SINTERED BODY OF ALUMINUM NITRIDE

Abstract: Sintered bodies of aluminum nitride are produced by adding, as a sintering aid, an oxide mixture consisting essentially of calcium oxide, yttrium oxide and one or more oxides of tungsten oxide and molybdenum oxide or a mixture of compounds (precursors) capable of being converted into said oxide mixture by firing to aluminum nitride as a main component in such proportions that total amounts of the sintering aid is in the range 0.1 to 20% by weight (calculated as oxides of CaO, Y₂O₃, WO₃, and MoO₃) based on the sum of the weight as the oxide forms of the sintering aid and aluminum nitride, molding the resulting composition and sintering the molded composition. The composition can be sintered at relatively low temperature. The black sintered bodies thus obtained are especially useful as materials for insulating substrate, heat sink, packaging for semiconductor device, etc., due to their high density and good thermal conductivity.

Description: 1. Field of the Invention The present invention relates to sintered bodies of aluminum nitride used as materials for insulating substrates, heat sinks, packages for semiconductor devices etc. More specifically, the present invention relates to sintered bodies of aluminum nitride having a high density and a high thermal conductivity which can be produced at reduced sintering temperatures. 2. Description of the Prior Art In recent years, with an increasing trend toward the high integration and high densification of LSI, there has been a great demand for insulating substrate materials having a good thermal conductivity. Heretofore, sintered alumina has been most widely used as a material for an insulating substrate. However, since the alumina substrate has a low thermal conductivity (about 20 W/m.K) and has a large thermal expansion coefficient as compared to silicon, it has many problems such as poor bondability, etc. Therefore, sintered bodies of aluminum nitride (AlN) which have a good thermal conductivity and a thermal expansion coefficient approaching that of silicon have attracted a great interest. In addition to these advantages, the aluminum nitride sintered bodies possess a high mechanical strength (40 - 50 kg/mm²) and good electrical properties, for example, a small dielectric constant. However, since aluminum nitride materials are poor in sinterability due to their strong covalent bond, it is difficult to obtain high density sintered bodies. For this reason, various ways of achieving high density sintered bodies of aluminum nitride have been tried up to date and, for example, there may be mentioned a hot press sintering method and a normal pressure sintering method, both methods using a sintering aid. The hot press sintering method is unfavorable because of poor productivity. In the normal pressure method, sintering aids, such as compounds of rare earth or alkaline earth elements have been used and sintered bodies of aluminum nitride having a high density and a high thermal conductivity have been obtained. However, such a known method requires high sintering temperatures exceeding 1800 °C and is unfavorable from the viewpoint of productivity. On the other hand, as means for improving productivity, Japanese patent application laid-open No. 61-117160 discloses a method for producing a low temperature sintered body of aluminum nitride in which rare earth oxide and alkaline earth oxide are added in combination. This method tends to cause uneveness in sintering and coloration and can not still provide satisfaction in thermal conductivity. Further, sintered bodies of aluminum nitride with a high thermal conductivity and a production method thereof are described in Japanese Patent Application laid-open Nos. 61 - 270 262 and 61 - 270 263, the sintered bodies consisting of boride or carbide of Groups 4a, 5a or 6a elements of the Periodic Table and aluminum nitride. In such circumstances, further improvements have been needed in order to obtain higher thermal conductivity. Previously, Inventors have found that addition of CaMoO₄ and/or CaWO₄ enables sintering at low temperature, as described in Japanese Patent Application No. 61 - 270 390 substantially corresponding to EP-A-267 623. However, the thermal conductivity of the resulting sintered bodies is, at most, on the order of about 100 W/m.K, as referred to in the other above Japanese Patent Application Laid-open Nos. 61-117 160, 61-270 262 and 61-270 263. Also, as a method recently proposed, addition of TiO₂ or ZrO₂ in combination with Sc₂O₃ is described in Japanese Patent Application Laid-open No. 62- 72 570. However, this method is economically unfavorable in view of the use of expensive Sc₂O₃ and the thermal conductivity of the resulting sintered bodies is at most about 100 W/m.K. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a sintered body of aluminum nitride having a high density and a high thermal conductivity. Another object of the present invention is to provide such a sintered body of aluminum nitride which can be obtained by sintering at relatively low temperatures. According to the present invention, there is provided a sintered body of aluminum nitride which is produced by adding, as a sintering aid, an oxide mixture consisting essentially of calcium oxide, yttrium oxide and one or more oxides of tungsten oxide and molybdenum oxide or a mixture of compounds (precursors) capable of being converted into the foregoing oxide mixture by firing to aluminum nitride as a main component in such proportions that the total amount of the sintering aid is in the range of 0.1 to 20% by weight (calculated as oxides of CaO, Y₂O₃, WO₃ and MoO₃) based on the sum of the weight as the oxides of the sintering aid and aluminum nitride, molding the resulting composition and sintering the molded composition. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the results of an X-ray diffraction analysis of the aluminum nitride sintered body according to the present invention; FIG. 2 is an electron micrograph showing the microstructure of the cross section of the sintered body set forth in FIG. 1; FIG. 3 is a composition diagram showing proportions of ingredients of sintering aids used in Compositions 1 to 14 of the present invention and Compositions 23 to 32 of Comparative Examples; FIG. 4 is a graph illustrating the relationship between the ratio of CaO : WO₃ and the thermal conductivity of aluminum nitride sintered bodies for Compositions 3, 4 and 5 according to the present invention and Compositions 25 and 29 (Comparative Examples); FIG. 5 a graph illustrating the relationship between the ratio of CaO : WO₃ and the thermal conductivity of aluminum nitride sintered bodies for Compositions 6, 7, 8 and 9 according to the present invention and Compositions 26 and 30 (Comparative Examples); FIG. 6 is a composition diagram showing proportions of ingredients of sintering aids used in Compositions 33 to 47 of the present invention and Compositions 56 to 66 of Comparative Examples; and FIG. 7 is a graph illustrating the relationship between the ratio of CaO : MoO₃ and the thermal conductivity of aluminum nitride sintered bodies for Compositions 39 to 42 according to the present invention and Compositions 59 and 63 (Comparative Examples). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be now described in detail hereinunder. Inventors' studies have started from the finding that addition of CaMoO₄, CaWO₄ or Ca₃WO₆ to AlN makes it possible to sinter at reduced temperatures and, after extensive experimentation, have found that when the composition of CaMoO₄, CaWO₄ or Ca₃WO₆ is used in combination with Y₂O₃, there can be obtained an aluminum nitride sintered body having a high density and high thermal conductivity without adversely affecting the low-temperature sinterability. Besides such a finding, it has been also found that when a combination of CaO and MoO₃ or a combination of CaO and WO₃ or a mixture of these combination is added in proportions outside the composition of CaMoO₄, CaWO₄ or Ca₃WO₆ together with Y₂O₃ to AlN, the resulting AlN sintered bodies have a high thermal conductivity and a high density without affecting unfavorably the above low-temperature sinterability. Aluminum nitride powder containing not greater than 3% of oxygen may be preferably used in the present invention. As examples of calcium compounds used as the sintering aid of the present invention, there may be mentioned CaO or calcium compounds, such as CaCO₃, Ca(NO₃)₂.4H₂O or CaC₂O₄.H₂O, which can be eventually converted to the CaO by firing. As the yttrium compounds, there may be mentioned Y₂O₃ or compounds, such as Y₂CO₃, Y(NO₃)₃, Y(CH₃COO)₃, or Y(C₂O₄)₃, which can be eventually converted to Y₂O₃ by firing. MoO₃ or Mo compounds (e.g., H₂MoO₄) capable of being converted to MoO₃ by firing may be used as the sintering aid. Also, composite compounds such as CaMoO₄ or Ca₃MoO₆ can be employed, while it is uncertain whether they yield oxides of the respective constituent metal elements by firing. Examples of tungsten compounds used in the present invention include WO₃ and tungsten compounds (e.g., H₂WO₄) capable of being converted to WO₃ by firing. Needless to say, composite compounds such as CaWO₄ or Ca₃WO₆ may be also employed, although it is uncertain whether they yield oxides of the respective metals. Aluminum nitride and the sintering aids set forth above are preferably employed in the form of small powder having an average particle diameter of not more than 10 µm, preferably not more than 5 µm. In the present invention, for the sake of convenience, the sintering aids are described as sintering aid powder, but liquid sintering aids are included in the invention When the aforementioned sintering aids, i.e. CaO or other calcium compounds (hereinafter referred merely as "Ca sintering aid"), Y₂O₃ or other yttrium compounds ("Y sintering aid"), MoO₃ or other molybdenum compounds ("Mo sintering aid") and WO₃ or other tungsten compounds ("W sintering aid") are added in total amounts of less than 0.1% by weight calculated in terms of CaO, Y₂O₃, MoO₃ and WO₃, the effects of these sintering aids can not be sufficiently achieved. On the other, an excess addition of the sintering aids exceeding 20% by weight in total detrimentally affects the properties of the resulting sintered bodies. For example, such an excess addition leads to reduction in the density and the thermal conductivity. Thus, the sintering aids are preferably added in a total amount of 0.1 to 20 % by weight calculated as CaO, Y₂O₃, MoO₃ and WO₃, based on the total weight of aluminum nitride and the sintering aids calculated as the oxides. The minimum amount of each sintering aid is 0.03% for Y₂O₃, 0.02% for CaO and 0.05% for WO₃ and/or MoO₃ and the total of the sintering aids is at least 0.1% by weight. Addition of the sintering aids is minimized in the required amounts, depending on the production abilities of the used apparatus and the properties required in the resulting sintered bodies, and such amounts are in the range of 0.2 to 15 % by weight. In aluminum nitride compositions intended to be sintered at sintering temperatures not exceeding 1800°C, the mixing ratio of the respective sintering aids is determined depending on the properties required in the intended sintered bodies. More specifically, in order to obtain black aluminum nitride sintered bodies which are needed for the use as semiconductor packaging materials or the like, the foregoing Ca, Y, Mo and W sintering aids are added in such proportions that the total amount of the Mo sintering aid and the W sintering aid preferably ranges from 10 to 90% by weight, more preferably 20 to 80% by weight (calculated in terms of MoO₃ and WO₃), based on the total amounts of the sintering aids of Ca, Y, Mo and W, calculated in terms of oxides (i.e., CaO, Y₂O₃, MoO₃ and WO₃). On a basis of the sum of the weight of the aluminum nitride powder and the weight as the foregoing oxide forms of the sintering aids, the total amount as MoO₃ and WO₃ of the Mo and W sintering aids is preferably not less than 0.1% by weight and more preferably not less than 0.3 % by weight. When the amounts of these sintering aids stray from this range, the resulting sintered products do not show black color and sintering can not be sufficiently effected. In order to obtain aluminum nitride sintered bodies having a high thermal conductivity, the amount of the Y sintering aid, calculated as Y₂O₃, is preferably in the ranges of 5 to 90 % by weight and more preferably in the range of 20 to 80 % by weight, when the total weight of the sintering aids, calculated in terms of CaO, Y₂O₃, MoO₃ and WO₃, is taken as 100% by weight. Further, the remaining sintering aids (i.e., Ca sintering aid, Mo sintering aid and W sintering aid) are added in such proportions (as CaO, MoO₃ and WO₃) that the weight ratio of CaO : (MoO₃ + WO₃) is preferably from 9:1 to 1:9 and more preferably from 4:1 to 1:4. Further, when the sum of the aluminum nitride powder and the sintering aids (as the foresaid oxide forms) is taken as 100% by weight, the amount of the Y sintering aid, calculated as Y₂O₃, is preferably in the range of 0.5 % by weight to 7 % by weight, and more preferably in the range of 1 to 5 % by weight. When the amount of the Y sintering aid falls outside the ranges, the desired high thermal conductivity can not be achieved and sintering can not be sufficiently achieved. Further, in order to achieve sintered bodies having a desired combination of uniform black color and high thermal conductivity, an overlapping range between the ranges specified above may be employed and the mimimum content of each component in the resulting sintered body is 0.5% for Y₂O₃, 0.05% for CaO and 0.3% for WO₃ and/or MoO₃ and the total amount of the sintering aids is at least 0.85%. Besides the aforesaid sintering aids, other components, for example, compounds of Ti, Zr, Fe, etc., may be added or included. Also, carbon or its compound may be included as an incidental component. Needless to say, the addition or inclusion of these components is permitted only within the ranges which do not detrimentally affect sinterability and other properties required in the sintered bodies. The reason why the sintered bodies of aluminum nitride according to the present invention have an improved thermal conductivity and a low-temperature sinterability is not yet certain. However, the following assumption is possible. With respect to the low-temperature sinterability, it is assumed that CaO-Al₂O₃ system and MoO₃ ( or WO₃)-Al₂O₃ system composite compounds having a low melting temperature are formed during the firing process and these compounds make possible the reduction of the sintering temperature. With respect to the high thermal conductivity, metallic Mo and W, Y₂O₃ - Al₂O₃ compound and CaO-Al₂O₃ compound, etc., were detected in the sintered body in our X-ray diffraction studies. Based on the experimental data, it is possible to assume that oxygen is released from MoO₃ or WO₃ during the sintering process and bonds to aluminum component to form Al₂O₃. The thus formed Al₂O₃ is converted, for example, to Y₂O₃-Al₂O₃ and thereby is fixed as grain boundary components. The thermal conductivity of the sintered body may also change depending on the ratio of CaO to (WO₃ + MoO₃), but the reason for this is not fully understood. FIG. 1 shows the results of the X-ray diffraction analysis and FIG. 2 is an electron micrograph of the sintered body of the present invention in which W sintering aid was used with Ca and Y sintering aids. As can be seen from these figures, the sintered body includes aluminum nitride ("A" in FIG. 1 and black, gray or white large grains in FIG. 2), metallic tungsten ("W" in FIG. 1 and white spherical fine grains in FIG. 2), and aluminum oxide containing rare earth elements ("Y" and "G" in FIG. 1 and white grain boundary phase of aluminum and yttrium composite oxide in FIG. 2) as main components and further unidentified components (unidentified phase "X" in FIG. 1) which are assumed as composite aluminum oxides containing alkaline earth and rare earth elements. As a further identified component, alkaline earth aluminum oxide is considered to be present in the sintered body. Although these figures are the experimental results for the W sintering aid, similar data can be also obtained using the Mo sintering aid. In the process for producing the sintered bodies of the present invention, sintering should be performed in a nonoxidizing atmosphere, for example, in an atmosphere of nitrogen gas, argon gas, helium gas, carbon monoxide gas, hydrogen gas, ammonia gas or a mixed gas thereof or in a vacuum. Particularly, an atmosphere of nitrogen gas is preferable. Sintering temperature is generally in the range of 1500 to 2100 °C and may preferably be in the range of 1550 to 1800 °C. Sintering is ordinarily conducted by a normal pressure sintering process but it can be also conducted under reduced pressure or by hot-press sintering. The present invention will now be described in detail with reference to Examples and Comparative Examples. In the Tables below, unless otherwise indicated, the amounts of sintering aids used in Examples and Comparative Examples are all given in percent by weight (calculated as CaO, Y₂O₃, WO₃ and MoO₃) on the basis of the sum of the weight calculated as the oxides, of the sintering aids and aluminum nitride. Composition 1 (Example of the present Invention) 1.0% by weight as CaO of calcium carbonate powder, 3.5% by weight of Y₂O₃ powder and 0.5% by weight of tungstic acid anhydride (WO₃) powder were added to aluminum nitride powder having an average particle diameter of 2 µm and containing 1.7% by weight of oxygen as an impurity and mixed together. Then, the resulting mixture was mixed with 2% by weight of a binder based on the weight of the mixture of the aluminum nitride and the sintering aids and granulated. The obtained mixture was molded under a pressure of about 1200 kg/cm² at room temperature to give a molded body with a diameter of about 25 mm and a thickness of about 5 mm. The molded body was heated to a temperature of 600 °C and held at that temperature for five hours in an atmosphere of nitrogen gas to remove the binder. Then, the body was placed in a boron nitride crucible and a sintered body of aluminum nitride was obtained by normal pressure sintering at 1700 °C for three hours. The properties of the sintered body thus obtained are given as Composition No. 1 in Table 1 below. Compositions 2 - 14 (Examples of the present Invention) Y₂O₃ powder, CaCO₃ powder and WO₃ powder were mixed in the proportions (calculated as CaO, Y₂O₃, and WO₃) as shown in FIG. 3 and were added in a total amount of 5.0% by weight (as CaO, Y₂O₃, and WO₃) to the aluminum nitride powder used in Composition 1. Sintered bodies of aluminum nitride were obtained from the respective compositions in the same manner as described in Composition 1. The properties of the resulting sintered bodies are shown as Composition Nos. 2-14 in Table 1. Compositions 15 - 20 (Examples of the present Invention) 50% by weight of Y₂O₃ powder, 20% by weight as CaO of CaCO₃ powder and 30% by weight of WO₃ powder were mixed to provide a powder mixture of the sintering aids. The mixture was added in given amounts to the aluminum powder employed in Composition 1 and sintered bodies were obtained in the same manner as described in Composition 1. The added amounts of the powdered mixture of the sintering aids and the properties of the sintered bodies thus obtained are shown in Table 1. Compositions 21 - 22 (Examples of the present Invention) A mixture of 2.5% by weight of Y₂O₃ powder and 2.5% by weight of Ca₃WO₆ (Composition 21) or a mixture of 3.5% by weight of Y₂O₃ powder and 1.5% by weight of CaWO₄ (Composition 22) was added to the aluminum nitride powder used in Composition 1. The percentages as Y₂O₃, CaO and WO₃ are given in Table 1. In accordance to the same procedure as set forth in Composition 1, the resulting mixtures were molded and the binder component was removed. The thus treated mixtures were put into a crucible of aluminum nitride and sintered bodies of aluminum nitride were obtained by normal pressure sintering at a temperature 1700°C for 15 hours in an atmosphere of nitrogen. The properties of the aluminum nitride sintered bodies thus obtained are given in Table 1. Compositions 23 - 32 (Comparative Examples) Sintering aids having compositions shown as Nos. 23 to 32 in FIG. 3 were added in total amounts as their oxides of 5.0% by weight to the aluminum nitride powder employed in Composition 1 and comparative sintered bodies of aluminum nitride were obtained in the same way as set forth in the above Examples. The properties of the sintered bodies thus obtained are given in Table 1 (Compositions Nos. 23 to 32) With respect to Compositions 2, 4 and 13 and Comparative Compositions 24 and 29, further studies were made by changing the sintering temperatures, as shown in Table 2. The properties of the sintered bodies thus obtained are given in Table 2. As is apparent from Table 2, the sintered bodies according to the present invention were suffciently densified and had a high thermal conductivity even when sintering was performed at 1650°C. In FIG. 3, percentages of Y₂O₃ of the sintering aids lying on a line joining 25 and 29 and the sintering aids lying on a line joining 26 to 30 are constant for each line. Using these sintering aid compositions, the dependence on CaO : WO₃ ratio of thermal conductivity was examined and the results are shown in FIGS. 4 and 5. It can be seen from these figures that thermal conductivity can be improved in the CaO : WO₃ ratio ranging from 9:1 to 1:9. Particularly, as can be seen from FIG. 4, a very high thermal conductivity is achieved in Composition 4 having a CaO : WO₃ molar ratio of about 3 : 1 and it is assumed that at this ratio Ca₃WO₆ is formed during the firing process and the thus formed Ca₃WO₆ makes a great contribution to the increase of the thermal conductivity. In Tables 1 and 2, the mark "O" at the column "Color" shows sintered bodies having a good black appearance and blank shows sintered bodies having a different color, such as white, gray or dark gray, or unevenly black-colored sintered bodies. Composition 33 (Example of the present Invention) 1.0% by weight as CaO of calcium carbonate powder, 3.5% by weight of Y₂O₃ powder and 0.5% by weight of molybdic acid anhydride (MoO₃) powder were added to aluminum nitride powder having an average particle diameter of 2 µm and containing 1.7% by weight of oxygen as an impurity and mixed together. Then, the resulting mixture was mixed with 2% by weight of a binder based on the sum of the weight of the aluminum nitride and the weight (as oxides) of the sintering aids and granulated. The obtained mixture was molded under a pressure of about 1200 kg/cm² at room temperature to give a molded body with a diameter of about 25 mm and a thickness of about 5 mm. The molded body was heated to a temperature of 600 °C and held at this temperature for five hours in an atmosphere of nitrogen gas to remove the binder. Then, the body was placed in a boron nitride crucible and a sintered body of aluminum nitride was obtained by normal pressure sintering at 1700 °C for three hours in an atmosphere of nitrogen to provide a sintered body of aluminum nitride. The properties of the sintered body are given as Composition No. 33 in Table 3 below. Compositions 34 - 55 (Examples of the present Invention) Y₂O₃ powder, CaCO₃ powder and MoO₃ powder or CaMoO₄ powder were added in quantities (calculated as CaO, Y₂O₃, and MoO₃) as given in Table 3 to the aluminum nitride powder used in Composition 33 and sintered bodies of aluminum nitride were obtained in the same manner as described in Composition 33. With respect to Compositions 34, 37 and 46, CaMoO₄ powder and Y₂O₃ powder were mixed in amounts (in terms of Y₂O₃, CaO and MoO₃) as shown in FIG. 6. In Compositions 35, 36, 38 to 45 and 47, Y₂O₃ powder and CaCO₃ powder and MoO₃ powder were used in proportions shown in FIG. 6. Numbers (33 to 47) shown in FIG. 6 correspond to the numbers of the Compositions in Table 3, respectively, and the sintering aids were added in a total amount of 5.0% by weight (as CaO, Y₂O₃, and MoO₃). In Compositions 48 to 55, sintering aids were added in proportions shown in Table 3. Compositions 52 to 55 are sintered bodies employing MoO₃ in combination with WO₃. The properties of the resulting sintered bodies are shown as Composition Nos. 34 - 55 in Table 3. Compositions 56 - 66 (Comparative Examples) Sintering aids having compositions shown as Nos. 56 to 66 in FIG. 6 were added in total amounts of 5.0% by weight to the aluminum nitride powder used in Composition 33 and comparative sintered bodies of aluminum nitride were obtained in the same way as set forth in the above Examples. The properties of the sintered bodies thus obtained are given in Table 3 (Compositions Nos. 56 to 66) With respect to Compositions 36 and 40 and Comparative Compositions 58 and 59, further studies were made by changing the sintering temperatures, as shown in Table 4. The properties of the sintered bodies thus obtained are given in Table 4. As is apparent from Table 4, the sintered bodies according to the present invention were sufficiently densified and had a high thermal conductivity even when sintering was performed at 1650°C. In FIG. 6, percentages of Y₂O₃ in sintering aids lying on a line joining 59 and 63 are the same and using these sintering aids, the dependence of thermal conductivity on the ratio of CaO : MoO₃ ratio was examined and the results are shown in FIG. 7. It can be seen from these figures that thermal conductivity can be improved when the CaO : MoO₃ ratio is within the range of 9:1 - 1:9. In Tables 3 and 4, external appearance of the resulting sintered bodies are indicated as referred to in Tables 1 and 2. In the following Examples, composite oxides such as CaWO₄, Ca₃WO₆ or CaMoO₄ were used as sintering aids. Composition 67 (Example of the present Invention) 40% by weight of calcium tungstate (CaWO₄) powder and 1.0% by weight yttrium oxide powder were added to aluminum nitride powder having an average particle diameter of 2 µm and admixed. Then, the resulting mixture was molded under a pressure of about 1200 kg/cm² at room temperature to give a molded body and then fired at a temperature of 1700 °C for three hours in an atmosphere of nitrogen gas to provide a sintered body of aluminum nitride. The sintered body thus obtained was dense, had an excellent light-shielding ability and showed black color. The density and the thermal conductivity of the sintered body were measured and the results are given in Table 5 below. A green sheet having the same composition as Composition 67 was formed using a doctor blade and was fired at 1700 °C in an atmosphere of a mixed gas of nitrogen and hydrogen. There was obtained a black sintered aluminum nitride substrate substantially free of warpage. Compositions 22, 68 - 73 (Examples of the present Invention) Alkaline earth tungstate powder (CaWO₄) and rare earth oxide powder were mixed and were added in appropriate proportions to the aluminum nitride powder used in Composition 67 so as to obtain Compositions 22, 68 to 73. Sintered bodies of aluminum nitride were obtained from the resulting mixtures in the same way as set forth in the above Example 67 except that sintering was conducted at 1600 to 1800 °C. Density and thermal conductivity were measured for the respective sintered bodies. The results obtained are shown in Table 5, together with the kind and amounts of the used sintering aids, the sintering conditions and the color of the respective sintered bodies. As can be seen from Table 5, the sintered bodies of Compositions 22 and 67 to 73 had a good black appearance free of uneven sintering and uneven coloration and this is due to a uniform dispersion of metallic tungsten fine particles in the resulting sintered bodies. Further, since the sintered bodies had a high volume resistivity of at least 10¹² ohm.cm, they are desirable as insulating materials. Further, when tungsten conductors are bonded onto the unsintered bodies of Compositions 22 and 67 to 73 by co-firing process, there could be obtained conductors having a much higher electrical conductivity without forming tungsten boride which is formed in the conventional aluminum nitride sintered body containing boride. Compositions 74 (Example of the present Invention) 4.0 % by weight of Ca₃WO₆ powder and 0.1% by weight of Y₂O₃ powder were added aluminum nitride powder with 2 µm in average particle diameter and the obtained mixture was molded under a pressure of about 1200 kg/cm² at room temperature. The molded body was fired at 1700 °C for three hours in an atmosphere of nitrogen to provide a sintered body of aluminum nitride. The sintered body thus obtained was dense and had a superior light-shielding ability and an excellent surface smoothness. Further, it had a good black color. The density and thermal conductivity of the sintered body were measured and the results given in Table 5. Compositions 21, 75 - 91 (Examples of the present Invention) Ca₃WO₆ powder and Y₂O₃ powder were added in amounts shown in Table 5 to the aluminum nitride powder employed in Composition 74 and there could be obtained sintered bodies by sintering at temperatures shown in Table 5 in accordance with the same procedure described in Composition 74. Density and thermal conductivity were measured for the respective sintered bodies. The results obtained are shown in Table 5, together with the color of the respective sintered bodies. The sintered body exhibited good electrical insulating properties (volume resistivity of at least 10¹² ohm.cm). In the above Compositions, a comparison of the thermal conductivities of Compositions 21 and 81 indicates that the holding time at a firing temperature considerably affects thermal conductivity. Namely, both Compositions contains the same amounts of Ca₃WO₆ (2.5%) and Y₂O₃ (2.5%) and their firing temperatures are both 1700 °C. However, the holding times at the firing temperatures are different (i.e., 15 hours for Composition 21 and 3 hours for Composition 81)and this difference leads to the different results in their thermal conductivity (i.e., 165 and 145). Low thermal conductivity of Composition 91 is due to the low firing temperature of 1550 °C. Further, in comparison of the thermal coductivities of Compositions 67 and 78 in which Y₂O₃ powder was used in the same amount (1.0%) and sintering conditions (1700 °C × 3 hours) were the same, it becomes apparent that Ca₃WO₆ results in a higher thermal conductivity than CaWO₄. Composition 92 (Comparative Example) A comparative sintered body was prepared in the same way described in Composition 67 except that tungsten oxide powder was added in an amount of 3.0% by weight to the aluminum nitride powder used in Composition 67 and sintering was conducted at 1900 °C. The sintered body thus obtained was black in color and its thermal conductivity was as low as 55W/m.K. Composition 93 (Comparative Example) A comparative sintered body was prepared in the same way as described in Composition 67 except that yttrium oxide powder was added in an amount of 5.0% by weight to the aluminum nitride powder used in Composition 67 and sintering was conducted at 1800 °C. The sintered body thus obtained had a light transmitting property and light brown in color. In some portions, uneven coloration was observed. Composition 94 (Comparative Example) A comparative sintered body was prepared in the same way as set forth in Composition 67 except that 4.0% by weight of tungsten oxide powder and 5.0% by weight of yttrium oxide powder were added to the aluminum nitride powder used in Composition 67 and firing was conducted at 1700 °C. The sintered body thus obtained was not dense. Composition 95 (Comparative Example) A comparative sintered body was prepared in the same way as described in Composition 67 except that calcium tungstate powder was added in an amount of 4.0% by weight to the aluminum nitride powder used in Composition 67 and firing was conducted at 1700 °C. The sintered body thus obtained was black in color and its thermal density was as low as 100 W/m.K. Composition 96 (Comparative Example) A comparative sintered body was prepared in the same way as set forth in Composition 67 except that 4.5% by weight of calcium carbonate powder and 2.5% by weight of yttrium powder were added to the aluminum nitride powder used in Composition 67 and sintering was carried out at 1700 °C. The sintered body thus obtained had a light transmitting ability and was unevenly colored in light gray. Composition 97 (Comparative Example) A comparative sintered body was prepared from aluminum nitride powder in the same way as set forth in Composition 74 except that sintering was carried out at 1800 °C for three hours without using any sintering aid. The resulting sintered body was white in color and was hardly densified. Composition 98 (Comparative Example) A comparative sintered body was prepared in the same way described in Composition 74 except that CaCO₃ powder was added in an amount of 1.0% by weight (calculated as CaO) to the aluminum nitride powder used in Composition 74 and sintering was conducted at 1800 °C for 3 hours. The sintered body thus obtained was gray in color and unevenly sintered portions were observed in the sintered body. Further, small pores were observed on the surface of the sintered body. Composition 99 (Comparative Example) A comparative sintered body was prepared in the same way as described in Composition 74 except that yttrium oxide powder was added in an amount of 1.0% by weight to the aluminum nitride powder used in Composition 74 and sintering was conducted at 1800 °C for three hours. The sintered body thus obtained had a slight light transmittance and was light brown in color. In some portions, uneven coloration was observed. Composition 100 (Comparative Example) A comparative sintered body was prepared in the same way as described in Composition 74 except that WO₃ powder was added in an amount of 3.0% by weight to the aluminum nitride powder used in Composition 74 and sintering was conducted at 1800 °C for three hours. The sintered body thus obtained was light brown in color and uneven sintering and coloration were observed. Composition 101 (Comparative Example) A comparative sintered body was prepared in the same way as set forth in Composition 74 except that 4.0% by weight of CaWO₄ powder and 1.0% by weight of Y₂O₃ powder were added to the aluminum nitride powder used in Composition 74 and sintering was conducted at 1600 °C for three hours. The aluminum nitride sintered body thus obtained was black. Composition 102 (Comparative Example) A comparative sintered body was prepared in the same way as set forth in Composition 74 except that 5.0% by weight of Ca₃WO₆ powder was added to the aluminum nitride powder used in Composition 74 and sintering was conducted at 1600 °C for three hours. The aluminum nitride sintered body thus obtained was black. Density and thermal conductivity were measured for the respective sintered bodies of aluminum nitride obtained from Comparative Compositions 92 to 102 as stated in the foregoing Examples of the present invention and the results obtained are given in Table 5. For comparison, the data of Composition 66 are also shown in the same Table. It is apparent that most of the comparative sintered bodies required high sintering temperatures and their thermal conductivity was low as compared to the sintered bodies of the present invention. As described above, the sintered bodies of aluminum nitride in accordance with the present invention have a high density and a good thermal conductivity and, if necessary, the sintered bodies can be uniformly colored in black to achieve a superior light shielding ability. Further, the aluminum nitride sintered bodies of the present invention can be produced at relatively low sintering temperatures of not higher than 1800 °C and this is industrially very advantageous from the viewpoint of productivity and cost. Such advantages not only make the sintered bodies suitable for practical use in insulating substrates, heat sinks, packages for semiconductor devices, but also lead to further expanded applications of them, for example, in co-fired type substrate application where substrate materials are co-fired with conductor materials to be bonded.

Claims: A sintered body of aluminum nitride produced by adding, as a sintering aid, an oxide mixture consisting essentially of calcium oxide, yttrium oxide and one or more oxides of tungsten oxide and molybdenum oxide or a mixture of compounds (precursors) capable of being converted into said oxide mixture by firing to aluminum nitride as a main component in such proportions that total amounts of said sintering aid is in the range 0.1 to 20% by weight (calculated as oxides of CaO, Y₂O₃, WO₃ and MoO₃) based on the sum of the weight as said oxides of the sintering aid and said aluminum nitride, molding the resulting composition and sintering the molded composition. A sintered body of aluminum nitride as claimed in Claim 1 in which said mixture of said compounds contains one or more of CaWO₄, Ca₃WO₆ and CaMoO₄. A sintered body of aluminum nitride as claimed in Claim 1 in which the total amount of said sintering aid is in the range of 0.2 to 15% by weight (calculated as said oxides of CaO, Y₂O₃, WO₃ and MoO₃) based on the sum of the weight as said oxides of said sintering aid and aluminum nitride. A sintered body of aluminum nitride as claimed in Claim 1 in which the minimum amounts of said sintering aids are 0.03% for Y₂O₃, 0.02% for CaO and 0.05% for one or more of WO₃ and MoO₃. A sintered body of aluminum nitride as claimed in Claim 3 in which the minimum amounts of said sintering aids are 0.5% for Y₂O₃, 0.05% for CaO and 0.3% for one or more of WO₃ and MoO₃. A sintered body of aluminum nitride as claimed in Claim 4 in which the total amount of one or more of WO₃ and MoO₃ is at least 0.1% by weight. A sintered body of aluminum nitride as claimed in Claim 4 in which the total amount of one or more of WO₃ and MoO₃ is at least 0.3% by weight. A sintered body of aluminum nitride as claimed in Claim 4 in which Y₂O₃ is in the range of 0.5 to 7% by weight. A sintered body of aluminum nitride as claimed in Claim 4 in which Y₂O₃ is in the range of 1 to 5% by weight.

Inventors: KURATANI, SHUSEI, MIZUNO, SHINYA, NISHIYAMA, SATOSHI, SAKURAMOTO, HISASHI, UNO, KOHICHI
Applicants: NARUMI CHINA CORPORATION
Application year: 1988
Main classification: C04B35
IPC codes: C04B35