Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2001-06-22
2003-05-27
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S689000, C428S697000, C428S698000, C075S229000, C075S230000, C075S236000
Reexamination Certificate
active
06569524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a composite material having high thermal conductivity, which is used as a heatsink material for semiconductor devices, and a method for producing the same.
2. Description of the Prior Art
Generally speaking, in order to radiate heat generated from incorporated semiconductor elements, a heatsink made of a high thermal conductivity material is attached to a semiconductor device. The standard of physical properties required for a heatsink material is such that the thermal conductivity is equivalent to or more than that of Cu (395 W/m·K) and the coefficient of thermal expansion is lower than that of Cu (16.9×10
−6
/°C.).
Conventionally, Al
2
O
3
and AlN whose coefficient of thermal expansion is approximate to that of a semiconductor element have been used as a heatsink material in spite of their comparatively low thermal conductivity because conventional electronic components incorporating semiconductor elements such as semiconductor lasers, microwave elements, etc., have generated only a slight amount of heat.
Recently, however, in line with an increase in the amount of information, semiconductor elements have been increased in size, and the output thereof has been highly increased. As a result, an increase in the amount of heat generation causes a problem. For Example, AlN has consistently been used till recently because it has comparatively satisfactory thermal conductivity as well as the coefficient of thermal expansion similar to that of Si and InP, but AlN can no longer meet further increase in the output and size of semiconductor elements in view of its thermal conductivity. Therefore, recently, in order to incorporate these semiconductor elements having high output, heatsink materials having remarkably excellent thermal conductivity are demanded.
Further, in view of the coefficient of thermal expansion, AlN is not suitable as a heatsink material for semiconductor elements that are composed of material, such as GaAs, which has a large coefficient of thermal expansion. In detail, the coefficient of thermal expansion of various types of semiconductor materials is expressed in terms of ×10
−6
/°C. (hereinafter expressed in terms of ppm/°C.), wherein Si is 4.2, InP is 4.5, and GaAs is 5.9 or so. Therefore, it is recommended that the coefficient of thermal expansion of heatsink materials is close to these figures. In addition, a heatsink material preferably should have a low Young's modulus so that the generation of thermal stress is reduced.
Although the material having the highest thermal conductivity is diamond and c-BN (cubic boron nitride), their coefficient of thermal expansion is very low, wherein that of diamond is 2.3 ppm/°C. and that of c-BN is 3.7 ppm/°C., and the Young's modulus of these materials is very high to be 830 through 1050 GPa. Therefore, a large thermal stress occurs when brazing a heatsink and a semiconductor element together or between the heatsink and semiconductor element when being used as a device, wherein a breakage is likely to occur.
Recently, various types of composite materials such as Al—SiC, in which ceramic and metal are composed together, have been proposed as a heatsink material having a low coefficient of thermal expansion as well as comparatively high thermal conductivity. However, since the thermal conductivity of Al is as low as approx. 238 W/m·K at room temperature, there exists an upper limit in the thermal conductivity of a composite material including Al, which results in the failure to meet the recent requirements for high thermal conductivity such as described above. The composite material cannot meet recent requirements in order to achieve the high thermal conductivity as described above. It may be considered that metals having high thermal conductivity such as Cu (395 W/m·K at room temperature) and Ag (420 W/m·K at room temperature) are used instead of Al. However, since the wettability thereof with SiC is very inferior, the high thermal conductivity that is inherent in Cu and Ag cannot be sufficiently displayed.
Japanese Unexamined Patent Publication No. 11-67991 discloses a diamond-Ag based or diamond-Cu based composite material as a heatsink material having improved wettability with Cu and Ag. According to the disclosed invention, a diamond powder and Ag—Cu—Ti based powder are blended together and molded, and then are heated at a higher temperature than the melting point of the resultant alloy. This allows Ti constituents to diffuse on the surface of diamond grains and to react to form a TiC film on the surface (sintering method). Since the TiC has good wettability with Cu or melted Ag, the phase boundaries of the diamond grains and the metal are adhered close to each other, whereby high thermal conductivity can be obtained.
Also, an infiltration method is disclosed in Japanese Unexamined Patent Publication No. 10-223812 as a method for producing such a diamond-Ag based or diamond-Cu based composite material. In this method, after diamond powder and Ag—Cu—Ti based powder are blended and molded, the molded body is heated at a higher temperature than the melting point of the corresponding alloy to form a TiC layer on the surface of diamond grains. After that, the molded body is further heated to elute and volatilize the Ag constituents and Cu constituents, thereby producing a porous body. Impregnating the porous body with an Ag—Cu alloy produces a composite material having a higher relative density and a higher thermal conductivity than that obtained by the sintering method.
However, there are common problems in the case of the above-described diamond-Ag based and diamond-Cu based composite materials: (1) the diamond is remarkably expensive, (2) the diamond has very high hardness, which allows a large thermal stress to remain at the phase boundary of bonding between the composite material and semiconductor element due to a high Young's modulus as described above, and (3) metal molding dies are remarkably worn when a blended powder including diamond is molded. Resultantly, the cost of a diamond-Ag based and diamond-Cu based composite material becomes very high, and it is very difficult to employ the same in practical applications. Also, (4) even if the infiltration method is employed, a problem still remains, it is difficult to make the diamond-Ag based and diamond-Cu based composite materials completely dense.
SUMMARY OF THE INVENTION
The present invention was developed in views of these situations, and it is therefore an object of the invention to provide, without the use of expensive diamond, a composite material that is favorable when it is used as a heatsink material while it is low cost, has a low coefficient of thermal expansion and has comparatively high thermal conductivity.
In order to achieve the above-described object, the invention provides a high thermal conductivity composite material consisting of a first constituent composed of composite carbon grains, composite carbon fibers, or composite carbide grains, which have a coating layer formed on the surface thereof, and a second constituent composed of a metal including silver and/or copper; wherein the coating layer formed on the surface of the composite carbon grains, composite carbon fibers, or composite carbide grains, which are the first constituent, is composed of carbide of at least a type of metal selected from the group consisting of 4A group elements, 5A group elements, and 6A group elements of the periodic table. The high thermal conductivity composite material has a relative density of 70% or more, thermal conductivity of 220 W/m·K or more at least in a specified direction at room temperature, and a mean coefficient of thermal expansion of 5 through 15×10
−6
/°C. from room temperature to 200° C. at least in a specified direction.
In the above-described high thermal conductivity composite material according to the invention, where the first constituent is made of composite carbon grains having a coating layer formed o
Kawai Chihiro
Nakata Hirohiko
Blackwell-Rudasill G. A.
Jones Deborah
Sumitomo Electric Industries Ltd.
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