Patent Publication Number: US-2011058340-A1

Title: Method of forming a multilayer substrate core structure using sequential microvia laser drilling and substrate core structure formed according to the method

Description:
This is a Continuation Application of Ser. No. 11/769,852 filed Jun. 28, 2007, which is presently pending. 
    
    
     FIELD 
     Embodiments of the present invention relate generally to the field of multilayer substrate core structure fabrication, and, in particular, to methods for fabricating such a board by laser drilling microvias therein. 
     BACKGROUND 
     A multilayer substrate core structure (MPWB) may be conventionally fabricated by first providing a copper clad core. The copper clad core (CCL) may be a laminate that is copper clad on one or two sides depending on application needs. An example of such a fabrication process is shown in  FIGS. 1-8 . As seen in  FIG. 1 , a two sided CCL  101  is first provided including an insulating laminate  12  and top and bottom copper films  140  and  160 , respectively. As seen in  FIG. 2 , the top and bottom copper films  140  and  160  are pre-patterned according to pre-determined interconnect patterns to be provided onto the laminate  120 , such as by way of etching, to provide patterned copper films  150  and  170 . Thereafter, as seen in  FIG. 3 , dielectric layer, such as ABF layers  190  and  210  (Ajinomoto Build-Up Film), are laminated onto the patterned copper films  150  and  160 , and, as seen in  FIG. 4  to provide a first intermediate laminate  180 . As seen in  FIG. 4 , the first intermediate laminate  180  is then provided with through holes  201  by way of mechanical drilling and des-mearing to provide a second intermediate laminate  220 . The de-smearing involves using a desmear solution to process the board to dissolve and remove any smears caused by drilling. As seen in  FIG. 5 , the through-holes  201  and the top and bottom surfaces of the intermediate laminate  220  are then plated to provide a plated intermediate laminate  240  with plated through holes  260 . As seen in  FIG. 6 , the plated intermediate laminate  240  may be subjected thereafter to PTH plugging with a conductive material  250  such as copper to yield a plugged intermediate laminate  280 . In a next stage, as shown in  FIG. 7 , the plugged intermediate laminate  280  may be lid plated with a conductive material such as copper to provide lids  270  and  290  on a top and bottom surface thereof, the plating occurring on the top and bottom plating existing on laminate  280  of  FIG. 6  to yield a lid plated intermediate laminate  300 . Thereafter, the copper existing at the top and bottom surfaces of lid plated intermediate laminate  30  is patterned, such as by way of etching, to yield the wiring board  320  as shown in  FIG. 8 . 
     Prior art substrate are typically built on the base of a thick core (for example one having a thickness of about 0.7 mm (not including any build-up or conductive layers). The prior art core build up process can be lengthy. Taking a four layer core as an example, the macro process stages of a prior art fabrication process may include all of: core baking and cleaning, core copper patterning, copper roughening, ABF lamination, plated through hole drilling, desmear, copper plating, copper roughening, plated through hole plugging, surface flattening, copper plating, and finally, copper patterning. However, mechanical plated through hole drilling can be the most expensive single process in the fabrication of a prior art multilayer substrate core structure. The need for plugging as explained above can add more to the manufacturing costs according to the prior art. 
     Disadvantageously, substrate core structures for substrate core structures according to the prior art can be costly, and can carry high manufacturing costs as a result of the use of mechanical drilling technology. These costs can skyrocket where the substrate core structures are miniaturized and scaled for future applications. In addition, mechanical drilling is not suitable for producing holes smaller than about 150 microns. 
     The prior art fails to provide a cost-effective, expedient and reliable method of providing a multilayer substrate core structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-8  show stages of forming a substrate core structure according to the prior art; 
         FIGS. 9   a - 9   h  show stages of forming a substrate core structure according to a first embodiment; 
         FIGS. 10   a - 10   h  show stages of forming a substrate core structure according to a second embodiment; 
         FIG. 11  is a schematic view of an embodiment of a system incorporating a substrate core as shown in either of  FIG. 9   h  or  10   h.    
     
    
    
     For simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Where considered appropriate, reference numerals have been repeated among the drawings to indicate corresponding or analogous elements. 
     DETAILED DESCRIPTION 
     In the following detailed description, a method of fabricating a substrate core structure, such as a substrate core structure, a substrate core structure formed according to the method, and a system including the substrate core structure, are disclosed. Reference is made to the accompanying drawings within which are shown, by way of illustration, specific embodiments by which the present invention may be practiced. It is to be understood that other embodiments may exist and that other structural changes may be made without departing from the scope and spirit of the present invention. 
     The terms on, above, below, and adjacent as used herein refer to the position of one element relative to other elements. As such, a first element disposed on, above, or below a second element may be directly in contact with the second element or it may include one or more intervening elements. In addition, a first element disposed next to or adjacent a second element may be directly in contact with the second element or it may include one or more intervening elements. In addition, in the instant description, figures and/or elements may be referred to in the alternative. In such a case, for example where the description refers to Figs. X/Y showing an element A/B, what is meant is that Fig. X shows element A and Fig. Y shows element B. In addition, a “layer” as used herein may refer to a layer made of a single material, a layer made of a mixture of different components, a layer made of various sub-layers, each sub-layer also having the same definition of layer as set forth above. 
     Aspects of this and other embodiments will be discussed herein with respect to  FIGS. 9   a - 11  below.  FIGS. 9   a - 9   h  show stages for the fabrication of a multilayer substrate core structure according to a first method embodiment involving subtractive patterning of the conductive layers, and  FIGS. 10   a - 10   h  show stages for the fabrication of a multilayer substrate core structure according to a second method embodiment involving semi-additive patterning of the conductive layers, such as for FLS (fine line and space) routing.  FIG. 11  shows a system incorporated a multilayer substrate core structure according to an embodiment. The figures, however, should not be taken to be limiting, as it is intended for the purpose of explanation and understanding. 
     Referring to  FIGS. 9   a  and  10   a , method embodiments include providing a starting insulating layer  10 . The starting insulating layer may include any one of well known core insulating/dielectric materials, such as, for example, glass epoxy resin or bismaleimide-triazine (BT), or ABF. Preferably, the starting insulating layer comprises a fiber reinforced glass epoxy resin. According to one embodiment, as shown in  FIGS. 9   a  and  10   a , the starting insulating layer  10  may include an initial conductive layer  12  thereon, such as one made of copper, or one made of silver or nickel. In the shown embodiments of  FIGS. 9   a  and  10   a , the starting insulating layer  10  may be part of a conventional copper clad core or CCL  14 . In the first embodiment, the initial conductive layer  12  may, for example, have a thickness between about 50 and about 70 microns, whereas, in the second embodiment as shown in  FIG. 10   a , the initial conductive layer  12  may have a thickness between about 1 micron and about 2 microns. 
     Referring next to  FIGS. 9   b  and  10   b , embodiments include laser drilling a first set of via openings  14  through the starting insulating layer  10  as shown. In the shown embodiments, the vias extend to the conductive layer  12 . For laser drilling, a carbon dioxide gas laser beam, an ultraviolet laser beam or an excimer laser beam may be used. For example, an embodiment contemplates using a carbon dioxide laser on a glass fiber reinforced starting insulating layer having a power range between about 1 to about 10 mJ, and a pulse width between about 1 and about 100 ms. The determination of laser drilling parameters would be, among others, a function of the material being laser drilled, its thickness, and the dimensions of the via to be provided. 
     Referring next to  FIGS. 9   c  and  10   c , embodiments include filling the first set of via openings  14  with a conductive material  16  to provide a first set of conductive vias  18  as shown. According to a preferred embodiment, provision of the conductive material  16  may be effected by way of selective fast electroless plating. Preferably, the conductive material  16  includes copper, but it may also include nickel and/or silver. In the embodiment of  FIG. 10   c , the provision of the conductive material  16  may also be effected by way of selective fast electroless plating. As is well known, selective fast electroless copper plating may be achieved by using an electroless plating solution which includes a catalyst therein, that is, a substance present in an amount such that a substantially increased deposition rate of the material to be plated is obtained in comparison with a solution where the catalyst is not present, the catalyst further having the property of depositing itself only on the copper area of the initial conductive layer in order to allow the provision of selective fast electroless plating. 
     Referring next to  FIGS. 9   d  and  9   e  on the one hand, and to  FIGS. 10   d  and  10   e  on the other hand, method embodiments include providing a first patterned conductive layer  19  on one side of the starting insulating layer  10 , and a second patterned conductive layer  20  on another side of the starting insulating layer  10 . According to the first embodiment as shown in  FIGS. 9   d  and  9   e , providing the first patterned conductive layer includes patterning the initial conductive layer  12  by way of etching, and providing the second patterned conductive layer includes providing a second conductive layer  24  onto a side of the starting insulating layer  10  opposite the side including the initial conductive layer  12 , and then patterning the second conductive layer  24 , such as, for example, by way of etching. Preferably, the second conductive layer  24  is provided by laminating the second conductive layer  24  onto the starting insulating layer  10 . According to the second embodiment as shown in  FIGS. 10   d  and  10   e , providing the first patterned conductive layer  19  includes removing, such as, for example, by way of etching, the initial conductive layer  12  after filling the first set of via openings  14 , and then providing the first patterned conductive layer  19  and the second patterned conductive layer  20  by using a semi-additive process. Removal of the initial conductive layer  12  may be preferably effected using a quick-etch process as would be recognized by one skilled in the art. A semi-additive process is a well known process according to which, for example, a desmear treatment may be performed as necessary to roughen the surfaces of the starting insulating layer  10 , and thereafter electroless plating may be performed onto the starting insulating layer  10  to form an electroless plating film (not shown), such as a electroless copper plating film, on the starting insulating layer  10 . Then, a photoresist may be deposited on the electroless plating film, which photoresist may then be exposed to light and developed, whereby a resist pattern may be formed while leaving, on the starting insulating layer  10 , a non-mask region corresponding to the pattern of the first and/or second patterned conductive layer. By means of electroplating, the electroless plating film may be used as a seed layer to stack an electroplated film in the non-mask regions. The resist pattern may then be removed by etching, and thereafter the electroless plating film which was till then covered with the resist pattern, may be removed by etching. In this way, the first and second patterned conductive layers  19  and  20  may be formed according to the second embodiment as shown in  FIG. 10   e.    
     Referring next to  FIGS. 9   f  and  10   f , method embodiments include providing a supplemental insulating layer  26  onto the first patterned conductive layer  19 . In some embodiments, as shown in  FIGS. 9   f  and  10   f , an additional supplemental insulating layer  28  may be provided onto the second patterned conductive layer  20  as shown. A supplemental insulating layer according to embodiments may include the same material as the one used for the starting insulating layer as noted above. According to an embodiment, provision of a supplemental insulating layer according to embodiments may include laminating the supplemental insulating layer onto a corresponding patterned conductive layer. 
     Referring next to  FIGS. 9   g  and  10   g , method embodiments include providing a second set of conductive vias  30  through the first supplemental insulating layer  26 , and providing a supplemental patterned conductive layer  36  onto an exposed side  32  of the supplemental insulating layer  26 , where the second set of conductive vias  30  contact the first patterned conductive layer  19  on one side thereof, and the supplemental patterned conductive layer  36  at another side thereof. Providing the second set of conductive vias  30  may include laser drilling a second set of via openings  34  through the supplemental insulating layer  26 , the second set of via openings extending to the first patterned conductive layer. Thereafter, the second set of via openings  34  may be filled with a conductive material such as copper, silver and/or nickel, in order to provide the second set of conductive vias  30 . Laser drilling may be accomplished for example in the same manner as described above in relation to  FIGS. 9   b  and  10   b , and the filling of the conductive material into the second set of via openings may further be accomplished, for the embodiment of  FIG. 9   g , for example in the same manner as described above with respect to  FIG. 9   c  (by way of fast electroless plating), and for the embodiment of  FIG. 10   g , for example in the same manner as described above with respect to  FIG. 10   c  (by way of selective fast electroless plating). Provision of the supplemental patterned conductive layer  36  may be accomplished, for the embodiment of  FIG. 9   g , for example in the same manner as described above with respect to  FIG. 9   e  (by way of lamination of a conductive layer followed by etching of the same), and for the embodiment of  FIG. 10   g , for example in the same manner as described above with respect to  FIG. 10   e  (by using a semi-additive process). 
     Optionally, referring still to  FIGS. 9   g  and  10   g , according to a method embodiment, the supplemental insulating layer  26  is a first supplemental insulating layer, and the method embodiment includes providing a second supplemental insulating layer  28  onto the second patterned conductive layer  20 , providing a third set of conductive vias  38 , and providing a second supplemental patterned conductive layer  40  onto an exposed side  41  of the second supplemental insulating layer  28 . According to the latter method embodiment, the third set of conductive vias  38  contacts the second supplemental patterned conductive layer  28  at one side thereof, and the second patterned conductive layer  20  at another side thereof. Provision of the second supplemental insulating layer  28  may be effected according to an embodiment in the same manner as described for the first supplemental insulating layer  26  above with respect to  FIGS. 9   f  and  10   f . In addition, the provision of the third set of conductive vias  38  may be accomplished, for the embodiment of  FIG. 9   g , for example in the same manner as described above for the first set of conductive vias  18  with respect to  FIG. 9   c  (fast electroless plating), and for the embodiment of  FIG. 10   g , for example in the same manner as described for the first set of conductive vias  18  above with respect to  FIG. 10   c  (by way of selective fast electroless plating). 
     According to embodiments, the laser drilling of via openings results in laser drilled via openings which may exhibit, as shown in  FIGS. 9   h  and  10   h , a conical configuration, and further in eventual conductive vias which may extend sequentially in each given layer of a substrate core structure, as opposed to extending through a total thickness of the substrate core structure as in the case of prior art printed through holes. The above layer-by-layer or sequentially disposed configuration of conductive vias according to embodiments allows the provision of staggered vias as shown. 
     Although the substrate core structure structures shown in  FIGS. 9   h  and  10   h , respectively, show only two supplemental insulating layers, three sets of conductive vias, and four sets of patterned conductive layers, it is noted that embodiments are not so limited, and include within their ambit the provision of as many supplemental insulating layers, corresponding sets of conductive vias, and corresponding sets of patterned conductive layers as necessary in order to arrive at a desired substrate core structure structure. The provision of the various elements noted above, including the supplemental insulating layers, sets of conductive vias, and sets of patterned conductive layers may be effected as noted above either with respect to the first embodiment as shown in  FIGS. 9   a - 9   h , or with respect to the second embodiment as shown in  FIGS. 10   a - 10   h . In addition, although the second embodiment is described in relation to a preference of providing an initial conductive layer  12  onto the starting insulating layer  10 , the second embodiment is not so limited, and includes the shown process flow in  FIGS. 10   a - 10   h  without the initial conductive layer  12 . 
     Referring now to  FIGS. 9   h  and  10   h , a method embodiment may include subjecting a combination of the starting insulating layer  10 , the first set of conductive vias  18 , the first patterned conductive layer  19 , the second patterned conductive layer  20 , the one or more supplemental insulating layers  26  and  28 , the one or more additional sets of conductive vias (such as, for example, the second and third sets of conductive vias  30  and  38 ), and the one or more supplemental patterned conductive layers  36  and  40  to hot pressing in order to bond the conductive vias to pad sections of the patterned conductive layers. Hot pressing may be effected according to any one of well known methods for hot pressing, as would be recognized by one skilled in the art. Preferably, with increasing pressure, the temperature needed to form metallurgical bonding may be greatly reduced for embodiments, such as, for example, from about 400 degrees Celsius down to about 150 degrees Celsius to a range that the material of the starting insulating layer and supplemental insulating layers may endure. Preferably, the maximum temperature in hot pressing according to embodiments would not exceed about 260 degrees Celsius. 
     Advantageously, embodiments provide a method to enable building multilayer substrate core structures using laser drilled via openings optionally metallized by fast electroless metal plating. Embodiments address a new multilayer substrate core structures and a method of building the same in which the costly plated through hole structure is replaced with the low cost laser drilled microvias. Depending on the need for patterning fineness, two different method embodiments are proposed, as described above in relation to  FIGS. 9   a - 9   i  on the one hand, and to  FIGS. 10   a - 10   i  on the other hand. The first method embodiment as described by way of example in relation to  FIGS. 9   a - 9   i  correspond to subtractive patterning for thick conductive layers such as thick copper (for example copper having a thickness between about 50 and about 70 microns) and for moderate line and space features (for example, line and space features above about 30 microns). The second method embodiment as described by way of example in relation to  FIGS. 10   a - 10   i  corresponds to a semi additive patterning (SAP) process for thin conductive layers such as thin copper (e.g., a copper conductive layer having a thickness of less than about 2 microns) for fine line and space features (for example, line and space features less than about 30 microns). Embodiments effectively address among other things the problems of: (1) high cost prior art substrate core structures which use mechanical drilling technology by replacing the costly mechanically drilled plated through holes with low cost laser drilled microvias; (2) the need to laser drill through conductive layers such as copper by achieving low cost, shorter processing times, and high reliability of a laser drilled via core structure that obviates the need to laser drill through any of the conductive layers. An embodiment provides a lower cost method of fabrication than prior art methods by not only replacing the prior art plated through hole regime with laser drilled microvias, but also by reducing the core dielectric material thickness (by virtue of the generally reduced via dimensions and line and space features possible according to embodiments). In addition, laser drilling according to embodiments allows for higher starting insulating layer connection density as compared with prior art structures, owing to small via sizes and pitches, thus allowing for smaller sized vias and smaller pitches, in this way leading to an improved design and to scalable miniaturization at low cost. Laser drilling features high alignment accuracy (15 um POR), and through-put (up to about 2000 vias/sec), a wide range of possible via sizes (such as between about 50 microns and about 300 microns), and low cost (about 2 cents per one thousand vias). The combination of high alignment accuracy and small via size make possible via pitches as low as about 150 microns, those pitches being much less than typically plated through hole pitches of about 400 microns. Moreover, embodiments lead to substrate core structure substrate structures having potentially smaller form factors (by virtue of potentially smaller pitches, pad sizes, via dimensions), and a potentially smaller z-height (by virtue of potentially finer routing through the thickness of the insulating layers, which may lead to thinner insulating layers and/or the use of a smaller amount of insulating layers). Embodiments further make possible flexible core routing, to the extent that embodiments are not limited to vias which extend straight through the thickness of the totality of the insulating layers (as in the case of plated through holes of the prior art), but rather make possible a large number of possibilities with respect to through core routing configurations because the vias are provided in each of the insulating layers separately. The above advantageously leads to an improved substrate core structure design and potential performance benefits as compared with the prior art. In addition, via filling according to an embodiment using selective fast electroless copper plating on copper pads but not on dielectric areas may be advantageously enabled by a proper choice of catalysts so that only the copper area is seeded with the catalyst. While conventional electroless plating speeds, such as electroless copper plating speeds, can be slow (about 4 to about 5 microns/hour), high speed or fast electroless plating solutions, such as fast electroless copper plating solutions, can allow plating at a speed as high as 2 microns per hour. Additionally, according to an embodiment, the use of a hot press advantageously allows the formation of a reliable metallurgical bond between the vias (whether copper, nickel or silver) and the pad, such as a copper pad. 
     Referring to  FIG. 11 , there is illustrated one of many possible systems  900  in which embodiments of the present invention may be used. In one embodiment, the electronic assembly  1000  may include a substrate core structure structure, such as structure  100  of  FIG. 9   h  or structure  200  of  FIG. 10   h . Assembly  1000  may further include a microprocessor. In an alternate embodiment, the electronic assembly  1000  may include an application specific IC (ASIC). Integrated circuits found in chipsets (e.g., graphics, sound, and control chipsets) may also be packaged in accordance with embodiments of this invention. 
     For the embodiment depicted by  FIG. 11 , the system  900  may also include a main memory  1002 , a graphics processor  1004 , a mass storage device  1006 , and/or an input/output module  1008  coupled to each other by way of a bus  1010 , as shown. Examples of the memory  1002  include but are not limited to static random access memory (SRAM) and dynamic random access memory (DRAM). Examples of the mass storage device  1006  include but are not limited to a hard disk drive, a compact disk drive (CD), a digital versatile disk drive (DVD), and so forth. Examples of the input/output module  1008  include but are not limited to a keyboard, cursor control arrangements, a display, a network interface, and so forth. Examples of the bus  1010  include but are not limited to a peripheral control interface (PCI) bus, and Industry Standard Architecture (ISA) bus, and so forth. In various embodiments, the system  90  may be a wireless mobile phone, a personal digital assistant, a pocket PC, a tablet PC, a notebook PC, a desktop computer, a set-top box, a media-center PC, a DVD player, and a server. 
     The various embodiments described above have been presented by way of example and not by way of limitation. Having thus described in detail embodiments of the present invention, it is understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many variations thereof are possible without departing from the spirit or scope thereof.