Aluminum nitride sintered body, method for manufacturing the same, and ceramic circuit board

Abstract: Disclosed is an aluminum nitride sintered body which has a high thermal conductivity and a high strength and which can be manufactured through low-temperature, short-time sintering. This aluminum nitride sintered body has an average grain size of aluminum nitride grains of 2 µm or less, a thermal conductivity of 80 W/m·K or more, and a relative density of 98% or more.

Description: The present invention relates to an aluminum nitride (AℓN) sintered body, a method for manufacturing the same, and a ceramic circuit board including the AℓN sintered body as an insulating layer. An aluminum nitride (AℓN) sintered body is used as a radiating circuit board for mounting semiconductors since its thermal conductivity is higher than that of alumina or the like and its thermal expansion coefficient is close to that of silicon (Si). The AℓN sintered body also has excellent characteristics, such as a high strength at high temperatures and a low reactivity with molten metals. Therefore, the AℓN sintered body is beginning to be applied to various other fields. Recently, as researches for increasing the thermal conductivity of the AℓN sintered body have advanced, 200-W/m·K class sintered bodies and packages have been able to be obtained in many places. To achieve a high thermal conductivity in the AℓN sintered body, it is considered necessary to decrease the heterophase amount in grain boundaries for the purpose of decreasing phonon scattering in the grain boundaries, thereby increasing the grain size. For this purpose, it has been assumed necessary to sinter an AℓN powder at a high temperature around 1,800°C for a long time period to thereby grow grains of the sintered body to have a grain size of several µm to between 10 and 20 µm. However, the growth of crystal grains accompanies a problem of a decrease in mechanical strength. This decrease in the mechanical strength of the AℓN sintered body is a serious problem in applying the sintered body to a circuit board or a package for mounting semiconductor chips. A high thermal conductivity of the AℓN sintered body is achieved by adding a sintering agent, such as an alkaline earth metal compound or a rare earth element compound, which traps impurity oxygen of AℓN crystal grains, and sintering the resultant material at a high temperature. However, the AℓN sintered body whose thermal conductivity is raised by such a method is poor in mechanical characteristics. A circuit board for mounting semiconductor chips must have an enough strength to withstand a thermal stress or other mechanical stresses, as well as a high thermal conductivity. For this reason, the advent of an AℓN sintered body having both a high thermal conductivity and a high strength has been desired strongly. Since, however, a high thermal conductivity and a high strength are contradictory characteristics from a conventional commonsense point of view, almost no AℓN sintered bodies having both the characteristics have been reported. The mechanism of sintering and the relationship between the thermal conductivity and the strength of the AℓN sintered body will be described in detail below. Generally, an aluminum oxide impurity unavoidably contained in an AℓN powder reacts with a sintering agent, such as a rare earth element compound, to produce aluminates during sintering. These aluminates produce a liquid phase during the sintering, remaining in the edges and the triple points of AℓN grain boundaries. In other words, the aluminum oxide impurity is trapped in a grain boundary phase, and the purity of the AℓN crystal itself increases. As an example, the use of Y 2 O 3 as a sintering agent produces a Y-Aℓ-O-based composite oxide, such as 3Y 2 O 3 ·5Aℓ 2 O 3 or Y 2 O 3 ·Aℓ 2 O 3 , and the use of CaO as a sintering agent forms a Ca-Aℓ-O-based composite oxide, such as CaO·Aℓ 2 O 3 or 2CaO·Aℓ 2 O 3 . It is assumed that liquid phase sintering of AℓN progresses to increase its density because of the presence of such a composite oxide grain boundary phase. As can be predicted from the eutectic points of the above individual composite oxides, the sintering temperature for obtaining the AℓN sintered body is approximately 1,800°C when the rare earth-based sintering agent is used. Although the sintering temperature when the alkaline earth metal-based sintering agent is used is slightly lower than that when the rare earth-based sintering agent is used, there is no large difference between the two temperatures. When a long-time sintering in a reducing atmosphere at a temperature of 1,800°C or more is combined with the addition of the above sintering agent, the thermal conductivity further increases to make the manufacture of an AℓN sintered body of about 270 W/m·K possible. This is so because a grain boundary phase composition changes during the high-temperature, long-time sintering, reducing solid solution oxygen contained in AℓN grains. Since, however, the high-temperature, long-time sintering also causes growth of crystal grains simultaneously, the mechanical strength decreases from 40 to 50 kg/mm 2 to 30 kg/mm 2 . As described above, a high thermal conductivity and a high strength are contradictory characteristics in the AℓN sintered body, so it has been very difficult to improve both the characteristics at the same time. As for applications of the AℓN sintered body, on the other hand, techniques to form the sintered body into various parts and to mass-produce the sintered body have been studied in recent years. In particular, decreasing the manufacturing cost is an urgent necessity to widen the range of applications of the AℓN sintered body, and so a demand has arisen for sintering at a low temperature of 1,700°C or less. In such low-temperature sintering, however, it has been impossible to obtain an AℓN sintered body having both a high thermal conductivity and a high strength. EP-A-286 690 discloses an aluminum nitride and a semiconductor substrate formed therefrom. An aluminum nitride sinter body, which is highly heat-conductive, is described which comprises 95.5 to 99.8 wt. % of an aluminum nitride crystal particle phase with an average particle size of 2 to 10 µm and the balance substantially consists of a dysprosium oxide phase having a density of 99 % or more of the theoretical density. 30 wt. % or more of the oxide phase exists at triple points between the aluminum nitride crystal particles. By forming an alumina type oxide layer on the sinter body and providing thereon an Ni and/or Cu plating layer with a vacuum deposited layer provided between them a semiconductor substrate having a high strength of joining with solder can be formed. EP-A-270 233 concerns dense sintered bodies of nitride materials made of AℓN and Si 3 N 4 which can be sintered near the theoretical densities at temperatures which are at least 200°C lower than the temperature required for the pure materials. Densification of these bodies is accomplished by the addition of a metal fluoride of Al, Ba, Ca, Sr, Y, the rare earth elements of the lanthanide group and mixtures thereof. Sintered bodies of AℓN exibiting very high thermal conductivity are described, which are prepared by sintering with a metal fluoride of Ba, Ca, Sr, Y, the lanthanide rare earth metals and mixtures thereof. EP-A-235 682 discloses an aluminum nitride sintered body having a conductive metallized layer on the surface of the sintered body. The metallized layer comprises at least one element selected from Mo, W and Ta and at least one element selected from the groups IIa, IIIa, IIIb, IVa of the Periodic Table, the rare earth elements and the actinide elements. By simultaneously sintering several insulating ceramic bodies together with the metallized conducting layers, a multilayer interconnection can be prepared. It is an object of the present invention to provide an aluminum nitride (AℓN) sintered body which has a high thermal conductivity and a high strength and which can be manufactured by performing low-temperature, short-time sintering. It is another object of the present invention to provide a method for manufacturing an AℓN sintered body with a high thermal conductivity and a high strength by sintering at a low temperature for a short time period. It is still another object of the present invention to provide a circuit board including an insulating layer with a high thermal conductivity and a high strength. According to the present invention, there is provided an aluminum nitride sintered body having an average grain size of aluminum nitride grains of 1.8 µm or less, a thermal conductivity of 80 W/m·K or more, and a relative density of 98% or more. In addition, according to the present invention, there is provided a method for manufacturing an aluminum nitride sintered body, comprising the steps of: adding a halogen-containing rare earth element compound, as a sintering agent, to an aluminum nitride powder with an average primary grain size of 0.05 to 0.3 µm and an impurity oxygen amount of 0.1 to 2.5 wt%, and molding the resultant material to form a molded body; and sintering the molded body in a non-oxidizing atmosphere at 1,000 to 1,600°C. Furthermore, according to the present invention, there is provided a ceramic circuit board comprising: an insulating layer consisting of an aluminum nitride sintered body with an average grain size of aluminum nitride grains of 1.8 µm or less, a thermal conductivity of 80 W/m·K or more, and a relative density of 98% or more; and a conductor layer formed on the insulating layer and containing, as a main constituent, one element selected from the group consisting of platinum, palladium, nickel, tungsten, and molybdenum. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view showing the structure of a multilayered wiring ceramic circuit board of one embodiment according to the present invention; Fig. 2 is a graph showing the relationship between the shrinkage ratio of a sintered body, which is applied to the ceramic circuit board of the embodiment according to the present invention, and the sintering temperature; and Fig. 3 is a graph showing the relationship between the volume resistivity of a sintered body, which is applied to the ceramic circuit board of the embodiment according to the present invention, and the addition amount of a powder mixture of an AℓN powder and a YF 3 powder in the manufacture of the sintered body. An aluminum nitride (AℓN) sintered body according to the present invention has an average grain size of AℓN grains of 1.8 µm or less, a thermal conductivity of 80 W/m·K or more, and a relative density of 98% or more. Such an AℓN sintered body according to the present invention has a very high thermal conductivity and also exhibits a high strength since it has a dense structure with a relative density of 98%. In addition, since the average grain size of the AℓN grains of this AℓN sintered body is 1.8 µm or less, the strength of the AℓN sintered body further increases, and its surface smoothness becomes very high. Therefore, the AℓN sintered body according to the present invention is suitable as particularly an insulating layer of a circuit board or the like on which a fine conductor layer is formed. The AℓN sintered body according to the present invention is preferably manufactured by method (1) explained below. AℓN Sintered Body Manufacturing Method (1) First, a sintering agent is added to an AℓN powder with an average primary grain size of 0.05 to 0.3 µm and an impurity oxygen amount of 0.1 wt% to 2.5 wt% (inclusive), and the resultant material is mixed by using a ball mill or the like. Kneading and granulation are performed to prepare a raw material while adding a binder if necessary, and the raw material is molded to form a molded body. The average primary grain size of the AℓN powder is limited to the above range for the reasons explained below. That is, if the average primary grain size of the AℓN powder is less than 0.05 µm, not only handling of the material may become difficult but also the shrinkage ratio during sintering of the molded body may increase. If, on the other hand, the average primary grain size of the AℓN powder exceeds 0.3 µm, it may become difficult to perform low-temperature sintering at 1,600°C or less, particularly 1,550°C or less. The average primary grain size of the AℓN powder more preferably ranges between 0.05 and 0.15 µm. The impurity oxygen amount of the AℓN powder means an amount which is obtained immediately before sintering and which will effectively take part in sintering. The impurity oxygen amount of the AℓN powder is limited to the above range for the reasons explained below. That is, if the impurity oxygen amount is less than 0.1 wt%, low-temperature sintering at 1,600°C or less, particularly 1,550°C may become difficult to perform. On the other hand, if the impurity oxygen amount exceeds 2.5 wt%, it may become difficult to obtain an AℓN sintered body with a high thermal conductivity. Usable examples of the sintering agent are alkaline earth metal compounds such as oxides, carbides, fluorides, carbonates, oxalates, nitrates, and alkoxides of alkaline earth metals such as Ca, Ba, and Sr, and/or rare earth element compounds such as oxides, carbides, fluorides, carbonates, oxalates, nitrates, and alkoxides of rare earth elements such as Y, La, and Ce. The mixing amount of the alkaline earth metal compound and/or the rare earth element compound as the sintering agent preferably ranges between 0.5 to 10 parts by weight, as the amount of an alkaline earth metal element and/or a rare earth element, with respect to 100 parts by weight of the AℓN powder. The material consisting of the AℓN powder and the sintering agent described above may contain an AℓN powder with an average grain size larger than the average grain size defined above. The material may also contain, in addition to the AℓN powder and the sintering agent, oxides, carbides, fluorides, carbonates, oxalates, and nitrates of transition metals such as Ti, W, Mo, Ta, and Nb in an amount of 0.05 to 1 wt%, as an amount of the transition metal, with respect to the AℓN powder, in order to perform coloring or increase the strength if necessary. Furthermore, to lower the sintering temperature, the material may contain an aluminum compound, such as aluminum oxide or aluminum fluoride, or a silicon compound, such as silicon oxide or silicon nitride, in an amount of 1 wt% or less with respect to the AℓN powder. As the molding method, mold press, hydrostatic press, or sheet forming can be adopted. Subsequently, the molded body is heated in a non-oxidizing atmosphere such as a nitrogen gas flow to remove the binder and sintered in a non-oxidizing atmosphere at 1,000 to 1,600°C, preferably 1,300 to 1,600°C, and more preferably 1,300 to 1,550°C, thereby manufacturing an AℓN sintered body. It is desirable that the above sintering step be performed in a nitrogen gas atmosphere with the molded body contained in a vessel (sintering vessel) which consists of AℓN or graphite. The sintering time is preferably 10 minutes (inclusive) to 90 hours (inclusive). According to this method, it is possible to manufacture an AℓN sintered body with an average AℓN grain size of 1.8 µm or less, a high thermal conductivity of 80 W/m·K or more, and an improved strength resulting from fine AℓN grains, through the low-temperature sintering at 1,600°C or less, preferably 1,550°C or less. AℓN Sintered Body Manufacturing Method (2) First, a sintering agent is added to an AℓN powder mixture consisting of 10 to 70 parts by weight of an AℓN powder with an average primary grain size of 0.05 to 0.3 µm and 30 to 90 parts by weight of an AℓN powder with an average primary grain size of 0.5 to 1.0 µm. The resultant material is mixed by using a ball mill to prepare a material powder, and the material powder is then molded to form a molded body. When the sintering properties and thermal conductivity are taken into account, the impurity oxygen amount of the AℓN powder is preferably 8 wt% or less, and more preferably 0.4 to 6 wt%. The mixing ratio of the AℓN powder with an average primary grain size of 0.05 to 0.3 µm contained in the above AℓN powder mixture is limited for the reasons explained below. That is, if the mixing ratio of the above predetermined AℓN powder is less than 10 parts by weight, sintering at a low temperature of 1,600°C or less may become difficult to perform. If, on the other hand, the mixing ratio of the above predetermined AℓN powder exceeds 70 parts by weight, the shrinkage ratio during sintering may increase. The type and the mixing amount of the sintering agent can be similar to those explained in the manufacturing method (1). In addition, like in the manufacturing method (1), the material powder consisting of the AℓN powder and the sintering agent is, if necessary, allowed to contain oxides, carbides, fluorides, carbonates, oxalates, and nitrates of various transition metals for the purpose of performing coloring and increasing the strength, or various aluminum compounds, silicon compounds, and silicon for the purpose of decreasing the sintering temperature, in respective predetermined amounts. The material powder is preferably prepared by adding a binder and performing kneading and granulation. Examples of the molding method are mold press, hydrostatic press, and sheet forming. Subsequently, the molded body is heated in a non-oxidizing atmosphere, e.g., in a nitrogen gas flow to remove the binder, and sintered in a non-oxidizing atmosphere at 1,000 to 1,600°C, preferably 1,300 to 1,600°C, and more preferably 1,300 to 1,550°C, thereby manufacturing an AℓN sintered body. It is desirable that the sintering step be performed in a nitrogen gas atmosphere with the molded body contained in a vessel (sintering vessel) consisting of AℓN, graphite, or BN. The sintering time is preferably 10 minutes (inclusive) to 100 hours (inclusive). This method also can manufacture an AℓN sintered body with an average AℓN grain size of 1.8 µm or less, a high thermal conductivity of 80 W/m·K or more, and an improved strength derived from fine AℓN grains, through the low-temperature sintering. In particular, since this method uses an AℓN powder mixture consisting of 10 to 70 parts by weight of an AℓN powder with an average primary grain size of 0.05 to 0.3 µm and 30 to 90 parts by weight of an AℓN powder with an average primary grain size of 0.5 to 1.0 µm, the packing properties can be improved significantly when the AℓN powder mixture is molded after being added with the sintering agent. Consequently, the shrinkage ratio in the sintering step can be controlled, and the use of the fine AℓN powder with an average primary grain size of 0.05 to 0.3 µm makes it possible to set the sintering temperature at a low temperature of 1,600°C or less. In addition, the AℓN powder mixture containing the AℓN powder with an average primary grain size of 0.5 to 1.0 µm in addition to the fine AℓN powder is improved significantly in handling properties as compared with an AℓN powder consisting of only a very fine powder of AℓN, and this can improve working properties. The AℓN sintered body according to the present invention preferably has an average grain size, a thermal conductivity, and a relative density falling within the above respective ranges and at the same time contains a halogen in its grain boundary phase. More specifically, it is preferable that the AℓN sintered body contain both LnOX and Ln x Al y O 3(x+y)/2 (wherein Ln represents a rare earth element and X represents a halogen element) in the grain boundary phase. The halogen element content of the grain boundary phase is 0.01% to 1%. The AℓN sintered body having the grain boundary phase containing halogen can be manufactured by low-temperature sintering because the melting point of the grain boundary phase is lowered. Consequently, AℓN grains are reduced in size to have a uniform grain size, and this improves the mechanical strength. The AℓN sintered body which satisfies the above conditions of the average grain size, the thermal conductivity, and the relative density and which contains a halogen element in its grain boundary phase can be manufactured by the following methods. (a) The AℓN sintered body is manufactured by sintering a molded body consisting of an AℓN powder and a halogen-containing rare earth element compound (sintering agent) in an inert atmosphere at 1,000 to 1,600°C, preferably 1,300 to 1,600°C, and most preferably 1,300 to 1,550°C. The molded bodies used in method (a) above are manufactured by mixing constituent components of their respective material powders by using a ball mill or the like, kneading and granulating the resultant mixtures to prepare material powders while adding a binder as needed, and molding the material powders. The AℓN powder constituting each molded body has an average primary grain size of preferably 0.05 to 0.3 µm, and more preferably 0.1 to 0.3 µm, and an impurity oxygen amount of 0.1 to 2.5 wt%, and more preferably 0.3 to 2.0 wt%. If the average primary grain size of the AℓN powder is less than 0.05 µm, molding may become difficult to perform. If the average primary grain size of the AℓN powder exceeds 0.3 µm, the sintering properties of AℓN may be degraded. If the impurity oxygen amount is less than 0.1 wt%, the properties of AℓN may change during molding. If the impurity oxygen amount exceeds 2.5 wt%, the final thermal conductivity may decrease. Usable examples of the halogen-containing rare earth element compound mixed in the molded body in the method (a) are rare earth element halides such as YF 3 and TbCℓ, rare earth element acid halides such as YOF and LaO x I l-x , and chalcogen halides. Assuming that the amount (the amount of an element) of the halogen-containing rare earth element compound mixed in each of the above molded bodies is (A) and the amount of the AℓN powder is (B), A/(A + B) is preferably less than 0.3, and more preferably 0.001 to 0.2. As the molding method, mold press, hydrostatic press, or sheet forming can be adopted. Sintering of each molded body need not be performed in a reducing atmosphere, but it is possible to use, e.g., a heating element, an oven member, and a box member consisting of carbon-based materials, various ceramic materials, and metal materials. Examples of the atmosphere for sintering the molded bodies are inert gases such as nitrogen gas, a rare gas, and carbonic acid gas. The gas phase pressure during the sintering of the molded bodies is preferably 0.01 to a value less than 1 atm, and more preferably 0.1 atm to 0.8 atm. If the vapor phase pressure is less than 0.01 atm in method (a), the halogen evaporates hard from the molded body, the surface of the resultant AℓN sintered body may be roughened. On the other hand, if the vapor phase pressure is 1 atm or more, excess halogen remains in the molded body to result in unsatisfactory sintering properties. If, however, the sintering temperature is 1,550°C or less, the vapor phase pressure can take any given value. The sintering time of the molded bodies is preferably at least 10 minutes or more. If the sintering time is less than 10 minutes, it may become difficult to obtain a sufficiently densified AℓN sintered body. If sintering is performed for 72 hours or more at a sintering temperature of 1,550°C or more, AℓN grains may grow excessively to decrease the strength of the sintered body. It is preferable that the sintering step in method (a) be performed under conditions in which the molar ratio of a rare earth element to oxygen is 1 or less. A ceramic circuit board according to the present invention comprises a ceramic insulating layer consisting of an AℓN sintered body with an average grain size of aluminum nitride grains of 1.8 µm or less, a thermal conductivity of 80 W/m·K or more, and a relative density of 98% or more, and a conductor layer formed on the insulating layer and containing, as a main constituent, one element selected from the group consisting of platinum (Pt), palladium (Pd), nickel (Ni), tungsten (W), and molybdenum (Mo). The ceramic circuit board according to the present invention has a remarkable performance derived from the characteristics of the AℓN sintered body that constitutes the ceramic insulating layer. That is, this insulating layer has a high thermal diffusivity (thermal conductivity), a high strength, and a high heat resistance. In addition, the conductor layer does not peel from the insulating layer and is free from defects such as oxidation and disconnection. This makes it possible to realize a ceramic circuit board which exhibits a high circuit performance. The ceramic circuit board according to the present invention is manufactured by properly selecting conditions and materials used in accordance with the type of a conductor metal as will be described below. (1st Step) A sintering agent is added to an AℓN powder, and the resultant powder is mixed well. This material powder is further added with a binder, and the resultant material is kneaded and dispersed in a predetermined solvent. After the viscosity of the material is controlled to a predetermined value, granulation and grain dressing are performed to prepare a slurry. Subsequently, the slurry is sheeted by, e.g., a doctor blade process and dried at a temperature of about 200°C, thereby forming a green sheet. Like in the AℓN sintered body manufacturing method (1) mentioned earlier, the AℓN powder is an AℓN powder with an average primary grain size of 0.05 to 0.3 µm and an impurity oxygen amount of 0.1 wt% to 2.5 wt%. If the average primary grain size of the AℓN powder is less than 0.05 µm, there is a possibility that not only handling of the AℓN powder becomes difficult but also the shrinkage ratio increases when the green sheet is sintered. If, on the other hand, the average primary grain size of the AℓN powder exceeds 0.3 µm, it may become difficult to perform low-temperature sintering at 1,600°C or less, preferably 1,600°C or less, and most preferably 1,550°C or less. The average primary grain size of the AℓN powder is more preferably 0.05 to 0.15 µm. If the impurity oxygen amount of the AℓN powder is less than 0.1 wt%, low-temperature sintering at 1,600°C or less, preferably 1,550°C or less may become difficult to perform. On the other hand, if the impurity oxygen amount of the AℓN powder exceeds 2.5 wt%, it becomes difficult to obtain an AℓN sintered body, i.e., a ceramic insulating layer with a high thermal conductivity. As the AℓN powder, it is also possible to use an AℓN powder mixture consisting of 10 to 70 parts by weight of an AℓN powder with an average primary grain size of 0.05 to 0.3 µm and 30 to 90 parts by weight of an AℓN powder with an average primary grain size of 0.5 to 1.0 µm, as in the AℓN sintered body manufacturing method (2) described above. The use of this AℓN powder mixture can significantly improve packing properties when the AℓN powder mixture is molded by adding a sintering agent. Consequently, the shrinkage ratio in the sintering step (to be described later), can be controlled. In addition, the use of the AℓN powder with an average primary grain size of 0.05 to 0.3 µm makes it possible to set the sintering temperature at a low temperature of 1,600°C or less. Furthermore, the AℓN powder mixture containing the AℓN powder with an average primary grain size of 0.5 to 1.0 µm in addition to the fine AℓN powder is improved significantly in handling properties as compared with an AℓN powder consisting of only a very fine powder of AℓN, and this can improve working properties. When the sintering properties and thermal conductivity are taken into consideration, the impurity oxygen amount of the AℓN powder used as the powder mixture is 0.1 to 2.5 wt%. As the sintering agent, an alkaline earth metal compound and/or a rare earth element compound similar to those used in the AℓN sintered body manufacturing method (1) described above can be used. The mixing amount of the sintering agent preferably ranges between 0.5 and 10 wt% with respect to the material powder. If the mixing amount of the sintering agent is less than 0.5 wt%, an unsatisfactory thermal conductivity may result. If, on the other hand, the mixing amount of the sintering agent exceeds 10 wt%, there is a possibility that the resultant AℓN sintered body, i.e., the ceramic insulating layer is prevented from being densified, leading to a decrease in the thermal conductivity. The mixing amount of the sintering agent most preferably ranges from 3 to 7 wt%. Examples of the binder are an acrylic binder and a PVB-based binder. Examples of the solvent are an alcoholic solvent such as n-butanol, methylisobutyl, and toluene. As in the AℓN sintered body manufacturing method (1), the slurry may contain, in addition to the AℓN powder and the sintering agent, oxides, carbides, fluorides, carbonates, oxalates, and nitrates of transition metals such as Ti, W, Mo, Ta, and Nb in an mount of 1 wt% or less, as an amount of the transition metal, with respect to the AℓN powder, in order to perform coloring and increase the strength as needed. In addition, in order to decrease the sintering temperature, the slurry may also contain aluminum compounds, such as aluminum oxide and aluminum fluoride, silicon compounds, such as silicon oxide and silicon nitride, or silicon, in an amount of about 1 part by weight or less with respect to 100 parts by weight of the AℓN powder. (2nd Step) A conductor paste prepared by adding a binder and a solvent to a metal powder of Pt, Pd, Ni, W, or Mo and kneading the resultant mixture is coated on at least one surface of the AℓN green sheet obtained in the above first step by using a screen printing process or the like, thereby forming a desired conductor paste pattern. As the binder contained in the conductor paste, a cellulose-based binder or an acrylic binder is used. Examples of the solvent are terepneol, toluene, and ethanol. The conductor paste may contain, as a filler, the AℓN powder used in the first step, a powder of an alkaline earth metal compound and/or a rare earth element compound as a sintering agent, or a powder mixture of the AℓN powder and a sintering agent. The use of this conductor paste makes it possible to match the shrinkage ratios of the AℓN green sheet and the conductor paste pattern with each other during sintering performed in the third step (to be described later). The above filler consisting of the AℓN powder is mixed at a ratio of preferably 0.01 to 70 vol%, and more preferably 0.1 to 50 vol% with respect to the total amount of the metal powder and the filler contained in the conductor paste. The filler consisting of the sintering agent is mixed at a ratio of 0.1 to 10 vol% with respect to the total amount of the metal powder and the filler contained in the conductor paste. For the purpose of further improving the low-temperature sintering properties of AℓN, the conductor paste may contain a small amount of Aℓ 2 O 3 or WO 3 . (3rd Step) The green sheet on which the conductive paste pattern is formed is heated in a non-oxidizing atmosphere, e.g., in a nitrogen gas flow to remove the binder, and sintered in a non-oxidizing atmosphere at a temperature of 1,000 to 1,600°C, preferably 1,300 to 1,600°C. Subsequently, grinding, polishing, formation of thin film circuits, plating, and pin formation are performed on the surface as needed, manufacturing a ceramic circuit board in which a conductor layer which serves as signal lines and power lines and contains Pt, Pd, Ni, W, or Mo as its main constituent is formed on the surface of the ceramic insulating layer consisting of the AℓN sintered body. According to the above method, the green sheet from which the binder is removed, and the conductive paste pattern are sintered at the same time at a low temperature of 1,600°C or less. This makes it possible to manufacture a high-performance ceramic circuit board including the ceramic insulating layer consisting of the AℓN sintered body which has an average AℓN grain size of 2 µm or less and a high thermal conductivity of 80 W/m·K or more and which is improved in strength due to a decrease in the size of AℓN grains. In particular, the use of the fine AℓN powder with an average primary grain size of 0.05 to 0.3 µm makes sintering at a low temperature (1,600°C or less) for a short time period possible, reducing the manufacturing cost. In addition, low-melting metals such as Pt, Pd, and Ni can be applied extensively as the conductor metal as well as high-melting metals such as W and Mo. A ceramic circuit board with a multilayered wiring structure can also be formed in accordance with the above method. That is, AℓN green sheets are formed according to the first step described above, and a plurality of via holes for connecting layers are formed at predetermined positions of each green sheet. As a method for forming the via holes, it is possible to use, e.g., a mechanical method using a punch, a die, or a punching machine, or a laser processing method. Subsequently, in accordance with the above second step, predetermined patterns of a conductor paste are printed on the individual AℓN green sheets and filled into the via holes, and these green sheets are stacked to one another such that the positions of the via holes formed in the green sheets are aligned properly, and were hot-pressed. The resultant stacked structure is then sintered at a temperature of 1,600°C or less in accordance with the third step, manufacturing a ceramic circuit board with a multilayered wiring structure in which the individual conductor layers are electrically connected through the via holes. In the manufacture of such multilayered ceramic circuit boards, the addition of an AℓN powder, a powder of a sintering agent, or a powder mixture of an AℓN powder and a sintering agent, as a filler, to the conductor paste can increase the adhesion properties between the conductor layers and the ceramic insulating layers particularly in the via holes, and this prevents occurrence of cracks. In the ceramic circuit board according to the present invention, the AℓN sintered body constituting the ceramic insulating layer preferably has a predetermined average AℓN grain size, a predetermined thermal conductivity, and a predetermined relative density and at the same time contains a halogen in its grain boundary phase, like the AℓN sintered body as has been described above. The AℓN sintered body most preferably contains LnOX and Ln x Al y O 3(x+y)/2 (wherein Ln represents a rare earth element and x represents a halogen element) in the grain boundary phase. The halogen element content in the grain boundary phase is 0.01% (inclusive) to 1% (inclusive). The AℓN sintered body constituting the insulating layer of the above ceramic circuit board can be obtained through low-temperature sintering because the melting point of the grain boundary phase of the AℓN sintered body is lowered. As a result, the grain sizes of the AℓN grains are reduced to become a uniform grain size. This realizes a ceramic circuit board including an insulating layer consisting of an AℓN sintered body with an improved mechanical strength. The above ceramic circuit board is manufactured by introducing a halogen to AℓN in a given step of the AℓN sintered body manufacturing method mentioned earlier. For example, in the first step, a material powder is prepared by using a halogen-containing rare earth element compound as a sintering agent, or by adding a halogen or a halogen compound to a sintering agent consisting of an AℓN powder, an alkaline earth metal compound and/or a rare earth element compound. Subsequently, a green sheet is formed from a slurry consisting of the material powder, and, in the third step, sintered in an inert atmosphere at 1,000 to 1,600°C, preferably 1,300 to 1,600°C, and most preferably 1,300 to 1,550°C. The result is a ceramic circuit board having an insulating layer containing a halogen as described above. This ceramic circuit board can also be manufactured by performing sintering in the presence of a halogen gas or a halogen compound gas in an inert atmosphere at 1,000 to 1,600°C, preferably 1,300 to 1,600°C, and most preferably 1,300 to 1,550°C. The AℓN powder contained in the material powder has an average primary grain size of preferably 0.05 to 0.3 µm, and an impurity oxygen amount of 0.1 to 2.5 wt%, and more preferably 0.3 to 2.0 wt%. If the average primary grain size of the AℓN powder is less than 0.05 µm, the formation of the green sheet may become difficult to perform. If the average primary grain size of the AℓN powder exceeds 0.3 µm, the sintering properties of the AℓN green sheet may deteriorate. If the impurity oxygen amount is less than 0.1 wt%, the properties of AℓN may change during the molding of the green sheet. If the impurity oxygen amount exceeds 2.5 wt%, the final thermal conductivity may decrease. In the technique to introduce a halogen to the insulating layer consisting of the AℓN sintered body, the above-mentioned method of manufacturing the AℓN sintered body containing a halogen in the grain boundary phase is applied for factors other than the AℓN powder, e.g., the type of a halogen-containing rare earth element compound to be used as a sintering agent, the amount of a halogen element to be introduced, and the gas phase during sintering. The ceramic circuit board according to the present invention may include an insulating layer consisting of an AℓN sintered body containing a halogen in an AℓN grain boundary, and a conductor layer containing a halogen compound. Since the insulating layer and the conductor layer are bonded with a high strength, a ceramic circuit board with a higher performance is realized. As the conductor metal constituting the conductor layer, W or Mo is particularly suitable. The ceramic circuit board including the above particular insulating layer and conductor layer is manufactured as follows. That is, in the formation of the ceramic insulating layer in the first step, a powder mixture added with a fluoride of a rare earth element or a fluoride of an alkaline earth metal as a sintering agent is used. In the formation of the conductor layer in the second step, a conductor paste prepared by adding a powder of a fluoride of an alkaline earth metal and/or a fluoride of a rare earth element to a W powder with an average grain size of 0.1 to 1.0 µm or an Mo powder with an average grain size of 0.1 to 5.0 µm is used. Examples of the fluoride of a rare earth element used in the formation of the insulating layer (first step) and the formation of the conductor layer (second step) are preferably YF 3 , ScF 3 , and fluorides of lanthanoid series elements, and most preferably YF 3 , YOF, LaF 3 , LaOF, CeF 3 , and CeOF. Examples of the fluoride of an alkaline earth metal are preferably CaF 2 , SrF 2 , and BaF 2 , and most preferably CaF 2 . It is desirable that the same fluoride be mixed in the insulating layer and the conductor layer. In the formation of the conductor layer, the average grain size of the W powder or the Mo powder is limited to the particular range for the reasons explained below. If the average grain size is smaller than the lower limit of the above range, the powder becomes difficult to handle, and this may make pasting of the powder difficult to result in imperfect printing. If the average grain size exceeds the upper limit of the above range, there is a possibility that the shrinkage ratio of the conductor layer becomes excessively small to be unable to match the shrinkage ratio of the AℓN insulating layer, producing defects, such as cracks or warpage, in the circuit board finally obtained. In the manufacture of the ceramic circuit board using the powder mixture and the conductor paste as described above, a powder of a sintering agent for AℓN, such as a fluoride of a rare earth element and/or fluoride of an alkaline earth metal, which has a low melting point and a high reactivity is added to the conductor paste containing W or Mo as its main constituent. Therefore, low-temperature sintering at a temperature of 1,600°C or less can be performed. More specifically, since the above fluoride exists continuously, i.e., fluorine atoms are distributed uniformly in both the AℓN insulating layer and the conductor layer during the low-temperature sintering, the adhesion between the two layers is improved to match the shrinkage ratios of the two layers. This consequently improves the strength in the interface between the insulating layer consisting of the AℓN sintered body and the conductor layer in the ceramic circuit board obtained after the sintering, preventing peeling and disconnection of the conductor layer. This also suppresses occurrence of cracks in the insulating layer and occurrence of warpage of the circuit board. Especially when the AℓN powder is added together with the fluoride powder to the conductor paste, the shrinkage behavior of the insulating layer consisting of the AℓN sintered body and that of the conductor layer become closer to each other during sintering, and this further suppresses occurrence of cracks and the like. In addition, although a detailed mechanism is still unknown, since the fluoride-based sintering agent is used, fluorine atoms constituting the fluoride mixed during the preparation of the conductor paste remain in the AℓN insulating layer and the conductor layer of the ceramic circuit board obtained after sintering, as compared with a case in which a general oxide-based sintering agent such as Y 2 O 3 is used. For example, when YF 3 is mixed during the preparation of the conductor paste, it is assumed that it remains in the form of YOF. Because of the presence of these fluorine atoms, textures of F which is contained in W or Mo of the conductor layer and F which is contained in AℓN continue even in the circuit board obtained after sintering, forming a conductor layer with a high interface strength (adhesion strength). This greatly improves the performance of the circuit board. According to the present invention, there is provided a ceramic circuit board particularly defined below by the use of a fluoride of a rare earth element or a fluoride of an alkaline earth metal in the formation of the insulating layer, and the use of a paste prepared by adding a powder of a fluoride of an alkaline earth metal and/or a fluoride of a rare earth metal to a W powder or an Mo powder with a predetermined average grain size in the formation of the conductor layer. According to the above ceramic circuit board the adhesion properties between the ceramic insulating layer and the conductor layer are excellent since fluorine exists continuously in the two layers. This suppresses defects such as peeling of the conductor layer and ensures a sufficient strength. This circuit board therefore exhibits a high thermal diffusivity (thermal conductivity) and a high heat resistance resulting from the application of AℓN as an insulating material. In addition, since defects in the conductor layer such as peeling and disconnection are prevented, the circuit board has a high circuit performance. If the average primary grain size of the AℓN powder is less than 0.05 µm, processing such as formation of sheets becomes difficult to perform, and this may increase the impurity oxygen amount in the final AℓN sintered body, making it impossible to obtain a high thermal conductivity. In addition, since the sheet density decreases to increase the shrinkage ratio during sintering, position control for circuits on the sheet surface tends to be difficult to perform. On the other hand, if the average primary grain size of the AℓN powder exceeds 0.3 µm, there is a possibility that the sintering properties become too low to densify the AℓN sintered body sufficiently. The average primary grain size of the AℓN powder is 0.05 to 0.3 µm. If the impurity oxygen amount of the AℓN powder is less than about 0.1 wt%, the sintering properties may become too low to densify the AℓN sintered body satisfactorily. If the impurity oxygen amount of the AℓN powder exceeds 2.5 wt%, there is a possibility that a high thermal conductivity cannot be obtained in the AℓN sintered body. The impurity oxygen amount of the AℓN power more preferably ranges between 0.3 and 2.0 wt%. The amount of the sintering agent mixed together with the AℓN powder can be set at about 0.1 to 20 wt% with respect to the total weight of the AℓN powder and the sintering agent powder. If the mixing amount of the sintering agent is less than 0.1 wt%, there is a possibility that no high thermal conductivity can be achieved in the AℓN sintered body and the sintering properties are degraded. A mixing amount of the sintering agent exceeding 20 wt% may also make it impossible to achieve a high thermal conductivity. The mixing amount of the sintering agent more preferably ranges from 0.2 to 10 wt%. The present invention will be described in more detail below by way of its examples. However, these examples are described for better understanding of the present invention and do not particularly limit the invention. Example 1 5 wt% of a YF 3 powder with an average grain size of 0.2 µm and a purity of 99.9% were added to an AℓN powder with an average primary grain size of 0.2 µm and an impurity oxygen amount of 2.3 wt%, and an acrylic binder and an organic solvent were added to the powder mixture. After being mixed and degassed, the resultant material was formed into a sheet 0.8 mm in thickness by a tape casting process. Subsequently, the sheet was cut, and two of them were stacked and hot-pressed to manufacture a molded body with dimensions of 100 mm × 100 mm × 1.6 mm. The molded body was then heated in the atmosphere at 600°C to remove the binder. Subsequently, the molded body was degreased and was then sintered in a nitrogen gas atmosphere at 0.8 atm and 1,500°C for 12 hours, thereby manufacturing an AℓN sintered body. Example 4 An AℓN sintered body was manufactured following the same procedures as in Example 1 except that an AℓN powder with an average primary grain size of 0.3 µm and an impurity oxygen amount of 2.0 wt% was used. Comparative Example 1 An AℓN sintered body was manufactured following the same procedures as in Example 1 except that sintering was performed in a nitrogen gas atmosphere at 1 atm and 1,800°C for 48 hours. Measurements of the density, the thermal conductivity, the four-point flexural strength, and the average AℓN grain size were performed for each of the AℓN sintered bodies according to Examples 1 to 5 and Comparative Example 1. The results are shown in Table 1 presented later. Note that these characteristics were obtained by test methods explained below. (1) Density of sintered body The density was measured by an Archimedes method. The relative density was calculated from the density measured and the ratio of constituent phases obtained by X-ray diffraction. (2) Thermal conductivity The thermal diffusivity of each sintered body was measured by using a laser flash type thermal conductivity measuring device manufactured by Shinku Riko K.K. The measurement was performed at a room temperature of 21°C ± 2°C. In addition, the specific heat of each sintered by was measured by using a DSC available from Seiko Denshi K.K. The heat conductivity was calculated by multiplying the thermal diffusivity, the specific heat, and the density. (3) Four-point flexural strength Each sintered body was cut into samples with dimensions of 40 mm × 4 mm × 1.2 mm, and the four-point flexural strength of each sample was measured in accordance with JIS1601. Note that in Table 1 below, the four-point flexural strength is represented by an average value of 40 samples of each sintered body. (4) Average grain size of AℓN grains The average grain size was obtained by an intercept process by observing the break surface of each sintered body by using SEM. That is, a straight line was drawn on an SEM photograph, and the average grain size was calculated by dividing the length of the straight line by the number of AℓN grains traversing the line and multiplying the quotient by 3/2. In this case, the measurement was performed by using 50 or more AℓN grains traversing the straight line so as to minimize statistical errors. Density, g/cm 3 Thermal conductivity, W/m·K 4-point flexural strength, kg/mm 2 Average grain size (µm) of AℓN grains of sintered body Example 1 3.27 220 45 1.5 Example 4 3.27 215 48 1.3 Comparative Example 1 3.27 240 27 15.2 The AℓN sintered bodies according to Examples 1 to 5 and Comparative Example 1 were also measured by X-ray diffraction. As a result, the AℓN sintered body of Example 1 was found to be contained YOF, Y 2 O 3 and Y 3 Aℓ 5 O 12 in amount of 2.5 wt%, 1.8 wt% and 2.1 wt%, respectively, in addition to AℓN. In Example 4, in Example 4, YOF, Y 2 O 3 and Y 3 Aℓ 5 O 12 were found to be contained in amount of 2.0 wt%, 2.2 wt% and 1.8 wt%, respectively, in addition to AℓN. On the other hand, when it comes to Comparative Example 1, YOF, Y 2 O 3 and Y 3 Aℓ 5 O 12 were found to be contained in amount of 0 wt%, 0.02 wt% and 0.01 wt% or less, respectively, in addition to AℓN. Example 6 5 parts by weight of a YF 3 powder with an average grain size of 0.1 µm and a purity of 99.9% were added to 95 parts by weight of an AℓN powder with an average primary grain size of 0.08 to 0.2 µm and an impurity oxygen amount of 2.5 wt%, and the powder mixture was milled and mixed by using a ball mill to prepare a material powder. Subsequently, 7 wt% of an acrylic binder were added to the material powder, the mixture was granulated, and the granulated powder was molded into a powder compact at a uniaxial pressure of 500 kg/cm 2 . The resultant powder compact was then heated up to 700°C in a nitrogen gas atmosphere to remove the acrylic binder. Subsequently, the powder compact was placed in an AℓN vessel and sintered in a nitrogen gas atmosphere at 1 atm and 1,500°C for three hours in a carbon heater oven, manufacturing an AℓN sintered body. The density, the thermal conductivity, the four-point flexural strength, and the average grain size of AℓN grains of the AℓN sintered body according to Example 6 were measured following the same procedures as in Example 1, and its constituent phases were also measured by X-ray diffraction. The result was that the density was 3.29 g/cm 3 , the thermal conductivity was 170 W/m·K, the four-point flexural strength was 46 kg/mm 2 , and the average grain size was 1.1 µm. In addition, YOF, Y 2 O 3 , and Y 3 Aℓ 5 O 12 were detected in amount of 2.8 wt%, 1.2 wt% and 2.3 wt%, respectively, as the constituent phases other than AℓN, and a small amount of an unknown phase was also contained. Comparative Example 2 3 parts by weight of a Y 2 O 3 powder with an average grain size of 0.2 µm and a purity of 99.9% were added to 97 parts by weight of an AℓN powder with an average primary grain size of 0.4 to 0.7 µm and an impurity oxygen amount of 1.0 wt%, and the powder mixture was milled and mixed by using a ball mill to prepare a material powder. Subsequently, 5 wt% of an acrylic binder were added to the material powder, the mixture was granulated, and the granulated powder was molded into a powder compact at a uniaxial pressure of 500 kg/cm 2 . The resultant powder compact was then heated up to 700°C in a nitrogen gas atmosphere to remove the acrylic binder. Subsequently, the powder compact was placed in an AℓN vessel and sintered in a nitrogen gas atmosphere at 1 atm and 1,800°C for three hours in a carbon heater oven, manufacturing an AℓN sintered body. The density, the thermal conductivity, the four-point flexural strength, and the average grain size of AℓN grains of the AℓN sintered body according to Comparative Example 2 were measured following the same procedures as in Example 1, and its constituent phases were also measured by X-ray diffraction. The result was that the density was 3.31 g/cm 3 , the thermal conductivity was 170 W/m·K, the four-point flexural strength was 32 kg/mm 2 , and the average grain size was 6.5 µm. The constituent phases other than AℓN were Y 3 Aℓ 5 O 12 and YAℓO 3 . The AℓN sintered body of Comparative Example 2 maintained a high thermal conductivity, but its strength was low because of its large grain size. The reason for this is assumed that the grain size of the AℓN powder used was large and sintering was performed at a high temperature, i.e., these conditions were unpreferable. Comparative Example 3 An AℓN sintered body was manufactured following the same procedures as in Comparative Example 2 except that the sintering step was performed in a nitrogen gas atmosphere at 1 atm and 1,450°C for three hours. The density, the thermal conductivity, the four-point flexural strength, and the average diameter of AℓN grains of the AℓN sintered body manufactured were measured following the same procedures as in Example 1, and its constituent phases were also measured by X-ray diffraction. As a result, the density was 2.1 g/cm 3 , i.e., the sintered body was not densified sufficiently, and the thermal conductivity was very low, 15 W/m·K. The four-point flexural strength was 5.1 kg/mm 2 , and the average grain size was 0.6 µm. The constituent phases other than AℓN were Y 3 Aℓ 5 O 12 and Y 2 O 3 . The AℓN sintered body according to Comparative Example 3 was poor in both the thermal conductivity and the strength. The reason for this is estimated that the grain size of the AℓN powder used and the sintering conditions were inadequate. Comparative Example 4 3 parts by weight of a YF 3 powder with an average grain size of 0.1 µm and a purity of 99.9% were added to 97 parts by weight of an AℓN powder with an average primary grain size of 0.6 ± 0.2 µm and an impurity oxygen amount of 1.0 wt%. Thereafter, the resultant powder mixture was sintered following the same procedures as in Example 9 to manufacture an AℓN sintered body. The green density was 1.84 g/cm 3 . The density of the resultant AℓN sintered body was 2.21 g/cm 3 , i.e., the sintered body was not densified well, and its thermal conductivity was as low as 51 W/m·K. The reason for this is assumed that the grain size of the AℓN powder was large and the sintering conditions were unpreferable. Example 10 An AℓN sintered body was manufactured following the same procedures as in Example 9 except that an AℓN powder mixture consisting of 60 parts by weight of an AℓN powder with an average primary grain size of 0.05 to 0.08 µm and an impurity oxygen amount of 1.8 wt% and 40 parts by weight of an AℓN powder with an average primary grain size of 0.6 ± 0.2 µm and an impurity oxygen amount of 1.0 wt% was used. The green density was found to be 1.80 g/cm 3 . The density, the linear shrinkage ratio, and the thermal conductivity of the obtained AℓN sintered body were evaluated following the same procedures as in Example 9. Consequently, the density was 3.29 g/cm 3 , the linear shrinkage ratio was 18.5%, and the thermal conductivity was 155 W/m·K. Example 15 In Example 15, a ceramic circuit board with a multilayered wiring structure using a low-melting metal, such as Pd, as a conductor metal, and AℓN as an insulating material was manufactured. 5 wt% of a YF 3 powder, as a sintering agent, were added to an AℓN powder with an average primary grain size of 0.08 µm, and the resultant powder mixture was wet-mixed in n-butanol for about 48 hours by using a ball mill. This powder mixture was dispersed together with an organic binder, such as a polycaprolactone-based or polyacrylate-based resin, which is readily decomposed in a neutral atmosphere, into an alcoholic solvent, thereby preparing a slurry with a viscosity of approximately 3,000 to 7,000 cps. The slurry was then coated on an organic film according to a casting film formation process, forming a plurality of green sheets with even thicknesses of 10 to 200 µm. Subsequently, a plurality of via holes for connecting layers were formed at predetermined positions of the green sheets by using a punch and a die. A conductor paste prepared by adding a binder and a solvent to a low-resistance powder of Pd and kneading the resultant material was printed on one surface of each green sheet and filled into the via holes so as to form a predetermined pattern by using a screen printing process, thereby forming a conductor layer. Subsequently, a plurality of green sheets on each of which a conductor layer as described above was formed were stacked such that the positions of the via holes were aligned appropriately, and were subjected to hot-pressing. Thereafter, the end portions of the resultant stacked body were cut to obtain a predetermined shape. The stacked body thus obtained was subjected to degreasing (binder removal) at a temperature of about 400 to 600°C and sintered in a nitrogen atmosphere at 1,500°C for six hours, manufacturing a ceramic circuit board with a multilayered wiring structure in which the individual conductor layers were connected through the via holes. Fig. 1 is a view showing the sectional structure of the ceramic circuit board with a multilayered wiring structure manufactured by the above method. A multi-layered ceramic circuit board 1 is formed by stacking a plurality of ceramic insulating layers 11 consisting primarily of an AℓN polycrystalline body. Conductor layers 12 containing Pd as the main constituent and having respective predetermined patterns constituted by signal lines, power sources, and the like are formed on the surfaces of the ceramic insulating layers 11. Via holes 13 are also formed at predetermined positions of the ceramic insulating layers 11. The conductor layers 12 formed on the surfaces of the ceramic insulating layers 11 are electrically connected through a conductor metal filled inside the via holes 13. Die pads 14 and bonding pads 15 for mounting LSI chips (not shown) are formed on the surface of the uppermost one of the ceramic insulating layers 11. In addition, I/O pads 16 for input/output of signals are formed on the lower surface of the lowermost one of the ceramic insulating layers 11 at positions corresponding to the via holes. Various characteristics of the multilayered ceramic circuit board 1 with the above structure were evaluated. The result was that the specific resistance was 10 11 Ω·cm or more, the dielectric constant was 8.7 (1 MHz), and the dielectric loss was 1 × 10 -3 or less (1 MHz). The multilayered ceramic circuit board 1 also exhibited electrical characteristics equivalent to those of a conventional ceramic circuit board having insulating layers, which are manufactured by sintering a material powder consisting of an AℓN powder and a Y 2 O 3 powder, and conductor layers consisting of W. In addition, the ceramic insulating layers 11 consisting of AℓN sintered body were densified sufficiently, and no pores were found. The average primary grain size of the AℓN grains was measured by an intercept process and found to be 1.5 µm. In this multilayered ceramic circuit board 1, heat generated by LSI chips mounted on the die pads 14 when the LSI chips are driven is diffused efficiently to the ceramic insulating layers 11 through the die pads 14. Consequently, the heat generated by the LSI chips can be radiated well through the circuit board. In the above multilayered ceramic circuit board 1, the ceramic insulating layers 11 had good adhesion properties with respect to the conductor layers 12 and the conductor metal filled in the via holes 13, and this suppressed occurrence of cracks. In addition, since no oxide-based sintering agent such as Y 2 O 3 was used, excessive oxidation of the conductor layers 12 and the like was prevented. Example 16 In Example 16, a ceramic circuit board was manufactured by using a conductor paste added with an AℓN powder containing a YF 3 powder. Approximately 25 vol% of an AℓN powder containing 3 wt% of YF 3 were added to a powder of Pd. 100 parts by weight of this powder mixture were added with 3 parts by weight of ethylcellulose, as an organic binder, and 15 parts by weight of butylcarbitol and 100 parts by weight of methylethylketone, as a solvent. The resultant material was kneaded by using a ball mill, a stirrer, and a three-roll mill, thereby preparing a conductor paste. The conductor paste was then printed into a predetermined pattern by a screen printing process on an AℓN green sheet about 500 µm in thickness formed following the same procedures as in Example 15, thereby forming a conductor layer. Subsequently, the resultant structure was degreased by heating in an N 2 flow at about 600°C and was sintered in an N 2 flow at about 1,400°C for six hours, manufacturing a ceramic circuit board. The conductor layer constituting the ceramic circuit board thus manufactured was subjected to powder X-ray analysis. As a result, an Aℓ-Pd alloy and a Y-Pd alloy were formed in the sintered conductor layer, but the electrical resistance did not particularly increase. No oxide of Pd was found. As described above, the ceramic circuit board of Example 16 had a high circuit performance with a low electrical resistance. Example 17 In Example 17, the characteristics of a conductor layer and an insulating layer of a ceramic circuit board manufactured by using W as a conductor metal and AℓN as an insulating material were evaluated. 10 vol%, 35 vol%, and 60 vol% of a powder mixture consisting of 3 wt% of a YF 3 powder with an average grain size of 1.2 µm and 97 wt% of an AℓN powder with an average grain size of 0.6 µm and an impurity oxygen amount of 1.0 wt% were added to a W powder with an average grain size of 0.7 µm, respectively, and the individual resultant powder mixtures were mixed and milled sufficiently by using a ball mill. Separately, a powder mixture consisting of only an AℓN powder and a YF 3 powder was mixed and milled in the same manner as described above. 5 parts by weight of paraffin as a binder were added to 100 parts by weight of each of the above powder mixtures having four different compositions, and each resultant material was uniaxially pressed at a pressure of 500 kg/cm 2 to form a powder compact with dimensions of 30 mm × 30 mm × 10 mm. Subsequently, the binder was removed from these powder compacts, and each resultant powder compact was placed in an N 2 atmosphere in a carbon heater oven and sintered by heating up to 1,300°C, 1,400°C, 1,500°C, and 1,600°C at a rate of 500°C/hour. Each of the powder compact was oven-cooled with a 0-minute hold time and a six-hour hold time, thereby forming twenty type sintered bodies. Note that the sintered body formed by using the W powder corresponds to a conductor layer of a circuit board, and the sintered body formed by using the powder mixture consisting only of the AℓN powder and the YF 3 powder corresponds to an insulating layer of a circuit board. The shrinkage ratios of the resultant AℓN sintered bodies were measured. Fig. 2 shows the relationship between the shrinkage ratio and the sintering temperature. In Fig. 2, a sintered body (an insulating layer) of sample No. 1 consists of 97 wt% of AℓN powder and 3 wt% of YF 3 . A sintered body (a conductor layer) of sample No. 2 consists of 90 vol% of a W powder and 10 vol% of a powder mixture. A sintered body (a conductor layer) of sample No. 3 consists of 65 vol% of the W powder and 35 vol% of the powder mixture. A sintered body (a conductor layer) of sample No. 4 consists of 40 vol% of the W powder and 10 vol% of the powder mixture. Note that the powder mixture consists of 97 wt% of AℓN powder and 3 wt% of YF 3 . In addition, in accordance with the same procedure as in Example 17, a sintered body was formed by milling, molding and sintering a W powder along with an average grains size of 0.7 µm, and the shrinkage ratio of the sintered body was measured. The relationship between the shrinkage ratio and the sintering temperature is also shown in Fig. 2 as sample No. 5. The volume resistivity was also measured for each sintered body obtained by a hold time of six hours after heating up to 1,600°C. Fig. 3 shows the relationship between the volume resistivity and the addition amount of the powder mixture of the AℓN powder and the YF 3 powder. The results shown in Fig. 2 reveal that in a sintering temperature range between 1,300 and 1,600°C, each conductor layer (Nos. 2 to 4) formed by using the W powder and the powder mixture consisting of the AℓN powder and the YF 3 powder presented a shrinkage ratio closer to that of the AℓN sintered body formed (No. 1) by using only the powder mixture consisting of the AℓN powder and the YF 3 powder than the conductor layer (No. 5) formed by using only the W powder. This demonstrates that in the manufacture of a circuit board, if a conductor layer is formed by using a powder mixture of W and YF 3 and an insulating layer is formed by using a powder mixture of AℓN and YF 3 , the shrinkage ratios of the two layers can be matched. Example 18 Powder mixtures were prepared by mixing conductor components and additive components in accordance with the formulation shown in Table 2 below. The resultant powder mixtures were milled, molded, and sintered following the same procedures as in Example 17 and in accordance with the conditions shown in Table 2, thereby forming sintered bodies a to k corresponding to a conductor layer of a circuit board. The shrinkage ratio and the volume resistivity of each resultant sintered body were measured. The measurement results are also shown in Table 2. As can been seen from Table 2, the conductor layers formed by adding YF 3 to a conductor metal such as W or Mo stably exhibited substantially constant shrinkage ratios and very low volume resistivities even when the compositions and the sintering conditions varied. As has been described above, according to the present invention, there are provided an aluminum nitride (AℓN) sintered body with a high thermal conductivity and a high strength, and a method capable of manufacturing the AℓN sintered with these characteristics through low-temperature, short-time sintering. According to the present invention, there is also provided a ceramic circuit board which uses the AℓN sintered body as an insulating layer, which has a high thermal conductivity and a high strength, and which can be manufactured by simultaneous sintering at a low temperature for a short time period.

Claims: An aluminium nitride sintered body, characterized in that: an average grain size of aluminium nitride grains is not more than 1.8 µm; the grain boundary phase of said aluminium nitride grains contains LnOX and Ln x Al y O 3(x+y)/2 , wherein Ln represents a rare earth element and X represents a halogen element, said LnOX being contained in said grain boundary phase in an amount of 0.01 to 1% by weight in terms of the halogen element; and said sintered body has a thermal conductivity of not less than 80 W/m.K and a relative density of not less than 98%. A method for manufacturing an aluminium nitride sintered body according to claim 1, characterized by comprising the steps of: adding a halogen-containing rare earth element compound, as a sintering agent, to an aluminium nitride powder with an average primary grain size of 0.05 to 0.3 µm and impurity oxygen amount of 0.1 to 2.5% by weight and molding the resultant material to form a molded body; and sintering said molded body in a non-oxidizing atmosphere at 1000 to 1600 °C. The method according to claim 2, characterized in that said aluminum nitride powder contains 0.3 to 2.0 wt% of impurity oxygen. The method according to claim 2, characterized in that said halogen-containing rare earth element compound is YF 3 . The method according to claim 2, characterized in that said halogen-containing rare earth element compound is YOF. The method according to claim 2, characterized in that the sintering temperature is 1,300 to 1,600°C. The method according to claim 2, characterized in that the sintering temperature is 1,300 to 1,550°C. The method according to claim 2, characterized in that a gas phase pressure in the sintering step is 0.01 atm to a value less than 1 atm. A ceramic circuit board, comprising: an insulating layer (11) made of an aluminium nitride sintered body; and a conductor layer (12) formed on said insulating layer (11) and containing, as a main constituent, one element selected from the group consisting of platinum, nickel, palladium, tungsten and molybdenum, wherein said aluminium nitride sintered body forming said insulating layer (11) is characterized in that: the average grain size of aluminium nitride grains is not more than 1.8 µm; the grain boundary phase of said aluminium nitride grains contains LnOX and Ln x Al y O 3(x+y)/2 , wherein Ln represents a rare earth element and X represents a halogen element, said LnOX being contained in said grain boundary phase in an amount of 0.01 to 1% by weight in terms of the halogen element; and said sintered body has a thermal conductivity of not less than 80 W/m.K and a relative density of not less than 98%. The circuit board according to claim 9, characterized in that said conductor layer (12) contains aluminum nitride and a halogen-containing rare earth element compound or a halogen-containing alkaline earth element compound. The circuit board according to claim 10, characterized in that the halogen-containing compound is at least one compound selected from YF 3 , YOF, LaF 3 and CaF 2 .

Inventors: HORIGUCHI AKIHIRO, KASORI MITSUO, OH-ISHI KATSUYOSHI, UENO FUMIO, HORIGUCHI, AKIHIRO, KASORI, MITSUO, OH-ISHI, KATSUYOSHI, UENO, FUMIO, HORIGUCHI, AKIHIRO, C/O INTELLECTUAL PROPERTY DIV., KASORI, MITSUO, C/O INTELLECTUAL PROPERTY DIVISION, OH-ISHI, KATSUYOSHI, C/O INTELLECTUAL PROPERTY DIV, UENO, FUMIO, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1993
Main classification: C04B35
IPC codes: H01B3, C04B35, H01L23