Abstract:
A method of selective metallization on a ceramic substrate includes selectively forming an active brazing material on a predetermined area of a surface of a ceramic substrate, attaching the metal layer to the ceramic substrate with the active brazing material, performing a brazing process on the active brazing material, forming an etching stop layer on the metal layer and performing an etching process, and removing the etching stop layer. The method can be applied to a severe environment, and the conchoidal fracture between the ceramic substrate and the metal layer can also be avoided. The present invention not only simplifies the process but also improves the product yield.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to methods for selective metallization on a ceramic substrate, and, more particularly, to a method of forming a metal layer within a selected area of a ceramic substrate by a brazing process. 
         [0003]    2. Description of Related Art 
         [0004]    Two methods are generally used for selective metallization on a ceramic substrate. For the metal of copper, one is realized with selectively etching a copper layer after a direct bonded copper (DBC) process is performed, and the other is done with selective copper deposition or by selectively etching a copper layer after a direct plated copper (DPC) process is performed on the whole substrate. 
         [0005]    As shown in  FIGS. 1A to 1E , cross-sectional views illustrating a method for the metallization of a ceramic substrate by using the DBC process according to the prior art are provided. The method is performed under a high temperature and specific oxygen content. As shown in  FIG. 1A , a copper layer  1  is provided, and the surface of the copper layer  1  is oxidized to form a cuprous oxide layer  11 . As shown in  FIG. 1B , the copper layer  1  and a ceramic substrate  2  are bonded by Cu—Al—O chemical bonds provided by a eutectic layer  12  which is composed of the cuprous oxide layer  11  after cooling. A ceramic substrate  5  directly bonded with copper is thus formed. As shown in  FIGS. 1C and 1D , an etching resist  3  is formed on the ceramic substrate  5 , and a portion of the copper layer  1  that is not covered by the etching resist layer  3  is etched and removed. As shown in  FIG. 1E , the etching resist  3  is removed, so as to complete the selective metallization process of the ceramic substrate  5 . 
         [0006]    However, the DBC process encounters problems. For example, since the desired eutectic temperature for the bonding of copper and ceramic is very close to the melting point of the copper itself, the DBC process must work in a very narrow temperature range, in order to prevent the copper from melting when the ceramic substrate is bonded to the copper layer. Therefore, in a batch production process it is difficult to keep the furnace in inconsistent atmosphere and temperature at different positions, which results in a yield issue on the product. Currently, the metallized ceramic substrate with eutectic bonding is made of aluminum oxide that has low thermal conductivity. This kind of ceramic substrate is rarely made of aluminum nitride (AlN) or silicon carbide (SiC) that have high thermal conductivity because it is difficult to be bonded with the copper layer, due to the lack of wettability or capability of forming Cu—Al—O bonds. Thus, the application of the DBC metallized ceramic substrate for high thermal conductivity or high heat dissipation is extremely restricted. In addition, since the DBC process adopts Cu—Cu2O eutectic bonding, such that no other metal material can be bonded with the ceramic substrate except copper. On the other hand, the conchoidal fracture exists between the ceramic substrate and the copper layer in the conventional DBC process, which is not along the lattice and is irregularly broken, mainly due to the internal stress caused by the mismatch thermal expansion, and affects the reliability and lifetime indirectly. 
         [0007]    Referring to  FIGS. 2A to 2E , cross-sectional views illustrating a method of fabricating a copper plating substrate according to the prior art are provided. The method of performing selective metallization with DPC is as follows. As shown in  FIG. 2A , an adhesion layer/seed layer  4  is formed on the ceramic substrate  2 . As shown in  FIG. 2B , a resist layer  6  for the prevention of metal deposition is formed on the adhesion layer/seed layer  4 . A metal layer  10  is deposited directly on the portion of the adhesion layer/seed layer  4  that is not covered by the resist layer  6  by a copper plating process, as shown in  FIG. 2C . As shown in  FIGS. 2D and 2E , the resist layer  6  is removed, and a surface micro-etching process is performed to complete the selective metallization of the ceramic substrate. 
         [0008]      FIGS. 3A to 3E  are cross-sectional views illustrating another method of fabricating a copper plating substrate according to the prior art. As shown in  FIG. 3A , an adhesion layer/seed layer  4  is formed on the ceramic substrate  2 . As shown in  FIG. 3B , a copper plating process is performed on the surface of the ceramic substrate  2 , and a copper layer  1  is formed on the adhesion layer/seed layer  4  to generate the DPC substrates  5 . As shown in  FIG. 3C , an etching stop layer  3  is formed on the DPC  5 . As shown in  FIG. 3D , a portion of the copper layer  6  and the adhesion layer/seed layer  4  that is not covered by the etching stop layer  3  is etched and removed. Finally, the etching stop layer  3  is removed and the selective metallization of the ceramic substrate is formed, as shown in  FIG. 3E . 
         [0009]    However, the disadvantages exist in performing the selective metallization in the DPC. For example, the copper layer and the ceramic substrate are bonded with the adhesion layer, and the adhesion layer is physically bonded by sputtering or evaporating the titanium (Ti) or titanium tungsten (TiW), thus the adhesion strength is not superior as chemical bonding, and can not be used in the situation of high temperature or large temperature difference. In addition, forming the copper layer (DPC process) by plating will significantly affect the production capability due to time-consuming for the plating deposition process. the obtained thickness of copper layer formed by plating will vary substantially. This is because the current density distribution is significantly affected by the design of plating tank, the resist pattern, and the edge effect of the ceramic substrate. Furthermore, the materials used may be restricted in the plating field, only copper or nickel can be used, and thereby the ceramic substrate can not be bonded with other metals. 
         [0010]    Therefore, how to provide a process of metallized ceramic substrate with high bonding strength, capable of solving the limitations to the applied environment and material selection of the conventional ceramic substrate and metal layer, and reducing the situation of conchoidal fracture caused by the inner stress between the ceramic substrate and the metal layer, is the issue has to be faced by persons skilled in the art. 
       SUMMARY OF THE INVENTION 
       [0011]    In view of the above drawbacks of the prior art, the object of the present invention is to provide a tightly bonding between the ceramic substrate and the metal layer using brazing technology. 
         [0012]    To achieve the objects above and other objects, the present invention provides a method of selective metallization on a ceramic substrate, comprising: forming an active brazing material on a predetermined area of a surface of the ceramic substrate; attaching a metal layer to the surface of the ceramic substrate with the active brazing material and performing a brazing process on the active brazing material; forming an etching stop layer on the predetermined area of the metal layer and etching the metal layer; and removing the etching stop layer. 
         [0013]    In the present invention, the active brazing material is formed on the ceramic substrate, but not limited thereto. In another embodiment, a layer of active brazing material can be also selectively formed on the copper layer and then the copper layer is attached to the ceramic substrate, and subsequent brazing and etching process are performed. 
         [0014]    In an embodiment of the present invention, the active brazing material has active metal with a specific proportion. 
         [0015]    In another embodiment of the present invention, the active brazing material is formed on the surface of the ceramic substrate by a printing, spray coating, or lamination. 
         [0016]    In another embodiment of the present invention, the etching stop layer corresponds to the active brazing material formed on the surface of the ceramic substrate. 
         [0017]    The present invention further provides a method of selective metallization on a ceramic substrate, comprising: performing a predetermined depth etching on a predetermined area of a metal layer, so as for forming an etching area and a reserved area on the metal layer; forming an active brazing material on the reserved area of the metal layer; attaching the metal layer with the active brazing material to the ceramic substrate and performing a brazing process on the active brazing material; and etching the metal layer for removing the metal on the etching area. 
         [0018]    In an embodiment of the present invention, the metal layer is copper, aluminum or stainless steel. 
         [0019]    Compared to the prior art, the present invention provides a method of selective metallization on the ceramic substrate. A brazing process is performed with the active brazing material to increase the bonding reliability between the ceramic substrate and the metal layer. Also, since the electroplating or eutectic bonding is not used in the present invention, materials of the ceramic substrate and the metal layer will not be limited as in the prior art and the process may be applied to the environment with high temperature or large temperature difference. In addition, the problem of conchoidal fracture or poor adhesion between the ceramic substrate and the metal layer in the prior art can be avoided. The present invention not only can simplify the process, but also can improve product yield. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIGS. 1A to 1E  are cross-sectional views illustrating a method for the DBC process according to the prior art; 
           [0021]      FIGS. 2A to 2E  are cross-sectional views illustrating a method of fabricating a DPC substrate according to the prior art; 
           [0022]      FIGS. 3A to 3E  are cross-sectional views illustrating another method of fabricating a DPC substrate according to the prior art; 
           [0023]      FIGS. 4A to 4E  are cross-sectional views illustrating a method of forming a selected metal on a ceramic substrate according to a first embodiment of the present invention; and 
           [0024]      FIGS. 5A to 5D  are cross-sectional views illustrating a method of forming a selected metal on a ceramic substrate according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    It is to be understood that both the foregoing general descriptions and the detailed embodiments are exemplary and are, together with the accompanying drawings, intended to provide further explanation of technical features and advantages of the invention. 
         [0026]    The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects can be apparent to those skilled in the art after reading the disclosure of this specification. The present invention can also be performed or applied by other different embodiments. The details of the specification may be on the basis of different points and applications, and numerous modifications and variations can be devised without departing from the spirit of the present invention. 
       First Embodiment 
       [0027]    Referring to  FIGS. 4A to 4E , cross-sectional views illustrating a method of forming a selected metal on a ceramic substrate according to a first embodiment of the present invention are provided. The method produces a selectively metallized structure of a ceramic substrate with high reliability by a brazing process. 
         [0028]    As shown in  FIG. 4A , a ceramic substrate  20  is provided. An active brazing material  40  is selectively formed on a predetermined area of the ceramic substrate  20 . The active brazing material  40  is selectively coated on some areas of the ceramic substrate  20 . The active brazing material  40  may be nickel-based or silver-based brazing material, and the active brazing material  40  has an active metal with a specific proportion, such as titanium (Ti), zirconium (Zr) or niobium (Nb), which is helpful to increase the wettability of the active brazing material  40  on the surface of the ceramic substrate  20 . In addition, the active brazing material  40  may be brazing material, such as paste, powder or foil, and the coating process may be performed by a printing, spray coating, or lamination. 
         [0029]    As shown in  FIG. 4B , the metal layer  10  is attached to the surface of the ceramic substrate  20  with the active brazing material  40 . Then, a brazing process is performed on the active brazing material  40 , such that the ceramic substrate  20  and the metal layer  10  may produce highly reliable bonding at an area having the active brazing material  40  to form a structure with the metal covering on the ceramic substrate. 
         [0030]    As shown in  FIG. 4C , an etching stop layer  30  is formed on the metal layer  10 . The etching stop layer  30  corresponds in position to the active brazing material  40  formed on the surface of the ceramic substrate  20 . In other words, one side of the metal layer  10  has the active brazing material  40  and the etching stop layer  30  is formed at the relative position of the other side of the metal layer  10 . 
         [0031]    As shown in  FIG. 4D , the metal layer  10  is selectively etched. The etching stop layer  30  formed on the metal layer  10  works as a protective layer with specific patterns generated by dry film and lithography process. Thus, an exposed portion of the metal layer  10  unprotected by the etching stop layer  30  is etched and removed. The etched and removed area is the area not coated by the active brazing material  40 . 
         [0032]    As shown in  FIG. 4E , the etching stop layer  30  is removed, after the metal layer  10  is etched, to complete the selective metallization of the ceramic substrate. 
         [0033]    In addition to commonly used alumina, the foregoing ceramic substrate  20  may also be used with aluminum nitride or silicon carbide. In addition to the common copper, the metal layer  10  may also be made of aluminum or stainless steel and the like. In the DBC structure shown in  FIGS. 1A to 1E , bonding for the metal layers may be Cu—Cu2O eutectic bonding. Thus, except copper, no other metal material can be used for bonding to the substrate. Also, for the copper plating method shown in  FIGS. 2A to 2E  and  3 A to  3 E, because the metal has to be produced by plating, material selection may also be limited to the one that is capable of electroplating. More common materials are only copper or nickel. Therefore, the present invention performs brazing with the active brazing material, such that the ceramic substrate  20  and the metal layer  10  are more flexible in selecting the material. 
         [0034]    Further, because the active brazing material  40  is not easily removed by etching, the present invention is characterized in selectively coating the active brazing material  40  at a particular area, such that the metal layer  10  at an area not coated by the active brazing material  40  is removed. The ceramic substrate  20  having the removal of the metal layer  10  will not have brazing material residue thereon, and the selective metallization of the ceramic substrate is completed. 
         [0035]    According to another embodiment of the present invention, as shown in  FIGS. 4A and 4B , the active brazing material  40  can also be first coated on the metal layer  10 , then the ceramic substrate  20  and the metal layer  10  are bonded, and the selective metallization of the ceramic substrate is also completed. 
       Second Embodiment 
       [0036]    Referring to  FIGS. 5A to 5D , cross-sectional views illustrating a method of forming a selected metal on a ceramic substrate according to a second embodiment of the present invention are provided. 
         [0037]    As shown in  FIG. 5A , a metal layer  10  having an etching area  102  and a reserved area  101  is provided. The metal layer  10  is selectively etched through dry film, photoresist or other methods. Thus, the etched etching area  102  has a certain depth, and the reserved area  101  will be bonded to the ceramic substrate  20 . 
         [0038]    As shown in  FIG. 5B , an active brazing material  40  is formed on the reserved area  101  of the metal layer  10 . The active brazing material  40  may be nickel or silver-based active brazing material. The active brazing material  40  containing an active metal with a specific proportion, such as titanium, zirconium or niobium, may be a brazing material in paste, powder or foil, and coated on the reserved area  101  by printing, spray coating or lamination. 
         [0039]    As shown in  FIG. 5C , the metal layer  10  having the active brazing material  40  is attached to the ceramic substrate  20 , and subsequently the active brazing material  40  is performed the brazing process, such that the ceramic substrate  20  and the metal layer  10  are bonded with high reliability at area having the active brazing material  40 . 
         [0040]    As shown in  FIG. 5D , the etching step of the metal layer  10  is performed. The metal layer  10  is etched to remove the etching area  102  of the metal layer  10 . The metal layer  10  is selectively etched. The etched portion refers to the etching area  102  which is not coated by the active brazing material  40 , such that the surface without the active brazing material  40  attached to the metal layer  10  is exposed, in order to complete the selective metallization of the ceramic substrate. 
         [0041]    The etching step of the metal layer  10  can also be comprehensive etching to the metal layer  10 . The etched portion refers to the etching area  102  and the reserved area  101 . As the metal layer of the reserved area  101  is thicker for the bonding to the ceramic substrate  20 , under equal etching rate, the etching area  102  will be fully removed after the comprehensive etching, and the reserved area  101  bonded to the ceramic substrate  20  will remain the metal layer  10  with a specific thickness. The selective metallization of the ceramic substrate is thus completed. 
         [0042]    Similarly, the aforementioned ceramic substrate  20  may be alumina, aluminum nitride or silicon carbide, and the metal layer  10  may be copper, aluminum, stainless steel or other materials. Compared to the conventional process, the present invention performs brazing with the active brazing material, such that the ceramic substrate  20  and the metal layer  10  are more flexible in selecting the material. 
         [0043]    In addition, according to another embodiment of the present invention, as shown in  FIGS. 5B and 5C , the active brazing material  40  can also be coated on the ceramic substrate  20 , and the ceramic substrate  20  and the metal layer  10  can be bonded to complete the selective metallization of the ceramic substrate of the present embodiment. 
         [0044]    By the process described in the present invention, as the operating range of the brazing temperature is greater, the yield of the batch production is improved, and by using the brazing process, the ceramic substrate and the metal layer are more flexible in selecting the material, and thus will not have limitations as in the prior art. In addition, the reliability is better by brazing bonding and the problems of the conchoidal fracture between the ceramic substrate and the metal layer or poor adhesion can be solved, and thus also can be used under the environment of a high temperature or large temperature difference. Finally, since the metal layer with a specific thickness is bonded to the ceramic substrate, the problem of uneven thickness will not occur. 
         [0045]    In summary, the method of selective metallization on the ceramic substrate of the present invention can tightly bond the metal layer and the ceramic substrate by the brazing process. Compared to the prior art, the better process of selective metallization on the ceramic substrate and higher product yield are provided. 
         [0046]    The above embodiments are illustrated to disclose the preferred implementation according to the present invention but not intended to limit the scope of the present invention. Accordingly, all modifications and variations completed by those with ordinary skill in the art should fall within the scope of present invention defined by the appended claims.