CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-66475, filed on Mar. 24, 2011, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to methods of manufacturing semiconductor devices. 
       BACKGROUND 
       [0003]    Examples of a method for mounting a semiconductor element on a circuit board or the like by flip-chip bonding include a method for soldering a semiconductor element, a method in which conductive particles are sandwiched between electrode terminals so as to be in contact with each other and are fixed with resin so as to be coupled, and a similar method. 
         [0004]    Japanese Laid-open Patent Publications Nos. 04-309474 and 05-131279 disclose the related art. 
       SUMMARY 
       [0005]    According to an aspect of the embodiments, a method of manufacturing a semiconductor device includes: forming a first layer including crystals by processing a surface of a first electrode of a semiconductor element; forming a second layer including crystals by processing a surface of a second electrode of a mounting member on which the semiconductor element is mounted; reducing a first oxide film present over or in the first layer and a second oxide film present over or in the second layer at a first temperature, the first temperature being lower than a second temperature at which a first metal included in the first electrode diffuses in a solid state and being lower than a third temperature at which a second metal included in the second electrode diffuses in a solid state; and bonding the first layer and the second layer to each other by solid-phase diffusion. 
         [0006]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0007]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  illustrates an exemplary method for manufacturing a semiconductor device; 
           [0009]      FIGS. 2A and 2B  illustrate an exemplary method for manufacturing a semiconductor device; 
           [0010]      FIGS. 3A to 3D  illustrate an exemplary electron diffraction pattern; 
           [0011]      FIGS. 4A and 4B  illustrate an exemplary method for manufacturing a semiconductor device; 
           [0012]      FIGS. 5A and 5B  each illustrate an exemplary surface of an electrode; 
           [0013]      FIGS. 6A ,  6 B, and  6 C illustrate an exemplary method for manufacturing a semiconductor device; 
           [0014]      FIG. 7  illustrates an exemplary semiconductor element; 
           [0015]      FIGS. 8A and 8B  each illustrate an exemplary sample; 
           [0016]      FIGS. 9A and 9B  each illustrate an exemplary fracture; 
           [0017]      FIG. 10A  illustrates an exemplary relationship between bonding temperature and die shear strength; 
           [0018]      FIG. 10B  illustrates an exemplary relationship between bonding temperature and percentage of bulk fractures; 
           [0019]      FIGS. 11A to 11D  each illustrate an exemplary chip sample; 
           [0020]      FIGS. 12A and 12B  illustrate an exemplary electrode; and 
           [0021]      FIGS. 13A and 13B  illustrate an exemplary electrode. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]    For example, when the pitch between electrodes is fine, the soldering of semiconductor elements may be difficult. When a semiconductor element is thin, a junction is fixed to reduce the warpage until cooling is finished, whereby treatment time may be increased. 
         [0023]    Anisotropic conductive films prepared by dispersing conductive particles in filmy insulating resins are used to couple specific semiconductor elements such as drivers for liquid crystal displays (LCDs). The reliability of connection may be low at a temperature not lower than the glass transition temperature (Tg) of each insulating resin. 
         [0024]    Thermocompression bonding may damage circuits in semiconductor elements because high temperature and high pressure for solid-phase diffusion are applied to electrodes. 
         [0025]    After surfaces of electrodes are planarized by chemical mechanical polishing (CMP), the electrode surfaces are activated by argon plasma or the like in a vacuum and solid-phase diffusion bonding is performed at low temperature (surface activation bonding). When performing surface activation bonding at a temperature at which circuits in semiconductor elements are not damaged, sufficient bonding strength may not be obtained. The use of expensive vacuum equipment may cause an increase in cost. 
         [0026]      FIG. 1  illustrates an exemplary method for manufacturing a semiconductor device. 
         [0027]    In an operation S 1 , surfaces of electrodes of a semiconductor element and surfaces of electrodes of a mounting member for mounting the semiconductor element are machined, whereby a microcrystalline layer with a reduced grain size due to machining is provided on a surface of each electrode. The electrodes may include at least one of, for example, Cu, Sn, Al, and Ni. Cu, Sn, Al, and Ni may be likely to be oxidized. A material for forming the electrodes of the semiconductor element may be different from a material for forming the electrodes of the mounting member. The mounting member includes, for example, a lead frame, a circuit board, or the like. 
         [0028]      FIGS. 2A and 2B  illustrate an exemplary method for manufacturing a semiconductor device. As illustrated in  FIG. 2A , for example, a semiconductor element  10  includes a circuit section  11  and electrodes  12  and a resin  13  is embedded between the electrodes  12 . As illustrated in  FIG. 2B , surfaces of the electrodes  12  and the resin  13  are cut off with a diamond turning tool  15  including a base section  15   a  and a cutting section  15   b.  A microcrystalline layer including a large number of dislocations is formed on a surface of each electrode  12 . The mounting member may be treated in substantially the same or similar way. The microcrystalline layer may have a thickness of about 100 nm. 
         [0029]      FIGS. 3A to 3D  illustrate an exemplary electron diffraction pattern. The electron diffraction pattern illustrated in  FIGS. 3A to 3D  may be an electron diffraction pattern of the cut-off electrodes illustrated in  FIG. 2A .  FIG. 3A  illustrates a figure corresponding to a transmission electron microscope (TEM) photograph of the cut-off electrodes.  FIG. 3B  illustrates an electron diffraction pattern of a site P 1  illustrated in  FIG. 3A .  FIG. 3C  illustrates an electron diffraction pattern of a site P 2  illustrated in  FIG. 3A .  FIG. 3D  illustrates an electron diffraction pattern of a site P 3  illustrated in  FIG. 3A . With reference to  FIG. 3A , a large number of dislocations are present in a region close to the surface. As illustrated in  FIGS. 3B ,  3 C, and  3 D, the crystal orientation of a region closer to the surface is more disordered. Although being not illustrated, the dislocation density of each electrode is substantially uniform before cutting. The electron diffraction patterns of the sites P 1  and P 2  may be substantially the same as the electron diffraction pattern of the site P 3 . A microcrystalline layer with a reduced grain size due to cutting may be present on a surface of each cut-off electrode. 
         [0030]    Machining may be grinding, sand blasting, or the like in addition to cutting. 
         [0031]    In an operation S 2  illustrated in  FIG. 1 , after the machining of the electrode surfaces, the electrode surfaces are reduced at a temperature lower than a temperature at which solid-phase diffusion occurs in the electrodes.  FIGS. 4A and 4B  illustrate an exemplary semiconductor device-manufacturing method. As illustrated in  FIG. 4A , for example, a circuit board  31  including electrodes  32  and the semiconductor element  10  including the electrodes  12 , are placed on a stage  22  placed in a housing  21 . A formic acid gas is introduced into the housing  21  and the housing  21  is heated to 120° C. The surfaces of the electrodes  12  and the electrodes  32 , which have the microcrystalline layers formed by machining, are reduced. 
         [0032]    When the formic acid gas is used to perform reducing treatment, the treatment temperature may be 100° C. to 150° C. When the treatment temperature is lower than 100° C., a reducing reaction may not proceed. When the treatment temperature exceeds 150° C., the number of crystals larger than fine crystals present on a surface of each electrode may be increased because the fine crystals are recrystallized.  FIGS. 5A and 5B  each illustrate an exemplary surface of an electrode. The electrode surface illustrated in  FIG. 5A  may be a surface of a reduced electrode. In  FIG. 5A , reducing treatment is performed at 120° C. In  FIG. 5B , reducing treatment is performed at 180° C. Grain boundaries between relatively large crystals formed by recrystallization may be present in the electrode surface illustrated in  FIG. 5B . 
         [0033]    Formic acid, hydrogen radicals, or a carbon monoxide gas may be used as a reductant. When such hydrogen radicals are used, the treatment temperature may be 25° C. to 150° C. When the treatment temperature exceeds 150° C., the number of crystals larger than fine crystals present on the surface of each electrode may be increased because the fine crystals are recrystallized. When such a carbon monoxide gas is used, the treatment temperature may be 50° C. to 150° C. When the treatment temperature is lower than 50° C., a reducing reaction may not proceed. When the treatment temperature exceeds 150° C., the number of crystals larger than fine crystals present on the surface of each electrode may be increased because the fine crystals are recrystallized. 
         [0034]    In an operation S 3  illustrated in  FIG. 1 , after reducing treatment, the electrodes  12  of the semiconductor element  10  are aligned with the electrodes of the mounting member at a temperature lower than a temperature at which solid-phase diffusion occurs in the electrodes.  FIGS. 6A ,  6 B, and  6 C illustrate an exemplary semiconductor device-manufacturing method. As illustrated in  FIG. 6A , for example, the electrodes  12  of the semiconductor element  10  are horizontally aligned with the electrodes  32  of the circuit board  31  in such a manner that the electrodes to be bonded to each other are arranged opposite to each other. A microcrystalline layer  12   b  is located closer to the surface of each electrode  12  than a base section  12   a  of the electrode  12 . A microcrystalline layer  32   b  is located closer to the surface of each electrode  32  than a base section  32   a  of the electrode  32 . The microcrystalline layer  12   b  and the microcrystalline layer  32   b  are arranged opposite to each other. 
         [0035]    In an operation S 4  illustrated in  FIG. 1 , the electrodes are bonded to each other by solid-phase diffusion in such a manner that voltages are applied between the electrodes and the electrodes are heated to a temperature at which solid-phase diffusion occurs. Solid-phase diffusion bonding may be performed in a non-oxidizing atmosphere, for example, in a vacuum or in an inert gas atmosphere. For example, the electrode  12  and electrodes  32  illustrated in  FIG. 4B  are brought into contact with each other, whereby the microcrystalline layer  12   b  and the microcrystalline layer  32   b  contact with each other as illustrated in  FIG. 6B . Pressurizing and heating are performed and metal atoms in the microcrystalline layer  12   b  and metal atoms in the microcrystalline layer  32   b  diffuse in a solid state. A temperature at which solid-phase diffusion occurs may be about 150° C. to 250° C. As illustrated in  FIG. 6C , the boundary between the electrodes  12  and  32  disappear and a connection member  30  including a bonding section  30   a  is formed in a region corresponding to a region in which the microcrystalline layers  12   b  and  32   b  are present. When the electrodes include substantially the same kind of metal, solid-phase diffusion tends to occur as a grain size reduces. Therefore, high bonding strength may be obtained at a temperature lower than or substantially equal to the temperature of surface activation bonding performed using bumps planarized by chemical mechanical polishing. 
         [0036]    Since solid-phase diffusion bonding is performed between the reduced microcrystalline layers, high bonding strength may be obtained even at relatively low temperatures. In surface activation bonding performed using bumps planarized by chemical mechanical polishing, for example, heating is performed at about 250° C. to 300° C. and sufficient bonding strength may not be obtained. In the solid-phase diffusion bonding of the reduced microcrystalline layers, sufficient bonding strength may be obtained by heating at about 150° C. to 250° C. 
         [0037]    Since no solder is used, no barrier metal such as Ti or Ni is used; hence, costs and man-hours may be reduced. Since similar metals are bonded to each other, the formation of voids due to alloying is reduced, whereby high reliability may be achieved. 
         [0038]    The semiconductor element may be, for example, a large-scale integration (LSI) chip, a memory, or a transistor such as a GaN high electron mobility transistor (HEMT).  FIG. 7  illustrates an exemplary semiconductor element. The semiconductor element illustrated in  FIG. 7  may be a GaN HEMT  42  mounted on a circuit board  41 . The circuit board  41  and the GaN HEMT  42  are coupled to each other with a source connection member  43   s,  a drain connection member  43   d,  and a gate connection member  43   g.  The source connection member  43   s,  the drain connection member  43   d,  and the gate connection member  43   g  are coupled to a source, drain, and gate, respectively, of the GaN HEMT  42 . The circuit board  41  may include, for example, a copper-clad laminate. 
         [0039]      FIGS. 8A and 8B  each illustrate an exemplary sample. For example, a chip sample  51   a  illustrated in  FIG. 8A  may be prepared by cutting, reduction, alignment, and solid-phase diffusion bonding as described above. For example, a chip sample  51   b  illustrated in  FIG. 8B  may be prepared by CMP instead of cutting and by reduction, alignment, and solid-phase diffusion bonding. 
         [0040]    As illustrated in  FIG. 8A , the chip sample  51   a  and a circuit board sample  61   a  are bonded together. In the preparation of the chip sample  51   a , conductive layers  53  and an insulating layer  54  are formed on a surface of a S 1  substrate  52  and bumps  55   a  are formed on the conductive layers  53 . The surface of each bump  55   a  is cut off, whereby a microcrystalline layer is formed. In the preparation of the circuit board sample  61   a,  conductive layers  63  and an insulating layer  64  are formed on a surface of a S 1  substrate  62  and a plate bump  65   a  is formed on the conductive layers  63 . The surface of the plate bump  65   a  is cut off, whereby a microcrystalline layer is formed. A Cu electrode may be used as a bump material. Since the hardness of the Cu electrode is higher than the hardness of an Au electrode, a thick microcrystalline layer may be formed. As for cutting, a fly cutting process using a single-crystalline diamond turning tool may be used. In the fly cutting process, all workpieces, for example, bumps, formed on a wafer are machined at substantially the same speed, whereby a microcrystalline layer on the surface of each bump may have substantially a uniform thickness. An R-shaped diamond turning tool with a nose diameter of 10 mm may be used. The edge of the turning tool may have a nose radius of 50 nm to 300 nm. 
         [0041]    As illustrated in  FIG. 8B , the chip sample  51   b  and a circuit board sample  61   b  are bonded together. In the preparation of the chip sample  51   b , conductive layers  53  and an insulating layer  54  are formed on a surface of a S 1  substrate  52  and bumps  55   b  are formed on the conductive layers  53 . Surfaces of the bumps  55   b  are subjected to CMP. In the preparation of the circuit board sample  61   b,  conductive layers  63  and an insulating layer  64  are formed on a surface of a S 1  substrate  62  and a plate bump  65   b  is formed on the conductive layers  63 . A surface of the plate bump  65   b  is subjected to CMP. In CMP, a hydrogen peroxide slurry and an abrasive pad made of polyurethane may be used. 
         [0042]    The chip samples  51   a  and  51   b  may have a size of 5 mm×5 mm×0.6 mm. In the chip sample  51   a,  392 of the bumps  55   a  are arranged on a peripheral section of the chip sample  51   a.  In the chip sample  51   b,  392 of the bumps  55   b  are arranged on a peripheral section of the chip sample  51   b.  The pitch between the bumps  55   a  and the pitch between the bumps  55   b  may be 40 μm. The bumps  55   a  and  55   b  may have a size of 25 mm×25 mm×0.008 mm. The circuit board samples  61   a  and  61   b  may have a size of 5 mm×5 mm×0.6 mm. The plate bumps  65   a  and  65   b  may have a size of 10 mm×10 mm×0.6 mm and may be arranged one by one. In order to avoid errors due to misalignment, a large plate bump may be used. The accuracy of alignment may not be taken into account depending on a plate bump used. 
         [0043]    The chip samples  51   a  and  51   b  are reduced at a temperature of 120° C. for 30 minutes using a formic acid gas. Since the plate bumps  65   a  and  65   b  are used, alignment may be simply performed. In solid-phase diffusion bonding (thermocompression bonding), two different bonding temperatures may be used, the bonding time may be set to 30 minutes, and the bonding pressure may be set to 300 MPa. 
         [0044]    The chip samples  51   a  and  51   b  are subjected to a die shear test and bonding interfaces are observed. In the die shear test, for example, the shear strength is measured and the fracture mode percentage is investigated.  FIGS. 9A and 9B  each illustrate an exemplary fracture.  FIG. 9A  illustrates a bulk fracture in which a breakage  70  is caused in one of the bumps  55   a  or  55   b.    FIG. 9B  illustrates an interface fracture in which a breakage  70  is caused at the interface between one of the bumps  55   a  or  55   b  and the plate bump  65   a  or the  65   b,  respectively. In the investigation of the fracture mode percentage, the percentage of the number of bulk fractures in the sum of the number of the bulk fractures and the number of interface fractures. In the observation of a bonding interface, a region near the bonding interface is processed with a focused ion beam (FIB) and a cross section including the bonding interface is observed with a scanning electron microscope (SEM). 
         [0045]      FIG. 10A  illustrates an exemplary relationship between bonding temperature and die shear strength.  FIG. 10B  illustrates an exemplary relationship between bonding temperature and percentage of bulk fractures. As illustrated in  FIG. 10A , the chip sample  51   a  has a shear strength that is about two times that of the chip sample  51   b  at a bonding temperature of 200° C. and 250° C. As illustrated in  FIG. 10B , in the chip sample  51   a,  the percentage of bulk fractures is close to 100% at a bonding temperature of 200° C. and 250° C. In the chip sample  51   b,  the percentage of bulk fractures is low at a bonding temperature of 200° C. In the chip sample  51   a,  high bonding structure may be obtained by solid-phase diffusion bonding at about 200° C. 
         [0046]      FIGS. 11A to 11D  each illustrate an exemplary chip sample.  FIGS. 11A to 11D  may be illustrations corresponding to SEM photographs.  FIG. 11A  illustrates a chip sample  51   a  prepared at a bonding temperature of 200° C.  FIG. 11B  illustrates a chip sample  51   a  prepared at a bonding temperature of 250° C.  FIG. 11C  illustrates a chip sample  51   b  prepared at a bonding temperature of 200° C.  FIG. 11D  illustrates a chip sample  51   b  prepared at a bonding temperature of 250° C. Circles illustrated in  FIGS. 11A to 11D  indicate the presence of voids. As illustrated in  FIG. 11A , most bonding interfaces are lost, slight bonding interfaces are observed, and slight voids may be present at a bonding temperature of 200° C. As illustrated in  FIG. 11B , a small number of voids are present and bonding interfaces are, however, lost at a bonding temperature of 250° C. Since voids adjacent to bonding surfaces migrate due to solid-phase diffusion and recrystallization, voids may be scattered. Bonding interfaces may be lost due to the recrystallization of fine crystals. As illustrated in  FIGS. 11C and 11D , in the chip sample  51   b,  bonding interfaces are observed in wide regions independently of bonding temperature. The number of microcrystalline layers is reduced by the action of a treatment solution after CMP and therefore no recrystallization may occur. 
         [0047]      FIGS. 12A and 12B  each illustrate an exemplary electrode.  FIGS. 12A and 12B  may correspond to TEM photographs of electrodes of a chip sample  51   a.    FIGS. 13A and 13B  each illustrate an exemplary electrode.  FIGS. 13A and 13B  may correspond to TEM photographs of electrodes of a chip sample  51   b.  A region illustrated in  FIG. 12B  and a region illustrated in  FIG. 13B  may correspond to a quadrangle illustrated in  FIG. 12A  and a quadrangle illustrated in  FIG. 13A , respectively. As illustrated in  FIGS. 12A and 12B , in the chip sample  51   a,  a region including fine crystal grains is present in a surface section. As illustrated in  FIGS. 13A and 13B , large crystal grains are present over the whole chip sample  51   b.  The possibility of recrystallization depends on the difference between structures and therefore differences between bonding strengths may be caused. 
         [0048]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
A method of manufacturing a semiconductor device includes: forming a first layer including crystals by processing a surface of a first electrode of a semiconductor element; forming a second layer including crystals by processing a surface of a second electrode of a mounting member on which the semiconductor element is mounted; reducing a first oxide film present over or in the first layer and a second oxide film present over or in the second layer at a first temperature, the first temperature being lower than a second temperature at which a first metal included in the first electrode diffuses in a solid state and being lower than a third temperature at which a second metal included in the second electrode diffuses in a solid state; and bonding the first layer and the second layer to each other by solid-phase diffusion.