Light-shielding aluminium nitride sintered body and method of manufacturing the same

Abstract: A light-shielding aluminum nitride sintered body essentially consists of aluminum nitride crystal grains and 0.01 to 1.5 wt% (an element content in the case of a transition metal compound) of homogeneously dispersed transition metal particles and/or transition metal compound particles having an average particle size of 5 µm or less and has a thermal conductivity of 200 W/m·K or more.

Description: The present invention relates to a light-shielding aluminum nitride sintered body and a method of manufacturing the same and, more particularly, to a light-shielding aluminum nitride sintered body suitably used as a substrate of a circuit board and a method of manufacturing the same. An aluminum nitride (AℓN) sintered body has a thermal conductivity much higher than that of alumina, a good insulation property, and a thermal expansion coefficient close to that of silicon (Si). Therefore, it is attempted to apply an AℓN sintered body to a substrate of a circuit board for mounting a semiconductor device. U.S.P. No. 4,847,221 discloses an AℓN sintered body consisting of an AℓN single phase and a second phase containing 0.01 to 8,000 ppm of a rare earth element and 2,000 ppm or less of oxygen and having a thermal conductivity of 200 W/m·K or more. Since such an AℓN sintered body has a high thermal conductivity, an undesirable phenomenon which has not been encountered in conventional sintered bodies is caused. That is, a high thermal conductivity of the above AℓN sintered body is achieved by a high purity of the sintered body. An AℓN sintered body having a high purity, however, inevitably exhibits transparency as its essential property. The transparency is an undesirable characteristic depending on the field of applications. For example, when the above AℓN sintered body is used as a substrate of a circuit board, a semiconductor chip such as an IC chip or an LSI chip mounted on the upper surface of the circuit board may be irradiated with light transmitted through the lower surface of the circuit board to cause an erroneous operation by a photoelectric effect. In addition, since a slight color tone difference is conspicuous in a substrate consisting of a transparent AℓN sintered body, its outer appearance is degraded. In order to solve the above problems, U.S.P. No. 4,883,780 discloses an AℓN sintered body which is colored dark gray, black, or the like. This AℓN sintered body consists of (a) AℓN, (b) at least one member selected from the group consisting of a rare earth aluminum compound, an alkali earth compound, and a rare earth alkali earth aluminum compound, and (c) at least one member selected from the group consisting of Hf, Mo, W, and transition metals belonging to the groups Va, VIIa, and VIII of the periodic table. The AℓN sintered body contains 0.01 to 18 wt% (oxide content) of the component (b) and 0.01 to 15 wt% of the component (c). Since, however, at least one of a rare earth aluminum compound, an alkali earth compound, and a rare earth alkali earth aluminum compound as the component (b) is present in an AℓN grain boundary or the like of such an AℓN sintered body, the thermal conductivity of this AℓN sintered body is unsatisfactory, i.e., a maximum of at most about 110 W/m·K. As described above, it is difficult to obtain an AℓN sintered body having both a high thermal conductivity and a satisfactory light-shielding property by the conventional techniques. US-A-4 877 760 discloses an AℓN-based sintered body with a high thermal conductivity produced by: preparing a compact of a material comprising 100 parts by weight of aluminum nitride and 0.1-10 parts by weight, calculated on metal element, of at least one component selected from the group consisting of elements of Groups 4a, 5a and 6a of the Periodic Table, and compounds thereof; and sintering said compact at a temperature ranging from 1500° to 2000° C. under a non-oxidizing atmosphere having a source of boron and/or carbon supply. The compounds include oxides, borides, nitrides and carbides. The elements include W, Mo, Ta and Ti. Oxides are converted to other compounds through sintering. EP-A-0342595 discloses a sintered body of aluminum nitride comprising (i) aluminum nitride as a main component and (ii) at least one component selected from the group consisting of Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Nd and Ho and compounds thereof in the total amounts of not greater than 1.0 wt.% and not less than 0.01 wt.% in terms of elements on the basis of sintered body, the sintered body being coloured. EP-A-0207465 discloses an aluminum sintered body comprising (a) aluminum nitride, (b) at least one compound selected from the group consisting of an aluminum compound of a rare earth metal, an aluminum compound of an alkaline earth metal, and an aluminum compound of a rare earth metal and an alkaline earth metal, and (c) at least one element selected from the transition elements consisting of Groups IVa, Va, VIa, VIIa and VIII of the periodic table, and/or at least one compound containing said element, and a process for preparing the same comprising mixing aluminum nitride with: (i) at least one of compound selected from the group consisting of a rare earth metal and/or an alkaline earth metal; and (ii) at least one of element selected from the group consisting of a transition element of Groups IVa, Va, VIa, VIIa and VIII of the period table, and/or at least one of a compound containing said element; and then molding and sintering the mixture. It is an object of the present invention to provide an AℓN sintered body having a high thermal conductivity and a good light-shielding property. It is another object of the present invention to provide a method of manufacturing the above AℓN sintered body having both a high thermal conductivity and a satisfactory light-shielding property by simple manufacturing steps. According to one aspect of the present invention, there is provided a light-shielding aluminum nitride sintered body consisting essentially of aluminum nitride sintered crystal grains and 0.01 to 1.5 wt % in terms of metal of transition metal particles and/or transition metal compound particles homogeneously dispersed in said aluminum nitride sintered crystal grains, and having a thermal conductivity of not less than 200 W/m.K. According to another aspect of the present invention, there is provided a method of manufacturing a light-shielding aluminum nitride sintered body, comprising the steps of: molding a raw material powder consisting of not more than 30 wt % in terms of alkaline earth or rare earth element of an alkaline earth compound powder and/or rare earth compound powder and 0.01 to 1.5 wt % in terms of metal of transition metal particles and/or transition metal compound particles, the balance of said raw material powder consisting of an aluminum nitride powder; and sintering said molded body in a reduction gas atmosphere consisting of a gas mixture of carbon gas and an inert gas at 1,800 to 2,000°C, for a time sufficient to remove the alkaline earth compound and/or rare earth compound. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1 to 4 are SEM photographs showing sections of AℓN sintered bodies manufactured by Example 1 and Comparative Examples 1 to 3, respectively; and Figs. 5 to 8 are graphs showing diffuse transmittance of the AℓN sintered bodies manufactured by Example 1 and Comparative Examples 1 to 3, respectively, with respect to light having a specific wavelength range. A light-shielding AℓN sintered body according to the present invention includes, as has been stated, transition metal particles and/or transition metal compound particles. Examples of the transition metal are Zr, Nb, Ta, Ti, W, Mo, Cr, Fe, Co, and Ni. Although a nitride, a carbide, an oxide, an oxycarbide, and an oxynitride of various types of transition metals can be exemplified as the compound, the compound most preferably has electrical conductivity. Examples of a transition metal compound having electrical conductivity are a nitride and a carbide of a metal selected from the group consisting of Zr, Nb, Ta, and Ti. The transition metal particles or the transition metal compound particles preferably have an average particle size of 5 µm or less because a coloring effect may become unsatisfactory if the average particle size of the transition metal particles of the transition metal compound particles is more than 5 µm. A dispersion amount of the transition metal particles and/or transition metal compound particles is limited for the following reason. That is, if the dispersion amount of the transition metal particles and/or transition metal compound particles is less than 0.01 wt%, an AℓN sintered body having a light-shielding property cannot be obtained. If the dispersion amount of the transition metal particles and/or transition metal compound particles is more than 1.5 wt%, an amount of the transition metal particles and/or transition metal compound particles dispersed in a grain boundary or the like of the AℓN crystal grains becomes too large to obtain an AℓN sintered body having a high thermal conductivity and degraded electrical characteristics. The dispersion amount of the transition metal particles and/or transition metal compound particles is more preferably 0.2 to 1 wt%. Note that the AℓN sintered body according to the present invention is allowed to contain 8,000 ppm or less, and preferably, 3,000 ppm or less of cation impurities of an alkali earth element and/or rare earth element and 2,000 ppm or less of impurity oxygen as components other than the AℓN crystal grains and the transition metal particles and/or transition metal compound particles. Another light-shielding AℓN sintered body according to the present invention essentially consists of aluminum nitride crystal grains and homogeneously dispersed transition metal particles and/or transition metal compound particles and has a thermal conductivity of 200 W/m.K or more. A method of manufacturing a light-shielding aluminum nitride sintered body according to the present invention includes, as has been stated, molding a raw material powder consisting of an alkali earth compound powder and/or rare earth compound powder and of transition metal particles and/or transition metal compound particles, the balance of the material powder consisting of an aluminum nitride powder. The AℓN powder contains preferably 0.1 to 2.5 wt%, and more preferably, 0.3 to 2.0 wt% of impurity oxygen and has an average particles size of preferably 1.5 µm or less, and more preferably, 0.1 to 1.2 µm. The impurity oxygen amount and the average particle diameter are limited for the following reasons. That is, if the impurity oxygen amount is less than 0.1 wt%, it is difficult to manufacture an AℓN sintered body having both a high density and a high purity. If the impurity oxygen amount exceeds 2.5 wt%, it is difficult to manufacture an AℓN sintered body having a high thermal conductivity. In addition, if the average particle size of the AℓN powder is more than 1.5 µm, a sintering property of the AℓN powder is degraded to make it difficult to manufacture an AℓN sintered body having both a high density and a high purity. The alkali earth compound and/or rare earth compound are converted into a composite oxide such as alkali earth aluminate, rare earth aluminate, or alkali earth/rare earth aluminate upon sintering and are removed from the system of the sintered body when sintering is finished. Examples of such an alkali earth compound are an oxide, a carbide, a fluoride, a carbonate, an oxalate, a nitrate, and an alkoxide of Ca, Ba, and Sr. Examples of the rare earth compound are an oxide, a carbide, a fluoride, a carbonate, an oxalate, a nitrate, and an alkoxide of Y, La, Ce, Nd, Dy, and Pr. Most preferable examples are compounds of Y, La, and Ce. An addition amount of the alkali earth compound powder and/or rare earth compound powder is limited for the following reason. That is, if the amount of the compound powders is more than 30 wt%, elements of the compounds remain in an AℓN grain boundary to degrade the high thermal conductivity of the AlN sintered body. The amount of the compound powders is more preferably 0.1 to 20 wt%. Examples of the transition metal compound are an oxide, a carbide, a fluoride, a carbonate, an oxalate, a nitrate, and an alkoxide of a transition metal. The amount of the transition metal particles and/or transition metal compound particles is limited for the following reasons. If the amount of the particles is less than 0.01 wt%, an AℓN sintered body having a light-shielding property cannot be obtained. If the amount is more than 1.5 wt%, not only an AℓN sintered body having a high thermal conductivity cannot be obtained, but also a sintering property of an AℓN sintered body is degraded. A more preferable amount of the transition metal particles and/or transition metal compound particles is 0.01 to 0.99 wt%. An example of the reduction gas is a gas mixture of carbon gas and an inert gas such as N₂, Ar, or He. The carbon gas may be supplied from a carbon heater or a carbon vessel of a sintering furnace or from a thermal decomposition gas of hydrocarbon gas such as methane supplied together with the inert gas. The sintering temperature is limited for the following reasons. That is, if the sintering temperature is less than 1,800°C, not only a sintering property of a molded body is degraded, but also the elements of the alkali earth compound powder and/or rare earth compound powder as a sintering assistant are not removed but remain in an AℓN grain boundary to decrease the thermal conductivity of the manufactured AℓN sintered body. If the sintering temperature is more than 2,000°C, AℓN in the molded body is sublimated to react with carbon gas in the atmosphere, thereby producing a large amount of a carbide and nitride to decrease the thermal conductivity of the manufactured AℓN sintered body. In addition, sintering is preferably performed for six hours or more if the sintering temperature is at the lower limit (1,800°C) of the above sintering temperature range and for 4 hours or more if it is at the upper limit (2,000°C). Since the AℓN sintered body according to the present invention described above essentially consists of AℓN crystal grains and 0.01 to 1.5 wt% (an element content in the case of a transition metal compound) of homogeneously dispersed transition metal particles and/or transition metal compound particles, it has both a high thermal conductivity and a good light-shielding property. That is, it is assumed that when a molded body consisting of an AℓN powder and an additive is sintered at a predetermined temperature, an aluminum oxide impurity inevitably contained in the AℓN powder reacts with the additive to produce an aluminate and causes a liquid phase reaction to increase the density of an AℓN sintered body. For example, if Y₂O₃ is used as an additive, a Y-Aℓ-O-based composite oxide such as 3Y₂0₃-5Aℓ₂O₃, Y₂O₃-Aℓ₂O₃, or 2Y₂O₃-Aℓ₂O₃ is produced in the AℓN sintered body. If CaO is used as an additive, a Ca-Aℓ-O-based composite oxide such as CaO-Aℓ₂O₃ or 2CaO-Aℓ₂O₃ is produced in the AℓN sintered body. Such an aluminate remains as a grain boundary phase in a grain boundary triple point. In other words, an aluminum oxide impurity contained in the AℓN powder is trapped by a grain boundary upon production of the aluminate, and the purity of AℓN crystals is increased. When the aluminate remains in the AℓN sintered body, however, the thermal conductivity of the AℓN sintered body is decreased in relation to the remaining amount. Therefore, sintering is performed in a reduction atmosphere for a long time period to remove the composite oxide from the system, thereby further increasing the purity, i.e., the thermal conductivity. An AℓN sintered body not containing such a composite oxide is known to essentially consist of an AℓN single phase by evaluation using X-ray diffractometry or an SEM. A mechanism of removing the composite oxide is assumed to have a relationship with a reduction-nitriding reaction and evaporation of the composite oxide. A speed at which the composite oxide transits outside the AℓN sintered body depends on the sintering atmosphere and is higher in an atmosphere using a carbon sintering vessel than in an atmosphere using an AℓN or BN sintering vessel. In this manner, known high-thermal conductivity AℓN sintered bodies are roughly classified into an AℓN sintered body consisting of an AℓN single phase and an AℓN sintered body in which an aluminate derived from an aluminum oxide impurity contained in an AℓN powder as a raw material remains. The AℓN sintered body according to the present invention essentially consists of AℓN crystal grains and 0.01 to 1.5 wt% (an element content in the case of a transition metal compound) of homogeneously dispersed transition metal particles and/or transition metal compound particles and therefore is a new product not containing an aluminate described above. Therefore, the AℓN sintered body of the present invention has both a high thermal conductivity of 200 W/m·K or more obtained by a high purity of AℓN and a light-shielding property obtained by the presence of the transition metal particles and/or transition metal compound particles. It is assumed that an AℓN sintered body having the above arrangement has a light-shielding property for the following reason. That is, when a predetermined amount of the transition metal particles and/or the transition metal compound particles are dispersed in AℓN crystal grains, light absorption occurs on the surfaces of the transition metal particles and/or the transition metal compound particles. The reason for this is assumed that the dispersed particles are electrically conductive. An electrically conductive substance such as a metal almost reflects light and therefore exhibits so-called metallic luster even through its surface layer absorbs light. If fine particles are present, reflection and absorption are infinitely repeated at a portion of the fine particles. In addition, many metal particles are black as is well known. It is assumed, therefore, that in an AℓN sintered body obtained by dispersing a predetermined amount of the transition metal particles and/or the transition metal compound particles in AℓN crystal grains, a light-shielding property is obtained by light absorption and multiple reflection on the grain surfaces. In addition, according to the present invention, a raw material powder consisting of a predetermined amount of an alkali earth compound powder and/or rare earth compound powder, a predetermined amount of transition metal particles and/or transition metal compound particles, and an aluminum nitride (AℓN) powder as the balance is molded, and the molded body is sintered in a reduction gas atmosphere at 1,800°C to 2,000°C. The sintering reaction progresses to increase the density and purity of the AℓN powder regardless of the presence of the transition metal grains and/or transition metal compound grains. In other words, the transition metal particles and/or transition metal compound particles have almost no effect on the increase in density and purity of AℓN in the above sintering process. As a result, an AℓN sintered body in which the purity of AℓN crystal grains is increased, and transition metal particles and/or transition metal compound particles are homogeneously dispersed in a grain boundary of the AℓN crystal grains can be manufactured. Although the AℓN sintered body manufactured by the method as described above is determined to consist of AℓN and a transition metal compound by evaluation using X-ray diffractometry or an SEM, it contains oxygen or another cation impurity at a level of several 100 ppm according to the results of elemental analysis. In addition, according to the results of calculation based on SEM photographs, while the AℓN crystal grains grow to an average grain size of 5 to 20 µm during sintering, the transition metal compound substantially maintains its shape as a raw material and causes almost no grain growth. Therefore, an AℓN sintered body having both a high thermal conductivity of 200 W/m·K or more and a light-shielding property obtained by the presence of the transition metal particles and/or transition metal compound particles in the grain boundary of the AℓN crystal grains can be manufactured. The present invention will be described in more detail below by way of its examples. Example 1 3 parts by weight (Y content; 2.38 parts by weight) of a Y₂O₃ powder having an average particle size of 0.1 µm and a purity of 99.9% and 0.5 parts by weight (Ti content; 0.30 parts by weight) of a TiO₂ particles having an average particle size of 1.0 µm and a purity of 99.9% were added to 96.5 parts by weight of an AℓN powder having an impurity oxygen amount of 1.0 wt% and an average primary particle size of 0.6 µm, and the resultant material was mixed by a ball mill to prepare a raw material powder. Subsequently, 5 wt% of an acrylic binder were added to the raw material powder, the mixture was granulated, and 12 g of the granulated powder were molded at a uniaxial pressure of 500 kg/cm² to manufacture a 30 mm × 30 mm × 7mm compact. Subsequently, the compact was heated up to 700°C in a nitrogen gas atmosphere to remove the acryl-based binder. Thereafter, the resultant compact was set in a carbon vessel, and the vessel was put in a carbon heater furnace to perform sintering in a 1-atm nitrogen gas atmosphere at 1,850°C for 48 hours, thereby manufacturing an AℓN sintered body. A piece of the obtained AℓN sintered body was milled, and its constituting phase was identified by X-ray diffractometry. As a result, as shown in Table 1 to be presented later, a constituting phase other than AℓN was only an unknown phase assumed to be TiN. Comparative Example 1 A compact manufactured following the same procedures as in Example 1 was set in a carbon vessel, and the vessel was put in a carbon heater furnace to perform sintering in a 1-atm atmosphere at 1,850°C for 2 hours, thereby manufacturing an AℓN sintered body. A constituting phase of the obtained AℓN sintered body was identified by X-ray diffractometry following the same procedures as in Example 1. As a result, as shown in Table 1 to be presented later, constituting phases other than AℓN were TiN and yttrium aluminum compounds YAℓO₃ and Y₄Aℓ₂O₉. Comparative Example 2 An AℓN sintered body was manufactured following the same procedures as in Example 1 except that a raw material powder was prepared by adding only 3 parts by weight (Y content; 2.38 parts by weigh) of a Y₂O₃ powder to 97 parts by weight of an AℓN powder similar to that used in Example 1 and mixing the resultant material by a ball mill. A constituting phase of the obtained AℓN sintered body was identified by X-ray diffractometry following the same procedures as in Example 1. As a result, as shown in Table 1 to be presented later, no constituting phase other than AℓN was found. Comparative Example 3 An AℓN sintered body was manufactured following the same procedures as in Example 1 except that a raw material powder was prepared by adding only 3 parts by weight (Y content; 2.38 parts by weight) of a Y₂O₃ powder to 97 parts by weight of an AℓN powder similar to that used in Example 1 and mixing the resultant material by a ball mill and sintering was performed for 2 hours. A constituting phase of the obtained AℓN sintered body was identified by X-ray diffractometry following the same procedures as in Example 1. As a result, as shown in Table 1 to be presented later, constituting phases other than AℓN were yttrium aluminum compounds YAℓO₃ and Y₄Aℓ₂O₉. A color tone of each of the AℓN sintered bodies of Example 1 and Comparative Examples 1 to 3 was checked. In addition, a density and a thermal conductivity of each of the AℓN sintered bodies of Example 1 and Comparative Examples 1 to 3 were measured by the following methods. These results are also shown in Table 1 to be presented later. (1) Density measurement of AℓN sintered body The density of each AℓN sintered body was measured by an Archimedes method. (2) Thermal conductivity measurement of AℓN sintered body A disc having a diameter of 10 mm and a thickness of 3 mm was cut out from each AℓN sintered body, and a thermal conductivity was measured at a temperature of 21°C ± 2°C by a laser flash method. Figs. 1 to 4 show SEM photographs of sections of the AℓN sintered bodies obtained by Example 1 and Comparative Examples 1 to 4, respectively. Figs. 5 to 8 show diffuse transmittance of the AℓN sintered bodies of Example 1 and Comparative Examples 1 to 3, respectively, with respect to light having a specific wavelength range. As is apparent from Table 1 to be presented later and the diffuse transmittance shown in Fig. 5, the AℓN sintered body of Example 1 has both a high thermal conductivity and a good light-shielding property. The AℓN sintered body of Example 1 has excellent characteristics because it is constituted by only AℓN crystal grains and a phase homogeneously distributed near a grain boundary of the grains and assumed to be TiN as is understood from the constituting phase shown in Table 1 and the SEM photograph shown in Fig. 1. As is apparent from Table 1 to be presented later and the diffuse transmittance shown in Fig. 6, however, although the AℓN sintered body of Comparative Example 1 has a good light-shielding property, its thermal conductivity is low. The thermal conductivity of the AℓN sintered body of Comparative Example 1 is low because the AℓN grains have but an insufficient impurity concentration, and also because yttrium aluminum oxide is present in addition to TiN near a grain boundary of AℓN crystal grains as is understood from the constituting phase shown in Table 1 and the SEM photograph shown in Fig. 2. As is apparent from Table 1 to be presented later and the diffuse transmittance shown in Fig. 7, although the AℓN sintered body of Comparative Example 2 has a high thermal conductivity, it has transparency. The AℓN sintered body of Comparative Example 2 has transparency because no constituting phase other than the AℓN crystal grains is contained and a purity is increased as is understood from the constituting phase shown in Table 1 and the SEM photograph shown in Fig. 3. As is apparent from Table 1 to be presented later and the diffuse transmittance shown in Fig. 8, the AℓN sintered body of Comparative Example 3 has not only a low thermal conductivity but also transparency. The AℓN sintered body of Comparative Example 3 has a low thermal conductivity and transparency because the AℓN grains have but an insufficient impurity concentration, and also because a large amount of yttrium aluminum oxide is present near a grain boundary of AℓN crystal grains as is understood from the constituting phase shown in Table 1 and the SEM photograph shown in Fig. 4. Examples 2 - 8 Seven types of AℓN sintered bodies were manufactured following the same procedures as in Example 1 except that raw material powders shown in Table 2 to be presented later were used. A color tone of each of the AℓN sintered bodies of Examples 2 to 8 was checked. As a result, each AℓN sintered body had a dark brown or gray color to exhibit a satisfactory light-shielding property. When these AℓN sintered bodies were subjected to X-ray diffractometry, Examples 2, 3, 4, and 5 to 8 contained, in addition to AℓN, phases assumed to be ZrN, NbC, TaC, and TiN, respectively. In addition, a density and a thermal conductivity of each AℓN sintered body were measured following the same procedures as in Example 1, and an electrical resistance, an insulation breakdown voltage, and a permittivity of these sintered bodies were measured by the following methods. The measurement results are shown in Table 2 to be presented later. (1) Measurement of electrical resistance of AℓN sintered body An electrical resistance was measured by applying a DC voltage of 100 V on the AℓN sintered body at room temperature in accordance with a two-terminal method. (2) Measurement of insulation breakdown voltage of Aℓ sintered body An insulation breakdown voltage was measured by applying a 50-Hz AC voltage on the AℓN sintered body in an insulating oil at room temperature at a boosting rate of 500 V/sec. (3) Measurement of permittivity of AℓN sintered body Permittivity of the AℓN sintered body was measured at room temperature and a frequency 10 GHz in accordance with a cavity resonance method. Examples 9 - 14 Six types of AℓN sintered bodies were manufactured following the same procedures as in Example 1 except that raw material powders shown in Table 3 to be presented later were used. A color tone of each of the AℓN sintered bodies of Examples 9 to 14 was checked. As a result, each AℓN sintered body had a dark brown or gray color to exhibit a satisfactory light-shielding property. When these AℓN sintered bodies were subjected to X-ray diffractometry, Examples 9, 10, 11, 12, 13, and 14 contained, in addition to AℓN, unknown phases assumed to be ZrN, NbC, TaC, TiN, metal W, and metal Mo, respectively. In addition, a density, a thermal conductivity, an electrical resistance, an insulation breakdown voltage, and a permittivity of each AℓN sintered body were measured by the methods described above. The measurement results are shown in Table 3 to be presented later. Examples 15 to 19 Five types of AℓN sintered bodies were manufactured following the same procedures as in Example 1 except that raw material powders having a component composition as in Example 1 were prepared by using AℓN powders having compositions as shown in Table 4 to be presented later and sintering was performed under the conditions shown in Table 4. Note that 6 g of a granulated powder were used in Example 19. A color tone of each of the AℓN sintered bodies of Examples 15 to 19 was checked. As a result, each AℓN sintered body had a dark brown or gray color to exhibit a satisfactory light-shielding property. When these AℓN sintered bodies were subjected to X-ray diffractometry, all of the AℓN sintered bodies contained an unknown phase assumed to be TiN in addition to AℓN. In addition, a density, a thermal conductivity, an electrical resistance, an insulation breakdown voltage, and a permittivity of each AℓN sintered body were measured by the methods described above. The measurement results are shown in Table 4 to be presented later. Example 20 A composition prepared by adding an acryl-based binder to a raw material powder similar to that used in Example 1 was formed into a sheet by a doctor blade method and cut into 1,000 sheets each having a size of 6.2 mm × 6.2 mm × 0.77 mm. Subsequently, these sheets were subjected to binder removal and sintering following the same procedures as in Example 1 to manufacture sheet-like AℓN sintered bodies. Comparative Example 4 A raw material powder was prepared by adding only 3 parts by weight (Y content; 2.38 parts by weight) of a Y₂O₃ powder to 97 parts by weight of an AℓN powder similar to that used in Example 1 and mixing the resultant material by a ball mill, and a composition was obtained by adding an acryl-based binder to the material powder. The obtained composition was formed into a sheet by a doctor blade method and cut into 1,000 sheets each having a size of 6.2 mm × 6.2 mm × 0.77 mm. Subsequently, these sheets were subjected to binder removal and sintering following the same procedures as in Example 1 to manufacture sheet-like AℓN sintered bodies. The presence/absence of a color variation was checked with a naked eye and by using a microscope for each of the sheet-like AℓN sintered bodies of Example 20 and Comparative Example 4. As a result, while a color variation was found in 270 out of 1,000 sheets in Comparative Example 4, it was found in only three out of 1,000 sheets in Example 20. As has been described above, according to the present invention, there is provided an AℓN sintered body which has both a high thermal conductivity of 200 W/m·K or more and a good light-shielding property and therefore can be suitably used as a substrate of a circuit board and a method of simply manufacturing such an AℓN sintered body at a high yield.

Claims: A light-shielding aluminum nitride sintered body consisting essentially of aluminum nitride sintered crystal grains and 0.01 to 1.5 wt % in terms of metal of transition metal particles and/or transition metal compound particles homogeneously dispersed in said aluminum nitride sintered crystal grains, and having a thermal conductivity of not less than 200 W/m.K. The sintered body according to claim 1, characterized in that an average particle size of said transition metal particles and/or transition metal compound particles is not more than 5 µm. The sintered body according to claim 1, characterized in that said transition metal compound has electrical conductivity. The sintered body according to claim 3, characterized in that said transition metal compound having electrical conductivity is a nitride or a carbide of a metal selected from the group consisting of Zr, Nb, Ta, and Ti. A method of manufacturing a light-shielding aluminum nitride sintered body, comprising the steps of: molding a raw material powder consisting of not more than 30 wt % in terms of alkaline earth or rare earth element of an alkaline earth compound powder and/or rare earth compound powder and 0.01 to 1.5 wt % in terms of metal of transition metal particles and/or transition metal compound particles, the balance of said raw material powder consisting of an aluminum nitride powder; and sintering said molded body in a reduction gas atmosphere consisting of a gas mixture of carbon gas and an inert gas at 1,800 to 2,000°C, for a time sufficient to remove the alkaline earth compound and/or rare earth compound. The method according to claim 5, characterized in that said aluminum nitride powder contains 0.1 to 2.5 wt % of impurity oxygen. The method according to claim 5, characterized in that an average particle size of said aluminum nitride powder is not more than 1.5 µm. The method according to claim 5, characterized in that said alkali earth compound is an oxide, a carbide, a fluoride, a carbonate, an oxalate, a nitrate, or an alkoxide of Ca, Ba, and Sr. The method according to claim 5, characterized in that said rare earth compound is an oxide, a carbide, a fluoride, a carbonate, an oxalate, a nitrate, or an alkoxide of a metal selected from the group consisting of Y, La, Ce, Nd, Dy, and Pr. The method according to claim 5, characterized in that said transition metal compound is an oxide, a nitride, a carbide, a fluoride, a carbonate, an oxalate, a nitride, or an alkoxide of a transition metal.

Inventors: GOTO SATO YOSHIKO C O INTELLEC, HORIGUCHI AKIHIRO C O INTELLEC, KASORI MITSUO C O INTELLECTUAL, SHUTOH MORITA NAOKI C O INTELL, UENO FUMIO C O INTELLECTUAL PR, GOTO SATO, YOSHIKO, C/O INTELLECTUAL PROPERTY DIV., HORIGUCHI, AKIHIRO, C/O INTELLECTUAL PROPERTY DIV., KASORI, MITSUO, C/O INTELLECTUAL PROPERTY DIVISION, SHUTOH MORITA, NAOKI, C/O INTELLECTUAL PROPE. DIV., UENO, FUMIO, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: C04B35
IPC codes: C04B35, H05K1