Silicon carbide based composite material and manufacturing...

Details Silicon carbide based composite material and manufacturing... Silicon carbide based composite material and manufacturing...

1999-09-08

2001-08-28

Mai, Ngoclan (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Consolidated metal powder compositions

C075S247000, C075S249000, C419S014000, C419S029000, C419S030000, C419S031000, C419S038000, C419S048000, C501S088000, C501S089000

Reexamination Certificate

active

06280496

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat spreader used for various devices and equipments, and more particularly to a silicon carbide based composite material having high thermal conductivity used as a heat spreader for a semiconductor device, as well as to a semiconductor device utilizing the same.
2. Description of the Background Art
There is a rapidly increasing demand in the market place for higher speed of operation and higher degree of integration of semiconductor devices recently. Accordingly, further improvement of thermal conductivity of a heat spreader for mounting semiconductor elements for the device has been required, so as to more efficiently radiate heat generated from the semiconductor elements. Further, in order to further reduce thermal strain between the semiconductor elements and other members (peripheral members) within the device arranged adjacent on the substrate, the heat spreader is required to have a coefficient of thermal expansion close to the elements and the members. More specifically, coefficients of thermal expansion of Si and GaAs which are commonly used for semiconductor elements are 4.2×10
−6
/° C. and 6.5×10
−6
/° C., respectively, and that of alumina ceramics used commonly as an enclosure member of the semiconductor device is about 6.5×10
−6
/° C., and therefore the heat spreader should desirably have a coefficient of thermal expansion close to these values.
Further, as the range of application of electronics equipment has been remarkably widened recently, semiconductor devices have come to be used in more various and wider applications. Among these, use in so-called semiconductor power device equipments including a high output AC converting equipment and frequency converting equipment has been increasing. In such devices, heat generation from a semiconductor element is several to several ten times (for example, several ten W) higher than from a semiconductor memory or a microprocessor. Therefore, a heat spreader used for such an equipment must have its thermal conductivity improved significantly, with the coefficient of thermal expansion adapted to have conformability with that of the peripheral members. Therefore, generally, the substrate has the following basic structure, for example. First, an Si semiconductor element is placed on an aluminum nitride (hereinafter referred to as AlN) ceramic substrate having high thermal conductivity as a first heat spreader. Thereafter, below the first heat spreader, a second heat spreader formed of a metal having high thermal conductivity such as copper is placed. Further, below the second substrate, a heat radiating mechanism which can be water-cooled or air-cooled is placed. By such a structure, heat is radiated without delay to the outside. Such a mechanism inevitably results in a complicated heat radiating structure. In this structure, assuming that AlN ceramics of about 170 W/m·K is used as the first heat spreader, the second heat spreader must let go the heat transmitted from the first substrate to the heat radiating mechanism therebelow. Therefore, the second substrate must have a high thermal conductivity of at least 200 W/m·K at a room temperature and low coefficient of thermal expansion of at most 10×10
−6
/° C., and more preferably, at most 8×10
−6
/° C. to attain conformability with the coefficient of thermal expansion with the first substrate.
Among the power devices, some devices generate large amount of heat when actually used, and when used with such devices, the heat spreader itself may be heated to 100° C. or higher. Therefore, sometimes it is required that the substrate has high thermal conductivity at such a high temperature. More specifically, one having thermal conductivity of at least 150 W/m·K at such a high temperature is required. The larger the capacity, the larger the size of the Si semiconductor element, and the larger must be the heat spreader on which the element is mounted. For example, a substrate used for a personal computer is about the size of 20 to 40 mm square at the largest, while a substrate exceeding the size of 200 mm square has been required for a power device with large capacity. Such a large substrate must have high dimensional precision at the time of packaging, and the precision must not be degraded even at a high temperature. More specifically, when the substrate warps or deforms at a high temperature, there would be a space at the interface between the substrate and the heat radiating mechanism (a radiator, a fin or the like) positioned therebelow, decreasing the efficiency of heat radiation. In the worst case, the semiconductor element may be damaged. Therefore, that the heat spreader surely has superior thermal conductivity at a high temperature is of critical importance.
Conventionally, a Cu—W based or Cu—Mo based composite alloy has been used for such a substance. Therefore, there has been a problem that the substrate costs considerably because of the expensive raw material, and that the subtrate is heavy. In view of the foregoing, various aluminum (hereinafter simply referred to as Al) composite alloys have been attracting attention as inexpensive and light materials. Among others, Al—SiC based composite alloy mainly consisting of Al and silicon carbide (hereinafter simply referred to as SiC) is a relatively inexpensive raw material, which is light weight and has high thermal conductivity. Commercially available pure Al itself and pure SiC itself have the densities of about 2.7 g/cm
3
and about 3.2 g/cm
3
respectively, and thermal conductivities of about 240 W/m·K and about 200 to 300 W/m·K, respectively. When the purity and defect density are further adjusted, thermal conductivities can be expected to be higher. Therefore, these are considered especially promising materials. Pure SiC itself and pure Al itself have coefficients of thermal expansion of about 4.2×10
−6
/° C. and about 24×10
−6
/° C., respectively, and it becomes possible to control the coefficient of thermal expansion in a wide range in the resulting composite material, which provides an additional advantage.
The Al—SiC based composite alloy and manufacturing method thereof are disclosed in (1) Japanese Patent Laying-Open No. 1-501489, (2) Japanese Patent Laying-Open No. 2-243729, (3) Japanese Patent Laying-Open No. 61-222668 and (4) Japanese Patent Laying-Open No. 9-157773. Reference (1) relates to a method of melting Al in a mixture of SiC and Al, and solidifying the same by casting process. References (2) and (3) both relate to infiltration of Al in voids or pores of a SiC porous body. Of these, reference (3) is directed to a so-called pressure infiltration process in which Al is infiltrated under pressure. Reference (4) is directed to a method in which a compact or a hot pressed compact of a mixed powder of SiC and Al is placed in a mold, and subjected to liquid phase sintering in vacuum, at a temperature not lower than the melting point of Al.
The inventors of the present invention proposed in (5) Japanese Patent Application No. 9-136164 (Japanese Patent Laying-Open No. 10-335538, laid-open on Dec. 18, 1998, corresponding to U.S. patent application Ser. No. 08/874,543), an aluminum-silicon carbide based composite material having thermal conductivity of at least 180 W/m·K obtained through liquid phase sintering. The composite material is obtained by compacting a mixture of SiC powder in the shape of particles, of 10 to 70 wt %, and Al powder, for example, and sintering the compact in a non-oxidizing atmosphere containing 99 vol % of nitrogen, with oxygen concentration of at most 200 ppm and dew point of not higher than −20° C., at a temperature of 600 to 750° C. Further, the inventors of the present invention also proposed in (6) Japanese Patent Application No. 9-93467 (Japanese Patent Laying-Open No. 10-280082, laid open on Oct. 20, 1998), a so-called net shape aluminum-silicon carbide based composite material of which dimension after s

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