Patent Publication Number: US-2010129961-A1

Title: Multi chip stacking with reliable joining

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     There are no cross-references related to this application. 
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor devices and more specifically to flip-chip bonding process for multi-chip stacking. 
     BACKGROUND OF THE INVENTION 
     Semiconductor chips, depending on their application, are manufactured in multi-stacks having layers of chips stacked upon each other. Proper bonding methods and materials are used to maintain proper conduction and connection between the stacked chips. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of multi chip stack bonding. A chip wafer is coated with a resin mixture. The chip wafer is heated until the coated resin mixture has solidified. The chip wafer is fragmented into individual chips and the individual chips are pre-stacked with alignment into a multi-chip stack. The pre-stacking process may be done at a temperature which is lower than the melting point of the bumps or solder balls of the individual chips. Pressure is applied to the multi-chip stack while heating the multi-chip stack in a joining process. The heating of the multi-chip stack is continued until the joining process is completed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of aspects of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the claims and drawings, in which like reference numbers indicate identical or functionally similar elements or steps. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears. 
         FIGS. 1A-1C  illustrate a chip wafer undergoing treatment and fragmentation in accordance with an aspect of an embodiment of the present invention. 
         FIGS. 2A-2B  illustrate a multi-chip stack being produced in accordance with an aspect of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1A , a chip wafer  100  undergoing treatment according to an aspect of an embodiment of the present invention is shown. Resin  102  is applied to chip wafer  100 . In one aspect or an embodiment of the present invention, epoxy resin is used and applied onto chip wafer  100 . The resin application may be done, for example, by using a spin coating method. In another aspect of an embodiment of the present invention, chip wafer  100  is spun at about 3000 rpm until resin  102  is uniformly distributed over chip wafer  100 . In another aspect of an embodiment of the present invention, resin  102  may contain resin hardener that belongs to a group such as carboxylic acid. In another aspect of an embodiment of the present invention, resin  102  may have a liquid encapsulation composition including a hardening accelerator and solvent. In yet another aspect of an embodiment of the present invention, resin  102  may be an imide series resin. In yet another aspect of an embodiment of the present invention, thermoplastic resin may be added to resin  102  in order to reduce brittleness of resin  102  during temporary hardening of resin  102 . 
     Referring now to  FIG. 1B , a chip wafer  100  undergoing additional treatment according to an aspect of an embodiment of the present invention is shown. Chip wafer  100 , having resin  102  uniformly distributed on it as shown in the previous Figure, is subjected to heating from heating source  104  for a period of time. The heating is continued until resin  102  has solidified. In an aspect of an embodiment of the present invention, resin  102  may be epoxy resin containing a hardening agent with a flux function that has a high melting point, has low solubility for a solvent to be added below the heating temperature of epoxy resin  102 , and has at least one aromatic series hydroxyl group and aromatic series carboxyl group. In this aspect of an embodiment of the present invention, a hardener having these characteristics helps avoid excessive reaction during the heating process shown in  FIG. 1B . 
     Referring now to  FIG. 1C , chip wafer  100  undergoing further treatment according to an aspect of an embodiment of the present invention is shown. Chip wafer  100 , following the solidification of resin  102 , is fragmented into individual chips. In one aspect of an embodiment of the present invention, the fragmentation may be done using a dicer, bonder  106  or similar tool, machinery or device. 
     Referring now to  FIG. 2A , a multi-chip stack  200  being produced in accordance with an embodiment of the present invention is shown. Individual chips  204   a - 204   d  previously produced by the fragmentation process of chip wafer  100  are stacked upon one another and chip substrate  202  as shown. In one aspect of an embodiment of the present invention, resin  102  may include spacer particles  206  that have a homogeneous grain size. In another aspect of an embodiment of the present invention, spacer particles  206  are of a specific grain size equivalent to the desired height between chips and the substrate. In yet another aspect of an embodiment of the present invention, spacer particles  206  may have a grain size that is smaller than the height of a chip&#39;s bump electrode or solder ball  208  before the chips are stacked. In yet another aspect of an embodiment of the present invention, minute particles such as glass may be added to resin  102  in order to reduce resin  102 &#39;s thermal expansion coefficient. These minute particles may have a low thermal expansion coefficient and have a grain size that is smaller than those of spacer particles  206 . 
     In yet another aspect of an embodiment of the present invention, spacer particles  206  may be inorganic particulates such as silica and aluminum nitride. In yet another aspect of an embodiment of the present invention spacer particles  206  may be cross-linked particles of sufficient elastic modulus that have a spacing function even at a joining temperature such as that of divinylbenzene. In yet another aspect of an embodiment of the present invention, spacer particles  206  may be shell particles with inorganic material for the core and a resin layer for the shell. In a further aspect of an embodiment of the present invention, resin  102  may include bumps or solder balls  208  which are used for bonding the different chip layers of multi-chip stack  200 . In a further aspect of an embodiment of the present invention, the grain size of spacer particles  206  may be in the range of 1 to 100 um. The grain size of spacer particles  206  may also be adjusted by a required bump height. In a further aspect of an embodiment of the present invention, if the shape of bumps or solder balls  208  before joining is a ball-type, the diameter of spacer particle  206  would be about 70% of the diameter of bumps  208 . 
     Pressure is applied by pressure source  210  while multi-chip stack  200  is subjected to heating by heating source  104 . Both pressure and heating may be applied by a thermo-compression device. III one aspect of an embodiment of the present invention, pressure may be applied using a flip chip bonder or device. In another aspect of an embodiment of the present invention, multi-chip stack  200  is subjected to a temperature which is lower than the joining temperature of bumps or solder balls  208 . When joining or stacking the chips, stacking is made possible without injecting or applying sealant. The joining process may entail sequentially stacking each individual chip  204   a - d , aligning the chips in the stack and thermo compressing the stack. In another aspect, the joining process may entail stacking individual chips  204   a - d  sequentially, and thermo compressing each stacked chip after the chip as been aligned onto the stack. 
     In one aspect of an embodiment of the present invention, because resin  102  has a flux function, no process of separate flux application and washing is required. In another aspect of an embodiment of the present invention, the temperature could be 20-40 degrees C. over the melting temperature of solder balls or bump  208 . Where lead-free solder such as Sn—Ag is used, the melting temperature is around 220 degrees C. 
     In one aspect of an embodiment of the present invention, individual chips  204   a - 204   d  may be stacked without having solder balls or bumps  208  melted. Bumps  208  are contacted physically once the resin is cured. In another aspect of an embodiment of the present invention, solder balls or bumps  2 () 8  are melted in the stacking process thereby contributing or assisting the bonding process. 
     Referring now to  FIG. 2B , a multi-chip stack  200  produced in accordance with an embodiment of the present invention is shown. Spacer particles  206  are shown maintaining their grain sizes while also maintaining the desired height between chips  204   a - d . Once resin  102  has cured, thermo compression by pressure source  210  and heating source  104 , respectively, is discontinued. Resin  102  completely covers the electrode surface between the chips in multi-chip stack  200  and has a sealing effect, which in turn prevents corrosion. The number of bumps or solder balls  208  crushed during the process is significantly reduced because of the shortened time used for mounting or stacking. 
     In one aspect of an embodiment of the present invention, solder balls or bumps  208  are melted and joined before the resin application and curing process. This results in obtaining a reliable joint between the chips in multi-chip stack  200 . 
     In another aspect of an embodiment of the present invention, multi-chip stack  200  is arranged or aligned to achieve good joining of many small bumps or solders balls  208  between the stacked chips. This is done in order to prevent chip dislocation. 
     In yet another aspect of an embodiment of the present invention, individual chips  204   a - d  are pre-stacked sequentially with pressure, heating and alignment. The temperature during this pre-stacking process may be lower than the melting point of bumps or solder balls  208 . As an example, if the multi-chip stack joining temperature is 260 degrees C., the pre-stacking temperature may be somewhere between 100 and 200 degrees C. In one aspect of an embodiment of the present invention, the pre-stacking temperature may be room temperature. Multi-chip stack is subjected to pressure and, upon joining, an elevated joining temperature which is maintained until the joining process is completed. In another aspect, the aligning and joining process may be performed for each individual chip. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.