JP2710049B2 - Method for producing high-purity ammonium molybdate crystal - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a high-purity ammonium molybdate crystal, and in particular, a hexagonal hexagon having good filterability and detergency and high molybdenum content. The present invention relates to a method for easily producing a high-purity ammonium molybdate crystal having a columnar shape. Ammonium molybdate crystals obtained by this method can be used to manufacture molybdenum materials for the electronics industry, such as molybdenum targets used in the fabrication of electrodes and wiring for semiconductor devices typified by gate electrodes and drain electrodes of VLSI / MOS devices. It is particularly useful as a raw material for (Prior art) Polysilicon has been conventionally used as electrodes and wirings of semiconductor devices, particularly gate electrodes and drain electrodes of VLSI / MOS devices. However, polysilicon gate electrodes and drains have been associated with high integration of devices. Signal propagation delay due to resistance of electrodes and the like has become a problem. On the other hand, it is desired to use a material having a high melting point as a gate electrode, a drain electrode, and the like for facilitating formation of a MOS element by a self-alignment method from the viewpoint of a manufacturing process. Under these circumstances, while research on refractory metal gate electrodes and drain electrodes having lower resistivity than polysilicon is progressing, research on refractory metal silicide electrodes with compatibility with the silicon gate process as the first is actively progressing. I am doing it. Molybdenum and molybdenum silicide are one of the promising and practically used refractory metals and refractory metal silicides. Thin films such as a molybdenum gate electrode and a drain electrode are mainly formed by a sputtering method, an electron beam evaporation method, or the like. In the sputtering method, a metal is released by bombarding a target plate with argon ions, and the released metal is deposited on a substrate facing the target plate. In the electron beam evaporation method, an evaporation source is dissolved by an electron beam, and generated vapor is deposited on a substrate. In any case, the purity of the produced film depends on the purity of the target plate and the evaporation source used (both are referred to as targets). Molybdenum targets are commercially available, often from 99.9 to 99.9
It is manufactured by powder metallurgy using 9% pure molybdenum powder as a raw material, and the target purity is largely determined by the molybdenum powder raw material. The situation is similar for molybdenum silicide.
A thin film such as a molybdenum silicide electrode is formed by using a molybdenum silicide target or a combination of a molybdenum target and a silicon target. Can be determined by the raw material molybdenum. 2. Description of the Related Art In semiconductor devices such as MOS devices, the requirements for impurity control are becoming increasingly severe. Incidentally, impurities affecting semiconductor devices such as MOS devices are roughly classified into the following three types: (1) alkali metals such as Na and K; (2) radioactive metals such as U and Th; and (3). Among transition metals such as Fe, alkali metals such as Na and K easily move in the gate insulating film and deteriorate the characteristics of the MOS interface, and radioactive metals such as U and Th cause the operation of MOS devices by emitting α rays. Critical impact on reliability. Therefore, in order to achieve high purification of molybdenum or molybdenum silicide thin films in semiconductor devices such as MOS devices, it is necessary to upgrade the purity of the target molybdenum powder itself, which is also the raw material molybdenum powder itself. In response to such a request, the present applicants have conducted many studies on the purification of molybdenum and have accumulated results. However, there is still much room for improvement, and with the development of semiconductor devices, development of a technique for stably and inexpensively producing a higher-quality molybdenum raw material has been desired. As one of molybdenum wet refining methods, the pH of a molybdenum-containing aqueous ammonia solution is adjusted to an alkalinity of 8.9 to 9.0, the adjusted solution is heated and concentrated, and ammonium paramolybdate crystals (APMo: (NH ₄ ) ₆ Mo ₇ O ₂₄ · nH ₂ O) or ammonium dimolybdate crystal (ADM: (NH ₄ ) ₂ Mo ₂ O ₇ · nH
₂ O) a method of precipitating. However, this method
The effect of purifying radioactive metals such as U and Th and alkali metals such as Na and K is not always sufficient. As still another method, there is a method of heating a molybdenum-containing acidic aqueous solution to crystallize molybdic acid crystals (MoO ₃ .nH ₂ O). This method is not problematic in that respect, as it can achieve a corresponding purification effect, but presents the following disadvantages: The obtained crystals are acicular, so that the filtration and washing properties are poor and the workability is poor. Lack of sex. The molybdenum content of the resulting crystals is low, approximately 25-30%. When the same crystal is reduced into molybdenum powder, it is partially sintered and is unsuitable as a powder for powder metallurgy. (Problems to be Solved by the Invention) In view of the above-mentioned drawbacks, an object of the present invention is to excel in refining effects of radioactive metals such as U and Th, and furthermore, alkali metals such as Na and K, and have excellent filterability and cleaning properties. An object of the present invention is to develop a technique for producing a molybdenum powder raw material crystal having good molybdenum content. (Means for Solving the Problems) To achieve the above object, the present inventors conducted many experiments on the molybdenum-containing acidic aqueous solution, and found that an appropriate amount of ammonium ion was present in addition to the hexagonal molybdenum crystals. A new finding was obtained that another crystal having a columnar shape was detected. Based on this knowledge, as a result of intensive studies, this crystal is a type of molybdate that is different from APMo or ADM described above and is judged to be (NH ₄ ) _0.15 MoO _3.08・ 0.4H ₂ O We have determined that it is an ammonium crystal. And these crystals are excellent in purification effect, good in filterability and washability, and have a molybdenum content.
It was as high as 40 to 45%, and it was confirmed that it was suitable for the purpose of the present invention. Under these circumstances, as a result of the desire to find a method for producing the ammonium molybdate crystals that do not contain acicular molybdate crystals, through strict control of the pH by adjusting the pH to an acidity of 0 or less, Successfully established. Based on this new finding, the present invention adjusts the pH of an ammonium ion-containing molybdic acid aqueous solution to an acidity of 0 or less, and heats the acidic ammonium ion-containing molybdic acid aqueous solution to form a hexagonal column-shaped ammonium molybdate. Provided is a method for producing a high-purity ammonium molybdate crystal, which comprises precipitating a crystal, and filtering and washing the crystal. Preferably, the aqueous solution of molybdic acid containing an acidic ammonium ion has a Mo concentration of 40 to 100 g / 1, a NH ₃ ⁻ / Mo molar ratio of 0.5 or more, and a T-acid / NH ₄ ⁻ concentration (N) ratio of 8 or less (T represents the total). Put in. Further, it has been found that since the ammonium molybdate crystals are soluble in aqueous ammonia, recrystallization from an aqueous solution of molybdic acid containing ammonium ions can be repeated. Accordingly, the present invention provides 2) adjusting the pH of the ammonium ion-containing molybdic acid aqueous solution to an acidity of 0 or less, and heating the acidic ammonium ion-containing molybdic acid aqueous solution to precipitate hexagonal columnar ammonium molybdate crystals. A method for producing high-purity ammonium molybdate crystals, comprising repeating the operation of filtering and washing the crystals, dissolving the generated ammonium molybdate crystals in aqueous ammonia, and recrystallizing the crystals under the above-mentioned conditions at least once. Also provide. JP-A-55-95625 and JP-A-55-149134 describe the production of ammonium molybdate by adding sulfuric acid to a molybdic acid aqueous solution containing ammonium ions. In other words, the former is intended to produce high-purity (reagent grade) ammonium molybdate having a low potassium content by heating industrial standard molybdenum trioxide in a roasting furnace and removing it at a temperature of 600 to 800 ° C. Cool at a cooling rate faster than 30 ° C./min to 400 ° C. or less, leaching the cooled molybdenum trioxide with hot water to remove potassium, and after solid-liquid separation operation, separate the separated molybdenum trioxide into an ammonium solution, preferably Describes digestion in an ammonium hydroxide solution to produce a purified ammonium molybdate solution. Ammonium molybdate can be obtained from a purified ammonium molybdate solution by known methods such as crystallization or acidification, and the ammonium molybdate can be calcined by known methods to produce reagent grade molybdenum trioxide. And However, the potassium concentration of the resulting product, even in the case of reagent grade molybdenum trioxide, is too high for the purposes of the present invention. There is no description about adjusting the pH to 0 or less. There is no description of the composition and crystal form of the ammonium molybdate to be precipitated, and, of course, no mention is made of excess or detergency. The latter aims at producing high-purity ammonium molybdate from an aqueous alkali solution containing vanadium and molybdenum components as obtained from a spent catalyst, and an alkali aqueous solution containing an alkali metal vanadate and an alkali metal molybdate. Was concentrated to crystallize crystals of alkali metal vanadate and alkali metal molybdate, and after filtration and separation, these crystals were dissolved in water and the pH was adjusted to 9.5.
It describes that the aqueous solution adjusted to 9.5 is subjected to metathesis by ammonium salt, and sulfuric acid is added to the metathesis filtrate at a predetermined ratio to hydrolyze. The purity of the obtained ammonium molybdate is up to 99.83%, which is still impure for the purposes of the present invention. There is no description about adjusting the pH to 0 or less. There is no description about the composition and crystal form of the ammonium molybdate to be precipitated, and no mention is made of the filterability or cleaning property. (Embodiment of the Invention) As a molybdenum raw material, commercially available molybdenum and molybdenum oxide (MoO ₃ ) powders, various molybdates, molybdates such as ammonium molybdate, and the like can be used. First, the raw material is dissolved using any of acid, alkali and water. in this case,
As the acid, any oxidizing acid such as nitric acid, sulfuric acid and hydrochloric acid may be used, but nitric acid which is easily purified by distillation or the like is preferably used. As the alkali, ammonia gas or aqueous ammonia is used in order to avoid alkali metal contamination, and as the water, ion-exchanged water or ultrapure water containing less alkali metal element is preferably used. In addition, from the point of purification of the solution, it is necessary to select the material not only to sufficiently separate undissolved solids and impurities by filtration, but also to make the equipment chemically resistant and free from contamination from the equipment itself. is required. As the material, for example, Teflon, polyethylene, polypropylene, transparent quartz, or the like is used. If necessary, an acid and aqueous ammonia are added to the molybdenum solution obtained as described above to prepare an ammonium ion-containing molybdic acid aqueous solution having a pH of 0 or less. Ammonium ion-containing aqueous molybdate is preferably are following condition range: Mo concentration 40~100g / 1 NH ₃ ^- / Mo molar ratio of 0.5 or more T- acid / NH ₄ ^- concentration (N) ratio of 8 or less pH The reason for setting it to 0 or less is that if it exceeds this, amorphous ammonium molybdate crystals having an irregular shape are likely to be crystallized, the intended purification effect of the present invention is excellent, the filterability and detergency are good, and molybdenum is excellent. This is because it becomes difficult to obtain an ammonium molybdate crystal having a high content. As mentioned above, the ammonium molybdate crystal obtained according to the present invention is different from APMo or ADM, and is determined to be (NH ₄ ) _0.15 MoO _3.08 · 0.4H ₂ O. This is a hexagonal columnar crystal, has good filterability and cleanability, and has a molybdenum content of 40 to 45% higher than the conventional one. The reason for adopting preferable conditions is as follows: The reason why the Mo concentration is 40 to 100 g / 1 is that productivity is poor when the Mo concentration is less than 40 g / 1, while the purification effect tends to slightly decrease when the Mo concentration exceeds 100 g / 1. . The reason why the NH ₃ ⁻ / Mo molar ratio is set to 0.5 or more is that if the molar ratio is less than 0.5, the crystals tend to be fine, and the filterability and the like deteriorate. T- acid / NH ₄ ^- 8 below Concentration (N) ratio of (T represents total) was, when more than 8, in addition to the ammonium molybdate crystals, poor yet molybdenum of filterability acicular This is because molybdate crystals having a low content are likely to be generated. The aqueous solution of molybdic acid containing acidic ammonium ions thus prepared is generally at least 60 ° C, preferably at 80 ° C.
By raising the temperature as described above, ammonium molybdate crystals precipitate. After heating for a predetermined time, the grown ammonium molybdate crystals are subjected to ordinary filtration and washing to obtain high-purity ammonium molybdate crystals. The good filtration and washing properties of the crystals greatly contribute to productivity and high quality. The generated ammonium molybdate crystal has a U / Mo concentration of at least 0.001 ppm or less and other impurities sufficiently reduced. Depending on the impurity content of the raw material, a single purification operation may not achieve the purpose. At this time, the obtained crystals are dissolved again in aqueous ammonia and the above operation is repeated at least once. Since the ammonium molybdate crystals according to the present invention are soluble in aqueous ammonia, recrystallization from a molybdenum-containing aqueous solution containing ammonium ions is possible many times. Thus, a high-purity ammonium molybdate crystal of Na / Mo <0.01 ppm K / Mo <0.01 ppm U / Mo <0.001 ppm is produced. Since the produced ammonium molybdate crystals are in a substantially anhydrous state, after drying, shortening or omitting the conventional calcination step and performing hydrogen reduction at a high temperature to obtain high-purity molybdenum powder. Can be. Another notable advantage is that no partial sintering occurs during reduction. In order to prevent external contamination, it is necessary to give due consideration to the crystal crystallization step and the handling of the obtained ammonium molybdate crystal, for example, inside a clean booth or the like. (Example 1) In ammonia water, the Mo concentration became 230 g / 1 and NH ₃ ⁻ / M
o Commercially available MoO ₃ (A) was dissolved such that the molar ratio was 1. This solution was filtered and then added to HNO ₃ to obtain an ammonium ion-containing acidic molybdic acid aqueous solution 20 adjusted to 6N so that the Mo concentration became 100 g / 1. This aqueous solution of acidic molybdic acid containing ammonium ions was heated and maintained at 90 ° C. for 1 hour to obtain ammonium molybdate crystals.
The obtained crystals were sufficiently washed with ultrapure water and filtered. HNo ₃ used in the above was a distilled product with Na <0.5 ppb, ammonia water with Na <10 ppb, and ultrapure water with Na <0.5 ppb. All devices used were made of transparent quartz and Teflon. Table 1 shows the analysis results of the obtained ammonium molybdate crystals. (Example 2) An ammonium molybdate crystal was obtained by using a commercially available product MoO ₃ (B) in the same manner as in Example 1. The obtained crystals were sufficiently washed with ultrapure water and filtered. Table 2 shows the analysis results of the obtained ammonium molybdate crystals. (Example 3) Ammonium molybdate crystals were obtained using a commercially available product MoO ₃ (A) in the same manner as in Example 1. The crystals were dissolved in aqueous ammonia, and the same operation as in Example 1 was performed three times in total. Table 3 shows the results of analysis of the obtained ammonium molybdate crystals after three purifications. The radiometal and alkali metal contents have been reduced to very low values and the transition metal content has also been reduced sufficiently to acceptable levels. (Effect of the Invention) According to the present invention, a high-purity ammonium molybdate crystal having good filterability and detergency and having a molybdenum content of 40 to 45%, which is higher than conventional ones, can be easily produced and used as a raw material. It contributes to quality improvement and cost reduction of molybdenum powder, and further, components for manufacturing electronic devices such as targets made from the powder.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Yoshiharu Kato               187-4 Usaba, Hanakawa-cho, Kitaibaraki-shi, Ibaraki               Japan Mining Co., Ltd.                (56) References JP-A-55-95625 (JP, A)                 JP-A-55-149134 (JP, A)                 JP-A-63-100020 (JP, A)                 Ohmushi-Three Others, "Iwanami Riken               , "3rd edition supplement, Iwanami Shoten Co., Ltd.               56-28-24, p. 636

Claims (1)

(57) [Claims] Adjusting the pH of the ammonium ion-containing molybdic acid aqueous solution to an acidity of 0 or less, heating the acidic ammonium ion-containing molybdic acid aqueous solution to precipitate hexagonal columnar shaped ammonium molybdate crystals, and filtering and washing the crystals. A method for producing a high-purity ammonium molybdate crystal, comprising: 2. The molybdic acid aqueous solution containing an acidic ammonium ion has a Mo concentration of 40 to 100 g / 1, a NH ₃ ⁻ / Mo molar ratio of 0.5 or more, and a T-acid / NH ₄ ⁻ concentration (N) ratio of 8 or less. For producing a high-purity ammonium molybdate crystal. 3. 2. The method for producing a high-purity ammonium molybdate crystal according to claim 1, wherein the U / Mo of the ammonium molybdate crystal is 0.01 ppm or less. 4. The pH of the ammonium ion-containing molybdic acid aqueous solution is adjusted to an acidity of 0 or less, the acidic ammonium ion-containing molybdic acid aqueous solution is heated to precipitate hexagonal columnar ammonium molybdate crystals, and the crystals are filtered and washed, A method for producing a high-purity ammonium molybdate crystal, characterized by repeating the operation of dissolving the generated ammonium molybdate crystal in aqueous ammonia and recrystallizing the crystal under the above-mentioned conditions at least once. 5. 5. The method for producing a high-purity ammonium molybdate crystal according to claim 4, wherein the produced ammonium molybdate crystal has Na / Mo <0.01 ppm K / Mo <0.01 ppm U / Mo <0.001 ppm.