WO2002088697A1

WO2002088697A1 - Production method for photoacoustic gas sensor-use gas diffusion filter

Info

Publication number: WO2002088697A1
Application number: PCT/JP2002/004284
Authority: WO
Inventors: Hisatoshi Fujiwara; Nobuaki Honda; Takashi Kihara
Original assignee: Yamatake Corporation
Priority date: 2001-04-27
Filing date: 2002-04-26
Publication date: 2002-11-07
Also published as: JP2002328115A

Abstract

A production method for photoacoustic gas sensor-use gas diffusion filter suitable for integral assembling into a small photoacoustic gas sensor and stable in characteristics, comprising the first step of etching an Si substrate (11) almost vertically from one surface to form recesses (12) as cavities in the gas sensor and almost vertically etching the Si substrate at a portion located on the outer side of the recesses from another surface to form a partition wall (18) with a specified thickness in the Si substrate, and the second step of anodizing the partition wall with a specified area excluding the partition wall masked to form a porous silicon layer (19) as a gas diffusion filter.

Description

Specification

Manufacturing method of gas diffusion filter for photoacoustic gas sensor

The present invention relates to a method for manufacturing a gas diffusion filter for a photoacoustic gas sensor that is suitable for being integrated into the cavity of a photoacoustic gas sensor.

Background art

A photoacoustic gas sensor detects the concentration of a specific type of gas, such as C02, in a mixed gas such as air by using the phenomenon that a specific type of gas absorbs infrared light of a specific wavelength and thermally expands. It is. In other words, when air is irradiated with infrared light having a specific wavelength and the intensity of which changes with time, the greater the concentration of CO 2 present in the air, the greater the thermal expansion and thermal contraction. Therefore, if this phenomenon is detected as a change in atmospheric pressure (sound pressure), it will be 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 (air) is introduced, a light source 2 that irradiates infrared rays into the cavity 1, and a cavity 1 as shown in FIG. It is provided with a microphone 3 that is provided as a part of the wall surface (ceiling surface) 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-sized photoacoustic gas sensor realized by using micromachining technology, the cavity 1 is formed by etching a Si substrate transparent to infrared light to form a predetermined space, Further, a structure is provided in which a gas flow passage 4 for introducing gas into the cavity 1 is provided. The gas passage 4 is usually provided with a gas diffusion filter 5. The gas diffusion filter 5 restricts the flow of gas, thereby maintaining the flow (replacement) of gas (air) between the inside and outside of the cavity 1 while maintaining the aforementioned infrared light. Sound pressure in the cavity 1 according to the thermal expansion of C 0 2 (gas) due to absorption Play a role in changing

That is, the gas diffusion filter 5 plays the following two roles. 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 infrared pulse irradiation, and to keep the inside of the cavity 1 substantially closed to produce sound. This function prevents pressure from being transmitted to the outside of cavity 1. The other is that it does not act as an airflow resistor against slow pressure changes (sound pressure) in the cavity 1 caused by changes in the external environment such as temperature and atmospheric pressure. This function keeps the air open.

Heretofore, as this kind of gas diffusion filter 5, a Teflon-based filter has been exclusively used. The gas diffusion filter 5 is incorporated in a spacer or the like forming a part of the cavity 1.

However, in the case of the gas diffusion filter 5 made of this kind of porous body, it cannot be denied that the filter characteristics vary widely. In addition, the workability of assembling into the cavity 1 (gas flow path 4) is poor. Therefore, in mass-producing high-quality photoacoustic gas sensors with stable sensor characteristics, a major problem remains in how to realize a gas diffusion filter with stable filter characteristics. In particular, when implementing a small photoacoustic gas sensor using micromachining technology, there is a problem of how to realize the gas diffusion filter. Disclosure of the invention

An object of the present invention is to provide a method for manufacturing a gas diffusion filter for a photoacoustic gas sensor which can easily mass-produce a gas diffusion filter having stable filter characteristics with good controllability and is suitable for being integrated into a small photoacoustic gas sensor. Is to provide. Another object of the present invention is to provide a compact photoacoustic gas sensor integrally provided with a gas diffusion filter.

In order to achieve the above object, a gas diffusion filter for a photoacoustic gas sensor according to the present invention is provided. Evening manufacturing method as described in claim 1

(1) The Si substrate is etched substantially vertically from one surface to form a concave portion that forms the cavity of the photoacoustic gas sensor,

(2) The Si substrate is etched substantially perpendicularly from the other surface of a portion located outside the concave portion to form a partition having a predetermined thickness on the Si substrate [first step],

(3) Next, a predetermined region excluding the partition is masked, and the partition is anodized to form a porous silicon layer forming a gas diffusion film [second step].

It is characterized by:

That is, according to the present invention, the Si substrate is deep-etched from both sides of the Si substrate with the gas diffusion filter formation site interposed therebetween to form a partition having a predetermined thickness at the gas diffusion filter formation site. Then, a predetermined current is passed through the partition from both sides of the Si substrate to anodic oxidize the partition, and only the Si of the portion forming the partition is made porous by this anodic oxidation to form a gas diffusion filter. It is characterized by.

Further, the present invention provides a

際 When the Si substrate is etched substantially perpendicularly from one surface to form a concave portion forming the cavity of the photoacoustic gas sensor, a portion of the concave portion facing the inner wall surface of the concave portion is also left at the same time, so that the concave portion of the concave portion is formed. While forming a wall facing the inner wall,

⑤ Thereafter, the Si substrate is etched substantially perpendicularly from the other surface of the Si substrate on the side opposite to the wall outside the recess to form a partition having a predetermined thickness on the Si substrate. First step],

所定 Masking a predetermined area excluding the partition wall and anodizing the partition wall to form a porous silicon layer forming a gas diffusion filter [Second Step]

You may do it.

At this time, as described in claim 3

⑦ Porous material forming the gas diffusion filter in the cavity formed by the concave portion An infrared light reflecting film such as Au or A1 is coated on the surface of the silicon layer and the cavity other than the infrared light introducing window [third step], and the infrared light irradiated into the cavity is irradiated with the infrared light. It is preferable to have a structure in which multiple reflections are made inside.

The formation of the porous silicon layer by anodic oxidation in the second step is performed, for example, by masking the non-etched surfaces on both sides of the Si substrate, immersing the Si substrate in a hydrofluoric acid solution, and The periphery of the substrate is sealed in a liquid-tight manner to prevent contact of the hydrofluoric acid solution between both surfaces of the Si substrate, and in this state, a predetermined current is applied between both surfaces of the Si substrate. Good. By the way, since the Si substrate itself is not easily etched by hydrofluoric acid, for example, it is necessary to coat a hydrofluoric acid-resistant insulating film on one side so that anodizing current does not flow easily, and then perform anodizing treatment in this state. You can do it.

According to the present invention, it is possible to easily produce a small-sized, stable, high-quality gas diffusion filter required for realizing a small-sized photoacoustic gas sensor. Furthermore, since a gas diffusion filter can be provided integrally with the Si substrate, which forms the cavity of the photoacoustic gas sensor, the practical advantage is great. However, since a plurality of gas diffusion filters can be mass-produced on a Si wafer at the same time, there are great industrial advantages such as a reduction in manufacturing cost. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a view for explaining a method for manufacturing a gas diffusion filter for a photoacoustic gas sensor according to an embodiment of the present invention. The concept of batch production of a photoacoustic gas sensor on a Si substrate (Si wafer) is shown. Figure showing

2A to 2G are diagrams showing a process of manufacturing the gas diffusion filter according to the first embodiment of the present invention, wherein a step of providing a protective film on the Si substrate and a step of forming an opening in the protective film Forming a recess in the Si substrate, selectively removing the protective film, A step of forming a partition on the Si substrate, a step of anodizing the partition of the Si substrate, and an anodized partition, respectively.

3A to 3I are diagrams showing a process of manufacturing a gas diffusion filter according to a second embodiment of the present invention, in which a step of forming a Si NX film on both sides of an SOI substrate, a lower Si Patterning the NX film, etching the back Si layer of the SOI substrate, patterning the upper Si NX film, etching the front substrate of the SOI substrate, selectively removing the SiO 2 film FIG. 4A to FIG. 4J show a step, a step of forming a partition wall, a step of anodizing the partition wall, and an anodized partition wall, respectively. FIGS. 4A to 4J show a gas diffusion filter according to the third embodiment of the present invention. FIG. 4 is a view showing a manufacturing process, a step of forming a protective film, a step of forming an opening in the protective film, a step of forming a shallow concave portion on the back surface of the Si substrate, a step of forming a resist film in the shallow concave portion, Etch Si substrate further in shallow recess Extent, shows the step of selectively removing the protective film to form a partition wall, the step of the partition wall anodizing, anodized septum, and the step of forming the infrared reflecting film respectively,

FIG. 5 is a diagram showing a schematic structure of the photoacoustic gas sensor. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a method for manufacturing a gas diffusion filter for a photoacoustic gas sensor according to an embodiment of the present invention will be described with reference to the drawings.

The gas diffusion filter for a photoacoustic gas sensor is formed integrally with the cavity 1 of the photoacoustic gas sensor or a spacer forming a part thereof. The cavities (spacers) 1 are mass-produced in batches using a micromachining technique such as etching the Si substrate 11 as shown in FIG. Is done.

The gas diffusion filter for a photoacoustic gas sensor according to the present invention is integrated with the cavity (spacer) 1 in the manufacturing process of such a cavity (spacer) 1. In general, the area where the gas diffusion filter is formed on the wall of the cavity 1 is anodized, and the area is made porous to form a gas diffusion filter. It is characterized by a silicon layer.

FIG. 2 shows step by step the manufacturing process of the gas diffusion filter according to the first embodiment of the present invention. The manufacturing method of the gas diffusion filter will be described with reference to FIG. 2. First, as shown in FIG. 2A, for example, a protective film made of a hydro-acid-resistant insulating film such as a SiNX film is formed on both sides of a Si substrate 11 having a thickness of 1 mm. 12 and 13 are provided. Then, as shown in FIG. 2B, a resist film 14 is formed on a protective film (SiNX film) 13 on the lower surface side of the Si substrate 11, and the resist film 14 is patterned. In the film (SiNX film) 13, for example, a square opening of 2 mm square is formed. The shape of the hole opened in the protective film (SiNX film) 13 is determined according to the size of the cavity 1 to be formed in the Si substrate 11.

Thereafter, using the protective film (SiNX film) 13 as a mask, the Si substrate 11 is vertically etched from the back side in the vertical direction, and as shown in FIG. 2C, the recess 15 forming the cavity 1 is formed on the back side of the Si substrate 11. [First step]. The recess 15 is formed to have a depth that leaves a thickness of about 0.3 to 0.5 xm on the bottom surface.

Next, a resist film 16 is formed on the surface side of the Si substrate 11, and the resist film 16 is patterned. In this patterning, for example, a 3 mm square area is removed so as to cover the area facing the concave section 15 and leave an area slightly larger than the size of the concave section 15 as the protective film (SiNX film) 12. This is performed by removing the resist film 16 at the site. Thereafter, using the resist film 16 as a mask, the protective film (SiNX film) 12 is selectively removed as shown in FIG. 2D.

Using the protective film (SiNX film) 12 as a mask, the Si substrate 11 is removed. 2D, a recess 1 形成 is formed on the front side of the Si substrate 11 outside the recess 15 forming the cavity 1, as shown in FIG. 2E. Process]. The recess 17 serves as a gas passage for the cavity 1 formed by the Si substrate 11. In addition, a partition 18 having a predetermined thickness is formed between the concave portion 17 and the concave portion 15 forming the cavity. The thickness of the partition wall 18 is determined by the mask pattern formed by the protective films (SiNX films) 12 and 13 described above, and is set according to the filter length required for a gas diffusion filter. You. It goes without saying that the order of forming the concave portions 15 and 17 may be reversed.

Thereafter, the Si substrate 11 is immersed in a hydrofluoric acid solution while the protective films (SiNX films) 12 and 13 are left to anodize the partition walls 18 [second step]. This anodization is performed, for example, on the peripheral surface of the Si substrate 11 so that the hydrofluoric acid solutions existing on both surfaces of the Si substrate 11 do not come into contact with each other as shown in FIG. 2F. The sealing is performed in a liquid-tight manner, and a current having a predetermined density is applied between a pair of electrodes provided on both surfaces of the Si substrate 11 in the hydrofluoric acid solution. Then, since the Si substrate 11 is insulated at the portions covered by the protective films (SiNX films) 12 and 13, the current flows through the partition 18 where the protective films (SiNX films) 12 and 13 do not exist. Flows. As a result, as shown in FIG. 2G, the Si portion forming the partition wall 18 is locally anodized and turned into a porous layer to form a porous porous silicon layer 19. When the partition 18 is anodized, a current may be applied while irradiating light depending on the anodizing conditions. When the partition wall 18 is anodized, the anodic oxidation current hardly flows through the inner wall surface without masking the inner wall surface of the concave portion 17. There is no danger of anodizing up to the inner wall surface. The partition 18 thus formed as the porous silicon layer 19 is used as a gas diffusion filter having a predetermined thickness integrally formed on the Si substrate 11 on which the cavity 1 is formed. Can be

Thereafter, for example, the back surface of the Si substrate 11 on which an optical filter (not shown) is formed is subjected to etching treatment, and individual chips are cut out from the Si substrate (Si wafer) 11 by dicing or the like, thereby integrally providing a gas diffusion filter. Multiple photoacoustic cells are required. At this time, the above-described protective films (SiNX films) 12, 13 may be removed.

As described above, according to the method of manufacturing a gas diffusion filter in which a gas diffusion filter (porous silicon layer 19) in which Si is made porous is integrally formed on the wall surface forming the cavity 1 of the Si substrate 11, as described above. The gas diffusion filter can be made easily and efficiently. In addition, the porosity of the Si substrate 11 by the anodic oxidation of the partition wall 19 can be controlled with good reproducibility only by controlling the anodic oxidation conditions such as the current density. Therefore, in combination with the thickness control (control) of the partition walls 19 by the deep etching of the Si substrate 11, a gas diffusion filter having required filter characteristics (pressure loss characteristics) can be manufactured with high quality and high mass productivity. . In particular, since the gas diffusion filter can be manufactured integrally with the cavity 1 by the micromachining technology for the Si substrate 11, there are advantages such as being extremely suitable for miniaturizing the photoacoustic gas sensor.

The method for manufacturing a gas diffusion filter according to the present invention can be similarly applied to a case where a photoacoustic gas sensor is realized using an SOI (Silicon on Insulator) substrate. In this case, for example, as shown in FIG. 3A, an SOI substrate 20 provided with Si layers 22 and 23 on both surfaces thereof via a Si〇2 film 21 is prepared. NX films 24 and 25 are formed respectively. Then, as shown in FIG. 3B, the SiNX film 25 is patterned using the resist film 26, and using the SiNX film 25 as a mask, as shown in FIG. Then, dip-etching is performed to reach a concave portion 26 forming a cavity 1 [first step]. Next, as shown in FIG. 3D, the SiNX film 24 on the upper surface side is patterned using a resist film 28, and using this resist film 28 as a mask, as shown in FIG. The Si layer 23 is deep etched until it reaches the Si02 film 21. Next, after selectively removing the SiO 2 film 21 as shown in FIG. 3F, a concave portion 29 serving as a gas passage is formed outside the cavity 1 as shown in FIG. 3G [second step]. . By the formation of the concave portion 29, the partition wall 30 having a predetermined thickness is formed between the concave portion 26 and the concave portion 26 forming the cavity 1 as in the previous embodiment.

Thereafter, the SOI substrate 20 is immersed in a hydrofluoric acid solution as shown in FIG. 3H, and a current having a predetermined density is applied between both surfaces of the SOI substrate 20 in the same manner as in the previous embodiment. Anodizing the Si forming the partition wall 30 [third step]. Then, the partition wall 30 is made porous as shown in FIG. 3I to form a porous silicon layer 31 serving as a gas diffusion filter.

In the case where a photoacoustic gas sensor is manufactured using the SOI substrate 20 in this way, a gas diffusion filter can be easily manufactured integrally with the photoacoustic gas sensor, and the same effects as those of the previous embodiment can be obtained. Is played. Further, in this embodiment, the depth of the deep etching of the Si layer 23 can be easily controlled by utilizing the Si02 film 21 constituting the SOI substrate 20, that is, the SiO 2 film 21 is used as an etching stop layer. Since it can be used as a material, it is possible to obtain effects such as facilitating the production management. Also, the surface of the SiO 2 film 21 etched off becomes an optically sufficient mirror surface, and thus is suitable for using the remaining Si layer 23 as an optical filter.

As described above, the Si substrate 11 or the SOI substrate 20 is de-etched to form the partition walls 18 and 30, and the partition walls 18 and 30 are anodized to form a gas comprising the porous silicon layers 19 and 31. When fabricating a diffusion filter, a light-reflecting film is formed on the inside of the cavity 1 facing the gas diffusion filter. It is also useful to provide a wall body.

In this case, as shown in FIG. 4, the manufacturing process is shown, and the formation of the protective film and the opening of the protective film shown in FIGS. 4A and 4B corresponding to FIGS. 2A and 2B, respectively. The formation is performed sequentially. Next, prior to forming the recess 15 forming the cavity 1 by deep etching the Si substrate 11 from the back side thereof, first, as shown in FIG. Form 15a. Then, again, a resist film 14a is formed on the back side of the Si substrate 11 as shown in FIG. 4D, and the Si substrate 11 is deeply etched from the back side using the resist film 14a as a mask. As shown in FIG. 7, a concave portion 15 forming the cavity 1 is formed. At the same time, a wall 40 for forming a light reflecting film is formed in the recess 15.

Thereafter, the protective film 12 is selectively removed in the same manner as in the previous embodiment (FIG. 4F). Next, the Si substrate 11 is deep-etched from the front surface side to allow gas flow as shown in FIG. A concave portion 17 forming a path is formed, and a partition wall 18 having a predetermined thickness is formed between the concave portion 17 forming a cavity 1 and the concave portion 17 forming a cavity 1. Then, as shown in FIG. 4H, the partition 18 is anodized, and the partition 18 is made porous. In this case, the current that has passed through the partition wall 18 flows into the hydrofluoric acid solution on the back surface side of the Si substrate 11, so that the above-described wall body 40 is not anodized. As a result, only the partition wall 18 between the concave portion 17 forming the gas passage and the concave portion 15 forming the cavity 1 becomes the porous silicon layer 19 forming the gas diffusion filter (FIG. 4I).

Thereafter, as shown in FIG. 4J, by depositing or sputtering Au, A1, or the like from the oblique lower side of the back surface of the Si substrate 11, a thick infrared light reflecting film 41 is formed on the inner surface of the wall body 40. Is done. At this time, the bolus-shaped partition wall 18 (porous silicon layer 19), which is a part of the inner wall surface of the cavity 1, is shielded from the source such as Au or A1 by the wall body 40. The infrared light reflection film 41 is not formed on the substrate. That is, the partition 18 (porous silicon layer 19) and the recess 1 Except for the surface forming the infrared light introducing window formed on the upper surface of 5, the infrared light reflecting film 41 can be formed on the entire inner wall surface of the cavity 1.

According to the photoacoustic gas sensor having the configuration including the infrared light reflecting film 41, the infrared light applied to the cavity 1 can be confined by multiple reflection in the cavity 1, This is convenient for increasing the gas detection sensitivity of the photoacoustic gas sensor. In addition, even though the infrared light reflecting film 41 is provided inside the cavity 1 in this manner, the infrared light to the surface of the partition wall 18 (porous silicon layer 19) forming the gas diffusion filter by the wall body 40 is formed. Since the formation of the reflection film 41 is prevented, the function of the gas diffusion filter is not impaired. Therefore, a highly practical photoacoustic gas sensor integrally provided with a gas diffusion filter can be realized. Particularly, in realizing a small-sized photoacoustic gas sensor, it is extremely suitable for realizing a small-sized, high-quality gas diffusion filter with stable filter performance.

Note that the present invention is not limited to the embodiment described above. For example, the porosity of the porous silicon layers 18 and 30 formed by anodic oxidation can be easily and accurately adjusted by controlling the anodic oxidation conditions. It can be easily adjusted by 11 etching. Therefore, according to the size (internal volume) of the cavity 1 and the filter characteristics (pressure loss) required for the gas diffusion filter, the conditions for the etching oxidation of the Si substrate 11 and the porous oxidation are controlled, respectively. The porosity and thickness of the silicon layers 18 and 30 are optimally set (this may be done by rubbing. In addition, the present invention can be implemented with various modifications without departing from the gist thereof.

Claims

The scope of the claims

1. The Si substrate is etched substantially perpendicularly from one surface to form a concave portion forming a cavity of the photoacoustic gas sensor, and a portion located outside the concave portion is substantially perpendicular to the Si substrate from the other surface. A first step of forming a partition having a predetermined thickness on the Si substrate by etching

A second step of forming a porous silicon layer forming a gas diffusion film by masking a predetermined region excluding the partition wall and anodizing the partition wall;

A method for producing a gas diffusion filter for a photoacoustic gas sensor, comprising:

2. In the second step, the non-etched surfaces on both sides of the Si substrate are respectively masked, and the Si substrate is immersed in a hydrofluoric acid solution and the periphery of the Si substrate is sealed in a liquid-tight manner. Preventing the contact of the hydrofluoric acid solution between both surfaces of the Si substrate, and in this state, performing a predetermined current between both surfaces of the Si substrate. 2. The method for producing a gas diffusion filter for a photoacoustic gas sensor according to claim 1.

3. Etching the Si substrate substantially perpendicularly from one surface to form a concave portion forming the cavity of the photoacoustic gas sensor and a wall facing the inner wall surface of the concave portion, and forming a wall facing the inner wall surface of the concave portion. A first step of etching the opposite side of the Si substrate substantially perpendicularly from the other surface to form a partition having a predetermined thickness on the Si substrate;

A second step of forming a porous silicon layer serving as a gas diffusion filter by masking a predetermined region excluding the partition wall and anodizing the partition wall;

A method for manufacturing a gas diffusion filter for a photoacoustic gas sensor, comprising:

4. The method further comprises a third step of coating an infrared light reflecting film on a surface of the cavity formed by the concave portion excluding a porous silicon layer serving as the gas diffusion filter and an infrared light introducing window. 4. The method for producing a gas diffusion filter for a photoacoustic gas sensor according to claim 3, wherein:

5. In the second step, the non-etched surfaces on both sides of the Si substrate are respectively masked, the Si substrate is immersed in a hydrofluoric acid solution, and the periphery of the Si substrate is sealed in a liquid-tight manner. Preventing the contact of the hydrofluoric acid solution between both surfaces of the Si substrate, and in this state, performing a predetermined current between both surfaces of the Si substrate. 5. The method for producing a gas diffusion filter for a photoacoustic gas sensor according to item 3 or 4.