ALUMINUM NITRIDE-BASED SINTERED BODY OF HIGH THERMAL CONDUCTIVITY

Abstract: An aluminum nitride-based sintered body having a high thermal conductivity and a total oxygen content of 0.01 to 20% by weight which is prepared by mixing a main component of aluminum nitride powder containing 0.001 to 7% by weight of oxygen with 0.01 to 15% by weight of at least one of the group consisting of a powder of a rare earth element and/or powder of a material containing the rate earth element (said 0.01 to 15% by weight being counted on the basis of the content of the rare earth element), and sintering said powder mixture.

Description: This invention relates to an aluminium nitride-based sintered body with high thermal conductivity and, more particularly, to an aluminium nitride-based sintered body of high thermal conductivity adapted, for example, as a high heat dissipating substrate for a semiconductor device. Aluminium nitride (AℓN) has high mechanical strength in a wide temperature range and has excellent chemical stability. Therefore, the AℓN has been applied as refractory material and can be applicable as a heat dissipating substrate for a semiconductor device by using its high thermal conductivity and high electric insulating property and so on. Since the AℓN generally has no melting point and decomposes at a higher temperature than 2200°C, the AℓN is normally used in the sintered form. The AℓN-based sintered body is commonly manufactured by the pressureless sintering method or hot pressing method. The pressureless sintering process comprises the steps of mixing the AℓN raw powder with a sintering aid such as a rare earth compound, for example, yttrium, or a material containing a rare earth element such as yttrium oxide, molding the raw mixture, and finally sintering the molded body. However, the AℓN-based sintered body manufactured by the above-mentioned conventional process has a lower thermal conductivity than the theoretical thermal conductivity (about 320 W/m.K) of AℓN. The hot pressing method is carried out by using the AℓN powder alone or a raw material prepared by adding the above-mentioned sintering aid to the AℓN powder. It is very difficult to manufacture a intricately shaped sintered body by said conventional hot pressing method. The thermal conductivities of an AℓN-based sintered body obtained by both the atmospheric sintering method and the hot pressing method are still lower than the theoretical value. Aluminum nitride-based sintered bodies formed from AℓN and a rare earth element or compound thereof (typically Y₂O₃ ) and in which the AℓN contains a relatively large amount of oxygen are disclosed in EP-A-0152545, EP-A-0114193, Chem. Abs. 88 , 1978, Abstract No. 196289; and Transactions and Journal of the British Ceramic Society, 70 , 1971, pp. 107-113 . Our EP-A-0075857 discloses the reduction of the oxygen content of AℓN powders, in admixture with CaO, SrO and/or BaO, using carbon. The object of this invention is to provide an aluminium nitride-based sintered body with high density and high thermal conductivity. To attain the above-mentioned object, this invention provides an aluminium nitride-based sintered body having a high thermal conductivity and a total oxygen content within the range of 0.01 to 20% by weight containing compounds of earth elements characterised in that it is prepared by mixing aluminium nitride powder containing 0.001 to 1% by weight of oxygen, with a rare earth element compound selected from the group consisting of oxides, carbonates and nitrates of a rare earth element, said rare earth element compound being capable of chemically fixing such oxygen of the aluminium nitride powder, to supply 0.01 to 15% by weight of rare earth element based on the aluminium nitride-based sintered body, and sintering said powder mixture at a temperature and pressure and under conditions maintaining the total oxygen content within the above range to obtain said sintered body and to chemically fix said oxygen of the aluminium nitride powder, the amount of total oxygen content in the sintered body being that due to said oxygen of the aluminium nitride powder and that supplied in combined form with said rare earth element by said rare earth element compound. The high thermal conductivity of the AℓN-based sintered body embodying this invention is supposedly derived from its specific structure as described below. The following description relates to the case where a rare earth oxide is used as a sintering aid. When an oxygen-containing AℓN-based material mixed with a powder of a rare earth oxide is molded and then sintered, the rare earth oxide reacts with the oxygen (normally present as aluminium oxide or aluminium hydroxide). As a result, a compound having a garnet structure (hereafter referred to as the "garnet phase"), which is expressed by a structural formula of 3Ln₂O₃ · 5Aℓ₂O₃ (Ln: rare earth element), is created at three or more grain crossing parts of an AℓN sintered body. Said garnet phase contributes to the sintering of the AℓN particles and the fixation of oxygen. If the oxygen content of the AℓN powder increases, then there remains some amount of oxygen which is not taken into the garnet phase. As a result, said remaining oxygen diffuses into the AℓN particles in the form of a solid solution or a polytype compound. The thermal conductivity of the insulator is governed by the diffusion of phonon. Therefore, with an AℓN-based sintered body containing the solid solution or the poly-type compound, the phonon is scattered in said regions leading to a decrease in the thermal conductivity of the AℓN-sintered body. In view of the above-mentioned drawback, the AℓN-sintered body embodying this invention is characterized in that the oxygen content of the AℓN powder is controlled to the aforementioned level of 0.001 to 1% by weight in proportion to the addition of the powder of a rare earth oxide, and chemically fixed with said oxygen content controlled to such a level as constitutes one component of aforesaid garnet phase. Therefore, the AℓN-based sintered body of the invention offers the advantages that the diffusion of oxygen into the AℓN particles is prevented, thereby minimizing the scattering of the phonon and improving the thermal conductivity of the subject AℓN based sintered body. It sometimes happens that a different compound phase having a perovskite structure (hereafter referred to as "the perovskite phase"), which has a structural formula of LnAℓO₃ (Ln: rare earth element), from the aforementioned garnet phase is created. With respect to said perovskite phase, the AℓN-based sintered body of this invention indicates the same behavior as in the case of the garnet phase. As previously described, the oxygen contained in the AℓN powder embodying this invention contributes to advance the sinterability of the AℓN powder. The oxygen content of the AℓN powder should be so controlled as to fall within the range of 0.001 to 1% by weight. The powder of a material containing the same rare earth element, acts as a sintering aid for the subject AℓN-based sintered body. The rare earth elements include, for example, Y, La, Ce, Pr, Nd and Sm. Y is a particularly preferred rare earth element. Materials containing such a rare earth element include, for example, oxides, carbonates, and nitrates thereof. Most preferred among the rare earth-containing materials is yttrium oxide (Y₂O₃). For the object of this invention, it is possible to apply a powder of one rare earth element or rare earth-containing material or a powder of a mixture of two or more of them. The powder of such a rare earth-containing material should be added to the AℓN powder at the rate of 0.01 to 15% by weight, preferably 0.14 to 11.3% by weight, or more preferably 0.28 to 9.5% by weight of rare-earth element (based on the aluminium nitride-based body). The following is the reason why the addition of said rare earth-containing material is limited to the above-mentioned extent. If the addition of said material containing a rare earth material, falls below 0.01% by weight of rare earth element (based on the aluminium nitride-based body), the sinterability of the subject AℓN-based sintered body will not be sufficient, presenting difficulties in manufacturing an AℓN-based sintered body with high density. Conversely, if the content of the material containing a rare earth material increases over 15% by weight of rare earth element (based on the aluminium nitride-based body), the relative quantity of the AℓN will decrease, reducing not only the thermal conductivity of AℓN-based sintered body, but also its fundamental properties, of high refractoriness and mechanical strength. If any of the aforementioned rare earth elements is added to a powder of AℓN, having a high total oxygen content, it is preferred to mix as large an amount of the rare earth element as can be controlled in order to fall within the aforesaid range of 0.01 to 15% by weight of reare earth element (based on the aluminium nitride-based body). When the AℓN-base sintered body is manufactured by the pressureless sintering method, the content of said rare earth element should preferably be controlled to fall within a range of 0.1 to 15% by weight. The reason why the total oxygen content of the AℓN-based sintered body embodying this invention is limited to the aforementioned ranged is that if said total oxygen content decreases from 0.01% by weight, the sinter ability of the AℓN-based sintered body with high density. Conversely, if said total oxygen content rises above 20% by weight, the grain boundaries of AℓN grains are contamin ated by the presence of unfixable oxygen, making it difficult to produce an AℓN-based sintered body of high thermal conductivity. A description may now be made of the pressureless sintering method and hot press sintering method applied in the manufacture of the AℓN-based sintered body of this invention. The pressureless sintering method comprises the steps of mixing AℓN powder containing 0.001 to 1% by weight of oxygen with a prescribed amount of a powder of a material containing the rare earth element; blending the mixed mass, for example, in a ball mill; adding a binder to the blended mass; subjecting said blended mass to kneading, pelletization and control of the pellet sizes; molding the blended mass thus treated in a metal mold, or by means of a still water press or sheet molding; heating the molded mass at a temperature around 700°C in streams of N₂ gas to remove the binder; and finally subjecting the molded mass to pressureless sintering in a graphite or AℓN vessel at a temperature of 1550 to 1950°C in an atmosphere of N₂ gas. During this pressureless sintering process, the aforesaid garnet phase or perovskite phase is created at a relatively low temperature of 100 to 1300°C while the temperature is stepped up. At a higher temperature of 1600 to 1950°C, the garnet phase or perovskite phases is melted, and liquid phase sintering occurs. In the case of the hot press sintering, the raw material mixture is subjected to hot pressing at a temperature of 1550 to 1800°C. This invention will become more apparent with reference to the following decription of the preferred embodiment. Examples 1 to 3 As shown in Table 1 below, three kinds of powder mixture were provided by adding different amounts of yttrium oxide powder having an average particle size of 1 micrometer to AℓN powders having different average particle sizes and oxygen contents. The respective mixture were blended by a wet process in a ball mill, thus preparing 3 kinds of powder mixtures each weighing 200 grams. Thereafter, with 7% by weight of paraffin. The blended masses were pelletized to provide 3 kinds of raw materials for the AℓN-based sintered body. These raw materials were subjected to cold molding with a pressure of 300 kg/cm² (29.4 MPa), producing three plate samples were heated up to 600°C in an atmosphere of nitrogen gas, allowed to stand in tact for 10 hours and later decreased. The plate samples were placed in an AℓN vessel and subjected to pressureless sintering shown in Table 1, thereby manufacturing 3 kinds of AℓN-based sintered bodies. The thermal conductivity of the respective AℓN-based sintered bodies was determined by the laser flash method at room temperature, the results being set forth in Table 1 below. Control 1 given in Table 1 below represents an AℓN-based sintered body manufactured in substantially the same manner as in Example 3 except that the raw material was prepared by adding 10% by weight of yttrium oxide powder to AℓN containing 20% by weight of oxygen and having an average particle size of 0.9 micrometer. A determination was made by X-ray diffraction of the structures of the AℓN-based sintered bodies produced in Examples 1 to 3. The AℓN-based sintered bodies of Examples 1 to 2 indicated a minute amount of the garnet phase and aluminium oxynitride phase. The AℓN-based sintered bodies of Example 3 showed the AℓN phase, garnet phase and aluminium oxynitride phase. The AℓN-based sintered body of Control 1 indicated the presence of a prominent amount of aluminium oxynitride. Examples 4 to 10 As shown in Table 2 below, AℓN powder containing 1% by weight of oxygen and having an average particle size of 1 micrometer and mixed with 3% by weight of Gd₂O₃, DY₂O₃, La₂O₃, Ce₂O₃, Pr₂O₃, Nd₂O₃ and Sm₂O₃ all had an average particle side of 1 micrometer. The raw powder mixture thus prepared was blended by a wet process in a ball mill for 10 hours, thus providing various kinds of powder mixtures each weighing 200g. 7% by weight of paraffin was added to each powder mixture. Thereafter the mass was pelletized, producing 7 kinds of raw materials for the AℓN-based sintered body. A determination was made of the density of all the AℓN-based sintered bodies thus obtained and also of the thermal conductivity of said sintered bodies in the same manner as in Example 1. As mentioned above, this invention can provide an aluminium nitride-based sintered body having a full density and extremely high thermal conductivity which can be effectively applied as a heat dissipating substrate for a semiconductor device.

Claims: An aluminium nitride-based sintered body having a high thermal conductivity and a total oxygen content within the range of 0.01 to 20% by weight containing compounds of rare earth elements characterised in that it is prepared by mixing aluminium nitride powder containing 0.001 to 1% by weight of oxygen, with a rare earth element compound selected from the group consisting of oxides, carbonates and nitrates of a rare earth element, said rare earth element compound being capable of chemically fixing such oxygen of the aluminium nitride powder, to supply 0.01 to 15% by weight of rare earth element based on the aluminium nitride-based sintered body, and sintering said powder mixture at a temperature and pressure and under conditions maintaining the total oxygen content within the above range to obtain said sintered body and to chemically fix said oxygen of the aluminium nitride powder, the amount of total oxygen content in the sintered body being that due to said oxygen of the aluminium nitride powder and that supplied in combined form with said rare earth element by said rare earth element compound. The sintered body according to claim 1, characterized in that said rare earth element is yttrium. The sintered body according to claim 2, characterized in that the rare earth element compound is yttrium oxide. A substrate of high thermal conductivity, having a total oxygen content within the range of 0.01 to 20% by weight and containing rare earth element compounds characterised in that it is prepared by mixing aluminium nitride powder containing 0.001 to 1% by weight of oxygen, with a rare earth element compound capable of chemically fixing the oxygen of the aluminium nitride powder, to supply 0.01 to 15% by weight of rare earth element based on the aluminium nitride-based sintered body, and sintering said powder mixture at a temperature and pressure and under conditions maintaining the total oxygen content within the above range to obtain said substrate and to chemically fix said oxygen of the aluminium nitride powder, the amount of total oxygen content in the substrate being that due to said oxygen of the aluminium nitride powder and that supplied in combined form with said rare earth element by said rare earth element compound.

Inventors: ANZAI, KAZUO, SHINOZAKI, KAZUO, TAKANO, TAKESHI, TSUGE, AKIHIKO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1984
Main classification: C04B35
IPC codes: C04B35