Aluminum nitride sintered body and semiconductor substrate thereof

A highly heat-conductive aluminum nitride sintered body comprising 95.5 to 99.8% by weight of an aluminum nitride grain phase having an average grain size of 2 to 10 .mu.m and the rest being substantially a dysprosium oxide phase and having a density of at least 99% of the theoretical density, at least 30% by weight of the oxide phase existing at the triple points of the aluminum nitride grains. By forming an alumina-based oxide layer on the sintered body and further forming on the oxide layer a plating layer of Ni and/or Cu via a vapor deposition layer, there can be obtained a semiconductor substrate having a high bonding strength to solder.

TECHNICAL FIELD 
The present invention relates to an aluminum nitride sintered body and a 
production process therefor and further to a semiconductor substrate using 
the sintered body and a production process therefor. 
BACKGROUND 
In recent years electronic equipment and apparatus have been improved so as 
to have a smaller size and a higher level of integration, and so there has 
arisen the very important task of removing the heat generated by various 
semiconductor devices including IC chips, mounted on electronic equipment 
and apparatuse. For the removal of the heat, various proposals have been 
made concerning part designing, circuit designing, materials and the like. 
Currently, Al.sub.2 O.sub.3 is in use as a material for most of the 
substrates for semiconductors such as high integration IC and the like. 
However, with the recent improvement of ICs to a higher integration and a 
higher operating speed and the resulting increase in the amount of heat 
released by IC chips, there has arisen a demand for a material of higher 
heat releasability. Hence, BeO, SiC, etc. have been investigated as a 
substrate material. While both BeO and SiC have a high heat conductivity 
of about 260 W/mk, BeO is disadvantageous in that it is expensive and its 
dust has toxicity, and SiC is not easy to produce because it is not 
sufficiently sintered under normal pressure and so it must be sintered by 
hot pressing. 
Hence, attention has been paid to aluminum nitride as a material for 
semiconductor substrates, having a higher strength than Al.sub.2 O.sub.3 
and BeO, capable of being sintered at normal pressure and having a high 
heat conductivity. However, commercially available aluminum nitride 
powders usually contain about 2 to 3.5% by weight of oxygen and, from such 
a powder, it is difficult to produce an aluminum nitride sintered body of 
high heat conductivity, as described in Journal of the Ceramics Society of 
Japan, Vol. 93, No. 9, 1985, Pages 517-522 and in Electronic Ceramics, 
Vol. 16, No. 3, March 1985, pages 22-27. Hence, there was proposed a 
process comprising mixing appropriate proportions of alumina, ash and an 
alkaline earth metal, an yttrium alloy or the like in a liquid dispersing 
medium and then sintering the mixture in nitrogen or an ammonia atmosphere 
to obtain an aluminum nitride sintered body of increased purity and 
accordingly of improved sintering density and improved heat conductivity 
(Japanese Patent Laid-Open No. 60-65768). 
There was also proposed an aluminum nitride sintered body obtained by 
adding to aluminum nitride for improving heat conductivity, boron nitride 
or an oxide of calcium, magnesium, aluminum, titanium, zirconium and/or a 
rare earth metal, preferably an yttrium oxide and then sintering the 
mixture (Japanese Patent Laid-Open No. 59-131583). 
These aluminum nitride sintered bodies, however, have a large variation in 
heat conductivity between production lots and products of high heat 
conductivity cannot be obtained stably. These sintered bodies have a 
further problem that an aluminum nitride sintered body of high heat 
conductivity can be obtained only when the aluminum nitride used as a 
starting material contains a very low concentration of oxygen. 
An aluminum sintered body can be used as a substrate for hybrid ICs whose 
patterning are not very fine, or as a package for high-integration logic 
circuit semiconductors. When the aluminum nitride sintered body is used as 
a substrate for hybrid ICs, even if a metallizing paste of a metal such as 
Mo, Mn or the like is directly printed on the surface of the sintered body 
and baked, the sintered body does not have a sufficiently high bonding 
strength to the metal. Therefore, it is desired that the bonding strength 
after baking be increased. 
When the aluminum nitride sintered body is used as a substrate for 
semiconductor packages, ordinarily a silicon chip is mounted on the 
aluminum nitride substrate, and the upper surface of the silicon chip is 
covered by a ceramic such as aluminum nitride or the like with a lead 
frame connected to the silicon chip projecting outside through the 
interface between the aluminum nitride substrate and the ceramic cover. In 
order to increase the adhesion of the aluminum nitride substrate and the 
ceramic cover, the opposing surface portions of the aluminum nitride 
substrate and the ceramic cover which are in contact with each other are 
metallized and further the lead frame is soldered. However, aluminum 
nitride is difficult to metallize because it is chemically stable. 
Further, the aluminum nitride sintered body, having poor water resistance, 
tends to react with water to form ammonia, thereby being eroded. 
In view of the above problems, it was attempted to laminate a metal such as 
silver, titanium, copper or the like on a substrate consisting of an 
aluminum nitride sintered body by means of vapor deposition to improve the 
wettability of the substrate. However, the metal layer thus formed by 
vapor deposition has a thickness of only about the order of .ANG. and it 
is very difficult to form the metal layer in a thickness of the order of 
.mu.m in order to secure a sufficient bonding strength to solder or a 
soldering material enabling the mounting of the devices. This is because 
when the metal layer is formed up to a thickness of the order of .mu.m, 
the internal stress becomes too large, making the metal layer highly 
likely to peel off. 
Hence, an object of the present invention is to provide an aluminum nitride 
sintered body having a heat conductivity and a mechanical strength which 
are both sufficiently high for use in semiconductor substrates. 
Another object of the present invention is to provide a process which can 
produce the above-mentioned aluminum nitride sintered body stably with 
substantially no variation in properties. 
A further object of the present invention is to provide a highly 
heat-conductive semiconductor substrate obtained by subjecting the 
above-mentioned aluminum nitride sintered body to a surface treatment so 
that a paste of a metal such as Mo, Mn or the like can directily be 
printed on the resulting sintered body and baked to form a circuit pattern 
strongly adhering to the sintered body. 
A still further object of the present invention is to provide a highly 
heat-conductive semiconductor substrate for packaging, having a strongly 
adhered metal layer formed by metallizing and so having improved 
wettability by solder or a soldering material. 
DISCLOSURE OF THE INVENTION 
The highly heat-conductive aluminum nitride sintered body according to the 
present invention comprises 95.5 to 99.8% by weight of an aluminum nitride 
grain phase having an average grain size of 2 to 10 .mu.m and the rest 
being substantially a dysprosium oxide phase, and has a density of at 
least 99% of the theoretical density, wherein at least 30% by weight of 
the oxide phase exists at the triple points of the aluminum nitride 
grains. 
Further, the process for producing a highly heat-conductive aluminum 
nitride sintered body according to the present invention comprises forming 
a dispersion of AlN particles containing a dysprosium alkoxide, 
hydrolyzing the dysprosium alkoxide to form a composite precipitate 
consisting of AlN particles and the hydrolysis product of the alkoxide 
adhering to the outer surfaces of the AlN particles, calcinating the 
composite precipitate to form a composite powder consisting of AlN 
particles and a fine powder of dysprosium oxide adhering to the outer 
surfaces of the AlN particles, and subjecting the composite powder to 
molding and then to sintering. 
Further, the semiconductor substrate according to the present invention 
comprises a highly heat-conductive aluminum nitride sintered body 
comprising 95.5 to 99.8% by weight of an aluminum nitride grain phase 
having an average grain size of 2 to 10 .mu.m and the rest being 
substantially a dysprosium oxide phase (at least 30% by weight of the 
oxide phase existing at the triple points of the aluminum nitride grains) 
and having a density of at least 99% of the theoretical density, and an 
alumina-based oxide layer having a thickness of 0.1 to 20 .mu.m, formed at 
the surface of the aluminum nitride sintered body. 
Further, the process for producing a semiconductor substrate according to 
the present invention comprises heating a highly heat-conductive aluminum 
nitride sintered body comprising 95.5 to 99.8% by weight of an aluminum 
nitride grain phase having an average grain size of 2 to 10 .mu.m and the 
rest being substantially a dysprosium oxide phase (at least 30% by weight 
of the oxide phase existing at the triple points of the aluminum nitride 
grains) and having a density of at least 99% of the theoretical density, 
at 950.degree. to 1,200.degree. C. for not longer than 30 minutes in an 
atmosphere having an oxygen partial pressure of 21% or below to form an 
alumina-based oxide layer at the surface of the sintered body. 
Further, the semiconductor substrate according to the present invention is 
a laminate comprising a highly heat-conductive aluminum nitride sintered 
body comprising 95.5 to 99.8% by weight of an aluminum nitride grain phase 
having an average grain size of 2 to 10 .mu.m and the rest being 
substantially a dysprosium oxide phase (at least 30% by weight of the 
oxide phase existing at the triple points of the aluminum nitride grains) 
and having a density of at least 99% of the theoretical density, a thin 
film layer of Ni and/or Cu having a thickness of 100 to 8,000 .ANG., 
formed on the surface of the sintered body, and a layer composed of Ni 
and/or Cu and having a thickness of 0.1 to 10 .mu.m, formed on the surface 
of the thin film layer. 
Further, the process for producing a semiconductor substrate according to 
the present invention comprises heat-treating a highly heat-conductive 
aluminum nitride sintered body comprising 95.5 to 99.8% by weight of an 
aluminum nitride grain phase having an average grain size of 2 to 10 .mu.m 
and the rest being substantially a dysprosium oxide phase (at least 30% by 
weight of the oxide phase existing at the triple points of the aluminum 
nitride grains) and having a density of at least 99% of the theoretical 
density, at 950.degree. to 1,200.degree. C. for not longer than 30 minutes 
in an atmosphere having an oxygen partial pressure of 21% or below to form 
an alumina-based oxide layer as a first layer at the surface of the 
sintered body, forming on the first layer a thin film layer composed of at 
least one metal selected from the group consisting of Ti, Cr, Mo and W and 
a thin film layer of Ni and/or Cu as a second layer and a third layer, 
respectively, according to a physical vapor deposition method, and forming 
on the third layer a layer of Ni and/or Cu as a fourth layer according to 
a plating method.

BEST MODE FOR CARRYING OUT THE INVENTION 
The highly heat-conductive aluminum nitride sintered body of the present 
invention comprises 95.5 to 99.8% by weight of an aluminum nitride grain 
phase having an average grain size of 2 to 10 .mu.m and the rest being 
substantially a dysprosium oxide phase. When the aluminum nitride grains 
have an average grain size of smaller than 2 .mu.m, the dysprosium oxide 
is not likely to gather at the triple points of the aluminum nitride 
grains. When the aluminum nitride grains have an average grain size of 
larger than 10 .mu.m, the sintered body has reduced mechanical strengths. 
The average grain size is preferably 3 to 7 .mu.m. 
The proportion of the aluminum nitride grain phase in the sintered body is 
95.5 to 99.8% by weight and the rest is substantially a dysprosium oxide 
phase. When the proportion of the oxide phase is less than 0.2% by weight, 
the sintered body is not sufficiently sintered. When the proportion is 
more than 4.5% by weight, the oxide phase is not likely to gather at the 
triple points of the aluminum nitride grains. The proportion of the 
aluminum nitride grain phase is preferably 96.5 to 98.5% by weight. 
The dysprosium oxide phase ordinarily contains Al in addition to Dy and is 
a glassy phase represented by a composition of DyAlO.sub.3 and/or Dy.sub.3 
Al.sub.2 (AlO.sub.4).sub.4. The proportion of Dy in the oxide phase is 
about 60 to 80% by weight, the proportion of Al is about 5 to 50% by 
weight, and the proportion of O is about 10 to 40% by weight. 
At least 30% by weight of the oxide phase gathers at the triple points of 
the aluminum nitride grains and the rest is scattered at the boundaries of 
the aluminum nitride grains. Since in general the heat conductivity of an 
aluminum nitride sintered body is reduced by the precipitation of an oxide 
phase on the boundaries of the aluminum nitride grains, the heat 
conductivity of the sintered body can be increased by gathering the oxide 
phase at the triple points of the aluminum nitride grains. However, the 
oxide phase inevitably exists at the grain boundaries to some extent and 
this is necessary to increase the mechanical strengths (e.g. bending 
strength) of the sintered body. Hence, the proportion of the oxide phase 
gathering at the triple points of the aluminum nitride grains is preferred 
to be 50 to 95% by weight. 
The aluminum nitride sintered body having such a composition and structure 
has a density of at least 99% of the theoretical density. When the density 
is smaller than 99%, the sintered body has no sufficient heat conductivity 
and has poor mechanical strengths even if the sintered body satisfies the 
compositional and structural requirements mentioned above. The density is 
preferably at least 99.4%, particularly at least 99.9% of the theoretical 
density. 
The aluminum nitride sintered body of the present invention further has a 
heat conductivity of at least 150 W/mk and a bending strength of at least 
30 kg/mm.sup.2 both at room temperature. 
The highly heat-conductive aluminum nitride sintered body can be obtained 
by mixing an aluminum nitride powder and a dysprosium oxide powder, 
molding the mixture to form a green body and then sintering the green 
body. In order to stably produce an aluminum nitride sintered body of high 
heat conductivity and high sintering degree, it is preferable that the 
dysprosium oxide powder be allowed to adhere to the surfaces of the 
aluminum nitride particles uniformly and thinly to form a composite powder 
and then the powder be molded and sintered. The AlN particles have an 
average particle size of 1 .mu.m or below and the dysprosium oxide powder 
adhering to the surfaces of the AlN particles has an average particle size 
of 0.2 .mu.m or below. When the AlN particles has an average particle size 
of larger than 1 .mu.m, the particles have poor sinterability. The average 
particle size of the AlN particles is preferably 0.3 to 0.8 .mu.m. When 
the dysprosium oxide powder has an average particle size of larger than 
0.2 .mu.m, it is difficult to uniformly disperse the glassy phase of 
dysprosium oxide formed in the sintering step in the sintered body, and so 
the ceramic obtained has reduced toughness. The average particle size of 
the dysprosium oxide powder is preferably 0.005 to 0.08 .mu.m. 
The above composite powder can be produced by preparing a dispersion of an 
AlN powder containing a dysprosium alkoxide and hydrolyzing the dysprosium 
alkoxide. As the dysprosium alkoxide, there are preferably used those 
soluble in organic solvents, such as ethoxide, isopropoxide, butoxide and 
the like. In order to dissolve the dysprosium alkoxide, any organic 
solvent can be used. Particularly preferable organic solvents are polar 
solvents such as lower alcohols (e.g. ethanol, isopropanol, normal 
butanol), ketones (e.g. acetone, methyl ethyl ketone), and esters (e.g. 
ethyl acetate, butyl acetate). 
The dispersion of an AlN powder is prepared by mixing a dysprosium alkoxide 
solution as mentioned above and a fine powder of AlN having a particle 
size of 1.0 .mu.m or below and a BET specific surface of at least 5 
m.sup.2 /g. This dispersion is mixed with a precipitating medium to 
hydrolyze the dysprosium alkoxide and thereby to obtain a dysprosium oxide 
(Dy.sub.2 O.sub.3). 
As the precipitating medium, there can be used water, ammonia water, an 
aqueous ammonium carbonate solution, an aqueous ammonium oxalate solution, 
etc. The use of a polar organic solvent as mentioned above for dissolution 
of the dysprosium alkoxide enables good dispersion of the AlN fine powder 
in the resulting solution and easy hydrolysis of the dysprosium alkoxide. 
The mixing of the AlN dispersion and the precipitating medium can be 
conducted by adding the precipitating medium to the AlN dispersion or by 
adding the AlN dispersion to the precipitating medium. 
The dysprosium oxide formed by hydrolysis takes a form of fine particles 
and adheres to the surfaces of the AlN particles. The composite 
precipitate thus obtained is collected by filtration and then dried. An 
evaporation method can be used for drying, but a spray drying method is 
preferable for drying in a large amount. 
The dried composite precipitate is calcinated at a temperature of 
400.degree. to 1,000.degree. C. As the calcination time, 30 to 150 minutes 
is usually sufficient. The calcination can be conducted in the atmosphere 
but it is preferable to conduct the calcination in an non-oxidizing 
atmosphere in order to prevent the oxidation of the composite precipitate. 
The composite powder obtained by calcination has a constitution in which 
the fine powder of Dy.sub.2 O.sub.3 adheres to the surfaces of the AlN 
particles. It can be molded using an ordinary die or press. The molding 
pressure is ordinarily 0.2 to 2 kg/cm.sup.2, and the molding time is 1 to 
20 seconds. The green body obtained is preferably further pressed 
isotropically. By the isotropic pressing, the sintered body to be formed 
later can have an increased density and increased mechanical strengths. 
The sintering of the green body can be conducted in N.sub.2 gas at a 
temperature of 1,750.degree. to 2,000.degree. C. according to the normal 
pressure sintering method. Besides, there may be used special sintering 
methods such as a hot pressing method, a hot isostatic pressing (HIP) 
method and the like. 
There can thus be obtained an aluminum nitride sintered body having the 
above mentioned properties. 
The highly heat-conductive semiconductor substrate of the present invention 
comprises an aluminum nitride sintered body as mentioned above and an 
alumina-based oxide layer formed on the surface of the sintered body. The 
alumina-based oxide layer consists mainly of two phases, namely, an 
.alpha.-Al.sub.2 O.sub.3 phase and a phase of a solid solution of 
dysprosium and aluminum oxides. Since the solid solution of dysprosium and 
aluminum oxides in the alumina-based oxide layer serves as a binder for 
the .alpha.-Al.sub.2 O.sub.3 particles, the oxide layer can have 
sufficient water resistance. The oxide layer has a further feature of 
having good adhesion to a metallizing layer formed by baking a metal 
paste. Therefore, the semiconductor substrate of the present invention can 
preferably be used as a substrate for hybrid ICs. The alumina oxide layer 
has a thickness of 0.1 to 20 .mu.m. When the thickness is smaller than 0.1 
.mu.m, the oxide layer is difficult to metallize and has insufficient 
adhesion to the metal layer formed by baking, causing peeling of the metal 
layer. When the thickness of the oxide layer is larger than 20 .mu.m, the 
resulting semiconductor substrate has reduced heat conductivity, making 
difficult the release of the heat generated by semiconductor devices and 
reducing the adhesion of the oxide layer to the underlying AlN sintered 
body. The thickness of the oxide layer is preferably 0.2 to 3 .mu.m. 
With respect to a metal pattern layer formed on the above semiconductor 
substrate which is suitable for hybrid ICs, it is formed by printing a 
paste of a metal such as Mo, Mn, W or the like in a desired pattern 
according to screen printing and then baking the paste at 1,300.degree. to 
1,500.degree. C. While the metal pattern layer formed directly on the AlN 
substrate has an adhesion strength of about 0.2 kg/mm.sup.2, the strength 
is improved to 5 kg/mm.sup.2 or more by providing the alumina-based oxide 
layer between the AlN substrate and the metal pattern layer. 
The alumina-based oxide layer is produced according to the following 
method. 
The alumina-based oxide layer is formed by heating the aluminum nitride 
sintered body in an atmosphere having an oxygen partial pressure of 21% or 
below, for example, air at a temperature of 950.degree. to 1,200.degree. 
C. for not longer than 30 minutes. 
The reason for heating in an oxidizing atmosphere containing 21% or less of 
oxygen is that when the atmosphere used has a higher oxygen partial 
pressure than 21%, the resulting oxide layer has a lot of pores 
communicating with each other, making it impossible to form a dense layer 
of sufficient water resistance. The oxygen pressure of the atmosphere used 
is desirably 15% or below. 
The reason for heating at 950.degree. to 1,200.degree. C. is that when a 
temperature lower than 950.degree. C. is used, no .alpha.-Al.sub.2 O.sub.3 
layer is formed, and when a temperature higher than 1,200.degree. C. is 
used, the resulting oxide layer has a lot of pores at the outer surface 
and the formation of a solid solution of dysprosium and aluminum oxides 
(this solid solution serves as a binder) is difficult. The heat treatment 
temperature is preferably 970.degree. to 1,150.degree. C. The reason for 
heating for not longer than 30 minutes is that when a time longer than 30 
minutes is used, the lots of pores formed at the outer surface of the 
oxide layer communicate with each other and the oxide layer has a 
thickness larger than 20 .mu.m making it impossible to fully utilize high 
heat conductivity inherently possessed by the aluminum nitride sintered 
body. 
Another highly heat-conductive semiconductor substrate of the present 
invention comprises an aluminum nitride sintered body, a vapor deposition 
layer formed thereon and a thick film layer formed on the vapor deposition 
layer according to plating or other appropriate methods so that the 
semiconductor substrate can have a large adhesion strength to solder or a 
soldering material. As mentioned previously, aluminum nitride itself is 
chemically stable and accordingly has a very low adhesion strength to a 
plating layer to be formed thereon. The present inventor has found that 
the plating layer can have a remarkably increased adhesion strength by 
forming beneath the plating layer a thin film layer according to a 
physical vapor deposition method. 
The plating layer consists mainly of Ni and/or Cu and can be formed 
according to an ordinary electroplating method. The underlying thin film 
layer is preferred to consist mainly of the same metal components as the 
plating layer and can be formed according to an ion plating method or a 
sputtering method. In order to increase the water resistance of the 
semiconductor substrate and the adhesion strength of the final laminate, 
it is preferable that an alumina-based oxide layer be formed first on the 
aluminum nitride sintered body. This oxide layer can be the same as the 
oxide layer previously mentioned. In order to further increase the 
adhesion strength between the alumina-based oxide layer and the thin film 
layer, it is preferable that a second thin film layer be formed between 
them. The second thin film layer preferably is composed of at least one 
metal selected from Ti, Cr, Mo and W all having good adhesion to the 
alumina-based oxide layer. 
An example of the semiconductor substrate having all of the above-mentioned 
layers is explained below. This semiconductor substrate is a laminate 
comprising a first layer consisting of an alumina-based oxide, a second 
layer composed of at least one metal selected from the group consisting of 
Ti, Cr, Mo and W, formed on the first layer, a third layer consisting of a 
thin film layer of Ni and/or Cu, formed on the second layer and a fourth 
layer composed of Ni and/or Cu, formed on the third layer. 
The first layer is the same alumina-based oxide layer as mentioned 
previously and has a thickness of 0.1 to 20 .mu.m, preferably 0.2 to 3 
.mu.m. 
The second layer has functions of increasing the adhesion strength between 
the first layer consisting of an alumina-based oxide and the third layer 
and further of preventing the solder or soldering material from 
penetrating into aluminum nitride. Therefore, the second layer is required 
to have a film thickness of at least 200 .ANG.. However, when the film 
thickness is larger than 5,000 .ANG., the internal stress or strain 
generated during the formation of the metal film remains, causing the 
formation of voids and the peeling of the layer. Hence, the film thickness 
of the second layer is 100 to 8,000 .ANG., preferably 500 to 5,000 .ANG.. 
The third layer is required to have a film thickness of at least 100 .ANG. 
in order to enable the uniform formation of the fourth layer. However, 
when the film thickness is larger than 8,000 .ANG., the internal stress or 
strain remains as in the case of the second layer, causing the formation 
of voids and peeling of the layer. Therefore, the film thickness of the 
third layer is 100 to 8,000 .ANG., preferably 500 to 5,000 .ANG.. 
The second layer and the third layer can each be formed according to a 
physical vapor deposition method such as an ion plating method, a 
sputtering method or the like. 
The fourth layer is formed by plating of the same metal as used in the 
third layer and is required to have a thickness of 0.1 to 10 .mu.m in 
order to secure a sufficient bonding strength to solder or a soldering 
material by forming an alloy therewith. When the thickness is smaller than 
0.1 .mu.m, no sufficient bonding strength is obtained and, when the 
thickness is larger than 10 .mu.m, the same problem exists as in the third 
layer. The fourth layer preferably has a thickness of 1 to 5 .mu.m. The 
fourth layer can be formed according to an electroplating method but can 
also be formed according to other methods such as an electroless plating 
method and the like. 
It is preferable that the upper surface of the fourth layer is provided 
with a plating composed of Au, Ag, Pd or Pt and having a thickness of 0.05 
to 5 .mu.m, in order to have better wettability by solder or a soldering 
material and to protect the fourth layer from oxidation. 
The semiconductor substrate thus formed has not only a large adhesion 
strength as a laminate but also high air tightness and high smoothness, 
and accordingly it is desirable for use as a substrate for packages of 
semiconductor devices such as large scale integrated circuits and very 
large-scale integrated circuits. 
The present invention is described in more detail below by way of the 
following Examples. 
EXAMPLE 1 
A commercially available aluminum nitride powder (oxygen content: 2.1% by 
weight) having an average particle size of 0.5 .mu.m and a Dy.sub.2 
O.sub.3 powder having an average particle size of 0.2 .mu.m were mixed at 
various mixing ratios shown in Table 1 and stirred for 24 hours in a 
plastic ball mill containing 500 cc of ethyl alcohol, using plastic balls. 
Each of the resulting mixtures was dried in vacuum and then sintered in 
N.sub.2 gas of 1 atm under the conditions shown in Table 1. The properties 
of the sintered bodies obtained are shown in Table 1. The samples No. 13 
and No. 14 were obtained using an AlN-Dy.sub.2 O.sub.3 composite powder 
produced according to the alkoxide method described below. That is, to an 
ethanol solution of dysprosium butoxide was added an AlN powder having an 
average particle size of 1.0 .mu.m or below and a BET specific surface 
area of 7 m.sup.2 /g, and they were made into a dispersion; this 
dispersion was slowly added to a water-ethanol solution to form a 
composite precipitate consisting of AlN particles and the hydrolysis 
product of a dysprosium butoxide covering the surfaces of the AlN 
particles; after the completion of hydrolysis reaction, the composite 
precipitate was dried in vacuum to obtain a dry powder of a composite 
precipitate; the dry powder was calcinated at 500.degree. C. to obtain an 
AlN-Dy.sub.2 O.sub.3 composite powder consisting of AlN particles and fine 
Dy.sub.2 O.sub.3 particles having particle sizes of 100 to 200 .ANG. and 
adhering to the surfaces of the AlN particles. A photograph by a 
transmission-type electron microscope (TEM), of the appearance of the 
composite powder is shown in FIG. 1. 
In sample No. 16 which is a conventional sample for comparison with the 
samples of the present invention, a Y.sub.2 O.sub.3 powder was used in 
place of the Dy.sub.2 O.sub.3 powder; the same AlN powder as used above 
and this Y.sub.2 O.sub.3 powder were mixed and vacuum-dried in the same 
manner as above; and the dry mixture was sintered in N.sub.2 gas of 1 atm 
at 1,900.degree. C. for 1 hour to obtain a sintered body of sample No. 16. 
Sample No. 15 which is also a conventional sample for comparison was 
obtained by using MgO as a sintering aid. The properties of the samples 
No. 15 and No. 16 are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Mixing Ratio Sintering Density 
Average Grain 
Proportion of 
Heat Bending 
Sample 
(wt. %) Temperature 
Time 
Ratio Size of Oxide Phase at 
Conductivity 
Strength 
No. AlN Dy.sub.2 O.sub.3 
(.degree.C.) 
(hr) 
(%) AlN (.mu.m) 
Triple Points (%) 
(W/mK) (Kg/mm.sup.2) 
__________________________________________________________________________ 
1 100 0 1900 1 82 2 0 50 10 
2 99.9 0.1 " " 93 4 40 62 15 
3 99.7 0.3 " " 99.9 4 90 203 30 
4 99 1 " " &gt;99.9 5 95 195 40 
5 97 3 " " " 6 90 186 50 
6 95 5 " " " 7 50 150 50 
7 93 7 " " " 8 25 82 50 
8 97 3 1800 " " 4 60 160 50 
9 99 1 " " &gt;99.4 3 65 178 45 
10 97 3 1750 " &gt;99.9 2 30 150 50 
11 97 3 1700 " 80 1.5 -- 40 10 
12 97 3 1900 8 &gt;99.9 11 95 185 20 
13* 97 3 1900 1 &gt;99.9 6 95 190 50 
14* 97 3 1800 " &gt;99.7 3 70 168 50 
15 Al.sub.2 O.sub.3 -0.5 wt % MgO 
1600 " &gt;99.8 3 -- 20 30 
16 AlN-3 wt % Y.sub.2 O.sub.3 
1900 " 99.5 5 86 130 45 
__________________________________________________________________________ 
Note: 
(1)*AlN powder and Dy.sub.2 O.sub.3 powder were mixed by the alkoxide 
method. 
(2) Sample Nos. 3-6, 8-10, 13 and 14 : Present Invention 
Sample Nos. 1, 2, 7, 11, 12, 15 and 16 : Comparative Example 
The photograph by scanning-type electron microscope (SEM) of samples No. 5 
and No. 10 are shown in FIG. 2A and FIG. 2B, respectively. In the 
photographs of FIG. 2A and FIG. 2B, there is seen the existence of an Dy 
and Al-containing oxide phase ((Dy, Al)(O,N)) at the boundaries of the AlN 
grains. As is clear from Table 1, sample No. 5 in which the proportion of 
the Dy-Al oxide phase existing at the triple points of the AlN grains is 
higher has a higher heat conductivity of 186 W/mK. As is clear from Table 
1, the proportion of the Dy-Al oxide phase existing at the triple points 
of the AlN grains differs depending on the amount of Dy.sub.2 O.sub.3 
added and the sintering temperature employed, but in general, the higher 
the proportion, the better the heat conductivity. 
EXAMPLE 2 
Using 2.5% by weight of a Dy.sub.2 O.sub.3 powder having an average 
particle size of 0.1 .mu.m as a sintering aid and 97.5% by weight of an 
AlN powder having an average particle size of 0.5 .mu.m, an aluminum 
nitride sintered body was obtained under the sintering conditions of 
1,850.degree. C. and 1 hour. This sintered body was heated for 30 minutes 
in the atmosphere at each temperature of 900.degree. C., 950.degree. C., 
1,000.degree. C., 1,100.degree. C., 1,200.degree. C. and 1,300.degree. C. 
to form an oxide layer at the surface of the sintered body. Each of the 
resulting oxide layers had the following average thickness. 
______________________________________ 
Heating temperature (.degree.C.) 
Average thickness (.mu.m) 
______________________________________ 
900 0 
950 0.1 
1000 0.7 
1100 2.1 
1200 5.6 
1300 8.4 
______________________________________ 
The oxide layer formed by heating at 1,300.degree. C., however, had a lot 
of pores. Therefore, it has poor water resistance and is not suitable for 
use in packages of semiconductor devices. 
The structures of oxide layers formed at the surface of the aluminum 
nitride sintered body at 900.degree. C., 1,000.degree. C., 1,100.degree. 
C., 1,200.degree. C. and 1,300.degree. C. were measured according to the 
X-ray diffractometry. The results are shown in FIG. 3, in which 2 
DyAlO.sub.3 and 3 Dy.sub.3 Al.sub.2 (AlO.sub.4).sub.3 are solid solutions 
of Dy and Al oxides. 
As is clear from FIG. 3, when the heating temperature was 900.degree. C., 
there was no formation of a layer consisting mainly of two phases, namely, 
an .alpha.-Al.sub.2 O.sub.3 phase and a phase of a solid solution of Dy 
and Al oxides, but when the heating temperature was 1,000.degree. to 
1,300.degree. C., there was formed an oxide layer consisting mainly of two 
phases, namely, an .alpha.-Al.sub.2 O.sub.3 phase and a phase of a solid 
solution of Dy and Al oxides. 
EXAMPLE 3 
On the aluminum nitride sintered bodies each having an oxide layer obtained 
in Example 2 (heating temperatures: 1,000.degree. C., 1,100.degree. C., 
1,200.degree. C. and 1,300.degree. C.), water resistance was examined in 
accordance with a pressure cooker test. 
Each sintered body was subjected to the conditions of 121.degree. C., 2 atm 
and a humidity of 100% to measure its weight increase due to corrosion. 
The relationship between the weight increase and the elapsed time is shown 
in FIG. 4. FIG. 4 includes, for comparison, the result of the untreated 
sample (the aluminum nitride sintered body having no oxide layer). 
As is clear from FIG. 4, the samples whose heat treatment temperatures were 
1,000.degree. to 1,200.degree. C. had a weight increase of about 0.7 
g/m.sup.2 or below, showing excellent water resistance. Meanwhile, the 
sample whose heat treatment temperature was 1,300.degree. C.-had a large 
weight increase, showing poor water resistance. This is presumed to be 
because the sample of 1,300.degree. C. heat treatment had an oxide layer 
consisting mainly of .alpha.-Al.sub.2 O.sub.3 and a solid solution of 
dysprosium and aluminum oxides but the layer was porous. Incidentally, the 
untreated sample had aluminum nitride exposed at the surface, and 
accordingly its water resistance was not good. 
EXAMPLE 4 
On the surface of a sintered aluminum nitride plate (10 mm.times.10 
mm.times.2 mm) produced under the same conditions as for the sample No. 4 
of Example 1, were formed according to an ion plating method a titanium 
layer having a thickness of 2,000 .ANG. (0.2 .mu.m) and a nickel layer 
having a thickness of 5,000 .ANG. (0.5 .mu.m), in this order. Formed 
thereon was a nickel electroplating layer having a thickness of 4 .mu.m 
and a gold electroplating layer having a thickness of 0.5 .mu.m, in this 
order to obtain a semiconductor substrate. The section of the 
semiconductor substrate is shown in FIG. 5 (a scanning-type electron 
micrograph). In FIG. 5, 1 is an aluminum nitride sintered body layer; 3 is 
a thin Ti layer; 4 is a thin Ni layer; 5 is a Ni plating layer; and 6 is a 
gold plating layer. 
EXAMPLE 5 
A sintered aluminum nitride plate (10 mm.times.10 mm.times.2 mm) produced 
under the same conditions as for the sample No. 5 of Example 1 was kept in 
the atmosphere at 1,100.degree. C. for 30 minutes to form at the surface 
an alumina-based oxide layer (a first layer) having a thickness of 2 
.mu.m. On this first layer were formed, according to an ion plating 
method, a titanium layer (a second layer) having a thickness of 2,000 
.ANG. (0.2 .mu.m) and a nickel layer (a third layer) having a thickness of 
5,000 .ANG. (0.5 .mu.m), in this order. Further, there was formed a nickel 
electroplating layer (a fourth layer) having a thickness of 4 .mu.m and a 
gold electroplating layer (a fifth layer) having a thickness of 0.5 .mu.m 
to complete a semiconductor substrate. The scanning-type electron 
micrograph of the section of the substrate is shown in FIG. 6. In FIG. 6, 
1 is an aluminum nitride sintered body; 2 is a first layer consisting of 
an aluminum-based oxide layer; 3 is a titanium ion plating film (a second 
layer); 4 is a nickel ion plating film (a third layer); 5 is a nickel 
electroplating layer (a fourth layer); and 6 is a gold plating layer (a 
fifth layer). 
Molten solder was placed on the above semiconductor substrate to evaluate 
the substrate's wettability by solder and the corrosion of the aluminum 
nitride sintered body by solder. FIG. 7 is a photograph showing the metal 
structure when solder was placed on the semiconductor substrate. As is 
clear from FIG. 7, Ni in the fourth layer 5 formed an alloy with Sn and Pb 
constituting the solder. That is, the gold plating layer formed to secure 
wettability disapperared entirely and the nickel constituting the fourth 
layer 5 reacted with solder to form an alloy layer. Therefore, it is 
believed that a sufficient bonding strength is secured between the 
substrate and solder. The layers from the first layer 2 to the fourth 
layer 5 had strong adhesion between the adjacent layers and no peeling was 
observed. Tin and lead constituting the solder stayed within the fourth 
layer 5 and did not reach the aluminum nitride sintered body layer 1. 
Therefore, there was no corrosion of the aluminum nitride sintered body 
layer 1 by solder. 
EXAMPLE 6 
Film layers as shown in Table 2 were formed on the sintered aluminum 
nitride plate (the sample No. 6) of Example 1. The alumina-based oxide 
layer (expressed as Al.sub.2 O.sub.3 in Table 2) was formed by heating the 
sintered body at 1,120.degree. C. for 25 minutes in the atmosphere; the 
thin Ti layer and the thin Ni layer were formed according to a sputtering 
method; and the thick Ni layer was formed according to an electroplating 
method. 
To the upper and lower surfaces of each of the thus formed semiconductor 
substrates were bonded aluminum pins via an epoxy resin to evaluate the 
bonding strength of each substrate. The results are shown in Table 2. 
TABLE 2 
______________________________________ 
No. Combination of Thin Layers 
Bonding Strength (kg/mm.sup.2) 
______________________________________ 
1 AlN/Al.sub.2 O.sub.3 
10 or above 
2 AlN/Al.sub.2 O.sub.3 /Ti 
7.5 
3 AlN/Al.sub.2 O.sub.3 /Ti/Ni 
7.4 
4 AlN/Al.sub.2 O.sub.3 /Ti/Ni/Au 
7.5 
5 AlN/Ti/Ni* 5.1 
6 AlN/Ni* 1.3 
______________________________________ 
Note: 
Layer thickness 
Al.sub.2 O.sub.3 : 3 .mu.m 
Ti: 1,000 
Ni: 4,000 
Ni*: 3 .mu.m 
As is clear from Table 2, the semiconductor substrates having film layers 
in the combinations of the present invention had a bonding strength of at 
least 5 kg/mm.sup.2. In particular, the substrates having an alumina-based 
oxide layer as a base layer and a thin Ti layer (and a thin Ni layer) as 
intermediate layer(s) had a high bonding strength of at least 7 
kg/mm.sup.2. In contrast, the substrate No. 6 obtained by forming a Ni 
plating layer directly on sintered aluminum nitride had a low bonding 
strength of 1.3 kg/mm.sup.2. 
EXAMPLE 7 
The sintered aluminum nitride plate (the sample No. 13) of Example 1 was 
heated in the atmosphere at 1,000.degree. C. for 25 minutes to form on 
the plate an alumina-based oxide layer having a thickness of 0.8 .mu.m. On 
the oxide layer was printed a paste having a composition of Mo and Mn in a 
circuit pattern, and baking of the paste was conducted at 1,450.degree. C. 
for 30 minutes. The adhesion strength of the metallizing layer thus formed 
was measured in accordance with a Sebastian method, and the result was 7.8 
kg/mm.sup.2. The adhesion strength was as low as 0.5 kg/mm.sup.2 when the 
metallizing layer was formed directly on the above plate without forming 
the alumina-based oxide layer. 
EXAMPLE 8 
Semiconductor substrates were formed in the same manner as in Example 4 
except that various thin film layers as shown in Table 3 were formed as 
the second layer in place of the Ti ion plating layer. The substrates were 
measured for bonding strength to solder and water resistance (shown by the 
weight increase observed when each substrate was left to stand for 200 
hours under the conditions of 121.degree. C., 2 atm and a humidity of 
100%). The results are shown in Table 3. 
TABLE 3 
______________________________________ 
Properties 
of Substrate 
Second layer Bonding Weight 
Compo- Thickness Method of 
Strength 
Increase 
No. sition (.mu.m) Formation 
(kg/mm.sup.2) 
(g/m.sup.2) 
______________________________________ 
1 Cr 0.2 Ion 7 or above 
1.5 
Plating 
2 Mo 0.2 Ion 7 or above 
1.4 
Plating 
3 W 0.2 Ion 7 or above 
1.4 
Plating 
4 Ti 0.3 Sputtering 
7 or above 
1.7 
______________________________________ 
As described above, an aluminum nitride sintered body of high heat 
conductivity and high strength can be produced stably according to the 
present invention. Further, the semiconductor substrate according to the 
present invention has good water resistance, good adhesion between the 
aluminum nitride sintered body and the metallizing layers formed thereon, 
and improved wettability by solder or a soldering material. 
APPLICATIONS IN INDUSTRY 
Since the aluminum nitride sintered body of the present invention has a 
high heat conductivity and high mechanical strengths, it is suitable for 
use as a semiconductor substrate. In particular, the substrate comprising 
such an aluminum nitride sintered body and an alumina-based oxide layer 
formed at the surface of the sintered body has a high adhesion strength to 
a metal layer formed thereon by applying a metal paste followed by baking, 
and so it is highly suitable as a substrate for hybrid ICs. On the other 
hand, the substrate comprising the aluminum nitride sintered body and a 
thick plating layer formed thereon via a vapor deposition layer has a high 
bonding strength to solder, is dense and highly air-tight, and has a 
smooth surface. Accordingly, it is suitable as a package for large scale 
integrated circuits and very large-scale integrated circuits.