SEMICONDUCTOR DEVICE

Object is to reduce the number of paths through which moisture enters in a semiconductor device. A semiconductor device includes a semiconductor element, a case housing the semiconductor element, a sealing material filled in the case, a low moisture permeable sheet covering the sealing material, and a lid covering an opening of the case. The low moisture permeable sheet is made of a low moisture permeable material having moisture permeability of 1 g/m2×24 Hr or less. The low moisture permeable sheet is interposed, at the peripheral edge thereof, between the case and the lid.

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/m2×24 Hr or less. A peripheral edge of the first low moisture permeable sheet is interposed between the case and the lid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a schematic cross-sectional view of a semiconductor device100according to Embodiment 1. As illustrated inFIG. 1, the semiconductor device100is a power semiconductor device having a structure in which semiconductor elements1being power semiconductor elements are housed in a case2and sealed with a sealing material3. Semiconductor elements1are 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 elements1are mounted on an insulating substrate4, and the insulating substrate4is mounted on a base plate6. The insulating substrate4is composed of an insulating layer4a, a circuit pattern4bformed on the upper surface of the insulating layer4a, and a circuit pattern4cformed on the lower surface of the insulating layer4a. The semiconductor elements1are bonded to the circuit pattern4bvia the bonding materials5, and the base plate6is bonded to the circuit pattern4cvia the bonding material7.

There are no restrictions on the materials of the insulating layer4aand the circuit patterns4band4c. The insulating layer4amay be composed of an inorganic ceramic material such as alumina (Al2O3), aluminum nitride (AlN), or silicon nitride (Si3N4). The circuit patterns4band4cmay be composed of, for example, copper or an alloy thereof, aluminum or an alloy thereof, and the like. As the bonding materials5and the bonding material7, 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 materials5and the material of the bonding material7may be the same or different from each other.

The material of the base plate6may 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 case2is also mounted on the base plate6, and the case2is adhered to the base plate6using an adhesive8(second adhesive).

The case2incorporates electrodes9used for connection with the outside. The semiconductor elements1, the circuit patterns4b, and the electrodes9are connected via metal wires10or directly to form an electric circuit. As the material of the electrodes9, 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 electrode9. As the material of the metal wires10, aluminum (Al), copper (Cu), alloys thereof, or the like is used.

The sealing material3is filled in the case2and seals the semiconductor elements1and the insulating substrate4on which the semiconductor elements1are mounted. The material of the sealing material3is an insulating resin such as a silicone resin or an epoxy resin.

Further, a low moisture permeable sheet11(first low moisture permeable sheet) made of a low moisture permeable material is provided in the case2so as to cover the sealing material3. In Embodiment 1, a flat plate-shaped low moisture permeable sheet11as illustrated inFIG. 2is used. The low moisture permeable material constituting the low moisture permeable sheet11has low moisture and gas permeability, such as a fluororesin such as polytetrafluoroethylene (PTFE), and the moisture permeability thereof is 1 g/m2×24 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 device100from being enlarged in size, the thickness of the low moisture permeable sheet11is preferably 3 mm or less, and more preferably 1 mm or less. The low moisture permeable sheet11may be in contact with the upper surface of the sealing material3.

The opening of the case2is closed by the lid12, and the lid12is adhered to the case2using an adhesive13(first adhesive). As illustrated inFIG. 1, the peripheral edge of the low moisture permeable sheet11is interposed between the case2and the lid12. The material of the case2and the lid12may be any material having electrical insulation, and may be formed of, for example, an epoxy resin or a polyphenylene sulfide (PPS) resin. Further, the adhesives8and13may 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 case2, the low moisture permeable sheet11and the lid12in the semiconductor device100ofFIG. 1will be described in detail. The lid12has a convex portion12ahaving a shape fitted to the opening of the case2, and the case2has a counterbore portion2aon the peripheral edge of the opening. Therefore, when the convex portion12aof the lid12is fitted into the opening of the case2, the convex portion12aand the counterbore portion2aface each other. The low moisture permeable sheet11extends over the counterbore portion2aof the case2, and the peripheral edge of the low moisture permeable sheet11is interposed between the convex portion12aand the counterbore portion2a.

The adhesive13for adhering the case2and the lid12is arranged in a portion where the low moisture permeable sheet11is not interposed (the outer portions of the counterbore portion2aand the convex portion12ainFIG. 1), and the thickness direction of the adhesive13is the same as the thickness direction of the low moisture permeable sheet11in the portion where the low moisture permeable sheet11is interposed between the case2and the lid12. Consequently, when the adhesive13is cured and shrunk, the counterbore portion2aof the case2and the convex portion12aof the lid12are attracted to each other, leading to the effect that the adhesion is strengthened between the counterbore portion2aand the convex portion12aand the low moisture permeable sheet11interposed therebetween can be obtained. In particular, if the low moisture permeable sheet11is made of an elastic material such as fluororesin, the low moisture permeable sheet11is elastically deformed and high adhesion between the counterbore portion2aand the convex portion12ais obtained.

According to the semiconductor device100of Embodiment 1, the sealing material3that seals the semiconductor elements1in the case2is covered with the low moisture permeable sheet11made of the low moisture permeable material, and the low moisture permeable sheet11is interposed between the case2and the lid12, thereby making a gap less likely to be formed between the case2and the low moisture permeable sheet11. Therefore, the number of paths through which moisture enters into the case2is reduced; therefore, moisture is prevented from entering the case2, improving dampproofness of the semiconductor device100. For example, even when a typical silicone-based adhesive (moisture permeability is about 10 g/m2×24 Hr to 100 g/m2×24 Hr) is used as the adhesive13for adhering the case2and the lid12, a sufficient moisture-proof effect can be obtained. Further, the low moisture permeable sheet11provides the high moisture-proof effect; therefore, narrowing the width of the region to which the adhesive13is applied can contribute to the downsizing of the semiconductor device100. In particular, when the power semiconductor element1is 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 sheet11may be roughened by providing irregularities on the surface of the low moisture permeable sheet11. By roughening the surface of the low moisture permeable sheet11, the adhesion between the low moisture permeable sheet11and the sealing material3is improved, and further suppression of paths through which moisture enters from being created can be expected. The roughening method of the low moisture permeable sheet11may 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 sheet11can be conceived.

Here, a method of manufacturing the semiconductor device100will be described. First, the base plate6, the bonding material7, the insulating substrate4, the bonding material5and the semiconductor elements1are 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 material5and the bonding material7or higher is given, thereby bonding each member together. Next, wiring is performed between the semiconductor elements1or between the semiconductor element1and the insulating substrate4by ultrasonic bonding of the metal wires10.

Subsequently, the adhesive8is linearly applied to the outer peripheral portion (adhesive portion with the case2) of the base plate6, the case2is placed on the adhesive8, and the adhesive8is subjected to thermosetting, thereby gluing the base plate6and case2together. After that, the electrodes9incorporated in the case2and the insulating substrate4are bonded by ultrasonic bonding.

Next, as illustrated inFIG. 3, the case2is filled with the sealing material3, and the sealing material3is subjected to thermosetting using an oven or the like to seal the semiconductor elements1. Then, as illustrated inFIG. 4, the sealing material3is covered with the low moisture permeable sheet11. At this point, the peripheral edge of the low moisture permeable sheet11is placed on the counterbore portion2aof the case2.

Then, as illustrated inFIG. 5, the adhesive13is applied linearly or in a dotted manner to the edge of the case2(the adhesive portion with the lid12). Then, as illustrated inFIG. 6, the convex portion12aof the lid12is fitted into the opening of the case2, and the opening of the case2is closed with the lid12. At this point, the convex portion12aof the lid12comes into contact with the upper surface of the peripheral edge of the low moisture permeable sheet11. Then, the adhesive13is subjected to thermosetting to adhere the case2and the lid12together. When the adhesive13cures and shrinks, the counterbore portion2aof the case2and the convex portion12aof the lid12are attracted to each other; therefore, the adhesion is strengthened between the counterbore portion2aand the convex portion12aand the low moisture permeable sheet11interposed therebetween. Through the above processes, the semiconductor device100illustrated inFIG. 1is completed.

FIG. 7is a schematic cross-sectional view of a semiconductor device according to Embodiment 2.FIG. 8is a top view and a cross-sectional view of a low moisture permeable sheet11according to Embodiment 2. In Embodiment 2, a low moisture permeable sheet11has a convex portion11aat a peripheral edge thereof, that is, a portion interposed between a case2and a lid12, and the case2is provided with a groove2binto which the convex portion11aof the low moisture permeable sheet11is to be inserted.

Further, in Embodiment 2, the convex portion11ais provided at the end portion of the low moisture permeability sheet11; therefore, the end portion of the low moisture permeability sheet11is L-shaped in cross-sectional view (As illustrated inFIG. 8, the low moisture permeable sheet11is U-shaped when viewed in the entire cross section of the low moisture permeable sheet11). Further, the groove2bof the case2is provided in the counterbore portion2a. Other configurations are the same as those in Embodiment 1 (FIG. 1).

Any method may be used as the method of providing the convex portion11aon the low moisture permeable sheet11. For example, the convex portion11amay be formed by a mold for molding the low moisture permeable sheet11, or the lower surface of the flat plate-shaped low moisture permeable sheet11as illustrated inFIG. 2may be cut to form the convex portion.11a.

According to the semiconductor device100of Embodiment 2, the positioning accuracy of the low moisture permeable sheet11with respect to the case2and the retention of the low moisture permeable sheet11are improved; therefore, in addition to the effect same as that of Embodiment 1, the effect that the humidity absorption tolerance of the semiconductor device100can be further improved can be obtained.

FIG. 9is a schematic cross-sectional view of a semiconductor device100according to Embodiment 3. As illustrated inFIG. 9, in the semiconductor device100of Embodiment 3, a low moisture permeable sheet14(second low moisture permeable sheet) is provided at the adhesive interface between a case2and a base plate6. The shape of the low moisture permeable sheet14is a frame shape in a plan view as illustrated inFIG. 10, and extends in the adhesive interface between the case2and the base plate6so as to surround a sealing material3. The low moisture permeable material constituting the low moisture permeable sheet14may be the same as a low moisture permeable sheet11covering the sealing material3, and the moisture permeability thereof is preferably 1 g/m2×24 Hr or less.

In order to prevent the semiconductor device100from being enlarged in size, the thickness of the low moisture permeable sheet14is preferably 3 mm or less, and more preferably 1 mm or less. Further, a plurality of frame-shaped low moisture permeable sheets14may be nested at the adhesive interface between the case2and the base plate6. Further, in Embodiment 3, a groove2cinto which a part of the low moisture permeable sheet14is inserted is formed at the adhesive interface with the base plate6in the case2, and a groove6ainto which a part of the low moisture permeable sheet14is inserted is also formed at the adhesive interface with the case2in the base plate6is formed. Therefore, the thickness of the low moisture permeable sheet14can be made larger than the thickness of the adhesive8that adheres the case2and the base plate6together. Further, the low moisture permeable sheet14extends inside the adhesive8. That is, the adhesive8is provided on both sides of the low moisture permeable sheet14. Other configurations are the same as those in Embodiment 1 (FIG. 1).

A method of manufacturing the semiconductor device100of Embodiment 3 will be described. First, the base plate6having the groove6a, the bonding material7, the insulating substrate4, the bonding material5and the semiconductor elements1are 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 material5and the bonding material7or higher is given, thereby bonding each member together. Next, wiring is performed between the semiconductor elements1or between the semiconductor element1and the insulating substrate4by ultrasonic bonding of the metal wires10. As a result, the configuration illustrated inFIG. 11is obtained.

Subsequently, as illustrated inFIG. 12, the adhesive8is linearly applied to both sides of the groove6ain the outer peripheral portion (adhesive portion with the case2) of the base plate6. Further, as illustrated inFIG. 13, the low moisture permeable sheet14is placed on the base plate6. At this point, the lower portion of the low moisture permeable sheet14is inserted into the groove6a. Further, as illustrated inFIG. 14, the case2having the groove2cis placed on the adhesive8. At this point, the upper portion of the low moisture permeable sheet14is inserted into the groove2c. Then, the adhesive8is subjected to thermosetting to adhere the base plate6and the case2together.

After that, as in Embodiment 1, the electrodes9incorporated in the case2are bonded to the semiconductor elements1or the insulating substrate4, the semiconductor elements1are sealed with the sealing material3, and after the sealing material3is covered with the low moisture permeable sheet11, the lid12is adhered to the case2. Through the processes, the semiconductor device100illustrated inFIG. 9is completed.

According to the semiconductor device100of Embodiment 3, moisture absorption through the adhesive8between the case2and the base plate6is reduced; therefore, in addition to the effect same as that of Embodiment 1, the effect that the humidity absorption tolerance of the semiconductor device100can be further improved can be obtained.

FIG. 15is a schematic cross-sectional view of a semiconductor device100according to Embodiment 4. The configuration of the semiconductor device100according to Embodiment 4 is different from the configuration of Embodiment 3 (FIG. 9) in that the thickness of a low moisture permeable sheet14provided at an adhesive interface between a case2and a base plate6is the same thickness as that of an adhesive8. The thickness of the low moisture permeable sheet14and the thickness of the adhesive8are the same; therefore, the groove2cprovided in the case2and the groove6aprovided in the base plate6in 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 device100of Embodiment 4 will be described. First, the base plate6, the bonding material7, the insulating substrate4, the bonding material5and the semiconductor elements1are 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 material5and the bonding material7or higher is given, thereby bonding each member together. Next, wiring is performed between the semiconductor elements1or between the semiconductor element1and the insulating substrate4by ultrasonic bonding of the metal wires10. As a result, the configuration illustrated inFIG. 16is obtained.

Subsequently, as illustrated inFIG. 17, the adhesive8is linearly applied to both sides of the installation position of the low moisture permeable sheet14in the outer peripheral portion (adhesive portion with the case2) of the base plate6. Further, as illustrated inFIG. 18, the low moisture permeable sheet14is placed on the base plate6. Unlike Embodiment 3, although no groove6afor inserting the low moisture permeable sheet14is provided in the base plate6, the position of the low moisture permeable sheet14is fixed by the paste-like adhesive8. Then, as inFIG. 19, the adhesive8is placed on the case2and the adhesive8is subjected to thermosetting to adhere the base plate6and the case2together.

After that, as in Embodiment 1, the electrodes9incorporated in the case2are bonded to the semiconductor elements1or the insulating substrate4, the semiconductor elements1are sealed with the sealing material3, and after the sealing material3is covered with the low moisture permeable sheet11, the lid12is adhered to the case2. Through the processes, the semiconductor device100illustrated inFIG. 15is 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.