Abstract:
A method of manufacturing a ceramic/metal composite structure includes the steps of: providing a ceramic substrate; forming a metal interface layer on the ceramic substrate; placing a copper sheet on the metal interface layer; heating the ceramic substrate, the metal interface layer and the copper sheet so that the metal interface layer forms strong bonds with the ceramic substrate and the copper sheet. A ceramic/metal composite structure is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a ceramic/metal composite structure and a method of manufacturing the same, and more particularly to a composite structure composed of an aluminum oxide layer and a copper layer, and a method of manufacturing the same. 
         [0003]    2. Related Art 
         [0004]    Heat is generated when an electronic component is operating with electrons flowing therethrough. The generation of heat increases the resistance and blocks the flows of the electrons so that the function of the electronic component is significantly influenced. When the technology of manufacturing the electronic component is greatly enhanced, a line width in the electronic component is getting smaller and smaller, and the line density in the electronic component is getting higher and higher. Thus, the heat generated by the electronic component is increased rapidly. Taking a central processing unit (CPU) of a computer as an example, the Pentium CPU only has to be equipped with the package with the heat dissipating capability of 16 W at its early stage. However, the heat generating power in the CPU produced in the year of 2004 has reached 84 W, and the heat generated in the CPU in the year of 2006 has reached 98 W. If the heat cannot be removed rapidly, the temperature of the CPU of the computer is rapidly increased so that the CPU of the computer can no longer operate. Thus, the heat dissipating ability of the substrate contacting with the CPU of the computer is a key factor for dominating whether the computer can operate normally or not. 
         [0005]    A typical power component, such as a solid relay, is similar to the CPU of the computer and generates a lot of heat. Thus, the power component also dissipates the heat rapidly through the substrate contacting therewith so that it can operate normally. 
         [0006]    Taking a light-emitting diode (LED) as another example, various color LEDs have been gradually developed, wherein the successful development of the white-light LED has attracted considerable attention. This is because the white-light LED can serve as a light source for an illumination lamp. The power consumption of a road lamp with LED light source is lower than that of a mercury lamp by 75% and is lower than that of a high pressure sodium lamp by 49%. So, the white-light LED advantageously has the low power consumption and can significantly save the energy. However, the white-light LED with the output power higher than 3 W has to be adopted in the application of the lamp used in the daily life and the applications such as the head light used in a vehicle. This white-light LED with the high output power also generates a lot of heat. However, the major barrier on the application of LED as the light source is that the LED cannot withstand the high temperature. Generally speaking, the temperature of the LED cannot exceed 90° C. If the temperature of the LED is higher than 90° C., the luminance thereof rapidly deteriorates. So, the rapid: heat dissipating ability of the heat dissipation substrate in contact with the LED has become a greatest challenge for determining whether the LED can become the illumination light source or not. This also specifies that the development of the heat dissipating substrate has played an important role on the applications of the LED as the light source. 
         [0007]    In order to satisfy the miniaturized requirement of the current 3 C electronic product, the substrate contacting with the CPU of the computer, or the power component or the LED has to satisfy the following fundamental requirements. 
         [0008]    First, the material must have a,high heat conductivity to dissipate the heat rapidly. 
         [0009]    Second, the material must have the high resistivity in order to prevent the high power electronic component from being short-circuited. 
         [0010]    Third, the substrate preferably has to be as thin as possible after the above-mentioned conditions are satisfied. 
         [0011]    Fourth, the substrate has to be used with the good reliability for a long time. This is because the high power electronic component, after being packaged, will encounter several tens of thousand times of on-off cycles, and the temperature of the substrate contacting with the high power electronic component is instantaneously increased and decreased therewith for several tens of thousand times. So, the reliability after the electronic component has been used for a long time is an extremely important requirement. This is absolutely associated with the bonding strength between the ceramic material and the metal material. 
         [0012]    At present, the heat dissipating mechanisms of the electronic components, which are frequently used, include heat dissipating fins and a heat pipe accompanied with fans in order to dissipate the heat generated by the high power electronic components. However, such heat dissipating structure has a larger thickness, and the applications in designing a miniaturized 3 C electronic product are thus hindered. 
         [0013]    In order to satisfy the requirements on heat dissipating and to keep the size small and the price low, all materials are searched and evaluated. Taking the copper as an example, the copper has a high coefficient of thermal conductivity, which can reach 380 W/mK. There are many choices, which can satisfy the second requirement of insulation, because most polymeric organic materials or ceramic materials can satisfy this requirement. In order to satisfy the requirement of heat dissipating, the ceramic material is the better choice under the consideration of the long-term reliability. Among the ceramic materials, aluminum oxide and aluminum nitride can provide the high thermal conductivity and the high insulating resistance. The aluminum oxide has a coefficient of thermal conductivity, which can reach 20 to 38 W/mK, while the aluminum nitride has a coefficient of thermal conductivity, which can reach 40 to 200 W/mK. The coefficient of thermal conductivity of the ceramic material has a wider range because the coefficient of thermal conductivity of the ceramic material is significantly influenced by the purity of the ceramic material and the type of the sintering additive used. Furthermore, the resistivity of each of the aluminum oxide and the aluminum nitride can be equal to or greater than 10 10  Ωm. Thus; the two ceramic materials have the excellent insulating properties. Furthermore, aluminum oxide and aluminum nitride have the low dielectric constant and the high dielectric strength, and are thus frequently used as the material for a substrate. 
         [0014]    However, the aluminum oxide is a solid, which has a high melting point (higher than 2000° C.) and has the coexisted covalent bonds and ionic bonds. The copper atoms are combined with metallic bonds, and the copper has the melting point, which is only 1083° C. So, it is a great challenge to bond the aluminum oxide and the copper together. According to the report of Beraud, C., Courbiere, M., Esnouf, C., Juve, D., Treheux, D., J. Mater. Sci., 24, 4545, 1989, there are two conventional methods for bonding the aluminum oxide with the copper. The first method is the solid state bonding method, and the second method is the liquid phase bonding method, as disclosed in U.S. Pat. No. 3,993,411. The treating temperatures of these two methods are higher than 1000° C. 
         [0015]    According to the research made by Seager et. al., (Seager, C. W., Kokini, K., Trumble, K., Krane, M. J. M., Scripta Materialia, 46, 395, 2002), the thick copper oxide is disadvantageous to the bonding between aluminum oxide and copper. After a long-term investigation, it is found that the substrate, formed by bonding an aluminum oxide sheet and a copper sheet together, cannot have the application value until the interface strength between the aluminum oxide sheet and the copper sheet reaches a very high level. This is because the aluminum oxide and the copper have different bonds, and the coefficient of thermal expansion of the copper (17×10 −6 K −1 ) is two times of the coefficient of thermal expansion of the aluminum oxide (8×10 −6 K −1 ). A formula derived by Selsing (Selsing, J., J. Am. Ceram. Soc., 44, 419, 1961) is listed in the following: 
         [0000]    
       
         
           
             σ 
             = 
             
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     α 
                     · 
                     Δ 
                   
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                    
                   T 
                 
                 
                   
                     
                       1 
                       + 
                       
                         v 
                         
                           Al 
                            
                           
                               
                           
                            
                           2 
                            
                           O 
                            
                           
                               
                           
                            
                           3 
                         
                       
                     
                     
                       2 
                        
                       
                         E 
                         
                           Al 
                            
                           
                               
                           
                            
                           2 
                            
                           O 
                            
                           
                               
                           
                            
                           3 
                         
                       
                     
                   
                   + 
                   
                     
                       1 
                       - 
                       
                         2 
                          
                         
                           v 
                           Cu 
                         
                       
                     
                     
                       2 
                        
                       
                         E 
                         Cu 
                       
                     
                   
                 
               
               . 
             
           
         
       
     
         [0000]    In the above-mentioned formula, Δα denotes a difference between coefficients of thermal expansion of the aluminum oxide and the copper, ΔT denotes a difference between the room temperature and the manufacturing temperature, ν denotes the Poisson&#39;s ratio and E denotes the elastic constant. The temperature, at which the aluminum oxide sheet and the copper sheet are joined together is over 1000° C., so the estimated thermal stress induced by the thermal expansion mismatch between aluminum oxide and copper after the high temperature joining process can reach several hundreds of MPa. This thermal stress is very large and significantly influences the bonding strength between the aluminum oxide sheet and the copper sheet. In addition, after the aluminum oxide sheet and the copper sheet are joined together, the composite substrate is packaged together with electronic components. Since the electronic component may be turned on and off for several tens of thousand times, de-bonding may be formed at the interface between the aluminum oxide sheet and the copper sheet if the bonding strength between the aluminum oxide sheet and the copper sheet is not high enough. Thus, the heat spreading ability is greatly reduced, which significantly influences the reliability after the high power electronic component is used for a long time. 
         [0016]    Thus, it is an important subject of the present invention to provide a ceramic/metal composite structure having a high bonding strength. 
       SUMMARY OF THE INVENTION 
       [0017]    It is therefore an object of the present invention to provide a ceramic/metal composite structure and a method of manufacturing the same in order to provide strong bonds between the ceramic and metal materials, and thus to provide the good heat dissipating and insulating properties for an electronic component under the considerations of the thinner dimension and the long-term reliability. 
         [0018]    The present invention achieves the above-identified objective by providing a ceramic/metal composite structure, which includes a ceramic substrate, a metal interface layer disposed on the ceramic substrate, and a copper sheet disposed on the metal interface layer. 
         [0019]    The present invention also provides a method of manufacturing a ceramic/metal composite structure. The method includes the following steps: providing a ceramic substrate; forming a metal interface layer on the ceramic substrate; placing a copper sheet on the metal interface layer; and heating the ceramic substrate, the metal interface layer and the copper sheet so that the metal interface layer forms strong bonds with the ceramic substrate and the copper sheet. 
         [0020]    The present invention further provides a ceramic/metal composite structure including a lower metal layer, a lower metal interface layer disposed on the lower metal layer, a ceramic substrate disposed on the lower metal interface layer, an upper metal interface layer disposed on the ceramic substrate, and an upper metal layer disposed on the upper metal interface layer. 
         [0021]    Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention. 
           [0023]      FIG. 1  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a first embodiment of the present invention. 
           [0024]      FIG. 2  is a flow chart showing a method of manufacturing the ceramic/metal composite structure according to the first embodiment of the present invention. 
           [0025]      FIG. 3  is a schematic top view showing a ceramic/metal composite structure according to a second embodiment of the present invention. 
           [0026]      FIG. 4  is a schematically cross-sectional view showing the ceramic/metal composite structure according to the second embodiment of the present invention. 
           [0027]      FIG. 5  is a schematic top view showing a ceramic/metal composite structure according to a third embodiment of the present invention. 
           [0028]      FIG. 6  is a schematically cross-sectional view taken along a line A-A of the ceramic/metal composite structure according to the third embodiment of the present invention. 
           [0029]      FIG. 7  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a fourth embodiment of the present invention. 
           [0030]      FIG. 8  is a schematic top view showing the ceramic/metal composite structure according to the fourth embodiment of the present invention. 
           [0031]      FIG. 9  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a fifth embodiment of the present invention. 
           [0032]      FIG. 10  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a sixth embodiment of the present invention. 
           [0033]      FIG. 11  is a schematic top view showing a ceramic/metal composite structure according to a seventh embodiment of the present invention. 
           [0034]      FIG. 12  shows a test result of an aluminum oxide substrate. 
           [0035]      FIG. 13  shows a test result of a composite structure composed of an aluminum oxide substrate and a single-stage pre-oxidized copper sheet. 
           [0036]      FIGS. 14 to 16  show test results of ceramic/metal composite structures according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same components. 
         [0038]      FIG. 1  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a first embodiment of the present invention. Referring to  FIG. 1 , the ceramic/metal composite structure of this embodiment includes a ceramic substrate  13 , a metal interface layer  14  and a copper sheet  15 . 
         [0039]    In this embodiment, the ceramic substrate  13  is composed of aluminum oxide or aluminum nitride. However, the present invention is not limited thereto. According to the following description, the ceramic substrate  13  may also be made of other materials. 
         [0040]    The metal interface layer  14  is disposed on the ceramic substrate  13 . The material of the metal interface layer  14  is selected from the group consisting of gold, beryllium, bismuth, cobalt, copper, iron, nickel, palladium, platinum, titanium, yttrium and alloys thereof. Alternatively, the material of the metal interface layer  14  is selected from the group consisting of nickel, a nickel alloy, copper and a copper alloy. The thickness of the metal interface layer  14  ranges from 0.1 microns to 10 microns, and preferably ranges from 1 micron to 5 microns. 
         [0041]    The copper sheet  15  is disposed on the metal interface layer  14 . The copper sheet  15  has one pattern or electric connection point (not shown) or multiple patterns or electric connection points (not shown). The metal interface layer  14  provides a wetting effect for the copper sheet  15  to the ceramic substrate  13  at a high temperature. In order to form the strong bonds, a coefficient of thermal expansion of the metal interface layer  14  ranges between coefficients of thermal expansion of the ceramic substrate  13  and the copper sheet  15 . Alternatively, the coefficient of thermal expansion of the metal interface layer  14  is greater than or equal to that of the ceramic substrate  13 , or is smaller than or equal to that of the copper sheet  15 . For example, the coefficient of thermal expansion of the metal interface layer  14  ranges from 8×10 −6 K −1  to 17×10 −6 K −1 . 
         [0042]      FIG. 2  is a flow chart showing a method of manufacturing the ceramic/metal composite structure according to the first embodiment of the present invention. As shown in  FIGS. 1 and 2 , the method of manufacturing the ceramic/metal composite structure of this embodiment includes the following steps. 
         [0043]    First, in step S 01 , a ceramic substrate  13  is provided. 
         [0044]    Then, in step S 02 , a metal interface layer  14  is formed on the ceramic substrate  13  by way of, for example, electroless plating, vapor deposition, coating, or the like. 
         [0045]    Next, in step S 03 , a copper sheet  15  is placed on the metal interface layer  14 . It is to be noted that a multi-stage pre-oxidizing process is performed on the copper sheet  15  at a temperature ranging from 50 to 700° C. before the copper sheet  15  is placed on the metal interface layer  14 . 
         [0046]    Then, in step S 04 , the ceramic substrate  13 , the metal interface layer  14  and the copper sheet  15  are heated to the temperature higher than 1000° C., for example, so that the metal interface layer  14  is simultaneously joined to the ceramic substrate  13  and the copper sheet  15  to form strong bonds. 
         [0047]    In order to enhance the interface strength and improve the reliability of the ceramic/metal composite structure, a multi-stage pre-oxidation process is developed so that oxide can be formed on a surface of the metal sheet. This oxide layer enables the metal sheet and the ceramic sheet to be successfully joined together at a temperature above the eutectic temperature and results in very strong bonds formed between the ceramic sheet and the metal sheet. Thus, the interface strength between the ceramic and metal sheets can be enhanced, and the reliability of the ceramic/metal composite structure after being used for many cycles of temperature rising and failing can be enhanced. In addition, the multi-stage pre-oxidation has the low temperature and the economic advantages, and the ceramic/metal composite structure formed after the multi-stage pre-oxidation has the excellent heat dissipating ability. 
         [0048]    The ceramic/metal composite structure of the present invention can provide the rapid heat dissipating ability and the good insulation ability. In order to enhance the interface strength and the long-term reliability of the composite structure, the proper multi-stage pre-oxidation may be performed. Embodiments will be illustrated to prove that the multi-stage oxidation can be performed at a lower temperature (the ratio of the absolute temperature of the highest temperature of the multi-stage pre-oxidation to the absolute temperature of the melting point of copper is smaller than or equal to 0.75). Because the pre-oxidation temperature is lowered, the cost can be lowered, and a heat spreading substrate with the high interface strength can be manufactured. A substrate composed of ceramic and metal sheets joined together may be regarded as the extension of this invention. 
         [0049]      FIG. 3  is a schematic top view showing a ceramic/metal composite structure according to a second embodiment of the present invention.  FIG. 4  is a schematically cross-sectional view showing the ceramic/metal composite structure according to the second embodiment of the present invention. Referring to  FIGS. 3 and 4 , the ceramic/metal composite structure of this embodiment includes a lower metal layer  11 , a lower metal interface layer  12 , a ceramic substrate  13 , an upper metal interface layer  14  and an upper metal layer  15 . 
         [0050]    In this embodiment, the lower metal layer  11  is a copper sheet. The thickness of the lower metal layer  11  ranges from 0.1 to 2 mm. The lower metal interface layer  12  is disposed on the lower metal layer  11 . 
         [0051]    The ceramic substrate  13  is disposed on the lower metal interface layer  12 . The thickness of the ceramic substrate  13  ranges from 0.1 to 3 mm. The material of the ceramic substrate  13  is selected from the group consisting of aluminum oxide, silicon oxide, aluminum nitride, silicon nitride, silicon carbide, glass and a glass-ceramic material. 
         [0052]    The upper metal interface layer  14  is disposed on the ceramic substrate  13 . The material of the metal interface layer  14  or  12  is selected from the group consisting of gold, beryllium, bismuth, cobalt, copper, iron, nickel, palladium, platinum, titanium, yttrium and alloys thereof. Alternatively, the material of the metal interface layer  14  or  12  is selected from the group consisting of nickel, a nickel alloy, copper and a copper alloy. The thickness of the metal interface layer  14  or  12  ranges from 0.1 to 10 microns, and preferably ranges from 1 micron to 5 microns. 
         [0053]    The upper metal layer  15  is disposed on the upper metal interface layer  14 . The upper metal layer  15  may be a copper sheet, on which various patterns or electric connection points are formed. The thickness of the upper metal layer  15  ranges from 0.1 to 2 mm. 
         [0054]      FIG. 5  is a schematic top view showing a ceramic/metal composite structure according to a third embodiment of the present invention.  FIG. 6  is a schematically cross-sectional view taken along a line A-A of the ceramic/metal composite structure according to the third embodiment of the present invention. As shown in  FIGS. 5 and 6 , this embodiment is similar to the second embodiment except that the upper metal layer  15  has a pattern, connection points, and an opening  16 . 
         [0055]      FIG. 7  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a fourth embodiment of the present invention.  FIG. 8  is a schematic top view showing the ceramic/metal composite structure according to the fourth embodiment of the present invention.  FIG. 7  is a cross-sectional view taken along a line B-B of  FIG. 8 . For the sake of clearness, a package material  22  is not shown in  FIG. 8 . Referring to  FIGS. 7 and 8 , the ceramic/metal composite structure of this embodiment further includes an electronic component  19  disposed on the opening  16  of the upper metal layer  15  and disposed over the ceramic substrate  13 . The electronic component  19  may be a central processing unit (CPU), a power component (e.g., a power transistor) or a light-emitting diode (LED). It is to be noted that the electronic component  19  is regarded as one portion of the ceramic/metal composite structure in the fourth embodiment of the present invention. 
         [0056]    In order to achieve the better thermal conducting effect, the electronic component  19  is placed over the ceramic substrate  13  through a thermal conductive adhesive or solder  20 . The thermal conductive adhesive  20  is formed by mixing an organic polymeric material with a metal or ceramic filler material. The metal or ceramic filler material is selected from the group consisting of silver particles, copper particles, aluminum particles, aluminum oxide particles, aluminum nitride particles, boron nitride particles or titanium boride particles. The solder is either a Sn alloy or an Ag alloy. The coefficient of thermal conductivity of the thermal conductive adhesive or solder  20  can typically reach above 3 W/mK. 
         [0057]    Alternatively, the ceramic substrate  13  in the opening  16  may also exist an upper metal interface layer (not shown). Therefore, the electronic component  19  is placed over the ceramic substrate  13  through the thermal conductive adhesive or solder  20  and the upper metal interface layer  14 . 
         [0058]    In order to control the electronic component  19  to work or not, the ceramic/metal composite structure of this embodiment further includes multiple wires  21  for connecting the electronic component  19  to the upper metal layer  15 . The package material  22  packages the electronic component  19  and the wires  21 . Most of the heat outputted from the electronic component  19  is conducted in a direction indicated by the bold arrow shown in  FIG. 7 . 
         [0059]      FIG. 9  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a fifth embodiment of the present invention.  FIG. 9  is similar to  FIG. 7 . As shown in  FIG. 9 , the upper metal layer  15  of this embodiment has an inclined surface  17  neighboring upon the opening  16 . When the electronic component  19  is the LED, the inclined surface  17  can reflect the secondary light ray of the electronic component  19  upwards to increase the light emitting efficiency of the electronic component  19 . 
         [0060]      FIG. 10  is a schematically cross-sectional view showing a ceramic/metal composite structure according to a sixth embodiment of the present invention.  FIG. 10  is similar to  FIG. 7 . As shown in  FIG. 10 , the upper metal layer  15  of this embodiment has a curved surface  18  neighboring upon the opening  16 . When the electronic component  19  is the LED, the curved surface  18  can also reflect the secondary light ray of the electronic component  19  upwards to increase the light emitting efficiency of the electronic component  19 . 
         [0061]      FIG. 11  is a schematic top view showing a ceramic/metal composite structure according to a seventh embodiment of the present invention. As shown in  FIG. 11 , many sets of electronic components may be arranged in an array and disposed over the ceramic substrate  13 . It is to be noted that the middle copper sheet  15 ′ is slightly different from its neighboring copper sheet  15  because the middle copper sheet  15 ′ is divided into four blocks to define the opening  16 ′ while the copper sheet  15  is divided into two blocks to define the opening  16 . 
         [0062]    The effects of the present invention will be described with reference to several examples. 
       FIRST EXAMPLE 6 
       [0063]    The aluminum oxide substrate, which has the purity of aluminum oxide of 96% and has the size of 32×23×0.5 mm, is tested by a universal testing machine (MTS-810, United States MTS Company) according to the 3-point bending method so that the load-displacement behavior of the substrate is obtained. Two lower supporting points to withstand the load are separated by 22.5 mm, and the load applying rate is 0.002 mm/second. The obtained load-displacement curve is shown in  FIG. 12 . As shown in  FIG. 12 , after the aluminum oxide substrate is applied with the load of 100 Newtons, an extremely small displacement is generated and the aluminum oxide substrate fails instantaneously. This phenomenon demonstrates a typical brittle failure. 
       SECOND EXAMPLE 
       [0064]    The aluminum oxide substrate, which is the same as that of the first example, is cleaned by acetone and then joined to two copper sheets, each of which has the size of 26×20×0.3 mm and is pre-oxidized at 1000° C. for 1 hour in an atmosphere with low oxygen partial pressure, to form a sandwiched structure in a temperature-calibrated oven at the high temperature of 1073° C. in nitrogen. The joined composite structure is applied with the 3-point bending test, which is the same as that of the first example, and the obtained load-displacement curve is shown in  FIG. 13 . The aluminum oxide/copper composite structure withstands the load of 150 Newtons, then the load rapidly decreases. The aluminum oxide substrate within the structure is broken after test, and a large portion of the interface between the aluminum oxide substrate and the copper sheet is separated, and only a small portion of the copper sheet is still joined to the aluminum oxide substrate. 
         [0065]    The above-mentioned example proves that the aluminum oxide/copper composite structure can withstand the external load higher than that withstood by the aluminum oxide substrate alone. However, as a larger load is applied, the aluminum oxide/copper composite structure cannot be used any more. As the copper sheet is pre-oxidized in multiple stages, the interface strength can be effectively enhanced. Illustrations will be made with reference to the following example. 
       THIRD EXAMPLE  
       [0066]    The aluminum oxide substrate the same as that used in the first example is coated with a nickel-phosphorus alloy layer (metal interface layer) by way of electroless plating. The thickness of this metal nickel-phosphorus alloy layer ranges from 2 to 4 microns. Then, the aluminum oxide substrate is joined to two copper sheets, each of which has a thickness of 0.3 mm and is pre-oxidized by using two stages of pre-oxidation at different temperatures ranging from 50° C. to 700° C. and in different atmospheres with different oxygen partial pressures. The high-temperature joining is carried out in a temperature-calibrated oven at 1058° C., in flowing nitrogen. During the high-temperature joining, the alloy thin layer joins the aluminum oxide sheet and the copper sheet together to form strong bonds therebetween. The temperature 1058° C. is lower than the eutectic point (1066° C.) between copper and copper oxide. So, it is proved that the alloy thin layer of this example can enhance the joining between the aluminum oxide and the copper. 
         [0067]    The joined aluminum oxide/copper composite structure is tested with the 3-point bending technique the same as that described in first example, and the obtained load-displacement curve is shown in  FIG. 14 . The aluminum oxide/copper composite structure withstands the load of 155 Newtons, and then the load only slightly decreases to 115 Newtons. Then, the composite structure can withstand the larger load up to 220 Newtons. Next, the load decreases, increases and then decreases. The test terminates when the displacement reaches 1.6 mm, and the composite structure is observed. At this time, the composite structure does not fail after the test, and the aluminum oxide substrate is not separated from the copper sheet, but has a small crack only in the aluminum oxide substrate. The copper sheet still tightly grasps the aluminum oxide sheet so that the completeness of the composite structure can still be kept. 
         [0068]    According to the load-displacement curve of  FIG. 14 , the aluminum oxide substrate forms a crack under the load of 155 Newtons. However, the aluminum oxide substrate is not separated from the copper sheet and thus can withstand the load continuously. At this time, the copper sheet has the plastic deformation. Then, when the load reaches 220 Newtons, the aluminum oxide substrate and the copper sheet have an extremely small separated interface at the first time. So, the load decreases. However, the interface separation amount is very small. If the load is continuously applied, the measured load increases when the interface crack stagnates. When the interface crack grows, the load decreases. After the displacement of 1.6 mm is reached, the composite structure can still have the completeness. The up-down load-displacement curve also shows that the aluminum oxide substrate and the copper sheet are well joined together. However, the chemical reaction products generated by the nickel-phosphorus alloy coating are not continuous. According to the research of Seager et. al. (Seager, C. W., Kokini, K., Trumble, K., Krane, M. J. M., Scripta Materialia, 46, 395, 2002), the discontinuous reaction product of aluminum oxide and copper is advantageous to the joining between aluminum oxide and copper. So, the nickel alloy coating is advantageous to the joining between aluminum oxide and copper. The alloy coating between aluminum oxide and copper can be in the form of a thin layer, points, columns, strips, a porous structure or a multi-layer structure. 
         [0069]    The example has proved that the thin nickel alloy layer between the aluminum oxide substrate and the copper sheet can enhance the strength of the composite structure. The coefficient of thermal expansion of nickel (13×10 −6 K −1 ) and an alloy thereof ranges from the coefficients of thermal expansion of aluminum oxide and of copper. So, the thermal stress between aluminum oxide and copper can be reduced. In addition, the nickel alloy can be simultaneously bonded to the aluminum oxide sheet and the copper sheet so that the interface strength is enhanced. So, the strength of the aluminum oxide/copper composite structure can be increased, and it is extremely advantageous to the increase of the lifetime of the high power electronic components. These components can then be used for several tens of thousand times of on-off cycles. 
       FOURTH EXAMPLE  
       [0070]    The aluminum oxide substrate the same as that used in the first example is coated with a nickel-boron alloy layer by way of electroless plating. The thickness of this metal nickel-phosphorus alloy layer ranges from 1 to 5 microns. Then, the aluminum oxide substrate is joined to two copper sheets, each of which has a thickness of 0.3 mm and is pre-oxidized by using two stages of pre-oxidation at different temperatures ranging from 100° C. to 600° C. and in different atmospheres with different oxygen partial pressures. The high-temperature joining is carried out in a temperature-calibrated oven at 1055° C. in flowing nitrogen. During the high-temperature joining, the alloy thin layer joins the aluminum oxide sheet and the copper sheet together to form strong bonds therebetween. The temperature 1055° C. is lower than the eutectic point (1066° C.) between copper and copper oxide. So, it is proved that the alloy thin layer of this example can enhance the joining between aluminum oxide and copper. 
         [0071]    The joined aluminum oxide/copper composite structure is tested with the 3-point bending technique the same as that described in the first example, and the obtained load-displacement curve is shown in  FIG. 15 . The aluminum oxide/copper composite structure withstands the load of 150 Newtons, and then the load only slightly decreases to 105 Newtons. Then, the composite structure can withstand the larger load up to 185 Newtons. Next, the load decreases, increases and then decreases. The test terminates when the displacement reaches 1.8 mm, and the composite structure is observed. At this time, the composite structure does not fail after the test, and the aluminum oxide substrate is not separated from the copper sheet, but has a small crack only in the aluminum oxide substrate. The copper sheet still tightly grasps the aluminum oxide sheet so that the completeness of the composite structure can still be kept. 
         [0072]    Similar to the nickel-phosphorus alloy coating in the third example, the nickel boron alloy coating is advantageous to the joining between aluminum oxide and copper. This example again proves that the thin nickel alloy layer between the aluminum oxide sheet and the copper sheet can enhance the strength of the aluminum oxide/copper composite structure. The coefficient of thermal expansion of the nickel alloy ranges between those of aluminum oxide and of copper. So, the thermal stress between aluminum oxide and copper can be reduced. The phenomenon that the copper wets the surface of the aluminum oxide is observed at the joining edge between aluminum oxide and copper (i.e., the welting angle between aluminum oxide and copper in this example is smaller than that in the second example), and it represents that the nickel boron alloy can enhance the wetting property of copper on the surface of aluminum oxide. So, the strength of the aluminum oxide/copper composite structure can be increased, and it is extremely advantageous to the increase of the lifetime of the high power electronic component. They can then be used for several tens of thousand times of on-off cycles. 
       FIFTH EXAMPLE  
       [0073]    The aluminum oxide substrate the same as that used in the first example is coated with a copper layer by way of electroless plating. The thickness of this metal alloy layer ranges from 2 to 4 microns. Then, the aluminum oxide substrate is joined to two copper sheets, each of which has a thickness of 0.3 mm and is pre-oxidized by using two stages of pre-oxidation at different temperatures ranging from 100° C. to 600° C. and in different atmospheres with different oxygen partial pressures. The high-temperature joining is carried out in a temperature-calibrated oven at 1056° C. in flowing nitrogen. During the high-temperature joining, the alloy thin layer joins the aluminum oxide sheet and the copper sheet together to form strong bonds therebetween. 
         [0074]    The joined aluminum oxide/copper composite structure is tested with the 3-point bending technique the same as that described in the first example, and the obtained load-displacement curve is shown in  FIG. 16 . The aluminum oxide/copper composite structure withstands the load of 190 Newtons, and then the load only slightly decreases to 90 Newtons. Then, the composite structure can withstand the larger load up to 160 Newtons. Next, the load decreases, increases and then decreases. The test terminates when the displacement reaches 0.8 mm, and the composite structure is observed. At this time, the composite structure does not fail after the test, and the aluminum oxide substrate is not separated from the copper sheet, but has a small crack only in the aluminum oxide substrate. The copper sheet still tightly grasps the aluminum oxide sheet so that the completeness of the composite structure can still be kept. 
         [0075]    This example proves that a thin copper layer coated on the aluminum oxide sheet or the copper sheet can enhance the strength of the composite structure. The existence of the copper layer decreases the wetting angle of the copper sheet on the aluminum oxide sheet so that the interface strength can be enhanced. So, the intensity of the aluminum oxide/copper composite structure can be increased by using the metal interlayer, and it is extremely advantageous to the increase of the lifetime of the high power electronic components. These components can then be used for several tens of thousand times of on-off cycles. 
         [0076]    In summary, the present invention discloses a composite structure, which is composed of copper and aluminum oxide and provides the basic heat dissipating and insulating ability. Because the copper has the extremely low resistivity of only 10 −4  Ωm, various patterns or electric connection points can be formed onto the copper sheet. The composite structure can be packaged with the high power electronic components. Both the good electrical connecting and heat dissipating properties can be achieved. In addition, the two-stage pre-oxidizing process is performed on the copper sheet to process the surface thereof with lower joining temperatures and short joining times. In addition, the composite structure has the high interface strength. So, it is extremely advantageous to the increase of the lifetime of the high power electronic components. They can then be used for several tens of thousand times of on-off cycles. 
         [0077]    While the present invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.