Patent Publication Number: US-2003230794-A1

Title: Semiconductor fabrication method and apparatus, and semiconductor device

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates generally to semiconductor fabrication methods, and more particularly to a bonding method. The present invention is also relates to a bonded intermediate product and an end product, like LSIs.  
       [0002] An LSI is generally experiences a bonding step of mounting a die or chip onto a lead frame or package, and then a sealing or packaging step of sealing them using resin. The bonding step has used such a jointing material as gold and resin for mounting of the die. FIG. 8 shows a schematic sectional view of a conventional LSI package  1  after the sealing step. In the LSI package  1 , as illustrated, an IC chip  12  mounted on a stage  10  is bonded to a lead frame  14  through gold balls  17  and wires  18 , and then sealed by plastic  20 . The gold ball  17  is formed at an end of the gold wire  18  and metal-jointed to a bonding pad  16  as a metal-coated electrode on the IC chip  12 , so as to connect the IC chip  12  to the gold wire  18 . The instant application sometimes refers to an interface between a bonding pad and a wire or an area including the interface as a bonding pad part, as enclosed by a broken line labeled by  16  shown in FIG. 8.  
       [0003] Recent trends of ecological protections have required LSIs to follow environmental conservation or afforestation. Use of unleaded solder has recently been proposed as one solution in mounting the package  1  onto a printed board. Lead is innocent in a metal state, but it has been pointed out that acid rain resulted from industrial waste etc. dissolves lead, possibly causing water pollution. Demands of unleaded solder have increased particularly in Europe. Although various types of unleaded solder have been proposed which include tin/silver alloy and tin/silver/cupper alloy, it becomes necessary to locate the package  1  under higher temperature conditions than the conventional in mounting the package  1  onto the printed board since the unleaded solder usually has a melting point (of, for example, about 260° C.) higher than that of the leaded solder (of, for example, about 225° C.).  
       [0004] The LSI package  1  is integrally formed with the plastic  20 , the IC chip  12  and bonding pad parts  16 . Thus, as the printed board becomes high temperature due to the unleaded soldering, the physical pressure applies to the bonding pad part  16  due to a difference in coefficient of thermal expansion between the plastic  20  and the IC chip  12 , causing a disadvantageous disconnection on the bonding pad part  16 . A description will be given of this problem with reference to FIGS. 8 and 9. Here, FIG. 9A is an enlarged sectional view of the boding pat part  16  shown in FIG. 8 at room temperature, and FIG. 9B is an enlarged sectional view of the bonding pad part  16  shown in FIG. 9A at high temperature. As shown in FIGS. 8 and 9, different coefficients of thermal expansion between the plastic  20  and the IC chip  12  generate thermal stresses F 1  and F 2 . Then, a difference of thermal stresses F 2 -F 1  applies as a distortion to an interface between the gold ball  17  and gold wire  18 , as shown in an arrow in FIG. 9B, possibly causing the disconnection. In addition, the disconnection lowers a yield of manufacture of the printed boards.  
       [0005] It is conceivable as one solution not to fill the plastic  20  on and around the bonding pad part  16  in filling the plastic  20  in packaging. Such a sealing method would result in complex working and increased fabrication cost.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006] Accordingly, it is an exemplified object of the present invention to provide a semiconductor fabrication method and apparatus, which may manufacture a package having improved heat resistance, and a semiconductor device having the package.  
       [0007] In order to achieve the above object, a semiconductor fabrication method as one aspect of the present invention includes the steps of bonding each of a plurality of bonding pads provided in a die and a circuit board using a wire, coating an interface between the bonding pad and the wire with a buffer material, and sealing the die and part of the circuit board using resin. According to this semiconductor fabrication method, the buffer material coats the interface. The wire in the buffer material is deformable like a linkage, thereby buffering a distortion caused by different coefficients of thermal expansion between the resin and die.  
       [0008] The method may further include the step of using unleaded solder to mount, onto a printed board, a package that is formed by the sealing step and includes the resin, circuit board and die. The package of the present invention particularly exhibits the heat resistance under the high temperature environment using unleaded soldering.  
       [0009] The buffer material may be a jelly material, a viscosity material, etc., have a melting point lower than the temperature in the sealing step, and be implemented, for example, as silicon rubber. The coating step may apply the buffer material onto each interface, onto a row of the bonding pads or onto the approximately entire surface of the die.  
       [0010] A semiconductor device as another aspect of the invention includes a die having a plurality of bonding pads and bonded to a circuit board through the bonding pads and wires, and a buffer material for coating an interface between the bonding pad in the die and the wire. According to this semiconductor device, the buffer material coats the interface, and thus the wire in the buffer material becomes deformable like a linkage.  
       [0011] A semiconductor device as still another aspect of the present invention includes a package including a die that has a plurality of bonding pads and is bonded to a lead frame through the bonding pads and wires, and a buffer material for coating an interface between the bonding pad in the die and the wire, and a printed board mounted with the package through unleaded soldering. This semiconductor device uses the unleaded solder, contributing to environment protection, while the package does not cause a disconnection between the bonding pad and wire, exhibiting good electric property. Various electronic apparatuses, such as computers, cell phones, audio-visual apparatuses, printers, etc., which have the above semiconductor device, constitute another aspect of the present invention.  
       [0012] A semiconductor fabrication apparatus of another aspect of the present invention includes an application part for applying a buffer material to a die that has a plurality of bonding pads and is bonded to a circuit board through the bonding pads and wires, at an interface between the bonding pad and wire, and a moving part for moving the application part such that the application part may apply the buffer material to the interface corresponding to each of the plurality of bonding pads. This semiconductor fabrication apparatus may also manufacture the above semiconductor device. The application part may apply the buffer material to each interface locally or the approximately entire surface of the die.  
       [0013] Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014]FIG. 1 is a flowchart for explaining a semiconductor fabrication method as one embodiment of the present invention.  
     [0015]FIG. 2 is a schematic sectional view corresponding to each step shown in FIG. 1.  
     [0016]FIG. 3 is a schematic perspective view for explaining a method of applying a buffer material onto each bonding pad part.  
     [0017]FIG. 4 is a schematic sectional view showing one exemplary structure of a coating applicator for applying the buffer material.  
     [0018]FIG. 5 is a schematic sectional view and plane view for explaining a variation of the application method shown in FIG. 3.  
     [0019]FIG. 6 is a schematic sectional view for explaining an effect of a package of the instant embodiment.  
     [0020]FIG. 7 is a perspective overview of a laptop personal computer as one example of an electronic apparatus mounted with an inventive printed board.  
     [0021]FIG. 8 is a schematic sectional view of a conventional LSI package after a sealing step.  
     [0022]FIG. 9A is an enlarged sectional view of a bonding pad part shown in FIG. 8 at room temperature, and FIG. 9B is an enlarged sectional view of the bonding pad part shown in FIG. 9A at high temperature. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0023] Referring now to accompanying drawings, a description will be given of a semiconductor fabrication method as one embodiment according to the present invention. Here, FIG. 1 is a flowchart for explaining a semiconductor fabrication method as one embodiment of the present invention. FIG. 2 is a schematic sectional view corresponding to each step in FIG. 1.  
     [0024] First, as shown in FIG. 2A, the IC chip  12  is adhered to the stage  10  (step  1002 ). Then, as shown in FIG. 2B, the IC chip  12  and the lead frame  14  are bonded using gold, aluminum and other wires  18  (step  2004 ).  
     [0025] As shown in FIG. 2C, the buffer material  30  is applied onto a bonding pad part (step  1006 ). The quantity of application of the buffer material  30  depends upon a size of the bonding pad part. The range and height of the applied buffer material  30  may be adjustable by adjusting temperature and viscosity. The buffer material  30  is made of a non-solid material, such as a jelly material and a viscosity material, has a melting point lower than the temperature in the sealing step, and may be made, for example, of silicon rubber. The buffer material  30  may be in a non-solid state at least in a mounting step (step  1012 ), which will be described later, and it may then turn to be in a solid state.  
     [0026] The coating applicator  40  for applying the buffer material  30  has a structure similar to that of a known dispenser, and includes, as shown in FIG. 4, a buffer-material supply  42 , a plunger or cylinder  46  for charging and discharging the buffer material  30 , a valve  44  provided between the buffer-material supply  42  and the cylinder  46  for controlling flow of the buffer material  30 , and a cylinder block  48  that supports the plunger  46 , accommodates the buffer material  30  and has a nozzle  49  at its top. For example, as the cylinder  46  goes up the valve  44  opens, thereby charging the flow-controlled buffer material  30  in the cylinder block  48  from the buffer-material supply  42 . Then the valve  44  closes as the cylinder  47  goes down, thereby applying the buffer material  30  filled in the cylinder block  48  onto the bonding pad part  16  from the tip of the nozzle  49  in the arrow direction. Here, FIG. 4 is a schematic sectional view showing one example of the coating applicator  40  for applying the buffer material  30 . Of course, the coating applicator is not limited to the structure shown in FIG. 4 but may use any structure known in the art.  
     [0027] The instant embodiment applies the buffer material  30  onto each bonding pad part  16 . FIG. 3 shows a schematic perspective view showing this state. Referring to FIG. 3, the nozzle  49  applies the buffer material  30  onto each bonding pad part  16  along a locus shown by a broken line P. FIG. 4 shows a moving part  50  of the nozzle  49 . The application timing is provided for each bonding pad part  16  in the instant embodiment, and the buffer material  30  is not applied between the bonding pad parts  16 . This application method may be implemented, for example, by providing FIG. 4 with a controller (not shown) for controlling drives of the cylinder  46  and the moving part  50 , and a sensor (not shown) for detecting a relationship between the nozzle  49  and the bonding pad part  16 . For instance, the controller moves the nozzle  49  along the locus P by moving the moving part  50 . The controller applies the buffer material  30  onto the bonding pad part  16  through the nozzle  49  by controlling the driving of the cylinder  46  when the nozzle  49  is located at a position corresponding to the bonding pad part  16  based on the sensor detection result. On the other hand, the controller may stop application by controlling driving of the cylinder  46  when the nozzle  49  is located between bonding pad parts  16  based on the sensor detection result.  
     [0028] Alternatively, the continuous application may be provided along the locus P. In this case, the buffer material  30  is applied between the bonding pad parts  16  and the bank of the buffer material  30  is formed along the locus P.  
     [0029] A description will be given of another embodiment of the present invention with reference to FIG. 5. Here, FIG. 5A is a schematic sectional view for explaining an application method of the other embodiment according to the present invention. FIG. 5B is a schematic plane view showing the IC chip  12  to which the buffer material  30  has been applied in accordance with this method. FIG. 5C is an XY sectional view in FIG. 5B.  
     [0030] The instant embodiment applies, through a nozzle  49   a , the buffer material  30  onto the approximately entire surface of the IC chip  12  at its center. The nozzle  49   a  serves substantially as the nozzle  49 , although its shape is different from the nozzle  49 . Preferably, the instant embodiment uses low viscosity material for the buffer material  30  so that the buffer material  30  may cover the entire surface on the IC chip  12 . As shown in FIG. 5A, when the buffer material  30  is applied to the center of the IC chip  12 , the capillarity between wires  18  swells the buffer material  30  on the bonding pad part  16 , as shown in FIG. 5C.  
     [0031] Then, as shown in FIG. 2D, the lead frame  14  connected to the IC chip  12  is attached to a molding die (not shown), and a molding die compound material (or plastic  20 ) is poured into the molding die at the temperature of 160° C. to 170° C. (step  1008 ). The molding die may be a multiple plunger molding die including a plurality of pairs of pot and plunger or a single plunger molding die including one pair of pot and plunger. As the temperature of the package  2  shown in FIG. 2D returns to the room temperature, the molding die compound hardens, completing the package  2  (step  1010 ). A package  2 A shown in FIG. 2E would be used for the application method shown in FIG. 5.  
     [0032] Next, as shown FIG. 2F, the package  2  (as well as the package  2 A) are mounted on the printed board  24  using the unleaded solder  22  by soldering the terminal  15  of the lead frame  12 . The unleaded solder  22  may use tin/silver alloy, tin/bismuth/silver alloy, tin/silver/cupper alloy, etc. These unleaded solders  22  have a melting point of about 260° C. higher than that of the widespread leaded solder. The inventive packages  2  and  2 A do not cause any disconnection even under high temperature environments. A description will now be given of the bonding pad parts  16  under the high temperature environment with reference to FIG. 6. Here, FIG. 6 is a schematic sectional view of the bonding pad part  16  in the packages  2  and  2 A in comparison with FIG. 9B.  
     [0033] As illustrated, the wire  18  is deformable between its interface  18   a  with the gold ball  17  and its interface  18   b  with the buffer material  30  is deformable like a linkage in the space h. More specifically, the buffer material  30  made of a jelly material in the package  2  may buffer the physical pressure caused by a difference of thermal stress F 2 -F 1 . The buffer material  3  made of a low-melting material in the package  2  may melt at high temperature during soldering, form the hollow space h, and buffer the physical pressure. This is true of the package  2 A similar to the package  2  when the buffer material  30  is made of a jelly material or a low-melting material. As a result, the wire  18  and gold ball  17  are not easily disconnect from each other, as shown in FIG. 9B, the instant embodiment may provide a heat resistant packages  2  and  2 A.  
     [0034] A printed circuit board onto which the package is mounted through unleaded solder  22  is applicable to various electronic apparatuses, such as computers, and its peripherals, cell phones, audio visual apparatuses, printers, etc. FIG. 7 shows a perspective overview of the laptop personal type computer  100  equipped with the printed circuit board  24 .  
     [0035] The instant invention thus provides a semiconductor fabrication method and apparatus, which may manufacture a package having improved heat resistance, and a semiconductor device having the package.  
     [0036] Further, the present invention is not limited to these preferred embodiments, and various modifications and modifications may be made in the present invention without departing from the spirit and scope thereof.