS technology, used as a readout
element, are offering new capabilities and new opportunities for cooled infrared FPAs. The exponential increase of new
functionalities on chip, like the active 2D and 3D imaging, the on chip analog to digital conversion, the signal processing
on chip, the bicolor, the dual band and DTI (Double Time Integration) mode ...is aiming to enlarge the field of
application for cooled IR FPAs challenging by the way the design activity.