JPH0227287B2 - KOJUNDOPARATANGUSUTENSANANMONIUMUKETSUSHONOSEIZOHOHO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing high-purity ammonium paratungstate crystals, particularly
The present invention relates to a method for producing high-purity ammonium paratungstate crystal suitable for use as a raw material for high-purity tungsten materials for the electronic industry, including electrodes and wiring of semiconductor devices such as gate electrodes and drain electrodes of VLSI/MOS devices. Background of the Invention Electrodes or wiring of semiconductor devices, especially MOS/
Polysilicon has traditionally been used as the gate electrode and drain electrode of LSI, but
As devices become more highly integrated, signal propagation delays due to the resistance of polysilicon gate electrodes and drain electrodes become a problem. On the other hand, in order to facilitate the formation of MOS elements by the self-alignment method, it is desired to use materials with high melting points for the gate electrode and the drain electrode. Under these circumstances, research is progressing on high-melting point metal gate electrodes and drain electrodes, which have lower resistivity than polysilicon, while research on high-melting point metal silicide electrodes, which prioritize compatibility with silicon gate processes, is actively progressing. be. Promising examples of such refractory metals and refractory metal silicides are tungsten and tungsten silicide. Taking tungsten as an example, the tungsten gate electrode and drain electrode are formed as thin films by a sputtering method, an electron beam evaporation method, or the like. The sputtering method is a method in which a target plate is bombarded with argon ions to release metal, and the released metal is deposited on a substrate facing the target plate. The electron beam evaporation method is a method in which an ingot evaporation source is melted by an electron beam and vapor deposition is performed.
In any case, the purity of the produced film depends on the purity of the target plate or evaporation source (hereinafter collectively referred to as target). Currently commercially available tungsten targets are manufactured by powder metallurgy using commercially available 99.9% pure tungsten powder as raw material, so their purity is equivalent to that of the raw material tungsten powder. On the other hand,
Polycrystalline silicon, SiO ₂ , etc., which are other constituent materials of MOSLSI, have a purity that is more than two orders of magnitude higher, so there is no way to apply tungsten to MOSLSI unless the impurity content is reduced by more than two orders of magnitude. . The same can be said of the tungsten silicide electrode. Tungsten silicide electrode is
The electrodes are formed using a tungsten silicide target or a combination of a tungsten target and a silicon target, but since silicon is available in high purity, the purity of the formed electrode depends on the tungsten source used. It can be said that. Incidentally, impurities that affect MOS devices can be broadly classified into the following three types. (1) Alkali metals such as Na (2) Radioactive elements such as U and Th (3) Transition metals such as Fe, especially alkali metals such as Na, easily move in the gate insulating film and deteriorate the MOS interface characteristics. ,or,
Radioactive elements are caused by alpha rays emitted from the element.
This has a fatal effect on the operational reliability of MOS devices.
Transition metals such as Fe also impair the reliability of MOS device operation. Therefore, in order to improve the purity of tungsten and tungsten silicide gate electrodes and drain electrodes, it is necessary to increase the purity of the raw material tungsten powder and, going back further, to improve the grade of the raw material for the tungsten powder itself. Ammonium paratungstate crystals are one of the useful raw materials for such tungsten powder. Conventional technology and its problems In general, wet refining methods for tungsten include:
There are methods to precipitate tungstic acid crystals in an acidic region and methods to precipitate ammonium paratungstate in a neutral region, but at present, industrially, the method of precipitating ammonium paratungstate crystals is preferable. Therefore, the latter method is generally adopted. The latter method is described in detail: (a) A method in which ammonium paratungstate is precipitated by dissolving tungstic acid in aqueous ammonia and adding an acid in the pH range of 6 to 8 to this mother liquor or concentrating this mother liquor. . (b) A method in which ammonium paratungstate is precipitated by dissolving ammonium metatungstate in water and adding aqueous ammonia to the mother liquor at a pH in the range of 6 to 8. There is. However, although the above method yields large crystals and is easy to handle, it is insufficiently effective in purifying alkali metals such as Na, radioactive metals such as U, and transition metals such as Fe. It is not sufficient as a raw material. SUMMARY OF THE INVENTION In view of the above circumstances, the present invention has been developed by using ammonium metatungstate as a raw material by a wet refining method.
Alkali metals such as Na, radioactive elements such as U, and Fe
The purpose of this study is to establish a method for producing high-purity ammonium paratungstate crystals with an extremely low content of transition elements such as. As a result of repeated studies aimed at the above-mentioned purpose, the present inventors have found that it is possible to incorporate a pre-purification step into the conventional method of adjusting the pH by adding ammonia water directly from an aqueous solution of ammonium metatungstate and heating the solution. It was concluded that this method is effective in refining Fe. In the preliminary purification step, the acid concentration of the aqueous solution of ammonium metatungstate is adjusted to precipitate water-soluble tungsten compound crystals, and after solid-liquid separation, the water-soluble tungsten compound crystals are purified again with water. It has been found that the best process is to obtain a purified ammonium paratungstate crystallization mother liquor by dissolving the ammonium paratungstate in water and filtering it. Thus, in the present invention, ammonium metatungstate is dissolved in water to produce a tungsten-containing aqueous solution, a high purity inorganic acid is added to the aqueous solution, a water-soluble tungsten compound is precipitated, and after solid-liquid separation,
The water-soluble tungsten compound is dissolved in water again to produce a purified ammonium paratungstate crystal precipitation mother liquor, the purified ammonium paratungstate crystal precipitation mother liquor is heated, and the pH is adjusted by adding ammonia water to produce paratungsten. Provided is a method for producing high-purity ammonium paratungstate crystals, which is characterized by precipitating ammonium acid crystals. DETAILED DESCRIPTION OF THE INVENTION The drawings are process diagrams of the present invention, and the following description will be made with reference to them. The present invention uses ammonium metatungstate as a starting material. Examples of alkali salts of metatungstic acid include ammonium metatungstate, sodium metatungstate, potassium metatungstate, etc. However, salts other than ammonium metatungstate are not so preferred because they contain large amounts of alkali metals. Practically, ammonium metatungstate is used. Ammonium metatungstate selected for the above reasons is first dissolved in pure water and filtered to create a tungsten-containing aqueous solution. An acid is added to the solution to control the acid concentration in the mother liquor. As the acid used, any of nitric acid, hydrochloric acid, sulfuric acid, etc. can be used, but nitric acid is preferred because it can be easily purified to a high degree by distillation or the like. The acid concentration after acid addition ranges from around 5 normal to 12
A range close to the specified value is preferable. A more preferable acid concentration is 8 to 12 normal, most preferably 9.5 to 11 normal. Regarding the tungsten concentration in the liquid, it is 50g/
The above is preferable. Next, the acid-adjusted solution is maintained at a temperature of 10 DEG to 50 DEG C. and stirred to precipitate crystals of a water-soluble tungsten compound. Above upper limit temperature 50℃
If the temperature is higher than this, some water-insoluble tungstic acid will precipitate, which is not preferable, and if the temperature is below 10°C, the crystal precipitation speed will be slow. The water-soluble tungsten compound crystals precipitated for a predetermined period of time are filtered by a conventional method to obtain water-soluble tungsten compound crystals. Next, the crystals of the water-soluble tungsten compound are again dissolved and filtered in ultrapure water or pure water to provide a mother liquor for precipitation of purified ammonium paratungstate crystals. The pre-purification stage is thus completed. In some cases, the above steps may be repeated one or more times, but for practical purposes this alone is sufficient. While heating the above ammonium paratungstate crystal precipitation mother liquor, ammonia water is added,
Adjust the pH of the liquid to a range of 6.0 to 8.0 to precipitate ammonium paratungstate crystals. The temperature during crystal precipitation of ammonium paratungstate is 50°C or higher, preferably 80°C or higher. When ammonium paratungstate crystals are precipitated at low temperatures, dodecahydrate is also precipitated. This 12-hydrate salt has strong deliquescent properties, making drying difficult. 50℃～70℃
In this case, pentahydrate salt precipitates out, and when the temperature exceeds 80°C, anhydrous salt precipitates out. Regarding the pH during precipitation, if it is lower than 6, crystals other than ammonium paratungstate will precipitate, and if it is higher than 8, the recovery rate of ammonium paratungstate will decrease, which is not preferable. The ammonium paratungstate crystals precipitated for a predetermined period of time are subjected to a normal process and then recovered as high-purity ammonium paratungstate crystals. To avoid contamination as much as possible, alkali glass should be avoided as the material for equipment or containers used in wet refining processes such as dissolution, percolation, and crystallization, or in distillation processes. A corrosion-resistant material that does not contain radioactive elements should be selected. Dissolution,
The water used in processes that directly affect product purity, such as dilution and washing, should be pure water, preferably with a specific resistance of 18 MΩcm.
The above ultrapure water (hereinafter simply referred to as ultrapure water) is preferable. Furthermore, the utmost care must be taken to prevent contamination of impurities from the surrounding environment such as the atmosphere. The high-purity ammonium paratungstate crystals obtained by the above method are dried and roasted to obtain anhydrous tungstic acid, and then subjected to hydrogen reduction at a high temperature to obtain high-purity tungsten powder. There are no particular restrictions on drying, roasting, and hydrogen reduction, and conventional methods can be used. Although the above explanation has been made with specific reference to the gate and drain electrodes of MOS/LSI or MOS/VLSI, the present invention is also useful as a raw material for tungsten materials for other electronic industry parts and even various ceramic articles. be. Effects of the Invention According to the present invention, alkali metals such as Na, K, etc.
80ppb or less, radioactive elements such as U 2ppb or less, and
A truly high-purity ammonium paratungstate crystal with less than 1 ppm of transition elements such as Fe can be obtained, which improves the quality of tungsten powder made from it, as well as parts for the electronic industry etc. made from it. Example A predetermined amount of commercially available ammonium metatungstate was collected in a transparent quartz beaker, dissolved in ultrapure water with a specific resistance of 18 MΩcm, and filtered through a quartz filter.
Thereafter, special grade concentrated nitric acid was added, and the aqueous solution adjusted to an acid concentration of 10N and a tungsten concentration of 300g was stirred at room temperature (20° to 30°C) for 24 hours to precipitate crystals of a water-soluble tungsten compound. The crystals of the water-soluble tungsten compound were filtered through a quartz filter and further washed with special grade concentrated nitric acid.
The crystals of the water-soluble tungsten compound were redissolved in the same ultrapure water as described above and filtered through a quartz filter to produce a mother liquor for precipitation of purified ammonium paratungstate crystals. Thereafter, the mother liquor was heated to 80°C and aqueous ammonia was added to adjust the pH to 7.1. The mixture was held for 20 hours to grow ammonium paratungstate crystals. Thereafter, the crystals were separated using a quartz filter to obtain crystals of high purity ammonium paratungstate. Table 1 shows the content of impurities in the high purity ammonium paratungstate. As is clear from Table 1, the high purity ammonium paratungstate crystals obtained in the present invention are
The content of alkali metals, radioactive elements, and transition elements is very small, and both the crystals of water-soluble tungsten compounds and the crystals of tungstic acid are large and easy to handle. High purity tungsten powder can be easily obtained by drying and roasting the above-mentioned high purity ammonium paratungstate crystals as a raw material for tungsten powder, followed by hydrogen reduction. 【table】

[Brief explanation of drawings]

The drawings are process charts showing the flow of this method.

Claims (1)

[Claims] 1. Ammonium metatungstate is dissolved in water to produce a tungsten-containing aqueous solution, a high-purity inorganic acid is added to the aqueous solution, a water-soluble tungsten compound is precipitated, and after solid-liquid separation, the water-soluble tungsten compound is precipitated. The tungsten compound is dissolved in water again to produce a purified ammonium paratungstate crystal precipitation mother liquor, the purified ammonium paratungstate crystal precipitation mother liquor is heated, and the PH is adjusted by adding ammonia water to produce ammonium paratungstate. A method for producing high-purity ammonium paratungstate crystals, characterized by precipitating crystals. 2 The high-purity inorganic acid added to the tungsten-containing aqueous solution produced by dissolving ammonium metatungstate in water is nitric acid, sulfuric acid, hydrochloric acid, or a mixed acid thereof, and the amount added is such that the acid concentration after addition is 5.
12. The method for producing high-purity ammonium paratungstate crystals according to claim 1, wherein the crystals are within the range of .about.12. 3. High purity paratungsten according to claim 1 or 2, wherein the heating temperature of the purified ammonium paratungstate crystal precipitation mother liquor is 50°C or higher, and the pH adjustment range by adding ammonia water is 6.0 to 8.0. Method for producing ammonium acid crystals.