In general, heat is an enemy for the IC chip and it is necessary that the internal temperature thereof does not exceed the maximum allowable junction temperature. The electric power consumption per operation area is large in the semiconductor device such as a power transistor or a semiconductor rectifier element. Therefore, the generated heat amount cannot be sufficiently released with only the heat amount released from a case (package) and a lead of the semiconductor device. It is feared that the internal temperature of the device may be raised to cause thermal destruction.
This phenomenon also occurs in the same manner in the IC chip which carries a CPU. The amount of heat generation is increased during the operation in proportion to the improvement in clock frequency. It is an important matter to make the thermal design in consideration of the heat release.
In the thermal design for preventing the thermal destruction or the like, element design or package design is made on condition that a heat sink having a large heat release area is secured to a case (package) of the IC chip.
In general, a metal material such as copper and aluminum, which has a good thermal conductivity, is used as a material for the heat sink.
Recently, the IC chip such as CPU and memory is in a trend that the IC chip itself has a large size in accordance 10, with the high degree of integration of the element and the enlargement of the element-forming area, while it is intended to drive the IC chip at low electric power for the purpose of low electric power consumption. When the IC chip has such a large size, it is feared that the stress caused by the difference in thermal expansion between the semiconductor substrate (silicon substrate or GaAs substrate) and the heat sink is increased, and that the peeling-off phenomenon and the mechanical destruction occur in the IC chip.
In order to avoid such an inconvenience, for example, it may be pointed out that the low electric power driving of the IC chip should be realized, and the heat sink material should be improved. The low electric power driving of the IC chip is realized in the level of not more than 3.3 V at present and the TTL level (5 V) which has been hitherto used as the power source voltage becomes obsolete.
As for the constitutive material for the heat sink, it is insufficient to consider only the thermal conductivity. It is necessary to select a material which has a coefficient of thermal expansion approximately identical with those of silicon and GaAs, which are used as the semiconductor substrate, while having a high thermal conductivity at the same time.
A variety of reports have been made in relation to the improvement of the heat sink material, including, for example, a case in which aluminum nitride (AlN) is used and a case Cu (copper)-W (tungsten) is used. AlN is excellent in balance between the thermal conductivity and the thermal expansion. Especially, the coefficient of thermal expansion of AlN is approximately coincident with the coefficient of thermal expansion of Si. Therefore, AlN is preferred as a heat sink material for a semiconductor device in which a silicon substrate is used as the semiconductor substrate.
Cu—W is a composite material having both of the low thermal expansion of W and the high thermal conductivity of Cu. Further, Cu—W is mechanically machined with ease. Therefore, Cu—W is preferred as a constitutive material for a heat sink having a complicated shape.
Other instances have been suggested, wherein metal Cu is contained in a ratio of 20 to 40% by volume in a ceramic base material containing a major component of SiC (conventional technique 1, see Japanese Laid-Open Patent Publication No. 8-279569), and wherein a powder-sintered porous member of an inorganic substance is infiltrated with Cu by 5 to 30% by weight (conventional technique 2, see Japanese Laid-Open Patent Publication No. 59-228742).
The heat sink material concerning the conventional technique 1 is produced in the powder formation in which a green compact of SiC and metal Cu is formed to produce a heat sink. Therefore, the coefficient of thermal expansion and the coefficient of thermal conductivity represent only theoretical values. It is impossible to obtain the balance between the coefficient of thermal expansion and the coefficient of thermal conductivity required for actual electronic parts etc.
The conventional technique 2 uses a low ratio of Cu with which the powder-sintered porous member composed of the inorganic substance is infiltrated. It is feared that a limit may arise to enhance the thermal conductivity thereby.
On the other hand, a composite material, which is obtained by combining carbon and metal, has been developed and practically used. However, such a composite material is used, for example, as an electrode for the discharge machining when the metal is Cu. When the metal is Pb, such a composite material is used, for example, as a bearing material. No case is known for the application as a heat sink material.
That is, in the present circumstances, the coefficient of thermal conductivity is at most 140 W/mK for the composite material obtained by combining carbon and metal, which cannot satisfy the value of not less than 160 W/mK required for the heat sink material for the IC chip.