Passive Radiometric Imaging SysteM – MILLIPRISM

We have developed a new type of detection diodes called Heterostructure Low-Barrier Diodes (HLBD). These are constituted by a nin structure presenting a triangular barrier made by molecular beam epitaxy. Various samples with barrier heights of 0.11 to 0.16 eV were produced. The barrier height is indeed a critical parameter on the sensitivity and impedance level of the diodes. Since the contacts on the n zones are of the ohmic type, a study has been carried out in order to optimize the latter, in order to reduce the parasitic series resistance of the diode as much as possible in order to obtain the highest cutoff frequencies. In parallel to this study the masks were designed by the partner MC2-Technologies. Different topologies have been implemented: diodes with coplanar access for probe measurements in dipole and quadripole configuration, diodes for «flip-chip« bonding. These different structures have been declined in four forms: without impedance matching and with impedance matching for frequencies: 90, 150 and 180 GHz.

The responsivity of the realized zero-bias HLBD is on average 2kV/W in the 80-220 GHz band without impedance matching and up to 5-6 kV/W at 90 and 175 GHz with impedance matching. The NEP (noise equivalent power) is in the range of 0.6 to 0.7 pW/Hz1/2 at 90 and 175 GHz respectively. This good performance is also obtained for relatively low video résistances of the order of 400 to 800 ohms, which facilitates the impedance matching both on the RF and on the detection side. The results are in the state of the art worldwide. A publication has been submitted concerning these results.

The HLBDs developed in the MILLIPRISM project are currently being tested at MC2-Technologies in configurations very close to the end-use in a millileter-wave camera? Encouraging results have been achieved. At the and of these tests, the Outlook will be defined.

S. Nadar et al., Lampin, High performance heterostructure low barrier diodes for sub-THz detection, submitted to IEEE Transactions on Terahertz Science and Technology

S. Nadar et al., Sub-THz zero-bias detector with high performances based on heterosctructure low barrier diodes (HLBD), Proceedings of 41st International Conference on Infrared, Millimeter and Terahertz Waves, IRMMW-THz 2016, Copenhagen, Denmark, September 25-30, 2016.

J.-F. Lampin et al., Millimeter wave passive radiometric imaging system (MILLIPRISM), WISG Janvier 2014.

J.-F. Lampin et al., Millimeter wave passive radiometric imaging system (MILLIPRISM), WISG Janvier 2015.

M. Thirault et al., High sensitivity zero bias thermoinic diodes with high cut-off frequency for millimeter wave and THz applications, SENSO 2014.

S. Nadar et al., Thermoinic diode based on AlGaInAs heterostructure for millimeter wave detection, 8th THz days April 2015.

S. Nadar et al., Heterostructure low barrier diode with high performances for sub-THz detection, 1ère reunion du GDR NanoTeraMIR (20-23 septembre 2015).

S. Nadar et al., Détecteur sub-THz zéro-bias à base de diode thermoïonique à hétérostructure de faible barrière, 15èmes Journées Nano, Micro et Optoélectroniques, JNMO 2016, Les Issambres, France, 30 mai-1 juin 2016.

The goal of the joint-research PRISM consortium, involving an academic research laboratory and a company, is to develop a small, lightweight and low-cost passive millimeter wave camera. This kind of cameras is a solution for the detection of hidden objects under cloth, fabrics, paper, cardboard, plastics, wood, plaster and bricks. This new technology will be essential for protecting the citizens and the infrastructures. A well-known example concerns airports. For this kind of applications the passive aspect of the proposed camera is an advantage compared to more standard approaches in test in airports: it will be more easily acceptable by the population (no radiations are emitted by the camera). It must be noted that such passive camera responds to one recommendation of the French ANSES (Agence nationale de sécurité sanitaire de l’alimentation, de l’environnement et du travail) in a report establish on the request of the French DGAC in July 2012.
The current technology is based on low noise amplification before detection at 90 GHz. These amplifiers have several drawbacks and the camera would become smaller, lighter and less expensive if a direct detection scheme could be used (no millimetre wave amplification). This is the goal of the PRISM project and it will require extremely sensitive detectors.

Two types of detector diodes will be studied:

- The first ones are zero-bias thermoionic diodes. They are based on a graded III-V semiconductor heterojunction. The potential profile can be easily varied thanks to a variation of thickness and composition by Molecular Beam Epitaxy. These compositions will be optimized in order to maximize the sensitivity and minimize the noise floor of the diode at room temperature and also at cryogenic temperature. Working at low temperature is an efficient way to improve the sensitivity of the diode and it is easily possible thanks to the fact that detectors dissipate almost no DC power. For all that, original cryocoolers could be used.

- The second studied diode will be based on tunnelling effect (backward diodes). These diodes based on InAs nanowires small bandgap pn junctions will be processed and analyses following the technological modifications. The optimization will be focused to maximize the sensitivity and at the same time minimize the input capacitance for a small video resistance.

In any case, the impedance matching of the detector is critical and must be as large as possible in bandwidth in order to increase the sensitivity of the detector. It will be studied for the two types of diodes. Increasing the frequency above 90 GHz will studied because it allows to increase the bandwidth and also to reduce the size of the optics of the camera.
The low noise low frequency amplifier that follows the detector is also a critical part of the system. It will be specially studied and optimized. One solution to reduce the effect of the noise of this amplifier is to add a modulation of the signal and a lock-in detection. A new modulation scheme is proposed in order to reduce the noise to a value close to the intrinsic noise of the detector.
The PRISM academic/industrial consortium has a strong expertise in high-frequency semiconductor devices design and processing, circuit design and system integration. The consortium is completed within its steering committee, by an engineer of the Service Technique de l'aviation civile (STAC), the French DGAC service in charge of the certification of security equipment for aviation security, which will provide inputs on regulator requirements as well as users needs in the field of aviation security which is one leading market for security products. This project should generate the detectors that will be used in the next generation of passive millimetre wave camera. Thanks to their smaller size, weight and cost they will be more generally used, not only in airports but also in all critical areas and in temporary events.