Patent Publication Number: US-6703261-B2

Title: Semiconductor device and manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 10/318,018 filed Dec. 13, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device and a method of manufacturing the same. Particularly, the invention is concerned with a technique which is effectively applicable to a manufacturing technique for a high frequency power amplifier (high frequency power amplifier module) for a base station in a radio communication system such as a cellular communication system. 
     SUMMARY OF THE INVENTION 
     In a radio communication system such as a cellular communication system, a portable telephone set (a portable terminal) is connected to a base station adjacent to a telephone network by a speaker&#39;s operation of the telephone set, then is connected successively to a single or plural base stations, and finally a portable terminal of the party being called is called (originating a call), thereafter the portable telephone set assumes a state which permits talking with the called party. In this case, the base station amplifies the received signal and transfers the thus-amplified signal. Such an amplification is performed by means of a high frequency power amplifier for a base station. 
     Having made studies about the cost reduction of a high frequency power amplifier for a base station, the present inventor reviewed a sealing structure (package structure) as a main cause of high cost. 
     FIGS. 28 to  39  illustrate a semiconductor device (hereinafter referred to as the “studied semiconductor device”) which the present inventor had studied prior to the present invention. The studied semiconductor device is a high frequency power amplifier (a high frequency power amplifier module). FIG. 28 is a plan view of the studied semiconductor device, FIG. 29 is a side view thereof, FIG. 30 is a plan view thereof with a cap removed, and FIG. 31 is a schematic sectional view of the studied semiconductor device shown in FIG.  30 . 
     A semiconductor device  70  is manufactured using a rectangular substrate  71 . The substrate  71  is formed of a metal (e.g., CuMo plate) which is superior in thermal conductivity so that heat generated from a semiconductor chip is transferred promptly to an installed radiation board or mounting substrate, thus serving also as a heat sink. 
     A quadrangular frame  72  is fixed by bonding (silver soldering) centrally onto the substrate, or heat sink,  71 , the frame  72  having a width smaller than the width of the heat sink  71  and having a length shorter than long sides of the heat sink  71 . The heat sink  71  serves as a source electrode. Screwing grooves  73  are formed centrally of both ends of the heat sink  71  at positions spaced apart from the frame  72 . 
     As shown in FIG. 31, the frame  72  comprises a frame-shaped ceramic base  74  and two frame-shaped ceramic sleeves  75  superimposed successively on the ceramic base  74 . A metallized surface layer  75   a  (the dotted region in FIGS. 28 and 30) is formed on the surface of the upper ceramic sleeve  75 . 
     Along the width of the heat sink  71  the ceramic sleeves  75  are superimposed one on the other at the same width as the width of the ceramic base  74  so that their inner and outer wall surfaces are in registration with the ceramic base, while in the long sides of the heat sink  71  the ceramic sleeves  75  are smaller in width than the ceramic base  74  and the surface of the ceramic base is exposed to both inside and outside of the ceramic sleeves  75 . 
     On the surface of the ceramic base  74  extending along the long sides of the heat sink  71  there is formed an electrically conductive metallized layer  76 . Consequently, the metallized layer  76  is exposed to the surface portions of the ceramic base  74  inside and outside the ceramic sleeves  75 . The metallized layer portions located inside the ceramic sleeves  75  serve as bonding posts  76   a  for wire connection, while the metallized layer portions located outside the ceramic sleeves  75  serve as lead connecting portions  76   b  for lead connection. 
     An inner end of a drain lead  77  formed of a broad metallic plate is fixed to the lead connecting portion  76   b  extending along one long side of the heat sink  71 , and an inner end of a gate lead  78  formed of a broad metallic plate is fixed to the lead connecting portion  76   b  extending along the other long side of the heat sink  71 . One corner of the drain lead  77  is cut off obliquely, serving as an electrode index  77   a  for recognition of a drain electrode (see FIG.  28 ). The metallic plates which constitute the drain lead  77  and the gate lead  78  are formed of Kovar or Fe—Ni alloy, having such characteristics as thermal expansion coefficient and thermal expansion coefficient difference of ceramic being small and has a resistance to a high temperature in silver soldering. 
     On an upper surface of the heat sink  71  located inside the frame  72  there are fixed a capacitor chip  79 , an SiMOSFET chip  80 , and a capacitor chip  81  side by side from the drain lead  77  toward the gate lead  78 . In the capacitor chips  79  and  81 , an oxide film (SiO 2  film) is formed on a silicon substrate and an electrode is formed thereon to afford a predetermined capacitance. The silicon substrate serves as one electrode, while the electrode on the oxide film serves as the other electrode. These chips are fixed by AuSi eutectic, which is satisfactory in heat radiating property and electric conductivity and high in reliability, to the heat sink  71  formed of a metallic plate, so that there is ensured an electric contact between the substrate-side electrode of each chip and the heat sink  71  and there is obtained a satisfactory heat radiating property. 
     As shown in FIG. 30, one end of each conductive wire  82  is connected to a drain electrode  80   d  on the SiMOSFET chip  80 , while an opposite end thereof is connected to an upper electrode  79   a  of the capacitor chip  79  at a position close to the drain lead  77 . Likewise, one end of each conductive wire  83  is connected to the upper electrode  79   a  of the capacitor chip  79 , while an opposite end thereof is connected to the bonding post  76   a  which is connected electrically to the drain lead  77 . Further, one end of each conductive wire  84  is connected to one end of a gate electrode  80   g  on the SiMOSFET  80 , while an opposite end thereof is connected to an upper electrode  81   a  of the capacitor chip  81  located close to the gate lead  78 . One end of each conductive wire  85  is connected to the upper electrode  81 a of the capacitor chip  81 , while an opposite end thereof is connected to the bonding post  76   a  which is electrically connected to the gate lead  78 . 
     The wires  81  to  85  are formed of aluminum wires. As to the wires connected to drain and gate electrodes, there is adopted a structure wherein plural wires are connected in parallel to ensure current capacity and high frequency characteristic (see FIG.  30 ). 
     As shown in FIG. 28, a cap  87  is fixed to the frame  72 . The cap  87  is formed of a ceramic plate. As shown in FIG. 29, in a peripheral portion on a back side of the cap  87 , a plating film  87   a  is formed of AuSn in a frame shape correspondingly to the ceramic sleeves  75 , and the ceramic sleeves  75  are fixed through the plating film  87   a . That is, the cap  87  is fixed to the frame  72  hermetically (hermetic seal structure) through AuSn alloy. 
     Along a side face of the frame  72  there is formed a side metallized layer  76   d , as indicated with dots in FIG.  29 . The side metallized layer  76   d  provides an electric connection between the heat sink  71  serving as a source electrode and the surface metallized layer  75   a , placing the surface metallized layer  75   a  at an equal potential to the source electrode. 
     Exposed portions of the heat sink  71 , drain lead  77 , gate lead  78 , surface metallized layer  75   a , side metallized layer  7   d , and metallized layer  76  are wholly plated with Au. 
     It is difficult to reduce the manufacturing cost of such a semiconductor device. More particularly, (1) a heat sink made of CuMo is closely similar in thermal expansion coefficient to a semiconductor chip formed of a ceramic material or Si and exhibits a package stress diminishing effect; besides, it is superior in heat radiating property, so it becomes possible to ensure a stable operation of the associated semiconductor device. However, the material cost is high and machining is troublesome, leading to an increase of cost. In the structure of the semiconductor device, moreover, the size of the heat sink used is large and hence an increase of cost results. 
     (2) Thick gold plating is needed for ensuring a required heat resistance in AuSi eutectic chip bonding, but a selective gold plating is difficult and there must be adopted a whole-surface plating method, with consequent increase of cost. It is necessary that the gold plating be performed to a thickness sufficient to ensure such a heat resistance as prevents the occurrence of discoloration of plating due to heating in chip bonding. For AuSi eutectic, in which heating is made to about 430° C., a plating thickness of 2 μm or more is needed, constituting one of causes of an increased cost. 
     (3) AuSn sealing work costs high. Besides, there is obtained a package of a hollow structure and a hermetic seal structure, so it becomes necessary to conduct a hermetic seal leak inspection and a movable dust particle inspection (dust particle inclusion inspection) as additional works, thus resulting in that the inspection cost further increases. 
     In more particular terms, since reliability depends on whether hermetic seal performance is good or not, it is necessary to conduct a hermetic seal leak inspection, which causes an increase of the assembling cost. Moreover, the management of oxygen control is important for ensuring a high hermetic seal performance, which causes deterioration of the sealing work efficiency and hence an increase of the assembling cost. 
     Further, if a hollow package is sealed with electrically conductive dust particles mixed therein, the dust particles move around the interior of the package under vibrations imparted to the package from the exterior. If the moving dust particles span the electrodes, there is a fear that an electrically semi-shorting defect may result, thus requiring the dust particle inclusion inspection as an essential condition. 
     On the other hand, a review of the semiconductor device  70  from the standpoint of electrical characteristics has revealed that the following problems are involved therein. 
     In the case of a base station for mobile telephone, the frequency is about 0.8 to about 2.1 GHz and an output as large as 60 to 250 W is required as the output of a final-stage power amplifying FET. A supply voltage of an amplifier which uses MOSFET (Metal Oxide Semiconductor Field-Effect-Transistor) made of Si as FET for output is about 28V, so in the case of an FET with an output of 125 W and a drain efficiency of about 50%, an average current is about 9 A and a peak current reaches 27 A which is about three times as large as the average current. 
     Since such a large current is handled, it is necessary to use a large-sized FET chip with a gate width of 20 cm or so, and input and output capacitances become as large as nearly 150 PF and 80 PF, respectively, resulting in the device being very low in impedance ACwise. For this reason, a high frequency loss increases at a frequency band of 1.5 GHz or higher. For avoiding this inconvenience, an internal matching circuit is included between FET chip and package electrode leads to increase the impedance of the lead portions. 
     FIG. 32 is an equivalent circuit diagram in which internal matching circuits are provided on an input side (gate side) and an output side (drain side), respectively. Between a gate electrode  80   g  of SiMOSFET chip  80  and an input terminal Pin is formed an input matching circuit by inductances L 1 , L 2  of wires  85 ,  84 , capacitance C 1  of the capacitor chip  81 , and input capacitance Ciss of SiMOSFET chip  80 . 
     Further, between a drain electrode  80   d  of SiMOSFET chip  80  and an output terminal Pout is formed an output matching circuit by inductances L 3 , L 4  of wires  82 ,  83 , capacitance C 2  of the capacitor chip  79 , and output capacitance Coss of SiMOSFET  80 . A source electrode  80   s  for the drain electrode  80   d , as well as one terminals of the capacitors, are connected to ground (GND). 
     With the input matching circuit, an impedance of several ten ohms is converted to a low impedance of 1 ohm or less, while with the output matching circuit, an impedance of 1 ohm or less is converted to an impedance of several ten ohms, thus permitting an efficient power amplification. 
     The wires  82  to  85  serve as appropriate inductances for a desired frequency characteristic and the wire length (wire loop shape) is designed so as to form an internal matching circuit. 
     However, for forming the internal matching circuit it becomes necessary to ensure an additional space for the capacitor chips  79  and  81 , which is disadvantageous to the needs for the reduction of size, and it is also necessary to perform a wire bonding work many times in a uniform loop shape, which causes a deteriorated yield in assembly. 
     Now, with reference to FIG. 33, a description will be given about a sectional structure of SiMOSFET  80  and a current path thereof. FIG. 33 illustrates a section and current path of a horizontal type SiMOSFET chip used for high frequency power amplification. 
     The horizontal SiMOSFET has a p +  silicon substrate  90  of a high concentration and a p −  epitaxial layer  91  of a low concentration which is formed on the p +  silicon substrate  90  for obtaining a required voltage proof. In a surface portion of the p −  epitaxial layer  91  are formed a channel-forming p layer  92  and a drain n +  layer  93  of a high concentration which is located at a position spaced a predetermined distance from the channel-forming p layer  92 . Further, an n −  drain offset layer  94  of a low concentration is formed in the surface layer portion of the p −  epitaxial layer  91  from the drain n +  layer  93  up to the channel forming p layer  92 . 
     The channel-forming p layer  92  is formed relatively deep and a source-forming n +  layer  95  of a high concentration is formed on the left-hand side of a surface portion of the channel-forming p layer  92 . The region from the left end of the source-forming high concentration n +  layer  95  up to the pn junction formed of both channel-forming p layer  92  and n −  drain offset layer  94  serves as a channel-forming region. 
     A p +  through diffusion layer  96  of a high concentration is formed throughout a depth extending from the left end of the source-forming high concentration n +  layer  95 , past the p −  epitaxial layer  91 , up to a surface portion of the p +  silicon substrate  90 . An insulating film (oxide film  97  is formed selectively on the surface of the p −  epitaxial layer  91 . The insulating film  97  extends from an intermediate position of the source-forming high concentration n +  layer  95  up to an intermediate position of the drain n +  layer  93 . At the right end of the insulating film  97  a drain electrode  80   d  is formed on the drain n +  layer  93 , and at the left end of the insulating film  97  a source wiring line  80   sa  is formed from the source-forming high concentration n +  layer  95  onto the p +  through diffusion layer  96 . 
     At a position deviated from the channel forming region, a source field plate  98  is provided on the insulating film  97  which overlies the n −  drain offset layer  94 . The source field plate  98  is fixed at a source potential and is improved in drain voltage proof by relaxing an electric field. 
     The insulating film  97  is thinned at its portion opposed to the channel-forming region, which thinned portion serves as a gate insulating film (oxide film)  97   a . A gate electrode  80   g  is formed on the gate insulating film  97   a  and is electrically connected to a gate wiring line  80   ga  which extends onto the insulating film  97 . 
     By a high accuracy by both ion implantation technique and diffuison technique, p- and n-type diffusion layers (regions) are formed, whereby an n-channel enhancement type MOSFET is formed. 
     In such a device structure, a thick-line arrow shown in FIG. 3 represents a current path, along which an electric current  99  flows through the drain electrode  80   d , drain n +  layer  93 , n −  drain offset layer  94 , channel-forming p layer  92 , source-forming high concentration n +  layer  95 , source wiring line  80   sa , p +  through diffusion layer  96 , and p +  silicon substrate  90 , and reaches the source electrode  80   s.    
     Current control is made by increasing or decreasing the gate potential, but since the horizontal FET is an enhancement type, it has a structure such that the drain current increases with an increase in gate potential and is cut off as the gate potential approaches 0V. Thus, there accrues an advantage that a minus power supply for gate bias which is necessary for a depletion type GaAsFET can be omitted and that therefore it is easy to implement a set circuit configuration. 
     The conventional high frequency power amplification using the horizontal FET is approximately class B amplification, in which a power component based on ON/OFF switching operation of a specified frequency is accumulated in a tank circuit constituted by both inductor and capacitor and a high frequency power is taken out by a matching circuit. Therefore, the higher the drain voltage and the larger the drain current capable of being flowed, the higher the output obtained in FET concerned, but for following up a high frequency operation it is necessary that the capacitance be small and mutual inductance (Gm) be large, so there is made such a chip design as affords high voltage proof, low capacitance, and high Gm. For attaining low capacitance and high Gm of FET chip, microminiaturization of gate length is necessary and is now a trend in FETs for high frequency amplification. 
     As a great factor of obstructing the attainment of high Gm there is mentioned source resistance. If an apparent Gm observed in an actual FET is assumed to be Gm′ (exteriorly measurable Gm), Gm′ can be represented by the following relationship between true Gm (=Gmint) and source resistance (=Rs): 
     
       
           Gm ′=( G mint)/(1 +Rs×G mint)  (1) 
       
     
     Thus, for improving the performance of FET, the resistance diminishing processings subsequent to the formation of source n layer (source-forming high concentration n +  layer  95 ) shown in terms of the foregoing current path are important, and the decrease of contact resistance between the source electrode (=Si substrate) and the heat sink is an important subject in the assembling process. More particularly, the AuSi eutectic chip bonding method is suitable, including the point of high heat radiating performance, and is applied while imposing such a condition as does not develop voids in AuSi eutectic. 
     Now, with reference to FIGS. 34 and 35, a description will be given below about problems involved in space margin which are an obstructing factor against the reduction in size of a heat sink as an expensive component. 
     FIG. 34 is a side view as seen from a side face of the package for explaining a space margin which is necessary for chip bonding of the SiMOSFET  80 . For chip bonding, the SiMOSFET chip  80  is chucked and conveyed by a chip bonding collet  100  and is pushed and scrubbed against a predetermined portion of the heat sink  71  heated to about 430° C. (Au foil now shown is affixed beforehand to the chip bonding portion of the heat sink to facilitate the formation of AuSi eutectic), allowing AuSi eutectic  111  to be developed for fusion-bonding. 
     In this case, scrubbing is necessary for preventing the generation of voids in AuSi, and the larger the size of chip, the greater the degree of scrubbing is needed. Since it is necessary for the collet  100  to embrace the FET chip, the size of the collet is larger than that of the FET chip  80 . 
     It is necessary that a space margin be ensured an amount sufficient to prevent abutment of a collet end against the ceramic base  74  when scrubbing width is added to the collet size. Further, it is necessary that a positional variation margin of the frame  72 , a ceramic size variation margin, and a collet position accuracy margin be added thereto. The total is about 1 mm. 
     FIG. 35 is a sectional view as seen from a package side face for explaining the space margin of a bonding posts  76   a  necessary for wire bonding. In wire bonding, with use of a wire bonding tool  113 , wire  112  is first-bonded (1st bonding) to an Al electrode (not shown) of the FET chip  80 , then is second-bonded (2nd bonding) to an Al electrode (not shown) of the capacitor chip  81  on the drain lead  77  side in the illustrated example. Next, with an electrode (not shown) of the capacitor chip  81  as the 1st bonding side, the wire is moved onto the bonding post  76   a  of the ceramic base  74  to which the drain lead  77  is fixed, and the wire  112  formed of Al is pushed against the bonding post  76   a  with the bonding tool  113  and is subjected to ultrasonic compression bonding to effect 2nd bonding. Thereafter, the bonding tool  113  is raised while allowing the wire  112  to be clamped by the bonding tool to tear off the wire from the 2nd-bonded portion. 
     In this case, as shown in FIG. 35, it is necessary to ensure such a margin as prevents abutment of an end portion of the bonding tool  113  and the wire  112  against a ceramic sleeve  75 . The larger the height of the ceramic sleeve  75 , or the obtuser the feed angle of the wire  112 , the larger is required the length of the bonding post  76   a , thus obstructing the reduction of size. 
     In a conventional ultrasonic Al wire bonding, the lower the Al wire feed angle, the smaller the variations in compression-bonded shape of Al and the higher the yield in assembly. For this reason, the lower the Al wire feed angle, the more desirable. On the other hand, however, it is necessary that the size of the bonding post  76 a be made large. As a result, there may occur an increase of cost due to an increase in package size, or an increase of electrode capacitance may exert an influence on characteristics. 
     Next, with reference to FIGS. 36 to  39 , the following description will be provided about the case where the semiconductor device  70  is attached to a set radiation plate and also about problems involved therein. FIG. 36 is a plan view of the semiconductor device  70  as screwed to a set radiation plate and FIG. 37 is a schematic sectional view thereof. 
     The semiconductor device  70  is mounted in the following manner. Screws  115  are inserted into screwing grooves  73  formed in both ends of the heat sink  71  and the substrate  71  is fixed to a mounting substrate  116  with the screws  115 . The drain lead  77  and the gate lead  78  are connected to predetermined wiring portions of the mounting substrate  116  through a bonding material (e.g., PbSn solder). In connection with fixing the heat sink  71  as a radiation board to the mounting substrate  116 , a tightening torque in the fixing work is prescribed for suppressing an increase in both thermal and electrical resistances. 
     In actual mounting, for preventing contact imperfection caused by the generation of an intermetallic compound between Au plating on the surfaces of drain and gate leads  77 ,  78  and PbSn solder, it is necessary to perform a work for removing Au plating from the leads before the mounting. Generally, it is necessary to conduct a preliminary soldering work of dipping the leads into a solder dipping vessel, allowing the Au plating to be diffused into the solder vessel. Such wasteful labor and time are required. 
     As a countermeasure the application of a selective Au plating method is considered so as not to plate the lead portions with Au. However, unlike the selective Au plating for such a material as a lead frame material capable of being conveyed continuously, close to a two-dimensional structure, masking is difficult for a ceramic package which is of a three-dimensional structure and which is basically conveyed one by one. Thus, the productivity of selective Au plating is low and whole-surface Au plating costs lower than selective Au plating. Even if Au plating is thinned for only the leads, there arises the problem of discoloration on heating in chip bonding, so Au plating is applied thick throughout the whole surface. 
     Next, problems involved in such screw mounting of the semiconductor device will be described with reference to FIGS. 37 to  39 . FIG. 37 is a schematic sectional view of the studied semiconductor device as screwed to a mounting substrate, FIGS.  38 ( a ) and  38 ( b ) are schematic diagrams showing the semiconductor device wherein a substrate is warped in a centrally depressed state, as well as a mounted state thereof, and FIGS.  39 ( a ) to  39 ( c ) are schematic diagrams showing the semiconductor device wherein a substrate is warped in a centrally raised state, as well as a mounted state thereof. 
     From the standpoint of heat radiating performance it is preferable that the substrate (heat sink)  71  be in such a flat state as shown in FIG.  37 . However, due to a bimetal effect inducted by thermal stresses among the metallic heat sink  71 , the ceramic frame  72  fixed to the heat sink  71 , and the metallic cap  87 , there actually occurs a concave warp wherein the substrate center is recessed as in FIGS.  38 ( a ) and  38 ( b ) or a convex warp wherein the substrate center is raised as in FIGS.  39 ( a ) to  39 ( c ). The heat sink  71  and the frame  72  are bonded together by silver solder  117 . In these figures the ceramic portion is not bent, but actually there also is a case where the ceramic portion is bent. 
     In the case where the heat sink  71  is concavely warped as in FIG.  38 ( a ), if the heat sink is screwed to the mounting substrate  116 , as in FIG.  38 ( b ), the heat sink which is in a concavely warped state is corrected to the flat side, so that a tensile stress is imposed on the frame  72  formed of a ceramic laminate and there easily occurs a crack C. 
     On the other hand, in the case of such a convex warp as shown in FIG.  39 ( a ), a compressive stress is imposed on the frame  72  formed of a ceramic laminate due to a screwing stress as in FIG.  39 ( b ), so the breakage of ceramic is difficult to occur. However, as shown in FIG.  39 ( c ), since a gap G is formed in the heat radiation path to the mounting substrate  116 , a thermal resistance Rth increases and there arises the problem that both reliability and heat radiating performance are deteriorated. 
     As a measure against such warps it is considered to use a sufficiently thick heat sink material so as to make the heat sink difficult to warp, and conduct the selection of a heat sink warp. In this case, however, the material cost increases, with consequent increase in the cost of the semiconductor device. 
     It is an object of the present invention to reduce the material cost, reduce the cost of an assembling work including a sealing work, reduce the inspection cost, and thereby reduce the semiconductor device manufacturing cost. 
     It is another object of the present invention to provide a semiconductor device in which a heat sink is difficult to warp. 
     The above and other objects and novel features of the present invention will become apparent from the following description and the accompanying drawings. 
     Typical inventions disclosed herein will be outlined below. 
     (1) A semiconductor device comprising: 
     a heat sink formed of a flat metallic plate having a high thermal conductivity; 
     a pair of screwing pieces having respective inner end portions connected with screws respectively to right and left ends of the heat sink on a main surface side of the heat sink, the screwing pieces further having outer end portions formed with through spaces for screwing; 
     one or plural semiconductor chips fixed to the main surface of the heat sink; 
     a seal member formed of an insulating resin, the seal member covering a back side opposite to the main surface of the heat sink and also covering the other portions of the two screwing pieces than the outer end portions; 
     a first lead having an outer end portion projecting from the seal member and further having an inner end portion which is positioned within the seal member and whose inner end is arranged near one side face of the heat sink; 
     a second lead having an outer end portion projecting from the seal member and further having an inner end portion positioned within the seal member and near another side face of the heat sink; and 
     a plurality of conductive wires for electrically connecting between the leads and predetermined electrodes of the predetermined semiconductor chip, or between the leads and predetermined electrodes of the predetermined semiconductor chip and also between predetermined electrodes of the predetermined semiconductor chip and predetermined electrodes of the predetermined semiconductor chip. 
     The heat sink is formed using a thick material difficult to deform, while the screwing pieces are formed using a flexible material easy to deform. Each of the screwing pieces is bent in one step at an intermediate portion thereof projecting from the seal member, then extends so that a back side thereof is positioned on a plane on which the back side of the heat sink is positioned, or on a plane close to the plane, the bent portion serving as a buffer portion adapted to deform under a stress. The inner end portions of the first and second leads are formed with engaging portions for engagement with the resin which constitutes the seal member. The first or the second lead is partially provided with a polarity identifying portion. The inner ends of the first and second leads are positioned in regions diverted from the heat sink. 
     A semiconductor chip with a field effect transistor formed thereon and semiconductor chips with capacitors formed thereon are fixed to the main surface of the heat sink in such a manner that the capacitor-formed semiconductor chips are positioned on both sides in the gate-drain direction of the FET-formed semiconductor chip. A drain electrode formed on an upper surface of the field effect transistor and an upper electrode of one of the semiconductor chips with capacitors formed thereon are connected with each other through the plural wires, and the upper electrode and the first lead are connected with each other through the plural wires. Further, a gate electrode formed on the upper surface of the field effect transistor and an upper electrode of the other capacitor semiconductor chip are connected with each other through the plural wires, and the upper electrode and the second lead are connected with each other through the plural wires. In this way there is constituted a high frequency power amplifier for a base station. 
     Such a semiconductor device is manufactured through the steps of: 
     providing a lead frame and a heat sink, the lead frame being constituted by a metallic plate having one or more product forming portions of a predetermined pattern, the heat sink being constituted by a flat metallic plate of a high thermal conductivity which is fixed to the product forming portions with screws; 
     fixing one or plural semiconductor chips to a main surface of the heat sink; 
     superimposing the heat sink on back sides of the product forming portions of the lead frame and fixing the heat sink to the lead frame with screws from a main surface side of the lead frame; 
     connecting between electrodes of the semiconductor chips and inner end portions of corresponding leads and also between predetermined electrodes of predetermined semiconductor chips, using conductive wires; 
     covering the main surface side of the heat sink and a predetermined portion of the lead frame with an insulating resin by one-side transfer molding to form an insulating seal member; and 
     cutting off an unnecessary portion of the lead frame, allowing a back side of the heat sink which is insulative to be exposed to a back side of the seal member and allowing the leads to be projected from side faces of the seal member, 
     the product forming portion each comprising: 
     a pair of screwing pieces having respective inner end portions connected with screws respectively to right and left ends of the heat sink on the main surface side of the heat sink, the screwing pieces further having respective outer end portions projecting to the exterior of the seal member, the outer end portions having respective through spaces for screwing which are used at the time of mounting the semiconductor device; 
     a first lead having an inner end portion which is positioned within the seal member and whose inner end is arranged near one side face of the heat sink and further having an outer end portion projecting from the seal member; and 
     a second lead having an inner end portion positioned within the seal member and near another side face of the heat sink and further having an outer end portion projecting from the seal member, 
     wherein in the step of cutting off the unnecessary portion of the lead frame, or thereafter, the leads projecting from the side faces of the seal member are bent in one step to form surface-mounted type leads. 
     In the above semiconductor device manufacturing method, the screwing pieces are cut so that there remain the through spaces for screwing, and the screwing pieces are bent in one step at intermediate portions thereof projecting from the seal member, then extend so that back sides thereof are each positioned on a plane on which the back side of the heat sink is positioned or on a plane close to the plane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective plan view showing the structure of a semiconductor device according to an embodiment (first embodiment) of the present invention; 
     FIG. 2 is a plan view thereof; 
     FIG. 3 is a front view thereof; 
     FIG. 4 is a side view thereof; 
     FIG. 5 is a schematic sectional view taken along line A—A in FIG. 2; 
     FIG. 6 is a schematic sectional view taken along line B—B in FIG. 2; 
     FIG. 7 is an enlarged sectional view showing a part of the semiconductor device of the first embodiment; 
     FIG. 8 is a plan view of a lead frame used in manufacturing the semiconductor device of the first embodiment; 
     FIG. 9 is a plan view of a substrate used in manufacturing the semiconductor device of the first embodiment; 
     FIG. 10 is a side view thereof; 
     FIGS.  11 ( a ) and  11 ( b ) illustrate the substrate with semiconductor chips fixed thereto in the manufacture of the semiconductor device of the first embodiment; 
     FIG. 12 is a plan view of the lead frame with the substrate attached thereto; 
     FIG. 13 is a plan view of the lead frame after the end of wire bonding in the manufacture of the semiconductor device of the first embodiment; 
     FIG. 14 is a schematic diagram showing how to perform the wire bonding; 
     FIG. 15 is a plan view of the lead frame after the end of sealing with resin in the manufacture of the semiconductor device of the first embodiment; 
     FIG. 16 is a schematic diagram showing in what state the semiconductor device of the first embodiment is manufactured as a device equipped with suspension leads by cutting the lead frame selectively; 
     FIG. 17 is a schematic diagram showing in what state the semiconductor device of the first embodiment is manufactured as a device not equipped with suspension leads by cutting the lead frame selectively; 
     FIGS.  18 ( a ) and  18 ( b ) are a diagram and a graph, respectively, showing how a gap formed between a package bottom of the semiconductor device and a lower surface of each suspension lead, as well as thermal resistance and package breakdown occurrence rate, are correlated with each other; 
     FIG. 19 is a perspective plan view showing the structure of a semiconductor device according to another embodiment (second embodiment) of the present invention; 
     FIG. 20 is a schematic sectional view thereof; 
     FIG. 21 is a schematic sectional view in another section of the semiconductor device of the second embodiment; 
     FIG. 22 is a plan view of a spacer used in a semiconductor device according to a modification of the second embodiment; 
     FIG. 23 is a schematic sectional view showing the structure of a semiconductor device according to another modification of the second embodiment; 
     FIG. 24 is a plan view of a semiconductor device according to a further embodiment (third embodiment) of the present invention; 
     FIG. 25 is a front view of thereof; 
     FIG. 26 is a plan view of a semiconductor device according to a still further embodiment (fourth embodiment) of the present invention; 
     FIG. 27 is a side view thereof; 
     FIG. 28 is a plan view of the studied semiconductor device which the present inventor had studied prior to the present invention; 
     FIG. 29 is a side view thereof; 
     FIG. 30 is a plan view thereof with a cap removed; 
     FIG. 31 is a schematic sectional view thereof; 
     FIG. 32 is an equivalent circuit diagram thereof; 
     FIG. 33 is a schematic sectional view of an FET chip used in the studied semiconductor device; 
     FIG. 34 is a schematic diagram showing a state of chip bonding in the manufacture of the studied semiconductor device; 
     FIG. 35 is a schematic diagram showing a state of wire bonding in the manufacture of the studied semiconductor device; 
     FIG. 36 is a plan view of the studied semiconductor device as screwed to a set radiation board; 
     FIG. 37 is a schematic sectional view thereof; 
     FIGS.  38 ( a ) and  38 ( b ) are schematic diagrams showing a mounted state of the studied semiconductor device in which a substrate is warped in a centrally depressed state; and 
     FIGS.  39 ( a ) to  39 ( c ) are schematic diagrams showing a mounted state of the studied semiconductor device in which a substrate is warped in a centrally raised state. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail hereinunder with reference to the accompanying drawings. In all of the drawings for illustrating the embodiments, portions having the same functions are identified by like reference numerals, and repeated explanations thereof will be omitted. 
     First Embodiment 
     FIGS. 1 to  18  are concerned with a semiconductor device according to an embodiment (first embodiment) of the present invention, of which FIGS. 1 to  7  are concerned with the structure of the semiconductor device and FIGS. 8 to  17  are concerned with the manufacture of the semiconductor device. In this first embodiment, a description will be given below about an example in which the present invention is applied to a high frequency power amplifier (a high frequency power amplifier module) for a base station, as a semiconductor device. 
     The semiconductor device of this first embodiment, i.e., a high frequency power amplifier module for a base station, indicated at  1 , has such a structure as shown in FIGS. 1 to  7 , of which FIG. 1 is a perspective plan view, FIG. 2 is a plan view, FIG. 3 is a front view, FIG. 4 is a side view, FIGS. 5 and 6 are schematic sectional views taken along lines A—A and B—B in FIG. 2, and FIG. 7 is a partially enlarged sectional view. 
     In appearance, as shown in FIGS. 2 to  4 , the semiconductor device  1  of this embodiment comprises a flat, rectangular, seal member (package)  2  formed of an insulating resin, a heat sink (substrate)  3  whose lower surface is exposed to a bottom of the seal member  2 , a pair of screwing pieces (suspension leads)  4  projecting respectively from both ends (short sides) in the longitudinal direction of the seal member  2 , a first broad lead  5  and a second broad lead  6  projecting respectively from long sides of the seal member  2 . The second lead  6  serves as a gate (G) lead. The lower surface of the heat sink  3  exposed to the bottom of the seal member  2  serves as both a heat radiating surface and a source electrode. 
     The seal member  2  is formed, for example, using an insulating epoxy resin by transfer molding. The seal member  2  has a structure which covers the four side faces of the heat sink  3  and in which a water admission path passing the interface between the heat sink  3  and the resin is made long to ensure a high reliability. Recesses  8  are formed centrally of short sides of the seal member  2  and toward the center of the seal member. The recesses  8  are draw-out traces of an upper mold for clamping the heat sink  3  and the screwing pieces  4  with upper and lower molds of a molding die and causing the heat sink  3  to come into close contact with the lower mold positively in transfer molding to prevent the occurrence of burrs caused by flowing-out of resin. 
     The heat sink  3  and the screwing pieces  4  are formed of metal, and within the seal member  2 , an inner end portion of each screwing piece  4  is superimposed on an upper surface of the heat sink  3  and is connected thereto with two screws  7  (see FIG.  1 ), so that the heat sink  3  and the screwing pieces  4  become equal in potential and can be used as source (S) terminals. Semiconductor chips are fixed to the heat sink  3  and the heat sink plays the role of dissipating the heat generated in the semiconductor chips, therefore, the heat sink  3  is formed of a flat metallic plate (e.g., CuMo plate) superior in thermal conductivity. Besides, the heat sink  3  is as thick as 2 mm for example to make its deformation difficult. On the other hand, the screwing pieces  4  are as thin as 0.15 mm for example and are flexible so as to serve as suspension leads. 
     Both screwing pieces  4  have screwing grooves  9  formed in respective outer end portions. With use of the paired screwing grooves  9  the semiconductor device  1  is fixed with screws to a mounting substrate such as a mother board. The inner end portions of the screwing pieces  4  are superimposed and fixed onto the main surface (upper surface) of the heat sink  3 , and the outer end portions of the screwing pieces  4  are mounted in contact with the mounting substrate. Therefore, as shown in FIG. 5, the screwing pieces  4  are bent in one step at their portions projecting from the seal member  2 . The bent portions, indicated at  10 , are easily deformable portions against a stress, i.e., suspension portions. 
     As shown in FIG. 7, a plating film  11  is formed on the whole surface of the heat sink  3 . The plating film  11  comprises Ni plating film as undercoat and Au plating film formed on the Ni plating film. The Ni plating film is superior in its adhesion to CuMo which constitutes the heat sink  3 . The Au plating film is formed as thick as about 3 μm so that it can withstand heat of a high temperature which is generated at the time of fixing semiconductor chips to the heat sink  3  by AuSi eutectic. 
     Each screwing piece  4  also has a plating film  12  formed on its surface. The screwing piece  4  is formed of an FeNi-base metal such as Kovar or  42  alloy. The plating film  12  comprises Ni plating film as undercoat and Au plating film formed on the Ni plating film. The Ni plating film is superior in its adhesion to the FeNi-base metal. The Au plating film is formed for improving the bondability of aluminum (Al) wire, but its thickness may be as small as 0.2 μm or so. 
     As shown in FIGS. 1 and 5 to  7 , three semiconductor chips  15 ,  16 , and  17  are fixed to the main surface portion of the heat sink  3  located between the pair of screwing pieces  4 . The semiconductor chips  15 ,  16 , and  17  are elongated in shape and with electrodes extending along their long sides. All the semiconductor chips  15 ,  16 , and  17  extend along the long sides of the heat sink  3 . The long sides of the seal member  2  and those of the heat sink  3  extend in the same direction and are opposed to each other. 
     A field effect transistor (MOSFET) is incorporated in the semiconductor chip  15 . As shown in FIG. 7, out of drain, gate, and source electrodes of MOSFET, a drain electrode  20  and a gate electrode  21  are formed on an upper surface of the semiconductor chip  15 , while a source electrode  22  is formed on a lower surface of the chip  15 . The drain and gate electrodes are elongated and extend along long sides of the chip in an opposed relation to each other. The source electrode  22  is fixed to the heat sink  3  through an AuSi eutectic layer  23 . The AuSi eutectic layer  23  is formed by affixing gold foil to a predetermined portion of the heat sink  3 , then pushing the semiconductor chip against the gold foil, followed by heating scrubbing. 
     The semiconductor chips  16  and  17  are fixed onto the main surface of the heat sink  3  on both sides of the semiconductor chip  15  spacedly a predetermined distance from the chip  15 . The semiconductor chips  16  and  17  have a structure in which capacitors are incorporated. As shown in FIGS. 6 and 7, one upper electrodes  25   a  and  25   b  are formed on upper surfaces of the semiconductor chips  16  and  17  respectively, while other lower electrodes  26   a  and  26   b  are formed on lower surfaces of the semiconductor chips  16  and  17  respectively. The lower electrodes  26   a  and  26   b  of the semiconductor chips  16  and  17  are fixed to the heat sink  3  through AuSi eutectic layers  27   a  and  27   b  respectively. As is the case with the semiconductor chip  15 , the AuSi eutectic layers  27   a  and  27   b  are formed by affixing gold foil to predetermined portions of the heat sink  3  and thereafter pushing the semiconductor chips against the heat sink  3 , followed by scrubbing under heating. As a result, the semiconductor chips  16  and  17  are fixed to the heat sink  3 . 
     The drain lead  5  and the gate lead  6  have a broad structure so as to stand face to face with the elongated semiconductor chips  15 ,  16 , and  17 . The drain electrode  20  of the semiconductor chip  15  and the upper electrode  25   a  of the semiconductor chip  16  located close to the drain lead  5  are electrically connected with each other through a conductive wire  31 . Further, the upper electrode  25   a  and the drain lead  5  are electrically connected with each other through a conductive wire  32 . Likewise, the gate electrode  21  of the semiconductor chip  15  and the upper electrode  25   b  (see FIG. 1) of the semiconductor chip  16  located close to the gate lead  6  are electrically connected with each other through a conductive wire  33 . Further, the upper electrode  25   b  and the gate lead  6  are electrically connected with each other through a conductive wire  34 . 
     As to the wires for connection between electrodes or between electrodes and leads, as shown in FIG. 1, plural wires of connections can be effected because the wire connections are parallel to one another and extend long. By this plural-wire connection it is possible to ensure desired current capacity and high frequency characteristics (e.g., drain efficiency and gain). As the wires there are used aluminum wires. The aluminum wires are each in the form of a wire loop so as to afford an appropriate inductance for a desired frequency characteristic and permit the formation of a desired internal circuit, whereby there is formed a high frequency power amplifier module constituted by the equivalent circuit shown in FIG.  32 . 
     As shown in FIG. 1, each lead portion positioned within the seal member  2  is provided with engaging portions  35  on both sides of the lead for engagement with the resin which constitutes the seal member. This is for preventing easy release of the leads  5  and  6  from the seal member  2 . Further, a corner portion of an outer end of the drain lead  5  is cut off obliquely to form a polarity identifying portion  36  so that the polarity of the electrode can be identified visually. The shape and structure of the polarity identifying portion  36  may be other shape and structure. 
     The leads  5  and  6  and the screwing pieces (suspension leads)  4  are formed by cutting a single lead frame in the manufacture of the semiconductor device. Therefore, the composition, thickness and surface treatment are common to them. 
     Next, with reference to FIGS. 8 to  17 , the following description is provided about the method of manufacturing the semiconductor device according to the first embodiment. In this first embodiment there are used a lead frame  40  shown in FIG. 8 and a heat sink (substrate)  3  shown in FIGS. 9 and 10. 
     The lead frame  40  is formed into a predetermined pattern by etching a metallic plate about 0.15 mm in thickness or by punching the plate with a precision press. The lead frame  40  used in this embodiment is of a multi-arrangement structure in which plural product forming portions  41  each for the formation of one product are arranged in a row. In this embodiment the number of the product forming portions  41  is five. The metallic plate is formed using an FE—Ni-base alloy such as Kovar or  42  alloy to match the heat sink  3  in expansion coefficient. On one side of the lead frame  40  are formed guide holes  43  to be used for conveyance or positioning of the lead frame. The guide holes  43  are positioned in opposition to the product forming portions  41  respectively. 
     As shown on the right end side of FIG. 8, each product forming portion  41  has a quadrangular frame  45 , a pair of cantilevered screwing pieces  4   a  projecting inwards from central portions of a pair of opposed sides of the frame  45 , and cantilevered leads  5   a  and  6   a  projecting inwards from central portions of the remaining pair of opposed sides. At a base portion of the frame  45  one side edge of the lead  5   a  is thinned obliquely to form a polarity identifying portion  36 . In a subsequent step of cutting the lead  5   a  the polarity identifying portion  36  is cut halfway thereof to form a polarity identifying portion  36  at a tip corner portion of the lead  5 . 
     Ni plating film is formed on the whole surface of the lead frame  40  and Au plating film is formed at least on surfaces of the screwing pieces  4   a  and the leads  5   a ,  6   a . Ni plating film is high in its adhesion to both Fe—Ni alloy and Au plating. In this first embodiment it suffices for the Au plating film to be as thin as 0.2 μm for example because it is only the connection of Al wires to the leads  5   a  and  6   a  that is required. The material cost is reduced by thinning the Au plating film. 
     All of the screwing pieces  4   a  and the leads  5   a ,  6   a  are eventually cut into screwing pieces  4  and leads  5 ,  6  at positions near base positions of the frame  45 . Therefore, in each screwing piece  4   a  is formed a hole  9   a  by a pattern which partially overlaps a screwing groove  9  so that the groove  9  is formed upon formation of the screwing piece  4 . Further, recessed engaging portions  35  are formed on both sides of an inner end portion of each of the leads  5   a  and  6   a.    
     A square space is present centrally of each product forming portion  41 . The heat sink (substrate)  3  comes to be positioned in the space. Screws are used in this embodiment for fixing the heat sink  3 . Therefore, guide holes  46  for insertion therein of the screws are formed in both side edge portions on an inner-end side of each screwing piece  4 . FIGS. 9 and 10 show a rectangular heat sink  3 , with tapped holes  47  being formed in the four corners of the heat sink correspondingly to the guide holes  46 . Means for fixing the heat sink  3  is not limited to the use of screws. There may be adopted welding, soldering, or riveting. 
     Semiconductor chips are fixed to the heat sink  3 . The semiconductor chips are each formed on a fragile silicon board, so for the prevention of damage to each semiconductor chip, it is required that the heat sink  3  be high in rigidity. It is also required for the heat sink  3  to be high in thermal conductivity because it also plays the role of dissipating the heat generated in the semiconductor chips to the exterior promptly. For this reason, the heat sink  3  is formed using such a metallic material as is highly compatible in thermal expansion coefficient with the semiconductor chips formed of silicon and superior in heat radiating property. For example, the heat sink  3  is formed of a flat CuMo plate about 2 mm thick. Ni plating film high in adhesion to CuMo is formed on the surface of the heat sink  3  and Au plating film is formed on the Ni plating film. The Au plating film is as thick as 3 μm or so. This is for making the semiconductor chips resistive to a high-temperature heat at the time of chip bonding for fixing the chips to the heat sink  3  by AuSi eutectic. 
     At the beginning of the assembling work in this first embodiment, as shown in FIGS.  11 ( a ) and  11 ( b ), three semiconductor chips  15 ,  16 , and  17  are fixed to the main surface of the heat sink  3  by AuSi eutectic layer. This fixing operation can be done easily by affixing gold foil to predetermined portions of the heat sink, then pushing the semiconductor chips against the gold foil, followed by scrubbing under heating (430° C. for example). In this way the semiconductor chip  15  with MOSFET formed thereon is fixed centrally and the semiconductor chips  16  and  17  with capacitors formed thereon are fixed on both sides of the chip  15  (see FIGS.  6  and  7 ). 
     This chip bonding work is performed by means of a chip bonding apparatus (not shown). Handling of each semiconductor chip is conducted using a tool called collet. The heat sink  3  is a flat plate having a flat main surface. Thus, unlike the heat sink  71  shown in FIG. 34, any obstacle, including the frame  72 , is not present on the main surface, so there is no fear of the collet striking against any obstacle even when it is scrubbed, thus permitting bonding for any portion of the main surface of the heat sink  3 . This leads to reduction of the bonding margin and hence reduction in size of the heat sink  3 . 
     Next, the heat sink  3  is fixed to the lead frame  40 , as shown in FIG.  12 . More specifically, the main surface of the heat sink  3  is positioned and superimposed on the back side of the lead frame  40 , then screws  7  are inserted into the guide holes  46  formed in the lead frame  40  and their tips are threaded into the tapped holes  47  of the heat sink  3 , whereby the heat sink is fixed to the lead frame  40 . At this stage, the discrete members are assembled into a continuous frame shape, thus permitting easy handling and permitting subsequent works to be done in high efficiency and accurately. 
     Next, wire bonding is conducted as shown in FIGS. 13 and 14. The wire bonding is carried out by means of an ultrasonic wire bonder using Al wire. More specifically, as shown in FIG. 14, the lead frame  40  with the heat sink  3  secured thereto is put on a stage  50  of the ultrasonic wire bonder and then a wire  52  held by a bonding tool  51  is connected by scrubbing induced by ultrasonic oscillation of the bonding tool  51 . For example, the drain electrode  20  of the semiconductor chip  15  on the heat sink  3  and the upper electrode  25   a  on the semiconductor chip  16  are connected with each other through wires  31 , while the upper electrode  25   a  and the lead  5   a  are connected with each other through wires  32 . 
     Connection of the wires  31  and  32  is performed as indicated with arrows a to j in FIG.  14 . To be more specific, the bonding tool  51  moves down like arrow a while holding the wire  52  with use of a clamper, pushes the tip of the wire  52  against the drain electrode  20 , then oscillates for connection (first bonding), thereafter moves upward, horizontally and then downward like arrows b, c, d, to connect an intermediate portion of the wire  52  to the upper electrode  25 a on the semiconductor chip  16  (second bonding). During such ascent, horizontal movement, and descent of the bonding tool  51 , the damper opens and the wire  52  is drawn out successively from the tip of the bonding tool  51 , so that the wire  31  is connected while describing a desired loop. 
     After the second bonding, the bonding tool  51  rises like arrow e, but the damper holds the wire  52  during this upward movement. As a result, the wire  52  is pulled and breaks at a position near the connection (second bonding portion) with the upper electrode  25   a.    
     Next, the bonding tool  51  moves like arrows f to j to effect connection of the wire  32  in which the upper electrode  25   a  on the semiconductor chip  16  is the first bonding portion and the tip of the lead  5   a  is the second bonding portion. As a result, the drain electrode  20  on the semiconductor chip  15  is connected to the lead  5   a  electrically. 
     Next, the stage  50  is turned 180°, then by the same wire bonding method as that described previously the gate electrode  21  of the semiconductor chip  15  and the upper electrode  25   b  of the semiconductor chip  17  are connected with each other through wires  33 , and the upper electrode  25   b  and the lead  6   a  are connected with each other through a wire  34 , to complete wire bonding. The wires  31  to  34  are in such connected states as shown in FIG.  1 . 
     Since the Al wire bonding work can be done at room temperature, the deterioration of the surface plating of the lead frame  40  is difficult to occur. Therefore, even if the Au plating film formed on the surfaces of leads  5   a  and  6   a  of the lead frame  40  is as thin as 0.2 μ or so, the problem of deteriorated bondability does not occur. If the Au plating film is as thin as 0.2 μm or so, it is possible to enhance the reliability because an intermetallic compound of AlAu cannot grow. There also accrues an actual advantage that the work for removing Au from the lead surfaces is not needed at the time of mounting performed by a user. 
     The wire bonding work can be done smoothly because an obstacle such as a frame which interferes with the bonding tool  51  in wire bonding and restricts the movement of the bonding tool is not present on the main surface of the heat sink  3 . 
     Next, one-side molding is performed by transfer molding. FIG. 15 is a plan view of the lead frame  40  in which the heat sink  3  and the wires  31  to  34  are covered with the seal member  2 . As shown in the same figure, a lead  5   a  projects from one long side of the seal member  2  which is flat and quadrangular in shape, a lead  6   a  projects from the other long side, and screwing pieces  4   a  project from short sides respectively. As a matter of course, holes  9   a  are positioned outside the seal member  2 . 
     In central edge portions of the short sides of the seal member  2  are formed recesses  8  which are depressed toward the center of the seal member. The recesses  8  are draw-out traces of an upper mold of a molding die which pushes the heat sink  3  and the screwing pieces  4   a  superimposed on the heat sink against a lower mold of the die in transfer molding. With the upper mold which leaves such draw-out traces, the screwing pieces  4   a  can be brought into close contact with the heat sink  3  and the back side of the heat sink can be put in close contact with the lower mold, whereby it is possible to prevent the leakage of resin. 
     Next, an unnecessary lead frame portion is removed to manufacture the semiconductor device. In the upper-stage portions of FIGS. 16 and 17, a quadrangular shape which surrounds each seal member  2  represents a cutting line  55  in a cutting die, while the lower-stage portions represent the semiconductor device  1  manufactured by cutting along the cutting line  55 . 
     Cutting and removal of an unnecessary lead frame portion are carried out by cutting the pair of screwing pieces  4  and leads  5   a ,  6   a  projecting from the seal member  2  rectilinearly in their width directions. 
     As shown in FIG. 16, by cutting the leads  5   a  and  6   a  at positions spaced a predetermined distance from edges of the seal member  2  there can be obtained leads  5  and  6  as external electrode terminals. In cutting the leads  5   a  and  6   a , the lead  5   a  is cut intermediate the polarity identifying portion  36  which becomes an oblique portion. As a result, an obliquely cut-out polar identifying portion  36  is formed at one corner of the outer end of the lead  5 . Further, the screwing pieces  4   a  are cut so as to cross the holes  9   a , whereby screwing pieces (suspension leads)  4  having screwing grooves  9  at respective outer ends can be formed, as shown in FIG.  2 . 
     In this case, the cutting positions of the screwing pieces  4   a  may be set close to sides of the seal member  2 , as shown in FIG. 17, whereby it is possible to prevent screwing grooves from being formed in the outer ends of the screwing pieces  4 . In the semiconductor device  1  thus fabricated, the short screwing pieces  4  projecting from the seal member  2  are not used in mounting. 
     In any case, the leads  5  and  6  serve as a drain lead  5  and a gate lead  6 , respectively. Further, the heat sink serves as a source electrode. The screwing pieces (suspension leads)  4  connected to the heat sink  3  can be used also as source electrodes (source leads). 
     Next, there are performed characteristic selection (electrical selection) and a mark forming work. Further, by bending in one step the pair of screwing grooves  4  projecting from side faces of the seal member  2  to form such gull wing-like leads as shown in FIG. 3, it is possible to manufacture such a semiconductor device  1  as shown in FIG. 3 in a plural number. 
     The back sides of the screwing pieces  4  are positioned on the same plane as the back side of the heat sink  3  or on a plane close thereto. This bending work may be done simultaneously with the cutting of the screwing pieces  4 . 
     FIGS.  18 ( a ) and  18 ( b ) are a diagram and a graph, respectively, showing how a gap (t) formed between a package bottom (the bottom of the heat sink  3 ) in the semiconductor device  1  manufactured by the method of the first embodiment and a lower surface of each screwing piece (suspension lead)  4  is correlated with a thermal resistance and a package breakdown occurrence rate. More specifically, FIG.  18 ( a ) is a schematic diagram of the semiconductor device  1 , showing the gap (t), and FIG.  18 ( b ) is a graph showing a correlation between the gap (t) and a thermal resistance as well as a package breakdown occurrence rate. 
     As is seen from the above graph, in the semiconductor device of the structure according to the present invention (the first embodiment) and in case of the gap (t) being minus (−), the larger the minus, the wider the gap from the mounting substrate at the time of mounting, with the result that the thermal resistance through the heat sink  3  increases rapidly. In case of the gap (t) being plus (+), the thermal resistance becomes constant because the heat sink  3  comes into contact with the mounting substrate. 
     In the semiconductor device of this first embodiment, by making the gap (t) plus, that is, by setting the lower surface of each suspension lead at a position higher than the back side (lower surface) of the heat sink  3 , i.e., at a retracted position from the back side of the heat sink  3 , it is possible to diminish the thermal resistance and also possible to make the package breakdown occurrence rate smaller than in the case of the studied semiconductor device shown in FIG.  28  and subsequent figures. In the graph of FIG.  18 ( b ), the measurement result on the semiconductor device of the structure according to the present invention has been obtained in a screwed state of the suspension leads  4  to the mounting substrate, while the measurement result on the structure of the studied semiconductor device has been obtained in a fixed state of the heat sink to the mounting substrate directly with screws. 
     The following effects are obtained by the semiconductor device and manufacturing method for the same according to the first embodiment. 
     (1) According to the structure of the semiconductor device, the heat sink for heat radiation is thick, while the screwing pieces to be used in mounting the semiconductor device are thin, whereby a stress induced in mounting is absorbed by deformation of the stepped portions of the thin screwing pieces. Therefore, the heat sink no longer undergoes such a deformation as warping and hence it is possible to prevent damage of the semiconductor chips fixed to the heat sink. 
     (2) Since the set screw space so far provided in the heat sink  3  can be omitted, it is possible to decrease the amount of the expensive CuMo material used and hence possible to reduce the cost of the semiconductor device  1 . 
     (3) Since the screwing pieces (suspension leads)  4  are thinner than the heat sink  3  and permit the selection of a material low in rigidity and superior in formability, a stress induced at the time of screwing in the mounting work can be absorbed by the bent portions  10  of the suspension leads  4 . As a result, the heat sink machining accuracy can be mitigated and it is possible to reduce the package cost. 
     (4) Since it is only the heat sink portion that is required to have heat resistance in AuSi eutectic chip bonding, it suffices for only the heat sink  3  to be thick plated with Au. That is, it is possible to decrease the amount of the expensive Au used an hence possible to reduce the cost of the semiconductor device  1 . 
     (5) In the resin molded package, in comparison with a ceramic package, both sealing material cost and machining cost are low; besides, hermetic seal inspection and dust particle inspection are not needed, thus making contribution to the reduction of cost and making it possible to reduce the cost of the semiconductor device  1 . 
     (6) Since the plating of the leads  5  and  6  is an extremely thin Au plating, an Au plating removing work is not required on the user side, whereby the reduction of mounting cost can be attained. If the surfaces of the leads  5  and  6  are plated with any other plating material than Au, e.g., solder, it becomes unnecessary for the user to perform the Au plating removing work, whereby it is possible to attain the reduction of the mounting cost. 
     (7) By adoption of the resin molded package it becomes easy to attain the reduction in size and high integration of the semiconductor device in comparison with the ceramic package. More particularly, in the semiconductor device having a ceramic package, the ceramic package is compelled to become large in size in order to avoid contact of the chip holding collet with inner wall surfaces of the package at the time of chip bonding. On the other hand, in the case of a resin molded package, the size thereof can be reduced because chip bonding and wire bonding are performed in the absence of a package and thereafter the resin molded package is formed. 
     (8) Since the Al wire bonding work can be done at room temperature, the plating on the surface of the lead frame  40  is difficult to occur. Therefore, even if the Au plating formed on the surfaces of the leads  5   a  and  6   a  of the lead frame  40  is as thin as 0.2 μm or so, there does not arise any problem related to bondability. With a thin Au plating film of 0.2 μm or so, an intermetallic compound of AlAu cannot grow and hence it is possible to enhance the reliability. There also accrues an actual advantage that the Au removing work from the lead surfaces is not needed at the time of mounting performed on the user side. 
     (9) The lead frame and the heat sink are used in this first embodiment, but the heat sink is screwed to each of the product forming portions  41 , so in the subsequent assembling work there can be utilized an assembling technique with the thus-assembled lead frame, whereby the assembling work can be done efficiently and consequently it is possible to reduce the semiconductor device manufacturing cost. 
     Second Embodiment 
     FIGS. 19 to  21  illustrate a semiconductor device according to another embodiment (second embodiment) of the present invention, of which FIG. 19 is a perspective plan view showing the structure of the semiconductor device, FIG. 20 is a schematic sectional view of the semiconductor device, and FIG. 21 is a schematic sectional view in another section of the semiconductor device. 
     According to the semiconductor device of this second embodiment, in the structure of the first embodiment there is adopted a double molded structure in which, as shown in FIGS. 19 to  21 , the semiconductor chips  15 ,  16 ,  17  and the wires  31  to  34  are covered with undercoat resin  56 , and the undercoat resin  56 , etc. are sealed with the seal member  2 . 
     The heat resistance of transfer molding resins presently in practical use is 150° C. or so, thus giving rise to the problem that it is impossible to cope with the junction temperature of Tj≧175° C. required of a power FET for a base station. Besides, there is a fear that a high frequency loss of a transfer molding resin may become unignorable against the market trend to an increase of operating frequency. 
     In view of this point, according to the structure of this second embodiment, the semiconductor chips  15 ,  16 ,  17 , and the wires  31  to  34  are covered with a potting type undercoat resin superior in heat resistance and with little high frequency loss. By such a double molded structure using the under coat resin the chip surfaces which become the highest in temperature in the semiconductor device and the bonding wire portion as a high frequency current path can be covered with a potting resin having high heat resistance and low loss characteristics can be covered with the potting resin. Thus, the structure in question is effective for a semiconductor device for a radio base station with a junction temperature as high as 150 to 200° C. Therefore, this structure is effectively applicable to a semiconductor device of a high frequency output of 50W or more. 
     For the prevention of wire breaking during the application of the undercoat resin it is preferable that the whole surfaces of the chips and the wires be covered with the undercoat resin. But in the case of the structure of the previous first embodiment, the undercoat resin flows down from the gap between the leads and the heat sink, making it impossible to cover the whole of the wires. 
     In this second embodiment, in view of the point just mentioned above, the inner ends of the leads  5  and  6  are positioned on the heat sink  3  to prevent flowing and loss of the undercoat resin  56 , as shown in FIG.  19 . In this case, for electrical insulation between the leads  5 ,  6  and the heat sink  3 , as shown in FIG. 20, spacers  57  each constituted by a doughnut-like insulator are sandwiched in between the screwing pieces  4   a  and the heat sink  3  to ensure insulation between the heat sink and the screwing pieces  4   a . Thus interposing the spacers  57  between the heat sink  3  and the screwing pieces  4   a  means that in the assembled state the screwing pieces  4   a  and the leads  5 ,  6  (leads  5   a ,  6   a ) are spaced apart a distance corresponding to the thickness of each spacer  57 . 
     Consequently, at the time of application of the undercoat resin, the resin which has flowed down between the inner end sides of the leads  5   a ,  6   a  and the wires  31  to  34  gets on the heat sink  3  and no longer flows away, but covers the wires  31  to  34 , further covers the semiconductor chips  15 ,  16 ,  17  and the leads  5   a ,  6   a.    
     According to such a structure wherein the spacers  57  are used to ensure gaps between the heat sink and the lead frame (leads  5   a ,  6   a , and the screwing pieces  4   a ), at the time of clamping with a molding die in transfer molding, the screwing pieces  4   a  ( 4 ) are deformed when the heat sink  3  is pushed against the lower mold of the die, and thus there is a fear that the clamping operation may not be carried out in a satisfactory manner. In this second embodiment, therefore, holes (upper mold through holes)  58  for passage therethrough of the upper mold are formed in the screwing pieces  4   a  to prevent deformation of the screwing pieces  4   a . That is, the holes  58  are heat sink pressing type windows. 
     In this second embodiment, the undercoat resin material and the resin material which forms the seal member can be selected in accordance with separate specifications, it is possible to easily improve the characteristics and reliability of the semiconductor device  1 . 
     FIG. 22 is a plan view of a spacer used in a semiconductor device according to a modification of the second embodiment. In the illustrated example, spacers  57  formed of an insulator and having an external shape almost equal to that of the heat sink  3  are interposed between the heat sink  3  and the leads  5   a ,  6   a  ( 5 ,  6 ) as well as the screwing pieces  4   a  ( 4 ). The spacers  57   a  are formed with tapped holes  59  for insertion therein of screws  7 . 
     Since the insulating spacers  57   a  have such a structure as illustrated in the figure, they are disposed also under the leads  5   a  and  6   a  ( 5 ,  6 ), so that the insulation between the leads  5 ,  6  and the heat sink  3  is ensured and the reliability of the semiconductor device  1  is improved. 
     According to this structure, moreover, vertical movements (fluttering) of the leads  5  and  6  are diminished, ultrasonic oscillation in wire bonding can surely be applied to the wire, thus permitting a highly accurate wire bonding. As a result, it is possible to provide a semiconductor device  1  with high wire connectability. 
     The leads of a semiconductor device used in a high frequency amplifier circuit constitute a part of a high frequency circuit, so an appropriate capacitance is required as the case may be. When such spacers  27   a  as shown in FIG. 22 are interposed between the leads  5 ,  6  and the heat sink  3 , they function as dielectrics and therefore a required capacitance can be obtained by combining lead area, lead-heat sink spacing, and lead-heat sink dielectric constant with one another, also permitting the improvement in efficiency of the high frequency amplification circuit. 
     FIG. 23 is a schematic sectional view showing the structure of a semiconductor device according to another modification of the second embodiment. In this modification there is proposed another structure for ensuring a gap between the lead frame and the heat sink. More specifically, as shown in the same figure, the portions of the screwing pieces  4   a  ( 4 ) where guide holes  46  are to be formed are subjected to embossing and are thereby formed lower than the portions adjacent thereto, and the lower portions, indicated at  60 , are fixed to the heat sink  3  with screws  7 , whereby the screwing pieces  4   a  and the leads  5   a ,  6   a  ( 5 ,  6 ) are made higher by a predetermined spacing from the main surface of the heat sink  3  to form gaps. 
     Third Embodiment 
     FIG. 24 is a plan view of a semiconductor device according to a further embodiment (third embodiment) of the present invention and FIG. 25 is a front view thereof. This third embodiment shows an example in which the drain and gate leads  5 ,  6  of the structures described in the first embodiment are each arranged side by side in a plural number. In this third embodiment, two sets of drain and gate leads  5 ,  6  are arranged side by side. With such a structure, it is possible to increase the output. 
     Fourth Embodiment 
     FIG. 26 is a plan view of a semiconductor device according to a still further embodiment (fourth embodiment) of the present invention and FIG. 27 is a side view thereof. In this fourth embodiment the present invention is applied to a ceramic package. 
     According to the semiconductor device, indicated at  1   a , of this fourth embodiment, in the studied semiconductor device, a metallic cap  87  is extended long on both end sides of the frame  72  constituted by a ceramic laminate, and extended portions  87   d  are bent in one step, then extend so that lower surfaces thereof are positioned on a plane coincident with or close to the back side of the heat sink  71 . In outer ends of the extended portions  87   d  are formed screwing grooves  73 . 
     The cap  87  is bonded through AuSn alloy  89  to the plating film  87   a  formed on an upper surface of the frame  72 . The cap  87  is formed so as to become equal in potential as the source electrode. Thus, the extended portions  87   d  are employable also as source electrodes. 
     Although the present invention has been described above on the basis of embodiments thereof, it goes without saying that the invention is not limited to those embodiments and that various changes may be made within the scope not departing from the gist thereof. Although in the above embodiments screws are used to mount the respective semiconductor devices, the invention is also applicable to semiconductor devices which are mounted by the reflow of solder. 
     The following is a brief description of effects obtained by typical inventions disclosed herein. 
     (1) It is possible to reduce the material cost, reduce the cost of assembling, including sealing, and further reduce the inspection cost, and hence possible to reduce the semiconductor device manufacturing cost. 
     (2) It is possible to provide a semiconductor device wherein the warp of a heat sink is difficult to occur, and hence possible to improve the reliability of mounting.