Abstract:
A semiconductor device including; a bottom plate having a laminated structure in which between a first and a second metal plates a third metal plate harder than these metal plates is clipped, a concave portion formed by removing a part of the first metal plate laminated on the surface of the bottom plate and the third metal plate laminated there under and expose the second metal plate, a semiconductor element arranged in the concave portion H, circuit board connected with the semiconductor element arranged on the surface of the bottom plate, circuit boards arranged on the surface of the bottom plate, a sidewall made of metal and fixed on the bottom plate surrounding the circuit boards and the semiconductor element, a metal lead provided so as to penetrate the side wall through an insulator, and a lid made of metal provided to block an opening formed by the sidewall.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-373405, filed on Dec. 24, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND TECHNOLOGY OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device, which has improved a heat dissipation property of a package for containing semiconductor dies.  
         [0003]     Semiconductor devices used in microwave frequency is so constructed that, for example, semiconductor dies for amplifying radio frequency signals and various circuit boards connected with the semi-conductor dies are arranged in a package. A package is composed of, a bottom plate for fixing semiconductor dies and circuit boards, a side wall provided on the periphery of the bottom plate so as to surround the semiconductor dies and circuit boards, and a lid for covering upper opening formed by the side wall.  
         [0004]     By the way, usually a plurality of power amplifying semiconductor dies is used in a semiconductor device for power use. Therefore, a high mechanical strength is required for a package for power use and the size becomes large because a plurality of semiconductor dies and circuit boards to be connected with them should be contained therein.  
         [0005]     Further, packages used for semiconductor device for power use, need to discharge heat generated by semiconductor dies. For this reason, such metal having high heat conductivity and relatively low price as copper is generally used. However, copper is soft and ductile, and packages having enough mechanical strength for power use cannot be obtained.  
         [0006]     Therefore material having a so called Copper-Molybdenum-Copper structure is used, in which molybdenum or tungsten plate harder than copper is interposed between two copper plates as support metal, to improve mechanical strength in a package for power use with high heat generation. Hereinafter, material having such structure is called as a CMC structure.  
         [0007]     Here, a conventional semiconductor device using a CMC structure package is explained referring to  FIG. 1A  and  FIG. 1B .  FIG. 1A  is a top view showing the package with its lid removed and  FIG. 1B  is a cross section showing a section along the line b-b in  FIG. 1A  seen along an arrow direction. In  FIG. 1A  and  FIG. 1B , the corresponding parts are assigned with same symbols.  
         [0008]     A package  32  is mounted on a metal plate  31 . The package  32  is composed of a bottom plate  32   a,  a sidewall  32   b , and a lid (not illustrated) for covering the opening at the upper portion of the sidewall  32   b . The bottom plate  32   a  has the CMC structure, formed in a rectangular-shape as a whole, for example. Namely, the bottom plate  32   a  has a laminated structure, in which a metal plate M harder than copper is interposed between two copper plates C 1  and C 2  as shown in  FIG. 1B . The metal plate M is, for example, molybdenum Mo or tungsten W. On opposing edges of the bottom plate  32   a , two screw holes  33  for fixing the package  32  on the plate  31  by screw are provided respectively.  
         [0009]     A sidewall  32   b  is provided on the bottom plate  32   a . The major portion of the sidewall  32   b  is made of metal and is formed as a whole rectangular frame in shape. At center portions of two opposing sides of the sidewall  32   b , an input metal lead  34   a  and an output metal lead  34   b  are provided, which penetrates through the sidewall  32   b . At portions surrounding the metal lead  34   a ,  34   b , insulator  35   a ,  35   b  such as ceramic member are provided in order to prevent the metal lead  34   a ,  34   b  from contacting with the metal portion of the sidewall  32   b . One end of the metal lead  34   a ,  34   b  projects inside the sidewall  32   b  and the insulator  35   a ,  35   b  are provided under the projection portion a 1 , b 1 .  
         [0010]     A semiconductor element  36  for microwave power amplification is mounted on a central portion of the bottom plate  32   a  surrounded by the sidewall  32   b . The semiconductor element  36  for microwave power amplification contains, for example, four semiconductor dies  36   a  to  36   d . On the input metal lead  34   a  side of the bottom plate  32   a , a power divider circuit  37  including an input matching circuit formed on the surface of the dielectric plate is located. On the output metal lead  34   b  side of the bottom plate  32   a , a power combiner circuit  38  including an output matching circuit is located. The semiconductor dies  36   a  to  36   d  are electrically connected with the power divider circuit  37  and with the power combiner circuit  38  by wires W. Also, the input metal lead  34   a  is electrically connected with the power divider circuit  37  and the power combiner circuit is electrically connected with the output metal circuit  34   b  by wires W.  
         [0011]     In the device thus constructed, a radio frequency big power signal is supplied through the input metal lead  34   a , for example. The radio frequency signal is divided into four by the power divider circuit  37  and each of divided t signal is amplified in its power by semiconductor dies  36   a  to  36   d . Then, the divided signals were combined by the power combiner circuit  38  and outputted through the output metal lead  34   b.    
         [0012]     When amplifying a radio frequency signal having a large power, heat is generated from the semiconductor  36   a  to  36   d . A part of heat generated is dissipated through the bottom plate  32   a  from the lower surface to the metal plate  31  below. At this time, heat is dissipated through the junction of the bottom plate  32   a  and the metal plate  31  from the lower surface of the bottom plate  32   a  to the metal plate  31  with a spread angle of more than 45°. Further, a part of the heat generated in the semiconductor dies  36   a  to  36   d  is radiated from the upper surface of the semiconductor dies  36   a  to  36   d  upward in the figure by a black body radiation. Here, major portion of the dissipated heat is a downward radiation. The semiconductor device containing semiconductor element and associated circuits in a package as mentioned above is disclosed in Japanese published patent application 2001-257234.  
         [0013]     In the conventional semiconductor device, the thermal expansion coefficient of the package becomes higher than that of the dielectric plate such as a ceramic plate, on which a power divider circuit or the power combiner circuit is formed, if the package is formed with copper itself. Therefore, the ceramic plate may have a crack by thermal shrinkage afterwards, if the power divider circuit or the power combiner circuit is soldered on the bottom plate of the package.  
         [0014]     On the other hand, the crack generated in the ceramic plate can be avoided, if the bottom plate of the package is formed as a CMC structure material, because a heat expansion coefficient of molybdenum is close to that of the ceramic plate composing the power divider circuit or the power combiner circuit. However, heat dissipation ability is decreased because thermal conductivity of molybdenum is as low as about 40% of that of copper and the heat conductivity of the bottom plate including the junction of molybdenum and copper is less than a half of that of copper.  
       SUMMARY OF THE INVENTION  
       [0015]     According to one aspect of the present invention, a semiconductor device is provided including a bottom plate having a first, a second and a third metal plates formed in a laminated structure, in which the third metal plate is harder than the first and the second metal plate and is interposed between a first and a second metal plates, a concave portion formed so as to expose the second metal plate laminated on the bottom plate by removing a part of the first metal plate and the third metal plate laminated on t he second metal plate, a semiconductor element provided in the concave portion, a circuit board connected with the semiconductor element and arranged on the surface of the bottom plate, a metallic side wall fixed on the bottom plate surrounding the circuit board and the semiconductor element, a metal lead, which penetrates the side wall through an insulator, and a metallic lid, which closes an opening formed by the side wall. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1A  is a plan view showing a structure of the conventional semiconductor device.  
         [0017]      FIG. 1B  is a cross section along the line b-b in  FIG. 1A .  
         [0018]      FIG. 2A  is a plan view showing a semiconductor device according to an embodiment of the present invent ion.  
         [0019]      FIG. 2B  is a cross section along the line b-b in  FIG. 2A .  
         [0020]      FIG. 3  is a partial cross sectional view for explaining another embodiment according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     A semiconductor device according to a first embodiment of the present invention will be explained referring to  FIG. 2A  and  FIG. 2B .  FIG. 2A  is a plan view showing a semiconductor device, in which a package is shown with a lid eliminated and  FIG. 2B  is a cross section along by a line b-b in  FIG. 2A . Here, the same symbols are assigned to corresponding parts shown in  FIG. 2A  and  FIG. 2B .  
         [0022]     A package  12  is fixed on a plate  11  formed with a metal material such as aluminum (Al) etc. The package  12  is composed of a bottom plate  12   a , a side wall  12   b , and a lid (not illustrated) for covering an opening formed by the side wall  12   b . The bottom plate  12   a  has a laminated structure in which a support metal plate M harder than copper is interposed between two copper plates C 1  and C 2  as shown in  FIG. 2B . The support metal plate M is molybdenum Mo or tungsten W, for example. The bottom plate  12   a  is about 1 mm thick, for example. The copper plate C 1  and C 2  and the support metal plate M have a substantially same thickness.  
         [0023]     A concave portion H is formed at nearly central portion of the bottom plate  12   a  surrounded by the side wall  12   b , which is formed downward from the surface of the bottom plate  12   a . The concave portion H has a depth that the bottom portion is formed at the same level as or lower than the level of the support metal plate M. That is, an area R is formed in a part of the bottom plate, where the upper copper plate C 1  and the support metal plate M are removed and the lower copper plate C 2  is exposed. Here, it is desirable that the surface of the lower copper plate C 2  is not removed as slight as possible. If the surface of the copper plate C 2  is removed too much, it may affect the mechanical strength of the bottom plate  12   a , wherein a twist may occur on the bottom plate  12   a  by a heat cycle.  
         [0024]     On each of a pair of opposing edge portions of the bottom plate  12   a  extending out of the side wall  12   b , two screw holes  13  are provided for fixing the package  12  on the plate  11  by screws.  
         [0025]     The side wall  12   b  is provided on the bottom plate  12   a . The major portion of the side wall  12   b  is made of metal, which is formed in a rectangular frame shape as a whole. On a central portion of each of a pair of opposing sides of the side wall  12   b , an input metal lead  14   a  and an output metal lead  14   b  are respectively provided, which penetrates through the side wall  12   b . On the penetrating portion of the metal lead  14   a  and  14   b , insulators  15   a  and  15   b  made of ceramic and the like are provided around the metal lead  14   a  and  14   b . One end portion of the metal lead  14   a  and  14   b  are protruding inside the side wall  12   b  and insulators  15   a  and  15   b  made of the ceramic are also provided under the protruding portion a 1 , b 1 , to form a micro strip line.  
         [0026]     Semiconductor element  16 , which is composed of four GaAs semiconductor dies  16   a  to  16   d  for micro wave power amplification, for example, are mounted in the concave portion H formed on the bottom plate  12   a . Semiconductor dies  16   a  to  16   d  are mounted on the lower copper plate C 2  exposed by removing the upper copper plate C 1  and the support metal plate M in the concave portion H by soldering or other suitable methods.  
         [0027]     A power divider circuit  17  including an input matching circuit formed on the surface of the dielectric plate is provided on the bottom plate  12   a  on the input metal lead  14   a  side of the semiconductor dies  16   a  to  16   d . Similarly a power combiner circuit  18  including an output matching circuit formed on the dielectric plate is provided on the bottom plate  12   a  on the output metal lead  14   b  side of the semiconductor dies  16   a  to  16   d . The semiconductor dies  16   a  to  16   d  are electrically connected with the power divider circuit  17  by wires W. The semiconductor dies  16   a  to  16   d  are electrically connected with and the power combiner circuit  18  by wires W. The power divider circuit  17  is electrically connected with the input metal lead  14   a  by a wire W. The power combiner circuit  18  is electrically connected with the output metal lead  14   b  by a wire W.  
         [0028]     In the semiconductor device described above, a radio frequency (hereinafter referred as RF) signal of high power, for example, is supplied to the input metal lead  14   a . The RF signal is divided into four signals by the power divider circuit  17  and each of the signals is amplified by the semiconductor dies  16   a  to  16   d , respectively. The signals amplified by the semiconductor dies are combined by the power combiner circuit  18  and are outputted through the output metal lead.  
         [0029]     When the RF signal is amplified to a high power signal by the semiconductor dies  16   a  to  16   d , heat is generated in the operating layer inside the semiconductor dies during their operation. A part of the heat generated in the semiconductor dies  16   a  to  16   d  is transmitted through the bottom plate  12   a  and radiated downward of the figure from its lower surface. Apart of the heat generated in the semiconductor dies  16   a  to  16   d  is also radiated upward by black body radiation.  
         [0030]     According to the embodiment described above, an excellent heat dissipation property is obtained because the bottom plate  12   a  forming the package is made of a material with CMC structure. That is, the semiconductor dies  16   a  to  16   d  are mounted in the concave portion H provided on the bottom plate  12   a , where the lower side copper plate C 2  is exposed. Therefore, the heat generated in the semiconductor dies  16   a  to  16   d  is transported rapidly from the lower side copper plate C 2  to the plate  11  without being blocked by the support metal M. Thus, excellent heat dissipation property is obtained.  
         [0031]     The heat dissipation property is also improved by locating the semiconductor element in the concave portion H, because the semiconductor dies  16   a  to  16   d  are close to the plate  11 .  
         [0032]     Here, the mechanical strength is not degraded even if a part of the support metal M is removed because the concave portion H formed on the bottom plate  12   a  has a small area compared with a total area of the bottom plate  12   a . Thus, a package with a sufficient mechanical strength for practical use can be obtained.  
         [0033]     Further, the semiconductor dies  16   a  to  16   d  does not have an adverse effect such as having cracks on the GaAs semiconductor substrate, because thermal expansion coefficient of the GaAs semiconductor substrate is relatively close to that of the copper plate C 2  on the lower side.  
         [0034]     Next, another embodiment according to the present invention will be explained referring to  FIG. 3 , which is a cross section showing a part of the semiconductor device. In  FIG. 3 , the same symbols are assigned to the parts corresponding to those shown in  FIG. 2A  and  FIG. 2B  and a duplicating explanation is omitted.  
         [0035]     The outer surface of the semiconductor dies  16   a  to  16   d  mounted in the concave portion H is covered with a heat-resistant insulator material  21  such as polyimide. Then, a gap formed by a periphery of the semiconductor element  16  and an inner wall of the concave portion H is filled with metal  22  such as Sn—Pb solder alloy. In more detail, polyimide resin is solved in such an organic solvent as thinner and is filled in the concave portion H until the semiconductor element  16  sinks in the solvent. Drying the organic solvent, a film is formed around the semiconductor element  16  having a thickness of nearly about 10 μm. The molten Sn—Pb solder alloy, which is then poured into the concave portion H, is cooled and the solidified metal thus fills the space formed in the concave portion H. Here, the wire W, which connects the semiconductor dies  16   a  to  16   d  with the power divider circuit  17  on the input side and with the power combiner circuit  18  on the output side, is also coated with a heat-resistant material such as polyimide resin. Thus, an electrical short circuit is prevented from occurring otherwise caused by a metal  22  filled on the semiconductor element  16 .  
         [0036]     According to the embodiment, far better heat dissipation is realized compared with the embodiment described referring to  FIG. 2A  and  FIG. 2B , in which the space in the concave portion H is filled with air, because the heat generated inside the semiconductor element  16  is transported not only from the bottom surface of the semiconductor element  16  but also from the whole periphery of the semiconductor element  16  to the bottom plate  12   a  via the metal  22  filled in the concave portion H.  
         [0037]     In the embodiment shown in  FIG. 3 , the concave portion H is entirely filled with the metal  22 . However, the concave portion H is not always necessarily filled entirely. An upper part of the concave portion H may be left unfilled or may be over filled with the metal  22  so that the metal  22  protrudes from the surface of the bottom plate  12   a.    
         [0038]     In the embodiments mentioned above, a plurality of semiconductor dies, the power divider circuit and the power combiner circuit are arranged in the package. However, the elements are not limited to them, but such passive elements as multilayer capacitor or coil may be arranged. There is no problem in the heat dissipation even if these passive elements are arranged in the region where the support metal M exists, because electricity consumption and accompanying heat generation by them is low.  
         [0039]     Further, in the embodiment mentioned above, the three layer CMC structure material is used as a material forming the bottom plate  12   a . However, a structure of five or more layers laminated may be used other than the three layer structure, if the structure having a plurality of a first kind of metal plates laminated includes a second kind of metal plate, which is harder than the first kind of metal plates and is sandwiched between the first kind of metal plates.  
         [0040]     Further, in the case of three layer CMC structure, sometimes bowing due to heat may arise to degrade flatness, if the thickness of two metals, made of copper for example, having the support metal there between is different. Therefore, it is preferable that the thickness of the two metals having the support metal there between is equal.