Internal compression bonded semiconductor device with a chip frame enabling a longer creepage distance

Abstract: The top-side (n-type) electrode and bottom-side (p-type) electrode of a Si chip (6) with a p-n junction are pressed against a Cu cathode electrode (4) and a Cu anode electrode (8) via Mo plates (5) and (7), respectively, thereby establishing electrical connection. The inner wall of a case (9) is round and the Si chip (6) is almost square. The top of the case (9) is covered with ceramic (2), for example. A washer (3) is a compression member. A chip frame 1 holds the Si chip (6) and Mo plates (5) and (7) in compression positions and simultaneously determines their locations within the case (9). Specifically, the side face of the Si chip (6) is not flush with the side face of each of the Mo plates (5) and (7). This enables the chip frame (1) to make the creepage distance longer. Since the chip frame is a single chip frame without any joint, the creepage distance between the anode and cathode electrodes is defined by part of the chip frame (1) that faces part of the surface of the Si chip (6) and parts of the surfaces of the Mo plates sandwiching the Si chip (6) between them.

Description: This invention relates to a power semiconductor device, and more particularly to an internal compression held semiconductor device, such as a semiconductor rectifier diode. A semiconductor rectifier diode is such that a p-n junction formed from, for example, silicon, is sealed in a case for the purpose of rectification. Semiconductor rectifier diodes with various capacities have been manufactured: for example, their rated current ranges from 0.1 to 3000 A and their rated voltage ranges from 1000 to 6000 V. According to the capacity, various types of cases, including a lead-mount type, flat type, and stud type, have been used. In general, it is known to the applicant that, with a conventional semiconductor device in the form of a stud-type Si rectifier diode, an electrode on one side and an electrode on the opposite side of an Si chip with a p-n junction are pressed against a Cu cathode electrode and a Cu anode electrode, respectively, each via a Mo plate, thereby establishing electrical connection. The Mo plates function as heat buffer plates that prevent distortion due to the difference in thermal coefficient between Si and Cu. One side of the Si chip is of the n-type and is electrically connected to the cathode electrode side and the other side is of the p-type and is electrically connected to the anode electrode side. Such a chip is as thin as 400 to 500 µm and requires suitable uniform compression. To meet this requirement, a chip frame for holding a Si chip in place and a guide frame for securing the Si chip-mounted chip frame in the correct place and guiding and positioning the Mo plate are provided as component parts for positioning the Si chip within the case. Specifically, in the guide frame, a quadrilateral opening into which a chip frame is fitted is made. Both of the chip frame and guide frame are insulating members. After the Si chip has been bonded to the chip frame with an adhesive to increase its pressure resistance, tests are done on the chip to evaluate the characteristics of the product. The evaluated chip, together with the chip frame, is fitted into the guide frame. The Si chip, together with the chip frame and guide frame, is housed in a case. As described above, in a conventional semiconductor device, the chip frame and guide frame are separate component parts. The reason for such a configuration is that manufacture semiconductor devices having different current ratings or case sizes are manufactured by using Si chips complying with a single standard. By making a plurality of openings in a guide frame, a plurality of Si chips with chip frames of the same ratings can be fitted into the openings, thereby compressing the Si chips in parallel. This makes it possible not only to offer good-quality Si chips at low cost but also to form a semiconductor device with a large current rating. When a semiconductor device is composed of a single Si chip, however, the configuration where the chip frame and guide frame are separate has a manufacturing cost problem: the number of component parts is great. Furthermore, with a semiconductor device with a large voltage rating using such a thin Si chip, a very large voltage difference is applied between the electrodes or heat buffer plates facing each other with a short distance between them. Thus, after long hours of use, there is a possibility that current leakage or breakdown will take place due to electrical discharge. The chip frame and guide frame are provided as insulating material between the anode electrode and cathode electrode. The chip frame and guide frame have surface portions that face part of the surface of the Si chip and parts of the heat buffer plates (Mo plates) that sandwich the Si chip between them. When a creepage distance is considered, there is a joint of the chip frame and guide frame between the anode electrode and cathode electrode. That is, the distance along the external periphery of the chip frame is the shortest creepage distance. To make the creepage distance longer, the Mo plate acting as a heat buffer plate was made thicker in the prior art. This prevented the aforesaid electrical discharge occurred after long hours of use. The thicker Mo plate, however, made it impossible to decrease the thermal resistance, thus failing to dissipate the heat generated at the p-n junction sufficiently. In the case of a semiconductor device with a high rated voltage, when the Mo plate is made thicker to secure the creepage distance, a heat dissipation problem becomes serious. As described above, the conventional internal compression held semiconductor device has a chip frame and a guide frame separate from each other, which leads to a large number of component parts, leaving room for improvement in terms of cost. Furthermore, to increase the reliability, it is necessary to make the heat buffer plate thicker to increase the creepage distance, which increases the thermal resistance of the heat buffer plate, causing the problem of preventing the heat generated within the semiconductor device from dissipating sufficiently. FR 2 083 557 A discloses a semiconductor device which relates to a power device having non-alloyed electrodes and houses a semiconductor chip between two electrodes in a case, comprising: a first conducting plate provided between one of said two electrodes and said semiconductor chip; a second conducting plate provided between the other of said two electrodes and said semiconductor chip; and an insulating chip frame which is provided so as to extend from the periphery of said semiconductor chip to the inner wall of said case and which positions said semiconductor chip, said first conducting plate, and said second conducting plate within the inner wall of said case, the lateral edges of the first and second conducting plates being in respective contact with the chip frame. The object of the present invention is to provide an internal compressing held semiconductor device which has a smaller number of component parts and a structure enabling the creepage distance to be made longer regardless of the thickness of a heat buffer plate, and makes it possible to reduce the thermal resistance of the heat buffer plate by making the buffer plate thinner. The foregoing object is accomplished by providing an internal compression held semiconductor device according to claim 1. The dependent claims show advantageous further developments of the semiconductor device of claim 1. With the present invention, use of a single chip frame enables the semiconductor chip and the first and second conducting plates to be housed with a specific distance away from the inner wall of the case, reducing the number of component parts used. Because the chip frame is a single chip frame without any joint, the shortest creepage distance between the first and second electrodes is defined by surface portions of the chip frame that face part of the surface of the semiconductor chip and parts of the surfaces of the first and second conducting plates sandwiching the semiconductor chip. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a sectional view of an internal compression held semiconductor device according to an embodiment of the present invention; FIG. 2A is a top view of a chip frame used for the internal compression held semiconductor device of the present invention; FIG. 2B is a sectional view taken along line 2B-2B of FIG. 2A; and FIG. 3 is a sectional view of an internal compression held semiconductor device according to another embodiment of the present invention. FIG. 1 is a sectional view of a stud-type Si rectifier diode according to a first embodiment of the present invention. One side (the top side) and the other side (the opposite side or the bottom side) of an Si chip with a p-n junction are pressed against a Cu cathode electrode 4 and a Cu anode electrode 8 via Mo plates 5 and 7, respectively, thereby establishing electrical connection. The Mo plates 5 and 7 function as heat buffer plates that prevent distortion due to the difference in thermal coefficient between Si and Cu. The figure shows a structure where the Si chip 6 is put in a forward direction, with the top side of the Si chip 6 being of the n-type and the bottom side being of the p-type. The inner wall of a case 9 is round and the Si chip 6 is almost square. The top of the case 9 is covered with ceramic 2, for example. A washer 3 is a compression member. In the present invention, a chip frame 1 holds the Si chip 6 and Mo plates 5 and 7 in compression holding places and simultaneously determines their locations with the case 9. Namely, the chip frame 1 has a structure into which a chip frame and a guide frame, which were separate from each other in the prior art, have been formed integrally. The chip frame 1 is formed of an insulating material, such as resin. The side face of the Si chip 6 is not flush with each of the side faces of the Mo plates 5 and 7. As a result, the creepage distance is longer because of the chip frame 1. That is, because the chip frame 1 is a single chip frame without any joint, the creepage distance between the anode and cathode electrodes is defined by part of the chip frame 1 that faces part of the surface of the Si chip 6 and parts of the surfaces of the Mo plates chip sandwiching the Si chip 6 between them. More specifically, one side of the Si chip 6 is in contact with the main surface of the Mo plate 5, the other side of the Si chip 6 is in contact with the main surface of the Mo plate 7, and the creepage distance is produced in such a manner that part of the side face and part of the main surface of the Mo plate 5, the side face and part of the other side of the Si chip 6, and part of the side face of the Mo plate 7 face the inside surface portion of the chip frame 1. Therefore, unlike a conventional equivalent, the chip frame 1 of the present invention has a structure very favorable for the suppression of electrical discharge with time between electrodes. FIG. 2A is a top view of a chip frame associated with the present invention. FIG. 2B is a sectional view taken along line 2B-2B of FIG. 2A. A case where the inner wall of the case 9 is round and a Si chip housed in it is almost square will be described. The chip frame 1 is round so as to fit in the inner wall of the case 9. The chip frame has three different cutaway sections in the direction of thickness. For example, the chip frame has a quadrilateral cutaway section 11 for holding the Si chip 6 in the middle in the direction of thickness. A quadrilateral cutaway section 12 for holding the Mo plate 7 contacting the surface of the p-type side of the Si chip 6 is made in one surface of the chip frame so as to penetrate the cutaway section 11. Furthermore, in the surface of the opposite side of the chip frame, for example, a cutaway section 14 for holding the Mo plate 5 touching the surface of the n-type side of the Si chip 6 is made so as to penetrate the cutaway section 11. Such a structure enables the chip frame 1 not only to hold the Si chip and the heat buffer plates 5 and 7, or the Mo plates, but also to determine their locations within the case 9. As a result, this structure helps reduce the number of component parts used, as compared with a conventional structure using a chip frame and a guide frame. Furthermore, the insulating chip frame 1 is provided between the Mo plate 5 and electrode 8 or between the electrode 4 and electrode 8, which face each other with a large potential difference between them. The Mo plate 5 or electrodes 4 and 8 to the inner wall of the case 9 are formed integrally. The integral formation makes the creepage distance longer as described earlier. Specifically, the Mo plate has one main surface and the other main surface, with the whole of one main surface in contact with the electrode 4 and the other main surface except for part of it in contact with the Si chip 6. The part of the other main surface of the Mo plate 5 faces the surface of the chip frame 1 in a manner that comes into contact with each other. The distance over which the two surfaces face each other forms part of the creepage distance and plays a major role in lengthening the creepage distance. It is desirable that part of the creepage distance should be greater than at least the thickness of the Mo plate 7. This helps make the Mo plate 7 thinner remarkably than in the prior art. In the prior art, since a chip frame and a guide frame were provided between the Mo plate 5 or electrodes 4, 8 and case 9, there was a possibility that a current path would develop along the joint between the chip frame and guide frame, which made it impossible to produce a longer creepage distance. To overcome the shortcoming, the creepage distance between the Mo plate 5 and electrode 8 was secured by making the Mo plate 7 thicker, thereby improving the reliability. With the present embodiment, however, use of the chip frame 1 enables the creepage distance between the Mo plate 5 and electrode to be made longer. As a result, of the electrodes facing each other, it is the distance between the electrode 4 and case 9 (the case 9 has the same potential as that of the electrode 8) that has the shortest creepage distance. The distance between the electrode 4 and case 9 is guaranteed from the beginning. For this reason, the creepage distance between the electrodes 4 and 8 should be made longer than the distance between the electrode 4 and case 9. This eliminates the need of making the Mo plate 7 thicker to the extent that the thickness of the Mo plate has an adverse effect on the reduction of the thermal resistance of the product. While in the embodiment, the case where the Si chip 6 has been put in a forward direction, the Si chip 6 can be put in the reverse direction by reversing the chip frame 1 (FIG. 3). Although the present invention has been explained taking a stud-type Si rectifier diode as example, it is not limited to this. For instance, the invention may be applied to an internal compression bonded semiconductor device with another type of case, such as a flat-type case. In this case, a chip frame 1 must be manufactured according to the shape of the case. Since a guide frame had to be manufactured according to the shape of the case in the prior art, an internal compression held semiconductor device with another type of case can be manufactured at a similar manufacturing cost to that of a conventional equivalent. Unlike a conventional equivalent, a semiconductor device of the present invention uses only a single chip frame 1 and therefore can be manufactured at low cost. It is desirable that the present invention should be applied to an internal compression held semiconductor device particularly composed of a single Si chip which is not required for separate inspection. As described so far, with an internal compression held semiconductor device of the present invention, the number of component parts can be reduced. Furthermore, since the reliability can be improved by making the creepage distance larger without being affected by the thickness of the heat buffer plate, the thermal resistance of the heat buffer plate can be decreased by making the buffer plate thinner.

Claims: An internal compression held semiconductor device which houses a semiconductor chip (6) between two electrodes (4, 8) in a case (9) and presses the chip against the electrodes, comprising: a first conducting plate (5) provided between one of said electrodes and said semiconductor chip (6); a second conducting plate (7) provided between the other of said electrodes and said semiconductor chip (6); and an insulating chip frame (1) which is provided so as to extend from the periphery of said semiconductor chip (6) toward the inner wall of said case (9) and which positions said semiconductor chip (6), said first conducting plate (5) and said second conducting plate (7) with specific distances from the inner wall of said case (9), the creepage distance between said two electrodes (4, 8) being defined by surface portions of said chip frame (1) that faces part of the surface of said semiconductor chip (6) and each of parts of the surfaces of said first conducting plate (5) and said second conducting plate (7), said parts of the surfaces of said first conducting plate (5) and said second conducting plate (7) including parts of the main surface facing the chip of at least one of said first and second conducting plates (5, 7) and parts of the lateral edges thereof, said parts and the chip frame (1) being in contact with each other. An internal compression held semiconductor device according to claim 1, characterized in that said first conducting plate (5) and said second conducting plate (7) function as heat buffer plates. An internal compression held semiconductor device according to claim 1 or 2, characterized in that said first conducting plate (5) has one main surface and another main surface, with the whole of said one main surface being in contact with one of said two electrodes (4, 8) and said other main surface being in contact with said semiconductor chip (6). An internal compression held semiconductor device according to any of the preceding claims, characterized in that said second conducting plate (7) has one main surface and another main surface, with the whole of said one main surface being in contact with the other of said two electrodes (4, 8) and said other main surface being in contact with said semiconductor chip (6). An internal compression held semiconductor device according to any of the preceding claims, characterized in that the part of the creepage distance formed by the contact between the part of other main surface of said first conducting plate (5) and the surface of said chip frame (1) is greater than the thickness of said second conducting plate (7). An internal compression held semiconductor device according to any of the preceding claims, characterized in that the creepage distance between said two electrodes (4, 8) is larger than the distance from the inner wall of said case (9) to the closer one of said two electrodes (4, 8). An internal compression held semiconductor device according to any of the preceding claims, characterized in that said semiconductor chip (6) has one main surface and another main surface, said one main surface containing n-type impurities and said other main surface containing p-type impurities, and one of said first and second conducting plates (5, 7) that is in contact with said other main surface being thinner than one of said first and second conducting plates that is in contact with said one main surface. An internal compression held semiconductor device according to claim 1, characterized in that said first conducting plate (5) has one main surface, another main surface and a side face, said second conducting plate (7) has one main surface, another main surface and a side face, said semiconductor chip (6) has one main surface, another main surface and a side face, one main surface of said semiconductor chip (6) being in contact with the other main surface of said first conducting plate (5), the other main surface of said semiconductor chip (6) being in contact with the whole of the other main surface of said second conducting plate (7), and said creepage distance being produced in such a manner that part of the side face and part of the other main surface of said first conducting plate (5), the side face and part of the other main surface of said semiconductor chip (6), and part of the side face of said second conducting plate (7) face the surface of said chip frame (1). An internal compression held semiconductor device according to claim 8, characterized in that said chip frame (1) has three cutaway sections of different shapes in the direction of thickness, the cutaway sections being composed of a first cutaway section that fits on the side face of said first conducting plate (5), a second cutaway section that fits on the side face of said semiconductor chip (6), and a third cutaway section that fits on the side face of said second conducting plate (7). An internal compression held semiconductor device according to claim 9, characterized in that the peripheral shape of said chip frame (1) is formed so as to fit in the inner wall of said case (9).

Inventors: HIYOSHI MICHIAKI, KOBAYASHI IKUKO, HIYOSHI, MICHIAKI, KOBAYASHI, IKUKO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: H01L23
IPC codes: H01L21, H01L23, H01L25