Method for preparing aluminum nitride ceramics having superior thermal conductivities
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for preparing aluminum nitride (AlN) ceramics, especially for preparing AlN ceramics having high thermal conductivities. 
     2. Description of the Prior Art 
     With the advances of high integration density and high output power of the recent semiconductor integrated circuits, the value of power consumption per unit area of a semiconductor chip is increasing. Unless the generated heat is sufficiently radiated, the integrated circuit produces a maloperation or eventully breaks down. The insulating substrates to mount integrated circuit chips, which have larger thermal conductivities have been demanded. 
     Heretofore, alumina ceramics has been widely used as the materials for the insulating substrates to mount semiconductor chips. However, as the power dissipated in a semiconductor integrated circuit increases, the thermal conductivity becomes to be insufficient. The insulating substrate having more improved thermal conductivity is still demanded. For the material of such insulating substrate, a large electrical resistance, a thermal expansion coefficient close to that of silicon and a high mechanical strength are required in addition to the high thermal conductivity. 
     As the material satisfying these requirements, the use of aluminum nitride ceramics has recently proposed. The aluminum nitride ceramics has a thermal expansion coefficient of about 4.3×10 -6  /°C. (average from room temperature to 400° C.) which is smaller than the thermal expansion coefficient, about 7×10 -6  /°C., of the alumina ceramics and which is close to the thermal expansion coefficient, 3.5 to 4.0×10 -6  /°C., of silicon. The mechanical strength of the aluminum nitride ceramics is about 50 Kg/mm 2  of a flexural strength and is strong compared to the alumina ceramics having that of 20 to 30 Kg/mm 2 . 
     The aluminum nitride ceramics is prepared by sintering aluminum nitride powder. However, since the aluminum nitride is covalent bond material, it is difficult to obtain densified ceramics. To sinter the aluminum nitride powder, appropriate sintering additives are used. Japanese published unexamined patent applications (TOKKAISHO) Nos. 54-100410, 50-23411 and 58-55377 use oxides such as calcium oxide (CaO), barium oxide (BaO) and strontium oxide as the sintering additives. However, those oxides do not cause reducing reaction. According to the findings of the present inventors, oxygen contained in the aluminum nitride powder deteriorates the thermal conductivity of the resultant aluminum nitride ceramics, and the use of oxides cannot remove the oxygen content to restrict improving the thermal conductivity. For example, when calcium oxide (CaO) is added to aluminum nitride powder with a content of 0.6 wt % and the mixture is sintered under a sintering temperature of about 1800° C. for two hours, the obtained aluminum nitride ceramics has a thermal conductivity between 40 to 50 W/mK. The best value of the thermal conductivity reported in the above-mentioned Japanese published unexamined patent applications were at most 70 W/mK. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method for preparing aluminum nitride ceramics having improved thermal conductivities. 
     The method for preparing aluminum nitride ceramics according to the present invention comprises steps of mixing aluminum nitride powder with 0.02 to 10 wt % of at least one acetylide, and sintering the mixture in non-oxidizing atomosphere. Such acetylide is preferably selected from acetylides of calcium (Ca), strontium (Sr), barium (Ba), sodium (Na) potassium (K), rubidium (Rb), cesium (Cs), copper (Cu), silver (Ag), magnesium (Mg), cadmium (Cd), mercury (Hg), zinc (Zn), aluminum (Al) and cerium (Ce). 
     According to the present invention, the sintering additive of a cetylide reacts with oxygen contained in the aluminum nitride powder during the sintering process to remove oxygen from obtained aluminum nitride ceramics. As a result, the content of oxygen in the aluminum nitride ceramics which deteriorates the thermal conductivity becomes very small. The prepared aluminum nitride ceramics can have a thermal conductivity larger than 70 W/mK. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a graph showing a relationship between a content of CaC 2  as a sintering additive and a thermal conductivity of prepared AlN ceramics; 
     FIG. 2 is a graph showing a relationship between a contents of SrC 2  as a sintering additive and a thermal conductivity of prepared AlN ceramics; and 
     FIG. 3 is a graph showing a relationship between a content of BaC 2  as a sintering additive and a thermal conductivity of prepared AlN ceramics. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The raw aluminum nitride of high purity is used. Although the purity higher than 98% is preferable, it is possible to use aluminum nitride having a purity of 95 to 98%. It is better to use powder of the raw aluminum nitride having the particle size smaller than 10 μm, preferably smaller than 2 μm. The acetylides to be used in the present invention as the sintering additive are of Ca, Sr, Ba, Na, K, Rb, Cs, Cu, Ag, Mg, Cd, Hg, Zn, Al and Ce. The acetylides of Ca, Sr and Ba are preferable. By the use of at least one of the acetylides with content between 0.02 and 10 wt %, the thermal conductivity of the prepared aluminum nitride ceramics is greatly improved. The acetylides actively react with oxygen contained in the raw aluminum nitride in a high temperature atomosphere, as a result of improving the thermal conductivity by reducing the content of oxygen in the prepared aluminum nitride ceramics. Here, the acetylides actively react with moisture, too. Therefore, the acetylides should be carefully mixed with the raw aluminum nitride. The solvent to be used to mix them should be non-water solvent. The atomosphere during the mixing process should be non-oxidizing such as nitrogen gas. The temperature should keep low. 
     The sintering is applied to the mixture in non-oxidizing atomosphere with a high temperature. If the sintering is done in an oxidizing atomosphere, the aluminum nitride is oxidized to lower both the density of prepared ceramics and the thermal conductivity. As the non-oxidizing atomosphere, nitrogen gas, helium gas, argon gas, carbon mono-oxide gas, hydrogen gas and vacuous atomosphere may be used. Among them, the use of nitrogen gas, argon gas, helium gas or vacuous atomosphere is convenient. The sintering temperature is between 1,500° and 2,000° C., and preferably between 1,600° and 1,900° C. The normal sintering (pressureless sintering) technique at normal pressure may be employed. The pressure sintering technique may also be employed. As the pressure sintering technique, two methods may be employed, the hot press sintering and HIP (Hot Isostatic Press) sintering. Especially, a very high thermal conductivity is obtained by the hot press sintering. 
     Several preferred embodiments will next be explained. 
     (Embodiment 1) 
     Several kinds of acetylides are mixed with aluminum nitride powder of particles having a mean diameter of two μm with a content of two wt %. The obtained mixtures are then formed with a pressure of 2,000 Kg/cm 2  at room temperature. The formed bodies are sintered under a nitrogen atomosphere in a sintering oven at 1,800° C. for two hours to obtain aluminum nitride ceramics. The thermal conductivities at room temperature of thus obtained aluminum nitride ceramics are shown in the table 1. The thermal conductivities equal to or higher than 80 w/mk are obtained at room temperature by adding several kinds of acetylides. 
     
                       TABLE 1                                                     
______________________________________                                    
                              thermal                                     
Sample                        conductivity                                
No.   Acetylides              (w/mk)                                      
______________________________________                                    
 1.   CaC.sub.2 (2 wt %)      140                                         
2.    SrC.sub.2 (2 wt %)      130                                         
3.    BaC.sub.2 (2 wt %)      120                                         
4.    Na.sub.2 C.sub.2 (2 wt %)                                           
                               90                                         
5.    K.sub.2 C.sub.2 (2 wt %)                                            
                               80                                         
6.    Rb.sub.2 C.sub.2 (2 wt %)                                           
                               90                                         
7.    CsC.sub.2 (2 wt %)       90                                         
8.    CuC.sub.2 (2 wt %)      110                                         
9.    Ag.sub.2 C.sub.2 (2 wt %)                                           
                              110                                         
10.   MgC.sub.2 (2 wt %)      100                                         
11.   CdC.sub.2 (2 wt %)       80                                         
12.   HgC.sub.2 (2 wt %)       80                                         
13.   ZnC.sub.2 (2 wt %)      110                                         
14.   Al.sub.2 C.sub.6 (2 wt %)                                           
                              110                                         
15.   Ce.sub.2 C.sub.6 (2 wt %)                                           
                               90                                         
16.   CaC.sub.2 (1 wt %) + SrC.sub.2 (1 wt %)                             
                              140                                         
17.   CaC.sub.2 (1 wt %) + BaC.sub.2 (1 wt %)                             
                              130                                         
18.   SrC.sub.2 (1 wt %) + BaC.sub.2 (1 wt %)                             
                              120                                         
19.   CaC.sub.2 (1.5 wt %) + Rb.sub.2 C.sub.2 (0.5 wt %)                  
                              100                                         
20.   CaC.sub.2 (0.5 wt %) + CuC.sub.2 (1.5 wt %)                         
                              100                                         
21.   CaC.sub.2 (1 wt %) + SrC.sub.2 (0.5 wt %) +                         
                              120                                         
      BaC.sub.2 (0.5 wt %)                                                
22.   CaC.sub.2 (1 wt %) + ZnC.sub.2 (0.5 wt %) +                         
                              110                                         
      MgC.sub.2 (0.5 wt %)                                                
23    No additive              30                                         
24.   CaO (0.6 wt %)           55                                         
______________________________________                                    
 Note:                                                                    
 Samples 23 and 24 are data to be compared.                               
 
    
     (Embodiment 2) 
     The acetylides of table 1 are mixed with aluminum nitride powder of particles having a mean diameter of two μm with different contents. The mixtures are formed with a pressure of 2,000 Kg/cm 2  at room temperature. Thus formed bodies are sintered in an oven of nitrogen gas atomosphere under conditions shown in table 2. The thermal conductivities of thus obtained aluminum nitride ceramics at room temperature are also shown in table 2. As apparent from table 2, the aluminum nitride ceramics having thermal conductivities equal to or higher than 60 W/mK at room temperature are obtained by adding acetylides. 
     
                                           TABLE 2                                 
__________________________________________________________________________
Acetylide    Sintering                                                    
                    Sintering                                             
                         Pressure at                                      
                                   Relative                               
                                        Thermal                           
Sample  Content                                                           
             Temperature                                                  
                    Time Hot Press Density                                
                                        Conductivity                      
No. Kind                                                                  
        (wt %)                                                            
             (°C.)                                                 
                    (h)  Sintering (kg/cm.sup.2)                          
                                   (%)  (w/mk)                            
__________________________________________________________________________
25  CaC.sub.2                                                             
        0.02 1900   4    200       98   70                                
26  SrC.sub.2                                                             
        0.01 1900   4    100       98   80                                
27  BaC.sub.2                                                             
        0.5  1900   1     0        97   60                                
28  CaC.sub.2                                                             
        1.0  1800   2     0        98   130                               
29  CaC.sub.2                                                             
        1.0  1800   2    200       99   160                               
30  ZnC.sub.2                                                             
        1.0  2000   2    100       98   90                                
31  Na.sub.2 C.sub.2                                                      
        2.0  2000   2     0        96   80                                
32  K.sub.2 C.sub.2                                                       
        4.0  2000   1    200       98   100                               
33  CsC.sub.2                                                             
        7.0  1900   1    400       98   90                                
34  CaC.sub.2                                                             
        10.0 1900   1     0        96   70                                
35  CaC.sub.2                                                             
        10.0 1800   2    100       98   80                                
36  CaC.sub.2                                                             
        2.0  1800   2     0        97   70                                
    BaC.sub.2                                                             
        2.0                                                               
37  CaC.sub.2                                                             
        3.0  1800   2     0        97   60                                
    Na.sub.2 C.sub.2                                                      
        4.0                                                               
38  CaC.sub.2                                                             
        2.0                                                               
    SrC.sub.2                                                             
        2.0  1800   2    100       98   70                                
    BaC.sub.2                                                             
        2.0                                                               
39  CaC.sub.2                                                             
        4.0                                                               
    SrC.sub.2                                                             
        4.0  1800   2    200       98   60                                
    BaC.sub.2                                                             
        2.0                                                               
__________________________________________________________________________
 
    
     (Embodiment 3) 
     Calcium acetylide (CaC 2 ) is mixed with aluminum nitride powder of particles having a mean diameter of two μm with several contents from 0.01 to 15 wt %. The mixtures are then formed at room temperature with a pressure of 2,000 Kg/cm 2 . Thus formed bodies are sintered in an oven of a nitrogen gas atomosphere at 1,800° C. for two hours to obtain aluminum nitride ceramics. The thermal conductivities of thus obtained aluminum nitride ceramics are shown in Table 3 and FIG. 1. By adding carbon acetylide with content between 0.02 and 10 wt %, the thermal conductivity becomes especially high and is equal to or higher than 70 W/mK. If the carbon acetylide is added with a content between 0.03 and 7 wt %, a thermal conductivity equal to or higher than 100 W/mK is obtained. 
     
                                           TABLE 3                                 
__________________________________________________________________________
    Contents of                                                           
             Sintering                                                    
                  Sintering                                               
                       Pressure at                                        
                             Relative                                     
                                  Thermal                                 
Sample                                                                    
    CaC.sub.2 in Mixture                                                  
             Temp.                                                        
                  Time Sintering                                          
                             Density                                      
                                  Conductivity                            
No. (wt %)   (°C.)                                                 
                  (Hours)                                                 
                       (Kg/cm.sup.2)                                      
                             (%)  (W/mK)                                  
__________________________________________________________________________
40  0.01     1,800                                                        
                  2    0     95   50                                      
41  0.02     1,800                                                        
                  2    0     97   70                                      
42  0.5      1,800                                                        
                  2    0     99   120                                     
43  1.0      1,800                                                        
                  2    0     98   130                                     
44  5.0      1,800                                                        
                  2    0     99   120                                     
45  10.0     1,800                                                        
                  2    0     97   70                                      
46  15.0     1,800                                                        
                  2    0     96   50                                      
__________________________________________________________________________
 
    
     (Embodiment 4) 
     Strontium acetylide (SrC 2 ) is mixed with aluminum nitride powder with contents from 0.01 to 15 wt %, similarly to the embodiment 3. Under the same conditions as the embodiment 3, the mixtures are formed and then the formed bodies are sintered to obtain aluminum nitride ceramics. The thermal conductivities at room temperature of obtained aluminum nitride ceramics are shown in Table 4 and FIG. 2. By adding strontium acetylide with contents between 0.02 and 10 wt %, the thermal conductivity of aluminum nitride ceramics becomes especially high and is equal to or higher than 60 W/mK. If the strontium acetylide is added with a content between 0.03 and 5 wt %, a thermal conductivity equal to or higher than 100 W/mK is obtained. 
     
                                           TABLE 4                                 
__________________________________________________________________________
    Contents of                                                           
            Sintering                                                     
                 Sintering                                                
                      Pressure at                                         
                            Relative                                      
                                 Thermal                                  
Sample                                                                    
    SrC.sub.2 in Mixture                                                  
            Temp.                                                         
                 Time Sintering                                           
                            Density                                       
                                 Conductivity                             
No. (wt %)  (°C.)                                                  
                 (Hours)                                                  
                      (Kg/cm.sup.2)                                       
                            (%)  (W/mK)                                   
__________________________________________________________________________
47  0.01    1,800                                                         
                 2    0     95   50                                       
48  0.02    1,800                                                         
                 2    0     97   60                                       
49  0.5     1,800                                                         
                 2    0     99   110                                      
50  1.0     1,800                                                         
                 2    0     98   130                                      
51  5.0     1,800                                                         
                 2    0     99   100                                      
52  10.0    1,800                                                         
                 2    0     97   70                                       
53  15.0    1,800                                                         
                 2    0     96   50                                       
__________________________________________________________________________
 
    
     (Embodiment 5) 
     Barium acetylide (BaC 2 ) is mixed with aluminum nitride powder with contents from 0.01 to 15 wt %, similarly to the embodiment 3. Under the same condition as the embodiment 3, the mixtures are formed and then formed bodies are sintered to obtain aluminum nitride ceramics. The thermal conductivities at room temperature of obtained aluminum nitride ceramics are shown in Table 5 and FIG. 3. By adding BaC 2  with content between 0.02 and 10 wt %, the thermal conductivity of aluminum nitride ceramics become especially high and is equal to or higher than 60 W/mK. If the barium acetylide is added with a content between 0.05 and 4 wt %, a thermal conductivity equal to or higher than 100 W/mK is obtained. 
     
                                           TABLE 5                                 
__________________________________________________________________________
    Contents of                                                           
             Sintering                                                    
                  Sintering                                               
                       Pressure at                                        
                             Relative                                     
                                  Thermal                                 
Sample                                                                    
    BaC.sub.2 in Mixture                                                  
             Temp.                                                        
                  Time Sintering                                          
                             Density                                      
                                  Conductivity                            
No. (wt %)   (°C.)                                                 
                  (Hours)                                                 
                       (Kg/cm.sup.2)                                      
                             (%)  (W/mK)                                  
__________________________________________________________________________
54  0.01     1,800                                                        
                  2    0     95   50                                      
55  0.02     1,800                                                        
                  2    0     97   60                                      
56  0.5      1,800                                                        
                  2    0     99   100                                     
57  2.0      1,800                                                        
                  2    0     98   120                                     
58  5.0      1,800                                                        
                  2    0     99   90                                      
59  10.0     1,800                                                        
                  2    0     97   70                                      
60  15.0     1,800                                                        
                  2    0     96   50                                      
__________________________________________________________________________
 
    
     (Embodiment 6) 
     The aluminum nitride ceramics obtained in accordance with the embodiment 3 are further subjected to the HIP sintering at the sintering temperature of 1,700° C., with a pressure of 1,000 Kg/cm 2  for one hour. As a result, the terminal conductivities at room temperature of the aluminum nitride ceramics are improved to 150 W/mK. 
     (Embodiment 7) 
     Calcium acetylide (CaC 2 ) is mixed with aluminum nitride powder having a purity of 98% and a mean particle diameter of 1 μm with a content of 1 wt % by using a solvent of alcohol. The mixture is dried under a dried nitrogen atomospher with high temperature. The dried mixture is formed at room temperature with a pressure of 2,000 Kg/cm 2 . The formed body is set in a hot press form made of carbon to sinter the body for two hours at a sintering temperature of 1,800° C., with a pressure of 200 Kg/cm 2  in a nitrogen atomosphere. 
     The obtained aluminum nitride ceramics has a relative density of 99%, a thermal conductivity of 160 W/mK, a thermal expansion coefficient of 4.3×10 -6  ° C., a resistivity larger than 10 13  ohm-cm and a flexural strength of 50 Kg/mm 2  at room temperature. It has a light transparency. The transparency is about 48% for light having wave-length from 4 to 6 μm and about 20% for light having wave-length from 0.2 to 6.5 μm. 
     As explained in the above in detail, the aluminum nitride ceramics prepared in accordance with the present invention has a high relative density and is superior in thermal conductivity, electrical insulating property, mechanical strength, and light transparency. Therefore, the aluminum nitride ceramics prepared in accordance with the present invention is usable as materials for crucibles, evaporation apparatus and temperature-resistant jigs, in addition to heat sink materials in semiconductor devices. Furthermore, by using the property of light transparency; the aluminum nitride ceramics prepared in accordance with the present invention is also applicable to windows of various apparatus.