Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
1998-08-28
2002-09-10
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S469000, C428S698000, C428S697000, C427S372200, C427S383100, C427S383500
Reexamination Certificate
active
06447923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a metallized silicon nitride ceramic having a metallizing layer of high bonding strength on the surface of a highly heat-conductive, mechanically-strong silicon nitride ceramic; to a fabricating process thereof; and to a metallizing composite for the process. In the invention, the term “silicon nitride ceramic(s)” as well as the term “Si
3
N
4
ceramic(s)” includes Si
3
N
4
-based ceramic(s), Si
3
N
4
-glass-based ceramic(s), and Si
6−x
Al
x
O
x
N
4−x
(Sialon)-based ceramic(s).
2. Description of the Background Art
Highly heat-conductive materials also superior in electrical insulation, mechanical strength, and antiwear quality are needed as a sliding part in mechanical equipment or as a circuit substrate in electronic devices. Ceramics are used in these fields as a material that meets the requirements.
An alumina, one of the ceramic materials, is usually used in these fields. Although superior in electrical insulation and antiwear quality, the alumina has low thermal conductivity and hence is not suited for use in electronic devices or in a sliding part of mechanical equipment where heat must be rapidly dissipated to prevent excessive temperature increase.
Recently, an aluminum nitride having a high thermal conductivity, therefore having high capability in heat dissipation, is increasingly used. But the aluminum nitride has poor antiwear quality and mechanical strength, hence has low reliability as a structural member. In response to this problem, Si
3
N
4
ceramics having a high mechanical strength and high thermal conductivity are being developed. And there is a high possibility to solve the problem as a ceramic package comprising a Si
3
N
4
-based ceramic with a bending strength of 30 kg/mm
2
or more as disclosed in the unexamined published Japanese patent application Tokukaihei 4-125950.
When a highly heat-conductive Si
3
N
4
ceramic is used as a semiconductor package or circuit substrate or is brazed to a metal part as a structural member of a machine, the surface of the ceramic must be metallized. For example, the unexamined published Japanese patent application Tokukaihei 9-69672 discloses several methods of surface metallization for Si
3
N
4
-based ceramics, such as a directly-uniting method of a highly heat-conductive Si
3
N
4
-based ceramic substrate with a sheet of copper circuit, an active-metal method to unite an Si
3
N
4
-based ceramic substrate with a sheet of metal circuit, and a method to metallize the surface of an Si
3
N
4
-based ceramic substrate by a high-melting-point metallization method.
However, the above-mentioned methods are incapable of forming a highly reliable, strongly-bonded metallizing layer on the surface of an Si
3
N
4
-based ceramic. Furthermore, the directly- uniting method of an Si
3
N
4
-based ceramic substrate with a sheet of copper circuit and the active-metal method disclosed in the Tokukaihei 9-69672 are limited in the application of the metallized Si
3
N
4
-based ceramics.
The high-melting-point metallization method disclosed in the Tokukaihei 9-69672, which bakes a metallizing composite consisting mainly of a high-melting-point metal and titanium or its alloys onto the surface of an Si
3
N
4
-based ceramic, gives a relatively weak bonding between the Si
3
N
4
-based ceramic and the metallizing layer. Therefore, products fabricated by this method are unreliable for adequate use in heavy-duty bonded parts, though this method provides adequate bonding strength for a copper circuit substrate such as disclosed in the Tokukaihei 9-69672.
SUMMARY OF THE INVENTION
To overcome the above-mentioned difficulties, the present invention provides a highly reliable metallized Si
3
N
4
ceramic having a metallizing layer of high bonding strength on the surface of a highly heat-conductive, mechanically-strong Si
3
N
4
ceramic; a fabricating process thereof; and a metallizing composite for the process.
For the above purpose, the metallized silicon nitride ceramic of the invention specifically has a metallizing layer, which contains 0.01 to 20% silicon by weight in terms of silicon, formed on the entire or a part of the surface of a silicon nitride ceramic having 0.01 to 10% free silicon by weight.
In the metallized silicon nitride ceramic of the invention, it is preferable that the silicon nitride ceramic have a thermal conductivity of 50 W/mK or more and a bending strength of 600 MPa or more, and that the metallizing layer contain 0.01 to 5% silicon by weight in terms of silicon, and that the contained silicon be free silicon.
A fabricating process for the metallized silicon nitride ceramic of the invention specifically comprises that first, silicon powders having an average particle size of 5 &mgr;m or less and powders of rare earth compounds are mixed to form a compact; second, the compact is heated in a non-oxidizing atmosphere including nitrogen to produce a silicon nitride ceramic having 0.01 to 10% unnitrided, or free, silicon by weight; and third, a metallizing paste is applied on the entire or a part of the surface of the silicon nitride ceramic to be baked in a non-oxidizing atmosphere so that a metallizing layer containing 0.01 to 20% silicon by weight in terms of silicon is formed.
The other fabricating process for the metallized silicon nitride ceramic of the invention specifically comprises that first, silicon powders having an average particle size of 5 &mgr;m or less and powders of rare earth compounds are mixed to form a compact; second, a metallizing paste is applied on the entire or a part of the surface of the compact; and third, the compact is sintered with heat in a non-oxidizing atmosphere including nitrogen to produce a silicon nitride ceramic having 0.01 to 10% unnitrided, or free, silicon by weight and to form at the same time a metallizing layer containing 0.01 to 20% silicon by weight in terms of silicon on the surface of the ceramic.
In either of the above-mentioned processes, the metallizing paste comprises a metallizing composite and a vehicle, which includes a binder resin and a solvent. In the invention, the metallizing composite has a metal component as its main ingredient and may include 0.01 to 20% silicon by weight in terms of silicon.
According to the present invention, a highly reliable metallized Si
3
N
4
ceramic may be produced, which comprises a highly heat-conductive, high strength bending and wear-resistant Si
3
N
4
ceramic having a strongly bonded metallizing layer thereon.
The metallized SiN
3N
4
ceramic of the invention is useful as a ceramic circuit substrate for transistors, a ceramic package, and other parts of semiconductor devices or as a machine's structural member in combination with a metal part. The ceramic may serve additionally to improve the performance of these devices and machines.
DETAILED DESCRIPTION OF THE INVENTION
Efforts were made to seek a means for improving the bonding strength between an Si
3
N
4
ceramic and a metallizing layer. Finally it was found that high bonding strength is attainable by combining an Si
3
N
4
ceramic having free silicon with a metallizing layer having free silicon and other silicon-containing components. The end result was a highly heat-conductive metallized Si
3
N
4
ceramic with exceptionally high mechanical strength.
In the invention, it is imperative that an Si
3
N
4
ceramic to be metallized contains 0.01 to 10% free silicon by weight. The free silicon in the Si
3
N
4
ceramic plays an important role in improving the bonding strength between the ceramic and the metallizing layer having silicon components. If the content is less than 0.01% by weight, the bonding between the ceramic and the metallizing layer is weakened; if more than 10% by weight, the mechanical strength and heat-resisting property of the Si
3
N
4
ceramic itself decreases.
It is desirable that the Si
3
N
4
ceramic have a thermal conductivity of 50 W/mK or more and a bending strength of 600 MPa or more. To obtain sufficient heat-dissipating power as a circuit substrate, a cera
Jones Deborah
McDermott & Will & Emery
Stein Stephen
Sumitomo Electric Industries Ltd.
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