Patent Publication Number: US-2012025362-A1

Title: Reinforced Wafer-Level Molding to Reduce Warpage

Description:
FIELD OF DISCLOSURE 
     This disclosure relates generally to electronic packaging, and in particular to a reinforcing material applied to a package for reducing warpage. 
     BACKGROUND 
     In electronic packaging, circuit devices in a chip are being manufactured smaller and lighter, but are required to perform greater functionality. These packages must also include a larger number of input and output connections. The semiconductor devices must also protect the chip from moisture and mechanical damage (e.g., cracking, warpage, etc.). As the chip performs more functions, however, greater power is consumed and more heat is generated. Also, as the size of the chip is reduced, the generated heat is required to dissipate from a smaller surface area. In a silicon chip, for example, it can be difficult to control the silicon surface and junction temperature. 
     In wafer level packaging, wafer warpage continues to be a concern. This is particularly true with thin dies or wafer stacking and assembly. Warpage can prevent successful assembly of a die-to-wafer stack because of the inability to maintain the coupling of the die and wafer. Also, during temperature cycling, thermal stresses can cause other mechanical defects such as cracking. One solution to the warpage problem is using a window wafer-based die-to-wafer process. The drawback to this solution is that the process can be very long and it is difficult to manufacture these packages at a high volume. Another possible solution is using a low coefficient of thermal expansion mold compound material between the die and wafer. However, there continue to be reliability issues with this type of wafer-level package such that it too cannot be produced at high volumes. 
     Therefore, it would be desirable to develop an electronic package and method of manufacturing the package which could overcome the warpage and mechanical defects of conventional electronic packages. In addition, it would be desirable for the electronic package to be manufactured at high volumes while also reducing the reliability concerns of conventional packages. 
     SUMMARY 
     For a more complete understanding of the present disclosure, reference is now made to the following detailed description and the accompanying drawings. In an exemplary embodiment, a method for forming an electronic package is provided. The method includes providing a wafer and coupling a die to the wafer. A mold compound material is applied to the wafer such that the mold compound material surrounds the die. The method further includes applying a reinforcing material to the mold compound material. The mold compound material is therefore disposed between the wafer and the reinforcing material. The reinforcing material can be glass and include mechanical properties similar to the wafer. 
     In one form of the method, the die is coupled to the wafer before applying the mold compound material. The wafer can also be mounted to a carrier and frontside bumping can be applied before mounting the wafer. After the wafer is mounted to the carrier, the wafer is later removed from the wafer. In doing so, the reinforcing material is applied before the wafer is removed from the carrier. The frontside bumping can be applied, however, after the wafer is removed from the carrier. 
     In another form of the method, a portion of the reinforcing material can be roughened before applying the reinforcing material. The thickness of the reinforcing material can be approximately between the thickness of the wafer and the combined thickness of the wafer, the die, and the mold compound material. 
     The reinforcing material can be prefabricated and then applied to the mold compound material. In addition, the reinforcing material can be applied to the mold compound material and then cured. The reinforcing material can be cured by using an adhesive material. In a different form of the present embodiment, a piston can be used for applying the reinforcing material to the mold compound material. A vacuum can also be used to apply the reinforcing material to the mold compound material. When coupling the die to the wafer, the die can comprise a plurality of die. 
     In another embodiment, an electronic package includes a stack of semiconductor die and a mold compound material for supporting the stack. The mold compound material can surround the stack. The package also can include a reinforcing material that has similar mechanical properties to the stack of semiconductor die. The mold compound material is disposed between the stack and the reinforcing material. The reinforcing material can be glass or silicon, and it can be applied non-continuously or as a layer. In addition, at least a portion of the reinforcing material has a roughened surface. The roughened surface can be disposed in contact with the mold compound material. Also, flip chip bumping can be completed on the front side of the stack of semiconductor die. 
     In a different embodiment, an electronic package assembly includes a stack of semiconductor die, a means for reducing warpage in the assembly, and a means for supporting the stack. The means for reducing has similar mechanical properties as the stack and the means for supporting surrounds the stack. The means for supporting is also disposed between the stack and the means for reducing. 
     In one form of this embodiment, the means for reducing can be glass or silicon. Also, at least a portion of the means for reducing can have a roughened surface. The roughened surface can be in contact with the means for supporting. In another form, the means for supporting can be an epoxy-based or silicon-based material. In addition, flip chip bumping can be completed on the front side of the stack of semiconductor die. This embodiment of the package can be incorporated into a device selected from a group consisting of a music player, a video player, an entertainment unit, a navigation device, a communications device, a personal digital assistant (PDA), a fixed location data unit, and a computer. 
     In another exemplary embodiment, a method of fabricating a reinforced electronic package is provided. The method includes providing a stack of semiconductor die and a reinforcing material. A mold compound material is applied to the stack such that the mold compound material surrounds a portion of the stack. The method further includes a step for reducing warpage in the package. The mold compound material can be disposed between the stack and the reinforcing material. In addition, the method includes mounting the stack to a carrier and applying frontside bumping after the stack is removed from the carrier. The stack can be removed from the carrier after the step for reducing warpage. In addition, a portion of the reinforcing material can be roughened. 
     In the above-described embodiments, a balanced system is achieved which reduces warpage. In at least one of the above-described embodiments, warpage can be reduced by 50-90% based on various glass thicknesses, mold compound material thicknesses, and glass properties. In addition, the above-described embodiments are more reliable and can be manufactured at high volume more easily than conventional packages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram of a first embodiment for forming an electronic package with improved reliability; 
         FIG. 2  is a schematic view of a wafer; 
         FIG. 3  is a schematic view of the wafer of  FIG. 1  mounted on a carrier; 
         FIG. 4  is a schematic view of a plurality of die coupled to the wafer of  FIG. 1 ; 
         FIG. 5  is a schematic view of a reinforcing material being coupled to a mold compound material and the wafer of  FIG. 1 ; 
         FIG. 6  is a schematic view of the electronic package after carrier demount; 
         FIG. 7  is a flow diagram of a second embodiment for forming an electronic package with improved reliability; 
         FIG. 8  is a schematic view of a wafer; 
         FIG. 9  is a schematic view of the wafer of  FIG. 8  mounted on a carrier; 
         FIG. 10  is a schematic view of a plurality of die coupled to the wafer of  FIG. 8 ; 
         FIG. 11  is a schematic view of a reinforcing material being coupled to a mold compound material and the wafer of  FIG. 8 ; 
         FIG. 12  is a schematic view of the electronic package after carrier demount; 
         FIG. 13  is a schematic view of the electronic package of  FIG. 12  including flip chip bumps; and 
         FIG. 14  is a block diagram showing an exemplary wireless communication system in which it may be advantageous to use a package having a reinforcing layer. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention is shown in  FIGS. 2-6  and an exemplary method for making such an embodiment is shown in  FIG. 1 . In this embodiment, a method  100  ( FIG. 1 ) is provided for forming an electronic package with improved reliability. The design of the electronic package includes increased stiffness to reduce warpage and other mechanical stresses. In block  102  of the method  100 , a wafer  202  is provided. The wafer  202  ( FIG. 2 ) can be formed of silicon, for example, or other known wafer materials. The wafer  202 , which can also be referred to as the Tier  1  wafer, can include front-end-of-the-line (FEOL) and back-end-of-the-line (BEOL) processing. Through-silicon vias, for example, can also be fabricated in the wafer  202 . 
     Referring to  FIG. 3  and block  104  of method  100 , solder bumps  306  can be formed on the frontside of the wafer  202 . The solder bumps  306 , which can be any conductive material, is used for coupling the package to a substrate, chip, or other device. Once the frontside bumping is achieved, an adhesive material  304  can be applied to the frontside of the wafer between the bumps  306 . The wafer  202  can then be mounted to a carrier  302 , such as a tape carrier, for further assembly and processing. The carrier  302  can provide sufficient stiffness to the wafer  202  for additional assembly processing. This can be important particularly when the wafer undergoes a thinning process. When a thin die, e.g., a die thickness of approximately 50 μm, is formed from the wafer, the die can bend and/or sustain damage due to material stress from various assembly processes. Thus, the carrier  302  provides improved stiffness and rigidity to the thin die, i.e. wafer  202 , during assembly processing. 
     Other assembly processes can include u-bumping, flip chip bumping, and backside processing. In block  106  of method  100  and  FIG. 4 , for example, a die  402  is coupled to the wafer  202 . An optional underfill layer  308  can be disposed between the die  402  and wafer  202  to enhance the reliability of the package. A plurality of u-bumps  404  couple the die  402  to the wafer  202 . The die  402  can be referred to as a Tier  2  die. As shown, there can be a plurality of Tier  2  dies  402  coupled to the wafer  202  through a plurality of u-bumps  404 . And, although not shown, additional dies can be stacked to achieve a desired package. 
     Referring to block  108  of method  100 , a wafer-level molding process is performed. A mold compound  502 , which can be, for example, an epoxy-based material, is applied to the package such that the plurality of dies  402  is substantially surrounded by the mold compound  502  and the backside of the wafer  202  is substantially covered by the mold compound  502 . The mold compound  502  provides rigidity to the package, but it is unable to provide sufficient stiffness required to prevent or significantly reduce warpage. Even applying a thicker layer of mold compound  502  is undesirable because it increases the overall height of the package and can be expensive. Thus, the thickness of the mold compound  502  is applied such that the mold compound  502  surrounds the plurality of dies  402  and covers the backside of the wafer  202 . 
     The mold compound  502  can be applied in several ways. In one embodiment, the mold compound  502  can be dispensed onto the backside of the wafer, spun in a circular motion to spread the mold compound  502  evenly, and then compressed. In the embodiment of block  110  and  FIG. 5 , the mold compound  502  is liquid-based and compressed using a piston  506 . Once the mold compound  502  is compressed to a desired thickness, it is cured at a curing temperature. The curing temperature can be between 150-250° C., for example. Depending on the type of mold compound, the curing temperature may vary. 
     Once the mold compound cures, a reinforcing material  504  can be applied to the mold compound  502 . In block  112 , a thin reinforcing material  504  is disposed on the backside of the mold compound  502 . The reinforcing material  504  can be glass, for example, or any other material that has similar mechanical and thermal properties as the wafer  202 . Since glass and silicon have similar properties such as stiffness and coefficient of thermal expansion (CTE), the two materials can advantageously balance or reduce stresses and warpage during assembly and use. 
     It is desirable for the reinforcing material  504  to have a small thickness so that the overall package height is minimal. The reinforcing layer  504  can be applied as a continuous or non-continuous layer on the mold compound  502 . Advantageously, the mold compound  502  can have a thickness between 100-400 μm and the reinforcing material  504  can have a thickness of up to about 400 These thicknesses are non-limiting, however, and different embodiments of an electronic package can include a mold compound  502  and reinforcing material  504  having thicknesses outside of these ranges. 
     The reinforcing material  504  can be prefabricated or applied to the mold compound and then cured. As described above, the mold compound  504  can be cured before the reinforcing layer  504  is applied. However, in an alternative embodiment, the prefabricated reinforcing material  504  can be used for compressing the mold compound  502 . For example, once the mold compound  502  is applied to the backside of the wafer  202 , the prefabricated reinforcing material  504  can compress the mold compound  502  to its desired thickness. In this example, once the mold compound  502  is compressed, it cures at the curing temperature in block  114  of method  100 . 
     The prefabricated reinforcing material  504  can be used solely or in combination with the piston  506  for compressing the mold compound. When used in combination, a vacuum can be created through channels  508  ( FIG. 5 ) defined in the piston  506  for coupling the reinforcing material  504  to the piston  506 . The combined reinforcing material  504  and piston  506  can apply a uniform amount of pressure to the mold compound  502  to achieve a desired mold compound thickness. In addition, the piston  506  provides support to the thin reinforcing material  504  during the compression process. 
     Once the mold compound  502  is compressed to the desired thickness, the mold compound  502  couples to the reinforcing material  504  and the vacuum pressure is released. The piston  506  can be removed from the reinforcing material  504  and the assembly is cured in block  114 . In block  116  of the method  100  and  FIG. 6 , the carrier  302  is removed from the wafer  202  and the wafer can be diced. The bumps  306  on the frontside of the wafer  202  can now be coupled to a chip or substrate. 
     In block  112 , the coupling of the mold compound  502  and reinforcing material  504  can be aided or promoted by a surface roughening process. The surface of the reinforcing material  504 , for example, can be roughened before coupling the mold compound  502  and reinforcing material  504 . There are several roughening processes or techniques that can be used. For example, the roughening process can be achieved by a dry or wet etching process. Plasma bombardment or etching is one form of a dry etching process that can be used. 
     Referring to  FIG. 7 , another exemplary embodiment of a method  700  is provided for forming an electronic package with improved reliability. Similar to the embodiment of  FIG. 1 , the method  700  fabricates the electronic package with increased stiffness to reduce warpage and other mechanical stresses. In block  702  of the method  700 , a wafer  802  is provided. The wafer  802  ( FIG. 8 ) can be formed of silicon, for example, or other known wafer materials. The wafer  802 , or Tier  1  wafer, can include front-end-of-the-line (FEOL) and back-end-of-the-line (BEOL) processing. Similar to the wafer  202  in  FIG. 2 , through-silicon vias can also be fabricated in the wafer  802 . 
     In  FIG. 9  and block  704  of method  700 , an adhesive material  904  can be applied to the frontside of the wafer  802 . The wafer  802  can then be mounted to a carrier  902 , such as tape, for further assembly processing. The carrier  902  provides stiffness and rigidity to the wafer  802  during the assembly process. As described above, this is important when the wafer undergoes a thinning process. When a thin wafer, e.g., a wafer thickness of approximately 50 μm, is formed, the wafer  802  can bend and/or sustain structural damage due to material stresses from various assembly processes. Thus, the carrier  902  provides protection to the thin wafer  202  during the assembly process. 
     Other assembly processes can include u-bumping and backside processing. In block  706  of method  700  and  FIG. 10 , for example, a die  1002  is coupled to the wafer  802 . Also, an optional underfill layer  1006  can be disposed between the die  1002  and wafer  802  to enhance the reliability of the package. A plurality of u-bumps  1004  couple the die  1002  to the wafer  802 . The die  1002  can be referred to as a Tier  2  die. As shown, there can be a plurality of Tier  2  dies  1002  coupled to the wafer  802  by the plurality of bumps  1004 . And, although not shown, additional dies can be stacked to achieve a desired package. 
     Referring to block  708  of method  700 , a wafer-level molding process similar to that described above in block  108  is completed. A mold compound  1102 , which can be an epoxy-based material, for example, is applied to the wafer  802  such that the plurality of dies  1002  is surrounded by the mold compound  1102 . The mold compound  1102  can provide stability to the thin wafer  802 , but it is unable to provide sufficient stiffness required to prevent or significantly reduce warpage. As described above, applying a thicker layer of mold compound  1102  is undesirable because it increases the overall height of the package and can be expensive. Thus, the mold compound  1102  is applied to a desired thickness such that the mold compound  1102  surrounds the plurality of dies  1002  and covers the backside of the wafer  802 . 
     The mold compound  1102  can be applied in several ways. In one embodiment, the mold compound  1102  can be dispensed onto the backside of the wafer  802 , spun in a desired motion to spread the mold compound  1102  evenly, and then compressed. In the embodiment of block  710  and  FIG. 11 , the mold compound  1102  is liquid-based and compressed using a piston  1106 . Once the mold compound  1102  is compressed to a desired thickness, it is cured at a curing temperature. The curing temperature can be between 150-250° C., for example. Depending on the type of mold compound  1102 , the curing temperature may vary. 
     In this embodiment, before the mold compound  1102  cures a reinforcing material  1104  can be applied to the mold compound  1102 . In block  712 , a thin reinforcing material  1104  is disposed on the backside of the mold compound  1102 . The reinforcing material  1104  can be glass, for example, or any other material that has similar mechanical and thermal properties as the wafer  802 . Since glass and silicon have similar mechanical and thermal properties such as stiffness and coefficient of thermal expansion (CTE), the two materials can advantageously balance or reduce stresses and warpage during assembly and use. In addition, it is desirable for the reinforcing material  1104  to have a small thickness so that the overall package height is minimal. 
     In another embodiment, the reinforcing material  1104  can be prefabricated, applied to the mold compound  1102 , and then the mold compound  1102  is cured. In this embodiment, the prefabricated reinforcing material  1104  can be used for compressing the mold compound  1102 . For example, once the mold compound  1102  is applied to the backside of the wafer  802 , the prefabricated reinforcing material  1104  can compress the mold compound  1102  to its desired thickness. In this example, once the mold compound  1102  is compressed, it cures at the curing temperature in block  714  of method  700 . 
     The prefabricated reinforcing material  1104  can be used solely or in combination with the piston  1106  for compressing the mold compound  1102 . When used in combination, a vacuum can be created through channels  1108  ( FIG. 11 ) defined in the piston  1106  for coupling the reinforcing material  1104  to the piston  1106 . As described above, the combined reinforcing material  1104  and piston  1106  can apply a uniform amount of pressure to the mold compound  1102  to achieve a desired mold compound thickness. In addition, the piston  1106  provides support to the thin reinforcing material  1104  during the compression process. 
     Once the mold compound  1102  is compressed to the desired thickness, the mold compound  1102  couples to the reinforcing material  1104  and the vacuum pressure is released. The piston  1106  can be removed from the reinforcing material  1104  and the assembly is cured in block  714 . 
     In block  712 , the coupling of the mold compound  1102  and reinforcing material  1104  can be aided or promoted by a surface roughening process. The surface of the reinforcing material  1104 , for example, can be roughened before coupling the mold compound  1102  and reinforcing material  1104 . There are several roughening processes or techniques that can be used. For example, the roughening process can be achieved by a dry or wet etching process. Plasma bombardment or etching is one form of a dry etching process that can be used. 
     In block  716  of the method  700  and  FIG. 12 , the carrier  902  is removed from the wafer  802 . Before the wafer  802  is diced, flip-chip bumps  1302  can be fabricated on the frontside of the wafer  802  in block  718 . As shown in  FIG. 13 , the bumps  1302  can be fabricated as flip chip bumps such that the package can be attached to a chip or substrate after the wafer  802  is diced. 
     In the embodiment of  FIG. 7 , the frontside bumping process in block  718  is completed after the carrier  902  is removed from the wafer  802 . This is the primary distinction between the first embodiment described in method  100  and the second embodiment described in method  700 . The second method  700  can be advantageous over the first method  100  because there is an increase in the margin for carrier/adhesive performance. The improved margin is related to the topography or surface flatness of the wafer. 
     In the first embodiment (e.g., method  100 ), the frontside bumps  306  can increase the surface height of the wafer  202  by 80-90 μm. This difference in surface height, or irregular surface flatness, can make it difficult to apply a substantially equal amount of adhesive along the surface of the wafer. In addition, if there are pockets or gaps along the wafer where there is an unequal amount of adhesive, it can be difficult to mount the carrier  302  to the wafer  202 . More particularly, when the carrier  302  is not effectively mounted to the wafer  202 , the carrier  302  provides less stiffness to the wafer  202 . Thus, the wafer  202  can bend, warp, or suffer other structural damage. To overcome this variation in surface height, the wafer  202  may require a stricter manufacturing process that includes tighter tolerances. For instance, in some embodiments, the surface variation of the wafer may not exceed 5 μm and this can be difficult to manufacture. 
     Conversely, in the second embodiment, the frontside bumping process is completed near the end of method  700 . In this instance, the adhesive material  904  can be applied more effectively to the surface of the wafer  802 . More importantly, the carrier  902  can more effectively be mounted to the wafer  802 . This improves the performance of the carrier  902  and provides a more desirable stiffness to the wafer  802  during the assembly process. 
     In addition, in the second embodiment, the wafer  802  can have a larger variation in surface height. For example, the wafer  802  can have a surface variation between 5-10 μm that does not require as stringent of a manufacturing process as in the first embodiment. The tolerances are not as strict and this can reduce the cost of manufacturing the wafer  802 . Also, because the carrier  902  is able to mount more easily to the wafer  802  in this embodiment, it is also able to demount from the wafer  802  more easily. 
     Although there is at least one distinction between the embodiments described in methods  100  and  700 , both methods can substantially reduce the warpage of the electronic package. In one non-limiting example, a study was performed on an electronic package similarly configured as the two embodiments shown in  FIGS. 6 and 13 . In this non-limiting example, warpage was measured on the bottom surface of the Tier  1  die (or wafer) after the carrier was demounted from the wafer. In the study, the thickness of the reinforcing material varied between 0-400 μm and the thickness of the mold compound varied between 100-400 μm. The Tier  2  die had a thickness of approximately 100 μm and the wafer or Tier  1  die had a thickness of approximately 50 μm. The mold compound and reinforcing material were cured at about 175° C. and 220° C., respectively. 
     In this non-limiting example, the warpage was reduced by 50-90% over conventional methods known in the art. The greatest reduction of warpage was measured when the thickness of the reinforcing material was similar to the thickness of the Tier  1  die. Regardless, however, of the different thicknesses of each material, the warpage was significantly reduced when the electronic package included the mold compound disposed between the wafer and the reinforcing material. When the electronic package was assembled without a reinforcing material, a substantial amount of warpage was measured. Thus, the reinforcing material can provide significant reduction to warpage over conventional electronic packages. 
       FIG. 14  shows an exemplary wireless communication system  1400  in which an embodiment of an electronic package with a reinforcing layer may be advantageously employed. For purposes of illustration,  FIG. 14  shows three remote units  1420 ,  1430 , and  1450  and two base stations  1440 . It should be recognized that typical wireless communication systems may have many more remote units and base stations. Any of remote units  1420 ,  1430 , and  1450 , as well as the base stations  1440 , may include an electronic package with a reinforcing layer such as disclosed herein.  FIG. 14  shows forward link signals  1480  from the base stations  1440  and the remote units  1420 ,  1430 , and  1450  and reverse link signals  1490  from the remote units  1420 ,  1430 , and  1450  to base stations  1440 . 
     In  FIG. 14 , remote unit  1420  is shown as a mobile telephone, remote unit  1430  is shown as a portable computer, and remote unit  1450  is shown as a fixed location remote unit in a wireless local loop system. For example, the remote units may be cell phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, or fixed location data units such as meter reading equipment. Although  FIG. 14  illustrates certain exemplary remote units that may include an electronic package with a reinforcing layer as disclosed herein, the package is not limited to these exemplary illustrated units. Embodiments may be suitably employed in any electronic device in which an electronic package with a reinforcing layer is desired. 
     While exemplary embodiments incorporating the principles of the present invention have been disclosed hereinabove, the present invention is not limited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.