The infrared focal plane detector can convert the infrared radiation emitted by people into measurable electrical signal output, and it is the core component of the infrared complete machine system. Infrared focal plane detectors can be divided into cooled and uncooled types.

 

The cooled detector integrates a low temperature refrigeration device to reduce the temperature of the detector and reduce thermal noise. Refrigerated detectors have the advantages of high temperature sensitivity and long detection distance, and are mainly used in high-end military equipment. The uncooled detector has no refrigeration device and works at room temperature. Although the sensitivity is not as good as that of the refrigerated detector, it has the advantages of low power consumption, long life and high cost performance. In recent years, the scale of uncooled infrared focal plane detector arrays has continued to increase, and the pixel size has continued to decrease, and many new technologies have appeared in the structure of the detection unit and its optimized design, readout circuit design, packaging form, etc. The performance can meet the needs of some military equipment and most civilian fields, and the application field has been greatly expanded.

 

The uncooled infrared focal plane detector is composed of many micro-electro-mechanical system (MEMS) micro-bridge structure pixels arranged two-dimensionally on the focal plane. Take measurements. The principle of pixel measurement of infrared radiation is to use the thermal effect of infrared radiation to convert infrared radiation energy into thermal energy by thermal materials, causing the temperature of the sensitive element to rise, resulting in a change in a certain physical parameter of the sensitive element. Some conversion mechanism converts to an electrical or visible light signal. Thermosensitive materials have a great influence on the detection sensitivity, that is, the noise equivalent temperature difference (NETD). According to the heat-sensitive materials used, uncooled infrared detectors are mainly divided into five categories: ① Pyroelectric type composed of triglyceride sulfate, lithium cholate, etc.; ② Thermopile composed of N-type and 'type polysilicon' type; ③ diode type composed of single crystal or polycrystalline PN structure; ④ thermistor type composed of silver oxide, amorphous silicon, etc.; ⑤ thermal capacitor type composed of bi-material film. Different types of uncooled detectors have their own advantages and disadvantages, but in general they have outstanding cost performance and are welcomed by the market.

 

The function of the readout circuit of the uncooled infrared focal plane detector is to output the change of the physical parameters of the sensitive element, such as the resistance value, as an electrical signal. Since the electrical signal generated by the infrared radiation irradiated on the focal plane is very small, it is easily interfered by noise, so the electrical noise of the readout circuit should be controlled as small as possible to avoid affecting the sensitivity of the detector. In addition, due to the deviation in the manufacturing process, the pixel array has non-uniformity, which makes the output signal of the detector show complex changes, which makes subsequent signal processing difficult. In addition to other basic functions, it is also necessary to effectively compensate for the non-uniformity of the pixel output signal.

 

The temperature change of the detection pixel after receiving the target infrared radiation is very weak. In order to maintain the heat above the pixel and avoid air heat exchange, it needs to be placed in a vacuum environment to work. Therefore, the vacuum packaging technology of uncooled infrared focal plane detector can be divided into chip level, wafer level, pixel level and so on. In comparison, wafer-level packaging is more suitable for high-volume and low-cost production, and can provide sufficiently cost-effective detectors for large-scale infrared thermal imaging markets (such as automotive, surveillance, handheld devices, etc.). Pixel-level encapsulation attempts to seal each pixel independently, and it is still in the research stage. As the technology gradually matures, the cost will be greatly reduced.

 

The pixel size of uncooled infrared focal plane detectors continues to decrease, and the size of the area array continues to increase. The pixel size refers to the size of a single detection element. The smaller the detection element, the better the imaging quality. The pixel size is generally 35um, 25um, 17um and so on, and 10um has appeared at present. The size of the area array is reflected in the number of pixels on the area array, that is, the resolution. The higher the separation rate, the clearer the imaging effect. Recently, the mainstream area array resolutions on the market are 160×120, 384×288, 320×240, 640×480, 640×512, etc., and the development and production of detectors with 1024×768 and larger area arrays have begun. In addition, the performance of non-uniformity correction and other digital image processing of such detectors has been further improved, and the application of low-cost packaging has also grown rapidly. These developments can significantly improve the imaging quality of detectors, improve reliability, and reduce volume and weight. and power consumption.