JPH07121830B2 - Method for manufacturing high thermal conductivity aluminum nitride sintered body - Google Patents

Description

The present invention relates to an aluminum nitride sintered body and a method for producing the same, and more specifically, to a nitride comprising an aluminum nitride single phase having high thermal conductivity. The present invention relates to a method for manufacturing an aluminum sintered body.

(Prior Art) Aluminum nitride (AlN) has many excellent characteristics such as high strength from normal temperature to high temperature, no leakage to molten metal, high electrical insulation, and high thermal conductivity. Has been attracting attention as a new material.

In recent years, application research on semiconductor substrates has been actively conducted, and the thermal conductivity of mass-produced AlN sintered bodies has been 40 to 60 W / m · k until several years ago.
However, it has been improved to ~ 200 W / m · k.

The high thermal conductivity of sintered aluminum nitride is achieved by using high-purity AlN.
The first factor is the possibility of mass production of raw materials, especially AlN powder with a low oxygen content.

AlN powder with low oxygen content was the main component, and by optimizing the sintering aid, it became possible to obtain an AlN sintered body with high thermal conductivity, but on the other hand, the oxygen content decreased and sinterability increased. In order to obtain a dense sintered body, it is necessary to sinter at a higher temperature than in the past.

That is, it can be said that the AlN powder having a large amount of oxygen in the past was excellent in sinterability although the sintered body obtained from the powder had a low high thermal conductivity.

When considering application to semiconductor mounting boards, it is possible to substitute for alumina substrates that are widely used at present, but in such a situation it is necessary to thoroughly reduce the cost, and the increase in the sintering temperature causes the manufacturing cost to rise. Is increased, which is not preferable.

On the other hand, when such an AlN sintered body is to be obtained by a method other than hot pressing, in order to prevent densification of the sintered body and solid solution of the impurity oxygen of the AlN raw material powder into AlN grains,
It is generally practiced to add a rare earth oxide, an alkaline earth metal oxide or the like as a sintering aid (JP-A-60-
127267, JP-A-61-10071, JP-A-60-71575 and the like. ) These sintering aids react with the impurity oxygen of the AlN raw material powder to form a liquid phase and achieve densification of the sintered body, and
Fixing this impurity oxygen as a grain boundary phase (oxygen trap)
However, it is considered to achieve high thermal conductivity.

By adding the sintering aid in this way, the sintered body is certainly densified and has a high thermal conductivity, but on the other hand, as a result, the remaining grain boundary phase (the secondary phase relative to the main phase AlN phase) )The presence of,
Due to the presence of oxygen that could not be completely trapped, AlN
The theoretical thermal conductivity of 320 w / mK was low.

Therefore, various attempts have been made for the purpose of improving the thermal conductivity of the AlN sintered body, but a satisfactory one has not been obtained yet.

(Problems to be Solved by the Invention) The present invention provides a method for producing an AlN sintered body which uses AlN having high purity and low oxygen content to improve its sinterability and has excellent thermal conductivity. The purpose is to do.

[Structure of Invention]

(Means and Actions for Solving Problems) The inventors of the present invention have found that the sintering aid to be added to the AlN powder, the sintering conditions,
As a result of conducting experiments and studies on the relationship between the thermal conductivity and the composition of the sintered body, the fine structure of the sintered body, etc., the following new matters were discovered and the present invention was completed.

That is, the present invention comprises AlN as a main component, to which i) an alkaline earth metal compound and / or a rare earth compound, and ii)
1550 to 2050 ° C in a reducing atmosphere containing a nitrogen gas in a compact containing 0.05 to 20% by weight of an additive made of an aluminum oxide (or an aluminum compound that changes into an oxide by firing) converted into each element. When fired at the temperature of 4 hours or more, the amount of the grain boundary phase decreased compared with the conventional aluminum nitride sintered body, and it was composed of AlN single phase with substantially no subphase, and it was extremely polycrystalline. It was found that an aluminum nitride sintered body having extremely high thermal conductivity can be obtained.

In the present invention, as the alkaline earth metal element, Ca, Br, Sr
However, Y, Sc, Dy, Ce are particularly effective as rare earth elements,
A compound composed of these elements, that is, an oxide, a fluoride, a nitride or a carbide is added.

The aluminum nitride sintered body obtained by such a method has a very high thermal conductivity of 200 W / mK or more as a polycrystalline body, and this sintered body was examined by X-ray diffraction and an electron microscope. Even when the constituent phases are observed, only AlN crystal grains are observed, and other phases are not observed.

The present invention is roughly divided into two components described below.

That is, (I) improvement of sinterability by addition of aluminum oxide, and (II) high thermal conductivity by long-time firing in a reducing atmosphere containing nitrogen gas.

First, the effect of adding aluminum oxide will be described.

AlN is a rare earth compound as an alkaline earth metal.
It has been known that it is effective as a sintering aid of and for increasing the thermal conductivity.

These additives react with impurity oxygen contained inevitably during AlN, for example, CaO · 2Al ₂ after sintering when the additive is CaO alkaline earth metal compound O _3, CaO · Al ₂ It is considered that it becomes a sub-phase such as O ₃ and becomes a product in which impurity oxygen is taken in, and the sintered body has high thermal conductivity.

Also, if sintering is performed with AlN without any such additives, the impurity oxygen reacts with AlN and the oxynitride (Al (8/3
+ X / 3) O _4-x N _x ) and / or polytype (Al ₉ O ₃ N ₇ ) and / or α-Al ₂ O ₃ etc. are generated, and even if densified by hot press sintering, thermal conductivity Is known to significantly reduce.

In general, in order to obtain an AlN sintered body having high thermal conductivity, it is a normal idea to prevent aluminum oxide from being mixed as a harmful impurity as much as possible. Alkaline earth metal compounds and rare earth compounds are effective for densification as sintering aids because they mainly react with oxygen impurities in the AlN raw material to form a liquid phase at the sintering temperature, and the liquid phase firing of AlN It is believed to advance the conclusion.

In such a sintering mechanism, the sinterability of the low oxygen content AlN raw material decreases because the amount of liquid phase generated during the sintering due to the reaction with the sintering aid as described above decreases. It is presumed that this is because it becomes difficult to proceed. Therefore, by adding aluminum oxide, the sinterability can be improved.

Next, a description will be given of how to increase the thermal conductivity by baking for a long time in a reducing atmosphere containing nitrogen gas. At present, its mechanism has not been completely elucidated, but according to the studies by the present inventors, it is presumed as follows.

For example, when Y is selected as the rare earth element, the impurity oxygen of the raw material powder is 3Y ₂ O ₃ .5Al ₂ O ₃ , Y ₂ O ₃ .Al ₂ O ₃ , 2Y ₂ O ₃ .Al ₂
It is trapped as a compound such as O ₃ and Y ₂ O ₃ . This state occurs at the early stage of sintering. In the firing process after this, the surface of the sintered body (rare earth element) -O compounds (e.g. Y ₂ O ₃₎ and / or (rare earth element) -Al-O compounds (e.g. 2Y ₂ O ₃ · Al
₂ O ₃ ) is reductively nitrided by a substance having a reducing action such as nitrogen gas and carbon gas and / or CO gas existing in the atmosphere, and for example, (rare earth element) -N compound (eg YN) and / or AlN Changes to.

Due to the reduction nitriding reaction on the surface of the sintered body, the (rare earth element) -O compound and / or the (rare earth element) -Al-in the sintered body.
A concentration gradient of O compound is generated, and this acts as a driving force for AlN.
Other subphases move to the surface of the sintered body through the grain boundaries. Finally, the sintered body becomes an AlN single phase that does not substantially contain other phases, and the thermal conductivity increases significantly. This is because the grain boundary phase, which has small thermal conductivity and worked as thermal resistance, is removed. Moreover, the particles of the sintered body grow by firing for a long time. This means that as the AlN particles grow, the number of grain boundaries that become the thermal resistance eventually decreases, resulting in a sintered body with small phonon scattering. In addition to the above-described removal of the sub-phase and grain growth, firing for a long time in a reducing atmosphere may slow down the AlN crystal grains, for example, it may be possible to increase the conductivity by reducing lattice defects.

The firing atmosphere is a reducing atmosphere containing nitrogen gas. The reducing atmosphere can be created by allowing one or more kinds of CO, H ₂ gas and C (gas and solid phase) to exist.

The simplest thing is that a carbon container can be used as the firing container. As such a baking container, the whole container is made of carbon, the whole container is made of carbon, and AlN plate, BN plate, W plate, etc. are laid on the place where the sample is set, and the container made of aluminum nitride has an upper lid. A carbon material or the like can be used. In the present invention, the carbon gas atmosphere means that the vapor pressure is 1 × 10 within the sintering temperature range of 1550 to 2050 ° C.
^-6 ~ 5 × 10 ^-2 Pa refers to the generated gas.

The internal volume of the container is such that the ratio of the internal volume to the volume of the aluminum nitride compact (internal volume / volume of the compact) is 1 × 10 ⁰ to 1
× 10 ⁷ is good. When a larger volume than this is used, the carbon vapor pressure in the vicinity of the sample is low, and the effect of removing the grain boundary phase by carbon becomes small.

Although it depends on the sintering temperature and the amount of the auxiliary agent added, it takes 4 hours or more to obtain the AlN single phase. It is more preferably 6 hours or longer, and further preferably 12 hours or longer.

The sintering temperature is preferably 1550 to 2050 ° C. 1550 ° C
When firing at a lower temperature, the generation of carbon gas from the firing container is reduced, and the grain boundary phase remains. 2050 ℃
When fired at a higher temperature, the vapor pressure of AlN itself increases,
In addition to the difficulty in densification, a secondary phase presumed to be a nitride may be formed in the sintered body, resulting in a decrease in thermal conductivity. The firing temperature is more preferably 1800 to 2000 ° C.
Furthermore, 1800 to 1950 ° C is preferable.

In the present invention, the total amount of the alkaline earth metal compound and / or the rare earth compound and the aluminum oxide is set to 0.05 to 20% by weight, since it is less than 0.05% by weight, the desired effect cannot be obtained, and 20% by weight. If it exceeds, the subphase may remain in the sintered body, resulting in a decrease in thermal conductivity.

Further, the alkaline earth metal compound and the rare earth compound are preferably oxides, fluorides, nitrides and carbides, but there is no problem even if they become these compounds during firing.

Further, as the aluminum oxide, it is possible to use Al ₂ O ₃ such as α-Al ₂ O ₃ and γ-Al ₂ O _3, or those which become these oxides during firing.

(Examples of the Invention) Next, examples of the present invention will be described.

Example 1 0.45% by weight of oxygen was contained as an impurity, and the average particle size was 3.
AlN powder of 0 μm (centrifugal sedimentation method, CAPA-500 used by Horiba Ltd., dispersion medium ethylene glycol) was added as an additive to Y ₂ O ₃ having an average particle size of 0.9 μm, 3.9% by weight in terms of Y, and α having an average particle size of 1 μm. ₂ % by weight of -Al ₂ O ₃ was added, and a raw material was adjusted by using a ball mill and mixing with n-butyl alcohol as a dispersion medium.

Then, 4% by weight of an organic binder was added to this raw material for granulation, and press molding was performed at a pressure of 500 kg / cm ² to produce 38 × 3.
8 × 10 mm compacts were used. The compact was heated to 700 ° C. in a nitrogen gas atmosphere to remove the binder. Furthermore, B
The degreased body was placed in a carbon container (calcining container A) in which an AlN plate coated with N powder was used as a bottom plate. At this time, the shape and size of the container A are 12 cmφ × 6.4 cm and the internal volume is 720 cm.
It is about ³ . That is, the ratio of the internal volume of the container A to the volume of the AlN compact is about 5 × 10 ¹ . Using this container, atmospheric pressure sintering was performed in a nitrogen gas atmosphere (1 atm) at 1900 ° C. for 96 hours. The density of the obtained AlN sintered body was measured. A disk having a diameter of 10 mm and a pressure of 3.3 mm was ground from the sintered body, and the thermal conductivity was measured by a laser flash method using this as a test piece (using TC-3000 manufactured by Vacuum Riko). The measurement temperature is 25 ° C.

Table 1 shows the sintering conditions and the characteristics of the obtained sintered body. In addition, X-ray diffraction of this sintered body (Rigaku Denki rotor flex RU-200, goniometer CN2173D5, radiation source Cu 50kv, 1
Fig. 1 shows the result of performing (using 00mA).
The M photograph is shown in FIG. 2 (using JSM-T20 manufactured by JEOL Ltd.).

Comparative Example 1 A sintered body was manufactured by the same method as in Example 1 except that α-Al ₃ O ₃ was not added. The obtained sintered body was used as Example-1.
Evaluation was conducted in the same manner as in, and the results are shown in Figure 4.

The density of the sintered body is as low as 3.119 kg / cm ³ , so the thermal conductivity is also 1.
It can be seen that it is as low as 95W / m ・ k.

Examples-2 to 20 Sintered bodies were manufactured in the same manner as in Example-1, except that the type and amount of various additives added to the AlN powder used in Example-1 were changed. The obtained sintered body was evaluated in the same manner as in Example-1, and the results are shown in Table-1.

Examples 21 to 42 Various fired bodies were obtained in the same manner as in Example-1, except that the AlN raw material powder, the type and amount of the additive, the firing conditions and the like were changed. The evaluation results of this sintered body are shown in Table-2.

Examples 43 to 46 Various sintered bodies were obtained in the same manner as in Example 1 except that the volume ratios of the firing container and the molded body were different. The evaluation results of the sintered body are shown in Table-3.

Example 47 Carbon container with BN plate as bottom plate (baking container B)
An AlN sintered body was manufactured in the same manner as in Example 1 except that was used. The same evaluation was performed and the results are shown in Table 3.

Example 48 An AlN sintered body was manufactured in the same manner as in Example 1 except that a container whose inside was entirely made of carbon (calcining container C) was used. The same evaluation was performed and the results are shown in Table 3.

Example 49 A carbon container (43 × 44 × 15 mm) used in Example 45 was filled with carbon powder having an average particle size of 0.02 μm, and a molded body similar to that of Example-1 was placed therein, 1900 ° C., It was baked for 96 hours. The obtained sintered body was evaluated in the same manner as in Example 1 and the results are shown in Table 3.

Comparative Examples 2 to 4 AlN degreased bodies obtained by the same method as in Example 1 except that the types and amounts of AlN powder and additives were different were set variously in sintering containers A, B and C, and the temperature was 1900 ° C. , 2 hr, then normal pressure sintering in a N ₂ atmosphere to obtain a sintered body. Table 4 shows the characteristics of these sintered bodies. Further, the result of X-ray diffraction using the sintered body of Comparative Example 2 is shown in FIG. 3, and the SEM photograph of the fractured surface of the sintered body is shown in FIG. From these results and the results of the same evaluation, it was found that a compound containing yttrium as a subphase was observed and it was not an AlN single phase, and the resulting thermal conductivity was a low value of 170 w / m · K or less.

Thus, when the sintering time is as short as less than 4 hours, it was found that the grain boundary phase was not sufficiently removed by using the carbon container, and it took a long time to obtain an AlN sintered body having high thermal conductivity ( It can be seen that sintering for 4 hours or more) is necessary.

Comparative Examples 5 to 7 An AlN degreased body obtained by the same method as in Comparative Example 2 was used. In Comparative Example 5, a container whose inside is entirely made of AlN (sintering container D),
In Comparative Example 6, a container whose inside is entirely made of alumina (sintering container E) and Comparative Example 7 is made of a container whose inside is entirely made of tungsten (sintering container F), are used in a N ₂ stream at 1900 ° C. for 96 hours. Pressure sintering was performed to obtain a sintered body. The characteristics of these sintered bodies are shown in Table-4. Further, the results of X-ray diffraction using the sintered body of Comparative Example 5 are shown in FIG. From these results and the evaluation results, it was found that a compound containing yttrium as a subphase was observed and it was not an AlN single phase. As a result, the thermal conductivity is also a relatively low value of 168 W / mK or less.

Thus, an AlN sintered body having a high thermal conductivity cannot be obtained even when a sintering container of which at least a part of the inside is made of carbon is not used, and the effectiveness of the carbon atmosphere can be seen.

Comparative Example 8 The AlN powder used in Comparative Example 2 was press-molded at a pressure of 500 kg / cm ² to obtain a green compact of 30 × 30 × 10 mm, which was placed in a carbon mold and placed in a nitrogen gas atmosphere. , Temperature 1900 ℃, 400kg
Hot press sintering was performed under a pressure of / cm ² for 1 hour to obtain a sintered body. The characteristics of this sintered body are shown in Table 4. The result of further X-ray diffraction is shown in FIG. From this result, an Al-O-N compound was observed as a sub-phase, and it was found that it was not an AlN single phase. As a result, the thermal conductivity was as low as 80w / mK.

As described above, in the addition of the rare earth and / or alkaline earth metal element compound, the impurity oxygen on the surface of the AlN raw material powder reacts with AlN, and an Al-O-N compound that impedes heat conduction is generated, so the addition is effective. I understand.

[Effects of the Invention] As described above, the aluminum nitride sintered body of the present invention is
It consists essentially of an AlN single phase and has excellent properties such as high purity and high thermal conductivity, and its industrial value is extremely large.

[Brief description of drawings]

FIGS. 1, 3, 5, and 6 are X-ray diffraction pattern diagrams of the sintered body, and FIGS. 2 and 4 show the crystal structure of the fractured surface of the sintered body (by SEM photograph). It is a figure. 1 ... AlN diffraction peak 2 ... Y-Al-O compound diffraction peak 3 ... Al-O-N compound peak 4 ... AlN grains 5 ... Y-Al-O compound (grain boundary phase)

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yoshiko Sato 1 Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa Within Toshiba Research Institute, Inc. Stock company Toshiba Research Institute

Claims (7)

[Claims] 1. An aluminum nitride powder having an impurity oxygen content of 0.9% by weight or less as a main component, to which (i) at least one of an alkaline earth compound or a rare earth compound is added in a total amount of 0.05 to 0.05 of each element conversion. Addition of 20% by weight, (ii) 5% by weight in terms of oxide of aluminum oxide or a compound that becomes aluminum oxide by firing
A method for producing a highly heat-conductive aluminum nitride sintered body, which comprises firing the added molded body at a temperature of 1550 to 2050 ° C for 4 hours or more in a reducing atmosphere containing nitrogen gas and carbon gas. 2. The method for producing a highly heat-conductive aluminum nitride sintered body according to claim 1, wherein the alkaline earth element is at least one of Ca, Sr and Ba. 3. The method for producing a highly heat conductive aluminum nitride sintered body according to claim 1, wherein the rare earth element is at least one of Y, Sc, Dy and Ce. 4. The method for producing a highly heat-conductive aluminum nitride sintered body according to claim 1, wherein the powder used for sintering has an average particle diameter of 2.1 μm or more. 5. A reducing atmosphere is embodied by including a firing container for producing a carbon gas and / or a substance producing a carbon gas during firing in the firing container.
Of manufacturing a high thermal conductivity aluminum nitride sintered body according to the item 6. The high heat as set forth in claim 1, wherein firing is carried out in a carbon container on which an aluminum nitride plate, a BN plate or a tungsten plate is laid as a sample stand on which a compact or a sintered compact is placed. Manufacturing method of conductive aluminum nitride sintered body. 7. High heat according to claim 6, characterized in that the volume ratio of the internal volume of the firing container to the compact or the sintered body is 1 × 10 ⁰ to 1 × 10 ^7. Manufacturing method of conductive aluminum nitride sintered body.