JP2530689B2 - Small oxygen electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] With respect to a small oxygen electrode, it is an object of the present invention to provide a small oxygen electrode which does not show deterioration of reproducibility and reliability when measured in an aqueous solution and which can be mass-produced at low cost. For the purpose, a substrate, a hole formed by anisotropic etching on the surface of the substrate, and an insulating film formed on the substrate through an insulating film are provided from the bottom of the hole to the surface of the substrate. Two
In a small oxygen electrode having a book electrode, an electrolyte-containing porous material filled inside the hole, and a gas-permeable membrane covering and closing the hole, the substrate is further at least on its back surface, It is configured to have a hydrophobic insulating film.

[Industrial applications]

The present invention relates to a small-sized oxygen electrode, and particularly to a small-sized oxygen electrode that can be mass-produced at low cost.

The small oxygen electrode can be advantageously used for measuring the dissolved oxygen concentration in various fields. For example, the biochemical oxygen demand (BOD) in water is measured from the viewpoint of water quality conservation, and the small oxygen electrode can be used as a measuring device for the dissolved oxygen concentration. Further, in the fermentation industry, it is necessary to adjust the dissolved oxygen concentration in the fermentation tank in order to efficiently advance the alcohol fermentation, and the small oxygen electrode of the present invention can be used as this measuring device. Furthermore, the small oxygen electrode can also be used in combination with an enzyme to form an enzyme electrode and measure the concentration of sugars, vitamins and the like. For example, glucose is catalyzed by an enzyme called glucose oxidase, which reacts with dissolved oxygen to oxidize to gluconolactone, which utilizes the fact that the amount of dissolved oxygen that diffuses into the oxygen electrode cell decreases. The glucose concentration can be measured from the consumed amount.

Thus, the small oxygen electrode of the present invention can be used in various fields such as environment measurement, fermentation industry, clinical medicine, etc. It is also very useful because it is disposable and low cost.

[Conventional technology]

The present inventors have been unable to downsize and mass-produce conventional oxygen electrodes made of glass, and therefore developed a new type of small oxygen electrode utilizing lithography technology and anisotropic etching technology, Applied for a patent (Japanese Patent Application No. 62-
No. 71739). This oxygen electrode is formed on the silicon substrate by an anisotropic etching through an insulating film over a hole formed by anisotropic etching.
The oxygen electrode has a structure in which a book electrode is formed, an electrolyte solution containing body is further housed inside the hole, and finally the upper surface of the hole is covered with a gas permeable film. This oxygen electrode is small in size, has little variation in characteristics, and can be mass-produced in a batch, so that the cost is low.

In the above-mentioned type of oxygen electrode, the electrolytic solution containing body was usually a gel impregnated with the electrolytic solution, for example, agarose gel. However, in the case of agarose gel, it was necessary to repeatedly inject into the many small holes on the silicon substrate once with a micropipette.

As a result of research to further improve this point and make it suitable for mass production, the present inventors have used polyacrylamide gel, and in a number of minute holes on a silicon substrate,
We have found a method that allows the gel to be injected all at once (Japanese Patent Application No. 62-
No. 148221). This small-sized oxygen electrode is manufactured by using a lithographic technique and an anisotropic etching technique to form a plurality of holes on a silicon substrate having electrodes formed in the holes, except for the holes. A type resist is coated, the substrate is dipped in a solution of a photopolymerizable monomer containing an electrolytic solution, preferably acrylamide, and each hole is filled with the solution and irradiated with ultraviolet rays to be cured, and the electrolytic solution is removed. It is characterized in that the contained porous carrier is formed in the hole. According to this manufacturing method, it is possible to form the porous carrier that selectively holds the electrolytic solution only in the minute holes that form the small oxygen electrode, and thus it is possible to mass-produce.

[Problems to be Solved by the Invention]

The method described in Japanese Patent Application No. 62-148221 is an epoch-making manufacturing method in that a very small oxygen electrode can be mass-produced. However, when this method was examined in detail, it was found that there was a problem that had to be solved in order to be put to practical use. In other words, when examining the characteristics of this small oxygen electrode, noise was observed, and it was found that the noise was large and almost equal to the absolute value of the current, and in some cases it was impossible to use it as an oxygen electrode. It was Also, for practical use, it is necessary to obtain the same current value for the same oxygen concentration, but in reality, small ones are on the order of nA, and large ones are on the order of μA. It was found that there were variations in the response values for the differences. Such noise or extreme variation in characteristics is extremely disadvantageous in terms of reproducibility and reliability.

Therefore, an object of the present invention is to provide an oxygen electrode that is small in size and low in cost and can be mass-produced without showing deterioration in reproducibility / reliability during measurement in an aqueous solution.

[Means for solving the problem]

The present inventors have found that the cause of the above-mentioned noise and variations in characteristics is poor insulation in an aqueous solution. It was also found that not only the side surface where the silicon substrate is exposed at the time of cutting into chips but also the back surface on which the SiO ₂ film is formed by thermal oxidation has poor insulation in water, which is insufficient. It was thought that the poor insulating property of SiO ₂ was due to the fact that it was hydrophilic and thus the substrate and the external aqueous solution were likely to be electrically short-circuited. In order to solve such a problem of insulation failure, the present inventors have found that a hydrophobic insulating film is formed on at least the back surface of the chip of the small oxygen electrode, and according to this, almost no noise is recognized. It was found that it was not possible, and the dispersion of response values was remarkably improved.

The present invention is, namely, a substrate, a hole formed by anisotropic etching on the surface of the substrate, and an insulating film formed on the substrate with an insulating film interposed therebetween. In a small oxygen electrode having two electrodes reaching the surface, an electrolyte-containing porous substance filled inside the hole, and a gas-permeable membrane covering and closing the hole, the substrate further comprises at least A small oxygen electrode having a hydrophobic insulating film on its back surface.

The small oxygen electrode of the present invention preferably has a hole formed on a silicon substrate by anisotropic etching, and two electrodes extending from the bottom of the hole to the surface of the substrate are formed via an insulating film. After being coated with a hydrophobic insulating film, the inside of the hole is filled with an electrolytic solution-containing porous substance, and a gas permeable film is formed by dip coating or the like.

In practicing the present invention, a metal substrate or a semiconductor substrate, particularly a silicon substrate can be advantageously used as the base material or substrate of the electrode body. The insulating film can be formed of a silicon oxide film or the like. The silicon oxide film can be easily formed by thermally oxidizing the substrate when the substrate is silicon, for example.

The two electrodes can be variously modified depending on whether the manufactured electrode is a polaro type or a galvanic type. For example, when manufacturing a polaro-type oxygen electrode, both electrodes are made of gold electrodes or platinum electrodes,
A voltage is applied between both electrodes during measurement. In the case of the polaro type, it is preferable to use a neutral aqueous solution such as 0.1 M potassium chloride aqueous solution which is unlikely to attack the underlying silicon oxide film. Further, as the anode electrode, a metal electrode such as lead or silver that is more likely to undergo a chemical reaction than gold or platinum is used, as the cathode electrode, an electrode such as gold or platinum is used, and as an electrolyte, for example, 1M potassium hydroxide is used. A galvanic oxygen electrode can be manufactured by using an alkaline aqueous solution such as an aqueous solution. The formation of such an electrode can be advantageously performed by a film forming method such as vacuum deposition or sputtering.

After forming the electrodes, a photoresist is preferably coated on the substrate except for the holes and the portions to be in electrical contact with the substrate. This photoresist is preferably a negative photoresist, such as OMR-83 manufactured by Tokyo Ohka. This type of photoresist is advantageous because it has the property of repelling the aqueous solution that is the raw material when filling the electrolyte-containing porous material only in the holes.

The formation of the electrolyte-containing porous chamber material is described in Japanese Patent Application No. 62-14, for example.
As described in No. 8221, the substrate is immersed in an aqueous solution of acrylamide containing an electrolytic solution, the substrate is slowly pulled up while being immersed in such a solution, and the substrate in this state is irradiated with ultraviolet rays to gel. Method, a method of forming an electrolytic solution-containing alginate gel, for example, a substrate coated with a photoresist is immersed in an electrolytic solution-containing sodium alginate aqueous solution to fill only the hole portion with the aqueous solution,
The substrate in this state is immersed in an aqueous solution of calcium chloride containing an electrolytic solution to gel the aqueous solution of sodium alginate to form a calcium alginate gel, or a sol-gel method used in the process of synthesizing glass, for example, a photoresist. A method in which the coated substrate is immersed in a mixed solution of metal alkoxide, water and an electrolytic solution to fill the hole with the mixed solution, the mixed solution is gelated by a sol-gel method, and the electrolytic solution is held by a gel, And the like.

As the electrolyte to be retained by the porous substance as the carrier, various substances such as potassium chloride and sodium sulfate can be used.

The gas-permeable film is, of course, hydrophobic and impermeable to aqueous solutions, but initially it is liquid and can be dip-coated or spin-coated. It is used as an electrode material, a substrate such as a silicon substrate, and an insulating film. It is also indispensable that the adhesiveness with the silicon oxide film is good and the electrolyte does not leak outside. Suitable gas permeable membrane materials include photoresists, preferably negative photoresists. Teflon (trade name)
Although the membrane is oxygen permeable, it does not have adhesion, so its use should be avoided.

[Work]

The small-sized oxygen electrode according to the present invention is a mass-produced small-sized oxygen electrode using the lithography technique and thin film forming technique used for forming a semiconductor integrated circuit, and in order to make it practically usable, A product with low noise and good reproducibility should be obtained. As a method therefor, the present invention is to apply a hydrophobic insulating film to the surface and side surface opposite to the side where the oxygen electrode is formed on the substrate to improve the insulating property.

According to the above, the cell forming the oxygen electrode will be completely electrically isolated from the external environment composed of the aqueous solution,
Although variations in response values and noise can be reduced, in the present invention, in order to perform the above operation more advantageously,
When the formation of the cathode and anode in the oxygen electrode is completed, a hydrophobic resin is applied to the back surface, and when the gas permeable film (also hydrophobic) is formed by dip coating, side insulation is also achieved. It is something to do.

〔Example〕

Next, a preferred example of the method for producing the small oxygen electrode according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a perspective view showing a preferred example of the small oxygen electrode according to the present invention. The oxygen electrode shown in the figure has a rectangular parallelepiped shape, the sensitive portion is covered with the gas permeable film 14, and a part of the electrodes 3A, 3B is exposed for connecting to an attached device. In the case of the present example, the electrodes 3A and 3B were both gold electrodes in order to have a polaro type.

The structure of the small oxygen electrode of FIG. 1 can be understood in detail from FIG. 2 which is a sectional view taken along the line II-II.
The silicon substrate 1 has a hole formed by anisotropic etching, and a silicon oxide film 2 is deposited as an insulating film on the entire surface thereof. Further, the back surface of the substrate is covered with a hydrophobic insulating film 7 which is hard to break. In the hole of the silicon substrate 1, two gold electrodes 3A and 3B are attached in pairs. As shown in FIG. 1, the gold electrodes 3A and 3B each have a part extending to the outside of the groove. Further, the holes of the silicon substrate 1 are filled with the electrolyte-containing porous substance (hereinafter, gel) 6. Further, the upper part of the hole is covered with a gas permeable film 14 so as to cover the entire surface of the upper part of the substrate 1 (excluding the exposed part in FIG. 1). Is covered.

The small oxygen electrode shown in FIGS. 1 and 2 can be advantageously manufactured, for example, by the manufacturing process sequentially shown in FIG. The main body after electrode formation shown in FIG. 3 (A) can be manufactured through the following steps. In the following description, for ease of understanding, a case where only one oxygen electrode is formed on one wafer is described, but in reality, a large number of small oxygen electrodes are formed at the same time. I want you to understand. In addition, here, the case where calcium alginate gel is used as the electrolyte-containing gel will be described.

1. Wafer cleaning A (100) surface 2-inch silicon wafer having a thickness of 350 μm was prepared and washed with a mixed solution of hydrogen peroxide and ammonia and concentrated nitric acid.

_2. Formation of SiO ₂ Film A silicon wafer was subjected to wet thermal oxidation to form a 0.8 μm thick SiO ₂ film on the entire surface thereof.

3. Formation of etching pattern Negative photoresist (Tokyo Ohka OMR-83, viscosity 6
0cP) was used to form a resist pattern for etching on the wafer.

4. Resist coating The same negative photoresist as used in the above process was coated on the back surface of the wafer and baked at 130 ° C. for 30 minutes.

5. Etching of SiO ₂ film The wafer was immersed in a 50% hydrofluoric acid: 50% ammonium fluoride = 1: 6 aqueous solution, and the exposed SiO ₂ portion not covered with the photoresist was removed by etching. Subsequently, the resist was removed with a sulfuric acid / hydrogen peroxide solution (2: 1) solution.

6. Anisotropic etching of Si Anisotropic etching of silicon was performed in a 35% potassium hydroxide aqueous solution at 80 ° C. Etching depth 300μm. After the etching was completed, the wafer was washed with pure water.

After the completion of this anisotropic etching, the SiO ₂ film used during etching was removed. This was performed in a 50% hydrofluoric acid: 50% ammonium fluoride = 1: 6 aqueous solution as in Step 5.

To grow an SiO ₂ film 7.SiO ₂ film surface forming the silicon wafer 1, 1. Following the same washing process and were wet thermal oxidation of the wafers. A SiO ₂ film 2 having a film thickness of 0.8 μm was formed.

8. Formation of chromium and gold thin film Following the chromium thin film (400Å, for adhesion of gold and substrate), a gold thin film (4000Å) was formed by vacuum evaporation.

9. Formation of resist pattern for electrode formation Negative photoresist (Tokyo Ohka OMR-83, viscosity 6
0cP) was used to form a resist pattern (not shown) for electrode formation on the SiO _{2 of} the wear 1.

10. Etching of Gold and Chromium The substrate on which the resist pattern was formed was immersed in the following gold and chromium etching solutions in this order, and the exposed gold and chromium portions were removed by etching. Furthermore, after washing with pure water, sulfuric acid / hydrogen peroxide water (2:
1) The resist was removed by the solution.

Gold etching solution: 4g KI and 1g I ₂ dissolved in 40ml water.

Chromium etchant: 0.5g NaOH and 1g K ₃ Fe (CN)
A solution of ₆ dissolved in 4 ml of water.

The state where the gold electrode is formed is shown in FIG. In the figure, 3A and 3B are electrodes, and 5 is a hole.

11. Formation of Hydrophobic Insulation Film on Back Side of Substrate Hydrophobic insulation film (KR-made by Shin-Etsu Silicone
282) was evenly applied and baked at 250 ° C. This insulating film is shown by 7.

12. Resist coating (Fig. 3 (B)) On the surface of the main body, except for the holes 5 and the pad portion for electrical contact, a negative photoresist (manufactured by Tokyo Ohka)
OMR-83, viscosity 60 cP) 4. This was carried out by applying a photoresist to the surface of the wafer, exposing and developing after prebaking.

13. Cutting out of substrate A large number of oxygen electrodes formed on the substrate were cut out into chips.

14. Hydrophilization in the hole The tip of the chip was immersed in a 1M sodium hydroxide aqueous solution. As a result, the areas not covered with the resist became hydrophilic.

15. Filling with calcium alginate gel (Fig. 3 (C)) The following two kinds of solutions were prepared for forming a gel containing an electrolytic solution.

Solution A: Sodium alginate dissolved in 0.1M potassium chloride aqueous solution. Sodium alginate concentration 0.2%.

Solution B: Calcium chloride dissolved in 0.1M potassium chloride aqueous solution. Calcium chloride concentration 5%.

When the chip of FIG. 3 (B) was dipped in the solution A and slowly pulled up, the negative photoresist film 4 was hydrophobic, so the solution A was repelled from the resist film and remained only in the holes 5. It was Next, when this chip was immersed in the solution B, the solution A instantly gelled. An electrolytic solution-containing gel 6 was obtained.

16. Coating of Gas-permeable Membrane (FIG. 3 (D)) The gas-permeable membrane 14 was coated on the electrolytic solution-containing gel 6 so as to cover the gel. In this example, as the gas permeable membrane,
The same negative photoresist used in Step 3 and the like was used. That is, a negative photoresist (made by Tokyo Ohka
OMR-83 (trade name), viscosity 60 cP) was applied by dip coating. This resist is immediately exposed and developed without prebaking, and then the small oxygen electrode body is left in pure water or saturated water vapor for a whole day and night to remove the thinner from the resist to complete the gas permeable film and sidewall insulating film. Let This resist film is also formed on the back surface, but this works to improve the insulating property.

〔The invention's effect〕

According to the invention, the part of the cell forming the oxygen electrode is
Since it is completely electrically isolated from the aqueous solution in the external environment, it is less likely to be affected by noise and variations in response values are reduced. Therefore, it is possible to provide a small-sized, low-price, mass-produced oxygen electrode that does not exhibit deterioration in reproducibility and reliability during measurement in an aqueous solution.

[Brief description of drawings]

FIG. 1 is a perspective view showing a preferred example of a small oxygen electrode according to the present invention, FIG. 2 is a sectional view taken along line II-II of the electrode of FIG.
3A to 3D are cross-sectional views sequentially showing the latter half of the manufacturing process of the small oxygen electrode shown in FIGS. 1 and 2. In the figure, 1 is a substrate, 2 is an insulating film, 3A and 3B are electrodes, 4 is a photoresist film, 5 is a hole, 6 is an electrolyte solution containing gel, 7 is a hydrophobic insulating film, and 14 is a gas permeable film. is there.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naomi Kojima 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Limited

Claims (1)

(57) [Claims] 1. A substrate, a hole formed by anisotropic etching on the surface of the substrate, and an insulating film formed on the substrate via an insulating film, and the surface of the substrate from the bottom of the hole. In the small oxygen electrode having two electrodes up to the above, a porous material containing an electrolyte solution filled in the hole, and a gas-permeable membrane covering and closing the hole, the substrate further comprises at least the Small oxygen electrode having a hydrophobic insulating film on the back surface.