Keiichi Kuniya1, Hideo Arakawa2, Tadashi Sakaue3, Hitoshi Minorikawa4, Kenji Akeyama5 and Tatsuji Sakamoto6
1Hitachi Research Laboratory, Hitachi Ltd., Hitachi. Present address: Research and Development Department, Hitachi Nuclear Engineering Co., Ltd., Hitachi
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.
However, molybdenum and tungsten have some problems: (1) Their coefficients of thermal expansion are not equal to that of the silicon wafer, (2) the thermal and electrical conductivities are not so high as desirable, and (3) their coefficients of thermal expansion do not meet that of insulating substrates such as a sintered alumina plate, which is used in a power module.
We have already reported the development of a new copper-carbon fiber composite which possesses the properties of copper, i.e., the excellent electrical and thermal conductivities, and the properties of carbon fiber, i.e., a small thermal expansion coefficient(1)--(7). The properties of these copper-carbon fiber composites can be adjusted within a certain range by changing the volume fraction, kind and/or arrangement of carbon fibers, whereby the thermal expansion coefficient can be adjusted to be approximately equal to that of silicon. One of the practical consequences of this work is that the composite can be soldered directly to silicon wafers.
This new composite was applied to several power semiconductor devices, for example, resin molded diode, button type diode, stud type diode, power module and IC ignitor module. The properties of these power semiconductor devices were compared favorably with those conventional devices using molybdenum or tungsten electrodes. In the thermal fatigue tests, no degradation in the electrical and mechanical characteristics of these devices were observed. It is concluded that the new composite electrode with carbon fibers satisfies all of the major requirements for the electrodes in power semiconductor devices.
carbon fiber, copper-carbon composite, fiber reinforced metal, power semiconductor device
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