JP2000026111A - High purity phosphoric acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce high purity phosphoric acid useful for removing a silicon nitride film in a semiconductor manufacturing stage by reducing the contents of impurities, i.e., Fe, Mn and Na, in phosphoric acid to specified values or lower respectively, at the time of expressing the concn. of H3PO4 in terms of a specified concentration. SOLUTION: The production of this phosphoric acid, which is intended for reducing the contents of impurities, namely, Fe, Mn and Na to <=25 ppb, <=3 ppb and <=40 ppb respectively at the time of expressing the concn. of H3PO4 in terms of 85 wt.% H3PO4, comprises: first, immersing a crystallization pipe through which a cooling medium consisting of a water-ethyleneglycol mixed liquid is passed, in a raw material phosphoric acid solution at about 15 to 30 deg.C or preferably about 18 to 27 deg.C, to deposit phosphoric acid hemihydrate crystals on the surface of the crystallization pipe, wherein considerable amounts of impurities are separated from the deposited crystals; withdrawing the crystallization pipe from the phosphoric acid solution to separate the phosphoric acid hyemihydrate crystals from the mother liquor; and sweating the separated crystals to remove impurities contained in the crystals on their insides and outsides and to produce the objective high purity phosphoric acid contg. the above specified amounts of Fe, Mn and Na, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to high-purity phosphoric acid. Furthermore, the high-purity phosphoric acid of the present invention contains Fe, M
Since a small amount of n and Na elements are mixed, when used for removing a silicon nitride film in a semiconductor manufacturing process, a preferable electronic material substantially does not contain impurities that deteriorate electrical characteristics of a microelement. Further, the high-purity phosphoric acid of the present invention can be suitably used as a raw material for an etching solution for metallic aluminum, an alumina etching solution for ceramics, and a phosphate glass for optical fibers because of the feature that the content of impurities is small.

[0002]

2. Description of the Related Art A conventional technique for purifying phosphoric acid by a crystallization method has been found in, for example, Japanese Patent Publication No. 44-14692. According to this, a method is disclosed in which a series of crystallization purification operations including a crystallization operation, a separation operation from a mother liquor, and a melting operation are repeated three times in order to purify phosphoric acid to a desired purity.

Oimura et al. [Report of Toyo Soda: 10, 2, 21]
(1966)] reveals the basic physical property data for purifying phosphoric acid by a crystallization operation, such as saturation solubility, correlation between supersaturation and growth rate, hygroscopicity of phosphoric acid hemihydrate crystals, and crystal sedimentation rate in phosphoric acid. However, the purity of the precipitated phosphoric acid hemihydrate crystal particles is not quantitatively discussed.

Aoyama et al., Proceedings of a C
On pages 413 to 420 of onference of Industrial Crystallization (1976), an application example in a fluidized bed crystallizer is shown. In this fluidized bed type crystallizer, uniform coarse particles are generated, so that the separation operation from the mother liquor is easy.In addition, the solution is cooled by an external heat exchanger provided in the fluidized bed, and crystallization is efficiently performed. It has the characteristic that the production rate can be increased by removing heat.

[0005] As described above, it is known to purify phosphoric acid by a crystallization operation. However, the purity of phosphoric acid obtained by these operations is not clearly described. Although the method of purifying by the method has been clarified, the effect of the method is not specified.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been completed as a result of intensive studies in view of such prior art. Various metal elements are dissolved or insoluble in a liquid state. Regardless, an object of the present invention is to provide high-purity phosphoric acid substantially equivalent to the limit of detection.

[0007]

That is, the present invention provides H ₃
Impurities contained when the concentration of PO ₄ is converted to 85% by weight relate to high-purity phosphoric acid, which is characterized in that Fe is 25 ppb or less, Mn is 3 ppb or less, and Na is 40 ppb or less.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
In the present invention, phosphoric acid is a mixed liquid of a component represented by the following general formula H ₃ PO ₄ and H ₂ O in an arbitrary ratio, and the concentration thereof is determined by titration with sodium hydroxide as shown in JIS K-1449. It is measured by the method.

In general, phosphoric acid has a low vapor partial pressure as compared with other mineral acids, so that it is not easy to purify by applying a distillation method, and it is substantially very difficult to purify industrially. Although it is a simple compound, the high-purity phosphoric acid according to the present invention is a phosphoric acid characterized by having a small content of Fe, Mn and Na, that is, iron, manganese and sodium as described above.

The phosphoric acid according to the present invention has an impurity amount of ppb
Since the measurement accuracy is actually a problem from the point of handling the level of 定義, it is defined as follows. That is, the value of the content of each element, that is, Fe, Mn, Na, etc. in the impurity component in phosphoric acid is obtained by injecting 10 μl of a sample diluted to a dilution ratio of 5 times (100 times for Na) into a carbon heating furnace, The phosphoric acid was removed by heating to 800 ° C., and the absorbance of light at the measurement wavelength determined by the element was compared with the standard sample in dilute hydrochloric acid at the atomization temperature according to the element shown in Table 1. To be measured and evaluated by the so-called flameless atomic absorption spectrometry. Further, the purity of phosphoric acid is expressed as a ratio of the mass of an element per unit mass of phosphoric acid. Fe, M by flameless atomic absorption spectrometry
Table 1 shows the lower detection limits of n and Na.

[0011]

[Table 1]

The lower limit of detection was calculated by the following equation.

(Equation 1) Here, the sample injection amount is 10 μl, and the sample density is 1 g / cm.
^{It was set to 3} .

The high-purity phosphoric acid according to the present invention has an impurity amount, particularly Fe, Mn and Na, of 25 ppb or less, 3 ppb or less and 40 ppb or less, respectively, determined by the above-mentioned measuring method.

Since phosphoric acid is highly corrosive to various metals, it is useful as a surface etching solution for metals and the like. However, the phosphoric acid according to the present invention is a high-purity phosphoric acid having a small amount of impurities of each of the above elements. It is most suitable as an etchant for metals, glass and the like used in the field of semiconductors and other precision industries.

Next, the high-purity phosphoric acid according to the present invention comprises:
It can be industrially produced by a process essentially consisting of the crystallization step (a) and the purification step of the phosphoric acid crystal (b) by perspiration.

The raw material phosphoric acid applicable in the present invention is:
There is no particular limitation, and it is industrial phosphoric acid obtained by a dry method or a wet method, and its concentration is 70% by weight or more. If the concentration is less than 70% by weight, the saturation temperature is low, and a large amount of energy is required to obtain an appropriate degree of supersaturation during crystallization, which is not industrially preferable. The upper limit is up to the saturation concentration (91.6% by weight). If it is higher than this, crystals other than phosphoric acid hemihydrate crystals may be precipitated, and the viscosity is further increased, and crystallization and separation become difficult, which is not preferable. . For the purpose of the present invention, it is preferable to use phosphoric acid as purified as possible and excluding suspended particles.

(A) Crystallization Step This step is a crystallization step in which phosphoric acid hemihydrate crystals are precipitated from the raw phosphoric acid. The temperature of the raw phosphoric acid is 15-30 ° C,
The temperature is preferably 18 to 27 ° C., and a circulation system is configured so that temperature control can be performed efficiently.

As described above, the crystallization of phosphoric acid involves immersing a crystallization tube through which a refrigerant has been circulated in a raw phosphoric acid solution to precipitate hemihydrate crystals on the surface of the tube.

In many cases, the crystallization tube has a double-cylindrical hollow structure made of, for example, hard glass, and the temperature of the tube surface is reduced to a predetermined supercooling temperature by flowing a coolant through the inside. A relatively simple one that can be adjusted may be used.

Therefore, the surface of the crystallizing tube does not have to be particularly cylindrical, and the temperature of the surface of the crystallizing tube is ± 1 ° C., preferably ± 0. If the temperature can be maintained at 05 ° C., a flat surface or another shape can be adopted.

The refrigerant is a mixed solution of water and ethylene glycol. Other refrigerants may be used as long as solids do not precipitate and have fluidity in the operating temperature range. By immersing such a crystallization tube in the raw phosphoric acid, the hemihydrate crystals precipitate on the surface of the tube from the supercooled state.

In the present invention, the supercooling temperature is -2 to
A range of −8 ° C. is preferable, and this is maintained by flowing the refrigerant in such a state. Here, the supercooling temperature is a value obtained by subtracting a saturation temperature represented by the following equation with respect to the concentration of phosphoric acid from the temperature of phosphoric acid.

(Equation 2) Here, the phosphoric acid concentration is represented by (% by weight).

Accordingly, the supercooling temperature takes a negative value in the crystallization operation, and takes a positive value in the sweating operation in the next purification step. The reason for setting the temperature is determined from the relative relationship between the yield of high-purity phosphoric acid and the efficiency of impurity removal by many experiments by the present inventors. In the crystallization step, the precipitated phosphoric acid hemihydrate crystals have been substantially free of impurities compared to the phosphoric acid used as the mother liquor, and the purification effect is remarkable depending on the crystallization conditions.

(B) Purification Step In this step, the hemihydrate crystals precipitated in the previous step are separated from the mother liquor by taking out the crystallization tube from the phosphoric acid solution.
In this step, phosphoric acid is further purified by sweating the crystal to remove impurities contained inside and outside the crystal. here,
Sweating is an operation in which the supercooling temperature of the mother liquor attached to the crystal surface separated from the mother liquor is kept within a predetermined range to melt a part of the surface of the crystal and drop it. The impurities taken into the crystal are removed as a drop solution. Accordingly, the supercooling temperature at this time is set from the relative relationship between the yield of phosphoric acid hemihydrate crystals and the effect of removing impurities, and in many cases is in the range of 0 to 10 ° C.

The amount of melting in the sweating operation is in the range of 10% by weight or more and 40% by weight or less when expressed as a ratio of the amount of phosphoric acid solution dropped to the mass of the precipitated phosphoric acid hemihydrate crystals. Preferably, the operation is performed in the range of 20% by weight or more and 35% by weight or less. According to the study of the present inventors, if the amount of melting is less than 10% by weight, the effect of perspiration is insufficient and it is difficult to obtain high-purity phosphoric acid. This is because the effect of the sweating operation is saturated.

The desired high-purity phosphoric acid can be sufficiently obtained by the perspiration of phosphoric acid crystals. If necessary, either before or after the perspiration, ultrapure water or the high-purity phosphoric acid of the product is used to crystallize the crystallization tube surface. A purification step for removing impurities by replacing and washing the phosphoric acid hemihydrate crystals precipitated on the substrate may also be added. In this way, the displacement washing is to contact a mixture of ultrapure water or high-purity phosphoric acid at an arbitrary ratio to the surface of the precipitated phosphoric acid hemihydrate crystal to remove the raw phosphoric acid adhering to the phosphoric acid hemihydrate crystal. In this operation, impurities contained in the phosphoric acid hemihydrate crystals can be removed as effectively as possible. In many cases, it is more reasonable to perform this operation after the sweating operation. Thus, according to the present invention, a high-purity phosphoric acid crystal is obtained, and then, if necessary, is dissolved in ultrapure water to produce a high-purity phosphoric acid solution industrially advantageously.

According to the method of the present invention for producing high-purity phosphoric acid, the raw material phosphoric acid is crystallized as hemihydrate crystals in a supercooled state at a predetermined temperature, whereby primary purified phosphoric acid hemihydrate crystals are obtained. Then, the impurities taken into the crystal are migrated to the crystal surface and concentrated by the sweating operation of the crystal, and the impurity and the impurity of the mother liquor attached to the crystal surface are dropped in the droplet. Removal results in a second purification.

Further, more effective purification can be achieved by performing a third purification, which is carried out by washing with water or high-purity phosphoric acid before and after the second purification, preferably thereafter, if desired. Purified phosphoric acid can be obtained.

[0029]

EXAMPLES The starting phosphoric acid used in the examples was prepared as follows. That is, a phosphoric acid (89% by weight as H ₃ PO ₄ ) produced by a dry method, which is a known method for producing phosphoric acid, is added to a polytetrafluoroethylene (PTFE) having a pore size of 0.8
μm membrane filter [Toyo Roshi Kaisha Co., Ltd .: T
080A047A].
m, and the number of particles having a particle size of 2 μm or more was removed. When the amount of the metal element in the phosphoric acid was measured by flameless atomic absorption spectrometry, Fe was 180 ppb, Mn was 50 ppb, and Na was 2 ppb.
It was 50 ppb. The 85% by weight raw phosphoric acid was prepared by diluting the 89% by weight raw phosphoric acid with ultrapure water.

Examples 1 to 3 2.5 kg of raw phosphoric acid adjusted to a predetermined concentration was placed in a hard glass container, the temperature was set to a predetermined supercooling temperature, and the raw phosphoric acid was circulated by a stirring pump. The distribution of the temperature and concentration of the raw phosphoric acid was reduced. On the other hand, outer diameter 2.5c
The operation was carried out under the following conditions using a double cylindrical crystallization tube having a length of 10 cm and a length of 10 cm. That is, the crystallization tube is immersed in the raw phosphoric acid, a coolant (a mixture of water and ethylene glycol) having a predetermined supercooling temperature is circulated inside the immersion tube, and phosphoric acid hemihydrate is placed on the heat transfer surface of the crystallization tube. The crystals were crystallized. Table 2 shows the conditions at this time. Table 2 also shows the results of measuring the mass of the precipitated crystals after the crystallization operation.

[0031]

[Table 2]

The rate at which the phosphoric acid hemihydrate crystallizes on the surface of the crystallizing tube varies depending on the supercooling temperature of the refrigerant, but is in the range of about 12 to 16 kg · cm ^-2 · hr ^-1 .

After a lapse of a predetermined crystal growth time, the crystallizing tube was taken out, and phosphoric acid was dropped and separated. Impurity components such as Fe, Mn and Na contained in the phosphoric acid hemihydrate crystals were measured by flameless atomic absorption spectrometry to determine the degree of purification of phosphoric acid in the crystallization step, and the results in Table 3 were obtained. Was done.

[0034]

[Table 3]

Next, after pulling up the crystallization tube to which the phosphoric acid hemihydrate crystals obtained in the crystallization step are attached, the refrigerant temperature during the sweating operation is adjusted to the temperature shown in Table 4, and this temperature or at least When kept in a gas phase maintained at a constant temperature within the range of ± 2 ° C., a part of the surface of the phosphoric acid hemihydrate crystal was melted and dropped together with the mother liquor attached to the crystal. The phosphoric acid hemihydrate crystals remaining on the surface of the crystallization tube were collected after the amount of the mixed solution dropped to the mass of the phosphoric acid hemihydrate crystals deposited and adhered to the surface of the crystallization tube became 30% by weight. And the component contents of Fe, Mn and Na contained in the phosphoric acid hemihydrate crystals were measured by flameless atomic absorption spectrometry. Table 4 shows the results.
Shown in

[0036]

[Table 4] Note: nd means undetectable amount. Numerical values in parentheses indicate values before perspiration (described in Table 3).

Examples 4 and 5 The supercooling temperature of the surface of the crystallizing tube was set at 5 ° C. for each of the purified phosphoric acid hemihydrate crystals obtained after sweating obtained in Examples 1 and 2. Ultrapure water and the high-purity phosphoric acid according to the present invention were subjected to replacement washing by spraying the phosphoric acid hemihydrate crystals deposited on the surface of the crystallizing tube. After this washing operation, the phosphoric acid hemihydrate crystals remaining on the surface were collected, and the contents of Fe, Mn, and Na contained in the phosphoric acid hemihydrate crystals were measured in the same manner as described above, and the results in Table 5 were obtained. Was done.

[0038]

[Table 5] Note: () shows the amount of impurities before perspiration before washing. Therefore,
Example 4 uses Example 1 and Example 5 uses the phosphoric acid hemihydrate crystals according to Example 2 after sweating.

[0039]

As described above, the high-purity phosphoric acid of the present invention, when used for removing a silicon nitride film in a semiconductor manufacturing process, has a low content of Fe, Mn and Na element components. This is a preferable electronic material that does not substantially include impurities that deteriorate the electrical characteristics of the microelement. Further, to put it further, this high-purity phosphoric acid has a feature that the content of impurities is small.
It is also a suitable material as a raw material for alumina etching solution for ceramics and phosphate glass for optical fibers.

Claims (1)

[Claims] When the concentration of H ₃ PO ₄ is converted to 85% by weight, the content of impurities is 25 ppb or less as Fe,
High-purity phosphoric acid, characterized in that it is 3 ppb or less as Na and 40 ppb or less as Na.