JPS5938743B2 - semiconductor equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor device such as a thyristor or a gate turn-off thyristor.

Conventionally, in a semiconductor device in which semiconductor elements are pressure-welded (hereinafter referred to as a pressure-welded semiconductor device), heat and Cylindrical metal stamp with high electrical conductivity 3,
4, the semiconductor element 5 is pressed into contact with the semiconductor element 5. Generally a semiconductor element 5
Since the thermal expansion coefficients of the metal stamps 3 and 4 are different, when a temperature change occurs in the semiconductor device, mechanical stress due to the bimetallic effect is applied to the semiconductor element 5, causing characteristic deterioration or destruction of the semiconductor device. To prevent such problems, metal plates 1 and 2 made of molybdenum or tungsten, which have a coefficient of thermal expansion similar to that of the semiconductor element 5, are inserted between the metal stamps 3 and 4 and the semiconductor element 5, as shown in FIG. . For example, in the case of a thyristor, as shown in Figure 1,
On the anode electrode side, a semiconductor element 5 having a PNPN four-layer structure is directly fixed to a tungsten plate 1 by an alloy method, and the tungsten plate 1 is further soldered to a cylindrical copper stamp 3 6.
are doing.

On the other hand, on the cathode electrode side, a molybdenum plate 2 is soldered 7 to a copper stamp 4 so that a semiconductor element 5 can be pressed into contact therewith. FIG. 2 shows cathode electrodes of semiconductor devices that have been widely used in the past, such as thyristors and diodes.

As shown in Figure 2, the cathode electrode 1 is electrically integrated, and even if there is some degree of uneven pressure contact, it is possible to obtain the same performance as when electrically uniform pressure contact is performed. I can do it. Therefore, deterioration of characteristics of semiconductor devices due to non-uniform pressure bonding has not been much of a problem in the past. In addition, in FIG. 2, 2 is a gate electrode. In recent years, high-power gate turn-off thyristors (abbreviated as GTOs) have been used as power switching elements with self-extinguishing ability to replace thyristors.

) and high-power transistors came into practical use.
FIG. 3 shows a cathode electrode of a GTO as an example of these semiconductor devices. As shown in FIG. 3, the electrode is divided into a number of mesa-shaped cathode emitter electrodes 1.
These many cathode emitter electrodes 1 are pressed together with a metal stamp, and each cathode emitter is connected to a single GTO.
(hereinafter referred to as cathode elements) are operated in parallel to gate turn off large currents. Therefore, by applying pressure to the entire cathode electrode uniformly, partial pressure contact,
It is necessary to prevent current imbalance between cathode elements due to differences in contact resistance. In addition, in FIG. 3, 2 indicates a gate electrode. Furthermore, as is well known, when a semi-infinite elastic body is pressed with a cylindrical rigid stamp, the stress σz in the direction perpendicular to the pressing surface can be expressed as σz=K-P/V7=FT. .

However, P: pressure contact force, a: radius of the rigid post, r: distance from the center of the rigid stamp, K: constant. As is clear from this equation, the theoretical stress distribution on the pressure contact surface between the semi-infinite elastic body and the rigid post becomes infinite at the periphery of the rigid post, and the stress within the pressure contact surface is extremely non-uniform. molybdenum, etc.
When a semiconductor element is pressed into contact with a metal plate that does not easily deform, the stress distribution within the pressure contact surface is considered to be close to this state. Further, in FIG. 1, the coefficient of thermal expansion of the molybdenum plate 2 is smaller than that of the copper stamp 4, so when the temperature of the semiconductor device rises, the peripheral edge of the molybdenum plate 2 warps toward the semiconductor element 5 due to the bimetallic effect.

In addition, since the thermal expansion coefficient of the semiconductor element 5 is smaller than that of the tungsten plate 1 in FIG.
Turn in the direction of. As a result of the synergy of the causes mentioned above,
In pressure-contact type semiconductor devices, stress tends to concentrate around the semiconductor elements housed therein. In fact, when GTO was pressed near the maximum guaranteed joint temperature, ring-shaped pressure marks were often observed around the cathode electrode, as shown in the shaded area 3 in FIG. 3.

In a GTO in which pressure traces of this shape occur, not only the maximum controllable gate turn-off current decreases significantly, but also
In thermal fatigue tests, short circuits between the gate and cathode electrodes tend to occur around the cathode electrodes.
The important properties of TO were adversely affected. These accidents have not been a problem with conventional press-contact type semiconductor devices. The present invention has been made to eliminate the above-mentioned drawbacks, and its object is to eliminate stress concentration occurring around the semiconductor element of a press-contact type semiconductor device.

Hereinafter, an embodiment of the present invention will be explained in detail with reference to FIG.
FIG. 4 shows an embodiment in which the present invention is applied to the cathode electrode side of a GTO. A ring-shaped groove 8 with an appropriate radius of curvature is dug in the copper stamp 4 that is in contact with the circumferential portion of the molybdenum plate 2 shown in FIG. It is set as. When such a groove 8 is provided in the copper stamp 4, a spring action can be imparted to the circumferential portion of the molybdenum disk 2, and deterioration of characteristics caused by concentration of stress in this portion can be prevented. Further, in FIG. 4, the circumferential portion of the molybdenum disk 2 is not in direct contact with the copper stamp 4, so that warping of the molybdenum disk due to the bimetal effect can be alleviated. FIG. 5 shows an example of the results of an experiment conducted to confirm the effects of the present invention.

In Fig. 3, if the number of cathode elements press-welded by metal posts is N, and the latching current of a single cathode element is IL8, then the latching current 1L during pressure-welding is La in the high-power GTO we have developed.・The relationship of ILe is established. Therefore, the change in n due to the change in pressure contact state is I
It can be estimated by measuring L. Fifth
The horizontal axis in the figure represents the pressure contact force, and the vertical axis represents the latching current. Curve A is the case where the GTO is directly pressed with a cylindrical copper stamp without using the molybdenum disk 2 shown in FIG. Since copper is more easily deformed than molybdenum, IL quickly reaches a saturation value when the pressure contact force increases, and it can be seen that each cathode element is pressure-welded uniformly.
Curve B is the GTO via the molybdenum disk 2 shown in Figure 1.
These are the experimental results when welded together by pressure. The IL value is not saturated unless the pressure contact force becomes considerably higher than that of curve A. This is considered to be a phenomenon that occurred as a result of the peripheral portion of the cathode electrode of the GTO being pressed more strongly than other portions. Curve C is the experimental result when the present invention shown in FIG. 4 is implemented. Curve C shows a trend quite similar to curve A. By implementing the present invention in this way, it was possible to obtain the same effect as when GTO is directly pressed with a cylindrical copper stamp. When the present invention was carried out, no ring-shaped press trace was found around the cathode electrode, as in the case of press contact using a copper stamp. Further, in the press-contact type GTO according to the present invention, no decrease in the maximum gate turn-off current was observed. The results of the thermal fatigue test were also good. As is clear from the above description, the following effects could be obtained by applying the present invention to a pressure contact type semiconductor device. 1) Since it is not necessary to press the semiconductor device with an unnecessarily large pressure contact force in order to prevent partial pressure contact, the mechanical stress on the semiconductor element can be reduced.

Therefore, the reliability of the semiconductor device can be improved.
2) Since it is possible to alleviate thermal stress caused by temperature rise of the semiconductor device, it is possible to prevent reliability deterioration due to thermal fatigue and the like.

3) Deterioration of characteristics due to partial pressure welding can be prevented.

4) Accidents such as short circuit between gate and cathode can be prevented.

For convenience, the effects of the present invention have been explained using a GTO, but it goes without saying that the above effects can also be achieved in pressure contact type semiconductor devices other than the GTO, such as transistors, thyristors, diodes, etc.

Other embodiments are shown in FIGS. 6 to 9.

FIG. 6 shows a case where the depth of the ring-shaped groove 8 provided around the periphery of the copper post 4 is made constant to make machining easier. Figure 7 shows the soldering of the molybdenum board and copper stamp 4.
This is a case where the thermal resistance is improved. FIG. 8 shows a case where the molybdenum disk 2 is directly attached to the copper stamp 4 without digging a groove for positioning the molybdenum disk 2 in the copper stamp 4. FIG. 9 shows a case where a groove 8 is dug in the outer periphery of the copper stamp 4 to impart a spring action to the circumferential portion of the molybdenum disk 2.
These embodiments can achieve the same effect as the case of FIG. In the embodiments shown in FIG. 4, FIG. 6, FIG. 7, FIG. 8, and FIG. 9, the metal disk 2 is assumed to be a disk, but even if the shape changes, the effect of the present invention is not affected.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a conventional semiconductor device, Figure 2 is a plan view of a cathode electrode of a thyristor, which has been widely used in the past.
Fig. 3 is a plan view of the cathode electrode of a high power gate turn-off thyristor, Fig. 4 is a sectional view for explaining an embodiment of the present invention, and Fig. 5 is a diagram for confirming the effect of the embodiment of Fig. 4. This is the result of an experiment conducted. 6 to 9 are cross-sectional views showing other embodiments of the present invention, respectively.

Claims (1)

[Scope of Claims] 1. A semiconductor element, a metal plate provided on at least one surface of the semiconductor element and having a coefficient of thermal expansion close to that of the semiconductor element, and a metal plate provided on at least one surface of the semiconductor element, and a metal plate provided on at least one surface of the semiconductor element, and a metal plate provided on at least one surface of the semiconductor element, and a metal plate provided on at least one surface of the semiconductor element. What is claimed is: 1. A semiconductor device comprising: a metal stamp for press-welding a semiconductor device, wherein a recess is provided at a peripheral end of the metal stamp on the metal plate side. 2. The semiconductor device according to claim 1, wherein a solder layer is interposed between the metal stamp and the metal plate.