Development of Copper-Carbon Fiber Composite for Electrodes of Power Semiconductor Devices

In power semiconductor devices, a supporting electrode made of materials such as molybdenum or tungsten is inserted between a silicon wafer and a copper block. The electrode functions as a means for alleviating thermal stress acting on the wafer, as well as a means for conducting electric current. A copper--carbon fiber composite suitable to be the supporting electrode has been developed. The properties of the composite structure are expected to vary depending on the orientation of the fibers. In the case of disk-shaped electrodes usually employed for power devices, the thermal expansion coefficient of the composite has to be isotropic at least on the surface contacting the silicon wafer. That is, two-dimensional isotropy is required. For this purpose, carbon fibers were embedded in a copper matrix in either a weaving, bidirectional, or spiral arrangement. The properties of these composites were adjusted within a certain range by changing the volume, kind, and/or arrangement of carbon fibers. This new composite was applied to a new resin molded diode. The properties of this new diode compared favorably with those of conventional diodes using molybdenum or tungsten electrodes. It is concluded that the new composite electrode with carbon fibers satisfies all of the major requirements for the electrodes in power semiconductor devices.