Patent Publication Number: US-2004045656-A1

Title: Method of fabricating a ceramic substrate with a thermal conductive plug of a multi-chip package

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of Invention  
       [0002] The present invention relates generally to a method of fabricating a ceramic substrate with a thermal conductive plug of a multi-chip package. More particularly, the present invention relates to a method of improving the fabricating process of the thermal conductive plug in the ceramic substrate in order to increase performance of the ceramic substrate of a multi-chip package.  
       [0003] 2. Description of the Related Art  
       [0004] The demand of advanced electronic technology requires electronic products to be made lighter, thinner, faster, and smarter while simultaneously making them more friendly, powerful, reliable, robust, and less expensive. Thus, the trend for electronic packages is to develop highly-integrated packaging structures. Various techniques have been developed for having a large number of input/output points (I/O) in a device. The ceramic substrate is being widely utilized due to the desirable properties. Because the ceramic substrate has similar thermal expansion coefficient to the semiconductor device, thus a larger area of a chip can be used on the ceramic substrate and the detaching problem between the chip and the ceramic substrate can be prevented. Further, the ceramic substrate dose not absorb the moisture from the air, therefore, the ‘pop-corn’ effect and the peeling problem during the packaging can be effectively prevented.  
       [0005] Due to the complicated functions of the chip, a lot of heat dissipating problems are still required to be resolved. One of the methods of dissipating heat is to form a plurality of thermal conductive plugs under the die pad of the substrate. The heat generated from the chip will be conducted through the die pad and thermal conductive plugs to a ground plate located on the other side of the substrate. The ground plate is thermally conducted to other thermal conductive devices so that the heat generated from the chip can then be dissipated out to the external environment.  
       [0006] The conventional method of fabricating the thermal conductive plug inside the ceramic substrate is to perform the step of forming the thermal conductive plugs after cofiring the ceramic substrate. However, since the hardness of the ceramic substrate is very high, and the ceramic substrate is very brittle plus its measurement is very thick. Therefore, the only way to drill the thermal conductive holes in the ceramic substrate is by a laser method. A stencil printing method is carried out to fill the metal paste into the thermal conductive holes and followed by cofiring the metal paste so that thermal conductive plugs are formed in the thermal conductive holes.  
       [0007] From the above-mentioned fabrication, the formation of the thermal conductive holes on the ceramic substrate has to be by laser. However, in order to penetrate through the high hardness of thick ceramic substrate, a lot of energy is required. The high power of a laser machine is very expensive. On the other hand, the size of the metal conductive plug will be reduced after cofiring the metal paste in the thermal conductive holes. After cofiring, the size of the thermal conductive holes remains unchanged and the size of the thermal conductive plugs are shrunken, therefore, voids will be formed in the thermal conductive holes. The thermal conductivity will be inefficient and those voids will lead to produce rents in the ceramic substrate. The worst is that the thermal conductive plugs will even fall out from the holes. As a result, specific materials for the metal paste have to be chosen to use so that the volume will not shrink but rather expand during the cofiring process. However, the fabrication is then limited to the selection of the materials of the metal paste.  
       SUMMARY OF THE INVENTION  
       [0008] It is an object of the present invention provides a method of fabricating a ceramic substrate with a thermal conductive plug of a multi-chip package that can effectively reduce the production cost by using a punching method to form the thermal conductive holes.  
       [0009] It is another object of the present invention to provide an improved method of fabricating a thermal conductive plug inside a ceramic substrate of a multi-chip package that can simplify the whole fabricating process. The method of the invention not only allow the thermal conductive holes to be completely filled with the metal paste but it also allow the thermal conductive plugs to be fabricated simultaneously while the cofiring process is proceeding on the ceramic substrate.  
       [0010] To achieve the foregoing and other objects and in accordance with the purpose of the present invention. A plurality of green tapes is provided. A machine punching method is carried out to forming a plurality of conductive openings and thermal conductive openings on green tapes. A maximum width of the thermal conductive opening is approximately between 20 milli-inches to 40 milli-inches. A metal paste is filled into the conductive openings and the thermal conductive openings. A heating process is performed to stack the green tapes together, wherein the metal paste inside the conductive openings of every green tape is in contact with its neighboring metal paste within the conductive openings of the green tapes. The metal paste inside the thermal conductive openings of the green tapes is in contact with each neighboring metal paste inside the thermal conductive openings. Cofire those green tapes and the metal paste to form a pre-substrate. The pre-substrate comprises an insulating structure, a plurality of thermal conductive plugs and conductive plugs. The insulating structure is formed by cofiring the green tapes so that a plurality of conductive plugs are formed due to the cofiring on the metal paste in the conductive openings. A plurality of thermal conductive plugs is also formed by cofiring the metal paste inside the thermal conductive openings. The pre-substrate further comprises a top surface and a bottom surface. A sputtering deposition is performed to form a first metal film on the top surface of the pre-substrate and a second metal film on the bottom surface of the pre-substrate. A plurality of die pads and conductive traces are formed by patterning the first metal film. The die pads in contacted with the thermal conductive plugs and the conductive traces in contacted with the conductive plugs. A plurality of chips is adhered on the die pads and is electrically connected to the conductive traces.  
       [0011] In one preferred embodiment of the invention, a polishing method is carried out to planarize the top surface and the bottom surface of the pre-substrate after cofiring the green tapes and the metal paste. A flat insulating layer is formed on the top surface and bottom surface of the pre-substrate during the planarizating process. A plurality of openings are formed on the insulating layer to exposure the conductive plugs and the thermal conductive plugs.  
       [0012] The pre-substrate further comprises a plurality of aligning-mark holes, which are formed into cavity-shaped holes and are used to align a mask layer to the pre-substrate. A fabricating method of the aligning-mark holes includes forming a plurality of aligning hole on a predetermined number of green tapes while forming the conductive openings and the thermal conductive openings on the green tapes. The green tapes are aligned to each other according to the aligning holes on the predetermined number of green tapes and are stacked together with the green tapes without the aligning holes. After cofiring the green tapes and the metal paste a plurality of aligning-mark holes is formed from the aligning holes.  
       [0013] Therefore, the heat generated from the chip is conducted through the die pad to the thermal conductive plug then to the second metal film. From the second metal film, the heat is then dissipated out to the external environment from the second metal film. Since, the ceramic substrate can dissipate heat easily to the outside, thus, a powerful chip can be utilized in the invention.  
       [0014] The foregoing fabrication disclosed that the conductive openings, the thermal conductive openings and the aligning holes are formed by the machine punching method. Therefore, these features can be fabricated on the green tapes simultaneously by one punching process. Thus, the fabrication process is simplified and much faster plus less costly. Further, the conductive plug, the thermal conductive plugs and the aligning-mark hole can also be fabricated simultaneously in the cofiring process. Thus, the fabrication process of the ceramic substrate with the thermal conductive plug is simplified. In addition, since the metal paste is filled into the conductive opening and the thermal conductive openings before heating process. Therefore, the material selection of the metal paste is more flexible. The thickness of the ceramic substrate can be controlled in accordance with the layer number of the green tapes. The size of circuit traces of the circuit fabrication can be accurately controlled by the sputtering deposition of forming the metal film and the photographic etching process. Moreover, the present invention also provides an easy method to fabricate the aligning hole so that the aligning hole can be used to align the mask layer during the photographic etching process.  
       [0015] Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016] The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
     [0017] FIGS.  1 - 12  are magnified cross-sectional views of fabricating a ceramic substrate with a thermal conductive plug of a multi-chip package in accordance with a preferred embodiment of the present invention.  
     [0018]FIGS. 13 and 14 are magnified cross-sectional views of fabricating a ceramic substrate with a thermal conductive plug in accordance with a second preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0019] FIGS.  1 - 12  illustrate magnified cross-sectional views of fabricating a ceramic substrate with a thermal conductive plug in accordance with a preferred embodiment of the present invention. Referring to FIG. 1, a plurality of green tapes  110  are provided (only one of the green tapes  110  is shown in FIGS.  1 - 3 ). The green tape  110  is made of materials consisting of low temperature glass, ceramic power, a dispersant and an adhesion. The dispersant is a material used for preventing the low temperature glass and the ceramic power from agglomerating together. The adhesion is a material used for making the green tape  110  to be adhesive.  
     [0020] Referring to FIG. 2, a plurality of conductive openings  112  (only one is shown in the FIGS.  2 - 4 ) and a plurality of thermal conductive openings  114  are formed on the green tape  110  by utilizing a machine pouching method. A maximum width of the conductive opening  112  is approximately between 5 milli-inches to 15 milli-inches, and a maximum width of the thermal conductive openings  114  is about 20 milli-inches to 40 milli-inches. A stencil printing method is carried out to fill the conductive opening  112  and the thermal conductive openings  114  with a metal paste  120  so that a structure of the green tape  110  shown in FIG. 3 can be formed. The metal paste  120  is made of materials including a low temperature glass material, metal power, a dispersant and an adhesion. The dispersant is a material used for preventing the low temperature glass and the ceramic power from agglomerating together. The adhesion is a material used for making the green tape  120  to be adhesive. The metal power can be selected from a metal group consisting of silver or gold or copper.  
     [0021] A stacking process is performed to interlap the plurality of green tapes  110  through a heat pressing method so that the metal paste inside the conductive openings  112  of every green tape  110  can be in conducted with each other. The metal paste inside the thermal conductive openings  114  of each green tape  110  is also in conducted with each other. A cofiring process is carried out to heat up the green tapes  110  and the metal paste  120  at a predetermined temperature for a certain period so that organic materials inside the green tapes  110  and the metal paste  120  such as the dispersant, the adhesion are all vaporized. The green tapes  110  and the metal paste  120  are then heated up to a higher temperature in order to cofire and solidify the low temperature glass material inside the green tapes  110  and the metal paste  120 . Therefore, a structure shown in FIG. 5 is formed and is used as a pre-substrate  130 . The pre-substrate  130  further comprises an insulating structure  136 , a plurality of thermal conductive plugs  134  and a plurality of conductive plugs  132 . The insulating structure  136  is formed by cofiring the green tapes  110  and the thermal conductive plugs  134  are formed by cofiring the metal paste  120  inside the thermal conductive openings  114 . The conductive plugs  132  are also formed by cofiring the metal paste  120  inside the conductive openings  112 . The pre-substrate  130  has a top surface  131  and a corresponding back surface  133 .  
     [0022] On the other hand, the circuit traces and the formation of plugs can all be fabricated during the stencil printing process, after stacking and heating process, the metal circuit traces can then be formed between each green tape  110 .  
     [0023] Referring to FIG. 6, showing a schematic top view of a pre-substrate. A dotted-line region  135  indicates a location for a die pad (not shown). The thermal conductive plugs  134  will be formed within the dotted-line region  135 . The pre-substrate  130  can carry a plurality of chips (not shown) through the multiple number of die pads. The conductive plugs  132  are formed outside the dotted-line region  135 . A plurality of aligning-mark holes  137  are formed and located on a peripheral region of the pre-substrate  130 . A schematic cross-sectional vies of the aligning-mark hole  137  is shown in FIG. 7.  
     [0024] Referring to FIG. 7, the machine punching method that is used to fabricate the conductive openings and the thermal conductive openings on the green tapes is also used to form the plurality of aligning holes  139  onto a predetermined number of green tapes  110 . During the stacking process, those green tapes  110  with the aligning holes  139  are aligned according the aligning holes  139  and stacked together. Those green tapes  110  with aligning holes  139  are then stacked together with other green tapes  110  without the aligning holes  139 . After the heating and filling with the metal paste processes, the aligning holes  139  are formed into the aligning-mark holes  137  in the pre-substrate  130 . Since those aligning-mark holes  137  are not drilled through holes, thus cavity-shaped of these aligning-mark holes  137  are induced in the pre-substrate  130 . A diameter of the aligning-mark holes  137  is approximately between 10 milli-inches to 25 milli-inches.  
     [0025] A planarazing process, such as a polishing method, is performed to planarize the top surface  131  and the back surface of the pre-substrate  130 . A sputtering deposition or an evaporation method is used to form a first metal film  140  on the top surface  131  of the pre-substrate  130  and a second metal film  150  on the bottom surface  133  of the pre-substrate  130  as shown in FIG. 8. In FIG. 9, the first metal film  140  is patterned by utilizing a photographic etching method to form a plurality of die pads  144  (only one is shown) and conductive wires  142  (only one is shown). The die pads  144  are in contacted with the thermal conductive plugs  134  and the conductive wires  142  are in contacted with the conductive plugs  132 .  
     [0026]FIG. 10 illustrates a schematic view of aligning a pre-substrate to a peripheral region of an aligning-mark hole during a photographic etching process. During the photographic etching process, a spin-on method is used to form a photoresist layer  160  on the first metal film  140 . Then, an exposure process is carried out to a mask layer  170  consisting of a die pad pattern (not shown) and a conductive-trace pattern (not shown) so that the die pad pattern and the conductive trace pattern can be formed. The mask layer  170  is properly aligned with the pre-substrate  130  through aligning an aligning hole  172  of the mask layer  170  to the aligning mark hole  137  of the pre-substrate  130 . So that the patterns of the mask layer  170  can be accurately transferred to the photoresist layer  160 . A photographic etching process is carried out to remove portions of the photoresist layer  160  that are not for forming the die pads and the conductive wires. Thus, remaining portions of the photoresist layer  160  are then used for forming the die pads and the conductive wires. An etching process is performed to remove a portion of the first metal film  140  that is exposed outside the photoresist layer  160 . The remaining portions of the photoresist layer  160  are then removed to form a structure shown in FIG. 9. In the preferred embodiment of the present invention, the second metal film  150  can be used as a circuit connection for the ground plate. However, the second metal film  150  can also be patterned by the photographic etching method to form a plurality of conductive wires for a complex circuit on the substrate.  
     [0027] Referring to FIG. 11, an alternative process can be chosen to form a protective layer  180  on the first metal film  140  by a spin-on method in order to protect the conductive wires  142  of the first metal film  140 . A plurality of openings  182  can be formed on the protective layer  180  by utilizing the photographic etching method. So that the die pad  144  and nodes (not shown) are exposure. Therefore, the ceramic substrate  210  is completely fabricated.  
     [0028] Referring to FIG. 12, an adhesive material  200  is utilized to adhere back surfaces  192  of a plurality of chips  190  to the die pads  144 , however, only one chip  190  and one die pad  144  is shown in FIG. 12. A wire bonding process is carried out to electrically connect bonding pads  194  to the nodes. The heat generated from the chip  190  can be conducted through the die pad  144  to the thermal conductive plugs  134  then to the second metal film  150 . Finally, the ceramic substrate  210  dissipates the heat to an external environment through the second metal film  150 . Therefore, the heat generated from the chip  190  can be dissipated quickly so that a more powerful chip can be utilized.  
     [0029] According to the fabricating process of the ceramic substrate with the thermal conductive plug shown in FIGS.  1 - 12 , the conductive opening  112 , thermal conductive opening  114  and the aligning holes  139  are all formed by the machine punching method. Therefore, these features  112 ,  114  and  139  can be fabricated on the green tapes  110  simultaneously by one punching process. Thus, the fabrication process is simplified and faster plus less costly. Further, the conductive plug  132 , the thermal conductive plugs  134  and the aligning-mark hole  172  can also be fabricated simultaneously during the cofiring process. Thus, a simple fabrication process of the ceramic substrate with the thermal conductive plug is induced. Since the metal paste  120  is filled into the conductive opening  112  and the thermal conductive openings  114  before the cofiring process. Therefore, the selection of materials for the metal paste  120  is unrestricted. From the above-described fabrication, the thickness of the ceramic substrate  210  can be controlled in accordance with the layer number of the green tapes  110 . The size of circuit traces of the circuit fabrication can be accurately controlled by the sputtering deposition of forming the metal film and the photographic etching process. In addition, the present invention provides an easy method to fabricate the aligning hole  137  so that the aligning hole  137  can be used to align the mask layer  170  during the photographic etching process.  
     [0030] From the above-disclosed embodiment, the polishing method is used to planarize the surfaces of the substrate. However, the present invention is not limited to the polishing method. As a matter of fact, other methods can also be utilized to planarize the surfaces of the substrate.  
     [0031] Referring to FIGS. 13 and 14, showing magnified cross-sectional views of fabricating a ceramic substrate with a thermal conductive plug in accordance with a second preferred embodiment of the present invention. FIG. 13, after completed the pre-substrate fabrication, a spin-on method is used to form a flat first insulating layer  320  on the top surface  312  and a flat second insulating layer  330  on the bottom surface  314  of the pre-substrate  310 . A photography method is used to form number of openings  322 ,  332  respectively on the first insulating layer  320  and the second insulating layer  330  in order to exposure the conductive plug  340 , thermal conductive plug  350  and the aligning-mark hole. Referring to FIG. 14. A first metal film  360  and a second metal film  370  are formed. A plurality of conductive traces  362  and die pads  364  are formed by photographing on the first metal film  360  and a protective layer  380  is also formed. The chip  390  is adhered onto the die pad  364  and is electrically connected to the ceramic substrate  302 .  
     [0032] The fabrication of the ceramic substrate with the thermal conductive plug consists of the following advantages:  
     [0033] 1. The heat generated from the chip is conducted through the die pad to the thermal conductive plug then to the second metal film. From the second metal film, the heat is then dissipated out to the external environment from the second metal film. Since, the ceramic substrate can dissipate heat easily to the outside, therefore, a powerful chip can be utilized in the invention.  
     [0034] 2. The conductive openings, the thermal conductive openings and the aligning holes all are all formed by the machine punching method. Therefore, these features can be fabricated on the green tapes simultaneously by one punching process. Thus, the fabrication process is simplified and faster plus less costly.  
     [0035] 3. The conductive plug, the thermal conductive plugs and the aligning-mark hole can also be fabricated simultaneously during the cofiring process. Thus, a simple fabrication process of the ceramic substrate with the thermal conductive plug is induced.  
     [0036] 4. Since the metal paste is filled into the conductive opening and the thermal conductive openings before heating process. Therefore, the selection of materials for the metal paste is less restricted.  
     [0037] 5. The thickness of the ceramic substrate can be controlled in accordance with the number of the green tapes used.  
     [0038] 6. The size of circuit traces of the circuit fabrication can be accurately controlled by the sputtering deposition of forming the metal film and the photographic etching process.  
     [0039] 7. The present invention also provides an easy method to fabricate the aligning holes so that the aligning holes can be used to align the mask layer during the photographic etching process.  
     [0040] Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.