JPS63265866A - Production of aluminum nitride sintered body - Google Patents

Abstract

PURPOSE:To obtain an AlN sintered body having particularly high heat conductivity by covering a molding obtd. by adding and mixing respective cyanamides such as Mg, Sr and/or Ba or Ca, etc., to and with AlN powder with packing powder prepd. by adding the above-mentioned cyanamides to AlN powder, and calcining. CONSTITUTION:The cyanamide compds. of Mg, Sr and Ba as sintering assistants to remove oxygen to the outside of the system and to improve the heat conductivity at the time of sintering are added and mixed to and with the AlN powder. The respective cyanamides of Mg, Sr and/or Ba or Ca and Mg, Sr and/or Ba are usable independently or in combination as said cyanamides. The molding obtd. by molding the above-mentioned mixture is then coated wit the packing powder prepd. by adding the above-mentioned cyanamides to the AlN powder and is calcined. The above-mentioned molding is otherwise calcined in a nonoxidative atmosphere. The desired AlN sintered body is obtd. by these calcination methods. The calcination temp. is in a 1,500-2,200 deg.C, more particularly preferably 1,600-2,100 deg.C range.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing an aluminum nitride sintered body, particularly a highly thermally conductive aluminum nitride sintered body.

[Prior art] In recent years, as electronic devices have become faster, more sophisticated, smaller, and lighter, the amount of heat generated per unit area of semiconductor devices has increased significantly, and this heat generation has made it difficult for semiconductor devices to operate properly. Problems are starting to arise that are hindering this.

In particular, there is a strong demand for insulating substrate materials with high thermal conductivity in high-density actual devices C, LSI, VLSI, microwave transistors for microwave communication and optical communication, laser diodes, and the like.

Traditionally, alumina sintered bodies have been commonly used as insulating substrate materials, but as for recent insulating substrate materials, alumina substrates have poor heat dissipation (thermal conductivity) and a thermal expansion coefficient that is lower than that of silicon. Too big in comparison. Therefore, there is a need to develop an insulating substrate material with good heat dissipation properties.

Such insulating substrate materials have the following characteristics: ■ Good thermal conductivity (high thermal conductivity); ■ High electrical insulation; ■ The coefficient of thermal expansion is close to that of silicon; ■ High mechanical strength. required.

By the way, aluminum nitride, which is seen as a promising substrate material with high thermal conductivity, has a coefficient of thermal expansion of approximately 4.3 x 1.
Approximately 7×10-'/'C of alumina sintered body at 0-'/'C
It is smaller than 3.5X10 due to the coefficient of thermal expansion of silicon.
'Close to -17℃. In addition, the mechanical strength is approximately 5 in terms of bending strength.
0 kg/mi2 and about 20 kg/m of alumina sintered body
It is known to have higher strength than m'. Furthermore,
It is a material with excellent electrical insulation properties, with an electrical resistance of 10+4 Ωchi or more. In addition, aluminum nitride sintered bodies are being actively researched and developed because their dielectric strength, dielectric constant, and other properties are equal to or higher than those of alumina sintered bodies.

However, since aluminum nitride is a material that is difficult to sinter,
When used alone, it is difficult to obtain a dense sintered body when sintered under normal pressure. For this reason, methods for producing sintered bodies using hot press methods, gas pressurization methods, and HIP methods that involve pressure and heating, as well as methods for producing sintered bodies using pressureless sintering with the addition of various compounds as sintering aids, have been investigated. ing. Among these manufacturing methods, the pressureless sintering method is suitable for mass production and is therefore a preferred method for manufacturing sintered bodies. Ba
d) discloses a method for producing an aluminum nitride sintered body in which strontium oxide (S ro ) or the like is added as a sintering aid. However, the thermal conductivity of the aluminum nitride sintered body obtained by this method is as low as 50 to 60 W/mK.
A method of manufacturing an aluminum nitride sintered body having even higher thermal conductivity is being sought.

In addition, JP-A-60-151280 describes a method of adding acetylide compounds such as Ca, Sr, and BJI as additives, and JP-A-60-188478 describes a method of adding Ca3N2, M93N2.5rsNz, BJI3
A method of sintering aluminum nitride by adding N as a sintering aid is disclosed. However, in these methods, the compound used as a sintering aid is very unstable in the air. For example, calcium acetylide in JP-A-60-151280 reacts violently with water and releases acetylene gas. Therefore, there is a risk of explosion in the air. Furthermore, if calcium acetylide with a particle size of 150 mesh or less is left in the air for one hour, it will completely decompose into Ca(OH)2, so care must be taken when handling it. In addition, the sintering aid tends to remain on the sintered body, and the electrical insulation properties tend to be slightly inferior. Furthermore, calcium nitride (average particle size 3μ) disclosed in JP-A-60-186478 instantly decomposes into Ca(OH)z when it comes into contact with air.This drawback is due to the presence of air or water. Although it seems possible to overcome this problem by handling the product without sacrificing it, in reality it is very difficult to prevent decomposition during each process of board manufacturing (pulverization, mixing, molding, degreasing, and baking), and special measures must be taken. This increases costs.

The above-mentioned hot press method and HIP method have the drawbacks of difficulty in producing large molded products with complex shapes, low productivity, and considerably high costs, whereas the pressureless sintering method Although it is easy to mass-produce and can reduce costs, it has the following drawbacks. As seen in Japanese Patent Publication No. 61-28629, the sintering aids used in the pressureless sintering method generally have a relatively high vapor pressure at high temperatures, and the sintering aids near the surface of the powder compact during sintering. The agent evaporates and volatilizes. As a result, the vicinity of the surface of the sintered body becomes less dense, and depending on the shape, the sintered body may be significantly deformed. In particular, when an alkali earth metal is used instead of an additive, the deformation is significant, and the deformation is more significant as the sintered body has a larger surface area per volume, such as a flat plate shape.

For this reason, the dimensional accuracy after sintering is not good, and it is necessary to adjust the dimensions, surface smoothness, etc. by cutting, polishing, or other processing on the unsintered portions of the surface. Since some parts remain where the density of the sintered body has not increased, it is difficult to obtain a sintered body with good thermal conductivity.

[Means for Solving the Problems] Therefore, the present invention provides aluminum nitride powder with magnesium cyanamide, strontium cyanamide and/or barium cyanamide, or calcium cyanamide and magnesium cyanamide, strontium cyanamide and/or barium cyanamide.
Alternatively, barium cyanamide is added and mixed, the resulting mixture is molded, and the resulting molded body is mixed with aluminum nitride powder and magnesium cyanamide, strontium cyanamide and/or barium cyanamide, or calcium cyanamide and magnesium cyanamide, strontium cyanamide and/or barium. A method for producing an aluminum nitride sintered body, characterized by adding cyanamide and covering it with nail powder and firing it, or firing the obtained molded body in a non-oxidizing atmosphere, or both at the same time. is to provide.

[Operation] Oxygen and grain boundaries in the co-aluminum nitride sintered body cause phonon scattering and reduce thermal conductivity.

Therefore, in the method of the present invention, which aims to improve thermal conductivity by removing oxygen from the system during sintering, M9, S (Ba
cyanamide compound is used. These can be used alone or in combination as magnesium cyanamide, strontium cyanamide and/or barium cyanamide, or calcium cyanamide and magnesium cyanamide, strontium cyanamide and/or barium cyanamide.

The above-mentioned cyanamide compound easily decomposes into metal, carbon, and nitrogen during sintering.

MCN2→M + C0N2 (In the formula, M represents a metal) Among these, carbon reacts with oxygen to form CO or C○2
The metal traps oxygen and densifies the compact through liquid phase sintering.Aluminum nitride sintering occurs because the grain boundary phase is easily evaporated due to sufficient sintering. Aluminum nitride particles grow as they scatter from the compact, and the grain boundary phase that causes impurities almost disappears, resulting in a highly thermally conductive aluminum nitride sintered body.

The present invention will be specifically explained below.

The method of the present invention uses M as a sintering aid in aluminum nitride powder.
By adding at least one cyanamide compound of g, Sr, and Ba or a cyanamide compound of these and Ca, the amount of oxygen in the compact can be reduced during firing, and as a result, the amount of oxygen in the aluminum nitride sintered compact is reduced. Thermal conductivity can be significantly increased. In particular, by increasing the total amount of sintering aids added to 0.01 to 7% by weight, the density of the sintered body increases and the thermal conductivity also increases to 60 W/mK (
(room temperature) or higher, which is larger than that of conventional aluminum nitride sintered bodies. If the amount of the sintering aid added exceeds 7% by weight in the case of a cyanamide compound of Sr or Ba, it is not preferable because the density of the sintered body decreases rapidly and the thermal conductivity decreases. It is thought that when the sintering aid is used in excess, carbon that does not contribute to the reaction remains between the grains of aluminum nitride and inhibits sintering.

The sintering aid used in the present invention reacts with moisture in the air and gradually changes into an oxide, but is stable in non-aqueous solvents.
The mixing and pulverizing operations after adding the sintering aid are preferably carried out in a non-aqueous solvent.

The mixing and grinding time varies depending on the particle size of the raw material powder, and takes from several hours to several tens of hours. At this time, when a non-aqueous solvent is used, it is desirable to quickly heat and dry the non-aqueous solvent under vacuum or reduced pressure after mixing and pulverizing.

The obtained mixed powder is press-pressed at room temperature to form a predetermined shape, and finally nitrogen gas, hydrogen gas, carbon oxide gas,
When fired in an M4 arsenic atmosphere, where high temperature firing is performed in a non-oxidizing atmosphere such as an inert gas or vacuum atmosphere,
Aluminum nitride is oxidized and a dense sintered body cannot be obtained.

It should be understood that the calcination can be carried out at a temperature range of 1500-2200<0>C, with a temperature range of 1600-2100<0>C being particularly preferred, although the temperature range is not limited thereto.

Further, the firing time can be varied depending on the firing temperature and the like.

The firing method used in the method of the present invention may be an ordinary pressure sintering method or a pressure sintering method. Examples of the pressure sintering method include a hot press method, a gas pressurization method, and a HIP method (hot isostatic pressing method). pressure sintering method)
etc. can be used. In particular, when fired using a hot press method, a highly thermally conductive aluminum nitride sintered body can be obtained.

However, the hot press method has the drawbacks of low productivity and high cost. On the other hand, the pressureless sintering method is easier to mass-produce and is less expensive, but because the sintering aid near the surface of the compact is dispersed and volatilized during firing, it The sintered body may be slightly deformed, or may be considerably deformed depending on the shape of the sintered body. Therefore, the dimensional accuracy after sintering may not be good, and in such cases, it is necessary to adjust the dimensions and the smoothness of the surface by processing such as cutting or polishing.

Therefore, when firing the above-mentioned molded body, the molded body is covered with powder (nail powder) having the same composition or a composition similar to that of the molded body, and then fired under normal pressure under the same firing conditions as above. be able to. As for the composition of the nail powder, aluminum nitride powder is used as the main ingredient, and the same chemical species as the sintering aid added to the compact is added. The amount added may be slightly excess to the same composition as the composition of the molded body, or a very small amount. However, if it is added in large excess, the concentration of the sintering aid will become too high, which will actually inhibit sintering. For,
The particle size of the nail powder is not particularly limited, although it is not preferable because the resulting sintered body becomes porous.
When the shape of the compact is relatively complex or when deformation during sintering is to be particularly suppressed, it is desirable to use a relatively fine nail powder. However, if the particle size is too fine, the nail powder may sinter firmly together. On the other hand, if the shape of the compact is simple, coarse particles will prevent nail powder from sticking to the sintered body, which is undesirable. This is preferable because the field easily scatters from the sintered body.

For the same purpose as the firing method described above, the molded body is fired in nitrogen or a mixed gas of nitrogen and an inert gas at a nitrogen pressure of 50 kg/am2 above normal pressure.

This method is also used to suppress the decomposition and volatilization of the sintering aid, but although some deformation may occur at low nitrogen pressures, it is sufficient to prevent the decomposition and scattering of the sintering aid. Therefore, the amount of deformation of the aluminum nitride sintered body can be considerably reduced. However, unlike the method described above, this method does not have to worry about the nail powder covering the compact sticking to the aluminum nitride sintered compact. However, if the nitrogen pressure exceeds 50 kg/c, 2 However, the effect of suppressing the decomposition and scattering of the sintering aid is not improved much, and it is not preferable because it is accompanied by equipment difficulties.

The above-mentioned firing means can be appropriately selected depending on the shape of the compact, the use of the resulting aluminum nitride sintered body, and the like.

[Example] The present invention will be further explained with reference to Examples below.

E1 Salmon Branch In this example, a mixture having the composition shown in Table 1 below was molded at a molding pressure of 2 tons/cII2, and the resulting molded product was sintered at normal pressure at 1850°C for 2 hours in a nitrogen gas atmosphere. By doing so, an aluminum nitride sintered body was obtained. The properties of the obtained aluminum nitride sintered body are also listed in Table 1.

Note that the surface of the obtained sintered body is slightly porous, and the relative density and thermal conductivity were measured for the sintered body obtained after cutting out a part with a uniform structure. In this example, a mixture having the composition shown in Table 2 below was molded at a molding pressure of 2 tons/c+112, and the resulting molded body was heated at 1850'C in a nitrogen gas atmosphere for 2 hours. An aluminum nitride sintered body was obtained by firing under normal pressure. The properties of the obtained aluminum nitride sintered body are also listed in Table 2.

In this example, an aluminum nitride sintered body was obtained using a firing method in which the compact was covered with nail powder. The compositions of the molded body and nail powder, and the properties of the obtained aluminum nitride sintered body are listed in Table 3 below. In addition, the molding pressure of the molded body is 2 tons/
Cm2, the firing temperature was 1850°C, and the firing atmosphere was a nitrogen gas atmosphere.

[Effects of the Invention] According to the method of the present invention, aluminum nitride powder does not contain oxygen as a sintering aid, and a relatively stable and easy-to-handle cyanamide compound is used to produce only high-purity aluminum nitride powder with a low oxygen content. First, even if a general-purpose aluminum nitride powder is used, an aluminum nitride sintered body with high thermal conductivity can be obtained. In addition, various firing methods can be appropriately selected to produce a homogeneous aluminum nitride sintered body without deformation.

Claims (1)

[Claims] 1. Adding and mixing magnesium cyanamide, strontium cyanamide and/or barium cyanamide, or calcium cyanamide and magnesium cyanamide, strontium cyanamide and/or barium cyanamide to aluminum nitride powder, and molding the resulting mixture. The obtained compact is mixed with aluminum nitride powder and magnesium cyanamide, strontium cyanamide and/or barium cyanamide or calcium cyanamide and magnesium cyanamide, strontium cyanamide and/or
Alternatively, an aluminum nitride sintered body is produced by adding barium cyanamide and covering it with nail powder and firing it, or firing the obtained molded body in a non-oxidizing atmosphere, or both at the same time. Production method. 2. Claim 1, wherein the total amount of magnesium cyanamide, strontium cyanamide and/or barium cyanamide, or calcium cyanamide and magnesium cyanamide, strontium cyanamide and/or barium cyanamide added to the mixture is 0.01 to 7% by weight. A method for producing an aluminum nitride sintered body as described in Section 1. 3. The non-oxidizing atmosphere is nitrogen, and the nitrogen pressure is above normal pressure5.
The method for producing an aluminum nitride sintered body according to claim 1, wherein the aluminum nitride sintered body has a weight of 0 kg/cm^2 or less.