Patent Description:
There has been known a ceramic substrate having a ceramic layer mainly formed of a glass ceramic and a conductor trace containing silver (Ag) as a main component. Such a ceramic substrate is formed by applying a conductor paste which is the pre-firing form of the conductor trace to a green sheet which is the pre-firing form of the ceramic layer and firing the green sheet carrying the conductor paste applied thereto. Such a ceramic substrate is also called a low temperature co-fired ceramic (LTCC) substrate.

When the ceramic substrate is formed by firing, the silver component of the conductor paste diffuses into the ceramic layer. This may cause, for example, formation of voids in the ceramic layer, deformation of the ceramic layer, and change of color of the ceramic layer. It is considered that diffusion of the silver component into the ceramic layer is accelerated by oxidation of the silver component of the conductor trace.

Patent Document <NUM> discloses a technique of preventing diffusion of the silver component into the ceramic layer by covering the surfaces of silver powder particles contained in the conductor paste with an antimony salt. Patent Document <NUM> discloses a technique of preventing diffusion of the silver component into the ceramic layer by adding silicon powder to the conductor paste.

<CIT> discloses a method for manufacturing a low temperature-baked ceramic substrate. The calcined powder in said document contains diopside crystals as main crystals.

However, the techniques disclosed in Patent Documents <NUM> and <NUM> cannot sufficiently prevent diffusion of the silver component of the conductor paste into the ceramic layer in some cases.

The present invention has been accomplished to solve the above-described problem. The present invention can be realized as follows.

In particular, it is provided a method having the features as defined in claim <NUM>. According to a further aspect of the invention, it is provided a method having the features defined in claim <NUM>. Further preferred embodiments of the method are defined in dependent claims <NUM> to <NUM>.

The present disclosure is not limited to the ceramic substrate and the method of manufacturing the same, and may be realized in various other forms. For example, the present disclosure can be realized as an apparatus including a ceramic substrate or a manufacturing apparatus for manufacturing a ceramic substrate. However, the scope of protection is defined by the appendant claims.

<FIG> is an explanatory view schematically showing a cross section of a ceramic substrate <NUM>. The ceramic substrate <NUM> is a low temperature co-fired ceramic (LTCC) substrate. At least a portion of a circuit for realizing a predefined function is formed on the ceramic substrate <NUM>. In the present embodiment, a circuit for transmitting a signal to electronic components, etc. is formed in the ceramic substrate <NUM>.

The ceramic substrate <NUM> includes ceramic layers <NUM> and <NUM> and a conductor trace <NUM>. The ceramic substrate <NUM> has a structure in which the plurality of ceramic layers <NUM> and <NUM> are laminated. In the present embodiment, the ceramic substrate <NUM> has unillustrated vias and unillustrated through holes in addition to the conductor trace <NUM> as conductors which form the circuit.

Each of the ceramic layers <NUM> of the ceramic substrate <NUM> is a first ceramic layer which is located adjacent to the conductor trace <NUM>. The ceramic layers <NUM> have electrically insulating properties. The ceramic layers <NUM> are mainly formed of a glass ceramic. In this description, "mainly formed of (a component)" means that the component accounts for at least <NUM> mass% of the entirety. In the present embodiment, each of the ceramic layers <NUM> is a ceramic layer which is formed by firing borosilicate glass powder and alumina (Al<NUM>O<NUM>) powder. Borosilicate glass is mainly composed of silicon dioxide (SiO<NUM>), alumina (Al<NUM>O<NUM>), and boron oxide (B<NUM>O<NUM>). A powder of at least one of a metal boride and a metal silicide is added to a ceramic paste which is the pre-firing form of the ceramic layer <NUM>. Therefore, the concentration of silicon atoms (Si) and/or boron atoms (B) in the ceramic layers <NUM> is higher than that in the ceramic layers <NUM>. In the present embodiment, each of the ceramic layers <NUM> has a thickness of about <NUM>.

Each of the ceramic layers <NUM> of the ceramic substrate <NUM> is a ceramic layer which sandwiches the corresponding ceramic layer <NUM> in cooperation with the conductor trace <NUM>. The ceramic layers <NUM> have electrically insulating properties. The ceramic layers <NUM> are mainly formed of a glass ceramic. In the present embodiment, like the ceramic layers <NUM>, each of the ceramic layers <NUM> is formed by firing borosilicate glass powder and alumina (Al<NUM>O<NUM>) powder. In the present embodiment, unlike the ceramic layers <NUM>, a powder of at least one of the metal boride and the metal silicide is not added to each of green sheets which are the pre-firing form of the ceramic layers <NUM>. Each of the ceramic layers <NUM> has a thickness sufficiently greater than that of the ceramic layers <NUM>.

The conductor trace <NUM> of the ceramic substrate <NUM> is mainly formed of silver (Ag). In the present embodiment, the conductor trace <NUM> contains silver (Ag) powder and borosilicate glass powder, and has electrical conductivity. In the present embodiment, the conductor trace <NUM> has a thickness of about <NUM>.

In the present embodiment, the conductor trace <NUM> is sandwiched between the two ceramic layers <NUM> and is located adjacent to the two ceramic layers <NUM>. In another embodiment, the conductor trace <NUM> may be sandwiched between the ceramic layers <NUM> and <NUM> and be located adjacent to the ceramic layers <NUM> and <NUM>. In still another embodiment, the ceramic substrate <NUM> may include two or more conductor traces <NUM> which are laminated together with other ceramic layers <NUM> and <NUM>.

<FIG> is a flowchart showing a method of manufacturing the ceramic substrate <NUM>. First, a ceramic paste which is the material of each ceramic layer <NUM> is prepared (step P110). The ceramic paste which is the material of each ceramic layer <NUM> is a paste in which a powder of at least one of the metal boride and the metal silicide is added to a raw material powder of a glass ceramic. In the present embodiment, the raw material powder of the ceramic paste is prepared by mixing borosilicate glass powder and alumina powder, which are inorganic components, at a volume ratio of <NUM>:<NUM>.

From the viewpoint of preventing diffusion of silver into the ceramic layers <NUM> and <NUM>, the metal boride to be added to the ceramic paste is preferably at least one of lanthanum hexaboride (LaB<NUM>), silicon hexaboride (SiB<NUM>), and titanium diboride (TiB<NUM>). From the viewpoint of preventing diffusion of silver into the ceramic layers <NUM> and <NUM>, the metal silicide to be added to the ceramic paste is preferably at least one of titanium disilicide (TiSi<NUM>), zirconium disilicide (ZrSi<NUM>), and tantalum disilicide (TaSi<NUM>).

From the viewpoint of preventing diffusion of silver into the ceramic layers <NUM> and <NUM>, the total amount of the metal boride and the metal silicide with respect to the amount of the inorganic component of the ceramic paste is <NUM> vol. % to <NUM> vol. % in the invention.

In the present embodiment, a mixture of borosilicate glass powder and alumina powder, both of which are inorganic components, is prepared for producing the ceramic paste which is the material of each ceramic layer <NUM>. Subsequently, a powder of at least one of the metal boride and the metal silicide, ethyl cellulose serving as a binder, and terpineol serving as a solvent are added to the mixture of the inorganic components. The resultant material is kneaded with a triple roll mill, whereby the ceramic paste is obtained.

After preparation of the ceramic paste, which is to become a ceramic layer <NUM> after firing (step P110), the ceramic paste is applied to a green sheet which is to become a ceramic layer <NUM> after firing (step P120). In the present embodiment, the ceramic paste is applied to the green sheet by means of screen printing.

As a preparation for applying the ceramic paste to the green sheet, the green sheet which is the material of each ceramic layer <NUM> is prepared. The green sheet is formed by mixing powders of inorganic components with a binder, a plasticizer, a solvent, etc., and forming the resultant mixture into the shape of a thin plate (sheet). In the present embodiment, powders of borosilicate glass and alumina which are inorganic components are weighed such that their volume ratio becomes <NUM>:<NUM> and the total weight becomes <NUM>. These powders are placed in a container (pot) formed of alumina. Subsequently, <NUM> of acrylic resin serving as a binder, a proper amount of methyl ethyl ketone (MEK) serving as a solvent, and a proper amount of dioctyl phthalate (DOP) serving as a plasticizer are added to the materials (powders) in the pot. The materials in the pot are mixed for five hours to thereby obtain a ceramic slurry. Then, a green sheet is made from the ceramic slurry using the doctor blade method. In the present embodiment, the green sheet has a thickness of <NUM>.

After preparing the green sheet, the ceramic paste is applied to the green sheet. In the present embodiment, the green sheet carrying the ceramic paste applied thereto is formed into a desired shape by means of punching.

After the ceramic paste has been applied to the green sheet (step P120), a conductor paste which is to become the conductor trace <NUM> after firing is applied to the ceramic paste having been applied to the green sheet (step P130). In the present embodiment, the conductor paste which is to become the conductor trace <NUM> after firing is prepared by mixing a binder, a plasticizer, a solvent, etc. into a powder of inorganic components; i.e., a mixture of powder of silver (Ag) and powder of borosilicate glass. In the present embodiment, after ethyl cellulose serving as a binder and terpineol serving as a solvent are added to the powder of inorganic components, the resultant material is kneaded with a triple roll mill, whereby the conductor paste is obtained. Subsequently, the conductor paste is applied to the green sheet by means of screen printing and hole-filling printing.

After applying the conductor paste to the green sheet (step P130), the green sheet carrying the ceramic paste and the conductor paste applied thereto is fired (step P140). Thus, the ceramic substrate <NUM> is complete.

In the present embodiment, a plurality of green sheets are laminated to form a laminate before firing the green sheets. In the present embodiment, a green sheet carrying the ceramic paste applied thereto is placed on another green sheet carrying the ceramic paste and the conductor paste applied thereto such that the ceramic paste side of the former comes into contact with the conductor paste side of the latter, whereby the laminate is formed. In the present embodiment, by cutting operation, the laminate is formed into a shape suitable for firing. In the present embodiment, the laminate is exposed to an atmosphere of <NUM> for <NUM> hours for debindering. In the present embodiment, after debindering, the laminate is fired by being exposed to an atmosphere of <NUM> for <NUM> minutes. After these steps, the ceramic substrate <NUM> is obtained.

When the green sheet carrying the ceramic paste and the conductor paste applied thereto is fired, oxygen near the conductor paste is consumed by oxidation of the additive component(s) (at least one of the metal boride and the metal silicide) contained in the ceramic paste. As a result, oxidation of the silver component contained in the conductor paste is prevented. Thus, diffusion of the silver component into the ceramic layers <NUM> and <NUM> is prevented. Since the ceramic paste contains the additive component(s), the concentration of silicon atoms (Si) and/or boron atoms (B) in the ceramic layers <NUM> is higher than that in the ceramic layers <NUM> depending on the amount of the additive component(s).

<FIG> is a table showing the results of an evaluation test. In the evaluation test whose results are shown in <FIG>, samples S01 to S09 of the ceramic substrate <NUM> were made through use of different ceramic pastes. In the table of <FIG>, the amount of the additive in the ceramic paste which is the pre-firing form of the ceramic layer <NUM> is shown as the amount (volume percent) of the additive with respect to the amount of the inorganic component of the ceramic paste.

The method of preparing the samples S01 to S08 is the same as that described with reference to <FIG>. The method of preparing the sample S09 is the same as that described with reference to <FIG> except that the metal boride and the metal silicide are not added to the ceramic paste.

A cross section of each sample was observed using a scanning electron microscope (SEM) and an electron probe micro analyzer (EPMA) to measure the distance of diffusion of silver into the ceramic layers <NUM> and <NUM>. The concentration of silver (Ag) at the interface between the ceramic layer <NUM> and the conductor trace <NUM> was used as a reference concentration, and the distance between the interface and a position in the ceramic layers <NUM> and <NUM> at which the concentration of silver (Ag) becomes half the reference concentration was measured at <NUM> points. The average of the measured distances was obtained as a silver diffusion distance.

Each sample was evaluated on the basis of the following criteria.

The evaluation results of the samples S01 to S03 and S09 show that diffusion of silver into the ceramic layers <NUM> and <NUM> can be prevented by adding one of lanthanum hexaboride (LaB<NUM>), silicon hexaboride (SiB<NUM>), and titanium diboride (TiB<NUM>), which are metal borides, to the ceramic paste which is the pre-firing form of the ceramic layer <NUM>.

The evaluation results of the samples S04 to S06 and S09 show that diffusion of silver into the ceramic layers <NUM> and <NUM> can be prevented by adding one of titanium disilicide (TiSi<NUM>), zirconium disilicide (ZrSi<NUM>), and tantalum disilicide (TaSi<NUM>), which are metal silicides, to the ceramic paste which is the pre-firing form of the ceramic layer <NUM>.

The evaluation results of the samples S01 to S08 show that diffusion of silver into the ceramic layers <NUM> and <NUM> can be sufficiently prevented when the total amount of the metal boride and the metal silicide with respect to the amount of the inorganic component of the ceramic paste which is the pre-firing form of the ceramic layer <NUM> is <NUM> vol. % to <NUM> vol.

According to the embodiment described above, at least one of the metal boride and the metal silicide added to the ceramic paste oxidizes during firing. Therefore, diffusion of the silver component of the conductor trace <NUM> into the ceramic layers <NUM> and <NUM> can be prevented. As a result, it is possible to prevent problems caused by diffusion of the silver component, for example, formation of voids in the ceramic layers <NUM> and <NUM>, deformation of the ceramic layers <NUM> and <NUM>, or change of color of the ceramic layers <NUM> and <NUM>. Thus, the quality of the ceramic substrate <NUM> can be improved.

The metal boride added to the ceramic paste may be at least one of lanthanum hexaboride (LaB<NUM>), silicon hexaboride (SiB<NUM>), and titanium diboride (TiB<NUM>). This prevents diffusion of the silver component from the conductor trace <NUM> into the ceramic layers <NUM> and <NUM>.

The metal silicide added to the ceramic paste may be at least one of titanium disilicide (TiSi<NUM>), zirconium disilicide (ZrSi<NUM>), and tantalum disilicide (TaSi<NUM>). This prevents diffusion of the silver component from the conductor trace <NUM> into the ceramic layers <NUM> and <NUM>.

When the total amount of the metal boride and the metal silicide with respect to the amount of the inorganic component of the ceramic paste is <NUM> vol. % to <NUM> vol. %, diffusion of the silver component from the conductor trace <NUM> into the ceramic layers <NUM> and <NUM> can be sufficiently prevented.

The present invention is not limited to the above-described embodiment and modifications, but may be embodied in various other forms within the scope of the appended claims.

In another embodiment, when the ceramic paste which is the pre-firing form of the ceramic layer <NUM> is prepared (step P110), before adding a binder and a solvent to the raw material powder, the powder of at least one of the metal boride and the metal silicide is caused to adhere to the surfaces of silver (Ag) powder particles by adding the powder of at least one of the metal boride and the metal silicide to the raw material powder. This prevents diffusion of the silver component from the conductor trace <NUM> into the ceramic layers <NUM> and <NUM> to a greater degree.

Claim 1:
A method of manufacturing a ceramic substrate (<NUM>) including a ceramic layer (<NUM>, <NUM>) mainly formed of a glass ceramic and a conductor trace (<NUM>) mainly formed of silver (Ag), comprising:
a step of preparing a ceramic paste in which a powder of at least one of a metal boride and a metal silicide is added to a raw material powder of the glass ceramic;
a step of applying the ceramic paste to a green sheet which is to become the ceramic layer after firing;
a step of applying a conductor paste which is to become the conductor trace after firing to the ceramic paste having been applied to the green sheet; and
a step of firing the green sheet carrying the ceramic paste and the conductor paste applied thereto,
wherein the raw material powder of the glass ceramic comprises borosilicate glass powder and alumina (Al<NUM>O<NUM>) powder,
wherein when the green sheet carrying the ceramic paste and the conductor paste applied thereto is fired, oxygen near the conductor paste is consumed by oxidation of at least one of the metal boride and the metal silicide contained in the ceramic paste, so that oxidation of the silver component contained in the conductor paste is prevented, and
wherein the total amount of the metal boride and the metal silicide with respect to the amount of the inorganic component of the ceramic paste is <NUM> vol. % to <NUM> vol. %, so that diffusion of the silver component from the conductor trace into the ceramic layers and can be sufficiently prevented.