Infrared technology application

At present, infrared technology, as a high-tech technology, is in keeping with the laser technology and plays a pivotal role in the military. Infrared imaging, infrared reconnaissance, infrared tracking, infrared guidance, infrared warning, infrared countermeasures, etc. are all important tactics and strategic means in modern and future wars. After the 1970s, military infrared technology was gradually transformed into the civilian sector. Infrared heating and drying technology is widely used in various industries and sectors such as industry, agriculture, medicine, and transportation. Infrared temperature measurement, infrared moisture measurement, infrared physiotherapy, infrared detection, infrared alarm, infrared remote sensing, and infrared anti-counterfeiting are advanced technologies that various industries are competing for. Infrared thermal imaging technology, which marks the latest achievements of infrared technology, together with radar and television constitutes the three contemporary sensing systems, especially the adoption of focal plane array technology, which will develop it into a gaze system comparable to the eye.

In 1672, Newton used a prism to split the sunlight (white light) into red, orange, yellow, green, cyan, blue, purple and other monochromatic lights, confirming that sunlight (white light) is composed of light of various colors. to make. In 1800, when British physicist FW Heller studied various shades of light from a hot point of view, he accidentally found a thermometer placed outside the red light of the light, which was higher than the other colored light. After repeated trials, this so-called high-temperature zone with the most heat is always outside the red light at the edge of the light strip. So he announced that in addition to visible light, the sun emits a "hot line" that is invisible to the human eye. This invisible "hot line" is located outside the red light and is called infrared. Such infrared rays, also called infrared radiation, refer to electromagnetic waves having a wavelength of 0.78 to 1000 μm. The part with a wavelength of 0.78 to 1.5 μm is called near-infrared, the part with a wavelength of 1.5 to 10 μm is called mid-infrared, and the part with a wavelength of 10 to 1000 μm is called far-infrared. The portion having a wavelength of 2.0 to 1000 μm is also called thermal infrared rays.

Infrared is one of the many invisible rays in the sun's rays. It is one of the most extensive electromagnetic radiation in nature. Its position in the continuous spectrum of electromagnetic waves is the area between radio waves and visible light. This kind of infrared radiation is based on the random movement of any object and its atoms in the normal environment, and it constantly radiates thermal infrared energy. The more intense the movement of molecules and atoms, the greater the energy of radiation; conversely, the smaller the energy of radiation.

Infrared characteristics

Infrared has a thermal effect: the dipole and free charge in the living body have a tendency to be aligned in the direction of the electromagnetic field under the action of the electromagnetic field. In this process, the molecules are excited, and the irregular movement of the atoms is intensified to generate heat. When the infrared radiation has sufficient strength, that is, it exceeds the heat dissipation capability of the living body, the local temperature of the irradiated body is raised, which is the thermal effect of infrared rays.

Theoretical analysis and experimental research have shown that not only infrared light is present in the sun, but also objects with high temperature and absolute zero (such as the human body) are constantly radiating infrared rays. It is ice and snow, because their temperature is also high and absolute zero, so they are constantly radiating infrared rays. Therefore, the most important feature of infrared is that it is ubiquitous in nature. That is to say, any "hot" object can radiate infrared rays although it does not emit light. Therefore, infrared rays are also referred to as heat radiation lines for short.

Another feature of infrared and visible light is that it is rich in color. Since the longest wavelength of visible light is 1 times the shortest wavelength (780 nm to 380 nm), it is also called an octave. The longest wavelength of infrared light is 10 times the shortest wavelength, that is, it has 10 octaves. Therefore, if the visible light can be expressed in seven colors, the infrared light may represent 70 colors, showing a rich color.

Infrared is an electromagnetic radiation that has similar characteristics to visible light, obeys the laws of reflection and refraction, and also has phenomena of interference, diffraction, and polarization. At the same time, it has particle properties, that is, it can be emitted and absorbed in the form of photons. In addition, infrared has some unique features that are different from visible light:

(1) Infrared is not sensitive to human eyes, so it must be received by an infrared detector sensitive to infrared rays;

(2) The quantum light energy of infrared light is smaller than that of visible light, for example, the energy of infrared photons with a wavelength of 10 μm is about 1/20 of that of visible light photon energy;

(3) The thermal effect of infrared light is much stronger than visible light;

(4) Infrared rays are more easily absorbed by substances, but for mist, long-wave infrared rays are easier to pass.

Development of infrared technology

19th century: Studying the infrared radiation of astronomical stars and applying infrared spectroscopy for material analysis. The 20th century: Infrared technology was first noticed by the military because it provided the possibility to observe and detect the radiation of military targets and conduct confidential communications in the dark.

Some experimental infrared devices, such as signal scintillators, search devices, etc., were developed during the First World War. On the eve of the Second World War, Germany: Infrared CRT; During the war: Germany, the United States: infrared radiation sources, narrow-band filters, infrared detectors, infrared telescopes, bolometers, etc.

After the Second World War: the former Soviet Union. After the 1950s, the US: the seeker on the Sidewinder missile and the infrared camera on the U-2 spy plane represented the level of military infrared technology at the time. The forward-looking infrared device (FLIR) has gained the attention of the military and is widely used: the airborne forward-looking infrared device can detect people, small vehicles and hidden targets over 1500m, and can distinguish cars at a altitude of 20,000 m, especially Detect submarines at depths of 40m underwater.

In the Gulf War, the role and power of infrared technology, especially thermal imaging technology, was fully demonstrated.

Infrared detection technology

2.1 infrared detector

2.1.1 Progress in physics and infrared detectors

Infrared radiation interacts with matter (material) to produce various effects. For more than 100 years, modern physics from the classic physics to the 20th century, especially the creation of disciplines such as quantum mechanics and semiconductor physics, to modern mesoscopic physics, low-dimensional structural physics, etc., are many and more Physical phenomena and effects of infrared detection.

2.1.1.1 Heat detector:

Thermal radiation causes a change in material temperature to produce a measurable output. There are a variety of thermal effects available for infrared detectors.

(1) a liquid mercury thermometer with a thermal expansion and contraction effect, a gaseous Golay cell;

(2) The Seebeck effect. It can be made into thermocouples and thermopiles, mainly for measuring instruments.

(3) Resonance frequency sensitivity to temperature can produce a quartz resonator non-cooling infrared imaging array.

(4) Thermistor effect of the electrical resistance or dielectric constant of the material - the temperature rise caused by the radiation changes the resistance of the material to detect the thermal radiation - the bolometer: the semiconductor has a high temperature coefficient and is the most widely used, often called " Thermistor". Superconducting detectors utilizing large changes in resistance near the transition temperature have attracted attention. If room temperature superconductivity becomes a reality, it will be the most striking detector of the 21st century.

(5) Pyroelectric effect: rapid temperature change changes the spontaneous polarization of the crystal, and the surface charge changes, which can be used as a pyroelectric detector. The heat detector generally does not need to be cooled (except for superconducting) and is easy to use and maintain, and has good reliability; the spectral response is independent of wavelength, and is a non-selective detector; the preparation process is relatively simple and the cost is low. However, the sensitivity is low and the response speed is slow. The main factor in the performance limitations of heat detectors is the design of thermal insulation.

2.1.1.2 Photodetector:

Infrared radiation photons excite unbalanced carriers (electrons or holes) in the semiconductor material, causing changes in electrical properties. Since the carriers do not escape from the body, the internal photoelectric effect is called. The quantum photoelectric effect is highly sensitive and the response speed is much faster than that of the heat detector. It is a selective detector. In order to achieve the best performance, it is generally required to work at low temperatures. Photodetectors can be divided into:

(1) Light guide type: also known as photoresistor. The incident photons excite the valence band electrons in the uniform semiconductor to enter the conduction band across the forbidden band and leave holes in the valence band, causing an increase in conductance, which is an intrinsic photoconductivity. The impurity level from the forbidden band can also excite photo-generated carriers into the conduction band or valence band, which is the photoconductivity of the impurity. The cutoff wavelength is determined by the ionization energy of the impurity. The quantum efficiency is lower than the intrinsic light guide and requires a lower operating temperature.

(2) Photovoltaic type: mainly the photovoltaic effect of the pn junction. Infrared photons with energy greater than the forbidden band amplitude excite electron-hole pairs in and around the junction region. The existing junction electric field causes holes to enter the p region, electrons enter the n region, and potential differences occur in the two portions. The external circuit has a voltage or current signal. Compared with the photoconductive detector, the detection limit of the photodetector back-image limit is greater than 40%; no external biasing electric field and load resistance are required, no power is consumed, and high impedance is obtained. These characteristics bring great benefits to the preparation and use of focal plane arrays.

(3) Light emission-Schottky barrier detector: Metal and semiconductor contact, typically PtSi/Si structure, forming a Schott ky barrier, infrared photons are absorbed by PtSi through the Si layer, and electrons gain energy to the Fermi level. The holes are left over the barrier to enter the Si substrate, and the electrons of the PtSi layer are collected to complete the infrared detection. Make full use of Si integration technology, easy to manufacture, with low cost, good uniformity, etc., can be made into large-scale (1024 × 1024 or even larger) focal plane array to compensate for the low quantum efficiency. There are strict low temperature requirements. With such detectors, thermal imaging cameras with good image quality have been produced at home and abroad. Pt Si/Si structure FPA is the first IRFPA made.

(4) Quantum Well Detector (QWIP): The two semiconductor materials A and B are alternately grown by a thin layer of artificial methods to form a superlattice, and at the interface thereof, a mutation can be carried out. The electrons and holes are confined in the low potential well A layer, and the energy is quantized, called a quantum well. The infrared detector can be used by the principle of energy level electronic transition in the quantum well. Since the 1990s, it has developed rapidly. The QWIP GaAs/AlGaAs focal plane of 512×512 and 64××480 scales has been produced to produce the corresponding thermal imager. Because only the electrodeization vector perpendicular to the superlattice growth plane acts on the incident radiation, the photon utilization rate is low; the concentration of the ground state electrons in the quantum well is limited by doping, the quantum efficiency is not high; the response spectrum is narrow; the low temperature is demanding. People are working hard to improve it, and it is expected to compete with mercury cadmium telluride detectors.

2.1.2 The rapid development of new technologies promotes the replacement of infrared detectors

Before the 1960s, most of the unit detectors were scanned and imaged, but the sensitivity was low, and the two-dimensional scanning system was complicated and cumbersome. Increasing the probe element, for example, a detector consisting of N elements, the sensitivity is increased by N1/2 times, an M×N array, and the sensitivity is increased by (M×N) 1/2 times. The increase in the number of elements will also simplify the scanning mechanism of the optical machine, and gaze at the focal plane array on a large scale, eliminating the need for optical scanning, which greatly simplifies the whole system. One of the important signs of modern detector technology entering the second and third generations is that the number of elements has increased greatly. Another aspect is the development of two-color and multi-spectral detectors that cover both bands at the same time. All developments are inseparable from the development and advancement of new technologies, especially semiconductor technology. Several landmark technologies are:

(1) Semiconductor precision lithography technology has enabled the rapid development of detector technology from cells to multi-line detectors, later known as first-generation detectors.

(2) Si integrated circuit technology The Si readout circuit is coupled with the large area array of photosensitive cells, and the so-called second-generation large-scale infrared focal plane array detector is born. Further, there are new varieties such as Z-plane and smart type smart detectors. The technology also induces an uncooled focal plane array that revitalizes once-cold-hot heat detectors.

(3) Advanced thin-layer material growth technology Molecular beam epitaxy, metal organic chemical vapor deposition and liquid phase epitaxy can repeat and precisely control the growth of large-area highly uniform materials, making it possible to prepare large-scale infrared focal plane arrays. It is also a prerequisite for the emergence of quantum well detectors.

(4) Micro-refrigeration technology High-performance detectors require low-temperature driving of the development of micro-refrigerators, and refrigeration technology has promoted the development and application of detectors.

The development of infrared detectors in China has been in existence since 1958 and has been in existence for more than 40 years. PbS, PbSe, Ge:Au, Ge:Hg, InSb, PbSnTe, HgCdTe, PtSi/Si, GaAs/AlGaAs quantum wells and pyroelectric detectors have been developed. With the emergence of low-dimensional materials, technologies such as nanoelectronics and optoelectronic integration are advancing with each passing day, and infrared detectors in the 21st century must have revolutionized. Physics and materials science are the main foundations for the development of modern technology. The rapid development of modern technology has a huge adverse effect on physics research.

4, high-performance infrared detector - mercury cadmium telluride detector

In 1959, Lawson et al. first made a variable bandgap Hg1-xCdxTe solid solution alloy, which provided an unprecedented degree of freedom in the design of infrared detectors.

Mercury cadmium telluride has three major advantages:

1) Intrinsic excitation, high absorption coefficient and high quantum efficiency (can exceed 80%) and high detection rate;

2) Its most attractive feature is to change the Hg, Cd ratio adjustment response band, can work in each infrared spectrum section and get the best performance. Moreover, the lattice parameters are almost constant, which is especially important for the preparation of new devices with composite forbidden heterojunction structures.

3) The same response band, the working temperature is higher, and the working temperature range is also wider.

Among the mercury cadmium telluride, the weak Hg-Te bond (about 30% weaker than the Cd-Te bond) can form a P or N type by heat treatment or a specific route, and can complete the transformation. Its electrical properties such as low carrier concentration, 2 minority carrier long life, 3 electron hole effective mass ratio (~10.0), high electron mobility, and 4 small dielectric constant are beneficial to detector performance.

The first generation of mercury cadmium telluride detectors was mainly a multi-element type. The United States used 60, 120 and 180 yuan photoconductive detectors as general components of thermal imaging cameras, while the United Kingdom developed SPRITE as a general-purpose component in the mid-1970s. SPRITE is a three-electrode photoconductive device that utilizes the unbalanced carrier sweep effect in the semiconductor. When the spot scanning speed matches the carrier bipolar drift speed, the detector realizes the signal delay while completing the radiation detection. Integration function. The performance of 8 SPRIET can be more than 100 yuan multi-detector. The structure, preparation process and subsequent electronics are greatly simplified. The prior art overcomes two drawbacks such as high speed scanning speed and limited spatial resolution.

In 1992, the first domestically produced general-purpose component high-performance thermal imager was born. The successful development of SPRITE detector is the key. By the early 1990s, the first generation of mercury cadmium telluride photoconductive detectors had completed technical appraisal, and the performance reached the world advanced level.

The SPRITE, 32 and 60-element detectors of the 211 weapons factory have been put into practical use and put into mass production, and the scale and market are expanding. Foreign countries have been mass-produced in the 1980s. Due to the difficulties in electrodes, dewar design and chillers, the first generation of mercury cadmium telluride detectors generally cannot exceed 200. Large mercury cadmium telluride photosensitive arrays and Si readout integrated circuits are separately prepared and optimized, and then electrically coupled and mechanically coupled to form a hybrid focal plane array, which is the second generation of mercury cadmium telluride detector.

At present, long-wave IRFPA of 256×256 or even 640×480 scale has been developed internationally. Medium wave infrared has been used for astronomical 1024×1024 scale. The typical product at this stage is the French 4N series 288×4 scanning FPA. The country is still in the research and development stage. Crystal cadmium cadmium mercury materials also have distinct weaknesses:

1) The phase diagram liquid line and the solid line are separated greatly, and the splitting causes the radial and longitudinal components to be uneven;

2) High Hg pressure makes large-diameter crystals difficult to grow and poor lattice structure integrity;

3) Repetitive production yield is low. The difficulty with thin film materials is that it is difficult to obtain a desired CdZnTe substrate material.

People are working on alternative substrates such as PACE (Producible Alternative to CdTe for Epitaxy) - I (HgCdTe / CdTe / Gems), PACE-II (HgCdTe / C dTe / GaAs) and PACE-III (HgCdTe / CdTe / Si) . Japan and France have also reported Ge substrates with the goal of matching the lattice of the MCT and facilitating coupling with the Si readout line. High-quality mercury cadmium telluride materials are difficult to prepare, have poor uniformity, special process technology, and low yield. Therefore, high cost has always been a major obstacle to IGFPA. People have never given up trying to find materials, but so far there is no new material that can exceed the basic advantages of mercury cadmium telluride. To meet the higher performance requirements of military applications, mercury cadmium telluride FPA is still the preferred detector.

5, non-cooling focal plane array (UFPA) infrared detector

Uncooled focal plane arrays eliminate expensive cryogenic refrigeration systems and complex scanning devices, and sensitive devices are dominated by thermal detectors. Breaking through the historically high cost of thermal imaging cameras, "changing the field of sensors." In addition, its reliability is greatly improved, maintenance is simple, and working life is prolonged, because cryogenic refrigeration systems and complex scanning devices are often the source of failure of infrared systems. The sensitivity of the non-cooling detector (D) is more than one order of magnitude lower than that of the low-temperature cadmium telluride, but it can be compensated with the large-scale focal plane array to compete with the first-generation MCT detector. It is sufficient for many applications, especially surveillance and night vision. The vast paramilitary and civilian markets are the areas in which it exerts its strengths. In order to avoid a large amount of investment, the silicon integrated circuit process is introduced into the development and production of low-cost, uncooled infrared detectors, and the signal processing of large-scale high-density arrays and propulsion systems is integrated, that is, large-scale focal plane array technology has great potential. Because of this, thermoelectric detectors with low unit performance are re-emphasizing and may become one of the most competitive detectors of the 21st century. There are two types of UFPA that are currently the fastest growing and promising:

(1) Pyroelectric FPA. The research of pyroelectric detectors was quite popular in the 1960s and 1970s. There are many kinds of materials, such as BST ceramics and lead strontium titanate (PST). The 328×240 barium titanate (BST) FPA introduced by TI in the United States has been formed, and NETD is better than 0.1K, which has many applications. There is also a 640×480 FPA in the plan. The development trend is to deposit a thin film of ferroelectric material on the silicon wafer to make a monolithic pyroelectric focal plane with high potential performance. It is expected to realize 1000×1000 array. High quality imaging.

(2) Microbolometer. It is a microbridge device (single-chip FPA) fabricated on an IC-CMOS silicon wafer by a deposition technique using Si3N4 with a high temperature coefficient of resistance and a high resistivity thermistor material Vox or α-Si. Receiving thermal radiation causes temperature changes to change resistance, DC coupling does not require a chopper, and only a semiconductor cooler is required to maintain its stable operating temperature. In the early 1990s, Honeywell first developed and developed 320×240 UFPA working at 8μm～14μm, and made a practical thermal image system. NETD has reached below 0.1K and is expected to reach 0.02K in the near future. This kind of FPA developed rapidly in the 1990s and became a hot spot. Compared with pyroelectric UFPA, the microbolometer uses a silicon integrated process with low manufacturing cost; good linear response and high dynamic range; good insulation between pixels with low crosstalk and image blur; low 1/f noise; and high frame rate and potentially high sensitivity (theoretical NETD up to 0.01K). Its bias power is limited by the power dissipation and the large noise bandwidth is not sufficient compared to pyroelectric.

Combined with the geometric relationship equations of the carrier and the target space motion, the simultaneous solution can solve the distance and velocity initial values ​​of the target, and then recursively solve the subsequent sampling points of the target according to formula (6), which is to satisfy the real-time solution. Calculated the foundation. Filtering algorithms can also be used for smoothing and extrapolation to improve accuracy.

The target distance and radial velocity can be solved by measuring the radiant flux of the target in two bands and two sampling moments by an infrared detector.

2.3 Infrared detection development prospects

In order to meet the development needs of infrared information acquisition technology, the United States and other developed countries are actively exploring the spectrum band. Seamless gap detection". In the microwave / millimeter wave, visible / medium wave infrared / long wave infrared and other band detection has made great progress.

2.3.1 Infrared focal plane array technology

First, in the developed countries such as the United States and France, the single-band infrared focal plane device technology based on the narrow band gap semiconductor hoof mercury material has matured. The focal plane device represented by 288×4 yuan long wave and 256×256 element medium wave has been basically It replaces the universal component of multi-element wire array. The 256×256-yuan hoofed mercury focal plane detector has been applied in engineering. It has been developed to a larger-scale gaze-type surface focal plane detector and two-color detector. The long-wave device has been reached. On the scale of 256×256 yuan, the medium and short wave devices have reached the scale of 512×512 yuan or even 2048×2048 yuan. The long-line array scanning focal plane has been highly valued for its requirements in space-to-ground observation, for different applications. Target, 1500 far infrared medium long wave, 3000 far infrared short wave, 4000 far infrared long wave and 6000 far infrared medium and short wavelength line focal plane device have come out.

Infrared focal plane device technology, not only to develop silicon Boltometer and ferroelectric material pyroelectric or thermal capacitive infrared focal plane devices based on material temperature characteristics, but also to develop thermomechanical strain-type infrared focal plane devices based on micro-optical electromechanical technology . At the same time, through the exploration of new materials, the excellent device structure and material properties are effectively combined to form a more powerful focal plane device, which forms one of the important development directions. For example, France's large infrared focal plane array (TRFPA) And dual-band infrared focal plane array technology and small pixel amorphous silicon uncooled detector technology research has made some progress.

2.3.2 Infrared Electron Physics

A focus of the development of infrared optoelectronics is the physical problems associated with infrared focal planes. For example, narrow bandgap semiconductor physics, which is mainly studied with hoof mercury, has been well developed, directly supporting the infrared focal plane technology represented by hoof mercury, and based on semiconductor microstructure and nanostructure band engineering. The band physics is also developing rapidly, and a new class of focal plane technology represented by quantum well infrared detectors and other quantum devices has been vigorously promoted. The new infrared detection application materials and physics research based on photoelectric physics have also changed in recent years. Very active. Infrared optoelectronic physics has become the main research to study the interaction between electromagnetic radiation and matter in the infrared band energy range, study the principles and mechanisms of infrared radiation and detection, explore new materials and devices, provide scientific basis for infrared optoelectronic technology and popular subjects for direct application. .

Up to now, the infrared early warning detection system integrates infrared detector technology, refrigeration technology and photoelectric technology, signal processing technology and automatic control technology, and works in a passive manner. It has good concealment and anti-interference ability. The target range is wide. It has gradually become one of the main means of warning against incoming threat targets.

2.3.3 Dual Band Detection

The single-band infrared detection system will reduce its detection capability and accuracy due to factors such as target camouflage and environmental interference. The dual-band detector can not only improve the system's ability to identify false targets, but also reduce the system's threshold level and improve the system. The detection range of the system, etc., so the dual-band detection has a good development prospect. For example, France. The Spiraer's infrared warning system for ships and the IRSCAN in the Netherlands use dual-band detection.

2.3.4 Compound detection

Since the infrared focal plane array technology has evolved from single-pixel monochrome to two-color, and is developing in the direction of three-color and four-color, it is expected that the capability of hyperspectral applications will be obtained by the year. The field test of the two-color gaze focal plane array has been At the same time, the multi-color focal plane of the spectral filter linear array can be used to detect the visible light to long-wave infrared, and the spectral range can be as many as several tens to hundreds. Boeing Aircraft Corporation's Electronic Systems and Missile Defense Department has made great progress in this area. Boeing/Rockwell's remote sensing device uses HgCdT-e multispectral infrared focal plane PACE-1 which has reached 1024×1024 yuan, Hawaiiyi 2 The 2048×2048-element array has been developed, and its pixel size is as small as 18×18.

A comprehensive composite photodetector system using radar, infrared, ultraviolet, laser and other technologies, and continuously expand its response spectrum range, reduce false alarm rate and improve multi-sensor data fusion capabilities to meet the needs of future battlefields. For example, the US Air Force Developed a composite alarm that detects infrared, visible, ultraviolet and RF threats simultaneously.