JPS6257576B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing alumina with an extremely low content of radioactive elements, and more specifically, to a method for producing alumina suitable for use in semiconductor memory device packages, etc. It is.

In a semiconductor memory device, a MOS transistor and a capacitor are used to inject charge into a memory cell, store it, and extract it from the memory cell, and detect information of 0 or 1 depending on the presence or absence of charge.
MOSRAM type devices are currently mainly used. This MOSRAM consists of 16K-bit memory cells on a silicon substrate several millimeters square, and it is predicted that the integration will increase to 64K-bits in the future. Along with this, each memory cell is becoming smaller and smaller, but this means that if a memory cell is accidentally hit by a radiation particle, a single alpha particle will cause a charge equivalent to the amount of charge in the memory cell. This means that there is a risk of generating electrical charges and causing information errors (soft errors). Therefore, various measures for preventing such soft errors are known in the field of semiconductor device manufacturing. That is,
Examples include applying alpha ray shielding coatings to memory cell package materials, or incorporating error correction circuits into semiconductor memory devices.
All of these not only increase the manufacturing cost of memory devices, but also pose obstacles to high integration. Therefore, the most desirable countermeasure is to reduce the alpha-emitting elements in the memory cell package material. However, there is currently no known technique for reducing the content of radioactive elements in the material of the package of a semiconductor memory device, which is one of the sources of radiation particles, to a desirable level. The present inventor has developed alumina suitable for use as a package material in semiconductor memory devices by significantly reducing the content of uranium and thorium in alumina, which is the main component of the package, compared to the content in the raw material during the alumina manufacturing process. Research was conducted with the aim of manufacturing it.

By the way, it is quite difficult to accurately measure alpha rays when the radiation flux is low, especially in the low region of 0.1 count/hr cm ² or less, and reliable measuring equipment is still lacking. Not developed. Therefore, in developing a method for producing alumina for semiconductor memory device packages, it was first necessary to determine a method for quantitatively understanding alpha rays. In this regard, the radioactive elements contained in alumina are uranium (U) and thorium (Th). Uranium (U) decays according to the uranium or actinium decay series, and thorium (Th) decays according to the thorium decay series, ultimately becoming stable lead. In the process of collapse,
One atom of U ²³⁸ produces eight alpha particles and one
It is determined by the decay series of each atom that U ²³⁵ emits seven alpha particles and thorium (Th ²³² ) emits six alpha particles.
In addition, there are radioactive elements Ra, Pa, Ac, etc. generated from nuclear fission of U and Th, but these are outside the scope of the present invention due to their origin. Therefore, even without measuring the alpha rays of alumina, if uranium or thorium is chemically analyzed, the number of emitted alpha ray particles can be calculated from the atomic weight of uranium or thorium contained in alumina. The accuracy of chemical analysis for uranium or thorium is 10 ^-8 to 10 ^-10 gr according to neutron activation analysis, and 10 ^-5 to ^{10 -10} gr for uranium according to fluorescence spectrometry. It has high analytical accuracy. Based on the above circumstances, the present inventor has determined that uranium of about 0.5 to 1.0 ppm and 0.02 to 1.0 ppm of uranium in the current alumina
Based on the idea that soft errors can be prevented by reducing the uranium content to about 0.2 ppm or less and the thorium content to about 0.02 ppm or less, compared to the thorium content of about 0.05 ppm, we developed an ultra-low uranium and thorium-containing alumina manufacturing method. Research was conducted.

An object of the present invention is to provide a method for producing alumina having an extremely low radioactive element content and suitable for use as a package material for semiconductor memory devices.

The method according to the present invention involves lightly firing aluminum hydroxide so that the proportion of α-alumina phase is 80% or less, and
Alumina particles with a BET specific surface area of 10-220 m ² /g are immersed in a dilute mineral acid solution to dissolve the radioactive element in the mineral acid solution, and then the alumina particles are separated from the mineral acid solution. It is characterized by

Hereinafter, the alumina manufacturing method will be explained step by step.

To industrially produce alumina, bauxite is used as a raw material and subjected to the treatment described below using the Bayer process. Bauxite, the raw material for the Bayer method, is
Although it varies somewhat depending on the production area and mining area, the materials used in Japan contain 3 to 5 ppm of uranium and 5.
Contains ~10ppm thorium. When such bauxite is dissolved in a caustic soda solution and the undissolved content (red mud) is precipitated, most of the uranium and thorium precipitate together with the red mud, but a portion migrates into the sodium aluminate solution. Aluminum hydroxide precipitated by hydrolysis from such a sodium aluminate solution contains about 0.4 ppm of uranium and about
Contains 0.02ppm thorium. Alumina produced by firing such aluminum hydroxide contains about 0.6 ppm of uranium and about 0.03 ppm of thorium.

According to the method of the present invention, alumina particles obtained by lightly calcining aluminum hydroxide are used as a raw material, and the raw materials are washed by immersing them in at least one mineral acid solution such as nitric acid, sulfuric acid, or hydrochloric acid. Uranium and thorium in the alumina particles are eluted into the mineral acid solution.

Next, the significance of immersion treatment in mineral acid solution and cleaning conditions such as concentration will be explained. The alumina raw material is lightly fired and has a large specific surface area.
When this is immersed in a mineral acid solution, uranium and thorium in the raw alumina particles are eluted into the mineral acid solution.
After soaking, the alumina is separated from the mineral acid solution as a residue by suitable solid-liquid separation means.

Among mineral acids, nitric acid has the greatest effect in removing uranium and the like, and can reduce the uranium content in raw alumina from 0.5 to 1.0 ppm to 0.04 to 0.3 ppm. Sulfuric acid has the second highest removal effect after nitric acid. Hydrochloric acid is less effective than these mineral acids. A low concentration of these mineral acids is preferable to a high concentration, for example 0.2~
A 0.5N dilute acid solution is used to clean the raw alumina particles. Next, there is no particular restriction on the temperature of the mineral acid solution;
It may be at room temperature or may be slightly heated. moreover,
Conditions for treating raw alumina particles with mineral acid solution may be determined as appropriate by taking into account the amount of washing and time; for example, 0.5 to 1.5 kg of raw alumina particles per mineral acid are treated with a mineral acid solution for 0.5 to 1 hour. It may be treated with mineral acid. The content of the mineral acid treatment according to the present invention will become clear from the above explanation.

Next, the physical properties or properties of the raw material alumina particles will be explained. In the industrial manufacturing method of alumina,
1000% aluminum hydroxide obtained by Bayer method
Alumina is obtained by firing at temperatures above ℃. In the method of the present invention, the proportion of α phase in alumina (gelatinization rate)
80% or less. When the alpha conversion rate exceeds 80%, the specific surface area of alumina decreases and hard alpha alumina crystals develop, so the properties of alumina are insufficient for radioactive elements to come into contact with acid solution and be dissolved and removed. becomes. Alumina is α−Al ₂ O ₃ (α
In the process of transformation to 100% alumina, alumina exists as intermediate forms such as γ-alumina and x-alumina. Since such alumina has a BET specific surface area of 10 to 220 m ² /g and has many pores due to light calcination of aluminum hydroxide, uranium etc. can be eluted and removed from the alumina particles by mineral acid treatment. suitable for

Next, the steps after mineral acid immersion treatment will be explained. The mineral acid in which uranium and the like are dissolved by mineral acid treatment is separated into solid and liquid by decantation or filtration, and the residual alumina particles are thoroughly washed with water and then dried. The content of uranium, etc. in the alumina thus obtained is reduced to 5 to 40% compared to the raw material alumina before treatment. Such alumina is sufficiently calcined at a high temperature of 1000° C. or higher, if necessary, to produce alumina particles with a high gelatinization rate.

In order to prepare alumina with a lower content of uranium etc. than the mineral acid treatment method described above, alumina particles obtained by the mineral acid treatment method are used as a raw material, and aluminum fluoride and/or boric acid and silicon oxide are added to the alumina particles as a raw material. It is possible to carry out a firing method of adding and mixing the contained particles as an additive and firing the mixture. Although the mechanism of deuranization in the firing process is still not fully elucidated, uranium is removed from aluminum fluoride and/or
Alternatively, it reacts with boric acid to form a volatile compound and vaporizes, but this vapor is difficult to desorb from the alumina powder, but is presumed to be adsorbed and fixed on the silicon oxide particles in the alumina powder. Ru. Excess aluminum fluoride or boric acid adheres to and reacts with silicon oxide-containing particles such as shamottu grains during the firing process, and the shamottu grains are separated from the calcined alumina particles in a subsequent classification process such as sieving of the calcined alumina particles. The amount of silicon oxide-containing particles such as siyamoto grains added (the amount of additive is based on the weight of the raw material as 100%) is
It is preferably 0.2 to 7.0% by weight in terms of SiO ₂ . If the amount added is less than 0.2% by weight, the removal effect of uranium and thorium will be insufficient, while if it exceeds 7% by weight, silicon oxide and accompanying substances will be mixed into the product alumina in an undesirable amount. The amount of chamotsu granules added is usually preferably 1 to 5% by weight.
The particle size of the silicon oxide-containing particles is preferably 5 to 30 mesh from the viewpoint of sieving and classification with product alumina particles and reaction with volatile compounds of uranium and thorium, aluminum fluoride, and boric acid. As the silicon oxide-containing particles, in addition to the above-mentioned silica, silica sand (stone), mullite grains, etc. can be used. Next, the amount of aluminum fluoride and boric acid added (individually or in combination) is 0.25~
The amount is preferably 1.0% by weight, preferably 0.3 to 0.6% by weight.
If the amount added exceeds 1.0% by weight, abnormal growth of alumina crystals may occur, or a portion of the additive may remain. Moreover, if it is less than 0.25% by weight, the effect is insufficient.

The temperature conditions for the firing process are 1250 to 1500℃, especially
It is preferable to select a temperature of 1,350 to 1,450°C because the reaction with the additive can be sufficiently carried out. Next, as a mode of firing, it is preferable to place alumina mixed with additives in a ceramic pot and fire it in a tunnel kiln from the viewpoint of reaction with the additives.
Good materials for the pot include alumina and mullite. According to the firing method described above, the concentration of uranium, etc. from 0.04 to 0.3 ppm in the raw material (alumina treated with mineral acid) can be further reduced to 0.03 to 0.2 ppm.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 Aluminum hydroxide produced by the Bayer method was fired in an electric furnace (light firing) to give a gelatinization rate of 60%.
BET specific surface area 145m ² /gr, uranium content
Alumina with a thorium content of 0.81 ppm and a thorium content of 0.024 ppm was obtained. This 300gr of light calcined alumina is used as raw material.
After immersing in 300 ml of 0.5N dilute nitric acid solution at room temperature for 1 hour, it was filtered, washed with water, and dried. The uranium content in the alumina thus obtained (gelatinization rate 40%) is
The thorium content was 0.07ppm, and the thorium content was 0.020ppm.

In this example, the alumina obtained by the above treatment was fired at a high temperature to produce alumina with a high gelatinization rate. 300gr of alumina, 3gr (1% by weight) of Shamotsu particles of 5-30 meshes, and boric acid.
1.5gr (0.5% by weight), placed in an alumina crucible (95% alumina), covered with a lid, and heated to 1450℃ at a heating rate (3℃/min) in an electric furnace. , and heated at 1450°C for 2 hours. After cooling, only alumina was separated from the mixture using an 80 mesh sieve. Analysis of the obtained alumina revealed that the uranium content was 0.03 ppm and the thorium content was 0.03 ppm.
It was 0.016ppm.

Example 2 Lightly calcined alumina was treated with mineral acid under the same conditions as in Example 1, except that 0.5N sulfuric acid was used instead of 0.5N nitric acid. The contents of uranium and thorium in the obtained alumina were each 0.20. and 0.021ppm.

Example 3 Lightly calcined alumina was treated with mineral acid under the same conditions as in Example 1 except that 0.5N hydrochloric acid was used instead of 0.5N nitric acid, and the uranium and thorium contents in the obtained alumina were each 0.31. and 0.022ppm.

Example 4 Lightly calcined alumina was treated with mineral acid under the same conditions as in Example 1, except that a mixed acid solution of equal amounts of 0.5N nitric acid and 0.5N sulfuric acid was used.The contents of uranium and thorium in the obtained alumina were are 0.05 and
It was 0.021ppm.

Example 5 Lightly calcined alumina was treated with mineral acid under the same conditions as in Example 1 except that the concentration of nitric acid was changed from 0.5N to 2N. The contents of uranium and thorium in the obtained alumina were 0.08 and 0.020, respectively. It was ppm.

Example 6 Lightly calcined alumina was treated with mineral acid under the same conditions as in Example 1 except that the immersion time was changed from 1 hour to 3 hours. The contents of uranium and thorium in the obtained alumina were the same as in Example 1. There was almost no change.

Example 7 Alumina was treated with mineral acid under the same conditions as in Example 1, except that a light calcined alumina raw material with a gelatinization rate of almost zero and a BET specific surface area of about 200 m ² /g was used. The contents of uranium and thorium in the alumina were 0.04 and 0.020 ppm, respectively.

Claims (1)

[Claims] 1 Lightly calcined aluminum hydroxide so that the proportion of α-alumina phase is 80% or less and the BET specific surface area is 10.
-220 m ² /g of alumina particles are immersed in a dilute mineral acid solution to elute the radioactive element into the mineral acid solution, and then the alumina particles are separated from the mineral acid solution. A method for producing alumina with an extremely low content of alumina.