Abstract:
A method of ultrasonic mounting can increase mounting efficiency by using high-frequency ultrasound and can also mount large semiconductor chips. The method ultrasonically bonds a semiconductor chip  52  to a substrate  50  using an ultrasonic mounting apparatus including a horn  15  that propagates ultrasonic vibration of an ultrasonic vibrator, the horn  15  being made of a ceramic that has a higher vibration propagation speed than metal. The method includes steps of disposing the substrate  50  on a stage  13 , disposing the semiconductor chip  52  on the substrate  50 , and placing the semiconductor chip  52  in contact with a convex part  15   a  provided on the horn  15  and applying ultrasonic vibration to bond the semiconductor chip  52  to the substrate  50.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method of ultrasonic mounting that bonds a semiconductor chip to a substrate and an ultrasonic mounting apparatus using the same. 
         [0003]    2. Related Art 
         [0004]    When flip-chip bonding and mounting a semiconductor chip on a circuit board, a method is used that places electrode terminals, such as bumps, of the semiconductor chip in contact with electrode terminals, such as pads, of the circuit board and applies ultrasonic vibration to the semiconductor chip to bond the electrode terminals of the semiconductor chip and the circuit board together. 
         [0005]    Conventionally, around 50 kHz is used as the frequency of the ultrasound applied to the semiconductor chip. 
         [0006]    However, the present inventors have discovered that a large amount of bonding energy is obtained when ultrasound with a high frequency of around 200 kHz is used, which means that the mounting efficiency can be increased. 
         [0007]    During ultrasonic mounting, vibration is applied from an ultrasonic vibrator to the semiconductor chip via a horn, and to transmit a large amount of vibration energy, the horn is provided with a convex part of a required width at a position corresponding to the loop (maximum amplitude point) of the vibration caused by the ultrasonic vibration and the semiconductor chip is placed in contact with this convex part to transmit the vibration energy. 
         [0008]    When high-frequency ultrasound is used, there is a corresponding reduction in wavelength. Accordingly, the width of the convex part provided corresponding to the maximum amplitude point inevitably becomes narrow, so that there is the new problem that only small semiconductor chips can be mounted. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention was conceived to solve the problems described above, and it is an object of the present invention to provide a method of ultrasonic mounting and an ultrasonic mounting apparatus using the same that can increase mounting efficiency by using high-frequency ultrasound and can also mount large semiconductor chips. 
         [0010]    A method of ultrasonic mounting according to the present invention ultrasonically bonds a semiconductor chip to a substrate using an ultrasonic mounting apparatus in which a horn that propagates ultrasonic vibration of an ultrasonic vibrator is made of a ceramic that has a higher vibration propagation speed than metal, the method including steps of: disposing the substrate on a stage; disposing the semiconductor chip on the substrate; and placing the semiconductor chip in contact with a convex part provided on the horn and applying ultrasonic vibration to bond the semiconductor chip to the substrate. 
         [0011]    An ultrasonic mounting apparatus according to the present invention includes a horn for propagating ultrasonic vibration of an ultrasonic vibrator and bonds a semiconductor chip to a substrate by placing the semiconductor chip in contact with a convex part of the horn and applying ultrasound, wherein the horn is formed of a ceramic that has a higher vibration propagation speed than metal. 
         [0012]    Stepped parts may be provided in walls of the convex part. 
         [0013]    A spacer, which is composed of a material that has a vibration propagation speed of an intermediate magnitude between a vibration propagation speed of the ultrasonic vibrator which is made of metal and a vibration propagation speed of the horn which is made of ceramic, may be interposed at a joint between the ultrasonic vibrator and the horn. 
         [0014]    A male screw for joining to the horn may be formed on the ultrasonic vibrator and a coating layer composed of a soft metal material such as copper or solder may be formed on the male screw. 
         [0015]    Another method of ultrasonic mounting according to the present invention ultrasonically bonds a semiconductor chip to a substrate using an ultrasonic mounting apparatus including a horn that propagates ultrasonic vibration of an ultrasonic vibrator, two convex parts being formed on the horn corresponding to maximum amplitude points that appear one wavelength apart and vibrate in the same direction due to the ultrasonic vibration, the method including steps of: disposing the substrate on a stage; disposing the semiconductor chip on the substrate; and inserting the semiconductor chip between the two convex parts of the horn and applying ultrasonic vibration from the two convex parts to bond the semiconductor chip to the substrate. 
         [0016]    Stepped parts that can be engaged by the edge parts of the semiconductor chip may be formed in walls of the two convex parts that face one another, the semiconductor chip may be inserted between the two convex parts so that the edge parts engage the stepped parts, and ultrasound may be applied while the semiconductor chip is pressed by surfaces of the stepped parts. 
         [0017]    The semiconductor chip may be inserted between the two convex parts via an elastic body. 
         [0018]    Another method of ultrasonic mounting according to the present invention ultrasonically bonds a semiconductor chip to a substrate using an ultrasonic mounting apparatus including a stage that propagates ultrasonic vibration of an ultrasonic vibrator, two convex parts being formed on the stage corresponding to maximum amplitude points that appear one wavelength apart and vibrate in the same direction due to the ultrasonic vibration, the method comprising steps of: disposing the substrate on the stage so as to be inserted between the two convex parts; disposing the semiconductor chip on the substrate; and applying ultrasonic vibration from the two convex parts to the substrate while the semiconductor chip is pressed by a pressing mechanism to bond the semiconductor chip to the substrate. 
         [0019]    Stepped parts that can be engaged by the edge parts of the substrate may be formed in walls of the two convex parts that face one another and the substrate may be inserted between the two convex parts so that the edge parts engage the stepped parts. 
         [0020]    Walls of the two convex parts that face one another may be formed as inclined surfaces and the substrate may be disposed between the two convex parts so as to be inserted between the inclined surfaces. 
         [0021]    The substrate may be inserted between the two convex parts via an elastic body. 
         [0022]    With the method of ultrasonic mounting and ultrasonic mounting apparatus according to the present invention, it is possible to increase mounting efficiency by using high-frequency ultrasound and to mount large semiconductor chips. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic diagram showing the entire construction of a mounting apparatus; 
           [0024]      FIG. 2  is a diagram useful in further explaining an ultrasonic bonding unit in the mounting apparatus shown in  FIG. 1 ; 
           [0025]      FIG. 3  is a diagram useful in explaining the relationship between a horn and an ultrasonic vibrator; 
           [0026]      FIG. 4  is an enlarged view of part of  FIG. 3 ; 
           [0027]      FIG. 5  is a graph showing the relationship between the vibration propagation speed due to the horn material and the effective tool size; 
           [0028]      FIGS. 6A and 6B  are diagrams useful in explaining a construction where stepped parts are provided in walls of the convex parts of the horn; 
           [0029]      FIGS. 7A and 7B  are diagrams useful in explaining warping of the convex parts; 
           [0030]      FIG. 8  is a diagram useful in explaining an embodiment where a spacer is interposed between the horn and the ultrasonic vibrator; 
           [0031]      FIG. 9  is a diagram useful in explaining an embodiment where a coating layer is formed on a male screw of the ultrasonic vibrator; 
           [0032]      FIG. 10  is a diagram useful in explaining an embodiment where two convex parts are provided on the horn; 
           [0033]      FIG. 11  is a diagram useful in explaining an embodiment where inclined surfaces are provided on the convex parts; 
           [0034]      FIG. 12  is a diagram useful in explaining an embodiment where the semiconductor chip is inserted via an elastic body; 
           [0035]      FIG. 13  is a diagram useful in explaining an embodiment where two convex parts are provided on the stage; 
           [0036]      FIG. 14  is a diagram useful in explaining an embodiment where inclined surfaces are provided on the convex parts; and 
           [0037]      FIG. 15  is a diagram useful in explaining an embodiment where the substrate is inserted via an elastic body. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    Preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. 
         [0039]      FIG. 1  is a schematic diagram showing one example of the entire construction of a flip-chip mounting apparatus  10 . 
         [0040]    Reference numeral  12  designates an ultrasonic bonding unit. The ultrasonic bonding unit  12  includes a stage  13  onto which a substrate is conveyed and a bonding tool  14  that is disposed above the stage  13 , holds a semiconductor chip on a lower surface thereof, and can move relatively toward and away from the stage  13 . 
         [0041]    The stage  13  is composed of a well-known XY table and can be moved in a desired direction within a horizontal plane by a driving unit, not shown. The XY table is constructed so as to be capable of being rotated within the horizontal plane about the vertical axis by a rotational driving unit, not shown. 
         [0042]    The bonding tool  14  is composed of a well-known ultrasonic bonding device, and includes a horn  15  for ultrasonic bonding and a pressing device  16  that is composed of a cylinder mechanism or the like that moves the horn  15  up and down. A semiconductor chip is held on a lower surface of the horn  15  by suction. 
         [0043]    A camera device  18  for position recognition is disposed so as to be capable of insertion between the stage  13  and the bonding tool  14 . The camera device  18  detects the positions of a substrate conveyed onto the stage  13  and a semiconductor chip held on the horn  15  of the bonding tool  14 , and aligns the substrate and the semiconductor chip by horizontally moving the stage  13  and/or rotating the stage  13  within the horizontal plane. 
         [0044]      FIG. 2  is a diagram useful in further explaining the ultrasonic bonding unit  12 . The ultrasonic bonding unit  12  is a well-known mechanism and therefore will be described in brief. 
         [0045]    Reference numeral  20  designates a pressing force control unit that controls the pressing device  16 ,  21  an ultrasonic vibrator,  22  an image processing unit,  23  a moving device that moves the camera device  18 ,  24  a movement control unit that controls movement by the moving device  23 ,  25  an alignment control unit that controls movement and rotation of the stage  13 , and  26  a main controller. 
         [0046]    By driving the moving device  23  using the movement control unit  24 , the camera device  18  is inserted between the substrate that has been conveyed onto the stage  13  and the semiconductor chip that is held on the horn  15  by suction. Image data from the camera device  18  is inputted into the image processing unit  22 , positional displacements between the substrate and the semiconductor chip are detected, and the stage  13  is moved and/or rotated by the alignment control unit  25  to correct any positional displacements, thereby aligning the substrate and the semiconductor chip. Next, the camera device  18  is withdrawn. After this, the pressing device  16  is driven by the pressing force control unit  20  to lower the horn  15  and apply a predetermined force to the semiconductor chip held on the lower surface of the horn  15  and ultrasound is applied from the ultrasonic vibrator  21  to the semiconductor chip to bond the semiconductor chip to the substrate. Driving control of the various control units is entirely carried out by a processing program set in the main controller  26 . 
         [0047]    In  FIG. 1 , reference numeral  35  designates a conveying unit for semiconductor chips. 
         [0048]    A large number of semiconductor chips are stored on a tray (not shown) and are supplied by a chip supplying stage  36 . Using a chip handler  38  that includes the suction nozzle  37  that can move up and down and horizontally, the semiconductor chips stored in the tray are held one at a time by suction on the suction nozzle  37  and are conveyed onto a mounting table  41  of a chip inverting stage  40 . 
         [0049]    The chip inverting stage  40  has a suction arm  42 . The suction arm  42  includes a suction nozzle  43  and is provided so as to be capable of being inverted by 180° by an inverting device  44  between a position located above the mounting table  41  and a position on an opposite side. The inverting device  44  is also provided so as to be capable of being moved back and forth by a driving unit, not shown, in a direction that approaches the mounting table  41  and a direction that approaches the horn  15 . 
         [0050]    The semiconductor chip is conveyed onto the mounting table  41  with a surface on which bumps are formed facing upwards. By holding the semiconductor chip conveyed onto the mounting table  41  by suction on the suction nozzle  43  of the suction arm  42 , inverting the suction arm  42 , and moving the semiconductor chip towards the horn  15 , the semiconductor can be held on the lower surface of the horn  15  by suction. The semiconductor chip therefore becomes held by suction on the horn  15  with the surface on which the bumps are formed facing downwards. 
         [0051]    It should be noted that the suction nozzle  43  is provided so as to be capable of being inwardly and outwardly projected (moved) by a mechanism, not shown, in a direction perpendicular to the suction arm  42  so that a semiconductor chip can be smoothly transferred between the mounting table  41  and the horn  15 . 
         [0052]    The substrate is conveyed onto the stage  13  by a substrate conveyor or the like, not shown. 
         [0053]    On the other hand, as described above, a semiconductor chip  52  is conveyed into the ultrasonic bonding unit  12  by the conveying unit  35  for semiconductor chips and is held by suction on the lower surface of the horn  15 . 
         [0054]    The camera device  18  is inserted between the substrate  50  conveyed onto the stage  13  and the semiconductor chip  52  held on the horn  15  and alignment of the substrate  50  and the semiconductor chip  52  is carried out as described above. 
         [0055]    Next, the camera device  18  is withdrawn and the horn  15  on which the semiconductor chip  52  is held by suction is lowered by the pressing device  16  so that the semiconductor chip  52  is pressed onto the substrate  50  with the required pressing force. After this, the ultrasonic vibrator  21  is operated and ultrasound is applied to the semiconductor chip  52  from the horn  15 . By doing so, bumps  52   a  of the semiconductor chip  52  are ultrasonically bonded to pads (not shown) of the substrate  50 . 
         [0056]      FIG. 3  shows the relationship between the horn  15 , the ultrasonic vibrator  17 , the semiconductor chip  52 , and the substrate  50  in the flip-chip mounting apparatus  10  described above.  FIG. 4  is an enlarged view of part of  FIG. 3 . Convex parts  15   a  are formed on an upper surface and a lower surface of the horn  15 . 
         [0057]    The ultrasonic vibration propagates as compressional waves inside the horn  15 . In this case, in principle, loops (maximum amplitude points) occur at both ends of the horn  15  and a plurality of other maximum amplitude points occur in intermediate part of the horn  15 . The convex parts  15   a  are formed with a required width at a position corresponding to such a maximum amplitude point. 
         [0058]    Such maximum amplitude points for the ultrasonic vibration naturally occur at intervals of one half of the wavelength. 
         [0059]    The positions of the maximum amplitude points of the compressional waves are positions at which the maximum vibration in the horizontal direction can be applied from the horn  15  to the semiconductor chip  52  and are positions where the ultrasonic energy can be transmitted to the greatest possible extent, and by providing the convex parts  15   a  of the required width at such positions, it is possible to carry out ultrasonic bonding of the semiconductor chip  52  efficiently. The width of the convex parts  15   a  extends across a maximum amplitude point and corresponds to a range where a substantially uniform amplitude value is obtained. 
         [0060]    The ultrasonic vibrator  17  is composed of metal, such as a titanium alloy, in which a piezoelectric element is incorporated. 
         [0061]    As in a conventional device, the horn  15  is formed of metal such as titanium alloy. 
         [0062]    The speed at which ultrasound propagates within a member is unique to the member, and is determined by the material used. 
         [0063]    However, the relationship between the propagation speed C, the frequency f, and the wavelength λ is C=f□λ. 
         [0064]    Accordingly, when the frequency is changed from 50 kHz to a high frequency of 200 kHz, the wavelength is quartered. Conventionally, if the horn  15  is formed of a metal such as a titanium alloy, when the frequency is 50 kHz, it is possible to set the width of the convex parts  15   a  at around 12 mm and bonding can be carried out for semiconductor chips that are around 12 mm in size. However, when the frequency is raised to 200 kHz, the wavelength is quartered so that the width of the convex parts  15   a  is also reduced to around one quarter, that is, the width can be set at only around 3 to 4 mm, so that large semiconductor chips can no longer be mounted. 
         [0065]    However, in the present embodiment, the horn  15  to which the ultrasonic vibration of the ultrasonic vibrator is propagated is formed of a ceramic that has a high vibration propagation speed compared to metal. 
         [0066]    The vibration propagation speed (m/sec) of various metals and ceramics are shown below. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Iron 
                 5,950 
                   
               
               
                   
                 A5052 
                 6,190 
               
               
                   
                 Titanium Alloy 
                 6,313 
                 (Ti—6Al—4V Alloy) 
               
               
                   
                 Zirconia 
                 7,036 
               
               
                   
                 Cermet 
                 9,086 
               
               
                   
                 Aluminum Nitride 
                 10,198 
               
               
                   
                 Silicon Nitride 
                 10,764 
               
               
                   
                 Sialon 
                 11,032 
               
               
                   
                 Alumina 
                 11,804 
               
               
                   
                 Silicon Carbide 
                 12,018 
               
               
                   
                 Single Crystal Sapphire 
                 12,624 
               
               
                   
                   
               
             
          
         
       
     
         [0067]    For the present invention, the expression “ceramics” includes zirconia and cermet. Aside from these, ceramics such as mullite, titania ceramics, and cordierite are effective. 
         [0068]    As described above, when ceramics are used, the vibration propagation speed is around double that of metal, and accordingly even when high frequency ultrasound with a frequency of 200 kHz is used, the width of the convex parts  15   a  of the horn  15  can be increased to around 8 mm, so that even large semiconductor chips can be mounted. 
         [0069]      FIG. 5  is a graph showing a model of the relationship between various materials of the horn and the vibration propagation speed and effective tool size (the width of the convex parts). It can therefore be understood that the tool size (the width of the convex parts) can be increased by using ceramics as the material of the horn  15 . 
         [0070]      FIGS. 6A and 6B  show an embodiment where stepped parts  19  are provided in the walled parts of the convex parts  15   a  of the horn  15  described above. 
         [0071]    In  FIGS. 7A and 7B , the convex parts  15   a  are simply provided on the horn  15 , but due to the provision of the convex parts  15   a , an amplitude component is produced in a height direction (Z direction) of the convex parts  15   a  and the convex parts  15   a  deform to become warped (see  FIG. 7B ), so that there is the problem that ultrasonic vibration cannot be transmitted uniformly to the semiconductor chip  52 . 
         [0072]    As shown in  FIG. 6B , by providing stepped parts  19  in the walls of the convex parts  15   a , there is the effect that warping is absorbed by the stepped parts  19  and the overall warping of the convex parts  15   a  is reduced. 
         [0073]      FIG. 8  shows an embodiment in which a spacer  27  is interposed at a joint of the horn  15  and the ultrasonic vibrator  17 . 
         [0074]    A material that has a vibration propagation speed of an intermediate magnitude between the vibration propagation speed of the ultrasonic vibrator  17  that is made of metal and the vibration propagation speed of the horn  15  that is made of ceramic is used for this spacer  27 . 
         [0075]    As one example, titanium alloy is used as the ultrasonic vibrator  17 , cermet is used as the spacer  27 , and alumina is used as the horn  15 . 
         [0076]    When the ultrasonic vibrator  17  is made of metal and the horn  15  is made of ceramic, there is a large difference in vibration propagation speed due to these materials, so that there is the risk of the ultrasonic vibration being reflected at the interface of the ultrasonic vibrator  17  and the horn  15  which reduces the transmissibility of the ultrasound, but this problem can be solved by interposing the spacer  27  that has an intermediate vibration propagation speed relative to the two parts. 
         [0077]      FIG. 9  shows an embodiment in which a coating layer composed of a soft metal material such as copper or solder is formed on a surface of a male screw  17   a  of the ultrasonic vibrator  17  used for joining the horn  15 . 
         [0078]    The ultrasonic vibrator  17  is integrated by screwing the male screw  17   a  into a female screw thread (not shown) of the horn  15 , and by providing a coating layer of a soft metal material on the surface of the male screw  17   a , gaps between the two parts are filled when the different materials are screwed together and the different materials can be connected so as to fit together well. 
         [0079]      FIG. 10  shows yet another embodiment. 
         [0080]    In this embodiment, a component with two convex parts  15   a ,  15   b , which correspond to maximum amplitude points P that appear at an interval of one wavelength and vibrate in the same direction due to the ultrasonic vibration, is used as the horn  15 . 
         [0081]    By disposing the substrate  50  on the stage  13 , disposing the semiconductor chip  52  on the substrate  50 , inserting the semiconductor chip  52  between the two convex parts  15   a ,  15   b  of the horn  15 , and applying ultrasonic vibration from the two convex parts  15   a ,  15   b , the semiconductor chip  52  is bonded to the substrate  50 . 
         [0082]    Here, the rear surface of the semiconductor chip  52  is set so as to not contact the horn  15 . Since the compressional waves propagate so that the convex parts  15   a ,  15   b  vibrate in synchronization (i.e., with the same phase), there is no risk of the semiconductor chip  52  being destroyed. 
         [0083]    Also, since the semiconductor chip  52  is inserted between the two convex parts  15   a ,  15   b  that are not half but one wavelength apart, mounting can be carried out for large semiconductor chips. The material of the horn  15  is not limited to ceramics, and a metal horn may be used. 
         [0084]    In the embodiment shown in  FIG. 11 , inclined surfaces  28  are formed at the walls of the convex parts  15   a ,  15   b  shown in  FIG. 10  that face one another, with the semiconductor chip  52  being disposed in the two convex parts  15   a ,  15   b  so as to be inserted between the inclined surfaces  28 . By doing so, it is possible to apply ultrasound while pressing the semiconductor chip  52  via the inclined surfaces  28  onto the substrate  50  with a predetermined pressing force. It should be noted that in place of the inclined surfaces  28 , stepped parts (not shown) that can engage edge parts of the semiconductor chip  52  can be formed, with the semiconductor chip  52  being inserted between the two convex parts  15   a ,  15   b  so that the edge parts engage the stepped parts and with ultrasound being applied while the semiconductor chip  52  is pressed by the stepped surfaces. 
         [0085]    In the embodiment shown in  FIG. 12 , the semiconductor chip  52  is inserted between the convex parts  15   a ,  15   b  shown in  FIG. 10  via an elastic body  29 . Ultrasound can be applied to the semiconductor chip  52  from the convex parts  15   a ,  15   b  via the elastic body  29 . 
         [0086]      FIG. 13  shows a construction where instead of applying ultrasound from a horn, the ultrasonic vibrator  17  is attached to the stage  13  and ultrasonic vibration is applied to the substrate  50  disposed on the stage  13  to bond the semiconductor chip  52 . 
         [0087]    In this embodiment, two convex parts  13   a ,  13   b  corresponding to maximum amplitude points P that appear one wavelength apart and vibrate in the same direction due to the ultrasonic vibration are formed in the stage  13 . 
         [0088]    The substrate  50  is disposed on the stage  13  so as to be inserted between the two convex parts  13   a ,  13   b , the semiconductor chip  52  is disposed on the substrate  50 , and while the semiconductor chip  52  is pressed by an appropriate pressing mechanism (not shown), ultrasonic vibration is applied to the substrate  50  from the two convex parts  13   a ,  13   b  so as to bond the semiconductor chip  52  onto the substrate  50 . 
         [0089]    In the embodiment shown in  FIG. 14 , inclined surfaces  28  are formed at the walls of the convex parts  13   a ,  13   b  shown in  FIG. 13  that face one another, with the substrate  50  being disposed between the two convex parts  13   a ,  13   b  so as to be inserted between the inclined surfaces  28 . It is therefore possible to apply ultrasound to the substrate  50  via the inclined surfaces  28 . It should be noted that in place of the inclined surfaces  28 , stepped parts (not shown) that can be engaged by edge parts of the substrate  50  can be formed, with the substrate  50  being inserted between the two convex parts  13   a ,  13   b  so that the edge parts engage the stepped parts. 
         [0090]    In the embodiment shown in  FIG. 15 , the substrate  50  is inserted between the convex parts  13   a ,  13   b  shown in  FIG. 13  via an elastic body  29 . Ultrasound can be applied to the substrate  50  from the convex parts  13   a ,  13   b  via this elastic body  29 .