Frédéric Pistone

HgCdTe APD-focal plane array performance at DEFIR

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.

HgCdTe APD-focal plane array development at DEFIR

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).

HgCdTe-based APD focal plane array for 2D and 3D active imaging: first results on a 320 x 256 with 30 µm pitch demonstrator

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.

Ultra-low power ADC on chip for high-performance IR detector

The InfraRed staring arrays developed by SOFRADIR are more and more compact and offer system solutions for wide range of IR wavebands. IR detectors have been taken to an even more advanced level of sophistication to achieve staring arrays high performances. Latest developments have also been focused on the silicon readout circuit. Digital conversion on chip is one of the recent progresses in this field of activity. In order to match each system requirements, on chip high performance ultra low power ADCs have been developed. Beyond the performance aspects, digital focal plane arrays can be considered as the first step towards low cost Dewar family, since they allow for a more simple electrical interface on Dewar designs and on chip image processing. Recent results concerning these new readout circuit architectures are presented in this paper.

New high-gain detectors for active imaging

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.

Latest developments in MCT for next generation of infrared staring arrays

This paper presents an overview of the very recent developments of the MCT infrared detector technology developed by CEA-LETI and Sofradir in France for next generation of applications. New applications require high sensitivity and dual band detectors. The Avalanche PhotoDiodes (APD) technology opens new interesting fields of investigation for low flux applications and fast detectors for laser imaging. 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, in order to gate the backscattered signal around the target. In partnership with Sofradir, CEA/LETI (France) has developed high quality MCT 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 presented in this paper is the first step in a road-map to address low flux infrared sensors in the next few years. In parallel, the development of dual-band infrared detectors has been the core of intense research and technological improvements for the last ten years. New TV (640 x 512 pixels) format detectors of 24μm pixel pitch is available. It is proposed with MWIR/MWIR or MWIR/LWIR dual band sensitivity integrated in dedicated tactical Dewars. At present, focused on pixel pitch reduction, Sofradir is carrying out optimization of the materials quality, photodiode design as well as flip-chip bonding process. 20 μm pixels have demonstrated dual color key performances (quantum efficiency, optical fill factor, and pixel operability) in accordance with mono-spectral structures. Results are presented in this paper.

Experimental characterization of an infrared focal plane array for flash laser radar imaging

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.

A comparative study of on chip decorrelation schemes for low power, high resolution infrared sensors

This paper discusses in-sensor decorrelation schemes for infrared applications. These applications require high image quality and low power consumption. Different scenarios involving analog and digital Haar discrete wavelet transform implementations are compared to classical quantization. High level modeling of image sensor shows that a simple decorrelation scheme can reduce column level power consumption while keeping high image quality.

Progress in MCT large staring array

The InfraRed staring arrays are more and more compact and offer system solutions in the different IR wavebands. The HgCdTe (Mercury Cadmium Telluride / MCT) material and process, as well as the hybridization technology, have been taken to an even more advanced level of sophistication to achieve these new staring arrays high performances. Latest developments allow progress at different stages of products offered by SOFRADIR. Uniformity of Focal Plan Arrays (FPA) is improved, read-out circuits propose new functions as the analogic to digital conversion, and the reliability of the whole dewar detector and cooler assembly is increased. New products take advantages on these progresses. In mid-wave (MWIR), 1280x1024 MCT detector available in a tactical dewar is presented.

High resolution staring arrays answering compact MW and LW applications

This paper overviews the electro-optical and thermal performances of different types of infrared detectors manufactured by Sofradir. The detector’s fabrication processes and detector’s performance are shortly described. New staring arrays are more compact and offer system solutions required by infrared market. Special attention is directed to some reliability advantages of new dewar design. Finally, the development trends for highest resolution infrared detector are discussed.