Infrared sensor

This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-287544 filed on Sep. 20, 2001 and Japanese Patent Application No. 2000-327288 filed on Oct. 26, 2000.

1. Field of the Invention

The present invention relates to an infrared sensor and, more particularly, to a design of a sensing area including thermocouples and an infrared absorbing film in the sensor.

2. Description of the related Art

In general, an infrared sensor includes a concavity made on a side of a semiconductor substrate and a plurality of sensing areas formed in a thin film area at the back side of the bottom of the concavity. The sensor makes it possible to quickly measure the temperature of an object without making contact with the object. Additionally, the sensing areas have a boundary with the substrate in the periphery of the thin film area which is susceptible to heat loss due to heat conduction of the substrate. Therefore, the sensing areas in the periphery of the thin film area have a greater thermal capacity than the sensing areas in the central part of the thin film. Thus, the proposed sensor has a sensitivity deviation among the sensing areas.

Thus, it is an objective of the present invention to provide an infrared sensor that has a reduced sensitivity deviation among the sensing areas. That is, a more consistent thermal conduction of the sensing areas is desired.

According to the present invention, it is possible to generate greater electromotive force in the sensing area having a boundary with the substrate by means described hereinafter.

More thermocouples are provided in the sensing areas having a boundary with the substrate than in the sensing areas having no boundary with the substrate. The difference in the number of thermocouples between those sensing areas compensates for the heterogeneity of heat transfer in the thin film area due to the heat loss of the sensing areas having a boundary with the substrate. Therefore, the sensitivity becomes homogeneous among the sensing areas.

A larger infrared absorbing film is formed in the sensing area having a boundary with the substrate as compared to the sensing area having no boundary with the substrate. The difference in size of the film between those sensing areas compensates for the heterogeneity of heat transfer in the thin film area caused by the heat loss of the sensing area having the boundary with the substrate. Therefore, the sensitivity becomes homogeneous among the sensing areas.

Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

The invention, together with additional objectives, features and advantages thereof, will be best understood from the following detailed description, the appended claims and the accompanying drawings in which:

FIG. 1 is a plan view of an infrared sensor according to a first embodiment of the present invention;

FIG. 2 is a cross-sectional view of the infrared sensor;

FIG. 3 is an enlarged, fragmentary, plan view of the infrared sensor in which a thermocouple is not shown; and

FIG. 4 is a plan view of an infrared sensor according to a second embodiment of the present invention.

The present invention will be described in detail with reference to various embodiments.

(First Embodiment)

An infrared sensor according to a first embodiment shown in FIGS. 1 and 2 is, for one example, utilized in an automatic air conditioner for measuring the inside temperature of a vehicle.

An infrared sensor chip includes a thin film area 3 made of a silicon oxide film 4 at the bottom of a concavity 2 that is made by selectively etching a portion of a single crystal silicon substrate 1 . Nine rectangular infrared absorbing films 7 are arranged in a three-by-three matrix, with equal spacing between the films, in the rectangular thin film area 6 . A plurality of polysilicon lines 5 a and a number of aluminum lines 5 b extend from the infrared absorbing film 7 to an outside of the thin film area 3 and are alternately disposed in parallel on the thermal silicon oxide film 4 . The polysilicon lines 5 a and the aluminum lines 5 b are connected in series to form a thermopile 10 consisting of a plurality of thermocouples 5 . A thermocouple 5 consists of a polysilicon line 5 a and an aluminum line 5 b. A contact point of the polysilicon line 5 a and the aluminum line 5 b make a hot junction and a cold unction, respectively, under the infrared absorbing film 7 on the silicon oxide film 4 outside the thin film area 3 . One thermopile 10 is formed under each of the nine infrared absorbing films 7 . A sensing area 8 includes a plurality of hot junctions formed in the thin film area 3 . A piece of dielectric film 6 rests on the hot junctions and the infrared absorbing film 7 covers the hot junctions with the dielectric film 6 sandwiched in-between, as shown in FIGS. 1 and 2. Specifically, nine sensing areas 8 are formed in the thin film area 3 . Aluminum pads 9 are formed at both ends of the thermopile 10 and connect electrically with the aluminum lines 5 b for picking up a signal generated by the thermopile 10 .

As shown in FIG. 3, the heat generated at sensing areas 8 b and 8 c adjacent to the silicon substrate 1 transfers to the silicon substrate 1 in the directions indicated by the arrows. The sensing area 8 b adjacent to sensing area 8 a, formed in the central part of the thin film area 3 , has one heat transfer direction while the sensing area 8 c situated diagonally to sensing area 8 a has two heat transfer directions. Consequently, the more boundaries the sensing area 8 has with the substrate 1 , the greater the heat loss by sensing area 8 . Therefore, the sensing area 8 b is more susceptible to heat loss in comparison with the sensing area 8 a, while sensing areas such as 8 c are more susceptible to heat loss than sensing areas 8 a and 8 b.

In the first embodiment, a group of two thermocouples, a group of three thermocouples and a group of four thermocouples are formed in the sensing area 8 a. Like groupings of thermocouples are formed in sensing area 8 b, and in each of the other sensing areas, respectively, to compensate for any heterogeneity of heat transfer in the thin film area 3 . Therefore, the sensing areas 8 b and 8 c generate greater and greater electromotive forces than sensing area 8 a does, so that sensitivity loss in each of the sensing areas 8 b and 8 c is suppressed. Thus, the difference in sensitivity between the sensing areas is reduced.

A fabrication method of the infrared sensor will now be described. The thermal silicon oxide film 4 is formed on a side of the silicon substrate 1 . A silicon nitride film may be used instead of the thermal silicon oxide film 4 . Desired patterns of the n-type polysilicon lines 5 a and the aluminum lines 5 b are defined on the silicon oxide film 4 to form the thermocouples 5 . The thermocouple 5 is a pair of two conductive or semi-conductive materials that have a different Seebeck coefficient from each other. In the first embodiment, the thermocouples 5 comprise the n-type polysilicon line 5 a and the aluminum line 5 b. The dielectric film 6 is deposited on the thermocouples 5 . A wire-bonding place is made by means of opening a part of the film 6 on the aluminum pad 9 . An anisotropic etching using an etchant such as KOH (Potassium Hydroxide) is applied to a partially masked surface on the semiconductor substrate 1 to form the concavity 2 and thereby form the thin film area 3 . The infrared absorbing film 7 made from gold black (made of small particles of gold) are deposited by vacuum evaporation on the thermocouples 5 with the dielectric film 6 sandwiched in-between.

The silicon substrate 1 is divided into each sensor chip by means of a method such as a dicing cut. The infrared sensor chip is assembled on a stem (not shown in a figure) with an adhesive such as silver paste. A bonding wire electrically connects the sensor chip and a pin. A cap (not shown in a figure) and the stem are welded together and sealed to finalize the sensor fabrication.

(Second Embodiment)

In the second embodiment of FIG. 4, the size of the infrared absorbing films 7 is a major difference from the first embodiment. The sensing area 8 a in the central part of the thin film area 3 has a smaller infrared absorbing film 7 compared to the sensing area 8 b adjacent to the sensing area 8 a, while the sensing area 8 b has a smaller infrared absorbing film 7 compared to the sensing area 8 c adjacent to the sensing area 8 b. Generally, the closer the sensing area 8 is to the periphery of the thin film area 3 , the larger the infrared absorbing film 7 is. Thereby, the sensing areas 8 b and 8 c generate greater and further greater electromotive forces than the sensing area 8 a does, respectively, so that sensitivity loss in the sensing areas 8 b and 8 c is suppressed. Thus, the difference in sensitivity between the sensing areas is reduced.

(Other Embodiment)

The first embodiment and the second embodiment may be combined in such a case that there are some limitations in the arrangement of the thermocouples 5 or in the size or the thickness of the infrared absorbing film 7 . Namely, both embodiments may be combined in such a manner that the closer the sensing area 8 is to the periphery of the thin film area 3 , the more thermocouples 5 and the larger infrared absorbing film 7 the sensing area 8 will have. Thereby, respective advantages of the two embodiments described hereinbefore are provided so that one embodiment compliments the other.

The above embodiment may be modified further within the spirit of the present invention. For example, in the embodiments described, the thermocouple 5 is a pair consisting of the n-type polysilicon line 5 a and the aluminum line 5 b, however, the thermocouple 5 may be a pair of two other conducting or semiconducting materials that have a different Seebeck coefficient from each other. In addition, in the first embodiment, a group of two thermocouples 5 , three thermocouples 5 and four thermocouples 5 are formed in the sensing area 8 a, in another sensing area 8 b and, still further, in other sensing areas 8 c, respectively, to compensate for the heterogeneity of heat transfer in the thin film area 3 . However, other numerical combinations of the thermocouples 5 may be assigned to the sensing areas 8 .

The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.