Abstract:
The present invention relates to a method to fabricate a flip chip substrate structure, which comprises: providing a carrier; forming a patterned resist layer on the surface of the carrier; forming sequentially a first metal layer, an etching-stop layer, and a second metal layer; removing the resist layer, forming a patterned first solder mask, and then forming at least one first circuit build up structure thereon; forming additionally a patterned second solder mask on the circuit build up structure; respectively removing the carrier, the first metal layer, and the etching-stop layer; and forming solder bumps on both sides of the circuit build up structure. The method increases integration and achieves the purpose of miniaturization. The method solves the problem of circuit layer multiplicity and process complexity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flip chip structure, the method to manufacture the same and, more particularly, to a flip chip substrate structure that applies to without plated through hole sand improves circuit integration, and a method to manufacture flip chip substrates with a streamlined process. 
     2. Description of Related Art 
     With the development of the IT industry, the research in the industry is gradually turning to multifunctional and high performance electronic products. To meet the demands for high integration and miniaturization of semiconductor packaging, the circuit boards providing circuit connections among active and passive components are evolving from double layer circuit boards to multi-layer circuit boards in order to expand available layout areas on circuit boards within limited spaces by interlayer connection techniques, so as to accommodate high circuit layout density. 
     The semiconductor packaging structures known in the art are fabricated by adhering a semiconductor chip on the top of the substrate, proceeding with wire bonding or flip chip packaging, and then mounting solder balls on the back of the substrate for electrical connection. Though a high pin quantity can be obtained, operations at higher frequencies or speeds are restricted due to unduly long lead routes and consequent limited performance. Besides, multiple connection interfaces are required in conventional packaging, leading to increased process complexity. 
     In the method to manufacture flip chip substrates, the fabrication of a substrate begins with a core substrate, which is then subjected to drilling, electroplating, hole-plugging, and circuit formation to accomplish the internal structure. A multi-layer substrate is then obtained through circuit build up processes, as the method to fabricate circuit build up multi-layer circuit boards shown in  FIG. 1A  to  FIG. 1E . Referring to  FIG. 1A , a core substrate  11  is first prepared, which is composed of a core layer  111  having a predetermined thickness and circuit layers  112  formed on the surface thereof. Meanwhile, a plurality of plated through hole  113  are formed in the core layer  111  to electrically connect the circuit layers  112 . Referring to  FIG. 1B , the core substrate  11  is subjected to a circuit build up process so as to overlay a dielectric layer  12  on the surface of the core layer  11 , wherein the dielectric layer  12  has a plurality of vias  13  that the circuit layer  112  are exposed to the vias  13 . Referring to  FIG. 1C , a seed layer  14  is formed by electroless plating or sputtering on the dielectric layer  12 , wherein a patterned resist layer  15  is formed on the seed layer  14 , and plural openings  150  are formed in the resist layer  15  to expose the portions of seed layer that are set to be a patterned circuit layer. Referring to  FIG. 1D , a patterned circuit layer  16  and plural conductive vias  13   a  are formed in the openings of the resist layer by electroplating, the resist layer  15  and the portions of seed layer  14  covered therebeneath are removed, such that a first circuit build up structure  10   a  is formed. Referring to  FIG. 1E , a second circuit build up structure is formed on the outer surface of the first circuit build up structure in the same manner, repeating the same circuit build up procedures to form a multi-layered substrate. 
     However, the aforementioned process begins with a core substrate, which is subjected to drilling, electroplating, hole-plugging, and circuit formation to form the internal structure. Then a multi-layer substrate is formed through a circuit build up process. The method has problems such as low integration, multiple layers, long leads and high resistance, rendering it less applicable to high-frequency semiconductor packaging substrates. Due to its multiple layers, the process procedures are complex and the process cost is higher. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing disadvantages, the object of the present invention is to provide a method for fabrication of a flip chip substrate structure, which is performed by removing a carrier to increase integration and streamline process procedures. 
     Another object of the invention is to provide a method for fabrication of a flip chip substrate structure, which reduces the substrate thickness and achieves the purpose of miniaturization. 
     To achieve the above-mentioned objects, still another object of the present invention is to provide a method for fabrication of a flip chip substrate structure, the steps comprising: 
     Providing a carrier; forming a resist layer on the carrier, wherein plural first openings are formed in the resist layer. Then sequentially form a first metal layer, an etching-stop layer, and a second metal layer in the first openings of the resist layer. Subsequently, remove the resist layer. Then form a first solder mask on the carrier surface and the second metal layer, wherein plural second openings are formed on the first solder mask and the surface of the second metal layer, which correspond to the second metal mask. Then at least one circuit build up structure is formed on the surface of the first solder mask. Subsequently, form a second solder mask upon the circuit build up structure, wherein plural third openings are formed on the second solder mask to expose the portions of circuits in the circuit build up structure that are to be electrically conductive pads. The carrier, the first metal layer, and the etching-stop layer is then removed, to expose the second metal layer in the second openings of the first solder mask as the electrically conductive pads of the other side. Finally, plural solder bumps are formed on the electrically conductive pads on both sides of the circuit build up structure. 
     According to the method for fabricating the flip chip substrate structure of the present invention, the etching-stop layer is at least one selected from the group consisting of iron, nickel, chromium, titanium, aluminum, silver, tin, lead, and the alloys thereof. If metals that do not easily oxidize are used for the etching-stop layer, for example, electroless plating gold or electroplating gold, its removal is not needed and can proceed with subsequent process. Further, prior to formation of the metal that does not easily oxidize, i.e. using the electroplating gold as the etching-stop layer, a protection layer can be first formed before formation of the etching-stop layer, removal of electroplating gold is not required, and the subsequent process can be conducted. The material of the protection layer is preferably at least one selected from the group consisting of nickel, chromium, titanium, copper/chromium alloys, and tin/lead alloys. More preferably, it is nickel. 
     According to the method for fabricating the flip chip substrate of the present invention, metal posts can be firstly formed on the electrically conductive pads on both sides of the circuit build up structure before formation of the solder bumps. 
     According to the method to fabricate the flip chip substrate of the present invention, further comprising a holding element, which is mounted upon the contour of the second solder mask to prevent the substrate from warping. 
     According to the method to fabricate the flip chip substrate of the present invention, there is no particular limitation to the material of the carrier, such as ceramics, metal, organic or inorganic materials; preferably it is metal, more preferably copper. 
     According to the method to fabricate the flip chip substrate of the present invention, there is no particular limitation to the material of the resist layer, but it is preferably photosensitive polymers such as dry-film or liquid photo resist, more preferably dry-film. There is no particular limitation to the method for formation of the first openings, but preferably it is exposure and development. 
     According to the method to fabricate the flip chip substrate of the present invention, there is no particular limitation to the material of the first solder mask, but it is preferably a photosensitive material. Besides, there is no particular limitation to the method to form the second openings of the first solder mask, but it is preferably by exposure and development. 
     According to the method to fabricate the flip chip substrate of the present invention, the first metal layer, the etching-stop layer, and the second metal layer are preferably formed by electroplating or electroless plating. The materials of the first metal layer and the second metal layer can be identical or different; preferably at least one selected from the group consisting of copper, nickel, chromium, titanium, copper/chromium alloy, and tin/lead alloys, and more preferably, it is copper. 
     According to the method to fabricate the flip chip substrate of the present invention, the procedures to form the at least one circuit build up structure comprise: 
     Forming a dielectric layer on the surfaces of the second metal layer and the first solder mask, and forming plural fourth openings in the dielectric layer, wherein at least one of the fourth openings corresponds to the second metal layer; forming a seed layer on the surfaces of the dielectric layer and the fourth openings; forming a patterned resist layer on the seed layer, and forming plural resist layer openings therein, wherein at least one of the resist layer openings corresponds to the second metal layer; electroplating an electroplating metal layer in the plural resist layer openings; removing the plural resist layers and the seed layer covered therebeneath, such that a desirable multi-layered circuit build up structure is obtained through the above-mentioned steps. 
     According to the method to fabricate the flip chip substrate of the present invention, the dielectric layer in the aforementioned procedures is at least one selected from the group consisting of photosensitive or non-photosensitive organic resins such as ABF (Ajinomoto Circuit build up Film), BCB (Benzocyclo-buthene), LCP (Liquid Crystal Polymer), PI (Poly-imide), PPE (Poly(phenylene ether)), PTFE (Poly(tetra-fluoroethylene)), FR4, FR5, BT (Bismaleimide Triazine), Aramide, and mixtures of epoxy resins and glass fibers. The seed layer serves as the current conductive routes in the following electroplating process. When it is at least one selected from the group consisting of copper, tin, nickel, chromium, titanium, copper/chromium alloy, and tin/lead alloy, it is formed by the method of sputtering, vapor deposition or electroless plating (or called chemical deposition). When conductive polymers are employed to form the seed layer, the seed layer is formed by spin coating, ink-jet printing, screen printing, or imprinting, wherein the conductive polymers are at least one selected from the group consisting of polyacetylene, polyaniline, and organic sulfur polymers. There is no particular limitation to the electroplating metal layer, preferably it is copper, nickel, chromium, palladium, titanium, tin/lead or the alloys thereof; more preferably, it is copper. 
     According to the method to fabricate the flip chip substrate of the present invention, there is no particular limitation to the material of the second solder mask, but preferably it is a photosensitive material. There is no particular limitation to the method to form the third openings of the second solder mask, but it is preferably formed by exposure and development. 
     According to the method to fabricate the flip chip substrate of the present invention, the solder bumps are preferably formed by electroplating or printing. The solder bumps can be at least one selected from the group consisting of copper, tin, lead, silver, nickel, gold, platinum, palladium, and the alloys thereof. According to the aforementioned method to fabricate the flip chip substrate of the present invention, the solder bumps are preferably formed by sputtering, vapor deposition or electroless plating (or called chemical deposition); more preferably, it is formed by electroplating. There is no particular limitation to the material of the metal posts, but preferably it is selected from the group consisting of copper, nickel, chromium, titanium, copper/chromium alloy, and tin/lead alloys. More preferably, it is copper. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A to 1E  is the cross-section of a prior art flip chip substrate having a core layer; 
       FIG.  2 A to  2 O′ is the cross-section of a flip chip substrate of one preferred embodiment of the present invention; 
       FIG.  3 A to  3 O′ is the cross-section of a flip chip substrate of another preferred embodiment of the present invention; and 
       FIG.  4 A to  4 O′ is the cross-section of a flip chip substrate of still another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Example 1 
     Referring to FIGS.  2 A- 2 O′, which illustrate the cross-section of one embodiment of the flip chip substrate structure of the present invention. 
     First, as shown in  FIG. 2A , a carrier  201  is provided, which is a metal plate, preferably a copper plate. Then, as shown in  FIG. 2B , a first resist layer  202  is formed on the carrier  201 , wherein the material of the resist layer  202  can be a dry-film, and plural first openings  203  are formed in the resist layer  202  by exposure and development, as shown in  FIG. 2C . 
     As shown in  FIG. 2D , a first metal layer  204 , an etching-stop layer  205  and a second metal layer  206  are formed sequentially by electroplating or electroless plating, wherein the materials of the first metal layer  204  and the second metal layer  206  are copper, and the material of the etching-stop layer  205  is at least one selected from the group consisting of iron, nickel, chromium, titanium, aluminum, silver, tin, lead and the alloys thereof. 
     Then referring to  FIG. 2F , a first solder mask  207  is formed on the surface of the second metal layer  206  and the carrier  201 , wherein plural second openings  208  are formed by exposure and development in the first solder mask  207 , the plural second openings correspond to the second metal layer  206 . 
     Referring to  FIG. 2G , a dielectric layer  209  is laminated on the surfaces of the first solder mask  207  and the second metal layer  206 , wherein the dielectric layer  209  is at least one selected from the group consisting of: photo-sensitive and non-photo-sensitive organic resins such as ABF (Ajinomoto Circuit build up Film), BCB (Benzocyclo-buthene), LCP (Liquid Crystal Polymer), PI (Poly-imide), PPE (Poly(phenylene ether)), PTFE (Poly(tetra-fluoroethylene)), FR4, FR5, BT (Bismaleimide Triazine), and Aramide, or mixtures of epoxy resins and glass fibers. Plural fourth openings  210  are formed by means of laser drilling or exposure and development in the dielectric layer  209 , wherein at least one of the fourth openings  210  corresponds to the positions of the second metal layer  206 . Note that De-smear processes must be performed to remove the smears generated in the fourth openings when laser drilling is employed. 
     As shown in  FIG. 2H , a seed layer  211  is formed on the surface of the dielectric layer  209  and the fourth openings  210 , which serves as a current conducting route during electroplating and comprises one selected from the group consisting of copper, tin, nickel, chromium, titanium, copper-chromium alloy, and tin-lead alloy, formed by a approach selected from the group consisting of sputtering, vapor deposition and electroless plating (or called chemical deposition). Besides, the seed layer  211  can comprise conductive polymers, which are one selected from the group consisting of polyacetylene, polyaniline, and organic sulfur polymers, and the seed layer  211  is formed by means of spin coating, ink-jet printing, screen printing, or imprinting. 
     As shown in  FIG. 2I , a patterned resist layer  212  is formed on the seed layer  211 , which is used to form plural resist layer openings  213  by exposure and development, wherein at least one resist layer opening  213  corresponds to the positions of the second metal layer  206 . Referring to  FIG. 2J , the plural resist openings  213  are electroplated with an electroplating metal layer  214 , the electroplating metal layer  214  is most preferably copper. 
     As shown in  FIG. 2K , then the resist layer  212  and the seed layer  211  covered therebeneath are removed, and a circuit build up structure  215   a  is obtained. Referring to  FIG. 2L , a multiple-layer circuit build up structure  215  is obtained through the aforementioned procedures, and a second solder mask  216  is coated on the surface of the multiple-layer circuit build up structure  215 , and plural third openings  217  are formed by exposure and development in the second solder mask  216  to expose the portions of circuits of the circuit build up structure  215  to be the electrically conductive pads  218 . 
     Then, as shown in  FIG. 2M , the carrier  201 , the first metal layer  204 , and the etching-stop layer  205  are removed by etching to expose the second metal layer  206  that will serve as electrically conductive pads  218 ′ on the other side. 
     Further referring to  FIG. 2N , solder bumps  219  are formed directly on the electrically conductive pads  218  and  218 ′, and the method to form the solder bumps  219  can be electroplating or printing. Alternatively, as shown in FIG.  2 N′, if needed, metal posts  220  can be formed first by electroplating in the second openings  217  of the second solder mask  216 , metal posts  220 ′ can be formed by electroplating under the second metal layer  206 , and the material of the metal posts  220  and  220 ′ is copper; then, solder bumps  219 ′ are formed respectively on the metal posts  220  and  220 ′, the method to form the solder bumps  219 ′ can be electroplating or printing, and the material of the solder bumps  219  and  219 ′ is one selected from the group consisting of copper, tin, lead, silver, nickel, gold, platinum, and the alloys thereof. 
     Finally, as shown in FIGS.  2 O and  2 O′, a holding element  221  is mounted upon the contour of the second solder mask  216 , which is used to prevent the substrate from warping. 
     Example 2 
     Please refer to FIG.  3 A to  3 O′ to see the cross-section of another embodiment of the flip chip substrate structure of the present invention. 
     First, as shown in  FIG. 3A , a carrier  301  is provided, which is a metal plate, preferably copper. Then, as shown in  FIG. 3B , a resist layer  302  is formed on the carrier  301 , the material of the resist layer  302  is dry-film, and plural first openings  303  are formed by exposure and development in resist layer  302 , as shown in  FIG. 3C . 
     As shown in  FIG. 3D , a first metal layer  304 , an etching-stop layer  305  and a second metal layer  306  are formed sequentially by electroplating or electroless plating in the first openings  303 , wherein the material of the first metal layer  304  and the second metal layer  306  is copper, the material of the etching-stop layer is a metal that does not oxidize easily, most preferably gold, and the method of formation can be electroless plating. Then the resist layer  302  is removed, as shown in  FIG. 3E . 
     Further referring to  FIG. 3F , a first solder mask  307  is formed on the carrier  301  and the surface of the second metal layer  306 , and plural second openings  308  are formed in the first solder mask  307  by exposure and development, the second openings  308  correspond to the second metal layer  306 . 
     Subsequently, referring to  FIG. 3Q  a dielectric layer  309  is laminated on surface of the first solder mask  307  and the metal layer  306 , the material of the dielectric layer is identical to that of Example 1 and therefore is not set forth herein. Plural fourth openings  310  are formed by means of laser drilling or exposure and development in the dielectric layer  309 , wherein at least one of the fourth openings  310  corresponds to the positions of the second metal layer  306 . Note that De-smear processes must be performed to remove the smears generated in the fourth openings  310  when laser drilling is employed. 
     As shown in  FIG. 3H , a seed layer  311  is formed on the surface of the dielectric layer  309  and the fourth openings  310 , which serves as a current conducting route during electroplating and comprises one metal selected from the group consisting of copper, tin, nickel, chromium, titanium, copper-chromium alloys, and tin-lead alloys, formed by a approach selected from the group consisting of sputtering, vapor deposition, and electroless plating (or called chemical deposition). Besides, the seed layer  311  can comprise conductive polymers, which are one selected from the group consisting of polyacetylene, polyaniline, and organic sulfur polymers, and the seed layer  311  is formed by means of spin coating, ink-jet printing, screen printing, or imprinting. 
     Subsequently, as shown in  FIG. 3I , a patterned resist layer  312  is formed on the seed layer  311 , which is used to form plural resist layer openings  313  by exposure and development, wherein at least one resist layer opening  313  corresponds to the positions of the second metal layer  306 . Referring to  FIG. 3J , the plural resist openings  313  are electroplated with an electroplating metal layer  314 , the electroplating metal layer  314  can be copper. 
     Then as shown in  FIG. 3K , the resist layer  312  and the seed layer  311  covered therebeneath are removed, such that a circuit build up structure  315   a  is obtained. Referring to  FIG. 3L , a multi-layered circuit build up structure  315  is obtained, and a second solder mask  316  is coated on the surface of the multi-layered circuit build up structure  315 , and plural third openings  317  are formed in the second solder mask  316  by exposure and development to expose the portions of the circuit build up structure  315  circuits that will serve as electrically conductive pads  318 . 
     Then, as shown in  FIG. 3M , the carrier  301  and the first metal layer  304  are removed by etching to expose the etching-stop layer  305  that will serve as electrically conductive pads  318 ′ on the other side. 
     Further referring to  FIG. 3N , solder bumps  319  are formed directly on the electrically conductive pads  318  and  318 ′, and the method to form the solder bumps  319  can be electroplating or printing. Alternatively, as shown in FIG.  3 N′, if needed, metal posts  320  can be formed first by electroplating in the third openings  317  of the second solder mask  316 , metal posts  320 ′ are formed on the surface of the etching-stop layer  305 , and the material of the metal posts  320  and  320 ′ is copper; then, solder bumps  319 ′ are formed respectively on the metal posts  320  and  320 ′, the method to form the solder bumps  319 ′ can be electroplating or printing, and the material of the solder bumps  319 ,  319 ′ is one selected from the group consisting of copper, tin, lead, silver, nickel, gold, platinum, and the alloys thereof. 
     Finally, as shown in FIGS.  3 O and  3 O′, a holding element  321  is mounted upon the contour of the second solder mask  316 , which is used to prevent the substrate from warping. 
     Example 3 
     Please refer to FIGS.  4 A to  4 P′ to see the cross-section of still another embodiment of the flip chip substrate structure of the present invention. 
     First, as shown in  FIG. 4A , a carrier  401  is provided, which is a metal plate, preferably copper. Then, as shown in  FIG. 4B , a resist layer  402  is formed on the carrier  401 , the material of the resist layer  402  is dry-film, and plural first openings  403  are formed by exposure and development in resist layer  402 , as shown in  FIG. 4C . 
     As shown in  FIG. 4D , a first metal layer  404 , a protection layer  405 , an etching-stop layer  406  and a second metal layer  407  are formed sequentially by electroplating or electroless plating in the first openings  403 , wherein the material of the first metal layer  404  and the second metal layer  407  is copper, the material of the etching-stop layer is a metal that does not oxidize easily, most preferably gold, and the method of formation can be electroplating. However, copper dissolves in the gold electroplating solution, so nickel must be electroplated as the protection layer  405  to protect copper from dissolving. Then the resist layer  402  is removed, as shown in  FIG. 4E . 
     Further referring to  FIG. 4F , a first solder mask  408  is formed on the surface of the carrier  401  and the second metal layer  407 , and plural second openings  409  are formed in the first solder mask  408  by exposure and development, the second openings  409  correspond to the second metal layer  407 . 
     Subsequently, referring to  FIG. 4G , a dielectric layer  410  is laminated on the surface of the first solder mask  408  and the second metal layer  407 , the material of the dielectric layer is identical to that of Example 1 and therefore is not set forth herein. Plural fourth openings  411  are formed by means of laser drilling or exposure and development in the dielectric layer  410 , wherein at least one of the fourth openings  409  corresponds to the positions of the second metal layer  407 . Note that De-smear processes must be performed to remove the smears generated in the fourth openings  411  when laser drilling is employed. 
     As shown in  FIG. 4H , a seed layer  412  is formed on the surface of the dielectric layer  410  and the fourth openings  411 , which serves as a current conducting route during electroplating and comprises one metal selected from the group consisting of copper, tin, nickel, chromium, titanium, copper-chromium alloys, and tin-lead alloys, formed by an approach selected from the group consisting of sputtering, vapor deposition and electroless plating (or called chemical deposition). Besides, the seed layer  412  can comprise conductive polymers, which are one selected from the group consisting of polyacetylene, polyaniline, and organic sulfur polymers, and the seed layer  412  is formed by means of spin coating, ink-jet printing, screen printing, or imprinting. 
     Subsequently, as shown in  FIG. 4I , a patterned resist layer  413  is formed on the seed layer  412 , which is used to form plural resist layer openings  414  by exposure and development, wherein at least one resist layer opening  414  corresponds to the positions of the metal layer  407 . Referring to  FIG. 4J , the plural resist openings  414  are electroplated with an electroplating metal layer  415 , the electroplating metal layer  415  can be copper. 
     Then as shown in  FIG. 4K , the resist layer  413  and the seed layer  412  covered therebeneath are removed, such that a circuit build up structure  416   a  is obtained. Referring to  FIG. 4L , a multi-layered circuit build up structure  416  is obtained, and a second solder mask  417  is coated on the surface of the multi-layered circuit build up structure  416 , and plural third openings  418  are formed in the second solder mask  417  by exposure and development to expose the portions of the circuit build up structure  416  circuits that will serve as electrically conductive pads  419 . 
     Then, as shown in  FIG. 4M , the carrier  401 , the first metal layer  404  and the protection layer  405  are removed by etching to expose the etching-stop layer  406  that will serve as electrically conductive pads  419 ′ on the other side. 
     Further referring to  FIG. 4N , solder bumps  420  are formed directly on the electrically conductive pads  419  and  419 ′, and the method to form the solder bumps  419  can be electroplating or printing. Alternatively, as shown in FIG.  4 N′, if needed, metal posts  421  can be formed first by electroplating in the third openings  418  of the second solder mask  417 , metal posts  421 ′ are formed on the surface of the etching-stop layer  406 , and the material of the metal posts  421  and  421 ′ is copper; then, solder bumps  420 ′ are formed respectively on the metal posts  421  and  421 ′, the method to form the solder bumps  420 ′ can be electroplating or printing, and the material of the solder bumps  420 ′ is one selected from the group consisting of copper, tin, lead, silver, nickel, gold, platinum, and the alloys thereof. 
     Finally, as shown in FIGS.  4 O and  4 O′, a holding element  422  is mounted upon the contour of the second solder mask  417 , which is used to prevent the substrate from warping. 
     In sum, the present invention solves the problems of low integration, too many layers, long leads and high resistance in packaging substrate having core substrate known in the art. The non-through hole structure increases circuit integration, streamlines the process, reduces thickness and achieves the purpose of miniaturization. 
     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.