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A bolometer is a temperature-sensitive electrical resistor. Its operation is based on its temperature rise caused by absorption of incident radiant energy. The change in temperature causes a change in electrical resistance, which is measured by an external electrical circuit. When the radiation is removed, the temperature of the bolometer returns to its initial value, which is determined by the ambient surroundings in which it is immersed. If the resistance increases with increasing temperature, such as is found with metals, the bolometer is said to have a positive temperature coefficient of resistance; if it decreases with increasing temperature, as is found in semiconductors under most operating conditions, it is said to have a negative temperature coefficient of resistance. Modern bolometer arrays employ a pixel structure originally developed by Honeywell, Inc. (see Fig. 4-1). It is termed a monolithic structure and is prepared by silicon micromachining. The detecting area is defined by a thin membrane, usually made of silicon nitride (Si 3 N 4 ), upon which is deposited a thin film of the detecting material, which is usually semiconducting vanadium oxide (VO x ). The membrane is supported above a silicon substrate by means of “legs” made of silicon nitride upon which is an electrically conductive film. The preparation of this structure employs a sacrificial layer of silicon dioxide, later removed, the thickness of which determines the height of the membrane above the substrate. A reflective layer on the substrate below the membrane causes incident infrared radiation that is not completely absorbed by the detecting material to be reflected back through the material, thereby increasing the amount absorbed. This is most effective when the spacing between the absorbing layer and the reflecting layer is one-fourth of the wavelength of the incident radiation.
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