The Materials of Uncooled Infrared Detectors

Infrared focal plane detector is the core of thermal imaging system, and it is the key to detect, distinguish and analyze thermal information of the target. It has a wide range of applications in various industries such as security monitoring, medical inspection, industrial inspection etc.

Infrared focal plane detectors can be divided into cooled infrared focal plane detectors and uncooled infrared focal plane detectors. The advantages of cooled infrared focal plane detectors are high sensitivity and low noise impact. They are mainly used in high-end applications.

The uncooled infrared focal plane detector does not need a cooling device, and can work at room temperature. Its volume and power consumption are greatly reduced. Although it is not as sensitive as cooled infrared focal plane detectors, the performance of uncooled infrared focal plane detectors can meet the technical needs of some high-end applications and most civilian fields.

As a bridge between the perception of infrared radiation and the output signal, the heat sensitive element is the core component of infrared detectors.

The thermal element materials of cooled focal plane detector are mainly 3 kinds: mercury cadmium telluride (MCT), quantum well (QWIP), superlattice (TSL).

For uncooled infrared detectors, the mainstream materials of the heat-sensitive element are mainly vanadium oxide (VOx) and amorphous silicon (α-Si).

1. Vanadium Oxide (VOx)

Uncooled vanadium oxide microbolometer was born in the late 1980s. Vanadium oxide has a variety of composite forms, such as VO2, V2O5, V2O3 and so on. Vanadium oxide material has a high TCR (about 2%/K~3%/K at room temperature). Its technology has been mature after years of development, and has been widely used in high-end microbolometer products.

2. Polysilicon (α-Si)

Research on polysilicon material detectors began in 1992, and the technology is now very mature. The TCR of polysilicon is equivalent to that of VOx, and it is also a widely used microbolometer material. The advantage is that it is fully compatible with standard silicon technology and the preparation process is relatively simple. However, since polysilicon is an amorphous structure, the 1/f noise presented is higher than that of VOx, so NETD is usually not as good as VOx materials.

3. Other Materials

The advantage of silicon diodes (SOI) is that their area can be made smaller than that of resistors, so smaller pixels can be made, but the area array is smaller, and it is usually used in the field of point temperature measurement.

There are also some materials that can be used in the manufacture of microbolometers, which have some excellent properties, but also have more obvious disadvantages. Titanium oxide film has low 1/f noise. It can be easily integrated with CMOS readout circuit, and has low thermal conductivity, but its TCR is only about 0.35%/K. So it has unstable performance and a certain limit of manufacturing process and life.

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