Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-03-16
2002-05-07
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S098600
Reexamination Certificate
active
06383962
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aluminum nitride sintered product and a process for its production.
2. Discussion of Background
An aluminum nitride sintered product has a theoretical thermal conductivity as high as 320 W/m·K, and it is excellent also in the mechanical strength and electrical properties at a level of alumina. Accordingly, it has recently been widely used as a substrate material for a semiconductor power module such as GTO (gate turn off thyristor) or IGBT (insulted gate bipolar transistor) which requires high levels of electrical insulating properties and heat dissipation properties. As common properties of aluminum nitride sintered products which are industrially used for semiconductor power modules, the thermal conductivity is from 130 to 200 W/m·K, and the three-point bending strength (hereinafter referred to simply as the bending strength) is from 30 to 40 kg/mm
2
. For such a semiconductor power module, a copper-bonded substrate is widely used, wherein a copper sheet is bonded to an aluminum nitride substrate via an active metal layer or the like. In this copper-bonded substrate, there is a substantial difference in thermal expansion between aluminum nitride and the copper sheet, and cracks are likely to form in the aluminum nitride sintered product by heat treatment at the time of mounting electronic elements on the copper-lined substrate or by heat cycle exerted when it is used as a semiconductor power module, whereby the reliability as a semiconductor power module tends to be impaired. Accordingly, as an aluminum nitride substrate for such a semiconductor power module, one excellent in the bending strength, is required.
Aluminum nitride sintered products having the above properties are mass produced by the following method. Namely, a sintering aid such as yttrium oxide and an organic binder are blended to an aluminum nitride material powder, and the blend is molded into a molded product by e.g. a doctor blade method or a press-molding method. Then, this molded product is heated in air or in a nitrogen atmosphere to remove the binder, and then the molded product is sintered in a nitrogen atmosphere under an ambient pressure to obtain a sintered product. The thermal conductivity of an aluminum nitride sintered product depends largely on the amount of oxygen contained in the aluminum nitride crystal grains. Namely, by reducing the oxygen content, it is possible to obtain an aluminum nitride sintered product having a high thermal conductivity. According by using an aluminum nitride material powder having a small oxygen content or by incorporating carbon to an aluminum nitride material powder and reacting the carbon with the contained oxygen during the sintering process in a nitrogen atmosphere to remove the contained oxygen, sintered products showing a high thermal conductivity at a level of 200 W/m·K are mass-produced.
Further, many attempts have been made for the purpose of improving the bending strength of the aluminum nitride sintered product. For example, (a) it has been attempted to improve the strength by incorporating a Si component to control growth of aluminum nitride crystal grains during sintering thereby to form a sintered product having fine, dense aluminum nitride crystal grains (JP-A-6-329474,etc.), and (b) it has been attempted to increase the strength by dispersing fine particles (nano particles) of titanium oxide or the like in the crystal grains and grain boundaries of the aluminum nitride sintered product (JP-A-4-132666).
However, these methods for improving the strength have problems respectively and have not been practically employed. For example, in the above method (a), the Si component present in the aluminum nitride sintered product tends to lower the thermal conductivity, whereby it tends to be difficult to obtain high strength and high thermal conductivity at the same time. On the other hand, in the above method (b), the step for preparation of nano particles, the step for uniformly dispersing nano particles in the sintered product, etc., are cumbersome, and it has been difficult to adopt such a method widely on an industrial basis.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above-mentioned problems of the prior art and to provide an aluminum nitride sintered product which can be produced constantly on an industrial basis, which has both high strength and high thermal conductivity and which is suitable for an aluminum nitride substrate for a semiconductor power module, and a process for its production.
The present invention provides an aluminum nitride sintered product which is made mainly of aluminum nitride and contains an yttrium compound in an amount of from 0.6 to 5 wt % as calculated as yttrium oxide, a vanadium compound in an amount of from 0.02 to 0.4 wt % as calculated as vanadium and carbon in an amount of from 0.03 to 0.10 wt % and which has a three-point bending strength of at least 45 kg/mm
2
and a thermal conductivity of at least 150 W/m·K, wherein crystal grains of aluminum nitride have an average grain size of at most 5 &mgr;m.
The present invention also provides a process for producing an aluminum nitride sintered product, which comprises molding a blend material prepared by blending a carbon material and a binder to a composition comprising from 0.6 to 5 wt % of yttrium oxide, from 0.02 to 0.4 wt %, as calculated as vanadium, of vanadium oxide and the rest being an aluminum nitride material powder, into a molded product of a predetermined shape, then heating the molded product in air to remove the binder from the molded product and then sintering the molded product having the binder removed therefrom, in a non-oxidizing atmosphere, wherein the content of the carbon material in the blend material is from 0.5 to 0.8 time by weight the amount of oxygen contained in the aluminum nitride material powder.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the aluminum nitride sintered product of the present invention, if the average grain size of aluminum nitride crystal grains exceeds 5 &mgr;m, the strength tends to deteriorate. The maximum grain size of the aluminum nitride crystal grains in the sintered product is preferably less than 15 &mgr;m.
In the present invention, the average grain size of the aluminum nitride crystal grains, is determined as follows. With respect to a fracture surface of a sintered product, a SEM photograph with 2,000 magnifications is taken, and an optional linear line (a length of about 120 &mgr;m) is drawn on the photograph. Then intersecting points of this linear line with grain boundaries of each crystal grain, are determined, and the length between the intersection points of each crystal grain is taken as the grain size of that crystal grain. Thus, the arithmetic average of grain sizes of the respective crystal grains is taken as the average grain size.
Further, the maximum grain size of crystal grains is determined in such a manner that with respect to a fracture surface of a sintered product, a SEM photograph with 1,000 magnifications, is taken, then, the maximum crystal grain is specified on the photograph (within an area of about 80×120 &mgr;m) and the maximum diameter of that crystal grain is taken as the maximum grain size.
The yttrium compound serves as a sintering aid and will remain in the sintered product. If its content is less than 0.6 wt % as calculated as yttrium oxide, the product tends to be porous, and the strength of the sintered product tends to be low. On the other hand, if its content exceeds 5 wt % as calculated as yttrium oxide, the yttrium compound tends to partially agglomerate on the surface of the sintered product, and the surface roughness tends to increase. Such an yttrium compound may, for example, be yttrium oxide or yttrium aluminum oxide (Y
4
Al
2
O
9
).
The vanadium compound serves to promote sinterability at the time of sintering and will remain in the sintered product. The vanadium compound may, for example, be V
2
O
3
,V
2
O
4
or V
2
O
5
. If i
Hiroi Atsuo
Kitamura Yukihiro
Obana Yoshiki
Ueki Mikio
Watanabe Kazunari
Asahi Techno Glass Corporation
Group Karl
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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