Patent Publication Number: US-6909182-B2

Title: Spherical semiconductor device and method for fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuing application of U.S. application Ser. No. 09/350,125 by Kohei TATSUMI et al., filed on Jul. 9, 1999 now U.S. Pat. No. 6,509,645 for “SPHERICAL SEMICONDUCTOR DEVICE AND METHOD FOR FABRICATING THE SAME”, the entire contents of which are hereby incorporated by reference, and for which priority is claimed under 35 U.S.C. §120. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor device and, more particularly, to a spherical semiconductor device comprising a spherical semiconductor element having one or more electrodes on its surface. The present invention relates also to a method for fabricating a semiconductor device and, more particularly, to a method for fabricating a spherical semiconductor device comprising a spherical semiconductor element having one or more electrodes on its surface. 
   2. Description of the Related Art 
   Recently, instead of conventional semiconductor devices fabricated by forming integrated circuits on silicon wafers, a spherical semiconductor element fabricated by forming an electric circuit on the surface of spherical silicon has been developed. This spherical semiconductor element has one or more electrodes on its surface. A semiconductor device having a variety of functions can be realized by combining spherical semiconductor elements having various functions. 
   Such a spherical semiconductor element cannot operate only by itself. It requires input/output means for electrical connection to the outside to exchange electrical signals with an external circuit or the like. Although spherical semiconductor elements have excellent functions, effective measures have not been found particularly for packaging. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a spherical semiconductor device having improved connectivity to the outside. 
   It is another object of the present invention to provide a method for fabricating a spherical semiconductor device having improved connectivity to the outside. 
   According to the present invention, a spherical semiconductor device comprises a spherical semiconductor element comprising one or more electrodes on a surface of the element and spherical conductive bumps formed at the positions of the electrodes. 
   According to an aspect of the present invention, said electrodes are arranged so as to contact a common plane. 
   According to another aspect of the present invention, the spherical bumps constituting a group to be connected to the outside, protrude above the spherical semiconductor element such that there is formed no gap or a predetermined gap between a plane or a spherical surface capable of contacting the group of spherical bumps, and the surface of the spherical semiconductor element. 
   According to another aspect of the present invention, each spherical bump is made of a refractory metal having a melting point of not less than 550° C. 
   According to another aspect of the present invention, each electrode is made of a material selected from the group consisting of aluminum, copper, and an alloy containing at least one of aluminum and copper, and each spherical bump is made of a material selected from the group consisting of gold, platinum, palladium, silver, copper, aluminum, nickel, and an alloy containing at least one of gold, platinum, palladium, silver, copper, aluminum, and nickel. 
   According to another aspect of the present invention, each spherical bump is made of a low-melting metal having a melting point of not more than 450° C. 
   According to another aspect of the present invention, each electrode is made of a material selected from the group consisting of aluminum, copper, and an alloy containing at least one of aluminum and copper, and each spherical bump is made of a material selected from the group consisting of lead, tin, indium, bismuth, zinc, an: alloy containing at least one of lead, tin, indium, bismuth, and zinc, and an alloy mainly containing one of gold-silicon alloy, gold-tin alloy, and silver-tin alloy. 
   According to another aspect of the present invention, at least one metal layer selected from the group consisting of titanium, tungsten, titanium-tungsten, nickel, chromium, gold, palladium, copper, and platinum is formed on each electrode. 
   According to another aspect of the present invention, each electrode is connected through the spherical bump formed thereon, to an electrode of a ceramics substrate, a film carrier, a silicon substrate, a printed circuit board, a lead frame, a semiconductor chip, or a spherical semiconductor element. 
   According to another aspect of the present invention, each spherical bump is made of a refractory metal and connected through a low-melting metal to an electrode of a ceramics substrate, a film carrier, a silicon substrate, a printed circuit board, a lead frame, a semiconductor chip, or a spherical semiconductor element, and the difference in melting point between the refractory metal and the low-melting metal is not less than 50° C. 
   According to another aspect of the present invention, each spherical semiconductor element is encapsulated with an encapsulating material. 
   According to another aspect of the present invention, each electrode has a shape selected from the group of a trapezoid, a polygon having at least five sides, and a circle. 
   According to another aspect of the present invention, each electrode has an area equivalent to the area of a circle having a diameter not less than 3% of a diameter of the spherical semiconductor element. 
   According to another aspect of the present invention, each spherical bump is made of a refractory metal coated with a low-melting metal. 
   According to the present invention, since spherical conductive bumps are formed at the positions of electrodes of a spherical semiconductor element, electrical connections to the outside can be easily and accurately made through the spherical bumps. 
   In particular, a group of spherical bumps to be connected to the outside are arranged to protrude above the spherical semiconductor element such that a predetermined gap is formed between a plane or a spherical surface capable of contacting the group of spherical bumps and the surface of the spherical semiconductor element. Since the spherical bumps thus protrude above the spherical semiconductor element, extremely superior bump joining properties can be obtained. 
   In case of melt joining, it can be performed by the wet effect of each bump melted even if there is formed no gap. 
   The surface of each spherical bump made of a refractory metal is coated with a low-melting metal. By setting the difference in melting point between the refractory and low-melting metals to 50° C. or more, preferably, 100° C. or more, the surface portion can be melted while the core remains solid during joining. So, a certain distance, i.e., a distance not less than the diameter of the core metal can be kept between the junction portions. 
   Each spherical bump may deform into a shape like a Rugby ball, or unevenly deform at its part, e.g., its junction portion. In order for the spherical bumps surely to protrude beyond an apex of the spherical semiconductor element, two or more layers of bumps may be used. 
   According to the present invention, since spherical conductive bumps are formed at the positions of electrodes of a spherical semiconductor element, electrical connections to the outside can be easily and accurately made through the spherical bumps. In this case, by arranging the spherical bumps to protrude above the spherical semiconductor element, extremely superior bump joining properties can be obtained. As a result, high reliability can be obtained when a semiconductor device comprising such a spherical semiconductor element is packaged or the like. 
   According to another aspect of the present invention, a method for fabricating a spherical semiconductor device having spherical bumps on surface electrodes of a spherical semiconductor element, comprises the steps of temporarily arranging conductive balls for forming the spherical bumps, on an arrangement substrate at positions respectively corresponding to said surface electrodes, and transferring the conductive balls onto the surface electrodes to join. 
   According to another aspect of the present invention, the conductive balls are transferred from the arrangement substrate to the surface electrodes while the position of each of the conductive balls on the arrangement substrate is regulated. 
   According to another aspect of the present invention, the conductive balls are transferred from the arrangement substrate to the surface electrodes such that a predetermined gap is formed between a surface of the arrangement substrate and a surface of the spherical semiconductor element. 
   According to another aspect of the present invention, the conductive balls are transferred onto and joined to the surface electrodes by thermo-compression bonding. 
   According to another aspect of the present invention, the conductive balls are transferred onto and joined to the surface electrodes by melting. 
   According to another aspect of the present invention, each conductive ball is transferred onto and joined to the corresponding surface electrode after one of the surface electrode and conductive ball is coated with a flux. 
   According to another aspect of the present invention, conductive balls are arranged on the arrangement substrate to correspond to electrodes of spherical semiconductor elements, and the conductive balls are transferred onto the spherical semiconductor elements at once from the arrangement substrate to form bumps. 
   The fabrication method according to the present invention uses an arrangement substrate having arrangement holes corresponding to surface electrodes of a spherical semiconductor element. Conductive balls are temporarily arranged on the arrangement substrate and then transferred onto the surface electrodes of the spherical semiconductor element, and thereby the conductive balls and the surface electrodes arc brought into contact with each other while they are aligned with each other. 
   In this case, since the surface of the semiconductor element is spherical, the position of each conductive ball may deviate during the transfer process if it is simply placed on the arrangement substrate for temporary arrangement. In the present invention, therefore, positional regulation is effected when each conductive ball on the arrangement substrate is brought into contact with a corresponding electrode. This affords a proper and reliable transfer operation for the conductive balls. 
   According to the present invention, in fabricating a semiconductor device comprising such a spherical semiconductor element, conductive balls are temporarily arranged on an arrangement substrate and then transferred onto the surface electrodes of the spherical semiconductor element, and thereby the conductive balls and the surface electrodes are brought into contact with each other while they are aligned with each other. It is, therefore, possible to form spherical bumps of the conductive balls and having excellent characteristics, and realize good electrical connections to an external circuit or the like through the spherical bumps. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing a semiconductor device according to the first embodiment of the present invention; 
       FIG. 2  is a representation for illustrating arrangements of spherical bumps in the semiconductor device according to the first embodiment; 
       FIG. 3  is a sectional view showing a packaging example of the semiconductor device according to the first embodiment; 
       FIGS. 4A and 4B  are sectional views showing another packaging example of the semiconductor device according to the first embodiment; 
       FIG. 5  is a sectional view showing another packaging example of the semiconductor device according to the first embodiment; 
       FIG. 6  is a sectional view showing another packaging example of the semiconductor device according to the first embodiment; 
       FIGS. 7A  to  7 C are views showing another packaging example of the semiconductor device according to the first embodiment; 
       FIGS. 8A  to  8 C are views showing another packaging example of the semiconductor device according to the first embodiment; 
       FIG. 9  is a plan view showing an arrangement of electrodes in the semiconductor device according to the first embodiment; 
       FIG. 10  is a perspective view showing a semiconductor device according to the second embodiment; 
       FIG. 11  is a representation for illustrating arrangements of spherical bumps in the semiconductor device according to the second embodiment; 
       FIG. 12  is a plan view showing the state that metal balls are temporarily arranged on an arrangement substrate in the semiconductor device according to the second embodiment; 
       FIG. 13  is a sectional view showing a metal ball temporarily arranged on the arrangement substrate according to the second embodiment shown in  FIG. 12 ; 
       FIG. 14  is a partially enlarged view of an arrangement hole of the arrangement substrate in a fabrication method of the semiconductor device according to the second embodiment; 
       FIG. 15  is a sectional view showing the state that metal balls are transferred in the fabrication method of the semiconductor device according to the second embodiment; 
       FIGS. 16A and 16B  are sectional views showing a packaging example of the semiconductor device according to the second embodiment; 
       FIG. 17  is a sectional view showing another packaging example of the semiconductor device according to the second embodiment; and 
       FIGS. 18A and 18B  are perspective views showing a construction for transferring metal balls onto spherical semiconductor elements at once in the fabrication method of the semiconductor device according to the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
   First Embodiment 
     FIG. 1  shows a semiconductor device according to an embodiment of the present invention. In this embodiment, spherical conductive bumps  10  are formed at the positions of electrodes of a spherical semiconductor element  1 . 
   The spherical semiconductor element  1  is fabricated by forming a desired electric circuit on the surface of a spherical silicon crystal material through fabrication steps. The fabrication steps mainly includes steps of cleaning a crystal material, forming oxide films, forming photoresist films, photolithographing by spherical exposure, patterning by developing, etching, etc. For the circuit formed through these steps, electrodes are formed for electrical connection to the outside. More specifically, an arrangement of electrodes is formed on the spherical surface of the spherical semiconductor element  1 . 
   Referring to  FIG. 1 , a group of spherical bumps  10  for making connection to the outside are formed on a circumference on the surface of the spherical semiconductor element  1 . These spherical bumps  10  are made by the manner of transferring conductive metal balls onto the electrode portions of the spherical semiconductor element  1 . These spherical bumps  10  have a common contact plane (that may be spherical) S. These spherical bumps  10  protrude above the spherical semiconductor element  1  such that a predetermined gap is formed between the contact plane S and the surface of the spherical semiconductor element  1 . 
     FIG. 2  schematically shows examples of arrangement of spherical bumps  10 . Each of the spherical bumps  10  contacting the contact plane S is joined to an electrode  2  formed on the surface of the spherical semiconductor element  1 . As shown in  FIG. 2 , a,gap G is formed between the contact surface S and the surface (having an apex P) of the spherical semiconductor element  1 . The spherical bumps  10  are disposed to protrude above the spherical semiconductor element  1  and form the gap G. This affords an effective margin for pressure deformation of the spherical bumps  10  when the spherical bumps  10  are pressed onto objects to join, and ensures proper bump joining. Note that bump joining is possible even when no gap is formed between the contact plane S and the surface of the spherical semiconductor element  1 , i.e., when G=0. 
   In another example shown in  FIG. 2 , no gap is formed between a contact surface S′ that includes the apex P, and the surface of the spherical semiconductor element  1 . In this state, however, proper bump joining becomes difficult. So, it is preferable that the level of arrangement of spherical bumps  10  meets the following expression:
 
 R−r ≦( r+R ) cos θ, (0≦θ≦2τ)
 
where R represents the radius of the spherical semiconductor element, r is the radius of each spherical bump, and θ is the angle between the line connecting the centers of the spherical semiconductor element and spherical bump, and the diameter extending through the apex P. The relation between the size r of each spherical bump and the position θ of the spherical bump is designed in accordance with the size of each electrode on the element surface and the necessary number of electrodes.
 
   The spherical bumps  10  can be formed on the electrodes  2  of the spherical semiconductor element  1  by thermo-compression bonding. In this case, each spherical bump  10  is made of a refractory metal material having a melting point preferably of not less than 550° C., more preferably of not less than 600° C. Particularly in case of each electrode  2  made of aluminum or copper, or an alloy containing one or more of those metals, each spherical bump  10  is made of gold, platinum, palladium, silver, copper, aluminum or nickel, or an alloy containing one or more of those metals. 
   Each spherical bump  10  may deform into a shape like a Rugby ball, or unevenly deform at its part, e.g., its junction portion. In order for the spherical bumps  10  surely to protrude beyond the apex P of the spherical semiconductor element  1 , two or more layers of bumps may be used that are stacked like “dumplings” or “rosary”. 
   Alternatively, the spherical bumps  10  can be formed on the electrodes  2  of the spherical semiconductor element  1  by melting. In this case, each spherical bump  10  is made of a low-melting metal material having a melting point preferably of not more than 450° C., more preferably of not less than 400° C. Particularly in case of each electrode  2  made of aluminum or copper, or an alloy containing one or more of those metals, each spherical bump  10  is made of lead, tin, indium, bismuth or zinc, or an alloy containing one or more of those metals, or an alloy mainly containing gold-silicon alloy, gold-tin alloy, or silver-tin alloy. 
   In the latter case, one or more metals selected from titanium, tungsten, titanium tungsten, nickel, chromium, gold, palladium, copper, and platinum are preferably formed on each electrode  2  in layers. The electrodes  2  made of aluminum or its alloy show bad wettability to a low-melting metal such as solder. For this reason, such a metal layer or layers as described above are provided on each electrode  2  as underlayer for giving good wettability and preventing diffusion or oxidation. 
   When the semiconductor device of this embodiment is packaged, its inner electric circuit is connected to an external circuit or the like through the spherical bumps  10  formed as described above. The electrodes  2  are then connected to electrodes of, e.g., a ceramics substrate, a film carrier, a silicon substrate, a printed circuit board, a lead frame, a semiconductor chip, or another spherical semiconductor element. Note that two or more spherical semiconductor elements may be connected to a substrate or the like after being connected to each other. 
     FIG. 3  shows an example of a BGA (Ball Grid Arrangement) package using a spherical semiconductor element  1  according to this embodiment. Referring to  FIG. 3 , each electrode  2  of the spherical semiconductor element  1  is connected to a printed circuit board  20  through the spherical bump  10  formed on the electrode  2 . The printed circuit board  20  connected to the spherical semiconductor element  1  is further connected to various electronic devices to exchange electrical signals with those devices. Note that two or more spherical semiconductor elements  1  may be packaged in a single BGA package. 
   When a semiconductor device according to this embodiment is packaged, its spherical semiconductor element  1  is preferably encapsulated with an encapsulating material  3  as shown in FIG.  3 . As the encapsulating material  3 , it is preferable to use an insulating material such as a resin or a mold compound containing a resin and filler. With this encapsulation, it is possible to protect the circuit surface of the spherical semiconductor element  1  or effectively to suppress thermal strain resulting from the difference in thermal expansion coefficient between the spherical semiconductor element  1  and the printed circuit board  20  or the like. 
   Referring to  FIG. 4A , spherical semiconductor elements  1  according to this embodiment arc connected to each other through some of spherical bumps  10  formed on their electrodes  2 , and mounted on a printed circuit board  20 . In this case, such spherical semiconductor elements  1  are preferably encapsulated as a whole with an encapsulating resin  3 , as shown in FIG.  4 B. 
     FIG. 5  shows an example of a QFP (Quad Flat Package) using a spherical semiconductor element  1  according to this embodiment. Referring to  FIG. 5 , each electrode  2  of the spherical semiconductor element  1  is connected to a lead frame  21  through the spherical bump  10  formed on the electrode  2 . This spherical semiconductor element  1  is also preferably encapsulated with an encapsulating material  3 . Also in such a semiconductor device, two or more spherical,semiconductor elements  1  may be connected to such a lead frame substrate. 
     FIG. 6  shows another example of packaging for a semiconductor device according to this embodiment. Referring to  FIG. 6 , each electrode  2  of a spherical semiconductor element  1  for a memory device is connected to a semiconductor chip  22  through the spherical bump  10  formed on the electrode  2 . 
   As shown in  FIG. 7A , two spherical semiconductor elements  1  according to this embodiment can be connected to each other through spherical bumps  10  of one of them. 
   Alternatively, two spherical semiconductor elements  1  according to this embodiment can be connected to each other through spherical bumps  10  of both of them, as shown in FIG.  7 B. In this case, the spherical bumps of each couple have been previously joined to each other. Also in this case, each spherical bumps  10  may deform after being pressed, as shown in FIG.  7 C. 
   As shown in  FIG. 8A , a spherical semiconductor element  1  according to this embodiment can be connected to a junction surface of, e.g., a printed circuit board  20  through spherical bumps  10   a  and  10   b  of different sizes. In this case, the spherical bumps  10   a  and  10   b  are concentrically arranged on the surface of the spherical semiconductor element  1 , as shown in FIG.  8 B. As the outer spherical bumps  10   a , two or more layers of spherical bumps can be used, as shown in FIG.  8 C. 
   Several examples of semiconductor device of the present invention have been explained together with several typical packaging manners. As described above, a semiconductor device according to the present invention is provided with spherical conductive bumps  10  formed at the positions of the electrodes  2  of each spherical semiconductor element  1 . So, electrical connections to the outside can be easily and accurately made through the spherical conductive bumps  10 . 
   In a spherical semiconductor element  1  according to the present invention, each electrode  2  preferably has a trapezoidal shape or a fan shape. The electrodes  2  constituting one connection group are arranged around a center such that the longer side of each electrode  2  is positioned outside, as shown in FIG.  9 . With such an arrangement, peeling resistance to external stress after joining can be increased. 
   Alternatively, each electrode  2  can be a polygon having five sides or more, or a circle (as plane figure). When the spherical bump  10  on each electrode  2  is pressed onto an object to join, such a shape of the electrode  2  makes it possible uniformly to disperse the load produced between the electrode  2  and spherical bump  10 , and so avoid stress concentration. So, the generation of strain or the like during bump joining process can be eliminated, and proper bump joining is ensured. 
   In a spherical semiconductor element  1  according to the present invention, each electrode  2  preferably has an area equivalent to that of a circle having a diameter which is 3% or more of the diameter of the spherical semiconductor element  1 . By thus setting the area of each electrode  2 , when the semiconductor device is put into practical use by packaging or the like, it is possible to obtain enough joining strength to resist the pressure load during bump joining process. Proper and good bump joining is ensured also in this respect. 
   In the above embodiment, a high-melting bump having a melting point of 600° C. or more may be formed on each electrode  2  of a spherical semiconductor element  1  and connected to an electrode of a ceramic substrate, a film carrier, a silicon substrate, a printed circuit board, a lead frame, a semiconductor chip, or another spherical semiconductor element through a low-melting metal having a melting point of 400□ or less. It may also be possible previously to form a refractory metal bump also on the other electrode to connect, and then to join the refractory metals on both electrodes through a low-melting metal. 
   Second Embodiment 
   Next, a fabrication method for a semiconductor device according to an embodiment of the present invention will be described. 
     FIG. 10  shows a semiconductor device according to an embodiment of the present invention. In this device, spherical conductive bumps  110  are formed at the positions of electrodes of a spherical semiconductor element  101 . 
   The spherical semiconductor element  101  is fabricated by forming a desired electric circuit on the surface of a spherical silicon crystal material through fabrication steps. The fabrication steps mainly includes steps of cleaning a crystal material, forming oxide films, forming photoresist films, photolithographing by spherical exposure, patterning by developing, etching, etc. For the circuit formed through these steps, electrodes are formed for electrical connection to the outside. More specifically, an arrangement of electrodes is formed on the spherical surface of the spherical semiconductor element  101 . 
   Referring to  FIG. 10 , a group of spherical bumps  110  for making connection to the outside are formed on a circumference on the surface of the spherical semiconductor element  101 . These spherical bumps  110  are made by the manner of transferring conductive metal balls onto the electrode portions of the spherical semiconductor element  101 . These spherical bumps  110  have a common contact plane (that may be spherical) S. These spherical bumps  110  protrude above the spherical semiconductor element  101  such that a predetermined gap is formed between the contact plane S and the surface of the spherical semiconductor element  101 . 
     FIG. 11  schematically shows examples of arrangement of spherical bumps  110 . Each of the spherical bumps  110  contacting the contact plane S is joined to an electrode  102  formed on the surface of the spherical semiconductor element  101 . As shown in  FIG. 11 , a gap G is formed between the contact surface S and the surface (having an apex P) of the spherical semiconductor element  101 . The spherical bumps  110  are disposed to protrude above the spherical semiconductor element  101  and form the gap G. This affords an effective margin for pressure deformation of the spherical bumps  110  when the spherical bumps  110  are pressed onto objects to join, and ensures proper bump joining. 
   In another example shown in  FIG. 11 , no gap is formed between a contact surface S′ that includes the apex P, and the surface of the spherical semiconductor element  101 . In this state, however, proper bump joining becomes difficult. So, it is preferable that the level of arrangement of spherical bumps  110  meets the following expression:
 
 R−r ≦( r+R ) cos θ, (0≦θ≦2τ)
 
where R represents the radius of the spherical semiconductor clement, r is the radius of each spherical bump, and θ is the angle between the line connecting the centers of the spherical semiconductor element and spherical bump, and the diameter extending through the apex P.
 
   For fabricating the spherical semiconductor device as described above that has the spherical bumps  110  formed on the electrodes  102  on the spherical semiconductor element  101 , an arrangement substrate is used which has holes in the arrangement corresponding to the electrodes  102  on the spherical semiconductor element  101 . Conductive metal balls for forming the spherical bumps  110  are temporarily arranged on this arrangement substrate, and then transferred onto the surfaces of the electrodes  102  of the spherical semiconductor element  101  to join. 
     FIG. 12  shows the state that the conductive metal balls  111  for forming the spherical bumps  110  are temporarily arranged on the arrangement substrate  120 . For forming the spherical bumps  110  arranged on a circumference on the surface of the spherical semiconductor element  101  as shown in  FIG. 10 , the metal balls  111  are temporarily arranged in the form of a circle as shown in FIG.  12 . 
   Referring to  FIG. 13 , each metal ball  111  is positioned and held by an arrangement hole  121  of the arrangement substrate  120 . In this example, each arrangement hole  121  is formed at the position along a circumference corresponding to each electrode  102  on the spherical semiconductor element  101 . The arrangement substrate  120  may be a flat plate. An opening portion  121   a  of each arrangement hole  121  is tapered. This taper makes the metal ball  111  stable, so the metal ball  111  can be accurately positioned and held. 
   Referring to  FIG. 14 , the taper angle α at the opening portion  121   a  of each arrangement hole  121  is designed to be within the range of preferably 10°&lt;α&lt;60°, more preferably, 30°−θ&lt;α&lt;60°−0, (θ&lt;20°). 
   An appropriate vacuum source (not shown) may be connected to the arrangement holes  121  of the arrangement substrate  120 . With the vacuum source, the metal ball  111  temporarily arranged on each arrangement hole  121  can be drawn by negative pressure, as indicated by the dotted line in  FIG. 13 , and held on the arrangement hole  121  by the sucking force. 
   The metal balls  111  can be transferred onto and joined to the surfaces of the electrodes  102  of the spherical semiconductor element  101  by thermo-compression bonding. In  FIG. 15 , the metal balls  111  are temporarily arranged in the form of a circle on the arrangement substrate  120 , as shown in FIG.  12 . The spherical semiconductor element  101  is moved down toward the metal balls  111 . The metal balls  111  and the electrodes  102  of the spherical semiconductor element  101  are brought into contact with each other while being aligned. The metal balls  111  can be transferred onto and joined to the electrodes  102  by pressing the metal balls  111  against the electrodes  102  with appropriately heating. The spherical bump  110  is thus formed on each electrode  102  of the spherical semiconductor element  101 . 
   In this example, each metal ball  111  is accurately positioned and held by the tapered opening  121   a  of the corresponding arrangement hole  121  of the arrangement substrate  120 , as shown in FIG.  13 . Each metal ball  111  can be properly and reliably transferred onto the corresponding electrode  102  by regulating the position of the metal ball  111  so as to be stable. 
   The metal balls  111  are transferred such that a gap G is formed between the surface  120   a  of the arrangement substrate  120  and the lowermost point (the apex P shown in  FIG. 11 ) of the spherical semiconductor element  101 . The gap G is determined by geometrical relations such as the arrangement position of the electrodes  102  and the size of the metal balls  111 . 
   When the metal balls  111  are transferred onto and joined to the electrodes  102  of the spherical semiconductor element  101  to form the spherical bumps  110 , the metal balls  111  can be drawn onto the arrangement holes  121  by vacuum. In this case, the metal balls  111  can be held on the lower side of the arrangement substrate  111 , so the above process can be performed in the reverse vertically positional relation. 
   The metal balls  111  can be transferred onto and joined to the electrodes  102  of the spherical semiconductor element  101  also by melting. In this case, each electrode  102  of the spherical semiconductor element  101  or each metal ball  111  is preferably coated with a flux. It is because an electrode made of an alloy of, e.g., aluminum shows bad wettability in general to a low-melting metal such as solder. Such flux coating as described above affords good joining properties. Such flux coating is useful also for removing solder oxide films and fixing the metal balls. 
   When the semiconductor device fabricated as described above is packaged, its inner electric circuit is connected to an external circuit or the like through the spherical bumps  110  formed as described above. The electrodes  102  of the spherical semiconductor element  101  are then connected to electrodes of, e.g., a ceramics substrate, a film carrier, a silicon substrate, a printed circuit board, a lead frame, a semiconductor chip, or another spherical semiconductor element. 
     FIG. 16A  shows an example of a BGA package using a spherical semiconductor element  101 . Referring to  FIG. 16A , each electrode  102  of the spherical semiconductor element  101  is connected to a printed circuit board  130  through the spherical bump  110  formed on the electrode  102 . The printed circuit board  130  connected to the spherical semiconductor element  101  is further connected to various electronic devices to exchange electrical signals with those devices. 
   When the semiconductor device fabricated as described above is packaged, its spherical semiconductor element  101  is preferably encapsulated with an encapsulating material  103  as shown in FIG.  16 A. As the encapsulating material  103 , it is preferable to use an insulating material such as a resin or a mold compound containing a resin and filler. With this encapsulation, it is possible to protect the circuit surface of the spherical semiconductor element  101  or effectively to suppress thermal strain resulting from the difference in thermal expansion coefficient between the spherical semiconductor element  101  and the printed circuit board  130  or the like. 
   Referring to  FIG. 16B , spherical semiconductor elements  101  are connected to each other through some of spherical bumps  110  formed on their electrodes  102 , and mounted on a printed circuit board  120 . In this case, such spherical semiconductor elements  101  are preferably encapsulated as a whole with an encapsulating resin  103 . 
     FIG. 17  shows an example of a QFP using a spherical semiconductor element  101 . Referring to  FIG. 17 , each electrode  102  of the spherical semiconductor element  101  is connected to a lead frame  131  through the spherical bump  110  formed on the electrode  102 . This spherical semiconductor element  101  is also preferably encapsulated with an encapsulating material  103 . 
     FIGS. 18A and 18B  show an example in which spherical bumps  110  are formed onto spherical semiconductor elements  101  at once. In this example, metal balls are arranged on an arrangement substrate so as to correspond to the arrangements of the electrodes of the spherical semiconductor elements, and then the metal balls are transferred onto the electrodes at once. In this manner, groups of metal balls can be transferred at once from one arrangement substrate. 
   More specifically, spherical semiconductor elements  101  are arranged on a holding substrate  200  such that the electrodes  102  of each spherical semiconductor element  101  face down, as shown in FIG.  18 A. Groups of metal balls  111  are temporarily arranged on an arrangement substrate  300  so as to correspond to the spherical semiconductor elements  101 . The metal balls  111  are accurately positioned by dimples or recesses  301  (see  FIG. 18B ) formed on the arrangement substrate  300 . While the electrodes  102  and the metal balls  111  are aligned with each other, the holding substrate  200  is overlaid on the arrangement substrate  300 . 
   An appropriate pressure is applied to the layers of the holding substrate  200  and arrangement substrate  300  to transfer the metal balls  111  onto the electrodes  102  and join the former to the latter. After this, the holding substrate  200  is pulled up, as shown in  FIG. 18B. A  spherical bump  110  is then formed on each electrode  102  of each spherical semiconductor element  101 . By forming the bumps on the spherical semiconductor elements  101  at once in this manner, the efficiency of fabricating spherical semiconductor devices can be greatly improved. 
   In the example described above, the arrangement of the spherical bumps  110  to be formed at the positions of the electrodes  102  of each spherical semiconductor element  101  is not limited to a circle as shown in  FIG. 10 , but other various arrangements can be employed. In any case, electrical connections to the outside can be easily and accurately made through spherical bumps  110  formed.