Patent Publication Number: US-11387186-B2

Title: Fan-out package with rabbet

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to and the benefit of U.S. Provisional Application No. 62/770,500, filed Nov. 21, 2018, entitled “FAN-OUT PACKAGE WITH RABBET”, the entire content of which is incorporated herein by reference. 
    
    
     FIELD 
     One or more aspects of embodiments according to the present disclosure relate to packaging, and more particularly to a fan-out package having a rabbet. 
     BACKGROUND 
     In applications in which an optoelectronic device is to be connected to an electronic integrated circuit, it may be advantageous, especially in high-speed applications, to position the parts close together so that the electrical path between them may be short. This may be challenging if, for example, the package of the electronic integrated circuit has a significantly larger envelope than the electronic integrated circuit die, and if there is potential mechanical interference between the optoelectronic device and the package of the electronic integrated circuit. 
     Thus, there is a need for an improved system and method for integrating an optoelectronic device and an electronic integrated circuit. 
     SUMMARY 
     According to an embodiment of the present invention, there is provided a system, including: a fan-out package, including: a first semiconductor die; a mold compound, covering the first semiconductor die on at least two sides; and an electrical contact, on a lower surface of the first semiconductor die, the fan-out package having a rabbet along a portion of a lower edge of the fan-out package. 
     In some embodiments, the vertical depth of the rabbet is between 10 and 500 microns. 
     In some embodiments, the horizontal depth of the rabbet is between 10 and 500 microns. 
     In some embodiments, the fan-out package further includes a redistribution layer, the redistribution layer being on a lower surface of the fan-out package. 
     In some embodiments, a portion of a vertical surface of the rabbet is an edge surface of the redistribution layer. 
     In some embodiments, the rabbet does not extend into the first semiconductor die. 
     In some embodiments, the system further includes: a second semiconductor die; and a shared support element, the second semiconductor die and the fan-out package both being secured to an upper surface of the shared support element. 
     In some embodiments, the system further includes an electrically conductive path between the first semiconductor die and the second semiconductor die, the electrically conductive path having a length less than 200 microns. 
     In some embodiments, the clearance between the second semiconductor die and the fan-out package is at least 2 microns. 
     In some embodiments, the clearance between the second semiconductor die and the fan-out package is at most 100 microns. 
     In some embodiments, an upper edge of the second semiconductor die extends into the rabbet. 
     In some embodiments, the system further includes a layer of underfill between the fan-out package and shared support element, the layer of underfill extending horizontally to the second semiconductor die. 
     In some embodiments, the underfill does not extend farther from the fan-out package than the part of the second semiconductor die most distant from the fan-out package. 
     According to an embodiment of the present invention, there is provided a method for fabricating a fan-out package, the method including: fabricating a carrier including: a layer of mold compound; a plurality of semiconductor dies, embedded in the mold compound; and a redistribution layer, on the semiconductor dies and the mold compound; cutting a first channel into the carrier, the first channel having a first width and a first depth, and extending between a first semiconductor die of the plurality of semiconductor dies and a second semiconductor die of the plurality of semiconductor dies; and cutting a second channel into the carrier, within the first channel, the second channel having a second width less than the first width and a second depth, from an upper surface of the carrier, greater than the first depth. 
     In some embodiments, the second channel has a depth equal to the thickness of the carrier and acts to separate a portion of the carrier on one side of the second channel from a portion of the carrier on the other side of the second channel. 
     In some embodiments, the first channel has a width exceeding a width of the second channel by between 30 and 100 microns. 
     In some embodiments, the first channel has a depth of between 30 and 100 microns. 
     In some embodiments, the method further includes: cutting a plurality of channels into the carrier to form a plurality of fan-out packages, the plurality of channels including the first channel and the second channel, forming a subassembly by: securing a first fan-out package, of the plurality of fan-out packages, to a shared support element, and securing a third semiconductor die to the shared support element, the subassembly including an electrically conductive path between the first semiconductor die and the third semiconductor die, the electrically conductive path having a length less than 200 microns. 
     In some embodiments, the method further includes: dispensing underfill between the first fan-out package and the shared support element, and damming, by the third semiconductor die, the underfill during the dispensing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present disclosure will be appreciated and understood with reference to the specification, claims, and appended drawings wherein: 
         FIG. 1  is a side cross sectional view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 2A  is a side cross sectional view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 2B  is a side cross sectional view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 2C  is a side cross sectional view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 3  is a side cross sectional view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 4A  is a top view of a carrier with a plurality of semiconductor dies, according to an embodiment of the present disclosure; 
         FIG. 4B  is a top view of a portion of a carrier with a plurality of semiconductor dies, according to an embodiment of the present disclosure; 
         FIG. 4C  is a bottom view of a fan-out package, according to an embodiment of the present disclosure; 
         FIG. 5A  is a side cross-sectional view of a carrier, according to an embodiment of the present disclosure; 
         FIG. 5B  is a side cross-sectional view of a carrier, according to an embodiment of the present disclosure; 
         FIG. 5C  is a side cross-sectional view of a carrier, according to an embodiment of the present disclosure; 
         FIG. 6A  is a perspective view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 6B  is a side cross sectional view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 6C  is top view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 7  is a perspective view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 8  is a perspective view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 9  is a side view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; 
         FIG. 10  is a side view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure; and 
         FIG. 11  is a perspective view of a fan-out package and a semiconductor die on a shared support element, according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a fan-out package with a rabbet provided in accordance with the present disclosure and is not intended to represent the only forms in which the present disclosure may be constructed or utilized. The description sets forth the features of the present disclosure in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the scope of the disclosure. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. 
     Referring to  FIG. 1 , in a subassembly including a fan-out package  105  (which may include a first semiconductor die  120 , e.g., a silicon CMOS chip) and a second semiconductor die  110  (e.g., an optoelectronic device, such as an electro-absorption modulator (EAM), a laser, or a photodetector) on a shared support element  115 , or “bottom wafer” (e.g., a substrate or another semiconductor die, such as a photonic integrated circuit (PIC)), it may be advantageous to minimize the length of an electrically conductive path (shown as an arrow in  FIG. 1 ) between the second semiconductor die  110  and the first semiconductor die  120  (e.g., a CMOS chip) in the fan-out package  105 . Such minimization may improve characteristic impedance, delay, electromagnetic interference (EMI), and electromagnetic compatibility (EMC). Allowing a significant gap to exist between the second semiconductor die  110  and the fan-out package  105 , as shown in  FIG. 2A , may be disadvantageous because it may result in a long electrically conductive path between the second semiconductor die  110  and the first semiconductor die  120  in the fan-out package  105 . 
     Moving one or both of the second semiconductor die  110  and the fan-out package  105  horizontally to reduce the separation between them may reduce the length of the electrically conductive path but (i) if the separation is made too small (e.g., less than 50 microns or comparable to the placement accuracy of the parts or the package size tolerance), a yield reduction may result, from occasional contact between the parts, and (ii) if adequate separation is maintained to avoid a yield reduction, the length of the electrically conductive path may remain undesirably large. The fan-out package  105  may be made to overhang the second semiconductor die  110  as shown in  FIG. 2C , but such a configuration may require significant thinning of second semiconductor die  110 , or tall copper pillar bumps  210 , or both, either of which may carry production challenges potentially resulting in a reduction in yield. In particular, high aspect ratio (&gt;1:1) copper pillar bumps may be challenging to fabricate, tall copper pillar bumps (e.g., taller than 100 microns) may require a pitch of more than 150 microns (whereas system requirements may dictate a pitch of 100 microns or less), thinning the second semiconductor die  110  to less than 50 microns may result in a yield reduction, and using copper pillar bumps that are less than 50 microns from the edge of the package may result in a yield reduction. Underfill (UF) overflow (or “bleed out”) (discussed in further detail below) may cause III-V performance degradation. 
     Referring to  FIG. 3 , in some embodiments, the fan-out package  105  includes, in addition to the first semiconductor die  120 , a layer of mold compound  305  (e.g., epoxy mold compound (EMC)) and a redistribution layer (RDL)  310 . A groove or rabbet  315  is cut into a lower edge of the fan-out package  105  to allow a portion of the fan-out package  105  to overhang the second semiconductor die  110  without using tall copper pillar bumps. As a result, a portion of the mold compound  305  may be cantilevered over the second semiconductor die  110  (i.e., it may overhang the second semiconductor die  110 ), as shown. A solder connection  320  may be used to secure each copper pillar bump (CPB) to metal patterns in the shared support element  115 . Similarly the EAM may be flip chip bonded to support element  115  using standard solder methods. The second semiconductor die  110  of the embodiment of  FIG. 3  may effectively act as an underfill dam (as discussed in further detail below) since it may be thicker than the die shown in  FIG. 2C , and it may also allow a smaller underfill keep out zone (KOZ), compared to the dies (the “EAM” dies) shown in  FIGS. 2A and 2B  since the second semiconductor die  110  of the embodiment of  FIG. 3  may be capable of damming the underfill just next to the rabbet. Improved damming of the underfill may reduce or prevent performance degradation of the second semiconductor die  110  due to bleedout. The edge of the keep out zone may be determined by the right edge of the second semiconductor die  110  in  FIG. 3 . 
     The mold compound  305  may include silica filler particles encased in and bound together with epoxy. The surfaces of the rabbet  315  may be rough, and may have characteristics depending on the process used to form the rabbet  315 . For example, if laser cutting is used, a rough surface over which portions of filler particle protrude above the surface of the epoxy may be formed, because the laser may have the effect of removing the epoxy while leaving the filler particles undisturbed, except that if the epoxy securing any of the filler particles is entirely or nearly entirely removed, the filler particle may also be removed. If a blade is used to form the rabbet, then it may cut through some of the filler particles, or it may tear some of the filler particles out of the epoxy, leaving voids in the epoxy at the surface of the rabbet. In the terminology used herein, the surfaces of the rabbet are defined as surfaces that define a volume into which no residual material of the mold compound  305  (i.e., neither epoxy nor silica filler) protrudes. 
       FIGS. 4A-C  show intermediate products and a final product, in the fabrication of a fan-out package  105 . Referring to  FIG. 4A , a plurality of first semiconductor dies  120  may be embedded in a sheet  405  of mold compound  305 . A redistribution layer (drawn transparent in  FIGS. 4A-4C , so that the first semiconductor dies  120  are visible) is deposited on the sheet  405  of mold compound  305  (and on the first semiconductor dies  120  embedded in it), to form a carrier  410 . In some embodiments, the carrier is square or rectangular instead of being round as shown. Referring to  FIG. 4B , a plurality of wide, shallow channels  415  are formed in the top surface of the carrier (e.g., using laser ablation or saw cuts), and a plurality of narrow, deep channels  420  are then formed (e.g., using a laser cut or a blade cut) in the wide, shallow channels  415 . The narrow, deep channels  420  may extend all the way through the carrier, serving to cut it apart into individual packages (or they may extend nearly all the way through, so that the packages may then be readily separated).  FIG. 4C  shows a resulting fan-out package  105 , with a region  425  of EMC that is cantilevered in the final assembly ( FIG. 3 ) and a region  430  that remains covered by the redistribution layer  310 . In this process what is referred to as the top surface of the carrier  410  becomes what is referred to as the bottom surface of the fan-out package  105 .  FIG. 5A  shows a side cross-sectional view of the carrier  410 ;  FIG. 5B  shows the carrier  410  after the wide, shallow channels  415  have been cut, and  FIG. 5C  shows the carrier  410  after the narrow, deep channels  420  have also been cut. In the embodiment of  FIGS. 5A-5C , the conductors on the top surface of the carrier  410  (which form conductors on the bottom surface of the fan-out package  105 , that are subsequently used to form connections to the shared support element  115 ) are micro bumps, which may be composed of, e.g., copper, solder, nickel, or gold. 
       FIG. 6A  shows a perspective view of a subassembly including a fan-out package  105  and a second semiconductor die  110  on a shared support element  115 , and  FIG. 6B  shows a side view of the subassembly with underfill (UF) dispensed into the gap between the fan-out package  105  and the shared support element  115 . The underfill material may be NCP (non-conductive paste) or NCF (non-conductive film). Thermal compression bonding (TCB) may be used to secure the fan-out package  105  to the shared support element  115 . The assembly process flow may include dispensing, e.g., non-conductive paste on the fan-out package  105  or on the shared support element  115 , placing the fan-out package  105  on the shared support element  115  (e.g., using a pick and place process), and bonding, using thermal compression bonding, copper pillars or gold stud bumps (that are on the bottom surface of the fan-out package  105 ) to the shared support element  115 . The thermal compression process may provide local heating that may also cure the non-conductive paste (or non-conductive film). 
     Referring to  FIG. 6C , the presence of second semiconductor dies  110  (labeled “DIE  1 ” in  FIG. 6C ) at the edge of (and extending into the rabbet  315  of) the fan-out package  105  (labeled “FOWLP” in  FIG. 6C ) may be advantageous in this process because the second semiconductor dies  110  may act as a dam to limit bleed out of the underfill. Initially, after the fan-out package  105  is placed on the shared support element  115  and before heat is applied (as part of the thermal compression process), the presence of the second semiconductor dies  110  may act as a dam to reduce the rate at which, e.g., the non-conductive paste flows out from under the fan-out package  105 . Moreover, once heat is applied and the temperature of the non-conductive paste increases, the viscosity of the non-conductive paste increases as it begins to cure, further slowing the rate at which it flows out from under the fan-out package  105  and spreads out away from the fan-out package  105 . In a system in which the second semiconductor dies  110  are more distant from the fan-out package  105  or absent entirely, the non-conductive paste may flow out from under the fan-out package  105  more rapidly before heat is applied, and the portion of the non-conductive paste that flows out from under the fan-out package  105  before heat is applied may remain relatively cool after heat is applied, causing its viscosity to remain lower and enabling it to spread over a greater area. The surface tension of the non-conductive paste may also accelerate the rate at which it spreads out on the shared support element  115 , if the top surface of the shared support element  115  is one that the non-conductive paste wets readily. In some embodiments, the underfill may extend no further away from the fan-out package  105  than the far edge of any of the second semiconductor dies  110 , as shown in  FIG. 6C , in which the stippled area is the underfill. Limiting the extent to which the underfill extends beyond the fan-out package  105  may be advantageous because the underfill, which may shrink when it cools, may adversely affect certain components that may be sensitive to the stress exerted by the underfill when it shrinks. 
       FIGS. 7-11  are additional perspective and side views of a subassembly including a fan-out package  105  and a second semiconductor die  110  on a shared support element  115 , in one embodiment. Each of  FIGS. 7-11  is drawn to scale, for one embodiment, with a scale bar in each of  FIGS. 7-11  indicating the scale used. It will be understood that the dimensions and relative dimensions shown in  FIGS. 7-11  (e.g., the vertical and horizontal depth of the rabbet, the clearance between the surfaces of the rabbet and the second semiconductor die  110 , and the like) may be varied, i.e., increased or decreased, by as much as a factor of two (i.e., changed by a factor having a value between 0.50 and 2.00) while preserving some or all of the benefits of the illustrated embodiments. 
     Some embodiments may enable high-speed signaling to 100 Gbps/ch and beyond, while mitigating current packaging technology challenges. Process challenges, such as thinning the second semiconductor die  110 , fabricating high aspect ratio CPBs, and high tolerance package dimensions, are reduced or mitigated. Utilizing the second semiconductor die  110  as an underfill dam in the package on wafer assembly process may result in an improvement of the component keep-out zone for UF bleed out during higher level assembly. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “under”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that such spatially relative terms are intended to encompass different orientations of the device in use or in operation, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” or “under” other elements or features would then be oriented “above” the other elements or features. Thus, the example terms “below” and “under” can encompass both an orientation of above and below. The device may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein should be interpreted accordingly. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. 
     Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of “1.0 to 10.0” or “between 1.0 and 10.0” is intended to include all subranges between (and including) the recited minimum value of 1.0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1.0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. 
     Although exemplary embodiments of a fan-out package with a rabbet have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that a fan-out package with a rabbet constructed according to principles of this disclosure may be embodied other than as specifically described herein.