Generally, infrared sensors are categorized into two types: quantum sensors using the photoelectric effect of semiconductor and thermal sensors using temperature increase due to the absorption of radiant heat. The former have high measurement accuracy and good responsiveness but have disadvantages such as their narrow wavelength range and unusability at room temperature. While the latter are inferior to quantum sensors in sensitivity and responsiveness, they have a simple structure, can sense a wide range of wavelength, and are usable even at room temperature.
Typical thermal sensors include thermocouple-type using a thermopile, pyroelectric-type using the pyroelectric effect of a piezoelectric substance, and bolometers using variations in resistance.
Bolometer-type infrared sensors are required to have high sensitivity and high responsiveness and be small and low-cost.
Patent Documents 1 and 2 disclose traditional thermistor bolometer-type infrared sensors.
Patent Document 1 discloses an infrared sensor where a temperature compensation thermistor chip and an infrared sensing thermistor chip are disposed with an insulator therebetween above a stem sealing a cap.
FIG. 1 is a drawing showing the configuration of the infrared sensor of Patent Document 1. This infrared sensor includes a stem 11, a cap 12, an infrared incident window 13, an insulating substrate 14, thermistor chips 15 and 16, a thermal and electrical insulator 17, pins 18, electrodes 19, and lead lines 20.
Patent Document 2 as a structure similar to Patent Document 1 discloses an infrared sensor where a base electrode film, a thermistor film and a temperature compensation part electrode film are formed on both surfaces of a flat insulating ceramic substrate.
Patent Document 1: Japanese Unexamined Utility Model Registration Application Publication No. 60-11042
Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-90073
In the configuration of Patent Document 1, the infrared sensing thermistor chip 15 is mounted above the temperature compensation thermistor chip 16 with the thermal and electrical insulator 17 therebetween. Thus, it is believed that the thermal resistance from the infrared sensing thermistor chip 15 to the stem 11 is high and that a large temperature difference can be made between the infrared sensing thermistor chip 15 and the temperature compensation thermistor chip 16 so that detection sensitivity can be increased. However, the heat sensing part is the infrared sensing thermistor chip 15 having a predetermined thickness of 0.5 mm×1.0 mm×0.2 mm and thus has a large heat capacity. Further, since the thermal and electrical insulator 17 is disposed between the infrared sensing thermistor chip 15 and the temperature compensation thermistor chip 19, sufficient thermal separation is not achieved and thus sufficient sensitivity is not easily obtained. Meanwhile, when the heat capacity of the infrared sensing thermistor chip 15 itself is small, the heat insulation effect of the thermal and electrical insulator 17 is effectively exhibited. To reduce the heat capacity of the infrared ray sensing thermistor chip 15, the thermistor chip 15 alone must be formed extremely thinly. In such a case, it is unrealistic to use air as the thermal and electrical insulator 17 and support the infrared sensing thermistor chip 15 by only the lead lines 20a and 20b. Further, the sensor must be a component with the pin terminals 18 and thus cannot be formed as a small, surface-mountable infrared sensor.
As for the configuration of Patent Document 2, a base electrode is formed on both surfaces of an alumina wafer, and a thin-film thermistor is formed on the base electrode by sol-gel process. In this case, the heat capacity can be reduced, since the infrared absorption film is formed as a thin film. However, the method of forming the infrared absorption film as a thin film by sol-gel process or the like includes complicated steps and thus not suitable for mass production. Further, cost reduction and miniaturization are difficult.
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