Method for producing ceramic substrate

A ceramic green sheet includes ceramic particles having a spherical diameter of 0.01-0.5 .mu.m of a primary particle as a ceramic component, an average degree of aggregation of 10 or less, wherein the ceramic green sheet contains at most 1 vol % of secondary particles having a diameter of 20 .mu.m or more and has a surface roughness (Ra) of 0.2 .mu.m or less. A method for producing a ceramic substrate having a thickness of 30 .mu.m or less includes the steps of: preparing a ceramic slurry by mixing a ceramic powder with an organic binder and at least one organic solvent, and adjusting a viscosity to be within the range of 100-10,000 mPa's; removing coarse aggregated particles from the ceramic slurry; molding the ceramic slurry into a ceramic green sheet by a reverse roll coater method; and firing the ceramic green sheet so that an average crystal grain size be 2 .mu.m or less.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
The present invention relates to a ceramic green sheet and a method for 
producing a ceramic substrate. The ceramic substrate is thin and hardly 
has surface defects, and it is used as a substrate for an electric 
circuit, a substrate for a multilayer circuit, a diaphragm, an elastic 
plate, etc. 
Japanese Patent Laid-Open 55-134991 discloses a method for producing a thin 
ceramic green sheet or substrate. In the method, a ceramic powder, an 
organic binder, an organic solvent and the like are mixed together so as 
to obtain a ceramic slurry. As shown in FIG. 6, the slurry 10 is subjected 
to filtering by a mesh net 60 in a filter container 50 using a suction 
bottle 70 and a vacuum tube 80 so as to remove aggregates or the like in 
the ceramic slurry and produce a filtered slurry 90. 
There has recently been a demand for a particularly thin ceramic substrate 
or ceramic green sheet having a thickness of, for example, at most 30 
.mu.m. However, suitable properties of ceramic particles constituting such 
a ceramic substrate or a green sheet has been unknown. Upon this, 
attention has been paid to the size of the ceramic particles so as to make 
the particles finer or the like using the conventional technique for 
ceramics. However, it has been difficult to make a product which hardly 
has surface defects, and many products had to be disposed of as defective 
goods. 
A ceramic slurry is prepared by mixing a ceramic powder with an organic 
binder and an organic solvent. Usually, the ceramic slurry inevitably 
contains coarse aggregated particles, dry coagulated grains, reaggregates, 
fine air bubbles, etc, besides impurities. The coarse aggregated particles 
and the like are imminent as defects in a ceramic green sheet and a 
ceramic substrate made from such a slurry. When the ceramic substrate is 
thin, such defects are prone to be actualized as a deterioration in the 
surface smoothness or the strength, occurrence of cracks, a deterioration 
in the dimensional stability, the electric insulation, the airtightness, 
etc. When a reverse roll coater method is employed, molding is conducted 
by a transferring process. Therefore, if the aforementioned coarse 
aggregated particles or the like are present in a ceramic slurry, a 
ceramic green sheet gains fatal defects on the surface. There has been a 
manufacturing problem of improving, particularly, a thin ceramic substrate 
having a thickness of at most 30 .mu.m, which is used as a substrate for 
an electric circuit, a substrate for a multilayer circuit, a diaphragm, an 
elastic plate, etc. 
Japanese Patent Laid-Open 55-134991 disclosed a method for removing coarse 
aggregated particles and the like, in which a net of 100-500 mesh is used 
for filtering a slurry. However, even a net of 500 mesh (25 .mu.m between 
each wire) cannot remove the coarse aggregated particles or the like of 25 
.mu.m or less. To make the finer mesh is very difficult, and it causes 
clogging for a short period of time. Therefore, such a fine mesh was not 
put to practical use at all. Further, there are some other problems in the 
aforementioned method, i.e., because suction filtering is employed, an 
organic solvent in the ceramic slurry is prone to evaporate, which causes 
occurrence of undesirable air bubbles and partially causes occurrence of 
coagulated dry substance. 
SUMMARY OF THE INVENTION 
The object of the present invention is for solving the aforementioned 
problems. 
The first aspect of the present invention provides a ceramic green sheet 
comprising ceramic particles having a spherical diameter of 0.01-0.5 .mu.m 
of a primary particle as a ceramic component, an average degree of 
aggregation of 1.0 or less, wherein said ceramic green sheet contains 1 
vol % or less of secondary particles having a diameter of 20 .mu.m or more 
and has a surface roughness (Ra) of 0.2 .mu.m or less. 
A ceramic green sheet of the present invention may be formed of a material 
having, as a main component of the ceramic particle, a partially 
stabilized zirconia, completely stabilized zirconia, alumina, or a mixture 
thereof, or a material which becomes one of these component after firing. 
The second aspect of the present invention provides a method for producing 
a ceramic substrate having a thickness of 30 .mu.m or less, comprising the 
steps of: preparing a ceramic slurry by mixing a ceramic powder with an 
organic binder and an organic solvent, and adjusting a viscosity to be 
within the range of 100-10,000 mPa's; removing coarse aggregated particles 
from the ceramic slurry; molding the ceramic slurry into a ceramic green 
sheet by a reverse roll coater method; and firing the ceramic green sheet 
so that an average crystal grain size is 2 .mu.m or less. 
Preferably, the ceramic slurry is put under pressure and filtered using a 
depth type filter having an average pore diameter of 100 .mu.m or less so 
as to remove coarse aggregated particles. 
In a suitable step for removing coarse aggregated particles, the ceramic 
slurry is preferably subjected to acceleration of at least 100 m/s.sup.2 
so as to separate coarse aggregated particles. 
The ceramic powder preferably has a spherical diameter of 0.01-0.5 of a 
primary particle and an average degree of aggregation of 10 or less. 
Here, the spherical diameter is a diameter of a sphere obtained by 
converting a value of a specific surface area (m.sup.2 /g) of a ceramic 
particle, and it is expressed by 6/.rho.S (.mu.m). In addition, .rho. 
denotes a theoretical density (g/cm.sup.3) of a ceramic particle, and S 
denotes a specific surface area (m.sup.2 /g) of a ceramic particle 
measured by BET method. 
The aforementioned average degree of aggregation can be obtained by 
dividing a value of the median of the diameter of secondary particles 
measured by a laser scattering method by the aforementioned spherical 
diameter of a primary particle. In this case, a sample to be supplied for 
the measurement of a particle diameter by the laser scattering method was 
obtained by weighing 30 ml of 0.2% solution of sodium hexametaphosphoric 
acid and 50 mg of a ceramic powder, putting them in a container, and 
dispersing them by a homogenizer (ultrasonic oscillator) for one minute. 
Incidentally, as an instrument for the measurement, there was used LA-700, 
which is a commercial product made by HORIBA, LTD. 
A sample for measuring a spherical diameter of a primary particle of a 
ceramic green sheet and a average degree of aggregation was obtained by 
subjecting the ceramic green sheet to heat treatment in which a heating 
temperature was slowly raised and the sheet was heated at the highest 
temperature of 350.degree. C. as shown in the profile in FIG. 5 under an 
oxidized atmosphere, thereby sufficiently decomposing and removing binders 
and the like. Thus, the powdered ceramic was obtained and measured in the 
same manner as described above. A rate of volume of secondary particles 
denotes a rate of total volume of particles each having a particle 
diameter of 20 .mu.m or more, which was measured by a laser scattering 
method. 
A depth type filter in the aforementioned method is desirably a porous 
ceramic filter or porous resin filter. An average pore diameter of the 
filter was measured by mercury press-fitting method. 
Suitable organic binders are (1) poly(vinyl butyral), (2) ethyl cellulose, 
(3) polyester acrylate such as polyethylacrylate, polybutylacrylate, and 
the like, (4) polyester methacrylate such as polymethylmethacrylate, 
polybutylmethacrylate, and the like. However, any organic binder which is 
soluble in an organic solvent can be used. 
The viscosity was measured using a LVT type of viscometer manufactured by 
Brook Field, Ltd. The conditions of the measurement were No. 3 rotor, the 
number of rotation of 12 rpm, a temperature of the sample ceramic slurry 
of 25.degree. C. 
Incidentally, a primary particle means an individual ceramic particle, and 
a secondary particle means a granule of aggregated primary particles.

DETAILED DESCRIPTION OF THE INVENTION 
A ceramic green sheet contains ceramic particles, as a ceramic component, 
having a spherical diameter of 0.01-0.5 .mu.m of a primary particle and an 
average deuce of aggregation of 10 or less. By specifying the spherical 
diameter of a primary particle and an average degree of aggregation to be 
in certain ranges, the ceramic green sheet becomes suitable to produce a 
ceramic substrate having a thin thickness. 
A spherical diameter of the primary particle is preferably within the range 
of 0.05-0.3 .mu.m, more preferably 0.1-0.2 .mu.m. When the spherical 
diameter is too large, it makes higher a firing temperature for producing 
a ceramic substrate or causes deterioration of a surface smoothness or 
strength of a ceramic substrate because crystal grains coarsely grow. When 
the spherical diameter is too small, mutual aggregating force among 
ceramic particles becomes strong and ceramic particles hardly disperse 
evenly. Thus, a ceramic green sheet having an even thickness cannot be 
obtained since a slurry suitable for molding cannot be obtained. Further, 
a coarse aggregated particle is prone to be formed, and drying shrinkage 
becomes uneven when a sheet is formed of a slurry, which is prone to cause 
a deformation and cracks. In addition, the dimensional stability after 
firing deteriorates because a coarse aggregated particle has a large 
difference in firing shrinkage rate in comparison with the other portion. 
The present invention is characterized in that an average degree of 
aggregation of ceramic particles is controlled. The average degree of 
aggregation is 10 or less, preferably 7 or less, more preferably 5 or 
less. When the average degree of aggregation is too high, a ceramic green 
sheet has an uneven thickness, deformation, cracks. Further, dimensional 
stability of a ceramic substrate deteriorates and a surface roughness 
becomes coarse when a ceramic substrate is produced. Thus, a material 
having such a condition is not suitable for producing a ceramic substrate 
having a thin thickness. 
Further, a particle having a high average degree of aggregation is prone to 
cause not only a defect on the surface of a ceramic substrate but also a 
defect such as a scratch. Therefore, in the present invention, secondary 
particles each having a particle diameter of 20 .mu.m or more should be 1 
vol % or less. Desirably, a secondary particle having a particle diameter 
of preferably 10 .mu.m or more, more preferably 5 .mu.m or more, is 1 vol 
% or less. 
A ceramic particle of the present invention may be a partially stabilized 
zirconia, a completely stabilized zirconia, alumina, spinel, mullite, 
silicon nitride, aluminum nitride, silicon carbide, titania, beryllia, or 
a material having a mixture of these as a main component. Particularly, a 
partially stabilized zirconia, a completely stabilized zirconia, alumina, 
or a material having a mixture of these as a main component is desirable. 
Among them, a material having a partially stabilized zirconia as a main 
component is more preferable. The zirconia is partially stabilized by 
adding a compound such as yttrium oxide to zirconia to make a tetragonal 
crystal or a mixed crystal mainly consisting of at least two crystal 
phases selected from cubic crystal, tetragonal crystal, and monoclinic 
crystal. Here, a partially stabilized zirconia means a zirconium oxide 
(zirconia) which crystal phase is partially stabilized so that crystal 
phase is transformed only partially when subjected to heat or stress. 
Incidentally, the aforementioned material may contain a sintering aid up 
to 30 wt %, for example, clay, silica, magnesia, a transition metal oxide, 
or the like. 
Further, it is also necessary that a ceramic green sheet of the present 
invention has a surface roughness Ra of 0.2 .mu.m or less, preferably 0.15 
.mu.m or less, more preferably 0.10 .mu.m or less. The surface roughness 
Ra was measured by a surface roughness gauge of Surfcom 470A manufactured 
by Tokyo Seimitsu Co., Ltd. Ra is an average of values of center line mean 
roughness obtained at 3 points under the conditions that a configuration 
of a tip diamond is conical of 5 .mu.mR 90.degree., a cut off value is 
0.80 mm, a tracing speed is 0.30 mm/s, and a length of measurement is 2.5 
mm. 
In the method for producing a ceramic substrate according to the present 
invention, the aforementioned ceramic powder is mixed with an organic 
binder and an organic solvent so as to prepare a ceramic slurry. The 
ceramic slurry has desirably a viscosity of 100-10000 mPa's, more 
preferably 500-2000 mPa's. The reason is that too high viscosity makes the 
removal of coarse aggregated particles difficult in the next step. For 
example, a high viscosity is prone to cause clogging when the slurry is 
filtered by a depth type filter, and a filtering efficiency decreases. In 
addition, in a step of separating coarse aggregated particles by 
acceleration, a moving speed of a coarse aggregated particle decreases, 
and therefore, an efficiency in separating coarse aggregated particles 
decreases. On the other hand, when the viscosity is too low, it becomes 
difficult to form a green sheet, and evenness of the ceramic slurry 
deteriorates. 
Filters can be classified into two kinds of a surface type and a depth type 
as shown in FIG. 3 depending on the structure of the media. A filter of a 
surface type is represented by a screen mesh constituted by stainless 
wires or the like and has a short media through which a slurry passes. 
This type of filter determines the size of particles which can be removed 
depending on a distance of each wire. Filtering small things having a 
small particle diameter as in the technical field of the present invention 
is prone to cause clogging, and the filter cannot be put to practical use. 
On the other hand, filtering by a filter of a depth type is called depth 
filtering or volume filtering. The filter has a thickness to some degree. 
A suitable embodiment for the method of the present invention uses the 
filter of a depth type so as to remove coarse aggregated particles. The 
embodiment is hereinbelow described with reference to FIGS. 1 and 2. 
A ceramic slurry which viscosity is adjusted is pressurized so as to be 
filtered in order to remove coarse aggregated particles in the ceramic 
slurry according to the flow shown in FIG. 1 by a filter 1 of a depth type 
of, for example, a porous ceramic filter or a porous resin filter. The 
filter apparatus shown in FIG. 1 includes a chamber 2, a slurry supply 
inlet 3, an outlet 4 for the separated slurry and the filter 1. The filter 
1 has an average pore diameter of 100 .mu.m or less, desirably 50 .mu.m or 
less, more desirably 30 .mu.m or less. The filter 1 for the present 
invention desirably has a thickness (d) of 5 mm or more. However, the 
thickness of the filter may be determined depending on the size of coarse 
particles to be filtered, the strength of the filter itself, etc. The 
pressure applied to the slurry is desirably within the range of 0.1-5.0 
Kg/cm.sup.2. If the pressure is too low, filtering of the ceramic slurry 
takes too much time. If the pressure is too high, pores of a filter are 
prone to be enlarged, the filter is prone to have breakage, or a particle 
having a larger diameter than a predetermined diameter is prone to pass 
through the filter. The ceramic slurry may be subjected to removal of 
bubbles under reduced pressure before and after filtering. 
In an embodiment of a method of the present invention, a depth type filter 
is used because it was found that the filter hardly caused clogging 
regardless of ability to remove rather smaller particles than a particle 
having an average pore diameter and gave high filtering efficiency. The 
reason why the filter hardly has clogging seems that pores of the 
filtering layer are complexly connected with each other between the front 
surface and the back surface, and a layer of slurry which is comparatively 
soft and which adsorbs inside pores blocks the flow of coarse aggregated 
particles. 
As a step of removing coarse aggregated particles of the present invention, 
there is recommended another embodiment i.e., a step of separating coarse 
particles by applying an acceleration of at least 100 m/s.sup.2 to the 
ceramic slurry. For example, an acceleration of 100 m/s.sup.2 is applied 
by centrifugation treatment as shown in FIG. 4. In FIG. 4, a slurry 10 
containing aggregated particles 20 and bubbles 30 is subject to 
centrifugation in container 5. The resultant slurry sediment is designated 
40 in FIG. 4. By subjecting a ceramic slurry put in a container to 
centrifugation treatment, aggregates or coarse aggregated particles such 
as a dried substance in the ceramic slurry sediment, adhere, and can be 
separated. In this case, the acceleration may be 100 m/s.sup.2 or more, 
desirably 1000 m/s.sup.2 or more, more desirably 5000 m/s.sup.2 or more. 
Further, centrifugation method can separate coarse aggregated particles 
regardless of an average degree of aggregation, while filtering using 
ceramic particles having an average aggregated degree of 10 or more 
sometimes causes clogging because of coarse aggregated particles and has 
difficulty in separating such particles. Incidentally, a ceramic powder to 
be used preferably has a spherical diameter of 0.01-0.5 .mu.m of a primary 
particle, and an average degree of aggregation of 10 or less because a 
proportion of ceramic particles to a composition of organic compound 
changes when a rate of sedimentation and adhesion increases. 
Then, in a step of molding a ceramic slurry into a ceramic green sheet of 
the present invention, a reverse roll coater method is employed. A doctor 
blade method, which is widely used as a method for molding a sheet, does 
not provide a thin green sheet having an even thickness. A trace of being 
scratched off remains on the surface as vertical lines, which cause 
deterioration of the strength or the surface smoothness. Therefore, a 
molding by a reverse roll coater method is necessarily employed so as to 
obtain a sheet scarcely having surface defects. 
Then, the aforementioned ceramic green sheet may be laminated with other 
green sheets and fired simultaneously or independently fired depending on 
use under conditions suitable for removing a binder, sintering, or the 
like as similar to the conventional method. In this case, it is necessary 
to fire the material so as to obtain a sintered body having an average 
crystal grain size of 2 .mu.m or less of a ceramic component. In the range 
in which an average crystal grain size is 2 .mu.m or less, a surface 
roughness Ra is preferably 0.30 .mu.m or less, more preferably 0.20 .mu.m 
or less. It is because a method for producing a ceramic substrate having 
thin thickness as in the present invention needs to be able to produce a 
ceramic substrate scarcely having surface defects and being excellent in 
surface smoothness. lncidentally, after a cross-section of the ceramic 
substrate was subjected to abrasion and mirror plane finishing, a grain 
boundary was exhibited by thermal etching according to a known method so 
as to calculate an average crystal grain size D(.mu.m)=.sqroot.(4S/.pi.n) 
by measuring for the number n of grains and the area S(.mu.m.sup.2) by an 
electron microscope. 
The aforementioned surface roughness Ra was obtained by the surface 
roughness gauge used for the measurement for the green sheet, and Ra is an 
average of values of center line mean roughness at 3 points under the 
conditions that a configuration of a tip diamond is conical of 5 .mu.mR 
90.degree., a cut off value is 0.80 mm, a tracing speed is 0.30 mm/s, and 
a length of measurement is 2.5 mm. 
In the present invention, as an organic binder and an organic solvent, 
which are used, and a plasticizer and a dispersant, which may be used for 
improving properties of a ceramic green sheet, the following binders, 
etc., are recommended. Suitable organic binders are poly(vinyl butyral), 
polyester methacrylate, ethyl cellulose, etc. However, any binder which is 
soluble in an organic solvent can be used. At least one organic solvent 
may be an alcohol such as methyl alcohol, ethyl alcohol, isopropyl 
alcohol, n-butyl alcohol, or the like, an aromatic hydrocarbon such as 
benzene, toluene, xylene, or the like, a ketone, such as methyl ethyl 
ketone, methyl isobutyl ketone, acetone, a general organic solvent such as 
trichloroethylene, tetrachloroethylene, or the like, or a mixed solvent 
thereof. A plasticizer may be a general plasticizer such as an ester 
phthalate, an ester sebacate, and an ethylene glycol. A dispersant may be 
a general dispersant such as a sorbitic fatty acid ester or a surface 
active agent. 
A ceramic substrate of the present invention has a relative density of 
generally 90% or more, preferably 95% or more, more preferably 98% or more 
in view of material properties such as strength, Young's modulus, or the 
like. 
A ceramic green sheet and a method for producing a ceramic substrate 
according to the present invention can be suitably applied to (1) a 
multilayer ceramic substrate, (2) an IC substrate, (3) a solid electrolyte 
for a fuel cell, (4) a diaphragm plate or an elastic plate such as various 
kinds of sensors, an actuator, a transmitter, an oscillator, a display, a 
microphone, a speaker, filter, and the like. Means of application for the 
aforementioned use are not only independently using the ceramic green 
sheet but also printing of the ceramic green sheet and laminating the 
sheets to be multilayered. When used as a diaphragm plate or an elastic 
plate, a ceramic green sheet is laminated on another ceramic green sheet, 
which is used as a substrate, so as to form a passive element, or an 
active clement. 
EXAMPLES 
Then, the present invention is hereinbelow described in more detail with 
reference to Examples. 
Example 1 
There was prepared a ceramic powder having the spherical diameter and the 
average deuce of aggregation shown in Tables 1 and 2. A partially 
stabilized zirconia of 100 weight parts containing yttrium oxide of 3 mol 
% as a ceramic powder was mixed with dioctyl phthalate of 4.5 weight parts 
as a plasticizer and sorbitic fatty acid ester of 2 weight parts as a 
dispersant, and toluene of 20 weight parts and isopropyl alcohol of 30 
weight parts as solvents in an alumina pot together with zirconia balls 
for 5 hours so as to be subjected to ball mill mixing. Then, there were 
added as an organic binder solution poly(vinyl butyral) resin of 9 weight 
parts and, toluene of 10 weight parts, isopropyl alcohol of 10 weight 
parts to the mixture in the alumina pot so as to being subjected to ball 
mill mixing for 30 hours. At this time, a viscosity of the ceramic slurry 
was adjusted to be 1500 mPa's. The slurry was filtered by a resin 
cylindrical filter 8 as shown in FIG. 2 (thickness d of 20 mm, outer 
diameter D.sub.o of 70 mm, inner diameter D.sub.i of 30 mm, width L of 
filter of 250 mm) under a pressure of 1 Kg/cm.sup.2. The thus obtained 
ceramic slurry was subjected to forming using a reverse roll coater so as 
to obtain a ceramic green sheet having a thickness of about 10 .mu.m. FIG. 
1 shows a typical cross-sectional view of a filtering portion of a 
cylindrical depth filter used here. Incidentally, the ceramic powder was 
obtained by calcining, grinding by ball mill, and heat treating a material 
obtained by coprecipitation method. A spherical diameter of a primary 
particle can be made larger as the calcining temperature becomes higher, 
and varied depending on conditions in the aforementioned coprecipitating 
conditions, for example, pH, temperature, concentration, etc. Further, the 
spherical diameter can be varied depending on conditions in the ball mill 
grinding step and in the step of heat treatment. An average degree of 
aggregation can be varied depending on treating conditions in the 
aforementioned ball mill grinding step or in the heat treating step. The 
thus obtained ceramic green sheet was fired at a predetermined temperature 
in the air so as to obtain a ceramic substrate having a thickness of 7 
.mu.m. 
A surface roughness Ra of the ceramic green sheet was measured. Further, 
the ceramic green sheet was calcined at 350.degree. C. so as to obtain a 
ceramic powder. A spherical diameter, an average degree of aggregation, 
and a volume rate of a secondary powder (.gtoreq.20 .mu.m)of the obtained 
ceramic powder were measured. The results are shown in Table 1. 
The number of defects larger than 10 .mu.m which were present on the 
surface of the ceramic substrate after firing (observed by a microscope), 
a surface roughness Ra of the ceramic substrate, and an average grain size 
are shown in Table 2. 
TABLE 1 
__________________________________________________________________________ 
Example 1 (1) 
Powder obtained by calcining 
a green sheet at 350.degree. C. 
Ceramic powder Ra Volume rate of 
Average 
of Average 
secondary particles 
Spherical 
degree 
green 
Spherical 
degree 
each having 
Sample 
diameter 
of sheet 
diameter 
of a diameter of 20 .mu.m 
No. (.mu.m) 
aggregation 
(.mu.m) 
(.mu.m) 
aggregation 
or more (%) 
__________________________________________________________________________ 
1 0.01 10 0.15 
0.01 10 0.9 
2 0.05 7 0.10 
0.05 7 0.0 
3 0.15 
4 10 0.16 10 
5 0.15 20 Slurry did not pass through. 
6 3 0.08 
0.15 3 0.0 
8 0.10 
9 0.30 5 0.30 5 
10 0.50 0.15 
0.50 
__________________________________________________________________________ 
*Comparative example 
TABLE 2 
__________________________________________________________________________ 
Example 1 (2) 
Treatment: Ceramic substrate 
Average 
Average 
Firing 
Number Average 
Spherical 
degree 
pore Temperature 
of Crystal grain 
Sample 
diameter 
of diameter 
(.degree.C.) .times. 
defects 
Ra size 
No. (.mu.m) 
aggregation 
(.mu.m) 
2 hrs (cm.sup.-2) 
(.mu.m) 
(.mu.m) 
__________________________________________________________________________ 
1 0.01 10 30 1350 40 0.20 
0.1 
2 0.05 7 1400 20 0.16 
0.3 
3 0.15 7 1450 0.18 
0.4 
4 10 30 0.26 
5 20 Slurry did not pass through. 
6 3 20 1450 3 0.20 
0.4 
7 30 6 
8 70 50 0.22 
9 0.30 5 30 1550 15 0.24 
0.9 
10 0.50 1650 30 0.30 
1.5 
__________________________________________________________________________ 
*Comparative Example 
Example 2 
A partially stabilized zirconia powder of 100 weight parts containing 
yttrium oxide of 3 mol % as a ceramic material was mixed with a dioctyl 
phthalate of 4.5 weight parts as a plasticizer, and sorbitic fatty acid 
ester of 2 weight parts as a dispersant, toluene of 20 weight parts, and 
isopropyl alcohol of 30 weight parts as solvents in an alumina pot with 
zirconia balls for 5 hours. Then, poly(vinyl butyral) resin of 9 weight 
parts, toluene of 1.0 weight parts, and isopropyl alcohol of 10 weight 
parts were added to the aforementioned mixture in the alumina pot so as to 
subject the mixture to ball mill mixing for 30 hours. At this time, the 
viscosity of the ceramic slurry was adjusted to be 1500 mPa's. Then, the 
slurry was put in a glass container with a lid and subjected to a 
treatment using a centrifugal separator for 30 minutes. Then, a ceramic 
green sheet having a thickness of about 10 .mu.m was formed by a reverse 
roll coater method. The thus obtained ceramic green sheet was fired at a 
predetermined temperature in the air so as to obtain a ceramic substrate 
having a thickness of 7 .mu.m. The results are shown in Tables 3 and 4. 
The method for evaluation was the same as Example 1. 
TABLE 3 
__________________________________________________________________________ 
Example 2 (1) 
Powder obtained by calcining 
green sheet at 350.degree. C. 
Ceramic powder Ra Volume rate of 
Average 
of Average 
secondary particles 
Spherical 
degree 
green 
Spherical 
degree 
having a diameter 
Sample 
diameter 
of sheet 
diameter 
of of 20 .mu.m or more 
No. (.mu.m) 
aggregation 
(.mu.m) 
(.mu.m) 
aggregation 
(%) 
__________________________________________________________________________ 
11 0.01 10 0.14 
0.01 10 0.6 
12 0.05 7 0.10 
0.05 7 0.0 
13 0.15 20 0.15 
0.15 20 0.8 
14 10 0.14 10 0.0 
15 3 0.05 3 
16 0.06 
17 0.08 
18 0.10 
19 0.30 5 0.30 5 
20 0.50 0.15 
0.50 
__________________________________________________________________________ 
TABLE 4 
__________________________________________________________________________ 
Example 2 (2) 
Treatment: 
Acceleration 
Ceramic substrate 
Average 
of Firing 
Number Average 
Spherical 
degree 
centrifugal 
Temperature 
of Crystal grain 
Sample 
diameter 
of separation 
(.degree.C.) .times. 
defects 
Ra size 
No. (.mu.m) 
aggregation 
(m/s.sup.2) 
2 hrs (cm.sup.-2) 
(.mu.m) 
(.mu.m) 
__________________________________________________________________________ 
11 0.01 10 6000 1350 20 0.20 
0.1 
12 0.05 7 1400 10 0.18 
0.3 
13 20 40 0.22 
0.4 
14 0.15 10 1450 15 0.20 
15 3 20000 0 0.15 
16 6000 1 0.18 
17 1000 6 0.20 
18 100 50 
19 0.30 5 6000 1550 8 0.25 
0.9 
20 0.50 1650 20 0.30 
1.5 
__________________________________________________________________________ 
Comparative Example 1 
A partially stabilized zirconia powder of 100 weight parts containing 
yttrium oxide of 3 mol % as a ceramic material was mixed with dioctyl 
phthalate of 4.5 weight parts as a plasticizer, and sorbitic fatty acid 
ester as a main component of 2 weight parts as a dispersant, toluene of 20 
weight parts, and isopropyl alcohol of 30 weight parts as solvents in an 
alumina pot with zirconia bails for 5 hours. Then, poly(vinyl butyral) 
resin of 9 weight parts, toluene of 10 weight parts, and isopropyl alcohol 
of 10 weight parts were added to the aforementioned mixture in the alumina 
pot so as to subject the mixture to ball mill mixing for 30 hours. At this 
time, a viscosity of the ceramic slurry was adjusted to be 1500 mPa's. 
Then, the slurry was filtered by a stainless screen of 325 mesh (distance 
between each wire is 44 .mu.m). A ceramic green sheet having a thickness 
of about 10 .mu.m was formed by a reverse roll coater. The thus obtained 
ceramic green sheet was fired at a predetermined temperature in the air so 
as to obtain a ceramic substrate having a thickness of 7 .mu.m. The 
results are shown in Tables 5 and 6. A method for the evaluation was the 
same as Example 1. 
TABLE 5 
__________________________________________________________________________ 
Comparative Example 1 (1) 
Powder obtained by calcining 
green sheet at 350.degree. C. 
Ceramic powder Ra Volume rate of 
Average 
of Average 
secondary particles 
Spherical 
degree 
green 
Spherical 
degree 
having a diameter 
Sample 
diameter 
of sheet 
diameter 
of of 20 .mu.m or more 
No. (.mu.m) 
aggregation 
(.mu.m) 
(.mu.m) 
aggregation 
(%) 
__________________________________________________________________________ 
27 0.01 10 0.55 
0.01 10 3.0 
28 0.05 7 0.40 
0.05 7 1.5 
29 0.15 10 0.50 
0.15 10 2.2 
30 20 1.50 20 4.5 
__________________________________________________________________________ 
TABLE 6 
__________________________________________________________________________ 
Comparative Example 1 (2) 
Ceramic substrate 
Average Firing 
Number Average 
Spherical 
degree Temperature 
of crystal grain 
Sample 
diameter 
of Treatment: 
(.degree.C.) .times. 
defects 
Ra size 
No. (.mu.m) 
aggregation 
Screen mesh 
2 hrs (cm.sup.-2) 
(.mu.m) 
(.mu.m) 
__________________________________________________________________________ 
27 0.01 10 325 1350 900 0.70 
0.1 
28 0.05 7 mesh 1400 200 0.55 
0.3 
29 0.15 10 1450 500 0.85 
0.4 
30 20 &gt;1000 
1.80 
__________________________________________________________________________ 
Example 3 
A ceramic green sheet and a ceramic substrate were prepared in the same 
manner as in Example 1 except for the following conditions. Incidentally, 
an average pore diameter of a resin cylindrical filter was 30 .mu.m. Three 
kinds of ceramic powders, i.e., (1) a completely stabilized zirconia 
powder (Sample No. 23 in Tables 7 and 8), (2) alumina powder (Sample No. 
21), and (3) a partially stabilized zirconia powder of 90 weight parts and 
alumina powder of 10 weight parts (Sample No. 22) were used. The results 
are shown in Tables 7 and 8. A method for the evaluation was the same as 
Example 1. 
TABLE 7 
__________________________________________________________________________ 
Example 3 (1) 
Powder obtained by calcining 
green sheet at 350.degree. C. 
Ceramic powder Ra Volume rate of 
Average 
of Average 
secondary particles 
Spherical 
degree 
green 
Spherical 
degree 
having a diameter 
Sample 
diameter 
of sheet 
diameter 
of of 20 .mu.m or more 
No. (.mu.m) 
aggregation 
(.mu.m) 
(.mu.m) 
aggregation 
(%) 
__________________________________________________________________________ 
21 0.10 2 0.06 
0.10 2 0.0 
22 0.14 3 0.08 
0.14 3 
23 0.20 5 0.10 
0.20 5 
__________________________________________________________________________ 
TABLE 8 
__________________________________________________________________________ 
Example 3 (2) 
Treatment: Ceramic substrate 
Average 
Firing 
Number Average 
Spherical 
Degree 
pore Temperature 
of crystal grain 
Sample 
diameter 
of diameter 
(.degree.C.) .times. 
defects 
Ra size 
No. (.mu.m) 
aggregation 
(.mu.m) 
2 hrs (cm.sup.-2) 
(.mu.m) 
(.mu.m) 
__________________________________________________________________________ 
21 0.10 2 30 1400 2 0.16 
0.3 
22 0.14 3 5 0.20 
0.4 
23 0.20 5 1450 10 0.5 
__________________________________________________________________________ 
Example 4 
A ceramic green sheet and a ceramic substrate were prepared in the same 
manner as in Example 2 except for the following conditions. Incidentally, 
an acceleration by centrifugal separation was 6000 m/s.sup.2. Three kinds 
of ceramic powders, i.e., (1) a completely stabilized zirconia powder 
(Sample No. 26 in Tables 9 and 10), (2) alumina powder (Sample No. 24), 
and (3) a partially stabilized zirconia powder of 90 weight parts and 
alumina powder of 10 weight parts (Sample No. 25) were used. The results 
are shown in Tables 9 and 10. A method for the evaluation was the same as 
Example 1. 
TABLE 9 
__________________________________________________________________________ 
Example 4 (1) 
Powder obtained by calcining 
green sheet at 350.degree. C. 
Ceramic powder Ra Volume rate of 
Average 
of Average 
secondary particles 
Spherical 
degree 
green 
Spherical 
degree 
having a diameter 
Sample 
diameter 
of sheet 
diameter 
of of 20 .mu.m or more 
No. (.mu.m) 
aggregation 
(.mu.m) 
(.mu.m) 
aggregation 
(%) 
__________________________________________________________________________ 
24 0.10 2 0.06 
0.10 2 0.0 
25 0.14 3 0.08 
0.14 3 
26 0.20 5 0.20 5 
__________________________________________________________________________ 
TABLE 10 
__________________________________________________________________________ 
Example 4 (2) 
Treatment: 
Acceleration 
Ceramic substrate 
Average 
of Firing 
Number Average 
Spherical 
degree 
centrifugal 
Temperature 
of crystal grain 
Sample 
diameter 
of separation 
(.degree.C.) .times. 
defects 
Ra size 
No. (.mu.m) 
aggregation 
(m/s.sup.2) 
2 hrs (cm.sup.-2) 
(.mu.m) 
(.mu.m) 
__________________________________________________________________________ 
24 0.10 2 6000 1400 0 0.17 
0.3 
25 0.14 3 1 0.20 
0.4 
26 0.20 5 1500 6 0.5 
__________________________________________________________________________ 
As described above, the first aspect of the present invention provides a 
ceramic green sheet which hardly has surface defects, has high surface 
smoothness, and has a thin thickness by controlling a spherical diameter 
and an average degree of aggregation of ceramic particles. 
The second aspect of the present invention provides a method for producing 
a ceramic substrate. In the method, a spherical diameter and an average 
degree of aggregation of ceramic particles are controlled, and coarse 
aggregated particles are removed in a ceramic slurry, which gives a thin 
ceramic substrate scarcely having surface defects and having surface 
smoothness.