U.S. patent application number 15/543290 was filed with the patent office on 2018-01-11 for method for manufacturing ceramic substrate, ceramic substrate, and silver-based conductor material.
The applicant listed for this patent is NGK SPARK PLUG CO., LTD.. Invention is credited to Masanori ITO, Tatsuya KATOH, Masaki KUTSUNA.
Application Number | 20180014408 15/543290 |
Document ID | / |
Family ID | 56405670 |
Filed Date | 2018-01-11 |
United States Patent
Application |
20180014408 |
Kind Code |
A1 |
KATOH; Tatsuya ; et
al. |
January 11, 2018 |
METHOD FOR MANUFACTURING CERAMIC SUBSTRATE, CERAMIC SUBSTRATE, AND
SILVER-BASED CONDUCTOR MATERIAL
Abstract
A method for manufacturing a ceramic substrate containing glass
includes a firing step in which an unfired silver-based conductor
material is disposed on an unfired ceramic layer and is fired. The
unfired silver-based conductor material contains at least one of a
metal boride and a metal silicide.
Inventors: |
KATOH; Tatsuya; (Nagoya-shi,
JP) ; ITO; Masanori; (Nagoya-shi, JP) ;
KUTSUNA; Masaki; (Ohbu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK SPARK PLUG CO., LTD. |
Nagoya-shi, Aichi |
|
JP |
|
|
Family ID: |
56405670 |
Appl. No.: |
15/543290 |
Filed: |
January 8, 2016 |
PCT Filed: |
January 8, 2016 |
PCT NO: |
PCT/JP2016/000082 |
371 Date: |
July 13, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 23/15 20130101;
H05K 3/4061 20130101; H05K 3/40 20130101; H05K 2201/017 20130101;
H05K 1/09 20130101; H05K 3/28 20130101; C03C 2204/00 20130101; C04B
2237/704 20130101; H05K 3/12 20130101; Y10T 428/12458 20150115;
C03C 10/0054 20130101; C04B 35/6303 20130101; H05K 3/4664 20130101;
C03C 4/14 20130101; H05K 1/092 20130101; H05K 3/4629 20130101; H05K
1/0306 20130101; H05K 1/0296 20130101; C04B 2237/40 20130101; H05K
3/321 20130101; H05K 2203/1126 20130101; H05K 2201/0769 20130101;
B32B 9/005 20130101; B32B 2307/202 20130101; H05K 3/285 20130101;
H05K 3/1291 20130101; Y10T 29/49163 20150115; H05K 3/46
20130101 |
International
Class: |
H05K 3/12 20060101
H05K003/12; B32B 9/00 20060101 B32B009/00; H05K 3/46 20060101
H05K003/46; H05K 3/40 20060101 H05K003/40; H05K 1/09 20060101
H05K001/09; H05K 1/03 20060101 H05K001/03 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 2015 |
JP |
2015-003819 |
Claims
1. A method for manufacturing a ceramic substrate containing glass,
comprising a firing step of firing an unfired ceramic layer and an
unfired silver-based conductor material disposed on the unfired
ceramic layer, wherein the unfired silver-based conductor material
contains a metal boride.
2. The manufacturing method according to claim 1, wherein the metal
boride is at least one of lanthanum hexaboride, silicon hexaboride,
titanium diboride, and tantalum diboride.
3. (Canceled)
4. The manufacturing method according to claim 1, wherein the
unfired silver-based conductor material contains the metal boride
and the amount of the metal boride with respect to the amount of
the inorganic components of the unfired silver-based conductor
material is greater than 3 vol. % and less than 20 vol. %.
5. The manufacturing method according to claim 1, wherein the
unfired silver-based conductor material contains a silver powder
and the metal boride is attached to surfaces of particles of the
silver powder in the silver-based conductor material.
6. A ceramic substrate comprising a ceramic layer and a wiring
layer of a sliver-based conductor which are formed by the firing
step according to claim 1.
7. A silver-based conductor material which is unfired and is fired
together with an unfired ceramic layer to form a wiring layer in a
ceramic substrate, wherein the unfired silver-based conductor
material contains a metal boride.
8. A method for manufacturing a ceramic substrate containing glass,
comprising a firing step of firing an unfired ceramic layer and
particles of an unfired silver-based conductor material disposed on
the unfired ceramic layer, wherein the particles of the unfired
silver-based conductor material are coated with at least one of a
metal boride and a metal silicide.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for manufacturing
a ceramic substrate, a ceramic substrate, and a silver-based
conductor material.
BACKGROUND ART
[0002] There has been known a multi-layer ceramic substrate fired
at a low temperature, which is also called a low temperature
co-fired ceramic (LTCC) substrate. Such an LTCC substrate is
usually manufactured by laminating a plurality of green sheets,
each having a wiring trace formed of an unfired conductor material,
and firing the green sheets (for example, see the following Patent
Documents 1 and 2, etc.).
PRIOR ART DOCUMENTS
Patent Documents
[0003] Patent Document 1: Japanese Patent Application Laid-Open
(kokai) No. H6-252524
[0004] Patent Document 2: Japanese Patent Application Laid-Open
(kokai) No. 2007-234537
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0005] A process of manufacturing a ceramic substrate using a
silver-based conductor material, not limited to the above-mentioned
LTCC substrate, has a problem of diffusion of silver contained in
the conductor material into the ceramic during firing. This may
cause formation of voids in the substrate, deformation of the
substrate, and change of the color of the substrate.
Conventionally, there have been proposed techniques of adding
various substances to the silver-based conductor material so as to
prevent diffusion of silver during firing. For example, Patent
Document 1 discloses a technique of coating the surfaces of the
particles of a silver-based conductor powder with an antimony salt
or an antimonate salt. Patent Document 2 discloses a technique of
adding a silicon (Si) powder to a conductor paste.
[0006] However, even when such a substance is added to the
silver-based conductor material, the effect of preventing silver
diffusion cannot be attained to a sufficient degree in the case
where a reaction caused by the added substance to prevent diffusion
of silver does not occur as expected at temperatures near the
firing temperature. Thus, there is still a room for improvement
regarding prevention of diffusion of silver during firing in the
process of manufacturing such a ceramic substrate.
Means For Solving the Problem
[0007] The present invention has been accomplished to solve at
least the above-described problem by employing a method different
from conventional ones. The present invention can be realized as
the following modes.
[0008] [1] One mode of the present invention is a method for
manufacturing a ceramic substrate containing glass. The
manufacturing method includes a firing step. The firing step may be
a step of firing an unfired ceramic layer and an unfired
silver-based conductor material disposed on the unfired ceramic
layer. The unfired silver-based conductor material may contain at
least one of a metal boride and a metal silicide. The manufacturing
method of this mode prevents diffusion of silver during firing
because at least one of a metal boride and a metal silicide is
added in the unfired silver-based conductor material. The unfired
silver-based conductor material may be disposed on a surface of an
unfired ceramic layer, between unfired ceramic layers adjacent to
each other, or in through holes formed in an unfired ceramic
layer.
[0009] [2] In the manufacturing method of the above-mentioned mode,
the metal boride may be at least one of lanthanum hexaboride,
silicon hexaboride, titanium diboride, and tantalum diboride. The
manufacturing method of this mode prevents diffusion of silver
during firing more effectively.
[0010] [3] In the manufacturing method of the above-mentioned mode,
the metal silicide may be at least one of titanium disilicide,
zirconium disilicide, tungsten disilicide, chromium disilicide,
molybdenum disilicide, and tantalum disilicide. The manufacturing
method of this mode prevents diffusion of silver during firing more
effectively.
[0011] [4] In the manufacturing method of the above-mentioned mode,
the unfired silver-based conductor material contains the metal
boride or the metal silicide. The amount of the metal boride or the
metal silicide with respect to the amount of the inorganic
components of the unfired silver-based conductor material may be
greater than 3 vol. % and less than 20 vol. %. The manufacturing
method of this mode prevents diffusion of silver during firing more
effectively and also prevents impurities from remaining in the
conductor of the substrate.
[0012] [5] In the manufacturing method of the above-mentioned mode,
the unfired silver-based conductor material contains a silver
powder and at least one of the metal boride and the metal silicide
may be attached to surfaces of particles of the silver powder in
the silver-based conductor material. The manufacturing method of
this mode prevents oxidation of silver during firing more
effectively. As a result, the effect of preventing diffusion of
silver into a ceramic layer improves.
[0013] [6] A second mode of the present invention is a ceramic
substrate. The ceramic substrate may include a ceramic layer and a
wiring layer of a sliver-based conductor which are formed by the
firing step according to any one of the manufacturing methods of
the above-mentioned mode. The ceramic substrate of this mode
prevents problems such as formation of voids in the ceramic
substrate, warpage of the ceramic substrate, change of the color of
the ceramic substrate, etc.
[0014] [7] A third mode of the present invention is a silver-based
conductor material which is unfired and fired together with an
unfired ceramic layer to form a wiring layer in a ceramic
substrate. The silver-based conductor material of this mode may
contain at least one of a metal boride and a metal silicide. The
silver-based conductor material of this mode prevents diffusion of
silver in a process of manufacturing a ceramic substrate. In the
silver-based conductor material of this mode, the metal boride may
be at least one of lanthanum hexaboride, silicon hexaboride,
titanium diboride, and tantalum diboride. In the silver-based
conductor material of this mode, the metal silicide may be at least
one of titanium disilicide, zirconium disilicide, tungsten
disilicide, chromium disilicide, molybdenum disilicide, and
tantalum disilicide.
[0015] All the plurality of constituent elements of each mode of
the present invention are not essential. In order to solve,
partially or entirely, the above-mentioned problem or yield,
partially or entirely, the effects described in the present
specification, a part of the elements may be properly modified,
deleted, or replaced with another new element, or the limitation
thereof may be partially removed. Also, in order to solve,
partially or entirely, the above-mentioned problem or yield,
partially or entirely, the effects described in the present
specification, a portion or all of the above-described technical
features contained in one mode of the present invention may be
combined with a portion or all of the above-described technical
features contained in other modes of the present invention to
thereby attain an independent mode of the present invention.
[0016] The present invention can be realized as various modes other
than a method for manufacturing a ceramic substrate, a ceramic
substrate, or a silver-based conductor material. For example, the
present invention can be realized as a method for firing a ceramic
substrate, a method for manufacturing a silver-based conductor
material, an apparatus for implementing those methods, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 Schematic view showing a structure of an LTCC
substrate.
[0018] FIG. 2 Flowchart showing steps of a process of manufacturing
the LTCC substrate.
[0019] FIG. 3 Explanatory view showing the results of an experiment
for checking an effect of preventing diffusion of silver by adding
an additive to a conductor paste.
[0020] FIG. 4 Explanatory views showing scanning electron
microscope (SEM) images of LTCC substrates and images showing the
silver concentration distribution profile in each of the LTCC
substrates.
MODES FOR CARRYING OUT THE INVENTION
A. Embodiment
[0021] FIG. 1 is a schematic view showing the structure of an LTCC
substrate 10 according to one embodiment of the present invention.
The LTCC substrate 10 which is a ceramic substrate is used, for
example, for electronic components, high-frequency modules, IC
packages, or printed wiring boards used in computers, communication
devices, etc. The LTCC substrate 10 has a multi-layer structure
formed by laminating a plurality of ceramic insulating layers 11.
Each of the ceramic insulating layers 11 is formed by low
temperature firing whose firing temperature is 1000.degree. C. or
lower.
[0022] Each of the ceramic insulating layers 11 has vias which are
through holes for disposing via electrodes 12. The LTCC substrate
10 has wiring layers including internal electrodes 13 and external
electrodes 14, each formed between ceramic insulating layers 11
adjacent to each other. The wiring layers are electrically
connected to one another through the via electrodes 12 formed in
the ceramic insulating layers 11.
[0023] In the LTCC substrate 10 of the present embodiment, each of
the electrodes 12 to 14 is formed of a silver-based conductor
material whose main component is silver. In this description, a
"main component" means a material component which accounts for at
least 50 mass % of the mixture. On the outermost surface of the
LTCC substrate 10 are disposed passive elements (resistors, etc.)
and active elements (ICs, etc.) which are connected to the external
electrodes 14. In this description, illustration and detailed
description of the passive elements and the active elements are
omitted.
[0024] FIG. 2 is a flowchart showing steps of a process of
manufacturing the LTCC substrate 10. The LTCC substrate 10 is
manufactured by firing an unfired ceramic material (green sheet)
and an unfired silver-based conductor material together at low
temperature.
[0025] In step 1, a green sheet which constitutes an unfired
ceramic layer containing ceramic particles and glass particles is
prepared. The green sheet is made by preparing a ceramic slurry by
mixing together inorganic components (including a glass powder and
an inorganic filler), a binder component, a plasticizer, and a
solvent, and forming the ceramic slurry into the shape of a sheet
using the doctor blade method or the like.
[0026] In step 2, a conductor paste which forms electrodes 12 to 14
and which is an unfired silver-based conductor material is
prepared. The conductor paste is made by mixing together a powder
of the silver-based material and a glass powder which are inorganic
components, and an organic solvent and a resin which is a varnish
component.
[0027] Notably, the inventor of the present invention has found
that adding at least one of a metal boride and a metal silicide
into the conductor paste as an inorganic component prevents
diffusion of silver contained in the conductor paste, or a silver
component of the conductor paste, into the ceramic insulating layer
during a firing step described later. It is considered that oxygen
present near the conductor paste is consumed by oxidation of the
metal boride or the metal silicide during the firing step, whereby
oxidation of the silver contained in the conductor paste is
prevented.
[0028] In step 2 of the present embodiment, an additive including
at least one of a metal boride and a metal silicide is added to the
conductor paste. For example, the following substances can be used
as an additive to be added to the conductor paste.
[0029] Examples of the metal boride include lanthanum hexaboride
(LaB.sub.6), silicon hexaboride (SiB.sub.6), titanium diboride
(TiB.sub.2), tantalum diboride (TaB.sub.2), niobium diboride
(NbB.sub.2), chromium diboride (CrB.sub.2), molybdenum boride
(MoB), zirconium diboride (ZrB.sub.2), tungsten boride (WB),
vanadium diboride (VB.sub.2), and hafnium diboride (HfB.sub.2).
Examples of the metal silicide include zirconium disilicide
(ZrSi.sub.2), titanium disilicide (TiSi.sub.2), tungsten disilicide
(WSi.sub.2), molybdenum disilicide (MoSi.sub.2), tantalum
disilicide (TaSi.sub.2), chromium disilicide (CrSi.sub.2), niobium
disilicide (NbSi.sub.2), iron disilicide (FeSi.sub.2), and hafnium
disilicide (HfSi.sub.2).
[0030] The metal borides and the metal silicides described above
are just examples. The additive may be a metal boride or a metal
silicide other than those described above. However, the metal
boride or the metal silicide used as an additive is preferably a
one which initiates a reaction with oxygen during the firing step
described later. In particular, the metal boride or the metal
silicide preferably has an oxidation temperature which is lower
than the firing temperature during the firing step 4 described
later. The "oxidation temperature" is a peak temperature at which
oxidation occurs, and is a value measured through
thermogravimetric-differential thermal analysis (TG-DTA).
Specifically, the oxidation temperature of the metal boride or the
metal silicide as an additive is preferably 800.degree. C. or
lower, and more preferably 700.degree. C. or lower. Also, the
oxidation temperature of the metal boride or the metal silicide as
an additive is preferably 400.degree. C. or higher, and more
preferably 500.degree. C. or higher. Diffusion of silver is
prevented if the oxidized silver is not wetted by the glass
material contained in the green sheet when the glass material
softens during the firing step. For this reason, the oxidation
temperature of the metal boride or the metal silicide as an
additive is preferably lower than the glass-transition temperature
of the glass material contained in the green sheet prepared in step
1.
[0031] The additive may be added in the form of powder, for
example, concurrently with or after the step of mixing the
inorganic components and the varnish components. Alternatively, the
additive may be added before mixing the inorganic components and
the varnish components. In this case, the additive is added in such
a manner that the surfaces of the particles of the silver-based
material contained in the inorganic components are coated with the
additive. For example, the silver-based material can be coated with
the additive by the following method. First, the additive is
dissolved or dispersed in an organic solvent (toluene, xylene, or
alcohol). Then, a powder of the silver-based material is dispersed
or suspended in the solution or dispersion of the additive. The
solvent is kept still for a predetermined time or stirred so as to
cause the additive to adhere to the surfaces of the particles of
the silver-based material. Coating the silver-based material with
the additive as described prevents oxidation of silver to a greater
degree, and improves the effect for suppressing silver diffusion.
The additive may be added to the conductor paste using a method
other than that described above.
[0032] The amount of additive with respect to the amount of the
inorganic components of the conductor paste is preferably more than
3 vol. %, more preferably more than 5 vol. %. This condition allows
the effect for suppressing silver diffusion to be attained more
reliably. The amount of additive with respect to the amount of
inorganic components of the conductor paste is preferably less than
20 vol. %, more preferably less than 18 vol. %. This condition
prevents impurities originating from the additive in the conductor
paste from remaining in the LTCC substrate 10 after firing.
[0033] In step 3, the above-described conductor paste is disposed
on the green sheet. Specifically, vias are formed in the green
sheet by a hole-making operation such as punching, and the vias are
filled with the conductor paste. A wiring trace is printed on each
surface of the green sheet by applying the conductor paste thereto
by means of screen printing or the like. After the wiring trace is
formed, a plurality of such green sheets are laminated to form an
unfired laminate.
[0034] In step 4, the unfired laminate is fired at a low
temperature. The firing temperature in step 4 may be a temperature
preset in accordance with the glass-transition temperature of the
material component of the green sheet prepared in step 1.
Specifically, the firing temperature in step 4 may be, for example,
approximately 750.degree. C. to 950.degree. C. After step 4, the
LTCC substrate 10 is completed. Passive elements and active
elements to be connected to the electrodes 14 are disposed on the
completed LTCC substrate 10.
[0035] As described above, in the LTCC substrate 10 of the present
embodiment, addition of the metal boride or the metal silicide to
the conductor paste in step 2 prevents diffusion of silver from the
silver-based conductor material to the ceramic insulating layer 11.
This prevents deterioration of electrical insulation of the ceramic
insulating layers 11 caused by diffusion of silver. Local change of
the color of the ceramic caused by a change in the composition of
the ceramic near the wiring trace as well as local deterioration of
the strength of the ceramic insulating layer 11 are also prevented.
In addition, acceleration of firing-caused contraction only near
the conductor paste is prevented, and formation of voids between
the electrodes 12 to 14 and the ceramic insulating layers 11 are
prevented.
[0036] FIG. 3 is an explanatory view showing the results of an
experiment for checking an effect of preventing diffusion of silver
by adding the additive to the conductor paste. This experiment
checked diffusion of silver into the ceramic insulating layers
using samples S01 to S18 (missing numbers: S04 and S16) of LTCC
substrates manufactured through use of a conductor paste containing
an additive and samples T01 to T03 of the LTCC substrates
manufactured through use of a conductor paste containing no
additive. The specific conditions for manufacturing samples S01 to
S18 (missing numbers: S04 and S16) and T01 to T03 are as
follows.
<Compositions of the Green Sheets>
[0037] For samples S01 to S03, S05 to S12, S18, T01, and T03, green
sheets containing an SiO.sub.2--B.sub.2O.sub.3--CaO glass and
alumina (Al.sub.2O.sub.3) were prepared. For samples S13 to S15,
S17, and T02, green sheets containing an SiO.sub.2--CaO--BaO--MgO
glass and alumina (Al.sub.2O.sub.3) were prepared.
<Procedure For Preparing the Green sheets>
[0038] (1) A powder of borosilicate-based glass whose main
components are silica (SiO.sub.2), alumina (Al.sub.2O.sub.3), and
boric acid (H.sub.3BO.sub.3) and a powder of alumina were put into
a pot formed of alumina such that their volume ratio became 60:40
and the total weight became 1 kg.
[0039] (2) Subsequently, 120 g of acrylic resin and proper amounts
of methyl ethyl ketone (MEK) serving as a solvent and dioctyl
phthalate (DOP) serving as a plasticizer were put into the pot
formed of alumina. The amounts of methyl ethyl ketone and dioctyl
phthalate were determined such that the desired levels of slurry
viscosity and sheet strength could be attained.
[0040] (3) The materials mentioned above were mixed for five hours
to thereby obtain a ceramic slurry.
[0041] (4) A green sheet with a thickness of 0.15 mm was made from
the ceramic slurry using the doctor blade method.
<Conductor Paste>
[0042] (1) Conductor pastes for samples S01 to S17 (missing
numbers: S04 and S16)
[0043] A mixture of the following inorganic components, varnish
components, and additive was kneaded with a triple roll mill,
whereby the conductor pastes for samples S01 to S17 were
prepared.
[0044] Inorganic components: a silver powder and a borosilicate
glass powder
[0045] Varnish components: ethyl cellulose resin and terpineol
solvent
[0046] Additive: any one of LaB.sub.6, SiB.sub.6, TiB.sub.2,
TaB.sub.2, ZrSi.sub.2, TiSi.sub.2, WSi.sub.2, CrSi.sub.2,
MoSi.sub.2, and TaSi.sub.2
[0047] The amount of additive with respect to the amount of the
inorganic components of the conductor paste was set to 15 vol. %
for samples S01 to S03, S05 to S10, and S13 to S15, 9 vol. % for
samples S11 and S17, and 3 vol. % for sample S12. The oxidation
temperatures shown in the table were measured through the TG-DTA
method.
[0048] (2) Conductor Paste For Sample S18
[0049] After the surfaces of the particles of the silver powder
which is an inorganic component were coated with SiB.sub.6 which is
an additive, the mixture of the above-described inorganic
components and varnish components was kneaded with a triple roll
mill, whereby the conductor paste for sample S18 was prepared. The
amount of additive with respect to the amount of the conductor
paste were 15 vol. %.
[0050] (3) Conductor Pastes For Samples T01 to T03
[0051] The conductor pastes for samples T01 and T02 were prepared
by the same method as that for samples S01 to S17 (missing numbers:
S04 and S16) except that no additive was added. The conductor paste
for sample T03 was prepared by the same method as that for samples
S01 to S17 (missing numbers: S04 and S16) except that in place of
the metal boride or the metal silicide, SiO.sub.2 was added as an
additive.
<Forming and Firing an Unfired Laminate>
[0052] (1) Vias were formed in the green sheet and filled with the
conductor paste. A wiring trace was formed on a surface of the
green sheet by applying the conductor paste thereto. A plurality of
such green sheets with the wiring trace formed thereon were
laminated to form an unfired laminate.
[0053] (2) Unfired laminates for samples S01 to S18 (missing
numbers: S04 and S16) and T01 to T03 were fired. The firing
temperature for samples S01 to S03, S05 to S12, S18, T01, and T03,
which used SiO.sub.2--B.sub.2O.sub.3--CaO green sheets, was set to
about 850.degree. C. The firing temperature for samples S13 to S15,
S17, and T02, which used SiO.sub.2--CaO--BaO--MgO green sheets, was
set to about 900.degree. C. The firing time was set to
approximately 60 minutes for all samples S01 to S18 (missing
numbers: S04 and S16) and T01 to T03.
[0054] The "silver diffusion distance" shown in FIG. 3 will be
described with reference to FIG. 4. Each of sections (A) to (G) of
FIG. 4 shows a scanning electron microscope (SEM) image of a cross
section of the LTCC substrate parallel to a direction of lamination
of the LTCC substrate and an image of the same cross section as the
SEM image captured by an electron probe micro analyzer (EPMA). In
each section of FIG. 4, the SEM image is shown on the upper side
and the image captured by EPMA is shown on the lower side. The
image captured by an EPMA (hereinafter, simply referred to as the
"EPMA image") shows the silver concentration distribution profile
of the LTCC substrate in colors in response to the level of silver
concentration. Sections (A) to (F) of FIG. 4 show the SEM images
and the EPMA images of samples S02, S03, and S05 to S08 which were
manufactured through use of different conductor pastes containing
SiB.sub.6, TiB.sub.2, ZrSi.sub.2, TiSi.sub.2, WSi.sub.2, and
CrSi.sub.2, respectively, as an additive. Section (G) of FIG. 4 has
the SEM image and the EPMA image of sample T01 which was
manufactured through use of a conductor paste containing no
additive. In the center of each SEM image and each EPMA image, the
internal electrode formed of the silver-based conductor extends in
the horizontal direction of the images. The EPMA image shown in
section (G) of FIG. 4 shows that silver diffused into a wide region
extending in the vertical direction of the image from the internal
electrode such that the silver concentration in the region is
approximately the same as that in the internal electrode. The
inventors of the present invention acquired the SEM images and the
EPMA images of predetermined polished cross sections of samples S01
to S18 (missing numbers: S04 and S16), and T01 to T03. The inventor
determined a "silver diffusion distance" for each sample from the
EPMA image of each sample. Specifically, the inventor used the
concentration of Ag at an electrode interface through which the
internal electrode is in contact with the ceramic insulating layer
as a reference concentration, and measured, at five points, the
distance from the electrode interface to a region in which the
concentration of Ag in the ceramic insulating layer becomes equal
to or less than half the reference concentration. The average of
the measured distances was used as the "silver diffusion
distance."
[0055] The silver diffusion distances were 30 .mu.m or less in all
samples S01 to S18 (missing numbers: S04 and S16) which were
manufactured through use of the conductor paste containing the
metal silicide or the metal boride as an additive. By contrast, the
silver diffusion distances were greater than 30 .mu.m in samples
T01 to T03 which were manufactured without use of the conductor
paste containing the metal silicide or the metal boride as an
additive. These results show that the metal silicide or the metal
boride added to the conductor paste prevented diffusion of silver
from the conductor material during firing.
[0056] If the same additive was added to the conductor paste,
diffusion of silver was prevented approximately to the same degree
(see samples S01 to S03 and samples S13 to S15, and samples S11 and
S17) irrespective of the composition of the green sheet. The test
results show that diffusion of silver was prevented to a great
degree in both the case where the additive was added to the
conductor paste in the form of powder and the case where the
additive was added to the conductor paste as a material for coating
the surfaces of the silver powder particles (see samples S01 and
S18).
[0057] Particularly, the silver diffusion distance was restrained
to a value smaller than 5 .mu.m in any of samples S01 to S03, S05,
S10 to S15, S17, and S18 in which one of LaB.sub.6, SiB.sub.6,
TiB.sub.2, TaSi.sub.2, and ZrSi.sub.2 was added to the conductor
paste as an additive in an amount greater than 3 vol. %. It should
be noted when SiB.sub.6 is used as an additive, SiO.sub.2 generated
by oxidation during firing remains in the ceramic insulating layer.
That is, in the case where SiB.sub.6 is used as an additive as in
sample S02, only a compound of the same composition as the compound
contained in the ceramic insulating layer remains in the ceramic
insulating layer. As a result, migration of impurities into the
ceramic insulating layer is prevented.
[0058] As described above, in the manufacturing process (FIG. 2) of
the present embodiment, the metal boride or the metal silicide
added to the conductor paste prevents diffusion of silver from the
conductor material during the firing step. Accordingly, the LTCC
substrate 10 manufactured according to the manufacturing process
can prevent various types of problems caused by diffusion of silver
from the conductor material during the firing step such as
formation of voids in the ceramic substrate, deterioration of the
ceramic substrate, etc.
B. Modifications
B1. Modification 1
[0059] In the above-described embodiment, a single type of metal
boride or a single type of metal silicide is added to the conductor
paste as an additive. However, both a metal boride and a metal
silicide may be added to the conductor paste as additives. A
plurality of types of metal borides may be added in combination as
additives. A plurality of types of metal silicides may be added in
combination as additives. Alternatively, one or more types of metal
borides may be added together with one or more types of metal
silicides as additives.
B2. Modification 2
[0060] In the above-described embodiment, in the process of
manufacturing the LTCC substrate, at least one of a metal boride
and a metal silicide is added to the conductor paste which is a
silver-based conductor material. However, in a process of
manufacturing a ceramic substrate other than the LTCC substrate,
the additive described above may be added to a silver-based
conductor material. For example, in a process of manufacturing a
ceramic substrate whose firing temperature is 1000.degree. C. or
higher, the additive described above may be added. The silver-based
conductor material containing at least one of a metal boride and a
metal silicide added thereto is not required be in the form of
paste, but may be, for example, in the form of powder.
B3. Modification 3
[0061] In the above-described embodiment, in preparation of the
green sheet, alumina is used as an inorganic filler. However, as an
inorganic filler used for preparation of the green sheet, a
material other than alumina may be used. As an inorganic filler,
for example, mullite can be used.
[0062] The present invention is not limited to the above-described
embodiment, examples, and modifications, but may be embodied in
various other forms without departing from the spirit of the
invention. For example, in order to solve, partially or entirely,
the above-mentioned problem or yield, partially or entirely, the
above-mentioned effects, technical features of the embodiments,
examples, and modifications corresponding to technical features of
the modes described in the section "SUMMARY OF THE INVENTION" can
be replaced or combined as appropriate. Also, the technical
feature(s) may be eliminated as appropriate unless the present
specification mentions that the technical feature(s) is
mandatory.
DESCRIPTION OF REFERENCE NUMERALS
[0063] 10 . . . LTCC substrate [0064] 11 . . . ceramic insulating
layer [0065] 12 . . . via electrode [0066] 13 . . . internal
electrode [0067] 14 . . . external electrode
* * * * *