Sintered aluminum nitride semi-conductor device
This is a division of application Ser. No. 373,150, filed 4/29/82, now U.S. Pat. No. 4,540,673. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a sintered aluminum nitride suitable for insulating substrates for integrated circuits, etc., and more particularly to sintered aluminum nitride having a high thermal conductivity, a high electrical resistivity, and a low coefficient of thermal expansion. 
     The semiconductor industry has made a remarkable progress in recent years and a number of circuit constituents such as semiconductor chips have been formed in increasingly higher density on an insulating substrate used in large scale integrated circuit and the like. Demands for devices having greater capacity with smaller size have also become keener and insulating substrates having a high thermal conductivity have been in demand. 
     As materials for such an insulating substrate, there has conventionally been employed sintered alumina. Since the sintered alumina does not have a satisfactory thermal conductivity and development of an insulating substrate having a higher thermal conductivity has been requested. The insulating substrate for such an application must satisfy the following requirements. 
     (1) high electrically insulating property, 
     (2) high thermal conductivity, 
     (3) its coefficient of thermal expansion is approximate to that of silicon, 
     (4) high mechanical strength, and 
     (5) low dielectric constant. 
     Sintered silicon carbide has been proposed to satisfy the foregoing requirements, but still has a problem in obtaining a high density sintered silicon carbide with a high electrical resistivity, and various improvements have been therefore proposed. That is, various sintering aids have been employed in order to obtain a high density sintered silicon carbide. U.S. Pat. No. 4,172,109, for example, discloses a sintered silicon carbide containing beryllium as a sintering aid. This prior art relates to a high strength material obtained by sintering powder of silicon carbide which contains 0.5 to 5 wt.% of excessive carbon. However, the sintered silicon carbide thus formed has small electric resistivity and cannot be used as an electrically insulating material. 
     On the other hand, U.S. Patent Application Ser. No. 203,554 filed Nov. 5, 1980 discloses sintered silicon carbide containing 0.1-3.5 wt.% beryllium, which has a thermal conductivity of at least 0.4 cal/cm.sec.°C. at 25° C., an electrical resistivity of at least 10 7  ohm.cm at 25° C. and a coefficient of thermal expansion of 3.3-4×10 -6  /°C. at 25° C. to 300° C. However, in that case, the firing temperature is as high as 1,850°-2,500° C., and a new material, which can be fired economically at a lower temperature and has a higher electrical resistance, has been in demand. 
     As a substitute for the sintered silicon carbide, sintered aluminum nitride has been regarded as important, because sintered aluminum nitride has a coefficient of about 5×10 -6  /°C., which is smaller than that of sintered alumina, about 7×10 -6  /°C., and is nearly as low as that of silicon, about 3.3×10 -6  /°C., a flexural strength of about 50 kg/mm 2 , which is much higher than that of sintered alumina, about 20 kg/mm 2 , and also has a good electrically insulating property. 
     The well known methods for preparing sintered aluminum nitride include (1) a reaction sintering method, (2) normal pressure sintering method, and (3) hot press sintering method [Journal of Material Science 4 (1969) 1045-1050; Journal of The American Ceramic Society-Discussions and Notes, 57 No. 9, 411-412]. The reaction sintering method is to sinter metallic aluminum molding while subjecting it to nitrizing reaction in a nitrogen gas atmosphere, where the rate of nitrizing reaction is dependent upon diffusion of nitrogen gas, so that, in the case of a thick molding, unreacted aluminum remains at the center part and the sintered molding is porous, so that it cannot be practically used as an electrically insulating material. 
     The normal pressure sintering method is to sinter a molding of a mixture of aluminum nitride powder and a powdery additive of yttrium oxide and a rare earth element oxide, or yttrium oxide and silicon dioxide, nickel and/or calcium oxide. 
     The hot press sintering method is to sinter a molding of a mixture of aluminum nitride powder and a powery additive of aluminum oxide, or yttrium oxide and silicon dioxide, or the like with heating under pressure. According to the known normal pressure sintering method and hot press sintering method, compact sintered aluminum nitride with a high mechanical strength, a high electrically insulating property and a low coefficient of thermal expansion can be obtained. However, the sintered aluminum nitride has a low thermal conductivity, usually 0.07 cal/cm.sec.°C. and at most 0.1 cal/cm.sec.°C. at room temperature. Thus, sintered aluminum nitride having a higher coefficient of thermal conductivity, if available, will be very useful as an insulating substrate material for large scale integrated circuits etc. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide sintered aluminum nitride having a high thermal conductivity, a high electrical resistivity and a low coefficient of thermal expansion, and also to provide a semi-conductor device using the same as an insulating substrate. 
     The present invention provides sintered aluminum nitride comprising at least 65% by weight of aluminum nitride and at least one of beryllium, a beryllium compound, lithium, and a lithium compound, and a semi-conductor device using the same as an insulating substrate. 
     Sintered aluminum nitride having a thermal conductivity of 0.2 cal/cm.sec.°C. or higher at 25° C., an electrical resistivity of 10 12  Ω.cm or higher at 25° C., an average coefficient of thermal expansion of 6×10 -6  /°C. or lower in a range of 25° to 300° C., and a density of at least 90% of the theoretical density of sintered aluminum nitride can be obtained by adjusting an amount of at least one of beryllium, beryllium compound, lithium, and lithium compound to be added, and also by adjusting firing conditions. 
     The present sintered aluminum nitride containing at least one of beryllium, a beryllium compound, lithium, and a lithium compound can be prepared by mixing 0.05 to 10% by weight of at least one of beryllium and a beryllium compound in terms of beryllium atom, or 0.5 to 5% by weight of at least one of lithium and a lithium compound in term of lithium atom, or 0.05 to 10% by weight of at least one of beryllium and a beryllium compound and 0.05 to 5% by weight of at least one of lithium and a lithium compound in terms of beryllium and, lithium atoms, respectively, where if the lithium or the lithium compound takes the major proportion, the sum total of at least one of the beryllium and the beryllium compound and at least one of the lithium and the lithium compound is not more than 5% by weight in terms of beryllium and lithium atoms, and if the beryllium content takes the major proportion, the sum total of at least one of the beryllium and the beryllium compound and at least one of the lithium and the lithium compound is not more than 10% by weight in terms of beryllium and lithium atoms, into aluminum nitride powder having an average particle size of 20 μm or less on the basis of aluminum nitride, compression-molding the resulting mixture, and sintering the resulting molding in a non-oxidizing atmosphere at a temperature of 1,600° to 2,000° C. under normal pressure, or hot-pressing the resulting mixture at a temperature of 1,600° to 2,000° C. under a pressure of 100 kg/cm 2  or higher in a non-oxidizing atmosphere, for a time enough to obtain at least 90% of theoretical density of sintered product. 
     A preferable beryllium content of the present sintered aluminum nitride is 0.05 to 10% by weight in terms of beryllium atom on the basis of aluminum nitride. To obtain a compact sintered product, not less than 0.05% by weight is preferable, while, to obtain a sintered product having a coefficient of thermal expansion of 6×10 -6  /°C., not more than 10% by weight is preferable. The thus obtained sintered product is particularly suitable for an insulating substrate for silicon semi-conductor element and other semi-conductor elements. 
     The beryllium compound to be added to the aluminum nitride powder includes, for exmple, beryllium nitrate, beryllium sulfate, beryllium carbonate, beryllium phosphate, beryllium hydroxide, beryllium halides such as beryllium chloride and beryllium fluoride, beryllium acetylacetonate, beryllium oxalate, beryllium carbide, beryllium boride, beryllium silicide, beryllium nitride, and beryllium oxide, which can be used alone or in combination. 
     A preferable lithium content of the present sintered aluminum nitride is at least 0.05% by weight in terms of lithium atom on the basis of aluminum nitride. To prevent dissipation of lithium or a lithium compound at the firing to obtain a compact sintered product, not more than 5% by weight is preferable. 
     The lithium compound includes, for example, lithium oxide, lithium halides such lithium chloride and lithium fluoride, lithium nitrate, lithium carbonate and lithium sulfate, which can be used alone or in combination. 
     If both beryllium or the beryllium compound and lithium or the lithium compound are contained in the present sintered product, their content is in a range of 0.05 to 10% by weight of at least one of beryllium and a beryllium compound and 0.05 to 5% by weight of at least one of lithium and a lithium compound in terms of beryllium and lithium atoms, respectively on the basis of aluminum nitride, where if the lithium or the lithium compound takes the major proportion in the content, the sum total of at least one of the beryllium and the beryllium compound and at least one of the lithium and the lithium compound is not more than 5% by weight in terms of beryllium and lithium atoms, and if the beryllium content takes the major proportion in the content, the sum total of at least one of the beryllium and the beryllium compound and at least one of the lithium and the lithium compound is not more than 10% by weight in terms of beryllium and lithium atoms. 
     The beryllium, beryllium compound, lithium and/or lithium compound can be mixed with aluminum nitride powder in the form of powder or solution. 
     The mixing can be carried out in any known apparatus or by any known means, and the resulting homogeneous mixture can be compression-molded into any desired shape at room temperature. 
     When at least one of Y 2  O 3 , MgO, Al 2  O 3  and SiO 2  is contained in aluminum nitride as a sintering aid in addition to beryllium, beryllium compound, lithium and lithium compound, these sintering aids act as impurities in the aluminum nitride, giving rise to disorder in the aluminum nitride crystal lattice and to consequent decrease in thermal conductivity. However, addition of beryllium, a beryllium compound, lithium or a lithium compound can improve the thermal conductivity. These sintering aids can reduce the amount of beryllium, a beryllium compound, lithium or a lithium compound to be added, but a sintered product having a higher thermal conductivity can be obtained if no such sintering aids are contained. It is preferable that the content of these sintering aids is not more than 5% by weight in their total, and is less than the total beryllium and lithium content in terms of atoms on the basis of aluminum nitride. 
     It is preferable that aluminum nitride powder has an average particle size of 20 μm or less, preferably 10 μm or less. 
     It is preferable to sinter a molding of a mixture of aluminum nitride powder with at least one of beryllium, a beryllium compound, lithium and a lithium compound in a non-oxidizing atmosphere, because aluminum nitride is oxidized in an oxidizing atmosphere, so that the desired sintered product is difficult to obtain. 
     Effective sintering temperature is 1,600° to 2,000° C., preferably 1,700° to 1,900° C. To obtain a compact sintered product, not less than 1,600° C. is preferable, whereas to prevent overfiring, not more than 2,000° C. is preferable. Sintering can be carried out according to the normal pressure sintering method or the hot press method. When sintering is carried out according to a hot press method of uniaxial compression type, shrinkage takes place only in the direction of compressing axis, and a sintered product having a high dimensional precision and a higher mechanical strength than that attained according to the normal pressure sintering method can be obtained. In the hot press method, the upper limit of a compression load depends upon the quality of a die to be used, but the desired sintered product can be obtained from any die, so long at it can withstand a load of at least 100 kg/cm 2 . 
     An optimum sintering time depends upon the particle size of raw material powder, species and amount of beryllium, beryllium compound, lithium or lithium compound, sintering temperature and an applying load, when applied at the sintering. Generally, a compact sintered product can be obtained for a short time when a molding of raw material powder of smaller particle size is sintered under a load, particularly a larger load, at a higher sintering temperature. 
     A binder can be used for preparing a molding and includes, for example, such organic materials as polyvinyl alcohol, phenol resin, silicone resin, etc., and particularly polyvinyl alcohol and silicone resin are preferable because they will leave no carbon in sintered products. 
     The sintered product is comprised of aluminum nitride grains and additive grains. The additive grains form the secondary phase in the sintered product. The dissolved content of the additive materials in the aluminum nitride is very small. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the relationship between the Be quantity in the sintered product and the relative density of the sintered product; 
     FIG. 2 is a diagram showing the relationship between the Be quantity and the thermal conductivity of the sintered product at 25° C.; 
     FIG. 3 is a diagram showing the relationship between the Be quantity and the electrical resistivity of the sintered product at 25° C.; 
     FIG. 4 is a diagram showing the relationship between the Be quantity and the coefficient of thermal expansion of the sintered product from 25° C. to 300° C.; 
     FIG. 5 is a sectional view of a conventional silicon semiconductor device; and 
     FIG. 6 is a sectional view of a silicon semiconductor device using the insulating substrate of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described more definitely with reference to embodiments thereof. 
     Example 1 
     Aluminum nitride powder having a purity of 99% and an average particle size of 2 μm was mixed with 0.03 to 30% by weight of beryllium oxide powder having an average particle size of 3 μm in terms of beryllium atom on the basis of aluminum nitride powder, and further admixed with 2 cc of an aqueous 3% polyvinyl alcohol solution per 40 g of the resulting powdery mixtures. Then, the mixtures were molded under the pressure of 1,000 kg/cm 2  at room temperature, and the resulting moldings were sintered in a furnace under a vacuum of 1×10 -3  to 1×10 -5  Torr. The moldings were heated by elevating a heating temperature from room temperature to 1,800° C. over about one hour, and kept at 1,800° C. for half an hour, and then left for cooling. The relations between the characteristics of the resulting sintered aluminum nitride products and beryllium content thereof are shown in FIGS. 1 to 4. 
     It is seen from FIGS. 1 to 4 that, when the beryllium content of sintered aluminum nitride is 0.05% by weight or higher, compact sintered products having a high thermal conductivity, i.e. 0.3 cal/cm.sec.°C. or higher at room temperature, an electrical resistivity of 10 12  Ωcm or higher at room temperature, a low coefficient of thermal expansion of 6×10 -6  /°C. below 10.5% by weight, and a density of more than 90% of theoretical density of sintered products (which will be hereinafter referred to as &#34;relative density&#34;) can be obtained. 
     Example 2 
     Moldings of mixtures of aluminum nitride powder having a purity of 99% with 3% by weight of beryllium oxide in terms of beryllium atom on the basis of aluminum nitride powder were prepared and sintered in the vacuum in the same manner as in Example 1, while changing the sintering conditions. Table 1 shows relations between the conditions for sintering the moldings and the relative density of sintered products. The sintered products having a relative density of 90% or higher had a thermal conductivity of 0.4 cal/cm.sec.°C. or higher at room temperature, an electrical resistivity of 10 12  Ωcm or higher at room temperature, and an average coefficient of thermal expansion of 4.2 to 4.3×10 -6  /°C. between room temperature and 300° C. 
     
                       TABLE 1                                                     
______________________________________                                    
Sintering Sintering             Relative                                  
temp.     time         Pressure density                                   
(°C.)                                                              
          (h)          (Kg/cm.sup.2)                                      
                                (%)                                       
______________________________________                                    
1550      1.0          --       78                                        
1600      1.0          --       93                                        
1600      0.5          --       90                                        
1800      0.5          --       100                                       
2000      0.5          --       96                                        
2000      0.2          --       98                                        
2100      0.5          --       85                                        
2100      0.2          --       89                                        
1550      1.0          300      85                                        
1600      0.5          300      98                                        
1600      0.5          100      95                                        
1600      0.5           50      87                                        
1800      0.2          300      98                                        
1800      0.5          300      100                                       
2000      0.5          300      98                                        
2000      0.2          300      98                                        
2000      0.2          100      97                                        
2100      0.5          100      83                                        
2100      0.2          100      88                                        
2100      0.2          300      86                                        
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     Example 3 
     Moldings of mixtures of aluminum nitride powder having a purity of 99% with 3% by weight of beryllium oxide in terms of beryllium atom on the basis of aluminum nitride powder were prepared and sintered in the same manner as in Example 1, while using various sintering atmospheres of argon gas, helium gas, nitrogen gas and hydrogen gas. The resulting sintered products had the same properties as those of the sintered products with 1% beryllium content in Example 1. 
     Example 4 
     Aluminum nitride powder having a purity of 99% was admixed with 0.03 to 10% by weight of one of metallic beryllium, beryllium nitrate, beryllium sulfate, beryllium carbonae, beryllium phosphate, beryllium hydroxide, beryllium chloride, beryllium fluoride, beryllium acetylacetonate, beryllium oxalate, beryllium carbide, beryllium boride, beryllium silicide, and beryllium nitride, in terms of atom, on the basis of aluminum nitride in the manner as in Example 1. When the beryllium content of the sintered products was 0.05% by weight or higher, compact sintered products having a relative density of 90% or higher were obtained, and had the similar properties as those of the sintered products obtained in Example 1. 
     Example 5 
     Aluminum nitride powders having a purity of 99% and different average particle sizes were admixed with 3% by weight of beryllium oxide powder in terms of beryllium atom on the basis of aluminum nitride, and sintered in the same manner as in Example 1. When the average particle size of the aluminum nitride powders was not more than 20 μm, sintered products having a relative density of 90% or higher were obtained. The compact sintered products having a relative density of 90% or higher had a thermal conductivity of 0.4 cal/cm.sec.°C. or higher at room temperature, an electrical resistivity of 10 12  Ω.cm or higher at room temperature, and an average coefficient of thermal expansion of 4.2-4.3×10 -6  /°C. between room temperature and 300° C. 
     
                       TABLE 2                                                     
______________________________________                                    
Average        Relative                                                   
particle size  density                                                    
(μm)        (%)                                                        
______________________________________                                    
0.5            99                                                         
1              99                                                         
2              99                                                         
6              97                                                         
10             95                                                         
16             92                                                         
20             90                                                         
24             81                                                         
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     Example 6 
     Aluminum nitride powder having a purity of 99% and an average particle size of 2 μm was admixed with 0.03 to 10% by weight of lithium carbonate having an average particle size of 5 μm in terms of lithium atom on the basis of aluminum nitride, and further admixed with 2 cc of an aqueous 3% polyvinyl alcohol solution per 40 g of the resulting powdery mixtures. Then, the resulting mixtures were molded under a pressure of 1,000 kg/cm 2  at room temperature, and the resulting moldings were sintered in a furnace under a vacuum of 1×10 -3  to 1×10 -5  Torr. The moldings were heated by elevating a heating temperature from room temperature to 1,800° C. over about one hour, and kept at 1,800° C. for half an hour, and then left for cooling. Table 3 shows the characteristics of the resulting sintered products. When the lithium content was 0.05 to 5% by weight in the resulting sintered product, the sintered products had a thermal conductivity of 0.2 cal/cm.sec.°C. or higher at room temperature, an electrical resistivity of 10 12  Ωcm or higher at room temperature, and a coefficient of thermal expansion of 5×10 -6  /°C. or less at room temperature, and were compact ones having a relative density of 90% or higher. 
     
                       TABLE 3                                                     
______________________________________                                    
                 Thermal            Coefficient                           
Lithium Relative conductivity                                             
                            Electrical                                    
                                    of thermal                            
content density  (cal/      resistivity                                   
                                    expansion                             
(wt. %) (%)      cm · s · °C.)                   
                            (Ω · cm)                       
                                    (× 10.sup.-6 /°C.)       
______________________________________                                    
0.03    73       0.04       5 × 10.sup.11                           
                                    4.2                                   
0.05    90       0.20       2 × 10.sup.14                           
                                    4.2                                   
0.1     95       0.33       5 × 10.sup.14                           
                                    4.2                                   
1       99       0.52       8 × 10.sup.14                           
                                    4.3                                   
3       99       0.47       8 × 10.sup.14                           
                                    4.3                                   
5       96       0.39       3 × 10.sup.14                           
                                    4.6                                   
10      87       0.08       7 × 10.sup.13                           
                                    5.7                                   
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     Example 7 
     Aluminum nitride powder having a purity of 99% was admixed with 1% by weight of lithium carbonate in terms of lithium atom on the basis of aluminum nitride and sintered in the same manner as in Example 6, while changing sintering conditions. Table 4 shows relations between the sintering conditions and the relative density of the resulting sintered products. Compact sintered products having a relative density of 90% or higher all had a thermal conductivity of 0.2 cal/cm.sec.°C. or higher at room temperature, an electrical resistivity of 10 12  Ωcm or higher at room temperature, and an average coefficient of thermal expansion of 4.2-4.3×10 -6  /°C. between room temperature and 300° C. 
     
                       TABLE 4                                                     
______________________________________                                    
Temp.    Time       Load     Relative density                             
(°C.)                                                              
         (hr)       (Kg/cm.sup.2)                                         
                             (%)                                          
______________________________________                                    
1550     1.0        --       69                                           
1600     1.0        --       93                                           
1600     0.5        --       91                                           
1800     0.5        --       99                                           
2000     0.5        --       98                                           
2000     0.2        --       95                                           
2100     0.5        --       82                                           
2100     0.2        --       88                                           
1550     1.0        300      85                                           
1600     0.5        300      96                                           
1600     0.5        100      95                                           
1600     0.5         50      86                                           
1800     0.2        300      98                                           
1800     0.5        300      99                                           
2000     0.5        300      99                                           
2000     0.2        300      99                                           
2000     0.2        100      96                                           
2100     0.5        100      89                                           
2100     0.2        100      85                                           
2100     0.2        300      89                                           
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     Example 8 
     Aluminum nitride powder having a purity of 99% was admixed with 1% by weight of lithium carbonate in terms of lithium atom on the basis of aluminum nitride and sintered in the same manner as in Example 6, while using a sintering atmosphere of argon gas, halium gas, nitrogen gas or hydrogen gas. The resulting sintered products all had the same characteristics as those of the sintered products having a lithium of 1% by weight as shown in Example 6. 
     Example 9 
     Aluminum nitride powders having a purity of 99% were admixed with 0.03-10% by weight of one of lithium oxide, lithium nitride, lithium hydride, lithium hydroxide, lithium chloride, lithium fluoride, lithium nitrate, and lithium sulfate in terms of lithium atom on the basis of aluminum nitride and sintered in the same manner as in Example 6. The resulting sintered products all had substantially same characteristics as those of the sintered products having corresponding lithium contents as shown in Example 6. 
     Example 10 
     Aluminum nitride powders having a purity of 99% and different average particle sizes were admixed with 1% by weight of lithium carbonate in terms of lithium atom on the basis of the aluminum nitride, and sintered in the same manner as in Example 6. Table 5 shows the relation between the average particle size of the aluminum nitride powder and the relative density of the sintered products. When the average particle size of aluminum nitride was not more than 20 μm, compact sintered products having a relative density of 90% or higher were obtained. The compact sintered products having a relative density of 90% or higher had a thermal conductivity of 0.2 cal/cm.sec.°C. or higher at room temperature, an electrical resistivity of 10 12  Ωcm or higher at room temperature and an average coefficient of thermal expansion of 4.2-4.3×10 -6  /°C. between room temperature and 300° C. 
     
                       TABLE 5                                                     
______________________________________                                    
Average        Relative                                                   
particle size  density                                                    
(μm)        (%)                                                        
______________________________________                                    
0.5            99                                                         
1              99                                                         
2              99                                                         
6              97                                                         
10             95                                                         
16             91                                                         
20             90                                                         
24             77                                                         
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     Example 11 
     Aluminum nitride powder having a purity of 99% was admixed with 1% by weight each of beryllium oxide powder as used in Example 1 and lithium carbonate as used in Example 6 in terms of atoms on the basis of aluminum nitride, and sintered in the same manner as in Example 1. A compact sintered product having a relative density of 99% or higher, a thermal conductivity of 0.55 cal/cm.sec.°C. at room temperature, an electrical resistivity of 8×10 14  Ωcm at room temperature, and a coefficient of thermal expansion of 4.3×10 -6  /°C. at room temperature was obtained. 
     Furthermore, sintered products were prepared in the same manner as above while using 0.05 to 10% by weight of beryllium oxide and 0.05-5% by weight of lithium carbonate while making the sum total of the beryllium oxide and the lithium oxide not more than 5% by weight in terms of beryllium and lithium atoms, when the amount of lithium carbonate took the major proportion and making the sum total of the beryllium oxide and the lithium carbonate not more than 10% by weight in terms of beryllium and lithium atoms, when the amount of beryllium oxide took the major proportion. The resulting sintered products had a relative density of 90% or higher, a thermal conductivity of 0.2 cal/cm.sec.°C. or higher at room temperature, an electrical resistivity of 10 12  Ω.cm or higher at room temperature, and a coefficient of thermal expansion of 5×10 -6  /°C. or lower at room temperature. 
     Application Example 
     As a definite example of the electrically insulating substrate in accordance with the present invention, a semi-conductor power module was produced using the sintered aluminum nitride of 1.0 wt.% beryllium content obtained in Example 1, and that of 1.0 wt.% lithium content obtained in Example 6. 
     FIG. 5 shows the section of a semiconductor power module assembly of the prior art. An organic insulating material 15 is interposed for insulation between a Cu conductor 14 and a Cu heat sink 16 while an aluminum substrate 17 is interposed for insulation between the heat sink 16 and a Cu metal support 18. A spacer 13 is interposed in order to mitigate strain due to the difference of thermal expansion coefficients between a silicon element 11 and the Cu heat sink 16. Reference numeral 12 denotes an aluminum lead wire while reference numeral 19 denotes a solder. 
     FIG. 6 is a sectional view of the module assembly using the insulating substrate in accordance with the present invention. The substrate 20 of the present invention has an extremely simple construction in which the substrate is directly brazed to the silicon element 11 via the solder 19, with or without metallizing the contact surface of the substrate. 
     According to the construction shown in FIG. 6, it is possible to replace the spacer 13, the organic insulating material 15, the Cu heat sink 16, the alumina substrate 17 and the metal support 18, each shown in FIG. 5, by the single substrate 20 shown in FIG. 6. 
     The abovementioned semiconductor device was held at -60° C. for 30 minutes, then at room temperature for 5 minutes and thereafter heated to 125° C. and held at that temperature for 30 minutes. When this heat cycle was applied 20 times to the semiconductor device of the prior art shown in FIG. 5, crack occurred on the alumina substrate and soldered positions peeled off. When the same heat cycle was applied 150 times to the semiconductor device of the present invention shown in FIG. 6, no abnormality was observed. 
     Comparative Example 
     Aluminum nitride powder having a purity of 99% and an average particle size of 2 μm was admixed with 10% by weight of yttrium oxide in terms of yttrium atom on the basis of aluminum nitride, and molded under pressure in the same manner as in Example 1. The resulting molding was sintered in a vacuum furnace at 1,800° C. under 1×10 -3  Torr for half an hour. A compact sintered product having a relative density of 99% was obtained and had a thermal conductivity of 0.07 cal/cm.sec.°C. at room temperature, an electrical resistivity of 10 11  Ω.cm at room temperature, and an average coefficient of thermal expansion of 5.0×10 -6  /°C. between room temperature and 300° C. The thermal conductivity was considerably lower than that of the present sintered products. 
     As is evident from the foregoing Examples, the present sintered aluminum nitride is compact and has a high thermal conductivity, a high electrical resistivity and a low coefficient of thermal expansion. Thus, the present sintered product is useful not only as an electrically insulating substrate material, but also as a heat-resistant, antioxidant or chemical-resistant material and also as a material having a high mechanical strength at a high temperature. Particularly the present sintered product has equivalent functions and effects to those of the sintered silicon carbide, and also has such additional merits as a firing temperature by 300°-500° C. lower than that of the silicon carbide and a higher electrical resistivity, and consequent economical saving in power and material consumption.