U.S. patent application number 09/835406 was filed with the patent office on 2001-10-04 for glass-ceramic wiring board.
Invention is credited to Ami, Norihiro, Horikoshi, Mutsumi, Ishihara, Shosaku, Okamoto, Masahide, Tanaka, Minoru, Yasuda, Akihiro.
Application Number | 20010025722 09/835406 |
Document ID | / |
Family ID | 12597357 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010025722 |
Kind Code |
A1 |
Ami, Norihiro ; et
al. |
October 4, 2001 |
Glass-ceramic wiring board
Abstract
In a ceramic wiring board which comprises a copper via, breaks
and defects of via interconnections resulting from enlargement of
copper particles in the interior of the via during sintering, are
prevented. For this purpose, alumina of mean particle diameter from
1 .mu.m to 4 .mu.m is disposed in the interior of the via
interconnection after sintering at an average interval of 7.4 .mu.m
or less.
Inventors: |
Ami, Norihiro; (Yokohama,
JP) ; Okamoto, Masahide; (Yokohama, JP) ;
Ishihara, Shosaku; (Chigasaki, JP) ; Tanaka,
Minoru; (Yokohama, JP) ; Horikoshi, Mutsumi;
(Machida, JP) ; Yasuda, Akihiro; (Hadano,
JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
12597357 |
Appl. No.: |
09/835406 |
Filed: |
April 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09835406 |
Apr 17, 2001 |
|
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|
09501683 |
Feb 10, 2000 |
|
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6248960 |
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Current U.S.
Class: |
174/257 ;
174/264; 257/E23.067; 257/E23.075; 428/209; 428/210; 428/323 |
Current CPC
Class: |
H01L 23/49883 20130101;
Y10T 428/24917 20150115; H01L 2924/00 20130101; H01R 12/523
20130101; H01L 23/49827 20130101; Y10S 428/901 20130101; Y10T
428/25 20150115; Y10T 428/24926 20150115; Y10T 29/49117 20150115;
H01L 2924/09701 20130101; H01L 2924/0002 20130101; Y10T 428/1314
20150115; Y10T 29/49163 20150115; Y10T 29/49165 20150115; H05K
1/092 20130101; H01L 2924/0002 20130101; Y10T 29/49155 20150115;
H05K 3/4629 20130101 |
Class at
Publication: |
174/257 ;
428/209; 428/210; 174/264; 428/323 |
International
Class: |
H05K 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 19, 1999 |
JP |
11-041042 |
Claims
What is claimed is:
1. A glass-ceramic wiring board, comprising: an insulating
substrate; a via disposed in said insulating substrate; and a via
interconnection filling the interior of said via, wherein said via
interconnection is sintered material having metal particles, and
has a cross-sectional area per one metal particle surrounded by a
metal particle boundary, appearing when a cross-section of said via
is etched, less than 2000 .mu.m.sup.2.
2. A glass-ceramic wiring board according to claim 1, wherein said
via interconnection filling said via is comprised of copper.
3. A glass-ceramic wiring board according to claim 1, wherein the
metal particles are copper particles.
4. A glass-ceramic wiring board, comprising: an insulating
substrate; a via disposed in said insulating substrate; and a via
interconnection filling the interior of said via, wherein said via
interconnection is a material having metal particles, having been
sintered at a temperature of at least 900.degree. C. to at most
1060.degree. C., and has a cross-sectional area per one metal
particle surrounded by a metal particle boundary, appearing when a
cross-section of said via is etched, less than 2000
.mu.m.sup.2.
5. A glass-ceramic wiring board according to claim 4, wherein said
via interconnection filling said via is comprised of copper.
6. A glass-ceramic wiring board according to claim 4, wherein the
metal particles are copper particles.
7. A glass-ceramic wiring board, comprising: an insulating
substrate; a via disposed in said insulating substrate; and a via
interconnection filling the interior of said via, wherein said via
interconnection is a material, having metal particles, sintered at
a temperature of at least 900.degree. C. to at most 1060.degree.
C., includes a metal oxide particle, and has a cross-sectional area
per one metal particle surrounded by a metal particle boundary,
appearing when a cross-section of said via is etched, equal to or
less than 2000 .mu.m.sup.2.
8. A glass-ceramic wiring board according to claim 7, wherein said
via interconnection filling said via is comprised of copper, and
said metal oxide is comprised of alumina.
Description
[0001] This application is a Divisional application of application
Ser. No. 09/501,683, filed Feb. 10, 2000.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a ceramic multilayer wiring board,
in particular a ceramic wiring board using copper as a via
interconnection, to a ceramic multilayer wiring board having a
suitable post-sintering via microstructure, and to a copper paste
for obtaining this microstructure.
[0003] Ceramic wiring boards having a multilayer structure are used
in electrical devices where modular wiring substrates are required
for high integration and high-speed processing, due to the need for
making fine interconnections. Copper is the material of choice for
these interconnections due to its low specific resistance.
[0004] As substrate used as a support for interconnections, an
inorganic material having glass as its principal component is used
as the glass can be sintered at the same time as the copper of the
interconnections. A borosilicate glass suitable for substrates is
described in detail in Japanese Patent Laid-Open No. Hei 8-333157.
Fillers which may be added are disclosed in Japanese Patent
Laid-Open No. Hei 8-18232.
[0005] Here, the method of manufacturing the substrate will be
briefly described.
[0006] Generally, the inorganic material is supplied in the form
substrate is manufactured by the well-known green sheet method.
This method consists of the following steps.
[0007] (1) Making a slurry of the powdered inorganic material using
an organic binder and a solvent.
[0008] (2) Forming this slurry into the shape of a sheet.
[0009] (3) Opening vias (through holes) in the sheet.
[0010] (4) Embedding an interconnection paste in the vias.
[0011] (5) Forming an interconnection or other pattern on the sheet
surface with the interconnection paste.
[0012] (6) Laminating these sheets with interconnection patterns
together under pressure.
[0013] (7) Heat treating the resulting laminate.
[0014] In the above-mentioned heat treatment process, the organic
binder in the laminate and the organic substance in the
interconnection paste decompose and are thus eliminated. At the
same time, the inorganic material in the laminate which is in a
powdered state of aggregation and the conducting metal in the
conducting paste are sintered and become finer.
[0015] However, if the organic binder remains in the sintered
compact, it will be converted to graphite, and the quality of the
substrate and wiring after sintering will deteriorate. For this
reason, sufficient binder removal time is generally allowed in the
sintering step, followed by a sintering period which has the main
purpose of increasing the fineness.
[0016] This classical type of heat treatment profile is disclosed
in Japanese Patent Laid-Open No. Hei 8-18232, etc. The binder is
removed in an atmosphere at about 800.degree. C. for 15 hours, and
the product is kept in an atmosphere at about 1000.degree. C. for 2
hours for sintering. Water vapor etc. is usually added to the
processing atmosphere during the above-mentioned binder
removal.
[0017] However, when copper is used for the metallic material of
the conductor, although sintering of the copper takes place
starting from approximately 600.degree. C., sintering of the glass
ceramics itself begins at a higher temperature. This difference of
sintering start times may causes serious problems in the substrate,
particularly in the conductor or at the interface between the
conductor and the ceramics, so in the case of copper paste, an
attempt is often made to adjust the sintering start temperature of
the ceramics.
[0018] As an example, a copper paste mixed with alumina of particle
size 0.1 .mu.m to 1 .mu.m is disclosed by Japanese Patent
Publication 2584911. Also, a copper paste comprising copper oxide
and glass frit is disclosed by Japanese Patent Laid-Open No. Hei
8-279666.
SUMMARY OF THE INVENTION
[0019] In producing a multilayer wiring board using the
above-mentioned green sheet method, in the case of an alumina and
copper mixture, it is difficult to disperse fine alumina of
particle size less than 1 .mu.m in the copper paste. For this
reason, it is difficult to obtain desired paste printing properties
required for processes such as embedding interconnection paste in
vias, or forming interconnections or other patterns.
[0020] Moreover, copper oxide tends to discharge copper ions in the
glass, and may produce a fine copper colloid in the ceramics
depending on the firing conditions. This causes deterioration of
the insulating properties of the ceramics, and decreased
hardness.
[0021] On the other hand, as the microinterconnections are formed
and via diameters reach about 50 .mu.m, a new problem may arise in
addition to the above-mentioned difference of sintering start
temperature. Specifically, if copper particles grow very large
during their growth when the substrate is fired, they will reach a
size of the same order as that of the via diameter. As a result,
after sintering, vias will be formed of several enlarged copper
particles, particle interfaces will break down due to the effect of
subsequent heat cycles, and breaks will tend to occur in the via
interconnections. Moreover, there is also the disadvantage that via
interconnections may fall out of the via holes of the ceramic
substrate.
[0022] As an example of one way of dealing with this copper
particle diameter problem after sintering, a copper paste mixed
with aluminum acid which generates alumina of sub-micron size in
the sinter is disclosed in Japanese Patent Laid-Open No. Hei
8-17217. However, as water vapor is generated simultaneously during
the alumina forming reaction, more voids than needed were produced
in the copper interconnections.
[0023] This invention aims to overcome the disadvantages of the
prior art by suppressing the size of copper particles in the via to
20 .mu.m or less, thereby reducing breaks in interconnections due
to fractures at interfaces of copper particles which grow during
sintering, and reducing the risk of fractured vias separating from
the ceramic substrate.
[0024] To achieve this objective, alumina having an average
particle size of 1 .mu.m to 4 .mu.m was distributed in sintered
copper at an interval of 7.4 .mu.m or less in terms of the average
distance between particle centers.
[0025] The reason why the copper particles grow large during
sintering is that the copper particle boundaries migrate through
the copper, fusing with the surface of the sinter body or with
other copper particle boundaries, and this leads to a decrease of
copper particle interfaces in the sintered copper.
[0026] By distributing alumina of average particle size 1 .mu.m to
4 .mu.m in the sintered copper at the aforesaid interval, the
copper boundaries can no longer migrate, the copper interfaces do
not decrease even at the high temperature of the sintering step,
and the copper particles remain in a fine state of division. As a
result enlargement of copper particles is prevented, and fractures
of via interconnections do not occur.
[0027] The inventors experimentally verified that migration of
particle boundaries in sintered copper was inhibited by alumina
particles having the aforesaid size in restricted shapes such as
vias. The details of these experiments will now be described.
[0028] The test substrate was an ordinary glass ceramic substrate
having vias of diameter 60 .mu.m and comprising 10-40 layers, these
layers being laminated so that the vias were vertically connected
with each other right through the substrate from one surface to the
other. It should be noted that the number of layers in the
substrate is not limited to the above, and it may comprise only one
layer.
[0029] Next, after sintering this substrate under sintering
conditions known in the art, it was cut so that the center line of
the via appeared on the surface. The cut surface was polished by
the ordinary method, and then etched so that the copper particle
boundaries could be clearly seen.
[0030] Next, for 500 or more vias observed in this cut surface, the
shapes of the copper particle boundaries therein were read by a
computer, and these shapes were accurately traced so as to
calculate the surface area of the copper particles.
[0031] The reason why, in evaluating the state of the via
interconnections formed inside the vias, the surface area of the
copper particles was used as a parameter instead of the diameter
which has usually been used in the past, is as follows.
[0032] In general, when a particle grows in a shape like a via, the
shape often has an aspect ratio largely different from 1, and if
the shape is assumed to be a circle, it is difficult to evaluate
the copper particles precisely. The inventors also considered not
only the average size of a particle, but also the maximum size.
This is because in considering enlargement reactions of particles,
when there are several thousand vias in the substrate, it is
difficult to assess the quality of the vias using only a simple
average. However, in this experiment, the surface area of each
copper particle in the observed via cross-section was measured and
recorded.
[0033] It was found that when the diameter of the via is 60 .mu.m,
if the cross sectional area of a copper particle exceeds 2000
.mu.m.sup.2, the copper particle interface will be formed from one
side edge to the other side edge of the via cross-section, and as
all the particles having a cross-sectional area exceeding 3000
.mu.m.sup.2 cut across the via cross-section, there is a very
strong possibility that particles having this cross-section will
lead to fracture of the via.
[0034] Therefore, the via was filled with a paste of copper
particles of diameter 0.5 .mu.m to 6 .mu.m with the objective of
making the cross-sectional area of all the copper particles in the
via, 2000 .mu.m.sup.2. The variation of particle diameter during
the sintering step was observed.
[0035] FIG. 1 shows the change in the average value and maximum
value of copper particle cross-sectional area in the sintering
step. As a result, the average value of the copper particle
cross-sectional area after processing for 10 hours in an atmosphere
at 850.degree. C. at which point binder removal is complete was 50
.mu.m.sup.2,and the maximum value within the observed range was
1000 .mu.m.sup.2. When 1000.degree. C. was reached, the cross
sectional area of a copper particle was 270 .mu.m.sup.2, and the
maximum value was 5000 .mu.m.sup.2. Also, at 1000.degree. C., the
average and maximum values of the cross sectional area of copper
particles increased with the heating time. In an atmosphere at
950.degree. C., the average value was 250 .mu.m.sup.2 and the
maximum value was 5000 .mu.m.sup.2 when left for 2 hours.
[0036] Hence, after binder removal, a significant increase in the
cross-sectional area of copper particles was found, and especially
at temperatures higher than 950.degree. C. The change of particle
cross-sectional area in the process occurs mostly in this
temperature region, but considering the behavior of the maximum
particle cross-sectional area, it was clear that the particle size
must be controlled from a temperature lower than 950.degree. C.
[0037] Next, FIG. 2 shows the variation of the cross sectional area
of the copper particles in the via in a paste wherein alumina is
added to the copper. The results apply to the case when the mean
particle diameter of the added alumina is 1 .mu.m, 2 .mu.m and 4
.mu.m, and the addition amount was varied from 2 vol % to 10.5 vol
% relative to the inorganic substance in the via.
[0038] It is clear from these results that, for all alumina
particle sizes from 1 .mu.m to 4 .mu.m, the cross-sectional area of
copper particles can be markedly decreased by adding alumina.
However, the decrease does not much depend on the addition amount
of alumina particles, the average value of cross-sectional area
lying in the range 100 .mu.m.sup.2-200 .mu.m.sup.2 and the maximum
value lying in the range 500 .mu.m.sup.2-1500 .mu..sup.2 whatever
the addition amount and particle size of alumina.
[0039] FIG. 3 shows the structure seen when observing the via
cross-section. When alumina is added, as shown in FIG. 3A, alumina
4 is present at the boundaries of copper particles 31 or in regions
where the copper particles 31 overlap (multiple points). The copper
particles are much finer than in the case shown in FIG. 3B, where
alumina is not added. Moreover, there are no longer any particles
which span the whole via from one side edge to the other side edge
of the via cross-section, as shown in FIG. 3 B.
[0040] Thus, by adding the alumina 4 having a particle size which
is easily dispersed in copper paste, the alumina 4 inhibits
particle boundary migration of the copper particles 31, and
therefore prevents enlargement of the copper particles.
[0041] The reason why the copper particle boundary stops in the
vicinity of the alumina is that the interface energy of the copper
changes near the alumina, as shown in FIG. 4.
[0042] Consider the case where a particle boundary 42 migrates from
a position 1 to a position 3 in the copper 41, and there are
alumina particles 43 at an intermediate position 2, as shown in
FIG. 4A. When the particle boundary 42 arrives at this position 2,
the cross-sectional area of the boundary 42 decreases by an amount
corresponding to the cross-sectional area Sb of the alumina 43, as
shown in FIG. 4B.
[0043] At the same time, an interface Sa is formed between the
copper 41 and the alumina 43. Hence, if there is a large difference
of interface energies between copper particles and between copper
particles and alumina particles, the interface energy of the
particle boundary 42 will either become very large or will vary at
the position 2 (FIG. 4C).
[0044] The change .DELTA.E of interface energy per alumina particle
may be expressed by the following equation.
.DELTA.E=Sb.multidot.Ecc-Sa.multidot.Eca (1)
[0045] where Sa=interfacial area between copper and alumina,
[0046] Sb=cross-sectional area of alumina,
[0047] Ecc=energy per unit cross-sectional area of copper-copper
interface,
[0048] Eca=energy per unit cross-sectional area of copper-alumina
interface
[0049] Substances which cause this change of interface energy are
not limited to alumina. This is because, if their interface energy
is different from the interface energy between copper and copper,
the energy at position 2 shows an extreme value whether the energy
difference is positive or negative.
[0050] When the change .DELTA.E of this energy is larger than the
energy which moves the boundary surface 42, the boundary surface 42
cannot move past the alumina 43. It moves inside the sintered
compact at the same speed as the alumina 43, but as the speed of
movement of the alumina 43 in the sintered compact is itself small,
the movement of the boundary surface 42 is consequently impeded by
the alumina 43.
[0051] It may therefore be said that a more correct understanding
of this phenomenon could be achieved by considering the relation
between numbers of alumina particles and interface surface area
rather than by considering the relation between alumina
concentration and copper particle cross-sectional surface area or
particle diameter.
[0052] The inventors measured the particle boundary length Lg
corresponding to the cross-sectional area A of a measured copper
particle, and calculated a copper particle boundary surface area
Sgv in one via. Sgv was calculated by multiplying the particle
boundary surface area Sv per unit volume of a crystalline
substance, given by equation 2 below, by the volume of a via which
is the subject of this experiment.
Sv=(4/.pi.).multidot.Lg/A (2)
[0053] Here, Lg is the particle boundary line length appearing in
the observed surface area, and A is the observed surface area.
[0054] Equation 2 is widely supported by researchers in the field,
and is for example reported in "Ceramic Processing" by Mizutani,
Ozaki, Kimura and Yamaguchi (Gihodo, 1985).
[0055] FIG. 5 shows the result of plotting the copper particle
boundary surface area Sgv in the aforesaid via against the number
of alumina particles in the via computed from the alumina
concentration. The plotted data can be approximated using a
straight line of slope 43 (1000 .mu.m.sup.2/1000). This means that
a migration of a 43 .mu.m.sup.2 particle boundary surface area can
be prevented per alumina particle, and if this region is assumed to
be a circle, it is equivalent to a diameter range of 7.4 .mu.m.
[0056] From the above discussion, the minimum amount of alumina
required to suppress the migration of a copper particle boundary,
or in other words, the enlargement of a copper particle in the via,
can also be computed. Specifically, this value means that alumina
particles are distributed in the via at an interval of 7.4
.mu.m.
[0057] It is known that in a particle disperse system, equation 3
shown below holds between the average value .lambda. of the
distance between the nearest particles and the particle number
density Nv (for example, R. T. DeHoff, F, N. Rhines et al,
"Measurement Morphology", Rogakuo Uchida (1983), etc., translated
by Makishima, Shinohara and Komori).
[0058] From this, if .lambda.=7.4 .mu.m, Nv may be calculated to be
4.2.times.10.sup.-4 (/.mu.m.sup.3).
.lambda.=0.554/(N v.sup.1/3) (3)
[0059] As stated above, it is not additive concentration but the
number density of added particles and their interval which are the
essential points of this invention.
[0060] The above measurements were made in order to understand the
phenomenon, and require a great amount of effort. In practice, the
volume of additive will probably be computed and manufacturing
carried out to obtain the desired interval and number density, so
the relation of the number density Nv of addition particles and
volume % of added particles such as alumina is given as equation 4.
In this equation 4, it is assumed that the added particles are
spherical. Moreover, if the required minimum number densities
4.2.times.10.sup.-4 (/.mu.m.sup.3) are given as examples for
particles of diameter 1 .mu.m, 2 .mu.m, 4 .mu.m, the corresponding
volumes are respectively 0.022, 0.18, 1.4 volume %.
C=100(.pi./6).multidot.d.sup.3.multidot.Nv (4)
[0061] Here, C is the volume % in the via of an added particle, d
is the diameter of the added particle, and Nv is the particle
number density.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] These and other features, objects and advantages of the
present invention will become more apparent from the following
description when taken in conjunction with the accompanying
drawings wherein:
[0063] FIG. 1 is a figure which describes the cross-sectional area
variation (average value and maximum value) of via copper particles
in the sintering step in this embodiment.
[0064] FIG. 2 is a figure which describes the cross-sectional area
variation (average value and maximum value) of via copper particles
after sintering relative to an added alumina concentration.
[0065] FIG. 3 is a figure which shows the cross-sectional structure
of via copper particles after sintering.
[0066] FIG. 3A is the case when alumina is added,
[0067] FIG. 3B is the case where alumina is not added.
[0068] FIG. 4 is a figure for describing the energy change when a
particle interface migrates in a copper sintered compact.
[0069] FIG. 5 is a figure showing change of copper particle
boundary surface area relative to alumina particle number in a via
after sintering.
[0070] FIG. 6 is a figure which describes the state in the via
after sintering according to this embodiment.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0071] Hereafter, the embodiments will be described referring to
the figures and tables.
[0072] FIG. 6 shows the method of manufacturing a multilayer
substrate having a via cross-sectional microstructure shown in FIG.
6. The manufacturing process used the ordinary green sheet method.
The green sheet itself was manufactured from a slip having the
composition shown in the following Table 1 using the doctor blade
method known in the art.
1 TABLE 1 Composition Weight % glass powder 22.4 mullite 27.6 water
33.8 isopropyl alcohol 8.0 dispersants 0.1 binders 8.1
[0073] The thickness of the green sheet described above was about
200 .mu.m. The composition of the glass is shown in Table 2. Both
the glass and mullite had particle diameters of 3 .mu.m.
2 TABLE 2 Composition Weight % SiO.sub.2 80 B.sub.2O.sub.3 13
Al.sub.2O.sub.3 3 Na.sub.2O 4
[0074] As a reference example, the glass was manufactured using a
composition disclosed in Japanese Patent Laid-Open No. Hei
8-333157, and the green sheet was manufactured using a composition
disclosed in Japanese Patent Laid-Open No. Hei 6-227855. This
showed that a via having the characteristics of this embodiment
could be formed even if the composition of the glass or green sheet
was changed, and therefore, the composition of the glass and green
sheet does not impose a limitation on the manufacturing method of
this embodiment.
[0075] Next, a via 1 of diameter 60 .mu.m was opened in the
aforesaid green sheet using a hole opening tool (commonly referred
to as a punch).
[0076] This via was then filled with paste. One example of the
paste used in this embodiment was obtained by adding 2.6 g of the
alumina powder 4 of 2.1 .mu.m mean particle diameter to 100 g of a
mull substance comprising 92 vol % copper of 3 .mu.m mean particle
diameter in a vehicle comprising ethyl cellulose and
2,2,4-trimethyl-1,3-pentadiol monoisobutyrate in a ratio of 1:9.
Next, 1.75 g of ethyl cellulose of viscosity 300000 mPa/s was added
so that the viscosity was, for example, about 400000 to 500000
mPa/s, the product was blended for about 60 minutes using a tub
paddle machine known in the art, and the resulting mixture was
homogenized in an ordinary vibrating stirrer.
[0077] Next, the aforesaid paste was embedded in the via 1 of
diameter 60 .mu.m according to the criteria for screen-stencil.
After filling the via with the paste, a predetermined intrasurface
interconnection 2 was printed on this sheet using copper paste.
This operation was repeated, and the resulting laminate of 25
layers was stuck together under pressure at 130.degree. C.
[0078] In sintering the laminate, as an example, it was kept for
approximately 10 hours in an atmosphere at 850.degree. C. while the
temperature was increased at a rate of 100.degree. C./hour 10
hours, left for 2 hours in an atmosphere at 1000.degree. C., and
cooled at an average rate of 200.degree. C./hour. When the laminate
was kept for approximately 10 hours in an atmosphere at 300.degree.
C. to 850.degree. C. during the aforesaid temperature raising step,
operations were performed in an atmosphere of water vapor
comprising nitrogen at 0.4 atmospheres in terms of partial
pressure, and in an atmosphere of pure nitrogen at other times.
[0079] After subjecting the substrate produced by the
above-mentioned method to various tests, the substrate was cut and
the state of the via 1 was confirmed. In particular, it was
verified that no fatal abnormalities such as disconnection of vital
interconnections occurred in the laminated substrate even if a load
of 3000 or more cycles were applied in a -50.degree. C./150 heat
cycle test.
[0080] FIG. 6 shows a schematic view of the via 1 in this
embodiment.
[0081] It should be noted the boundary lines of copper particles do
not appear clearly if the substrate is merely cut and its
cross-section polished, therefore it was immersed for several
seconds in an etching fluid comprising water, 28% ammonia, and 3%
aqueous hydrogen peroxide in a ratio of 50:50:1 by volume.
[0082] The via 1 was thus formed so that the interconnections 2
within the surface were connected between layers. The copper
particles 31 in the via were finer than the copper particles 32 in
the surface interconnections. Further, in the via 1, the alumina 4
was present at the boundaries of the copper particles 31 or in
regions where the particle boundaries overlapped (multiple points),
but were not present inside the copper particles 31.
[0083] The cross-sections of more than about 100 of the vias 1 were
observed, and of the copper particles 31 in the vias 1, none were
found to have a cross-sectional area exceeding 1500 .mu.m.sup.2. No
cracks were observed in the vias 3. When a chemical analysis was
performed on this via 1, the proportions of the copper 31 and
alumina 4 were respectively 94.1 and 5.9 in terms of volume %.
[0084] Other compositions different from the above were
manufactured as pastes filling the vias by the same manufacturing
method as that of this embodiment, and the same effect was
obtained. Specifically, the compositions of the pastes filling the
vias were obtained as follows. The alumina powder 4 of 2.1 .mu.m
mean particle diameter was added, together with ethyl cellulose of
viscosity 300000 mPa/s, to 100 g of a mull substance comprising 92
vol % copper of 3 .mu.m mean particle diameter in a vehicle
comprising ethyl cellulose and 2,2, 4-trimethyl-1,3-pentadiol
monoisobutyrate in a ratio of 1:9. In paste (a), the addition
amount of ethyl cellulose was 0.55 g, in paste (b), the addition
amount of ethyl cellulose was 4.8 g , and in paste (c), the
addition amount of ethyl cellulose was 3.21 g. In all of the above
via filling pastes (a), (b), (c), the state of the vias 1 after
sintering was identical to that shown in FIG. 6 of this embodiment,
and none of the copper particles 31 in the via had a
cross-sectional surface area exceeding 1500 .mu.m.sup.2.
[0085] It was moreover verified by chemical analysis that the vias
1 which had been filled with one of the via filling pastes (a), (b)
or (c) and sintered, respectively contained 2.0, 4.1, 10.5 of the
alumina 4 in terms of volume %.
[0086] As described above, if dispersible alumina particles of
suitable size are mixed with a paste of copper particles, and the
result is used to fill vias in a substrate and sintered, the sizes
of the copper particles in the vias do not grow as large as the via
diameter. Even if a load of 3000 or more -50.degree. C.
/150.degree. C. heat cycles is applied, it does not cause breaks in
interconnections due to fracture at copper particle interfaces.
Also, the problem of broken vias falling out of ceramic substrates
is thereby largely resolved.
[0087] Consequently, it was possible to construct a ceramic wiring
board having a multilayer structure having high reliability in
application to electronic instruments requiring a high degree of
integration and high-speed processing.
[0088] While we have shown and described several embodiments in
accordance with our invention, it should be understood that the
disclosed embodiments are susceptible of changes and modifications
without departing from the scope of the invention. Therefore, we do
not intend to be bound by the details shown and described herein,
but intend to cover all such changes and modifications that fall
within the ambit of the appended claims.
* * * * *