Patent Publication Number: US-2022223545-A1

Title: Semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a semiconductor device. 
     Description of the Background Art 
     A semiconductor device for power control having a structure in which semiconductor elements are sealed in a case is widely known. High heat resistance and high insulation engineering plastics such as polyphenylene sulfide (PPS) are used in many cases as the material of the case. Silicone based gel or epoxy resin is used as the sealing material to be filled in the case. Further, the case is provided with a lid for protecting the sealing material, and the material of the lid is typically the same as the material of the case. Hereinafter, the semiconductor elements and the semiconductor device for power control may be referred to as a “power semiconductor element” and a “power semiconductor device”, respectively. 
     With the shift to SiC devices for power semiconductor devices and the improvement of the temperature characteristics of Si devices, the market demand for power semiconductor devices is surging, and, for example, the humidity absorption tolerance, which has been used as a reference test, is being required as a guarantee of use. As a humidity absorption tolerance test, Temperature Humidity Bias (THB) test specified in JEITA ED-4701 102A is adopted, in which, for example, a storage test is conducted in which a specified voltage is applied in an environment of a temperature of 85° C. and a humidity of 85%. 
     As moisture permeation progresses in the power semiconductor device, the insulation resistance on the surface of the power semiconductor element and the surface of a highly insulating member such as an insulating substrate on which the power semiconductor element is mounted decreases, and this may shorten the life of the power semiconductor device. In particular, the termination portion of a power semiconductor element made of SiC is shrunk and is a portion to which a high electric field is applied; therefore, there is a considerable adverse effect due to the lowering of the insulation resistance of the termination portion from moisture absorption. 
     The main paths of moisture permeation and moisture absorption in the power semiconductor device having the above structure pass through the adhesive interface and the fitting portion of the case and the lid. For example, Japanese Patent Application Laid-Open No. 2014-150204 discloses a technique for arranging a sheet material on a sealing material of a semiconductor device in order to suppress intrusion of gas, moisture or the like, derived from the operating environment of the semiconductor device. 
     SUMMARY 
     In the technique of Japanese Patent Application Laid-Open No. 2014-150204, a gap is generated between the sheet material and the case, and there is a possibility that moisture may enter through the gap. 
     An object of the present disclosure is to suppress moisture from entering in the semiconductor device. 
     The semiconductor device includes a semiconductor element, a case housing the semiconductor element, a sealing material filled in the case in which the semiconductor element is housed, a first low moisture permeable sheet covering the sealing material, and a lid covering an opening of the case. The first low moisture permeable sheet is made of a low moisture permeable material having moisture permeability of 1 g/m 2 ×24 Hr or less. A peripheral edge of the first low moisture permeable sheet is interposed between the case and the lid. 
     These and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a semiconductor device according to Embodiment 1; 
         FIG. 2  is a perspective view of a low moisture permeable sheet according to Embodiment 1; 
         FIG. 3  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 1; 
         FIG. 4  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 1; 
         FIG. 5  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 1; 
         FIG. 6  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 1; 
         FIG. 7  is a schematic cross-sectional view of a semiconductor device according to Embodiment 2; 
         FIG. 8  is a top view and a cross-sectional view of a low moisture permeable sheet according to Embodiment 2; 
         FIG. 9  is a schematic cross-sectional view of a semiconductor device according to Embodiment 3; 
         FIG. 10  is a top view of a low moisture permeable sheet according to Embodiment 3; 
         FIG. 11  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 3; 
         FIG. 12  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 3; 
         FIG. 13  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 3; 
         FIG. 14  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 3; 
         FIG. 15  is a schematic cross-sectional view of a semiconductor device according to Embodiment 4; 
         FIG. 16  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 4; 
         FIG. 17  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 4; 
         FIG. 18  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 4; and 
         FIG. 19  is a schematic cross-sectional view illustrating a manufacturing process of the semiconductor device according to Embodiment 4. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a schematic cross-sectional view of a semiconductor device  100  according to Embodiment 1. As illustrated in  FIG. 1 , the semiconductor device  100  is a power semiconductor device having a structure in which semiconductor elements  1  being power semiconductor elements are housed in a case  2  and sealed with a sealing material  3 . Semiconductor elements  1  are Insulated Gate Bipolar Transistors (IGBT), Metal Oxide Semiconductor Field Effect Transistors (MOSFET) or the like formed of silicon (Si), silicon carbide (SiC), gallium nitride (GaN), or the like. 
     The semiconductor elements  1  are mounted on an insulating substrate  4 , and the insulating substrate  4  is mounted on a base plate  6 . The insulating substrate  4  is composed of an insulating layer  4   a , a circuit pattern  4   b  formed on the upper surface of the insulating layer  4   a , and a circuit pattern  4   c  formed on the lower surface of the insulating layer  4   a . The semiconductor elements  1  are bonded to the circuit pattern  4   b  via the bonding materials  5 , and the base plate  6  is bonded to the circuit pattern  4   c  via the bonding material  7 . 
     There are no restrictions on the materials of the insulating layer  4   a  and the circuit patterns  4   b  and  4   c . The insulating layer  4   a  may be composed of an inorganic ceramic material such as alumina (Al 2 O 3 ), aluminum nitride (AlN), or silicon nitride (Si 3 N 4 ). The circuit patterns  4   b  and  4   c  may be composed of, for example, copper or an alloy thereof, aluminum or an alloy thereof, and the like. As the bonding materials  5  and the bonding material  7 , solder or a solder alloy made of lead (Pb), tin (Sn) or the like, or a sintered material made of nano-silver or nano-copper particles is used. The material of the bonding materials  5  and the material of the bonding material  7  may be the same or different from each other. 
     The material of the base plate  6  may be a metal material such as copper, aluminum, a copper-molybdenum alloy (CuMo), or a composite material such as a silicon carbide-aluminum composite material (AlSiC) or a silicon carbide-magnesium composite material (MgSiC). 
     The case  2  is also mounted on the base plate  6 , and the case  2  is adhered to the base plate  6  using an adhesive  8  (second adhesive). 
     The case  2  incorporates electrodes  9  used for connection with the outside. The semiconductor elements  1 , the circuit patterns  4   b , and the electrodes  9  are connected via metal wires  10  or directly to form an electric circuit. As the material of the electrodes  9 , for example, a metal mainly composed of copper (Cu) or an alloy thereof is typically used. And, a plating layer such as Ni may be provided on the surface of the electrode  9 . As the material of the metal wires  10 , aluminum (Al), copper (Cu), alloys thereof, or the like is used. 
     The sealing material  3  is filled in the case  2  and seals the semiconductor elements  1  and the insulating substrate  4  on which the semiconductor elements  1  are mounted. The material of the sealing material  3  is an insulating resin such as a silicone resin or an epoxy resin. 
     Further, a low moisture permeable sheet  11  (first low moisture permeable sheet) made of a low moisture permeable material is provided in the case  2  so as to cover the sealing material  3 . In Embodiment 1, a flat plate-shaped low moisture permeable sheet  11  as illustrated in  FIG. 2  is used. The low moisture permeable material constituting the low moisture permeable sheet  11  has low moisture and gas permeability, such as a fluororesin such as polytetrafluoroethylene (PTFE), and the moisture permeability thereof is 1 g/m 2 ×24 Hr or less is desirable. The moisture permeability is defined by JIS Z 0208, “Humidity permeability test method for moisture-proof packaging material” and the like. 
     Further, in order to prevent the semiconductor device  100  from being enlarged in size, the thickness of the low moisture permeable sheet  11  is preferably 3 mm or less, and more preferably 1 mm or less. The low moisture permeable sheet  11  may be in contact with the upper surface of the sealing material  3 . 
     The opening of the case  2  is closed by the lid  12 , and the lid  12  is adhered to the case  2  using an adhesive  13  (first adhesive). As illustrated in  FIG. 1 , the peripheral edge of the low moisture permeable sheet  11  is interposed between the case  2  and the lid  12 . The material of the case  2  and the lid  12  may be any material having electrical insulation, and may be formed of, for example, an epoxy resin or a polyphenylene sulfide (PPS) resin. Further, the adhesives  8  and  13  may be a typical silicone-based adhesive or may be made of a low moisture permeable material such as an acrylic resin. 
     Here, the structure formed by the case  2 , the low moisture permeable sheet  11  and the lid  12  in the semiconductor device  100  of  FIG. 1  will be described in detail. The lid  12  has a convex portion  12   a  having a shape fitted to the opening of the case  2 , and the case  2  has a counterbore portion  2   a  on the peripheral edge of the opening. Therefore, when the convex portion  12   a  of the lid  12  is fitted into the opening of the case  2 , the convex portion  12   a  and the counterbore portion  2   a  face each other. The low moisture permeable sheet  11  extends over the counterbore portion  2   a  of the case  2 , and the peripheral edge of the low moisture permeable sheet  11  is interposed between the convex portion  12   a  and the counterbore portion  2   a.    
     The adhesive  13  for adhering the case  2  and the lid  12  is arranged in a portion where the low moisture permeable sheet  11  is not interposed (the outer portions of the counterbore portion  2   a  and the convex portion  12   a  in  FIG. 1 ), and the thickness direction of the adhesive  13  is the same as the thickness direction of the low moisture permeable sheet  11  in the portion where the low moisture permeable sheet  11  is interposed between the case  2  and the lid  12 . Consequently, when the adhesive  13  is cured and shrunk, the counterbore portion  2   a  of the case  2  and the convex portion  12   a  of the lid  12  are attracted to each other, leading to the effect that the adhesion is strengthened between the counterbore portion  2   a  and the convex portion  12   a  and the low moisture permeable sheet  11  interposed therebetween can be obtained. In particular, if the low moisture permeable sheet  11  is made of an elastic material such as fluororesin, the low moisture permeable sheet  11  is elastically deformed and high adhesion between the counterbore portion  2   a  and the convex portion  12   a  is obtained. 
     According to the semiconductor device  100  of Embodiment 1, the sealing material  3  that seals the semiconductor elements  1  in the case  2  is covered with the low moisture permeable sheet  11  made of the low moisture permeable material, and the low moisture permeable sheet  11  is interposed between the case  2  and the lid  12 , thereby making a gap less likely to be formed between the case  2  and the low moisture permeable sheet  11 . Therefore, the number of paths through which moisture enters into the case  2  is reduced; therefore, moisture is prevented from entering the case  2 , improving dampproofness of the semiconductor device  100 . For example, even when a typical silicone-based adhesive (moisture permeability is about 10 g/m 2 ×24 Hr to 100 g/m 2 ×24 Hr) is used as the adhesive  13  for adhering the case  2  and the lid  12 , a sufficient moisture-proof effect can be obtained. Further, the low moisture permeable sheet  11  provides the high moisture-proof effect; therefore, narrowing the width of the region to which the adhesive  13  is applied can contribute to the downsizing of the semiconductor device  100 . In particular, when the power semiconductor element  1  is a SiC element, the termination portion thereof is shrunk and is a portion to which a high electric field is applied, and there is a considerable adverse effect due to the lowering of the insulation resistance of the termination portion from moisture absorption; therefore, the above effect is effective. 
     The surface of the low moisture permeable sheet  11  may be roughened by providing irregularities on the surface of the low moisture permeable sheet  11 . By roughening the surface of the low moisture permeable sheet  11 , the adhesion between the low moisture permeable sheet  11  and the sealing material  3  is improved, and further suppression of paths through which moisture enters from being created can be expected. The roughening method of the low moisture permeable sheet  11  may be a physical method or a chemical method. For example, a method of providing irregularities on the surface of the mold for molding the low moisture permeable sheet  11  can be conceived. 
     Here, a method of manufacturing the semiconductor device  100  will be described. First, the base plate  6 , the bonding material  7 , the insulating substrate  4 , the bonding material  5  and the semiconductor elements  1  are stacked in this order, and the reflow is performed under reduced pressure or in a reducing gas atmosphere, and a thermal history of the melting temperature of the bonding material  5  and the bonding material  7  or higher is given, thereby bonding each member together. Next, wiring is performed between the semiconductor elements  1  or between the semiconductor element  1  and the insulating substrate  4  by ultrasonic bonding of the metal wires  10 . 
     Subsequently, the adhesive  8  is linearly applied to the outer peripheral portion (adhesive portion with the case  2 ) of the base plate  6 , the case  2  is placed on the adhesive  8 , and the adhesive  8  is subjected to thermosetting, thereby gluing the base plate  6  and case  2  together. After that, the electrodes  9  incorporated in the case  2  and the insulating substrate  4  are bonded by ultrasonic bonding. 
     Next, as illustrated in  FIG. 3 , the case  2  is filled with the sealing material  3 , and the sealing material  3  is subjected to thermosetting using an oven or the like to seal the semiconductor elements  1 . Then, as illustrated in  FIG. 4 , the sealing material  3  is covered with the low moisture permeable sheet  11 . At this point, the peripheral edge of the low moisture permeable sheet  11  is placed on the counterbore portion  2   a  of the case  2 . 
     Then, as illustrated in  FIG. 5 , the adhesive  13  is applied linearly or in a dotted manner to the edge of the case  2  (the adhesive portion with the lid  12 ). Then, as illustrated in  FIG. 6 , the convex portion  12   a  of the lid  12  is fitted into the opening of the case  2 , and the opening of the case  2  is closed with the lid  12 . At this point, the convex portion  12   a  of the lid  12  comes into contact with the upper surface of the peripheral edge of the low moisture permeable sheet  11 . Then, the adhesive  13  is subjected to thermosetting to adhere the case  2  and the lid  12  together. When the adhesive  13  cures and shrinks, the counterbore portion  2   a  of the case  2  and the convex portion  12   a  of the lid  12  are attracted to each other; therefore, the adhesion is strengthened between the counterbore portion  2   a  and the convex portion  12   a  and the low moisture permeable sheet  11  interposed therebetween. Through the above processes, the semiconductor device  100  illustrated in  FIG. 1  is completed. 
     Embodiment 2 
       FIG. 7  is a schematic cross-sectional view of a semiconductor device according to Embodiment 2.  FIG. 8  is a top view and a cross-sectional view of a low moisture permeable sheet  11  according to Embodiment 2. In Embodiment 2, a low moisture permeable sheet  11  has a convex portion  11   a  at a peripheral edge thereof, that is, a portion interposed between a case  2  and a lid  12 , and the case  2  is provided with a groove  2   b  into which the convex portion  11   a  of the low moisture permeable sheet  11  is to be inserted. 
     Further, in Embodiment 2, the convex portion  11   a  is provided at the end portion of the low moisture permeability sheet  11 ; therefore, the end portion of the low moisture permeability sheet  11  is L-shaped in cross-sectional view (As illustrated in  FIG. 8 , the low moisture permeable sheet  11  is U-shaped when viewed in the entire cross section of the low moisture permeable sheet  11 ). Further, the groove  2   b  of the case  2  is provided in the counterbore portion  2   a . Other configurations are the same as those in Embodiment 1 ( FIG. 1 ). 
     Any method may be used as the method of providing the convex portion  11   a  on the low moisture permeable sheet  11 . For example, the convex portion  11   a  may be formed by a mold for molding the low moisture permeable sheet  11 , or the lower surface of the flat plate-shaped low moisture permeable sheet  11  as illustrated in  FIG. 2  may be cut to form the convex portion.  11   a.    
     According to the semiconductor device  100  of Embodiment 2, the positioning accuracy of the low moisture permeable sheet  11  with respect to the case  2  and the retention of the low moisture permeable sheet  11  are improved; therefore, in addition to the effect same as that of Embodiment 1, the effect that the humidity absorption tolerance of the semiconductor device  100  can be further improved can be obtained. 
     Embodiment 3 
       FIG. 9  is a schematic cross-sectional view of a semiconductor device  100  according to Embodiment 3. As illustrated in  FIG. 9 , in the semiconductor device  100  of Embodiment 3, a low moisture permeable sheet  14  (second low moisture permeable sheet) is provided at the adhesive interface between a case  2  and a base plate  6 . The shape of the low moisture permeable sheet  14  is a frame shape in a plan view as illustrated in  FIG. 10 , and extends in the adhesive interface between the case  2  and the base plate  6  so as to surround a sealing material  3 . The low moisture permeable material constituting the low moisture permeable sheet  14  may be the same as a low moisture permeable sheet  11  covering the sealing material  3 , and the moisture permeability thereof is preferably 1 g/m 2 ×24 Hr or less. 
     In order to prevent the semiconductor device  100  from being enlarged in size, the thickness of the low moisture permeable sheet  14  is preferably 3 mm or less, and more preferably 1 mm or less. Further, a plurality of frame-shaped low moisture permeable sheets  14  may be nested at the adhesive interface between the case  2  and the base plate  6 . Further, in Embodiment 3, a groove  2   c  into which a part of the low moisture permeable sheet  14  is inserted is formed at the adhesive interface with the base plate  6  in the case  2 , and a groove  6   a  into which a part of the low moisture permeable sheet  14  is inserted is also formed at the adhesive interface with the case  2  in the base plate  6  is formed. Therefore, the thickness of the low moisture permeable sheet  14  can be made larger than the thickness of the adhesive  8  that adheres the case  2  and the base plate  6  together. Further, the low moisture permeable sheet  14  extends inside the adhesive  8 . That is, the adhesive  8  is provided on both sides of the low moisture permeable sheet  14 . Other configurations are the same as those in Embodiment 1 ( FIG. 1 ). 
     A method of manufacturing the semiconductor device  100  of Embodiment 3 will be described. First, the base plate  6  having the groove  6   a , the bonding material  7 , the insulating substrate  4 , the bonding material  5  and the semiconductor elements  1  are stacked in this order, and the reflow is performed under reduced pressure or in a reducing gas atmosphere, and a thermal history of the melting temperature of the bonding material  5  and the bonding material  7  or higher is given, thereby bonding each member together. Next, wiring is performed between the semiconductor elements  1  or between the semiconductor element  1  and the insulating substrate  4  by ultrasonic bonding of the metal wires  10 . As a result, the configuration illustrated in  FIG. 11  is obtained. 
     Subsequently, as illustrated in  FIG. 12 , the adhesive  8  is linearly applied to both sides of the groove  6   a  in the outer peripheral portion (adhesive portion with the case  2 ) of the base plate  6 . Further, as illustrated in  FIG. 13 , the low moisture permeable sheet  14  is placed on the base plate  6 . At this point, the lower portion of the low moisture permeable sheet  14  is inserted into the groove  6   a . Further, as illustrated in  FIG. 14 , the case  2  having the groove  2   c  is placed on the adhesive  8 . At this point, the upper portion of the low moisture permeable sheet  14  is inserted into the groove  2   c . Then, the adhesive  8  is subjected to thermosetting to adhere the base plate  6  and the case  2  together. 
     After that, as in Embodiment 1, the electrodes  9  incorporated in the case  2  are bonded to the semiconductor elements  1  or the insulating substrate  4 , the semiconductor elements  1  are sealed with the sealing material  3 , and after the sealing material  3  is covered with the low moisture permeable sheet  11 , the lid  12  is adhered to the case  2 . Through the processes, the semiconductor device  100  illustrated in  FIG. 9  is completed. 
     According to the semiconductor device  100  of Embodiment 3, moisture absorption through the adhesive  8  between the case  2  and the base plate  6  is reduced; therefore, in addition to the effect same as that of Embodiment 1, the effect that the humidity absorption tolerance of the semiconductor device  100  can be further improved can be obtained. 
     Embodiment 4 
       FIG. 15  is a schematic cross-sectional view of a semiconductor device  100  according to Embodiment 4. The configuration of the semiconductor device  100  according to Embodiment 4 is different from the configuration of Embodiment 3 ( FIG. 9 ) in that the thickness of a low moisture permeable sheet  14  provided at an adhesive interface between a case  2  and a base plate  6  is the same thickness as that of an adhesive  8 . The thickness of the low moisture permeable sheet  14  and the thickness of the adhesive  8  are the same; therefore, the groove  2   c  provided in the case  2  and the groove  6   a  provided in the base plate  6  in Embodiment 2 are not required to be provided, contributing to the improvement in the productivity and the reduction in processing cost. 
     A method of manufacturing the semiconductor device  100  of Embodiment 4 will be described. First, the base plate  6 , the bonding material  7 , the insulating substrate  4 , the bonding material  5  and the semiconductor elements  1  are stacked in this order, and the reflow is performed under reduced pressure or in a reducing gas atmosphere, and a thermal history of the melting temperature of the bonding material  5  and the bonding material  7  or higher is given, thereby bonding each member together. Next, wiring is performed between the semiconductor elements  1  or between the semiconductor element  1  and the insulating substrate  4  by ultrasonic bonding of the metal wires  10 . As a result, the configuration illustrated in  FIG. 16  is obtained. 
     Subsequently, as illustrated in  FIG. 17 , the adhesive  8  is linearly applied to both sides of the installation position of the low moisture permeable sheet  14  in the outer peripheral portion (adhesive portion with the case  2 ) of the base plate  6 . Further, as illustrated in  FIG. 18 , the low moisture permeable sheet  14  is placed on the base plate  6 . Unlike Embodiment 3, although no groove  6   a  for inserting the low moisture permeable sheet  14  is provided in the base plate  6 , the position of the low moisture permeable sheet  14  is fixed by the paste-like adhesive  8 . Then, as in  FIG. 19 , the adhesive  8  is placed on the case  2  and the adhesive  8  is subjected to thermosetting to adhere the base plate  6  and the case  2  together. 
     After that, as in Embodiment 1, the electrodes  9  incorporated in the case  2  are bonded to the semiconductor elements  1  or the insulating substrate  4 , the semiconductor elements  1  are sealed with the sealing material  3 , and after the sealing material  3  is covered with the low moisture permeable sheet  11 , the lid  12  is adhered to the case  2 . Through the processes, the semiconductor device  100  illustrated in  FIG. 15  is completed. 
     The embodiments can be combined, appropriately modified or omitted. 
     While the disclosure has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised.