Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2001-09-24
2003-11-04
Chen, Bret (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S249800, C427S307000
Reexamination Certificate
active
06641861
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heatsink and a fabrication method thereof. Particularly, the present invention relates to a heatsink on which is mounted a semiconductor element of relatively great heat generation such as a laser diode, a CPU (central processing unit), a MPU (microprocessor unit), a high frequency amplifier device, and the like, having a multilayer structure of a diamond layer and a metal layer, and a method of fabricating such a heatsink.
2. Description of the Background Art
The aforementioned high power semiconductor devices generate a great, amount of heat during operation. The heat generated by these semiconductor elements has become greater in accordance with improvement in the output and the operating frequency. The need of compact and light-weight electronic equipment is great in the industry while the packaging density of the semiconductor element is continuously increasing. The increase in the heat generation and packaging density of semiconductor elements implies more stringent requirements with respect to the heat radiation characteristic of the heatsink employed in modules in which high power semiconductor elements are mounted.
Regarding such heatsinks that require great heat dissipation, a semiconductor element is mounted on a heatsink formed of a material of high thermal conductivity to prevent the semiconductor element from becoming too hot. For heatsinks that incorporate high power semiconductor elements such as a high power transistor or microwave monothylic IC (MMIC) of great heat generation, beryllium oxide (BeO), for example, superior in thermal conductivity and dielectric property is conventionally used widely.
Diamond is known as the substance having the highest thermal conductivity. Research has been effected to apply diamond to the heatsink that is used for incorporating a semiconductor element.
As a heatsink employing diamond, development is in progress of a heatsink formed entirely of diamond, and a heatsink having a diamond film formed on a metal substrate.
Since natural diamond is precious and artificial diamond is costly, the cost of the heatsink will increase if the amount of diamond therein becomes greater. Therefore, a heatsink formed entirely of diamond is used with respect to a semiconductor element of high heat generation such as a high power laser only in the application where heat radiation is so insufficient that it prevents exhibition of proper performance when a substitute is used or in the application such as during the stage of research where the cost is not yet estimated. A heatsink having a diamond film formed on a metal substrate is used in products that must have the cost reduced.
By using a heatsink formed partially of metal, the cost can be decreased although the thermal conductivity is degraded in comparison to a heatsink formed only of diamond. Therefore, the cost and performance of the heatsink is substantially proportional. It can be said that a heatsink of higher thermal conductivity becomes more expensive. Therefore, there is a demand for an economical heatsink of high thermal conductivity.
In response to such demands, a heatsink of a multilayered structure with a thin diamond film formed on a metal of favorable thermal conductivity is disclosed in, for example, Japanese Patent Laying-Open No. 5-326767.
Conventionally, BeO superior in thermal conductivity has been widely used for the heatsink. However, the level of the heat radiation characteristic that is currently required has become so high that even BeO is even not sufficient. An approach has been made to reduce the thickness of the BeO substrate to reduce thermal resistance. However, it is difficult to process BeO per se. Furthermore, BeO is toxic. It can be said that reduction in the thickness has come to its limit.
As to the heatsink disclosed in the above publication, copper and copper-tungsten alloy which are metals of favorable thermal conductivity are mentioned as the substance of the substrate. These materials are suitable for the heatsink since the thermal conductivity thereof is high comparable to other metal materials the cost is relatively low.
However, there was problem that it is difficult to grow a thin diamond film on a substrate that includes copper in favorable adherence since the copper in the substrate does not produce carbide, does not absorb carbon, and is not occluded with carbon, as described in New Diamond, Vol. 10, No. 3 (34), pp. 26 and 27.
Copper has a high thermal expansion coefficient whereas diamond has a low thermal expansion coefficient. Therefore, there is a problem that the thin diamond film will peel off the substrate as the temperature of the heatsink becomes higher due to the difference in thermal expansion coefficient between copper and diamond.
If the difference in thermal expansion between the substrate and the diamond is small, warping in the diamond heatsink will not occur. Only stress will be generated within the thin diamond film even when the heatsink attains high temperature. However, the thermal expansion of copper or a sintered compact including copper is greater than that of diamond, resulting in the problem of warping in the heatsink.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a heatsink that can have a thin diamond film formed in good adherence on a substrate of favorable thermal conductivity.
Another object of the present invention is to provide a heatsink that can have occurrence of warping suppressed.
According to an aspect of the present invention, a heatsink includes a substrate of a sintered compact including Cu and W, and a thin diamond film layer formed on the surface of the substrate. The Cu content in the substrate is at least 5% by weight. In an X-ray diffraction chart obtained by irradiating a thin diamond film layer with an X-ray, the diffraction peak intensity of the (110) plane of W is at least 100 times the diffraction peak intensity of the (200) plane of Cu.
In such a heatsink, the amount of W at the surface of the substrate is relatively great whereas the amount of Cu at the surface of the substrate becomes relatively smaller. Therefore, the adherence between the substrate and the thin diamond film layer formed on the surface of the substrate is improved. As a result, the heat locally generated from the semiconductor element mounted on the thin diamond film layer can be promptly dissipated at the in-plane of the thin diamond film layer by virtue of the effect of the thin diamond film layer as a heat spreader (effect of heat dissipation) to be conveyed to the substrate. The thermal conductivity of the substrate is increased since the Cu content in the substrate is at least 5% by weight.
In an X-ray diffraction chart obtained by irradiating the thin diamond film layer with an X-ray, it is preferable that a peak of WC (tungsten carbide) appears. In this case, the adherence between the thin diamond film layer and the substrate is improved.
According to another aspect of the present invention, a heatsink includes a substrate of a sintered compact including Cu and W, and a thin diamond film layer formed on the surface of the substrate. The Cu content in the substrate is at least 5% by weight. In an X-ray diffraction chart obtained by irradiating the thin diamond film layer with an X-ray, the diffraction peak intensity of the (211) plane of W is at least 30 times the diffraction peak intensity of the (200) plane of the Cu.
In such a heatsink, the amount of W at the surface of the substrate becomes relatively greater whereas the amount of Cu at the surface of the substrate becomes relatively smaller. Therefore, the adherence between the substrate and the thin diamond film layer formed on the surface of the substrate is improved. As a result, the heat locally generated from the semiconductor element mounted on the thin diamond film layer is rapidly dissipated at the in-plane of the thin diamond film layer to be subsequently conveyed to the substrate by virtue of the effect of the t
Imai Takahiro
Meguro Kiichi
Saito Hirohisa
Yamamoto Yoshiyuki
Chen Bret
Fasse W. F.
Fasse W. G.
Sumitomo Electric Industries Ltd.
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