JP3995665B2 - Dielectric for ozone generator - Google Patents

Description

  The present invention relates to a dielectric for an ozone generator used in a discharge cell of a discharge type ozone generator.

  Discharge cells used in ozone generators are roughly classified into plate types and tube types. Each discharge cell has a pair of electrodes arranged with a gap, and a dielectric is arranged on the surface of at least one of the pair of electrodes so as to form a discharge space between the electrodes. ing. And ozone gas is produced | generated by distribute | circulating source gas, such as oxygen, in discharge space.

  Recently, in order not to expose the electrode made of metal to the discharge gap, a configuration in which a pair of dielectrics are arranged inside each of the pair of electrodes and a discharge gap is formed between the pair of dielectrics has been increasing. . Further, a multi-layer structure in which a plurality of units are stacked in the thickness direction as one unit is often used.

  The dielectrics in the discharge cell are roughly classified into a substrate type having rigidity in shape and a covering type in which the surface on the air gap side of the rigid electrode is coated. In the case of the coated type, uneven thickness distribution is unavoidable, which leads to uneven gaps in the discharge gap. Recently, substrate types such as ceramic plates that are hard and chemically strong have become mainstream. It is becoming.

  Meanwhile, while ozone generators are used in various chemical processing facilities, they have begun to be widely used in semiconductor manufacturing facilities. In the case of an ozone generator for manufacturing semiconductors used for oxide film formation, resist ashing, silicon wafer cleaning, etc., high cleanliness is required. Therefore, it is pure with very little contamination (metal impurities and particles). Ozone gas needs to be generated. For this purpose, high-purity oxygen gas is used as the raw material gas.

  Further, as the substrate-type dielectric described above, it can be said that a high-purity alumina substrate having high mechanical strength and excellent ozone resistance and sputtering resistance is preferable.

  It has already been widely known that when high-purity oxygen gas is used as the raw material gas, the ozone concentration of the ozone gas rapidly decreases immediately after the start of operation, and the predetermined performance is not achieved. In order to solve this problem, it is effective to add a small amount of catalyst gas to high-purity oxygen gas. As the catalyst gas, high-purity nitrogen gas that is easily available in the semiconductor manufacturing process is used. It is used a lot.

  The case where the dielectric is the above-described high-purity alumina substrate is no exception, and when the source gas is a high-purity oxygen gas, the performance as an ozonizer is hardly obtained. On the contrary, it has been found that even if nitrogen gas is mixed with oxygen gas, the ozone concentration does not increase sufficiently. More specifically, when a high-purity alumina substrate is disposed on both sides of the discharge gap, the effect of adding the catalyst gas cannot be obtained sufficiently.

  Attempts to increase the ozone concentration without using a catalyst gas have been made in various fields, and one of them is the use of titanium oxide in a dielectric described in Patent Document 1 and the like.

Japanese Patent Laid-Open No. 11-21110

  In Patent Document 1, when the dielectric is a coating type, the surface of the electrode on the discharge gap side is coated with a titanium oxide-containing substance by baking or the like. On the other hand, when the dielectric is a substrate type, a titanium oxide-containing substance is coated on the surface of the substrate by sputtering, ion plating, vapor deposition, or the like. In the case of the coating type, the coating layer of the titanium oxide-containing material is formed in multiple layers to prevent peeling due to the difference in thermal expansion coefficient from the electrode, and the titanium oxide content is gradually increased from the front side to the back side. It is recommended that the content be reduced, and the titanium oxide content should be as high as possible from the viewpoint of increasing the ozone concentration. At least the surface layer portion should have a Ti element content ratio of 10% by weight or more, preferably 30% by weight or more. Yes.

  When the dielectric is a substrate type, a coating layer of a titanium oxide-containing material is formed on the surface of the substrate. The titanium oxide content in the coating layer here is also considered to be as good as possible from the viewpoint of stabilizing the ozone concentration. Specifically, the Ti element amount ratio is 10% by weight or more, and titanium oxide alone is particularly desirable. ing.

  The coating of the titanium oxide-containing material is also effective for the alumina substrate. That is, when the surface of the alumina substrate is coated with a titanium oxide-containing substance, the ozone concentration is remarkably increased by the addition of the catalyst gas. However, if the surface of the alumina substrate is coated with a titanium oxide-containing material, the meaning of using the substrate is lost.

  That is, the dielectric directly faces the discharge gap and is directly exposed to electrical and chemical reactions in the discharge gap. For this reason, the surface on the discharge gap side of the dielectric must have high flatness in order to make the gap amount of the discharge gap uniform, and in terms of cleanliness and durability, it has ozone resistance and sputtering resistance. It must be excellent. Because of these requirements, a hard and chemically strong alumina substrate is effective. Titanium oxide is a kind of ceramics, but from the standpoint of mechanical and chemical performance, it has dielectric properties in discharge cells. It is not appropriate as a body.

  This is because titanium oxide is considerably inferior to alumina in terms of mechanical strength, ozone resistance, sputtering resistance, and the like.

For this reason, it is required to keep the content as low as possible even when titanium oxide is used. In the cited document 1, as described above, titanium oxide having a Ti element amount ratio of 10% by weight or more is required. . The Ti element amount ratio of 10% by weight or more reaches about 17% by weight or more in terms of the mixed amount of TiO ₂ , and 30% by weight or more reaches 50% by weight or more.

  If the surface of the alumina substrate is covered with such a brittle coating layer containing a large amount of titanium oxide, there is no point in using the substrate.

  An object of the present invention is to provide a dielectric for an ozone generator that uses a hard and chemically strong alumina substrate and can effectively extract the catalytic gas effect without impairing the advantages of the substrate. It is in.

  In order to achieve the above object, the present inventor planned to abandon the coating of the titanium oxide-containing material on the surface of the alumina substrate and to disperse the titanium oxide uniformly in the substrate itself. As a result, it has been found that the effect of increasing the ozone concentration by the catalyst gas can be effectively brought out with a surprisingly small amount that does not impair the original characteristics of the high-purity alumina substrate.

That is, even if titanium oxide is uniformly dispersed and contained in the substrate itself, if it is necessary to contain a large amount, a large amount of titanium oxide appears on the surface of the substrate. It is not different from the case of coating. Rather, the entire substrate becomes brittle due to the inclusion of a high concentration of titanium oxide-containing material, and the problem becomes greater than in the case of coating. However, a so-called bulk material obtained by mixing TiO ₂ powder with powder mainly composed of alumina (Al ₂ O ₃ ) and firing it into a plate shape contains a very small amount of about 10% by weight or less in terms of the mixed amount of TiO _2. Nevertheless, it has become clear that the catalyst gas effect is extremely remarkable.

The discharge body for an ozone generator of the present invention was developed based on such knowledge, and is a dielectric used for a discharge cell of a discharge type ozone generator, comprising an alumina substrate, and made of titanium oxide. It consists of a bulk material containing 0.006 to 6% by weight of Ti element. This titanium oxide content generally corresponds to 0.01 to 10% by weight in the mixing ratio of TiO ₂ powder in the substrate manufacturing stage.

  A first advantage of a bulk material in which titanium oxide is dispersed and mixed in an alumina substrate is that an ozone concentration can be effectively increased by a catalyst gas with a small amount of titanium oxide. Since the amount of titanium oxide used can be suppressed, it can be avoided that a large amount of alumina is exposed on the substrate surface and the purpose of using alumina is hindered. The purpose of using alumina is to ensure cleanliness by ozone resistance and sputtering resistance.

  The second advantage is that by dispersing a small amount of titanium oxide in an alumina substrate, it is possible to avoid surface flatness deterioration, which is a problem in coating, particularly thick coating, and unevenness in gap amount distribution due to this flatness deterioration. That is, it can be avoided. Recently, there is a strong tendency to reduce the gap amount from the viewpoint of ozone generation efficiency. Depending on the ozonizer, the gap amount has been reduced to the order of 0.1 mm or less, and the uniform gap amount distribution is an important technical issue. Incidentally, if the gap amount distribution is non-uniform, the performance deteriorates due to non-uniform discharge.

  The reason why titanium oxide dispersed and mixed in an alumina substrate is more effective for the catalytic gas effect than titanium oxide coated on the substrate surface is assumed as follows.

  Both the dispersed mixed titanium oxide and the coated titanium oxide are effective against the catalyst gas, but the former titanium oxide exists exclusively at the interface of the alumina grains, and there is almost no risk of peeling or dropping off from the substrate. On the other hand, since the latter titanium oxide is simply attached to the substrate surface, there is a great risk of peeling and dropping from the substrate, and it is necessary to attach a large amount in anticipation of this. For this reason, the latter titanium oxide inevitably increases the amount used, and the cleanliness is inevitably lowered. For this reason, the amount of use of the former titanium oxide can be reduced, and a high cleanliness can be secured.

  In addition, the present inventors consider the reason why titanium oxide is effective for improving the ozone concentration while alumina is not effective as follows.

  In recent ozonizers, reduction of the discharge gap has been pursued as one direction for improving ozone generation efficiency. By reducing the discharge gap, the cooling efficiency increases, and the ozone generation efficiency increases by increasing the energy distribution of 5 eV or more, which is advantageous for generating ozone.

  As a technical problem different from the pursuit of ozone generation efficiency from the ozonizer surface, there is a phenomenon that the ozone concentration does not increase when the source gas passing through the discharge gap is high-purity oxygen. For this, it is effective to add a catalyst gas such as nitrogen gas to the oxygen gas. Although there are various theories as the reason, a common idea is made in that the catalyst gas acts directly and indirectly on the discharge space which is an ozone generation field. However, according to this theory, even when the dielectric is alumina, the ozone concentration must be increased by adding the catalyst gas. However, in reality, as described above, the ozone concentration does not increase despite the addition of the catalyst gas.

  Under such circumstances, the present inventors paid attention to light emission of the catalyst gas in the discharge gap and modification of the dielectric surface due to the light emission as the reason for the effect of improving the ozone concentration by the catalyst gas. That is, there is a reaction to light as one of the physical differences between alumina and titanium oxide. FIG. 2 is a graph showing the relationship between light energy and interband transition intensity for alumina and titanium oxide. Alumina reacts only to light having a relatively short wavelength of 8 eV or more. In contrast, titanium oxide reacts to light having a relatively long wavelength of less than 8 eV.

  The relationship between the wavelength of light and energy is expressed by Equation 1, the energy when nitrogen gas is emitted in the discharge field is about 3 eV, the wavelength is about 400 nm, the energy when oxygen gas is emitted is around 2 eV, the wavelength Is around 600 nm.

  As can be seen from FIG. 2, the photoreactivity of alumina is high. However, the band is limited to high energy (short wavelength). On the other hand, the photoreactivity of titanium oxide is lower than that of alumina. However, the band is wide on the low energy (long wavelength) side. As a result, alumina does not react to the emission of nitrogen gas, and naturally no reaction can be expected to the emission of oxygen gas. In contrast, titanium oxide can be expected to react sufficiently to light emission of nitrogen gas, and may react to light emission of oxygen gas. Based on this fact, the inventors of the present invention have the following reasons for the ozone concentration improvement effect of the catalyst gas: light emission in the discharge field of the added catalyst gas, reaction / modification of the dielectric surface with respect to the light, and surface modification. We are looking to improve the ozone yield as a result of quality.

  The present inventors consider the following possibilities by further developing this idea. The dielectric is preferably one that responds to light having a relatively long energy and a long wavelength. Specifically, those that react with light longer than 200 nm including light emission of nitrogen gas and oxygen gas are preferable, and those that respond to light longer than 400 nm are particularly preferable. On the other hand, the catalyst gas is preferably one that has high emission energy and emits light having a short wavelength. If a catalyst gas that emits light of 8 ev or more (155 nm or less) is used and light is not absorbed by oxygen gas, the effect of the catalyst gas may be expected even when the dielectric is alumina.

  In short, the type of dielectric and the type of catalyst gas used are organically related through light emission in the discharge field, and the catalyst gas emits light of energy (wavelength) with which the dielectric reacts. When the catalyst gas is fixed to nitrogen or the like, it is necessary to use a dielectric that reacts to light emission of the catalyst gas. The energy when oxygen gas is emitted in the discharge field is around 2 eV, and the wavelength is around 600 nm. If a dielectric that reacts to light with a relatively long wavelength (600 nm or more) below this energy is used, the oxygen gas itself creates an effective environment for improving the ozone yield, and the possibility of eliminating the need for a catalyst gas is also possible. is there.

  As will be described later in the comparative test, the use of titanium oxide-containing alumina increases the ozone concentration without using nitrogen gas, if not enough. This is considered to be the result of the titanium oxide reacting with the emission of oxygen to create an environment favorable for the production of ozone. This fact is thought to support the theory of modification of the dielectric surface considered by the present inventors.

  In the discharge body for an ozone generator of the present invention, the purity of the alumina substrate is preferably higher from the viewpoint of cleanliness, preferably 80% or more, 90% or more, particularly 95% or more, and particularly preferably 99% or more. .

  The content of titanium oxide is 0.006 to 6% by weight in terms of the amount of Ti element because the ozone concentration does not increase sufficiently if it is less than 0.006% by weight, and the mechanical strength of the dielectric exceeds 6% by weight. This is because there is a shortage and problems arise not only in use but also in production. The titanium oxide content is affected by the purity of the alumina substrate, and the higher the purity, the more the content is limited. The ability to suppress the content of titanium oxide to a small amount is very preferable from the viewpoint of improving the purity of the alumina substrate. Therefore, it becomes possible to use a high-purity alumina dielectric such as 95% or more, 99% or more. is there. A particularly preferable Ti element amount ratio is 0.006 to 0.3% by weight mainly from the viewpoint of alumina purity.

As a component other than titanium oxide, a slight amount of SiO ₂ (silica), CaO (calcia), MgO (magnesia) and the like are contained as auxiliary materials (firing aids). The content of this auxiliary material (firing aid) also decreases as the purity of the main raw material alumina increases. In addition to the auxiliary raw materials, trace amounts of Na ₂ O, K ₂ O, Fe ₂ O _{3 and the} like are contained as inevitable impurities accompanying the use of the raw materials. Chromium oxides such as Cr ₂ O ₃ may be contained as an auxiliary material (firing aid), but this should be limited to 3% by weight or less in terms of the amount of Cr element from the viewpoint of suppressing metal contamination of ozone gas. Is desired.

  The thickness of the alumina substrate is preferably 0.05 to 1 mm. If this is too thin, the withstand voltage value will be low, and it will be difficult to ensure the required mechanical strength. If it is too thick, the distance between the electrodes increases and the discharge voltage increases.

  As the substrate structure, it is preferable from the viewpoint of preventing abnormal discharge that an electrode is formed as a thin film on the surface of the alumina substrate on the side opposite to the discharge gap. Examples of the material for the electrode film include Cu, Ag, Al, Au, and the like. The thickness of the electrode film is preferably 5 to 70 μm. If this is too thin, heat may be generated in a portion where the pattern width is narrow, and disconnection may occur. If it is too thick, there are many technical problems and it is difficult to form a uniform film thickness. As a method for forming the electrode film, metal foil adhesion, sputtering, vapor deposition, thermal spraying, screen printing, and the like are preferable from the viewpoint of uniform film thickness.

  As the substrate structure, it is preferable that the electrode film is coated on a portion other than the outer edge portion of the alumina substrate, and a flow hole for the source gas / ozone gas is provided at the outer edge portion of the alumina substrate so as not to contact the electrode film. With this substrate structure, the contact between the ozone gas and the electrode film can be completely avoided even though the ozone gas is discharged in a direction perpendicular to the substrate.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view of an ozone generator dielectric showing an embodiment of the present invention.

The dielectric 10 for ozone generators of this embodiment consists of a substantially square ceramic substrate. This ceramic substrate is, for example, an alumina plate having a purity of 80% or more. As other components, titanium oxide is contained in an amount of 0.006 to 6 wt. ), Contains a small amount of inevitable impurities. Such a dielectric 10 is manufactured by mixing a predetermined amount of Al ₂ O ₃ powder, TiO ₂ powder, various powders as auxiliary materials (firing aids) and a binder, forming them into a plate shape, and then firing them. Is done. The binder disappears in the firing stage.

  An electrode film 20 is formed on the surface of the dielectric 10 on the side opposite to the discharge gap by metal foil bonding, leaving the outer edge portion in a frame shape. The two outer edges of the four outer edges are provided with flow holes 11 and 11 for source gas / ozone gas, and the other two facing outer edges are for supplying and discharging refrigerant. The circulation holes 12 and 12 are provided. Each of the flow holes 11, 11 and 12, 12 is a long hole parallel to each side.

  One of the four corners is provided with a through hole 13 for drawing out the electrode. A conductive rod passes through the through hole 13. For electrical contact with the rod, the electrode film 20 is extended toward the corner so as to surround the through hole 13.

  In the ozone generator, the two dielectrics 10 and 10 are combined with a predetermined gap with the electrode film 20 facing the back side to form a discharge gap. A discharge cell is configured by stacking cooling bodies while sandwiching the dielectric unit therebetween. The cooling body is a ceramic plate having the same shape and size as that of the dielectric body 10 and is configured to allow the coolant to circulate. The two outer edges of the four outer edges are opposed to the material gas / ozone gas circulation holes. The other two opposing outer edges have flow holes for supplying and discharging refrigerant.

  In the constructed discharge cell, a raw material gas supply path is formed by continuous flow holes for raw material gas in the stacking direction, and an ozone gas discharge path is formed by continuous flow holes for ozone gas in the stacking direction. . Further, the refrigerant supply / discharge passages are formed by the circulation holes for supplying / discharging the refrigerant continuing in the stacking direction.

  In the operation of the ozone generator, high-purity oxygen gas is supplied as a source gas to the discharge cell. Further, the electrode film 20 formed on the back surface of one dielectric 10 in each dielectric unit is grounded, and a predetermined high voltage is applied to the electrode film 20 formed on the back surface of the other dielectric 10. The oxygen gas supplied to the discharge cell passes through the flow holes 11 for the source gas in each dielectric unit, flows between the dielectrics 10, 10, that is, into the discharge gap, is ozonized, and flows into the ozone gas flow holes 11. Through the discharge cell. Further, the refrigerant is supplied to the cooling bodies on both sides of the dielectric unit through the refrigerant supply / discharge circulation holes 12 and 12.

  In such an oson generator, the influence of the structure and component composition of the dielectric 10 on the ozone generation characteristics was investigated. The results will be described below.

As Conventional Example 1, an alumina substrate having a high purity of 99% was used as the dielectric. It contains no titanium oxide and is not coated. The thickness was 0.5 mm. The area of the discharge gap was 100 cm ² , and the gap amount was 0.1 mm (100 μm). An oxygen gas having a purity of 5N or more was used as a source gas. The stable ozone concentration was extremely low at 5 g / m ³ (N). Incidentally, the target ozone concentration is 200 g / m ³ (N).

As Conventional Example 2, 0.8% nitrogen gas was added to the oxygen gas. Despite the addition of a relatively large amount of nitrogen gas, the stable ozone concentration is still as low as 50 g / m ³ (N). That is, the effect of adding nitrogen gas is very insufficient.

As Conventional Example 3, the surface of the alumina substrate on the discharge gap side was coated with a titanium oxide-containing substance (TiO ₂ content 60 wt%) to a thickness of 1000 Å by vapor deposition. Oxygen gas added with 0.8% nitrogen gas was used. As the stable ozone concentration, a target value of 200 g / m ³ (N) was secured. However, as described above, such a coating inhibits the excellent properties (high cleanliness due to high ozone resistance, sputtering resistance, etc.) of the alumina plate and makes the discharge gap amount non-uniform.

As Example 1 of the present invention, an alumina substrate having a high purity of 99% or more and containing 0.006% by weight of titanium oxide in a Ti element amount ratio was used as a dielectric. Other conditions were the same as those in Conventional Examples 2 and 3. That is, oxygen gas containing 0.8% nitrogen gas was used as the source gas. Despite the very small amount of titanium oxide, a stable ozone concentration of 200 g / m ³ (N) was secured.

As Example 2 of the present invention, an alumina plate having a purity as high as 95% and containing 3% by weight of titanium oxide in a Ti element amount ratio was used as a dielectric. Other conditions were the same as in Example 1. A stable ozone concentration of 200 g / m ³ (N) was secured.

As Example 3 of the present invention, an alumina plate having a purity as high as 90% and containing 6% by weight of titanium oxide in a Ti element amount ratio was used as a dielectric. Other conditions were the same as in Examples 1 and 2. A stable ozone concentration of 200 g / m ³ (N) was secured.

As Comparative Example 1, an alumina plate having a purity as high as 99% or more and containing titanium oxide in an amount less than the range of the present invention (0.005 wt% in terms of Ti element amount) was used as a dielectric. The other conditions were the same as in Examples 1, 2, and 3. The stable ozone concentration was as low as 80 g / m ³ (N).

  As Comparative Example 2, an attempt was made to use an alumina plate having a high purity of 90% and containing titanium oxide in an amount larger than the range of the present invention (7% by weight in terms of Ti element amount) as a dielectric. The mechanical strength was insufficient and it was difficult to produce the dielectric itself.

As Example 4 of the present invention, an alumina plate having a purity as low as 80% and containing 6% by weight of titanium oxide in a Ti element amount ratio was used as a dielectric. The other conditions were the same as in Examples 1, 2, and 3. A stable ozone concentration of 200 g / m ³ (N) was secured. Incidentally, in the case of this low-purity alumina substrate containing a large amount of titanium oxide, a stable ozone concentration of about 100 g / m ³ (N) was ensured without the addition of nitrogen gas.

  From the above examples, it is clear that the inclusion of titanium oxide is effective in bringing out the effect of adding the catalyst gas. In each example, the amount of metal element such as Ti was measured by XPS (X-ray photoelectron spectroscopy).

It is a perspective view of an ozone generator dielectric showing an embodiment of the present invention. It is the graph which showed the relationship between light energy and the transition intensity between bands about an alumina and a titanium oxide.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Dielectric material 11 Flow hole for source gas and ozone gas 12 Flow hole for refrigerant supply / discharge 13 Through hole for electrode lead-out 20 Electrode film

Claims (6)

  A dielectric used for a discharge cell of a discharge type ozone generator, which is made of an alumina substrate and made of a bulk material containing 0.006 to 6 wt% of titanium oxide in a Ti element amount ratio. Dielectric for ozone generator.   The dielectric for ozone generator according to claim 1, wherein the alumina substrate has a purity of 80% or more. 2. The dielectric for an ozone generator according to claim 1, wherein the amount of Cr oxide in the alumina substrate is limited to 3 wt% or less in terms of a Cr element amount ratio.   The dielectric for ozone generator according to claim 1, further comprising an electrode film on a surface of the alumina substrate on a side opposite to the discharge gap.   The electrode film is coated on a portion other than an outer edge portion of the alumina substrate, and a flow hole for a source gas and an ozone gas is provided on the outer edge portion of the alumina substrate so as not to contact the electrode film. The dielectric material for ozone generators as described.   The dielectric for ozone generator according to claim 1, wherein the alumina substrate has a thickness of 0.05 to 1 mm.

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