Silicon nitride composite substrate

Details Silicon nitride composite substrate Silicon nitride composite substrate

2001-04-24

2003-07-29

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C428S704000, C428S698000, C501S097100, C501S097200, C501S097300

Reexamination Certificate

active

06599637

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a composite substrate such as a power module and the like comprising mounted exothermic electronic parts such as semiconductor elements, particularly, to a ceramic composite substrate which has a structure comprising a ceramic substrate connected with a metal layer, and has excellent heat radiation property, mechanical strength and heat-cycle-resistance property.
BACKGROUND ART
Conventionally, ceramic composite substrates obtained by connecting a metal plate composed mainly of copper or aluminum as an electric conductive layer to the surface of a ceramic substrate made of Al
2
O
3
, AIN, BeO and the like having electric insulation property have been widely used as constituent parts of various electric appliances.
Of these conventional ceramic composite substrates, those using an Al
2
O
3
substrate as a ceramic substrate cannot acquire excellent heat radiation property due to the low thermal conductivity of Al
2
O
3
, and those using a BeO substrate have high thermal conductivity and excellent heat radiation properties. The drawback, however, is that they are difficult to manage in production due to toxicity thereof. Composite substrates using an AIN substrate are excellent in heat radiation property because of the high thermal conductivity of AIN; however, their disadvantage is that they tend to crack by mechanical shock, and thermal load in repeated use under practical conditions due to the low mechanical strength of AIN.
On the other hand, ceramics containing mainly Si
3
N
4
are materials which generally show excellent heat resistance even under the atmosphere of a high temperature of 1000° C. or more, and have low thermal expansion coefficient and also excellent thermal shock resistance, in addition to an inherent high strength property. Consequently, the application of the ceramics as a high temperature structural material to various high temperature high strength parts has been tried.
Recently, studies have been done on a ceramic substrate to be used in a composite substrate, by utilizing the high strength property owned inherently by ceramics containing mainly Si
3
N
4
. For example, JP-B-No2698780, and JP-A No. 9-157054 disclose a trial in which insufficient thermal conductivity is compensated by enhancing the head radiation property of the whole circuit, in a composite circuit board comprising Si
3
N
4
substrates connected with a metal circuit plate, by making the thickness of the Si
3
N
4
substrate smaller than 1 mm.
However, it is believed that, even in a Si
3
N
4
substrate having higher strength than that of AIN, cracking also tends to occur by mechanical shock in installation and mounting or by thermal shock by a heat cycle similarly to the AIN substrate, and practical use of the Si
3
N
4
substrate is difficult when the substrate are excellent in heat radiation property because of the high thermal conductivity of AIN; however, their disadvantage is that they tend to crack by mechanical shock, and thermal load in repeated use under practical conditions due to the low mechanical strength of AIN.
On the other hand, ceramics containing mainly Si
3
N
4
, are materials which generally show excellent heat resistance even under the atmosphere of a high temperature of 1000° C. or more, and have low thermal expansion coefficient and also excellent thermal shock resistance, in addition to an inherent high strength property. Consequently, the application of the ceramics as a high temperature structural material to various high temperature high strength parts has been tried.
Recently, studies have been done on a ceramic substrate to be used in a composite substrate, by utilizing the high strength property owned inherently by ceramics containing mainly Si
3
N
4
. For example, JP-B No. 269870 and JP-A No. 9-157054 disclose a trial in which insufficient thermal conductivity is compensated by enhancing the head radiation property of the whole circuit, in a composite circuit board comprising Si
3
N
4
substrates connected with a metal circuit plate, by making the thickness of the Si
3
N
4
substrate smaller than 1 mm.
However, it is believed that, even in a Si
3
N
4
substrate having higher strength than that of AIN, cracking also tends to occur by mechanical shock in installation and mounting or by thermal shock by a heat cycle similarly to the AIN substrate, and practical use of the Si
3
N
4
substrate is difficult when the thickness of the substrate is small. The reason for this is, for example, that in a process of fabricating a ceramic composite substrate into an apparatus, the composite substrate must be fixed to the main part of the apparatus by screwing and the like. However, the occurrence of cracking by a pressing force by the screw and by shock in handling is inevitable even in a Si
3
N
4
substrate having excellent mechanical strength when the thickness thereof is small. When such cracking occurs, insulation failure occurs at the cracked part, and the composite substrate becomes unusable because of dielectric breakdown.
An object of the present invention is to provide a ceramic composite substrate which manifests no generation of cracking on the substrate even by mechanical shock or thermal shock, and has an excellent heat radiation property and heat-cycle-resistance property, in view of such conventional conditions.
DISCLOSURE OF INVENTION
The present inventors have studied and developed, for attaining the above-mentioned object, a Si
3
N
4
substrate material having high thermal conductivity and high strength, and found that when the ratio of the thickness of the Si
3
N
4
substrate to the thickness of the metal plate is set at a given value in a composite substrate obtained, fastening cracks and the like in a fabrication process can be dissolved, and heat-cycle-resistance property can be significantly improved, and that the heat radiation property of a composite substrate can be considerably improved by enhancing heat the conductivity of a Si
3
N
4
substrate, leading to the completion of the invention.
Namely, a ceramic composite substrate provided by the present invention comprises a silicon nitride ceramic substrate having a thermal conductivity of 90 W/m·K or more and a three-point flexural strength of 700 MPa or more, and a metal layer connected to one major surface thereof, and in the composite substrate, the thickness tc of the silicon nitride ceramic substrate and the thickness tm of the metal layer satisfy the relation formula: 2 tm≦tc≦20 tm.
Further, another silicon nitride composite substrate provided by the present invention comprises a silicon nitride ceramic substrate having a thermal conductivity of 90 W/m·K or more and a three-point flexural strength of 700 MPa or more, and metal layers connected to both major surfaces thereof, and in the composite substrate, the thickness tc of the silicon nitride ceramic substrate and the total thickness ttm of the metal layers on both major surfaces satisfy the relation formula: ttm≦tc≦10 ttm.
In the above-mentioned silicon nitride composite substrate of the present invention, the silicon nitride ceramic substrate before connection of the metal plate preferably is warped such that the major surface on which semiconductor elements are mounted forms a concave surface, and specific warp degree thereof is preferably in the range from 10 to 300 &mgr;m per 25.4 mm (inch) of the length of the substrate.
The silicon nitride ceramic substrate used in a silicon nitride composite substrate of the present invention contains a rare earth element in an amount of 0.6 to 10% by weight in terms of an oxide and at least one element selected from Mg, Ti, Ta, Li and Ca in an amount of 0.5 to 1.0% by weight in terms of an
Namely, a ceramic composite substrate provided by the present invention comprises a silicon nitride ceramic substrate having a thermal conductivity of 90 W/m·K or more and a three-point flexural strength of 700 MPa or more, and a metal layer connected to one major surface thereof, and in the composite substrate, the thickness tc of the silicon nitride

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