BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a coreless packaging substrate and a method for manufacturing the same, and, more particularly, to a light weighted and compact coreless packaging substrate, and a method for manufacturing the same. 
         [0003]    2. Description of Related Art 
         [0004]    As the electronic industry develops rapidly, researches move towards electronic devices with multifunction and high efficiency. Hence, circuit boards with lots of active and passive components and circuit connections thereon transfer from single-layered boards to multiple-layered boards so that the requirements such as integration and miniaturization in semiconductor packaging substrate can be met. Furthermore, interlayer connection technique is also applied in this field to expand circuit layout space in a limited circuit board and to meet the demand of the application of high-density integrated circuits. 
         [0005]    For manufacturing conventional semiconductor packaging structures, a chip is mounted on the top surface of a substrate first, and then connected thereto by wire bonding. Alternatively, the chip is connected with the substrate by flip chip technique. Subsequently, solder balls are disposed on the bottom surface of the substrate and electrically connected to a printed circuit board. However, even though the purpose of high quantity pin counts can be achieved through the method illustrated above, the electrical performance of a device operated in high frequency or at high speed can be unstable or limited due to the long paths of conductive circuits. Moreover, the complexity of the manufacturing process is relatively increased because many connective interfaces are required for conventional semiconductor packaging structures. 
         [0006]    In the method for manufacturing a flip-chip substrate, a packaging substrate is formed by providing a core board at first, and then followed by drilling, metal electroplating, plugging, circuit patterning, and so on to complete an inner structure. Subsequently, a multilayer substrate is afforded by built-up processes, as shown in  FIGS. 1A to 1E , which show a flowchart for manufacturing a built-up type multilayer substrate. In  FIG. 1A , a core board  11  is prepared first. The core board  11  includes a core layer  111  having a predetermined thickness, and a circuit layer  112  formed thereon. Meanwhile, the core layer  111  has a plurality of plated through holes (PTHs)  113  formed therein so that the PTHs  113  can be electrically connected to the circuit layer  112  on the core layer  111 . As shown in  FIG. 1B , the core board  11  is processed through a built-up process. The built-up process is illustrated as follows. First, a dielectric layer  12  is disposed on the surface of the core board  11 . The dielectric layer  12  has a plurality of vias exposing part of the circuit layer  112  serving as conductive pads  112   a . With reference to  FIG. 1C , a seed layer  14  is formed by electroless plating or sputtering on the surface of the dielectric layer  12 . Then, a patterned resist layer  15  is formed on the seed layer  12  so that the conductive pads  112   a  can be exposed by a plurality of openings  150  formed in the resist layer  15 . With regard to  FIG. 1D , conductive vias  16   a  and a patterned circuit layer  16  are formed by electroplating respectively in the vias and in the openings  150  of the resist layer  15 . The circuit layer  16  can be electrically connected to the conductive pads  112   a  by the connection of the conductive vias  16   a . Subsequently, the resist layer  15  and the seed layer  14  covered thereby are removed to afford a first circuit built-up structure  10   a . Referring to  FIG. 1E , a second built-up structure  10   b  is formed on the surface of the first built-up structure  10   a  in the same manner as the first built-up structure  10   a  so that a multilayer packaging substrate  10  is obtained. 
         [0007]    The above-mentioned manufacturing begins from provision of a core board, followed by drilling, metal electroplating, plugging, circuit patterning and so on to complete an inner structure, and finally to performing built-up processes to afford a multilayer packaging substrate. However, in the manufacturing illustrated above, there is a need to form PTHs by drilling and electroplating etc. Therefore, many circuit layout spaces are occupied by the PTHs because the diameter and the depth of each PTH are greater than those of each conductive via. Moreover, undesirable cross-talk, noises, or signal decay resulting from excessive length of signal transmitting pathway could easily occur. In order to solve the disadvantages arising from long signal transduction pathway, the design of the circuit layout is often dense on a chip disposition side electrically connected to a chip. In contrast, the density of the circuit layout on a solder ball disposition side connected to a printed circuit board could be sparse. For most of the packaging substrates, the numbers of the circuit layers on the both sides are identical. When the density of the circuit layout on the solder ball disposition side is too sparse, not only many layout spaces are idle, but also the number of laminated layers is increased. Because multiple circuit layers need to be included, manufacturing processes become more complex. In addition, the packaging substrate is hard to be used in high frequency because of long conductive circuits and high impedance. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the above-mentioned, the present invention provides a method for manufacturing a coreless packaging substrate comprising the following steps. First, a core board is provided. Then, a metal adhesive layer is formed on the surface of the core board. Subsequently, a patterned first solder mask layer is formed on the surface of the metal adhesive layer, wherein the first solder mask layer has a plurality of first openings. Further, a metal pillar is formed in each of the first openings, and a metal layer is formed on the surface of the metal pillars and part of the surface of the first solder mask layer. Furthermore, a circuit built-up structure is formed on the surfaces of the metal layer and the first solder mask layer, wherein the metal layer is embedded in the circuit built-up structure. Moreover, a patterned second solder mask layer is formed on the circuit built-up structure, wherein the second solder mask layer has a plurality of second openings exposing circuits of the circuit built-up structure, and the exposed circuits serve as second conductive pads. Finally, the core board and the metal adhesive layer are removed to expose the metal pillars serving as first conductive pads. 
         [0009]    Also, the present invention provides a coreless packaging substrate which can be manufactured by the foregoing method but is not limited thereto. 
         [0010]    The coreless packaging substrate in the present invention comprises: a circuit built-up structure, a first solder mask layer, and a second solder mask layer. A plurality of metal layers are embedded under one surface of the circuit built-up structure, and a plurality of second conductive pads are formed on the other surface of the circuit built-up structure. The first solder mask layer is disposed on the surface of the circuit built-up structure having the metal layers, which has a plurality of first openings exposing part of the metal layers. Each of the first openings has a metal pillar therein, and the metal pillars serve as first conductive pads. The second solder mask layer is disposed on the surface of the circuit built-up structure having the second conductive pads, which has a plurality of second openings to expose the second conductive pads. 
         [0011]    In the present invention, the first conductive pads and the second conductive pads can be bump pads or ball pads. While the first conductive pads are bump pads electrically connected to a chip, the second conductive pads in the other surface of the circuit built-up structure can be ball pads electrically connected to an electronic device such as a printed circuit board. On the other hand, while the first conductive pads are ball pads electrically connected to an electronic device such as a printed circuit board, the second conductive pads in the other surface of the circuit built-up structure can be bump pads electrically connected to a chip. 
         [0012]    In the method for manufacturing a coreless packaging substrate in the present invention, the metal adhesive layer is formed by electroplating or electroless plating. In addition, the metal adhesive layer is made of a metal having a melting point lower than that of the packaging substrate. Preferably, the metal can be Sn. Therefore, the metal adhesive layer can be removed preferably by thermomelting so as to be removed at the same time of removing the core board. 
         [0013]    In the processes for manufacturing a coreless packaging substrate in the present invention, the core board used preferably can be a copper clad laminate (CCL). 
         [0014]    The method for manufacturing a coreless packaging substrate in the present can further comprise forming a seed layer prior to form the metal pillars and the metal layer. The seed layer is mainly used as a conductive pathway of electric currents for follow-up processes, and can be made of a material selected from the group consisting of Cu, Sn, Ni, Cr, Ti, and Cu—Cr alloys. Herein, the seed layer is made by sputtering or electroless plating. 
         [0015]    In the method of the present invention for manufacturing a coreless packaging substrate, the metal pillars and the metal layer can be formed at the same time. In detail, a seed layer can be formed on the surface of the first solder mask layer and in the first openings. Subsequently, a patterned resist layer is formed on the first solder mask layer in order to expose the first openings. Then, electroplating is performed. Finally, the resist layer and the part of the seed layer covered by the resist layer are removed so that the metal pillars and the metal layer are formed at the same time. Besides, the metal pillars and the metal layer in the present invention can be preferably made of Cu. 
         [0016]    In the method of the present invention for manufacturing a coreless packaging substrate, the first openings in the first solder mask and the second openings in the second solder mask are formed preferably by photolithography process including exposing and developing. 
         [0017]    The circuit built-up structure of the present invention can comprise a dielectric layer, circuit layers disposed on the dielectric layer, and conductive vias formed in the dielectric layer. Besides, the circuit built-up structure of the present invention can be monolayer or multilayer. The circuit layers in the circuit built-up structure of the present invention, which also includes the second conductive pads formed from the circuit layers on the surface of the circuit built-up structure, and the conductive vias can be made of a material selected from the group consisting of Cu, Sn, Ni, Cr, Ti, and Cu—Cr alloys, but preferably is made of Cu. 
         [0018]    In conclusion, the present invention provides a solution to problems such as low circuit layout density, excessive circuit layers, long conductive lines and high impedance in a general packaging substrate having a core board. Additionally, the coreless packaging substrate of the present invention does not have through holes so as to achieve the purposes of advanced circuit layout density, reduced manufacture procedures, and decreased thickness of the packaging substrate. Therefore, the object of obtaining a lightweight and compact packaging substrate can be accomplished. 
         [0019]    Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIGS. 1A to 1E  are cross-sectional views of conventional packaging substrates; 
           [0021]      FIGS. 2A to 2E  show a flow chart for manufacturing a coreless packaging substrate in a cross-sectional view in a preferred example of the present invention; and 
           [0022]      FIGS. 3A to 3B  show part of a flow chart for manufacturing a coreless packaging substrate in a cross-sectional view in a preferred example of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    Because of the specific embodiments illustrating the practice of the present invention, a person having ordinary skill in the art can easily understand other advantages and efficiency of the present invention through the content disclosed therein. The present invention can also be practiced or applied by other variant embodiments. Many other possible modifications and variations of any detail in the present specification based on different outlooks and applications can be made without departing from the spirit of the invention. 
         [0024]    The drawings of the embodiments in the present invention are all simplified charts or views, and only reveal elements relative to the present invention. The elements revealed in the drawings are not necessarily aspects of the practice, and quantity and shape thereof are optionally designed. Further, the design aspect of the elements can be more complex. 
       Example 1 
       [0025]    With reference to  FIGS. 2A to 2E , there is shown a process flow for manufacturing a coreless packaging substrate in a cross-sectional view in the present example. 
         [0026]    As shown in  FIG. 2A , a core board  20  is provided first. In the present example, a copper clad laminate is used as the core board  20 . Then, a metal adhesive layer  21  is formed on the surface of the core board  20  by electroplating or electroless plating. The material of the metal adhesive layer  21  used in the present example is Sn. The melting point of Sn is at about 232° C., and that is lower than those of other materials used in the packaging substrate of the present example. Besides, the copper clad laminated used in the present example is beneficial to form the metal adhesive layer  21  thereon. 
         [0027]    Subsequently, a patterned first solder mask layer  22  is formed on the surface of the metal adhesive layer  21  as shown in  FIG. 2B . For example, the first solder mask layer  22  can be made of photoimagable polymer. A plurality of first openings are formed in the first solder mask layer by photolithography. Then, a seed layer (not shown) is formed by sputtering or electroless plating on the surface of the first solder mask layer  22  and in the first openings  221 . The seed layer can be made of a material selected from the group consisting of Cu, Sn, Ni, Cr, Ti, and Cu—Cr alloys, but preferably is made of Cu. Furthermore, a resist layer  23  is formed on the surface of the first solder mask layer  22 . A resist open area  231  corresponding to each of the first openings  221  is formed by photolithography. Herein, the resist layer  23  can be made of dry film or liquid photoresist. In the present example, dry film is used as the resist layer  23 . 
         [0028]    Further, a metal pillar  241  and a metal layer  242  are formed by electroplating or electroless plating respectively in each of the first openings  221  and in each of the resist open areas  231 . Then, the resist layer  23  and the part of the seed layer covered by the resist layer  23  are removed so that the structure as shown in  FIG. 2C  can be afforded. 
         [0029]    Furthermore, in  FIG. 2D , a circuit built-up structure  30  is formed on the surfaces of the metal layer  242  and the first solder mask layer  22 . The circuit built-up structure  30  comprises a dielectric layer  31 , circuit layers  32 , and conductive vias  33 . The circuit layers  32  are formed by photolithography of a resist layer (not shown) together with electroplating, and disposed on the dielectric layer  31 . The conductive vias  33  are formed in the dielectric layer  31  through forming vias (not shown) by laser ablation together with electroplating. Herein, the metal layer  242  is embedded in the dielectric layer  30  of the circuit built-up structure  30 . The conductive vias  33  can be electrically connected to the metal layer  242 . In addition, the circuit layers  32  and the conductive vias  33  can be made of a material selected from the group consisting of Cu, Sn, Ni, Cr, Ti, and Cu—Cr alloys. In the present example, Cu is used as the material of the circuit layers  32  and the conductive vias  33 . The dielectric layer  31  can be made of, for example, Ajinomoto Build-up Film (ABF). Subsequently, a second solder mask layer  25  is formed on the circuit built-up structure  30 . A plurality of second openings  251  are formed by photolithography on the second solder mask layer  25  so as to expose the circuit layers  32  of the circuit built-up structure  30 , and the exposed circuit layers  32  can serve as ball pads  51  which can be electrically connected to an electronic device such as printed circuit board. 
         [0030]    Finally, as shown in  FIG. 2E , the structure shown in  FIG. 2D  can be heated to melt the metal adhesive layer  21 . Due to the metal adhesive layer  21  having a melting point lower than those of the other materials used in the packaging substrate of the present example, the temperature can be raised to the point higher than the melting point of the metal adhesive layer  21  but lower than that being tolerated by the other materials in the packaging substrate so that the core board  20  can be removed after the metal adhesive layer  21  is melted. After that, chemical solutions can be used to clean and remove residues of the metal adhesive layer  21 . Surface treatment can be further performed on the metal pillars  241  to improve the performance of the packaging substrate. Posterior to removing the core board  20 , the metal pillars  241  of the circuit built-up structure formed in each of the first openings  221  can serve as a bump pad  41  capable of being electrically connected to a chip. Accordingly, the coreless packaging substrate of the present invention is manufactured. 
         [0031]    Conclusively, the coreless packaging substrate in the present example, as shown in  FIG. 2E , comprises: a circuit built-up structure  30 , a first solder mask layer  22 , and a second solder mask layer  25 . A plurality of metal layers  242  are embedded under one surface of the circuit built-up structure  30 , and a plurality of ball pads  51  are formed on the other surface of the circuit built-up structure  30 . The first solder mask layer  22  is disposed on the surface of the circuit built-up structure  30  having the metal layers  242 , which has a plurality of first openings  221  exposing part of the metal layers  242 . Each of the first openings  221  has a metal pillar  241  therein, and the respective metal pillar  241  serves as a bump pad  41 . The second solder mask layer  25  is disposed on the surface of the circuit built-up structure  30  having the bump pads  51 , which has a plurality of second openings  251  to expose the bump pads  51 . 
       Example 2 
       [0032]    With reference to  FIGS. 3A to 3B , there is shown a flow chart for manufacturing a coreless packaging substrate in a cross-sectional view in the present example. The manner of the present example is approximately similar to that of Example 1, but there are differences illustrated as follows. As shown in  FIG. 3A , the metal pillars  241  in the present example are used mainly for conduction to an electronic device such as printed circuit board in the follow-up processes. Positions exposed by the second openings  251  of the second solder mask layer  25  on the circuit layers  32  in the circuit built-up structure  30  are used for connection to a chip in the follow-up processes. Therefore, the first openings  221  formed in the first solder mask layer  22  are of the diameter larger than those of the second openings  251  formed in the second solder mask layer  25 . 
         [0033]    The subsequent steps are the same as those of the Example 1. After the core board  20  is removed in the present example, as shown in  FIG. 3B , each metal pillar  241  can serve as a ball pad  52  electrically connected to printed circuit board. Finally, the coreless packaging substrate of the present example can be afforded. 
         [0034]    Accordingly, the coreless packaging substrate of the present example is different from that of the Example 1, especially in that each metal pillar  241  embedded under one surface of the circuit built-up structure  30  serves as a ball pad  52  for conduction to printed circuit board, and the bump pads formed on the other surface of the circuit built-up structure  30  are electrically connected to a chip. 
         [0035]    In conclusion, the present invention provides a metal adhesive layer which has a melting point lower than that of the coreless packaging substrate thereof. This is why the core board adhered to the coreless packaging substrate of the present invention can be removed by using the above-mentioned property of the metal adhesive layer. Besides, in the circuit built-up structure of the packaging substrate in the present invention, the metal layers are embedded under one surface thereof. The solder mask layer is disposed on the surface having the metal layers and has a plurality of openings exposing part of the metal layers. Additionally, there is a metal pillar in each of the openings, and each metal pillar can serve as a bump or ball pad so as to be electrically connected to a chip or printed circuit board. Hence, in the present invention, not only the purposes (for an advance in circuit layout density and possession of a compact and light packaging substrate) can be achieved, but also the problems (such as large number of circuit layers and complexity of manufacturing processes) can be solved. 
         [0036]    Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the invention as hereinafter claimed.
A coreless packaging substrate and a method for making the same are disclosed in the present invention. The coreless packaging substrate is made by first providing a metal adhesion layer having a melting point lower than that of the substrate, and removing a core board connected with the substrate therefrom through melting the metal adhesion layer. In addition, the disclosed packaging substrate further includes a circuit built-up structure of which has the electrical pads embedded under a surface. The disclosed packaging substrate can achieve the purposes of reducing the thickness, increasing circuit layout density, and facilitating the manufacturing of the substrate.