BACKGROUND OF THE INVENTION 
     The present invention relates to a Cu plated ceramic substrate, a Pertier element to which the substrate is applied, and a method of manufacturing the substrate. 
     Cu plated materials are widely used in industrial fields. For example, in Cu plating solution, CuCN plating solution and copper pyrophosphate plating solution, a direct current is applied between Cu as anode and a workpiece as cathode to precipitate Cu on the workpiece, or Cu precipitated workpiece is made using electroless Cu plating solution. 
     The workpiece made of ceramic substrate is called Cu plated ceramic substrate, and the workpiece made of plastics is called a Cu plated plastic substrate. 
     Cu plated ceramic substrates are used as electronic parts such as semiconductor devices, small communication devices, relays, chip carriers and Pertier elements. 
     The surface of ceramics is treated with acid, sand blasts, etc. and is subjected to electroless Cu plating. Thereafter, Cu electroplating is applied. Alternatively, after another metal is applied on ceramic substrates with dry thin film process such as vapor deposition, Cu plating is applied. As another metal, Co, Nb, Ta, Ti, Cu—Ni—Cr alloy, Mo or W is used. 
     The following are known to manufacture a Cu plated ceramic substrate: 
     {circle around (1)} Cu film or plate is directly joined on a ceramic substrate as described in J. Am. Ceram. Soc. 61, 1978 by A. K. Varchneya and R. J. Petti. 
     {circle around (2)} Paste of conductive material such as Ag, Au, Cu and Ni is applied on a ceramic substrate by printing or other methods, and plating is applied thereon, or after firing at several hundred degree, plating is applied. 
     However, diameter of Cu particle precipitated by electroless copper plating is large, so that contact area between the Cu particle and irregular portions of the surface of the ceramic substrate is small. Anchoring effect is poor, and adhesibility between the Cu particle and the ceramic substrate is low. 
     Furthermore, the precipitated particles are large, and a recess becomes a space, to which treating chemicals are attached to decrease adhesibility. 
     In the latter, conductive paste is employed, and a diameter of a particle is as large as electroless Cu plating by twenty to thirty times, so that contact area becomes very small between the particle of the conductive material and ceramic substrate to lead poor anchoring effect and decreasing adhesibility. 
     A Pertier element comprises a module using Pertier effect which is exothermic or endothermic phenomenon except Joule heat at a contacting point of different conductors or semi-conductors when an electric current is applied to the point and which was found by J. C. A. Pertier in 1834. 
     The Pertier element is employed as a cooler for a semiconductor laser device used for optical communication. The semiconductor laser converts an optical signal by a wave length converter to a shape to which WDM (Wavelength Division Multiplexing) can be made to transmit it via an optical fiber 
     The signal in which wavelength division is multiplexed passes through the optical fiber, and is converted to the original optical signal by a wave length converter. A number of signals can be treated as a single signal and thus transmitted by a single optical fiber, thereby reducing the number of the optical fibers significantly. 
     The Pertier element is employed to cool a semiconductor laser device for optical communication. The narrower the range of w temperature for controlling wavelength of a signal for converting WDM becomes, the smaller a semiconductor device becomes. Thus, the Pertier element used therein becomes smaller, and it is difficult to make such an element by attaching known copper plates. Adhesive strength of the Pertier element made by such attachment needs a larger total area. Miniaturization of the element is limited. 
     To overcome such problem, plating must be used with photo-forming. With miniaturization, adhesive area between the ceramic substrate and conductor circuit significantly decreases, and it is very important to improve adhesive strength between them. 
     The Cu plated ceramic substrate according to the present invention is suitably used as substrate for the Pertier element for cooling a miniaturized semiconductor laser. 
     There are two known types of Pertier elements, one being a Cu plate or film which is punched by a press and bonded to a ceramic substrate, the other being a conductive paste which is applied to a ceramic substrate, a Cu plating being applied after firing of the substrate. 
     However, there are disadvantages in the known Pertier substrates. In the former, miniaturization is impossible, and organic compounds must be inserted as adhesive to lead low thermal conductivity and high consumption power which is expensive. In the latter, owing to particle diameter of the conductive paste, it provides low bonding strength with the ceramic substrate and low reliability. In both elements, owing to insertion of organic compounds, the compounds deteriorate to decrease bonding strength. 
     SUMMARY OF THE INVENTION 
     In view of the disadvantages in the prior art, it is an object of the present invention to provide a Cu plated ceramic substrate to increase adhesibility between a ceramic substrate and a Cu plating layer. 
     To achieve the object, according to one aspect of the present invention, there is provided a Cu plated ceramic substrate which comprises: 
     a ceramic substrate layer; 
     a dry thin-film Cr layer on the ceramic substrate layer; 
     a dry thin-film Au layer on the thin-film Cr layer; and 
     a Cu plating layer on the thin-film Au layer. 
     According to another aspect of the present invention, there is provided a method of manufacturing a Cu plated ceramic substrate, comprising the steps of: 
     forming a dry thin-film Cr layer on a ceramic substrate layer; 
     forming a dry thin-film Au layer on the thin-film Cr layer; and 
     plating the thin-film Au layer with Cu. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the invention will become more apparent from the following description with respect to embodiments as shown in the appended drawings wherein: 
     FIG. 1 is an enlarged sectional view of the first embodiment of a Cu plated ceramic substrate according to the present invention; 
     FIG. 2 is an enlarged sectional view of the second embodiment of a Cu plated ceramic substrate according to the present invention; and 
     FIG. 3 is an enlarged sectional view of a Pertier element to which the Cu plated ceramic substrate according to the present invention is applied. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a sectional view of the first embodiment of a Cu plated ceramic substrate according to the present invention. On one side of ceramic substrate layer  1 , a dry thin film Cr layer  2 , a dry thin film Au layer  3  and a Cu plating layer  4  are formed. 
     To overcome the foregoing disadvantages in the prior art, the dry thin-film Cr layer is applied on the ceramic substrate layer. By forming the dry thin-film Cr layer on the ceramic substrate layer by metal vapor deposition, Cr becomes fine particles to increase contact area with irregular surface of the ceramic substrate layer to increase anchoring effect, thereby improving adhesibility between the Cr particles and the ceramic substrate layer. 
     The Cr layer is formed by dry thin-film method, thereby avoiding corrosion by the Cr particles in known methods using chemical agents. 
     However, it is difficult to apply Cu plating directly on the dry thin-film Cr layer because Cr which provides high hydrophilicity resists Cu which provides high lipophilicity to decrease adhesibility. Further, Cr is oxidized by O 2  in air to put oxides between the Cr and Cu to decrease adhesibility. 
     In view of the features of the dry thin-film Cr layer, the inventors studied very hard, and found it very efficient to form the dry thin-film Au layer on the dry thin-film Cr layer. 
     To apply the dry thin-film Au layer on the dry thin-film Cr layer provides the following advantages. Au is a very stable metal which is never oxidized in air, does not form impurities which decrease adhesibility with Cu precipitated in the next Cu plating step, and diffuses into Cu at boundary of Au and Cu to increase anchoring effect and adhesibility. 
     It is very important to provide Au which contacts Cu in the present invention. 
     In the present invention, instead of the dry thin-film Au layer directly formed on the dry thin-film Cr layer on the ceramic substrate layer, another dry thin-film metal layer may be formed on the dry thin-film Cr layer, and the dry thin-film Au layer may be formed thereon. As another metal layer, Ni, Mo, W, Ti or Ag may be employed to apply a thin-film manufacturing process. In the present invention, the dry thin-film of Cr, Au etc. may be made by ordinary methods such as vacuum deposition, cathode sputtering and chemical vapor deposition (CVD). 
     FIG. 2 is a sectional view of the second embodiment of a Cu plated ceramic substrate according to the present invention. On both sides of a ceramic substrate layer  1 , a dry thin film Cr layer  2 , a dry thin film Au layer  3  and a Cu plating layer  4  are formed. 
     To prevent oxidation of the outermost Cu surface, one or combination of known methods such as Ni plating, Au plating, soft soldering and Sn—Sb plating may be applied, or the surface may be covered with organic compounds. 
     FIG. 3 is a schematic sectional view of a Pertier element  5  to which the Cu plated ceramic substrate  6  thus made as above is applied. 
     A semiconductor  7  is mounted at the ends to inner conducting circuits  8 , 8 , which are connected to the Cu ceramic substrates  6 , 6  according to the present invention respectively. Outer conducting circuits  9 , 9  are provided on the Cu plated ceramic substrates  6 , 6 . The Cu plated ceramic substrate  6  is mounted to the inner conducting circuit  8 , and the Cu plating layer  4  is mounted to the outer conducting circuits  9 . Between a ceramic substrate layer and a Cu plating layer of the Cu plated ceramic substrates  6 , a thin-film Cr layer and a thin-film Au layer are provided, thereby forming tightly adhesive structure. The outer conducting circuits  9 , 9  form an endothermic side A 1  and an exothermic side A 2 . An electric current is applied to the semiconductor  7  to result in generating endothermic reaction at one side and exothermic reaction at the other side, which are transmitted to the outside via A 1  and A 2 . The Cu plated ceramic substrate in FIG. 2 may be employed. 
     According to the Cu plated ceramic substrate, no organic compounds are inserted in the ceramic substrate, metal atoms are strongly anchored on the ceramic substrate as mentioned above, and the thin film Au layer and Cu are strongly bonded by plating, and owing to Au diffusion, the thin film Au layer is strongly adhered to the Cu plating layer. It provides high bonding strength with the ceramic substrate and no organic compounds to achieve good thermal conductivity. It can be manufactured by plating and photo-forming, thereby allowing it possible to manufacture a fine Cu plated ceramic substrate. Therefore, the Cu plated ceramic substrate according to the present invention is convenient as a substrate for Pertier element which attains both cooling and heating. 
     EXAMPLES 
     Examples according to the present invention will be described as below. Al 2 O 3 , AlN or SiC was used as ceramic substrate, and the surface of each ceramic substrate was cleaned by a ultrasonic cleaning machine and dried by hot wind of a clean bench. 
     Then, to apply chemical vapor deposition of a dry thin film manufacturing process, the cleaned ceramic substrate was put into a furnace of a vacuum vapor deposition machine, and Cr and Au were evaporated to form films to a determined thickness respectively while degree of vacuum is 1Pa±10% at temperature of 310±5° C. Cr—Au thin film ceramic substrates before Cu plating were made. Cr—Ni—Au thin film ceramic substrates were made as well. 
     Thus, electroless copper plating was made using electroless cooper plating solution of Work Metal Co., Ltd. at liquid temperature of 70±2° C., and copper electroplating was made at temperature of 25±1° C. using CuSO 4  of Adtech Japan Co., Ltd. By combination of the two methods, Cu plating layer of a determined thickness was made. 
     Peel or bonding strength was determined with respect to Cu plated ceramic substrates thus made, using AGS-50B of Shimazu Mfg., Co., Ltd. 
     The results are shown in Tables 1 to 3. Table 3 shows Ni thin-film layer between thin-film Cr and Au layers. The larger the numbers are, the higher bonding strength is. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Cr layer 
                 Au layer 
                 Cu layer 
                 Peel 
               
               
                   
                 Ceramic 
                 Thickness 
                 Thickness 
                 Thickness 
                 Strength 
               
               
                   
                 Substrate 
                 (Å) 
                 (Å) 
                 (μm) 
                 (kg/mm 2 ) 
               
               
                   
                   
               
             
             
               
                   
                 Al 2 O 3   
                 1500 
                 1500 
                 35 
                 0.793 
               
               
                   
                   
                 1000 
                 1500 
                 35 
                 0.820 
               
               
                   
                   
                 1500 
                 1000 
                 35 
                 0.585 
               
               
                   
                   
                 1000 
                 1000 
                 35 
                 0.679 
               
               
                   
                 AlN 
                 1500 
                 1500 
                 35 
                 0.857 
               
               
                   
                   
                 1000 
                 1500 
                 35 
                 0.845 
               
               
                   
                   
                 1500 
                 1000 
                 35 
                 0.712 
               
               
                   
                   
                 1000 
                 1000 
                 35 
                 0.722 
               
               
                   
                 SiC 
                 1500 
                 1500 
                 35 
                 0.767 
               
               
                   
                   
                 1000 
                 1000 
                 35 
                 0.802 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Cu 
                   
               
               
                   
                   
                   
                 Electroless 
                 electro- 
               
               
                   
                 Cr layer 
                 Au layer 
                 Cu layer 
                 plating 
                 Peel 
               
               
                 Ceramic 
                 Thickness 
                 Thickness 
                 Thickness 
                 thickness 
                 Strength 
               
               
                 Substrate 
                 (Å) 
                 (Å) 
                 (μm) 
                 (μm) 
                 (kg/mm 2 ) 
               
               
                   
               
             
             
               
                 Al 2 O 3   
                 1500 
                 1500 
                 0.3 
                 35 
                 0.745 
               
               
                   
                 1000 
                 1500 
                 0.3 
                 35 
                 0.760 
               
               
                   
                 1500 
                 1000 
                 0.3 
                 35 
                 0.642 
               
               
                   
                 1000 
                 1000 
                 0.3 
                 35 
                 0.631 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                 Electroless 
                 Cu electro- 
                   
               
               
                   
                 Cr layer 
                 Ni layer 
                 Au layer 
                 plating 
                 Peel 
               
               
                 Ceramic 
                 Thickness 
                 Thickness 
                 Thickness 
                 thickness 
                 Strength 
               
               
                 Substrate 
                 (Å) 
                 (Å) 
                 (μm) 
                 (μm) 
                 (kg/mm 2 ) 
               
               
                   
               
             
             
               
                 Al 2 O 3   
                 1500 
                 1500 
                 1500 
                 35 
                 0.830 
               
               
                   
                 1000 
                 1500 
                 1500 
                 35 
                 0.798 
               
               
                   
                 1500 
                 1500 
                 1000 
                 35 
                 0.721 
               
               
                   
                 1000 
                 1500 
                 1000 
                 35 
                 0.732 
               
               
                   
               
             
          
         
       
     
     COMPARATIVE EXAMPLES 
     A ceramic substrate is immersed in NaOH to roughen the surface, and cleaned by ultrasonic cleaning. Electroless copper plating and copper electroplating are made. 
     Cr vapor deposited samples before electroless copper plating were manufactured. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 Electroless 
                 Cu electro- 
                   
               
               
                   
                 Cr layer 
                 Cu layer 
                 plating 
               
               
                 Ceramic 
                 Thickness 
                 Thickness 
                 thickness 
                 Peel strength 
               
               
                 Substrate 
                 (Å) 
                 (μm) 
                 (μm) 
                 (kg/mm 2 ) 
               
               
                   
               
             
             
               
                 Al 2 O 3   
                 none 
                 0.3 
                 35 
                 0.145 
               
               
                   
                 none 
                 0.5 
                 35 
                 0.135 
               
               
                   
                 none 
                 0.8 
                 35 
                 0.152 
               
               
                   
                 1500 
                 0.3 
                 35 
                 0.381 
               
               
                   
                 1000 
                 0.3 
                 35 
                 0.364 
               
               
                   
               
             
          
         
       
     
     The Cu plated ceramic substrate according to the present invention formed a thin-film Cr layer on the ceramic substrate and an Au thin-film layer on the Cr thin-film layer, thereby achieving bonding strength of about 2.5 to 5.0 times as large as the prior methods. 
     Especially, if the Pertier element includes the substrate, the Cu plated ceramic substrate according to the present invention will meet with the requirements of bonding strength of over 0.400 kg/mm 2 . 
     The foregoing merely relate to embodiments of the invention. Various changes and modifications may be made by persons skilled in the art without departing from the scope of claims wherein:
A Cu plated ceramic substrate is used in a semiconductor. On a ceramic substrate layer, a thin-film Cr layer is put, and a thin-firm Au layer is put on the Cr layer. The Au layer is plated with Cu. By providing the Au and Cr layers between the ceramic plate and Cu layer, adhesibility is increased. A Pertier element which includes the Cu plated ceramic layer is employed in a semiconductor to absorb and generate heat efficiently.