Patent Application
20180014408
The present invention relates to a method for manufacturing a ceramic substrate, a ceramic substrate, and a silver-based conductor material.
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.).
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H6-252524
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2007-234537
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.
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.
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.
[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.
[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.
[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.
[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.
[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.
[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.
[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.
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.
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.
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.
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.
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.
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.
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.
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.
Examples of the metal boride include lanthanum hexaboride (LaB6), silicon hexaboride (SiB6), titanium diboride (TiB2), tantalum diboride (TaB2), niobium diboride (NbB2), chromium diboride (CrB2), molybdenum boride (MoB), zirconium diboride (ZrB2), tungsten boride (WB), vanadium diboride (VB2), and hafnium diboride (HfB2). Examples of the metal silicide include zirconium disilicide (ZrSi2), titanium disilicide (TiSi2), tungsten disilicide (WSi2), molybdenum disilicide (MoSi2), tantalum disilicide (TaSi2), chromium disilicide (CrSi2), niobium disilicide (NbSi2), iron disilicide (FeSi2), and hafnium disilicide (HfSi2).
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° C. or lower, and more preferably 700° C. or lower. Also, the oxidation temperature of the metal boride or the metal silicide as an additive is preferably 400° C. or higher, and more preferably 500° 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.
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.
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.
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.
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° C. to 950° 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.
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.
For samples S01 to S03, S05 to S12, S18, T01, and T03, green sheets containing an SiO2—B2O3—CaO glass and alumina (Al2O3) were prepared. For samples S13 to S15, S17, and T02, green sheets containing an SiO2—CaO—BaO—MgO glass and alumina (Al2O3) were prepared.
<Procedure For Preparing the Green sheets>
(1) A powder of borosilicate-based glass whose main components are silica (SiO2), alumina (Al2O3), and boric acid (H3BO3) 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.
(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.
(3) The materials mentioned above were mixed for five hours to thereby obtain a ceramic slurry.
(4) A green sheet with a thickness of 0.15 mm was made from the ceramic slurry using the doctor blade method.
(1) Conductor pastes for samples S01 to S17 (missing numbers: S04 and S16)
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.
Inorganic components: a silver powder and a borosilicate glass powder
Varnish components: ethyl cellulose resin and terpineol solvent
Additive: any one of LaB6, SiB6, TiB2, TaB2, ZrSi2, TiSi2, WSi2, CrSi2, MoSi2, and TaSi2
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.
(2) Conductor Paste For Sample S18
After the surfaces of the particles of the silver powder which is an inorganic component were coated with SiB6 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. %.
(3) Conductor Pastes For Samples T01 to T03
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, SiO2 was added as an additive.
(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.
(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 SiO2—B2O3—CaO green sheets, was set to about 850° C. The firing temperature for samples S13 to S15, S17, and T02, which used SiO2—CaO—BaO—MgO green sheets, was set to about 900° 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.
The “silver diffusion distance” shown in
The silver diffusion distances were 30 μm 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 μm 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.
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).
Particularly, the silver diffusion distance was restrained to a value smaller than 5 μm in any of samples S01 to S03, S05, S10 to S15, S17, and S18 in which one of LaB6, SiB6, TiB2, TaSi2, and ZrSi2 was added to the conductor paste as an additive in an amount greater than 3 vol. %. It should be noted when SiB6 is used as an additive, SiO2 generated by oxidation during firing remains in the ceramic insulating layer. That is, in the case where SiB6 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.
As described above, in the manufacturing process (
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.
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° 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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-003819 | Jan 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/000082 | 1/8/2016 | WO | 00 |
