JP2002328086A - Light source for acoustooptical gas sensor and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light source for an acoustooptical gas sensor of a simple structure, capable of efficiently guiding (irradiating) an infrared light outputted from a thin-film heater formed on an insulating thin-film bridge into a cavity of the acostooptiacal gas sensor. SOLUTION: There are provided a semiconductor substrate 11 where a recessed part 12 is formed on its surface, an insulating thin-film bridge 13 which is so provided as to stride the recessed part of the semiconductor substrate, and a thin-film heater 14 which is formed on the insulating thin-film bridge to output an infrared light. An infrared light reflecting film 15 of Au, Al, or the like is coated to enclose the thin-film heater, especially on the inside wall surface of the recessed part and/or on the rear surface of the substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source for a photoacoustic gas sensor suitable for being incorporated in a small-sized photoacoustic gas sensor realized by using micromachining technology, and a method of manufacturing the same.

[0002]

[Related Background Art] A photoacoustic gas sensor utilizes a phenomenon in which a specific type of gas absorbs infrared rays of a specific wavelength and thermally expands, thereby using a specific type of gas in a mixed gas such as air.
For example, it detects the concentration of CO ₂ . That is, when air is irradiated with infrared light having a specific wavelength and the intensity of which changes with time, the greater the CO ₂ concentration in the air, the greater the thermal expansion and contraction. Therefore, if this phenomenon is detected as a change in atmospheric pressure (sound pressure), it becomes possible to detect the CO ₂ concentration in the air. Incidentally, this type of photoacoustic gas sensor basically has a cavity 1 into which gas is introduced as shown in FIG.
The light source 2 includes a light source 2 that modulates infrared rays and emits a pulse, and a microphone 3 that forms part of the wall surface (ceiling surface) of the cavity 1 and that is sensitive to the sound pressure in the cavity 1.

Recently, attempts have been made to reduce the size of this type of photoacoustic gas sensor by applying semiconductor device manufacturing technology. For example, in a small photoacoustic gas sensor realized using micromachining technology,
The cavity 1 is formed by etching a Si substrate transparent to infrared light to form a predetermined space, and has a structure in which a gas passage 4 for introducing gas into the cavity 1 is provided. . Normally, the gas flow passage 4 restricts the flow of gas to maintain the flow (replacement) of gas (air) between the inside and the outside of the cavity 1 while maintaining the above-described infrared light. A gas diffusion filter 5 for changing the sound pressure in the cavity 1 according to the thermal expansion of the gas due to the absorption of the gas is provided. The gas diffusion filter 5 has a large number of fine ventilation holes, and maintains the flow (replacement) of gas (air) between the inside and the outside of the cavity 1 by restricting the flow of gas. However, it plays a role of changing the sound pressure in the cavity 1 according to the thermal expansion of the gas due to the absorption of the infrared light described above.

That is, the gas diffusion filter 5 has the following 2
Plays one role. One of them is to act as a large airflow resistor against a sudden pressure change (sound pressure) generated in the cavity 1 due to the irradiation of the infrared pulse, and to maintain the inside of the cavity 1 in a substantially sealed state, thereby maintaining the sound pressure. Is a function of preventing the light from transmitting to the outside of the cavity 1. The other one is
A function to keep the cavity 1 open to the outside air without acting as an airflow resistor against a slow pressure change (sound pressure) in the cavity 1 due to an external environment change such as temperature and atmospheric pressure. It is.

[0005]

By the way, in the small photoacoustic gas sensor described above, in order to sufficiently increase the detection sensitivity, the thermal expansion of the gas in the small volume cavity 1 due to the absorption of infrared light must be ensured. It is important that they occur. For this purpose, it is necessary to efficiently introduce the infrared light emitted from the light source 2 into the cavity 1 without waste.
However, this type of light source 2 is realized only by processing a Si substrate in the same manner as the cavity 1, and for example, a small-sized light source in which a thin film heater is formed on an insulating thin film bridge provided across a space formed by etching. Having a structure. Therefore, its thermal inertia is small, and the intensity (infrared density) of the infrared light output from the thin film heater is not always large.

The present invention has been made in view of such circumstances, and a purpose of the present invention is to efficiently transmit infrared light output by a thin film heater formed on an insulating thin film bridge to a cavity of a photoacoustic gas sensor. It is an object of the present invention to provide a light source for a photoacoustic gas sensor having a simple structure capable of being introduced (irradiated) into a small size photoacoustic gas sensor and capable of increasing its infrared irradiation density, and a method for manufacturing the same.

[0007]

In order to achieve the above object, a light source for a photoacoustic gas sensor according to the present invention comprises a semiconductor substrate having a concave portion formed on a surface thereof, and a bridge provided over the concave portion of the semiconductor substrate. A thin film heater formed thereon and outputting infrared light, wherein an infrared light reflecting film made of Au or Al is coated on the inner wall surface of the concave portion and / or the back surface of the substrate. It is characterized by doing.

Further, according to the method of manufacturing a light source for a photoacoustic gas sensor according to the present invention, after forming a thin film heater on a semiconductor substrate via an insulating layer having a predetermined width (first step), Is selectively etched on the surface of the semiconductor substrate to form a recess below the thin film heater, leaving the thin film heater as a bridge (second step). Thereafter, the semiconductor substrate is selectively etched. An infrared light reflecting film is formed on the inner wall surface of the concave portion by electrolytic plating (step 3).

Alternatively, an infrared light reflecting film is coated on the back surface of the semiconductor substrate by vapor deposition or the like (third step). At this time, for example, after the side surface of the semiconductor substrate is anisotropically etched and the back surface side of the semiconductor substrate is processed into a trapezoidal shape, or isotropically etched into a pot bottom shape, the entire trapezoidal surface or the pot bottom surface is covered. It is preferable to coat the infrared light reflection film.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A light source for a photoacoustic gas sensor according to an embodiment of the present invention and a method for manufacturing the same will be described below with reference to the drawings. FIG. 1 is a perspective view showing a schematic configuration of the light source for a photoacoustic gas sensor. Reference numeral 11 denotes a semiconductor substrate forming a housing of the light source. The semiconductor substrate 11 is, for example, a single crystal Si substrate having a size of about 4 mm square and a thickness of about 400 μm, and a structure in which a concave portion 12 having a size of about 1 mm square and a depth of about 200 μm is formed by etching. Having. An insulating thin-film bridge 13 made of SiO ₂ or the like is provided on the surface of the semiconductor substrate 11 so as to extend diagonally across the concave portion 12, and further outputs infrared light to the insulating thin-film bridge 13. Thin film heater 14
Is formed. The thin-film heater 14 is made of single crystal Si and has a thickness of, for example,
It is formed to have a size of about 0 × 200 μm ² .

The feature of the light source for a photoacoustic gas sensor according to this embodiment is that it is located below the thin-film heater 14 as schematically shown in FIG. 2 (a) and (b). The point is that an infrared light reflection film 15 made of Au, Al, or the like is provided on the entire surface of the concave portion 12. The infrared light emitted from the thin film heater 14 is reflected by the infrared light reflecting film 15 so that all (almost) of the infrared light is irradiated upward to the thin film heater 14.

Incidentally, as shown in FIG.
5 may be formed on the back side of the semiconductor substrate 11. In this case, as shown in FIG. 2D, the back side shape of the semiconductor substrate 11 is processed into a trapezoidal or hemispherical shape so as to surround the concave portion 12, and is formed over the entire area of the trapezoidal surface or hemispherical surface. You may do it. Even if the infrared light reflection film 15 is formed on the back surface side of the semiconductor substrate 11 in this manner,
Since the semiconductor substrate 11 made of the Si substrate is transparent to infrared light, the infrared light emitted from the thin film heater 14 is reflected upward by the infrared light reflection film 15.

Here, a method of manufacturing a light source for a photoacoustic gas sensor having such an infrared light reflecting film 15 will be described.
First, a Si substrate (Si wafer) is prepared, and an insulating thin film made of, for example, SiO ₂ is deposited and formed on the semiconductor substrate. Next, the insulating thin film is patterned to leave the insulating thin film only in a region where the insulating thin film bridge 13 is formed. Then, a thin film heater Si is formed on the insulating thin film.
Grow a layer (first step).

Thereafter, the entire surface is covered with a resist through a mask material, and the mask material is patterned by photolithography or the like to open the periphery of the insulating thin film bridge 13. Then, using the mask, the semiconductor substrate 1
The surface of 1 is selectively anisotropically etched to form a concave portion 12 below the insulating thin film (second step). More specifically, for example, a concave portion 12 having a trapezoidal cross section leaving an (111) plane as an etching plane by anisotropic etching of a Si substrate having a (100) plane as a main plane is shown in FIG. Formed. At this time, since the etching proceeds below the insulating thin film masked by the mask material, the insulating thin film remains as a bridge (insulating thin film bridge 13) straddling the recess 12 diagonally.

After the basic structure of the photoacoustic gas sensor light source is completed as described above, the infrared light reflecting film 15 made of Au, Al, or the like is formed on the entire inner surface of the recess 12 by electrolytic plating. It is formed to a thickness of about μm. If the infrared light reflecting film 15 is to be formed by vapor deposition or sputtering from above the semiconductor substrate 11, the insulating thin film bridge 13 hinders the portion of the concave portion 12 located immediately below the insulating thin film bridge 13. There is a possibility that the infrared light reflection film 15 will not be formed on the substrate. Therefore, this recess 12
As for the formation of the infrared light reflection film 15 on the inner surface of the substrate, it is desirable to employ electrolytic plating as described above.

However, when the infrared light reflecting film 15 is formed on the back surface of the semiconductor substrate 11 as described above instead of forming the infrared light reflecting film 15 in the concave portion 12, Au or It goes without saying that a technique such as evaporation or sputtering of Al or the like may be used. Further, in the case where the shape of the back surface of the semiconductor substrate 11 is processed prior to the formation of the infrared light reflection film 15 on the back surface of the semiconductor substrate 11, the back surface of the semiconductor substrate 11 may be chemically etched. In particular, when processing into a trapezoidal shape, the back surface side of the semiconductor substrate 11 is anisotropically etched,
What is necessary is just to make the side part be surrounded by the (111) plane.

Thereafter, if the infrared light reflecting film 15 is formed on the entire inner surface of the concave portion 12 or on the back surface of the semiconductor substrate 11 so as to surround the thin film heater 14, the above-mentioned Si substrate (Si
Individual chips from the wafer) (dicing)
A light source for a photoacoustic gas sensor is completed by providing a predetermined optical filter above the thin film heater 14 via a spacer.

According to the photoacoustic gas sensor light source manufactured as described above and having a structure in which the infrared light reflecting film 15 is provided on the inner surface of the concave portion 12 or on the back surface side of the semiconductor substrate 11, FIG. As schematically shown, the infrared light emitted from the thin film heater 14 and applied to the lower side thereof is:
The light is reflected upward by the infrared light reflection film 15. Therefore, even if the intensity of the infrared light emitted from the thin film heater 14 cannot be sufficiently increased due to the small size of the thin film heater 14, most of the infrared light is collected and the thin film heater 14 Through the optical filter 16 provided above, it is possible to irradiate the upper part without waste. As a result, it is possible to sufficiently increase the density of infrared light applied to a cavity (not shown) in the photoacoustic gas sensor, and a great effect can be obtained practically as an infrared light pulse irradiation light source for the photoacoustic gas sensor.

The present invention is not limited to the above embodiment. For example, the size of the light source for the photoacoustic gas sensor and the emission intensity (irradiation intensity) of infrared light from the thin film heater 14 may be determined according to the size of the cavity in the photoacoustic gas sensor, its gas detection sensitivity, and the like. . For example, the thin-film heater 14 generates heat at about 800 ° C. at 150 mW, and operates at 700 ° C. by driving at 100 Hz.
It suffices to be able to irradiate the modulated infrared light with the above temperature difference. The light source may be provided integrally with the photoacoustic cell including the chamber in the photoacoustic gas sensor, or may be provided as a separate component provided as a pair with the photoacoustic cell. .

In the case where the infrared light reflecting film 15 is formed on the inner wall surface of the concave portion 12 forming the lower space of the thin film heater 14,
It is not necessary to use a semiconductor substrate 11 that is transparent to infrared light. The insulating layer such as the SiN _x film is formed on the semiconductor substrate 11, it is also possible to form a thin film heater 14 on the insulating layer. In addition, the present invention can be variously modified and implemented without departing from the gist thereof.

[0021]

As described above, according to the present invention, the infrared light emitted by the thin film heater is reflected upward on the back side of the thin film heater formed in a bridge on the semiconductor substrate. Even if the light source itself is small and the intensity of the infrared light emitted by the thin film heater is limited, the infrared light reflecting film is provided. Can be irradiated. Moreover, it has a simple configuration, and can provide a great effect in practical use, such as being suitable for realizing a small-sized photoacoustic sensor using micromachining technology.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a basic structure of a photoacoustic sensor light source according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of forming an infrared light reflection film in the photoacoustic sensor light source according to the embodiment of the present invention.

FIG. 3 is a view schematically showing a light collecting action of infrared light by an infrared light reflecting film.

FIG. 4 is a diagram showing a schematic structure of a photoacoustic sensor.

[Explanation of symbols]

 Reference Signs List 11 semiconductor substrate (Si substrate) 12 concave portion 13 insulating thin film bridge 14 thin film heater 15 infrared light reflecting film 16 optical filter

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takashi Kihara 2-12-19 Shibuya, Shibuya-ku, Tokyo F-term (reference) 2G047 AA01 CA04 EA05 EA14 EA15 GD02 2G059 AA01 BB01 CC04 EE16 GG00 GG06 HH01 JJ02 JJ13

Claims (6)

[Claims] 1. A photoacoustic gas sensor light source for modulating and irradiating infrared light into a cavity into which a gas is introduced, comprising: a semiconductor substrate having a concave portion formed on a surface thereof; A light, comprising: a thin-film heater which is provided to form a bridge and outputs infrared light; and an infrared light reflecting film coated on an inner wall surface of the concave portion and / or a back surface of the substrate. Light source for acoustic gas sensors. 2. The light source for a photoacoustic gas sensor according to claim 1, wherein the semiconductor substrate is a Si substrate transparent to infrared light, and the infrared light reflecting film is made of a thin film of Au or Al. . 3. A first step of forming a thin film heater on a semiconductor substrate via an insulating layer having a predetermined width, and selectively etching a surface of the semiconductor substrate while leaving a portion where the thin film heater is formed. Forming a concave portion under the thin film heater leaving the thin film heater as a bridge;
And a third step of electroplating the semiconductor substrate to form an infrared light reflecting film on the inner wall surface of the concave portion. A method of manufacturing a light source for a photoacoustic gas sensor, the method comprising: 4. A first step of forming a thin film heater on a semiconductor substrate transparent to infrared light via an insulating layer having a predetermined width, and a surface of the semiconductor substrate except for a portion where the thin film heater is formed. Is selectively etched to form a recess below the thin film heater, leaving the thin film heater as a bridge.
And a third step of coating an infrared light reflecting film on the back surface of the semiconductor substrate. A method for manufacturing a light source for a photoacoustic gas sensor, comprising: 5. The coating of the infrared light reflecting film on the back surface of the semiconductor substrate, the side surface of the semiconductor substrate is etched to process the back surface side of the semiconductor substrate into a trapezoidal shape or a pot bottom shape, and then the trapezoidal surface is formed. 5. The method of manufacturing a light source for a photoacoustic gas sensor according to claim 4, wherein the method is performed over the entire area of the bottom surface of the pan. 6. The method according to claim 3, wherein in the second step, the surface of the semiconductor substrate is anisotropically or isotropically etched using the insulating layer on which the thin film heater is formed as a part of a mask. 5. The method for producing a light source for a photoacoustic gas sensor according to item 4.