JP2002328116A - Photoacoustic gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small photoacoustic gas sensor of simple structure enhanced in absorption efficiency of an infrared ray by detection-objective gas inside a cavity irradiated by the infrared ray of a limited intensity, and enhanced in gas detection sensitivity. SOLUTION: This sensor is provided with the cavity provided with a gas flow port 20 for introducing the outside atmospheric air and an infrared ray introducing window 30, a gas diffusion filter 40 comprising a porous silicon layer provided integrally with the cavity in the gas flow port, a microphone 50 for detecting sound pressure inside the cavity by providing the first corrugated electrode 51 serving as one portion of an inner wall face of the cavity, and a light source 60 for pulse-irradiating the infrared ray into the cavity via the infrared introducing window. In particular, an infrared ray reflecting film 70 comprising Au, Al or the like is coated on an area other than the gas flow port and the infrared ray introducing port in the inner wall face of the cavity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small-sized photoacoustic gas sensor having a simple structure and improved gas detection sensitivity.

[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. This type of photoacoustic gas sensor basically includes, as shown in FIG. 10, a cavity 1 into which gas (air) is introduced, a light source 2 for irradiating infrared light into the cavity 1, and a wall surface of the cavity 1. And a microphone 3 which is provided as a part (ceiling surface) and 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 further provided with a gas passage 4 for introducing outside air (gas) into the cavity 1. Having a structure. The gas passage 4 is usually provided with a gas diffusion filter 5. The gas diffusion filter 5 restricts the gas flow, thereby maintaining the gas (air) flow (replacement) between the inside and the outside of the cavity 1 while absorbing the infrared light. And changes the sound pressure in the cavity 1 according to the thermal expansion of CO ₂ (gas).

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]

However, when a small-sized photoacoustic gas sensor is manufactured by processing a Si substrate using a micromachining technique, various problems occur due to the limited internal volume of the cavity 1. Occurs. For example, there arise problems such as how to realize the gas diffusion filter 5 having desired filter characteristics (pressure loss) and how to enhance the efficiency of absorbing infrared light by CO ₂ in the cavity 1. . In addition, since the light source 2 is downsized in accordance with the downsizing of the photoacoustic gas sensor, generally, the infrared light intensity of the light source 2 also becomes small. For this reason, there arises a problem that the detection sensitivity decreases as the size of the photoacoustic gas sensor decreases.

The present invention has been made in view of such circumstances, and has as its object to reduce the absorption efficiency of infrared light by a gas to be detected in a cavity irradiated with infrared light of limited intensity. It is an object of the present invention to provide a small-sized photoacoustic gas sensor having a simple structure with a high gas detection sensitivity.

[0007]

In order to achieve the above object, a photoacoustic gas sensor according to the present invention is a small-sized one manufactured by processing a Si substrate or the like by using, for example, a micromachining technique, A cavity provided with a gas inlet and an infrared light introduction window for introducing a gas, a gas diffusion filter provided integrally with the cavity in the gas inlet, and a part of an inner wall surface of the cavity provided. A microphone for detecting the sound pressure in the cavity;
And a light source for irradiating the cavity with infrared light through the infrared light introduction window, particularly in an area excluding the gas flow opening and the infrared light introduction window on the inner wall surface of the cavity. It is characterized by coating with an infrared light reflection film.

Preferably, the cavity is formed by etching a Si substrate having a predetermined thickness to form a concave space having a predetermined internal volume, and anodic oxidizing a part of the wall to form a porous silicon layer forming the gas diffusion filter. It is realized as what was done. The microphone is provided with one of a pair of electrodes facing each other so as to cover the recessed space of the Si substrate, thereby forming a ceiling surface facing the infrared light introduction window of the cavity and integrally forming the ceiling with the cavity. Provided.

The infrared light reflecting film is made of, for example, a thin film of Au or Al, and has an inner wall surface excluding the gas diffusion filter made of the porous silicon layer of the Si substrate in which the concave space is formed, and a wall surface of the cavity. Is formed on the electrode surface of the microphone by vapor deposition or sputtering, for example, to a thickness of several μm. Also preferably, the microphone electrode on which the infrared light reflecting film is formed by forming a ceiling surface of the cavity has a waveform shape, and multiple-reflects the infrared light introduced from the infrared light introducing window. It is desirable to configure as follows.

[0010] Further, for the light source comprising, for example, a thin film heater formed on a bridge provided over a recess formed on the surface of the semiconductor substrate, the inner wall surface of the recess and / or the semiconductor An infrared light reflection film coated on the back surface of the substrate is provided, and the infrared light emitted from the thin film heater to the back surface side is reflected and condensed to irradiate the infrared light introduction window of the cavity. It is preferable to configure as follows.

At this time, the light source is thermally isolated from the cavity and provided so as to face the infrared light introducing window of the cavity so that undesired heat transfer from the light source to the cavity does not occur. Is desirable.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A photoacoustic gas sensor according to one embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing a schematic configuration of the photoacoustic gas sensor according to the embodiment. The photoacoustic gas sensor generally includes a gas inlet 20 for introducing outside air and an infrared light introduction window 3.
0, a gas diffusion filter 40 provided integrally with the cavity 10 in the gas flow opening 20, and a part of the inner wall surface of the cavity 10 provided. A microphone 50 for detecting the sound pressure in the cavity 10, and a light source 6 for irradiating the inside of the cavity 10 with a pulse through the infrared light introduction window 30.
0.

The cavity 10 and the microphone 5
The light source 60 and the light source 60 are manufactured by using a micromachining technique, for example, by etching a Si substrate having a predetermined thickness transparent to infrared light. In particular, the microphone 50 is manufactured integrally with the cavity 10 when the cavity 10 is manufactured. The method for manufacturing these components will be described later.

The cavity 10 is formed, for example, by anisotropically etching two Si substrates 11 and 12 each having a thickness of 500 μm, for example.
After forming the internal spaces 3, 14, the concave portion (hole portion) 1 is formed.
The three Si substrates 11 and 12 are abutted against each other.
The structure is integrated by semiconductor bonding between them. Then, the opening of one Si substrate 11 is closed, and the microphone 50 is closed.
Are formed integrally, and an optical filter forming an infrared light introducing window 30 is provided in the opening of the other Si substrate 12 to form an internal space having a predetermined internal volume.

The gas inlet 20 is formed, for example, on the joint surface of the Si substrate 12 on the side where the infrared light introducing window 30 is provided and the Si substrate 11 on the side where the microphone 50 is integrally formed. Is realized as a groove having a predetermined width. In particular, in this embodiment, the gas flow openings (grooves) 20
As described later, a gas diffusion filter 40 is formed integrally with a porous silicon layer by partially anodic oxidizing the Si substrate 12 as described later. The gas flow port 20 integrally provided with such a gas diffusion filter 40 is formed by, for example, providing a hole equivalent to the concave portions (holes) 13 and 14 on a Si substrate, thereby forming the cavity 10. It may be provided on a spacer (not shown) which is inserted between the two Si substrates 11 and 12 and forms a part of the cavity 10.

The microphone 50 is connected to the Si.
When the concave portion (hole portion) 13 is formed in the substrate 11, it is formed on the Si substrate 11. The microphone 50 includes a first electrode 51 provided so as to cover a concave portion (hole) 13 opened in the Si substrate 11, and the first electrode 51.
And a back plate 53 that supports the second electrode 52 from the upper surface side. The back plate 53 and the second electrode 52 are provided with a plurality of acoustic holes 54 communicating with the space between the first electrode 51 and the second electrode 52.

In the photoacoustic gas sensor basically configured as described above, the photoacoustic gas sensor according to this embodiment is characterized in that the gas passage 20 on the inner wall surface of the cavity 10 and the infrared light Introduction window 30
Is coated with an infrared light reflecting film 70 made of Au, Al, or the like. This infrared light reflection film 70
Is formed to a thickness of several μm by vapor deposition, sputtering, or the like, and reflects infrared light pulse-irradiated into the cavity 10 from the light source 60 through the infrared light introduction window 30 to form the cavity 10. It plays the role of lengthening the propagation optical path length of infrared light in the interior.

Particularly, in this embodiment, the inner wall surface of the cavity 10 is formed as a slope having a (111) plane by anisotropic etching of a Si substrate having a (100) plane as a main surface, for example. The electrode surface of the microphone 50 provided as the ceiling surface of the cavity 10 is also provided in a wavy shape using anisotropic etching of the Si substrate as described later. By providing infrared light reflecting films 70 on the inner wall surface of the cavity 10 forming the above slope and the electrode surface (the ceiling surface of the cavity 10) of the microphone 50 forming a waveform, respectively, the red light irradiated into the cavity 10 is irradiated with a pulse. External light is randomly and multiple-reflected in the cavity 10.

On the other hand, the light source 60 has a structure in which a thin film heater 64 is formed on, for example, an insulating thin film bridge 63 provided over a concave portion 62 formed on the surface of a Si substrate 61, for example. The inner wall surface of the concave portion 62 of the light source 60 is also coated with an infrared light reflection film 65 made of Au, Al, or the like. This infrared light reflection film 65 is formed by the thin film heater 64.
It plays a role of reflecting and condensing the infrared light emitted from the rear surface side of the cavity and irradiating the infrared light introduction window 30 of the cavity 10 with the infrared light.

Such an infrared light reflecting film 65 is formed by
2 is formed on the inner wall surface of the insulating thin film bridge 63 having the thin film heater 64 formed over the upper surface of the concave portion 62.
Is provided, it is ensured that the infrared light reflecting film 6 is also provided in a region immediately below the insulating thin film bridge 63 in the concave portion 62.
In order to provide 5, the infrared light reflection film 65 is preferably formed by electrolytic plating.

It should be noted that the infrared light reflecting film 65 is
Even when the thin film heater 64 is formed on the back surface side, the light condensing function for infrared light emitted from the thin film heater 64 to the back surface side can be similarly obtained. In this case, Au is provided on the back side of the Si substrate 61.
Or Al or the like may be deposited (or sputtered).
At this time, the side surface of the Si substrate 61 may be anisotropically etched, and the back surface side of the Si substrate 61 may be formed in a trapezoidal shape surrounding the thin film heater 64 or in a pot shape by isotropic etching. In addition, it is possible to enhance the efficiency of collecting infrared light by the infrared light reflection film 65 formed on the back surface side.

Here, the photoacoustic gas sensor having the above-described structure will be described together with the method of manufacturing each part. FIG.
(a) to (i) show the cavity 10 and the microphone 50.
Is shown in an exploded manner. This microphone 50 is connected to the first Si constituting the cavity 10.
It is formed integrally with the substrate 11. So first, cavity 1
For example, a Si wafer having a (100) plane as a main surface is prepared as a Si substrate 11 for forming a 0, and a thermal oxide film 15 is grown on the Si substrate 11 as shown in FIG. ,
A rectangular hole 16 is formed in the thermal oxide film 15 at a predetermined opening pitch by using photolithography.

The rectangular hole 1 provided in the thermal oxide film 15
For example, as shown in FIG. 3 (a), small holes 6 may be arranged vertically and horizontally at equal intervals, or a rectangular hole may be formed as shown in FIG. 3 (b). May be provided in parallel, or as shown in FIG. 3 (c), a frame shape in which the shape of the hole is gradually elongated from the center to the outside. In short, as described later, when the Si substrate 11 is anisotropically etched using the thermal oxide film 15 as a mask, a mask pattern may be formed so that unevenness having a uniform waveform shape is formed on the surface thereof. .

Thereafter, using the thermal oxide film 15 as a mask, the Si substrate 11 is anisotropically etched using KOH or TMAH (trimethylammonium hydride). Then, as shown in FIG.
A plurality of recesses 17 having a triangular cross section are formed on the surface of the microphone 5 in accordance with the above-described microphone pattern shape.
A waveform shape defining the shape of the first electrode 51 at 0 is formed on the surface of the Si substrate 11.

Next, the thermal oxide film 1 used as the mask
5 is removed once, and then the Si substrate 1 is removed as shown in FIG.
1 to protect the surface of the Si substrate 11 and to insulate the first electrode 51 from the Si substrate 11 over the entire surface of the Si substrate 11.
A thermal oxide film (not shown) such as O ₂ is formed. Then, a first electrode 51 made of, for example, Cr / Au / Cr is formed on this thermal oxide film (SiO ₂ ) to a thickness of 4/20/4 nm, and a photosensitive film used as a diaphragm on the first electrode 51 is formed. A polyimide (not shown) is formed to a thickness of 1 μm. That is, along one surface of the Si substrate 11 processed into the corrugated shape,
A first electrode 51 lined with a diaphragm is formed in a waveform shape insulated from the Si substrate 11.

Thereafter, on the upper surface of the first electrode 51,
As shown in FIG. 2D, Al serving as the sacrificial layer 55 is formed to a thickness of 2 μm. The sacrificial layer (Al) 55 plays a role in defining the distance (gap) between the second electrode 52 and the first electrode 51 in the microphone 50, and the thickness thereof is reduced by vacuum evaporation or sputtering. Formed under control.

Thereafter, as shown in FIG. 2E, a second electrode 52 made of, for example, Au / Cr is formed on the sacrificial layer (Al) 55, and the second electrode 52 is covered with the second electrode 52. 2 (f)
As shown in FIG. 7, a photosensitive polyimide forming the back plate 53 is formed to a thickness of about 10 to 20 μm. Note that the second
The electrode 52 is made of Au / Cr in consideration of the resistance at the time of removal of the sacrificial layer 55 by etching, which will be described later, but has a resistance to the etching liquid of the sacrificial layer (Al) 55. If so, it is of course possible to use other electrode materials.

Thereafter, a predetermined opening is formed by patterning the back plate 53 made of photosensitive polyimide, and the second electrode 52 is selectively etched by using the back plate 53 as a mask, thereby obtaining a structure shown in FIG. As shown in FIG. 7, a plurality of acoustic holes 54 that function to release air from the microphone 50 are formed. These acoustic holes 54 are also used for etching and removing the sacrificial layer (Al) 55 described above.

Next, a thermal oxide film 18 is grown on the back surface of the Si substrate 11, and a rectangular hole is formed in the thermal oxide film 18 using photolithography. And this thermal oxide film 1
Using the mask 8 as a mask, the Si substrate 11 is anisotropically etched from the back side using KOH or TMAH (trimethylammonium hydride) to expose the first electrode (diaphragm) 51, as shown in FIG. As shown, a recess (hole) 13 forming the cavity 10 is formed inside the Si substrate 11. Then, Au is sputtered into the cavity 10 from the back surface side of the Si substrate 11 so that the exposed surface of the first electrode (diaphragm) 51 and the inner wall surface of the Si substrate 11 where the cavity 10 is formed have a red thickness of several μm. An external light reflection film 70 is formed.

Thereafter, through the acoustic hole 54, FIG.
The sacrificial layer 55 is removed by etching as shown in FIG.
By forming a predetermined gap (space) between the first electrode 51 and the second electrode 52, the microphone 50 integrated with the cavity 10 is completed. By the way, Al
The sacrificial layer 50 made of is etched away by using, for example, a mixed solution of phosphoric acid and nitric acid (70 ° C.) as an etching solution.
This is performed using

On the other hand, the second
Although not shown, a thermal oxide film is grown on one surface of the Si substrate 12, and a rectangular hole is formed in the thermal oxide film using photolithography. This thermal oxide film corresponds to the thermal oxide film 18 formed on the back surface of the first Si substrate 11. Using this thermal oxide film as a mask, K
The second Si substrate 21 is anisotropically etched using OH or TMAH (trimethylammonium hydride) to form a concave portion (hole) 14 forming the cavity 10 inside the Si substrate 12. Then this second S
Au is sputtered on the inner surface of the
An infrared light reflecting film 70 having a thickness of several μm is formed on the inner wall surface of the concave portion (hole portion) 14 formed in the Si substrate 12.

In processing the second Si substrate 12 in this manner, for example, as shown in FIG. 4, a gas flow port 20 is formed on the joint surface side with the first Si substrate 11, and The gas diffusion filter 40 is integrally embedded in the gas flow port 20. Specifically, for example, as shown in FIG. 4A, the surface (back surface) of the second Si substrate 12 having the concave portion (hole) 14 opposite to the surface on which the gas diffusion filter 40 is formed (back surface) First, Al is deposited (or sputtered) and then heat-treated at about 500 ° C. in a vacuum. This Al film is used as an electrode contact at the time of anodic oxidation described later.

Next, the gas diffusion filter 4 of the Si substrate 12
For example, a photosensitive polyimide is spin-coated on the surface (surface) on the side where 0 is to be formed, and the photosensitive polyimide is selectively removed by irradiating the photosensitive polyimide with ultraviolet light.
The portion of the i-substrate 12 where the gas passage 20 is formed is exposed. Thereafter, using the photosensitive polyimide as a mask, the Si substrate 12 is anodized to make it porous, and FIG.
As shown in FIG. 3A, a porous silicon layer 41 forming a gas diffusion filter 40 is formed.

The selective anodic oxidation of the Si substrate 12 is as follows:
As described above, the Si substrate 12 on which the Al film is formed is mechanically covered or protected with a tape or the like, and the Si substrate 12 is used as an anode and a reference electrode (not shown) as a cathode. At the same time, it is immersed in a hydrofluoric acid solution and a current of a predetermined density is supplied to carry out the operation. At this time, it is preferable to remove the hydrogen generated during the anodic oxidation of the Si substrate 12 by adding ethanol to the hydrofluoric acid solution. Here, photosensitive polyimide is used as a mask for the Si substrate 12, but it goes without saying that a noble metal or other polymer material may be used.

By forming the porous silicon layer 13 by such partial anodic oxidation of the Si substrate 12,
The gas passage opening 20 in which the porous silicon layer 41 is integrally embedded on the upper surface of the i-substrate 12 is formed in a groove. At this time, it is desirable to adjust the length (the thickness of the filter) of the porous silicon layer 41 so that the gas diffusion filter 40 has desired filter characteristics.

In this case, for example, a resist such as SiO ₂ is applied on the porous silicon layer 41 and the resist film is patterned by photolithography or the like to leave a resist film only in a region where the gas diffusion filter 40 is to be formed. . Then, using the resist film (SiO ₂ ) as a mask, the porous silicon layer 41 is selectively etched by using a weakly alkaline etching solution composed of, for example, a 1% aqueous solution of NaOH, or by dry etching, as shown in FIG. The length of the porous silicon layer 41 (the thickness of the filter) is adjusted as shown in FIG. Thus, the gas diffusion filter 40 having desired filter characteristics is realized.

The gas diffusion filter 40 may be formed, for example, on the second Si substrate 12 constituting the cavity 10 as shown in FIG. That is, as shown in FIG.
On both surfaces of the i-substrate 12, protective films 21 and 22 made of a hydrofluoric acid-resistant insulating film such as a SiN _X film are provided. Then, as shown in FIG. 5B, a protective film (SiN _x film) 21 on the lower surface side of the Si substrate 12 is formed.
After forming a resist film 23 thereon and patterning the resist film 23, the resist film 23 is used as a mask to form a concave portion (hole portion) 14 forming the cavity 10 in the protective film (SiN _X film) 21. Make an opening. After a while
Using the resist film 23 as a mask, the Si substrate 12 is deep-etched in the vertical direction from the back side thereof, as shown in FIG.
As shown in (c), the cavity 1 is formed on the back side of the Si substrate 12.
A concave portion (hole portion) 14 forming 0 is formed.

Next, a resist film 24 is formed on the surface of the Si substrate 12, and the resist film 24 is patterned. This patterning is performed by the above-described recess (hole) 14.
This is performed by selectively removing the resist film 24 so as to cover the portion facing the substrate and to leave the protective film (SiN _x film) 22 in a region slightly larger than the size of the concave portion (hole portion) 14. Thereafter, selectively removing the protective film (SiN _X film) 22 to indicate the resist film 24 in FIG. 5 (d) as a mask.

Then, using the resist film 24 as a mask, the Si substrate 12 is deep-etched vertically from its surface side, and as shown in FIG.
(Hole) 1 forming the cavity 10 on the surface side of
The groove-shaped concave portion 25 is formed outside of the groove 4. The groove-shaped recess 25 forms a gas flow port 20 for the cavity 10 formed by the Si substrate 12.
Further, a partition wall 26 having a predetermined thickness is formed between the concave portion 25 and the concave portion (hole portion) 14 forming the cavity 10. Incidentally, the thickness of the partition 26 is defined by the pattern shape of the mask formed by the resist films 23 and 24 described above, and is set according to the filter length required for the gas diffusion filter 40.

Thereafter, the above-mentioned protective film (SiN _x film) 2
While leaving 1,22, the Si substrate 12 is immersed in a hydrofluoric acid solution to anodize the partition 26. As shown in FIG. 5 (f), the anodic oxidation is performed, for example, in such a manner that the hydrofluoric acid solutions existing on both surfaces of the Si substrate 12 do not come into contact with each other. Sealing is performed by applying a current of a predetermined density between a pair of electrodes provided on both surfaces of the Si substrate 12 in the hydrofluoric acid solution.

Then, this current is applied to the protective film (SiN _x film) 2
Since the Si substrate 12 is insulated at the portion covered with the first and second 22, the Si substrate 12 flows through the partition 26 where the protective films (SiN _x films) 21 and 22 do not exist. As a result, FIG.
As shown in (g), the Si portion forming the partition wall 26 is locally anodized to be porous and becomes a porous porous silicon layer 27.

When anodizing the partition 26, depending on the anodizing conditions, a current may be applied while irradiating light. When the partition wall 26 is anodized, the anodic oxidation current hardly flows through the inner wall surface without masking the inner wall surface of the groove-shaped concave portion 25 forming the gas flow passage 20. There is no danger of anodizing up to the inner wall surface of the gas flow path 25 (gas flow path 20). The partition wall 26 formed as the porous silicon layer 27 in this manner is used as a gas diffusion filter 40 having a predetermined thickness integrally formed with the Si substrate 12 forming the cavity 10.

When the infrared light reflecting film 70 is coated on the wall surface of the concave portion (hole portion) 14 forming the cavity 10 of the second Si substrate 12 in which the gas diffusion filter 40 is integrally formed as described above. Is, for example, FIG.
As shown in (g), a wall 28 is provided at a position facing the formation site of the gas diffusion filter 40, and the Si substrate 12
Au, Al, or the like may be deposited (or sputtered) from the obliquely lower side of the back side of the above. Incidentally, the wall 28 may be formed to have a predetermined height at the same time when the recess (hole) 14 is formed by etching.

Then, the gas diffusion filter 40 formed as a part of the inner wall surface of the cavity 10 is shielded from the source such as Au or Al by the wall 28, so that the infrared light reflection film No 70 is formed.
Therefore, the infrared light reflection film 70 can be formed on the entire inner wall surface of the cavity 10 except for the portion where the gas diffusion filter 40 is formed.

Thereafter, the second Si substrate 12 on which the gas diffusion filter 40 is integrally formed as described above, and the first Si substrate 12 on which the microphone 50 is integrally formed as described above.
The substrate 11 and the substrate 11 are attached to each other and joined and integrated. Further, an optical filter forming an infrared light introducing window 30 covering the opening of the second Si substrate 12 is joined to and integrated with one open surface of the second Si substrate 12, thereby forming a cavity 10 having a predetermined space (sample chamber). Is completed. In addition, regarding the gas diffusion filter 40, the first S in which the microphone 50 is integrally formed.
Of course, it can be provided on the i-substrate 11 side. In this case, as shown in FIG. 6, when the first Si substrate 11 is etched, if the above-described wall 28 is formed, the gas diffusion is performed on the entire inner wall surface of the cavity 10 including the wall 28. Except for the filter surface of the filter 40, the infrared light reflection film 70 of Au, Al, or the like can be coated.

Thus, according to the photoacoustic gas sensor configured as described above, the inside of the cavity 10 has the same structure as that of the gas acoustic outlet 20 and the infrared light introducing window 30 except that the gas diffusion filter 40 is integrally provided. The light source 60 is coated with the infrared light reflection film 70 made of Au, Al, or the like.
The infrared light applied to the inside of the cavity 10 is multiple-reflected inside the cavity 10. Therefore, the propagation optical path length of the infrared light in the cavity 10 can be made sufficiently long, and the gas (C
It becomes possible to effectively increase the absorption efficiency by O ₂ ). Therefore, even when the intensity of the infrared light pulsed from the light source 60 is limited, the thermal expansion of the gas (CO ₂ ) due to the absorption of the infrared light can be sufficiently increased,
The detection sensitivity of the microphone 50 can be sufficiently increased.

Further, as described above, since the detection sensitivity of the photoacoustic gas sensor can be increased by a simple configuration in which the inside of the cavity 10 is coated with the infrared light reflection film 70, the practical advantage is enormous. . In addition, since the coating of the inside of the cavity 10 with the infrared light reflection film 70 can be easily performed as described above, the industrial advantage is enormous also in this respect. Furthermore, the first electrode 5 of the microphone 50 forming the ceiling surface of the cavity 10
Since 1 has a waveform shape, infrared light can be reflected at random, and multiple reflection of infrared light in the cavity 10 can be effectively generated. Therefore, the first electrode 51 having the above-described waveform shape greatly contributes to enhancing the detection sensitivity.

Considering the various specifications required for a small-sized photoacoustic gas sensor manufactured using the micromachining technique as described above, the infrared light for thermally expanding a gas such as CO2 is considered. As a modulation frequency, in order to avoid a frequency region with much air conditioning related noise,
For example, it is desirable to set it to 100 Hz or more. For this purpose, the thin film heater 64 of the light source 60 is required to have a thermal time constant of 1.6 mSec or less.
The thermal time constant of the thin-film heater 64 is considered to be roughly proportional to a value obtained by dividing the heat capacity of the thin-film heater 64 by the degree of escapability of heat from the thin-film heater 64 (thermal conduction, convection, and radiation). I can do it. Therefore, in order to realize the above-described high-speed modulation of infrared light, it is necessary to reduce the heat capacity of the thin-film heater 64 as much as possible and to increase the degree of easiness of heat dissipation.

Incidentally, regarding the heat capacity, the thin film heater 6
By reducing the thickness of 4, the surface area can be reduced while maintaining its surface area. However, increasing the degree of escape of heat leads to an increase in power consumption, which is not preferable from the viewpoint of reducing the thermal time constant. Therefore, from the viewpoints of high-speed driving, high efficiency, and long-term stability, if the thin-film heater 64 is formed of single-crystal silicon as shown in the above-described embodiment, for example, the thermal time constant is 1 mSe
c and a light source 6 that generates heat at about 800 ° C. at 150 mW
0 can be realized. Further, according to such a thin film heater 64, it becomes possible to obtain pulsed light (infrared light) having a temperature difference of 700 ° C. or more when driven at 100 Hz.

On the other hand, the frequency response due to the escape of heat in the cavity 10 is determined depending on the internal volume of the cavity 10 by heat transfer or heat conduction. Accordingly, in the small-sized photoacoustic gas sensor as shown in the above-described embodiment, the internal volume of the cavity 10 is sufficiently small, and the frequency response may be handled by heat transfer. When the frequency characteristics of the sound pressure in the cavity 10 due to the escape of heat are replaced with an electric system, the frequency characteristics can be represented by, for example, an equivalent circuit as shown in FIG.
The thermal time constant T of the air within 0 becomes a first-order low-pass filter.

However, the thermal time constant T is represented by the heat capacity C of air,
It is indicated by a multiplier R for escaping heat to the Si substrate (housing),
R and C are respectively R = 1 / hS, C = Cvρ
Given as V. In the above equation, h is the natural convection heat transfer coefficient of air, S is the area of the inner wall surface in the cavity 10, Cv is the specific heat of constant volume of air, ρ is the density of air, and V is the internal volume of the cavity 10.

The frequency response to a change in sound pressure in the cavity 10 can be schematically represented as a first-order low-pass filter characteristic with the straight lines A and B asymptotically as shown in FIG. Incidentally, the cutoff frequency fc indicated by the low-pass filter characteristic is given as fc = 1 / 2πRC. And even if the values of R and C are changed,
The cutoff frequency fc itself does not change,
Only the level of the straight line A changes, and the characteristic indicated by the straight line B does not change. Therefore, the light source 60
If the driving frequency is set to be equal to or higher than the cutoff frequency fc, the sound pressure for the driving frequency changes along the straight line B. Further, in order to enhance the characteristic itself represented by the straight line B and increase the sound pressure change (sensitivity), the energy change ΔIR of the infrared light corresponding to the current i in the electric system.
There is no other way but to increase E.

When the cavity 10 is regarded as a 1 mm square cube, its thermal time constant T is 0.0.
7 Sec. From T = CvρV / hS, fc = 1 / 2πT, the cutoff frequency fc is obtained as 2.3 Hz. Therefore, if the light source 60 is modulated and driven at 70 Hz or more as described above, even if the internal volume of the cavity 10 is small, the change in sound pressure can be detected with sufficiently high sensitivity. Also, since noise is inversely proportional to the modulation drive frequency, it can be said that good S / N can be detected.

On the other hand, with respect to the gas diffusion filter 40 described above, it is required that the internal pressure in the cavity 10 can be sufficiently increased, and that the gas between the outside air and the cavity 10 can be quickly replaced. Is required to be able to be shut off. Incidentally, the holding performance of the internal pressure in the cavity 10 is a primary high-pass filter having a time constant T determined by the pressure loss coefficient K of the gas diffusion filter 40 and the internal volume V of the cavity 10. Is given by an equivalent circuit as shown in FIG. The pressure loss coefficient K of the gas diffusion filter 40 can be set as a resistance in an equivalent circuit, and the parameter of the internal volume V of the cavity 10 is the acoustic compliance obtained by dividing the bulk modulus of air by the internal volume V. Can be considered and placed as a capacitor in an equivalent circuit.

The gas diffusion filter 4 used in a small photoacoustic gas sensor having a small internal volume V of the cavity 10
In the case of 0, the time constant T becomes small, and the cutoff frequency becomes, for example, about 83 Hz, which is quite close to the pulse driving frequency of 100 Hz of the infrared light. In addition, the performance of retaining the internal pressure in the cavity 10 is also deteriorated. Therefore, in order to solve such a problem, it is necessary to use the gas diffusion filter 40 having a high pressure loss coefficient K. Incidentally, the pressure loss coefficient K is inversely proportional to the area of the gas diffusion filter 40. Therefore, as shown in the above-described embodiment, a part of the Si substrate 12 forming the cavity 10 is anodized to make it porous, and the gas diffusion filter 40 can be made small as a porous silicon layer integrated with the cavity 10. For example, it is possible to realize a desired filter characteristic by increasing the pressure loss coefficient K.

As shown in the above discussion, according to the present invention, a small photoacoustic gas sensor having a cavity 10 with a small internal volume is realized by, for example, etching a Si substrate using micromachining technology. However, the basic performance can be sufficiently maintained. In addition, an infrared light reflection film 70 is formed on the inner wall surface of the cavity 10, and the infrared light introduced into the cavity 10 is multiply reflected to increase the absorption efficiency of the infrared light by gas (CO ₂ ). , The detection sensitivity can be increased.

The present invention is not limited to the above embodiment. For example, the manufacturing method of each part constituting the photoacoustic gas sensor and the shape and size thereof may be determined according to the specifications. The means for forming the infrared light reflection film 70 is not particularly limited. Also, the light source 60 may be formed integrally with the cavity 10. In short, various modifications can be made without departing from the scope of the invention.

[0058]

As described above, according to the present invention, since the infrared light reflecting film coated on the inner wall surface of the cavity is provided, the infrared light introduced into the cavity is multiply reflected to obtain the gas (CO). ₂ ) It is possible to increase the absorption efficiency of infrared light by means of ₂ ), and it is possible to realize a small-sized photoacoustic gas sensor with a simple configuration in which the sound pressure change due to thermal expansion is increased and the detection sensitivity is increased. The benefits are significant.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a schematic configuration of a photoacoustic gas sensor according to an embodiment of the present invention.

FIG. 2 is an exploded view showing an example of a manufacturing process of the cavity 10 and the microphone 50 in the photoacoustic gas sensor shown in FIG.

FIG. 3 is a diagram showing an example of a mask used to form a first electrode 51 of a microphone 50 into a waveform.

4 is an exploded view showing an example of a manufacturing process of the gas diffusion filter 40 in the photoacoustic gas sensor shown in FIG.

FIG. 5 is an exploded view showing another example of the manufacturing process of the gas diffusion filter 40 in the photoacoustic gas sensor shown in FIG.

FIG. 6 is a sectional view showing a schematic configuration of a photoacoustic gas sensor according to another embodiment of the present invention.

FIG. 7 is an electrical equivalent circuit showing the thermal characteristics of the cavity.

FIG. 8 is a diagram showing a frequency response characteristic with respect to a change in sound pressure in a cavity.

FIG. 9 is an electrical equivalent circuit showing characteristics of a gas diffusion filter.

FIG. 10 is a diagram showing a general configuration of a photoacoustic gas sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Cavity 20 Gas inlet 30 Infrared light introduction window (optical filter) 40 Gas diffusion filter 50 Microphone 51 First electrode 60 Light source 64 Thin film heater 70 Infrared light reflection film

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takashi Kihara 2-12-19 Shibuya, Shibuya-ku, Tokyo F-term (reference) 2G047 AA01 CA04 EA15 GB11 GD02 2G059 AA01 BB01 CC04 DD12 DD13 GG08 HH01 JJ02 KK08 LL02 LL03

Claims (6)

[Claims] A cavity provided with a gas passage for introducing outside air and an infrared light introduction window; a gas diffusion filter provided integrally with the cavity at the gas passage; and an inner wall surface of the cavity. A microphone that is provided as a part and detects a sound pressure in the cavity; a light source that irradiates infrared light modulated into the cavity through the infrared light introduction window; A photoacoustic gas sensor comprising: a gas flow opening; and an infrared light reflecting film coated in a region excluding an infrared light introduction window. 2. The cavity is formed by etching a Si substrate having a predetermined thickness to form a concave space, and anodizing a part of a wall surface thereof to form a porous silicon layer constituting the gas diffusion filter. The microphone according to claim 1, wherein one of a pair of electrodes facing each other is provided so as to cover a concave space of the Si substrate, and a ceiling surface facing the infrared light introduction window of the cavity is formed. The photoacoustic gas sensor according to any one of the preceding claims. 3. The infrared light reflecting film is made of a thin film of Au or Al, and has an inner wall surface except for a gas diffusion filter made of the porous silicon layer of the Si substrate in which the concave space is formed, and a wall surface of the cavity. The photoacoustic gas sensor according to claim 2, wherein the photoacoustic gas sensor is provided on an electrode surface of the formed microphone. 4. The photoacoustic gas sensor according to claim 3, wherein an electrode of the microphone, which forms a ceiling surface of the cavity and is coated with the infrared light reflecting film, has a waveform shape. 5. The thin film heater formed on a bridge provided over a concave portion formed on the surface of the semiconductor substrate, and the light source is coated on an inner wall surface of the concave portion and / or a back surface of the semiconductor substrate. The photoacoustic gas sensor according to claim 1, further comprising an infrared light reflection film, wherein the photoacoustic gas sensor is provided so as to be thermally isolated from the cavity and to face the infrared light introduction window of the cavity. 6. A cavity provided with a gas passage for introducing outside air and an infrared light introduction window, a gas diffusion filter provided integrally with the cavity at the gas passage, and a filter of the gas diffusion filter. A wall body integrally formed in the cavity so as to face in parallel with the surface, an area excluding the gas flow port and the infrared light introduction window on the inner wall surface of the cavity, and infrared light reflection coated on the wall body A photoacoustic gas sensor comprising a film.