Two spectral range microbolometer

Топология микроболометрического тепловизионного детектора Астрон-64017-1

Two spectral range microbolometer

A photoreceiver (PR) is the “heart” of any thermal imaging device and determines its main tactical and technical characteristics. The principle of operation of thermal detectors is based on a change in the electrical characteristics of the receiver material due to the energy of absorbed thermal radiation. Over the past three decades, significant progress has been made in the creation of uncooled infrared thermal detectors, which are close in characteristics to photon detectors but at a significantly lower cost. One of the actively developed types of thermal detectors are microbolometers.

The need for the development and mass production of domestic thermal imaging photoreceivers became especially urgent after the ban on their supplies from abroad. In 2016, ASTROHN Technology Ltd decided to solve this problem and started production of microbolometric detectors cased in vacuum-tight domestic ceramics VK-94. Due to the new topology of the multiplexers, it was possible to significantly increase the PR sensitivity (up to 40 mK) and to begin mass production of microbolometers operating in the traditional range of 8-14 µm. Since 2016, all thermal imaging devices for security systems have been manufactured at the enterprise on the basis of in-house Astrohn-38425-1 and Astrohn-64025-1 microbolometric detectors with a sensor element size of 25 μm. In 2017, the production of a 17 μm sensor element was launched.

The next step was the development of a microbolometric detector, sensitive in two spectral ranges of 3-5 µm and 8-14 µm, by ASTROHN Technology Ltd specialists. To create a microbolometer operating in two ranges, it is necessary to reduce the thickness of the layers of silicon nitride and vanadium oxide. This will limit the amount of heat capacity of the pixel, as well as increase its speed. With a decrease in the thickness of the layers of silicon nitride and vanadium oxide, the coefficients are equalized, uniformity of absorption is achieved, and the speed of the microbolometer increases. The developed microbolometric detector is structurally an array of substrate-based pixels. A control and reading pixel circuit is formed on the substrate. The pixel, on the other hand, is a membrane suspended above the substrate. The reflector itself is located under the membrane. The membrane contains three thin layers. The first layer is silicon nitride. A second – vanadium oxide layer detects incident radiation. The third – infrared absorbing layer is made of V-VI metals. The distance from the membrane to the reflector is approximately 1.0-2.8 μm, and the exact value of the gap is determined by the results of optical calculation for the required absorption range. To prevent heat loss due to the thermal conductivity of the surrounding gas – an array of pixels is evacuated and sealed in the case. The case itself has a window transparent only to detectable radiation.

Changing the thickness of the constituent layers of the membrane of the microbolometric detector and the gap between the membrane and the reflector ensures required absorption characteristics of the microbolometric detector in the ranges of 3-5 μm and 8-14 μm. When the radiation flux to the bolometer is interrupted, the pixel membrane cools rapidly due to its low heat capacity. This contributes to the normal operation of the pixel without the use of special cooling. With good cooling performance, the performance of pixels also increases. Due to the heterogeneity of the absorption by the pixel over the entire spectral range, image distortion will be minimal.

The technology developed in the laboratory of ASTROHN Technology Ltd allows processing 25 to 150 mm plates. There are all technological processes for the manufacture of microbolometric matrices. The linear dimensions of trace elements are from 0.6 µm, and the size of nanoelements is 10 nm. Microbolometric detectors are manufactured with the use of the operations of sequential film deposition, photolithography, and sequential etching well known in microelectronics. The specialists of the laboratory of ASTROHN Technology Ltd developed a technological route for creating microbolometric matrices and modules. The 3-14 μm absorption study methods are infrared spectroscopy, probe microscope, and optical profilometer.

Currently, the specialists of ASTROHN Technology Ltd have managed to develop the design of a microbolometric detector pixel of equal absorption coefficients in two spectral ranges – 3-5 µm and 8-14 µm, to increase the speed by reducing the heat capacity of the pixel through reduced thickness of the constituent layers, to increase absorption using nano-sized films of V-VI metals as an absorber of IR radiation, and to achieve a high temperature sensitivity of <25 mK for an uncooled detector.

ASTROHN Technology Ltd plans to release in 2018 the first in Russia microbolometer capable of operating in two spectra (3-5 μm and 8-14 μm), and then the first module in Russia for two IR-image spectra of 3-5 µm and 8-14 µm.