Infrared sensor

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to a sensor that absorbs infrared rays and obtains a signal by thermoelectric conversion, and more particularly to a thermal infrared sensor.

【0002】

[Conventional technology]

In recent years, various techniques for producing a thermal infrared sensor by using semiconductor microfabrication have been developed. This infrared sensor absorbs infrared rays with a thin film temperature-sensitive film having a heat insulating structure, and converts the temperature rise due to the infrared rays into changes in electrical resistance or electromotive force by thermoelectric conversion elements (heat-sensitive elements) such as thermistors and pyroelectric materials. However, a signal proportional to the dose of infrared rays to be measured is obtained. As a material for forming an infrared absorbing film provided on the surface of a temperature-sensitive film, a NiCr thin film or a gold-black (gold-black) thin film is generally often used.

【0003】

[Problems to be Solved by the Invention]

However, the absorption rate of this infrared absorbing film changes depending on the film quality and film thickness, and at present, the maximum is about 50% in the case of NiCr, which is the most effective. Since the detection part of the sensor is originally small, the obtained signal strength is small, the signal is stronger, and a noise margin can be obtained. Therefore, in order to supplement the detection efficiency, it has been proposed that a microlens for condensing light is provided in front of the sensor in order to increase the amount of light received, but this complicates the structure, causes optical axis deviation, and focuses. There is a problem that misalignment occurs, so it cannot be said that it is appropriate for manufacturing. Therefore, an object of the present invention is to provide an infrared sensor having a higher infrared absorption efficiency than the conventional one.

【0004】

[Means for solving problems]

In order to solve the above problems, the configuration of the present invention comprises a first infrared absorbing film that absorbs infrared rays emitted from the outside, a heat-sensitive element that obtains an electric signal as a resistance change or electromotive force by thermoelectric conversion, and infrared rays incident. An infrared sensor in which a second infrared absorbing film is provided on the heat sensitive element on the side, and infrared rays reflected without being absorbed by the first infrared absorbing film are the first infrared sensor on the heat sensitive element. It is incident on the second infrared absorbing film. Further, the configuration of the related invention is characterized in that the heat sensitive element is either a thermistor or a pyroelectric element or a thermocouple covered with an insulating layer by NiCr or a gold-black infrared absorbing film. A further related invention is that when the length of the second infrared absorbing film of the heat sensitive element is h, the first infrared absorbing film around the heat sensitive element is attached to the surface of the heat sensitive element. , 0 <α <(π / 4) α, tilt angle [Number 1]

((π / 2) -α) [Radian] It has an inclination of the angle indicated by, and the inclined portion length k of the first infrared absorbing film is [Number 2]

k = hcos (2α) / sin (α) It is greater than or equal to the value indicated by (see Fig. 6).

【0005】

[Action and Effect of Invention]

Due to the structure of the first infrared absorbing film surrounding the heat-sensitive element, the infrared rays that are reflected without being absorbed even if they hit the first infrared absorbing film hit the second infrared absorbing film on the heat-sensitive element again after reflection. Infrared rays, which have been dispersed and did not contribute to detection, will now contribute to the signal, and the infrared detection efficiency will increase. In particular, since infrared rays are concentrated in the central part where the heat-sensitive element is located, the rate at which the thermoelectric conversion material receives heat directly increases, and the detection efficiency is improved.

【0006】

[Example]

Hereinafter, the present invention will be described based on specific examples. FIG. 1 is a schematic structural cross-sectional view of an infrared sensor having a temperature-sensitive film 5 having a wedge-shaped structure to which the present invention is applied. As in the conventional case, this infrared sensor has a cavity in the Si substrate 7 and is irradiated with infrared rays. The portion is formed as a temperature-sensitive film 5 of a thin film supported by a support layer 4 which is an insulating layer. As shown in FIG. 1, the temperature-sensitive film 5 which has been flat in the past has a wedge-shaped structure having a V-shaped cross section, and a film of a thermoelectric conversion material is formed as a heat-sensitive element 6 at the bottom portion thereof. The first infrared absorbing film 1 is provided on the inclined surface portion of the temperature sensitive film 5, the second infrared absorbing film 1'is provided on the central bottom surface portion via the insulating layer 2, and the heat sensitive element 6 is provided below the second infrared absorbing film 1'. There is. Since FIG. 1 is a schematic diagram, the actual dimensional proportions are not shown, and the film thicknesses of the temperature-sensitive films are not as shown in FIG.

【0007】

Infrared rays that are off-center and incident from above first hit the first infrared absorbing film 1 and are partially absorbed, and the rest are reflected, but again they hit another part of the temperature-sensitive film, most of which hits the bottom part. , The second infrared absorbing film 1'there is absorbed again, and the remaining infrared rays reflected further hit the first infrared absorbing film 1 at another place and are absorbed. As shown in FIG. 2, the inclination of this wedge-shaped shape is an angle at which the infrared ray absorbing film is absorbed three times, so that the absorption efficiency is significantly improved. Of course, the area that is reflected twice and hits three times is limited, but at the three incident positions off-center shown in Fig. 2 (a) to (c), it hits the infrared absorbing film three times. Since these three locations are located at approximately equal intervals from the center to the periphery, the infrared rays incident on the entire temperature-sensitive film 5 are at least multiple times, roughly three times. It can be seen that it corresponds to the infrared absorbing films 1 and 1'. If it is calculated simply assuming that 50% is absorbed by one reflection, 50 + 25 + 12.5 = 87.5% is absorbed by three times, and the absorption efficiency is considerably improved.

【0008】

A temperature-sensitive film having such a wedge-shaped structure is formed as follows. First, as shown in FIG. 3A, a wedge-shaped groove is formed on the Si substrate by using anisotropic etching. Since this portion becomes a portion that forms a temperature-sensitive film, anisotropic etching is performed so that the cutting angle of this groove is a predetermined desired angle. Next, in this groove, a sacrificial layer 32 for forming a cavity for heat insulation is formed as shown in FIG.₃N₄Form film 33. Next, a thermoelectric conversion element (heat sensitive element) 37 is formed at the bottom of the groove of the support layer 33, an Al wiring 34 is formed as an electrode, an insulating layer 35 is formed on the pattern, and the first is on the insulating layer 35. And a second infrared absorbing film 36 is formed (Fig. 3 (c)). Then, the infrared absorbing film 36 of the necessary part is left, and the etching hole 38 is formed by leaving the beam part that supports the temperature-sensitive film with the substrate, and the sacrificial layer 32 formed earlier and the Si substrate part below it are selected by selective anisotropic etching. A cavity is formed, and finally, a heat-insulating structure in which a temperature-sensitive film is floated from a substrate is formed to complete an infrared sensor. Note that FIG. 3 is also a schematic view and does not reflect accurate dimensions, and the first and second infrared absorbing films 36 are not independent configurations but are formed as continuous constituent films in the same process. ..

【0009】

FIG. 6 is a diagram for explaining in detail the relationship of the inclination angle of the first infrared absorbing film with respect to the heat sensitive element, and infrared rays radiated from far away from the heat sensitive element are substantially perpendicular to the heat sensitive element. Therefore, in response to such parallel incidents, the infrared rays that did not directly hit the heat-sensitive element hit the first infrared absorbing film around the heat-sensitive element, and then the reflected portion hits the heat-sensitive element again. The configuration when has is shown. When this inclination angle is formed on both sides, the further reflected infrared rays hit the first infrared ray absorbing film on the opposite side again. The tilt angle of the first infrared absorbing film with respect to the bottom surface is shown by Equation 1. The required length of the inclined portion of the first infrared absorbing film is given by Equation 2, and is determined by the length of the second infrared absorbing film of the bottom portion provided on the infrared incident side. Of course, infrared rays that are not vertically incident on the thermal element deviate from this relationship, but at least the portion that is irradiated and reflected by the surrounding infrared absorbing film corresponds to the vicinity of the thermal element. If the light receiving area of the temperature-sensitive film is large and the infrared rays received are effectively heated, it can be considered that the sensitivity is increased accordingly. Therefore, the opening area of this temperature-sensitive film is shown in Fig. 6 (b). This is the case when hcos (2α) shown in is the maximum, but since such an inclination angle is α 0, the length of k actually diverges to infinity, and the length of k is Since it is formed by the etching process described above, it cannot be made very long. Further, when α becomes π / 4 or more, the reflected light is parallel to or does not hit the surface of the heat sensitive element. Therefore, it is desirable that α is in the range of (π / 8) <α <(π / 6) in order for the reflected light to hit the heat sensitive element appropriately.

【0010】

FIG. 4 shows a Wheatstone bridge circuit using a thermistor as a heat sensitive element of the infrared sensor having the structure of the embodiment of the present invention. The thermistor is R1 in Fig. 4, which is irradiated with infrared rays, and this resistance value changes and is detected as a bridge output. Figure 5 shows a comparison of the outputs of this infrared sensor and the infrared sensor of the conventional structure. As can be seen from Fig. 5, the result was that about twice as many signals were obtained under the same conditions. R2 to R4 are reference resistors and are composed of board parts or external resistors other than the sensor part shown in Fig. 1.

【0011】

The structure in which the reflected infrared rays hit the infrared absorbing film again is not limited to the wedge-shaped structure shown in FIG. 1, and may be a temperature-sensitive film such as a parabolic type or a polygon mirror shape. Further, of course, the effect of the present invention is the same even if the circuit configuration of the infrared sensor is other than that shown in FIG.

【0012】

As described above, the temperature-sensitive film having a structure capable of reabsorbing the reflected infrared rays of the present invention greatly increases the absorption efficiency of the absorbed infrared rays, and can obtain an infrared sensor with higher sensitivity and easier handling.

[Simple explanation of drawings]

[Figure 1]

FIG. 3 is a schematic structural sectional view of the infrared sensor of the present invention having a temperature-sensitive film having a wedge-shaped structure.

[Figure 2]

Explanatory drawing which shows the state of the reflection of the infrared ray incident at each position.

[Fig. 3]

The explanatory view of the process of forming a temperature-sensitive film of a wedge-shaped structure.

[Fig. 4]

Sensor application circuit diagram.

[Fig. 5]

FIG. 5 is a comparison diagram of sensor output between the present invention and the conventional structure.

[Fig. 6]

The explanatory view of the inclination of the infrared sensitive film of this invention.

[Explanation of symbols]

1 First infrared absorbing film 1'Second infrared absorber film 2 Insulation layer (SiO)₂) 3 Al wiring 4 Support layer (Si₃N₄) 5 Temperature sensitive membrane (1, 2, 3, 4, 6) 6 Thermoelectric conversion element (heat sensitive element) 7 Si board 31 Si substrate 32 sacrificial layer 38 etching hole