Infrared Focal Plane Array Detectors: «Post Pandemic» Development Trends. Part I
The review deals with infrared detectors of thermal imaging technology. The devices are in demand in systems and complexes of civil and medical thermography, security and fire surveillance, personal night vision and security systems. A comparison of photonic and thermal detectors of various types by different world manufacturers is presented. An expert forecast of changes in market growth dynamics and trends of its post-pandemic development is given.
N. A. Kulchitsky 1, 2, A. V. Naumov 3, V. V. Startsev 3
Moscow Technological University (Moscow Institute of Radio, Electronics and Automatics, MIREA), Moscow, Russia
State Scientific Center of the Russian Federation, NPO ORION JSC, Moscow, Russia
Astron Design Bureau JSC, Lytkarino, Moscow Region, Russia
The review deals with infrared detectors of thermal imaging technology. The devices are in demand in systems and complexes of civil and medical thermography, security and fire surveillance, personal night vision and security systems. A comparison of photonic and thermal detectors of various types by different world manufacturers is presented. An expert forecast of changes in market growth dynamics and trends of its post pandemic development is given.
Keywords: thermal imagers, bolometers, photon detectors
Received on: 08.04.2020
Accepted on: 20.04.2020
Since its inception, the market for infrared (IR) thermal imaging equipment has grown, primarily, due to its military applications. Today, the military sector still provides some growth for the market, but its development paradigm has changed. Now the main growth in the market is provided by the sectors of civil and medical thermography, security and fire surveillance, personal night vision systems and local security niches (municipal, private, etc.). The devices using thermal imagers make it possible to detect in conditions of poor visibility, to detect people with high temperature in the crowd (Fig. 1) [1, 2].
According to the “pre-pandemic” forecast of Maxtech International (USA) and today’s estimates of the authors, the market for infrared systems (civil and military) amounted to 10.5 billion dollars in 2017, and could reach 20 billion dollars by 2025. Due to the pandemic, we have restated the Maxtech International’s forecasts upwardly for the medium term. It seems to us that the recession in this sector will be short-term, even if it happens. (Fig. 2).
of Infrared Detectors
Thermal imaging devices can be classified into two groups: more and less sensitive. Photonic detectors (cooled and uncooled) are used in the devices that are more sensitive; thermal (uncooled) detectors (microbolometers) are used in the devices that are less sensitive. In this article, we consider only some properties and characteristics of the currently widely used detectors, as well as possible prospects for the development of the market for their use in connection with the post-pandemic situation.
The infrared cameras recreate the image of a warm object using signals from primary thermal radiation detectors. Infrared energy from scene objects is focused by the optical system onto an IR detector. It transmits information to an electronic image processing system, which then transmits it to a standard video screen. In any detector, the absorbed electromagnetic radiation leads to the appearance or change of an electrical signal [1, 2]. This absorbed radiation excites or heats the electronic or lattice subsystems of the detectors, which leads to changes in their physical properties or changes in the electron energy distribution. As a result, the movement of charged carriers changes. Such changes entail a change in the physical parameters of the detectors, which fixes the detector.
In photon detectors (which are mainly semiconductor detectors), the radiation is absorbed directly by the material sensitive to a given radiation wavelength. The mechanism of the interaction of radiation with the electrons of the detector materials is schematically shown in Fig. 3. Conventionally, we divide electrons into those associated with the atoms of the crystal lattice (located in the valence band, the “intrinsic” detectors), those associated with impurity atoms (the “extrinsic” or impurity detectors) and free carriers (free-carriers detectors) located inside the valence or conduction bands, and also those in the metal located near the metal-semiconductor interface. The principle of operation of photoemissive Schottky detectors (SBD – Schottky Barrier Diode detectors) is based on a change in the mobility of the latter. That is, photon detectors react only to photons whose energy exceeds certain threshold values, e. g., the semiconductor band gap (“intrinsic” detectors), the transition energy in quantum wells (QWs), quantum dots (QDs), and superlattices (SL), barrier height qνb in detectors based on Schottky diodes. The number of charge carriers generated in a photon detector due to the absorption of incident radiation can be measured directly in the form of a voltage or current signal. The response of quadratic photon detectors is proportional to the number of absorbed photons (absorbed intensity is proportional to the square of the amplitude of the electric field).
The principle of operation of thermal detectors is based on a change in the electrical characteristics of the detector due to the energy of absorbed thermal radiation. The incident radiation heats the material, which leads to electronic mobility and, accordingly, to a change in resistance. In this case, there is no direct interaction of photons with the electrons of the material [1, 2].
Almost until the end of the 20th century, the development of IR technologies was determined by the dominant contribution of photon detectors. A significant disadvantage of IR photon detectors is the need for cryogenic cooling. This is due to the requirement to prevent thermal generation of charge carriers. It is a source of noise that limits the parameters of radiation detectors. The second revolution in infrared vision began in the last decades of the 20th century. Over the period from the late 1970s – early 1990s, several companies began to produce uncooled heat detectors operating on various principles of detecting heat fluxes (IR radiation). Such a technological breakthrough made it possible to create large-format arrays of infrared radiation detectors.
Compared with photon detectors, thermal detectors in the second half of the twentieth century were used to a lesser extent. The reason was that they worked relatively slowly (response time τ > 5 ∙ 10–2 s), and their sensitivity was lower. However, the transition to the release of sensitive pixels of smaller sizes has significantly reduced response time. Today, the thermal time constant τ can be about ~20 ms or less. Since the late 1970s, a significant shift has been noted in the market due to progress in increasing the number of elements in linear and array detectors. This made their release much more cost-effective, primarily due to the use of silicon read-out integrated circuits (ROICs). Integration of such circuits with different types of detectors allowed creating array photodetectors (APDs). There are up to 108 IR detectors in the APD IR‑arrays, which corresponds to the number of sensitive receptors in the human eye (~ 2 ∙ 108).
Since the 2000s, microbolometers have already dominated the market for detectors for uncooled and relatively inexpensive APDs. To date, the number of produced arrays of thermal detectors is several times larger than the IR‑arrays of all other types taken together. This situation will continue in the future (Fig. 4). The cost of multifunctional devices based on uncooled bolometers in industrial production is two orders of magnitude lower than the cost of photon arrays [3, 4]. You can compare the degree of technological readiness of APDs for implementation in industry on a conventional scale of various types of photodetectors using table 1 .
PHOTON COOLED DETECTORS
A typical design of a cooled photon detector is shown in Fig. 5. A hybrid photodetector assembly including an array of photosensitive elements coupled to a silicon integrated read-out circuit is mounted in a vacuum housing. APD cooling is provided by a microcryogenic cooling system (MCS) integrated with the APD housing and operating according to the Stirling cycle.
The basis of APD are the semiconductor photosensitive materials. Their composition varies depending on the required range of spectral sensitivity. The cadmium-mercury-tellurium triple semiconductor compound (CdHgTe) is used for the spectral ranges of 1–2.5 microns; 3–5 microns; 8–14 microns. The double semiconductor compound indium antimonide (InSb) is for the spectral range of 3–5 microns. Indium gallium arsenide ternary semiconductor compound (InGaAs) is used for the spectral range of 0.4–2.3 microns, the structures with quantum wells (QWIP) are used for the spectral ranges of 3–5 microns; 8–14 microns.
For highly sensitive and long-range thermal imaging devices, the APDs made of cadmium-mercury-tellurium (CdHgTe) and indium antimonide (InSb) semiconductor compounds are used. Currently, approximately equal amounts of both CdHgTe-based and InSb-based are produced (Fig. 7). We expect this ratio to continue in the medium term.
The CdHgTe remains the main material of the APDs of the long-wavelength spectral range. CdHgTe dominates for military applications where high sensitivity and speed are required. Today, high results have been achieved in obtaining high-quality epitaxial CdHgTe structures on various semiconductor substrates. The highest-quality structures are grown on CdZnTe substrates matched with the CdHgTe by the lattice constant. Sophisticated and multi-stage technology for CdHgTe producing involves deep purification of the initial Cd, Hg and Te, the synthesis of HgTe and CdTe compounds, and the obtaining of CdHgTe. Currently, the main industrial method for the manufacture of epitaxial layers in leading world companies producing multi-element and array photodiodes is the method of liquid-phase epitaxy on a cadmium-zinc-tellurium (CdZnTe) substrate. The advantages of this method are the relatively low cost and high productivity of the equipment, automatic surface cleaning at the initial stage of growth, additional purification from impurities during growth, and uniform composition over the area. However, large-area substrates made of CdZnTe remain expensive products with poorly reproducible characteristics.
The high cost of CdZnTe makes it possible to develop epitaxy on alternative substrates, from gallium arsenide, silicon, germanium, gallium antimonide, and some others. In this regard, such technologies are being developed everywhere. The large difference in the parameters of the crystal lattices, the chemical and structural inconsistencies of the CdHgTe on Si, GaAs, and Ge make the task of producing APDs based on the CdHgTe / Si, GaAs, Ge structures with suitable parameters extremely difficult .
Currently, the main trend is the decrease in overall dimensions and power consumption of photoelectronic modules. The APD format of 640 × 512 elements with a pitch of 15 microns is currently the main format and, apparently, in terms of price-quality ratio for the next 5–10 years, it will remain so. The leading developers of APDs as a commercially available megapixel format reached 1280 × 1024 elements. Currently, the prices for such arrays are quite high, which does not allow hardware developers to make a massive transition to it. However, by 2025, such a transition will occur. A large number of companies work abroad in the direction of the development and production of photoelectronics products. Among them AIM Infrared Modules (Germany), BAE Systems (USA), Brandywine photonics LLC (USA), CalSensors Inc. (USA), EGIDE USA (USA), China Germanium Co. Ltd. (China), FLIR Systems (USA), SCD (Israel), Raytheon Vision Systems (USA), RICOR (Israel), Selex ES (United Kingdom), Thales Cryogenics (France), Lynred (France), Spectrolab Inc. (USA) and others.
Reducing the pitch and increasing the format is a universal trend for practically all the milestones of the global developers and manufacturers of infrared multifunction devices. Leonardo (Great Britain) has already reached a pitch of 8 microns for a megapixel array of the medium-wave infrared range. Moreover, Lynred (France) this year will reach a pitch of 5–7 microns for such arrays (Fig. 8). Reducing the pitch and increasing the format leads to a significant increase in the range of recognition of objects [5–7].
The second part of the review will deal with cooled APDs for the spectral range of 3–5 microns, 8–12 microns, uncooled APDs and the development of their market.
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Kulchitsky Nikolai Alexandrovich, Doctor of Technical Sci., e-mail: email@example.com, Professor, Moscow Technological University (MIREA), Chief Specialist, SSC RF, JSC Orion Scientific-Production Association, Moscow, Russia.
ORCID ID: 0000-0003-4664-4891
Naumov Arkady Valerievich, engineer-analyst, ASTROHN Technology Ltd,
https://astrohn.ru, Lytkarino, Moscow region, Russia.
Startsev Vadim Valerievich, Cand. of Technical Sciences, ASTROHN Technology Ltd, https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID ID: 0000-0002-2800-544X