Patent Description:
Some circuit substrates include a ceramic substrate mainly formed of alumina, mullite, silicon nitride, aluminum nitride, a glass ceramic, or the like.

There has been known a circuit substrate which includes a ceramic substrate and in which a conductor trace mainly formed of silver (Ag) and a coating layer are formed on a surface of the ceramic substrate. The coating layer contains a glass component and covers the conductor trace (refer to Patent Document <NUM>). The coating layer is also referred to as an overcoat glass layer. The conductor trace and the coating layer are formed by applying a conductor paste and a glass paste to a surface of the ceramic substrate and firing them. A ceramic laminated device including an dielectric ceramic and an Ag electrode is described in <CIT>. A conductive paste wherein a glass powder is added for preventing the diffusion of the conductive metal when it is fired, is described in <CIT>. A photosensitive tape having a ceramic multi circuit layer is described in <CIT>.

The technique disclosed in Patent Document <NUM> has the following problem. When the conductor trace and the coating layer are formed by firing, the silver component of the conductor trace diffuses into the coating layer, which may deteriorate electrical insulation performance of the coating layer and change the color of the coating layer. It is considered that diffusion of the silver component into the coating layer is accelerated by oxidation of the silver component of the conductor trace.

The present invention has been accomplished to solve the above-described problem. In particular, it is provided a method for manufacturing a circuit substrate, having the features defined in claim <NUM>. Further preferred arrangements are defined in the dependent claims.

According to the present invention, at least one of the metal boride and the metal silicide added to the conductor paste oxidizes during firing. Therefore, diffusion of the silver component of the conductor trace into the coating layer can be prevented. As a result, it is possible to prevent deterioration of the electrical insulation performance of the coating layer and change of the color of the coating layer. Thus, the quality of the circuit substrate can be improved.

According to a further development, the metal boride added to the conductor paste may be at least one of lanthanum hexaboride (LaB<NUM>), silicon hexaboride (SiB<NUM>), and titanium diboride (TiB<NUM>). This mode prevents diffusion of the silver component from the conductor trace into the coating layer.

According to a further development, the metal silicide added to the conductor paste may be at least one of zirconium disilicide (ZiSi<NUM>) and tantalum disilicide (TaSi<NUM>). This mode prevents diffusion of the silver component from the conductor trace into the coating layer.

According to a further development, the total amount of the metal boride and the metal silicide with respect to the amount of the inorganic component of the conductor paste may be <NUM> vol. % to <NUM> vol. This mode can sufficiently prevent diffusion of the silver component from the conductor trace into the coating layer.

According to a further development, the step of preparing the conductor paste may include a step of causing the powder of at least one of the metal boride and the metal silicide to adhere to surfaces of powder particles of silver (Ag). This mode prevents diffusion of the silver component from the conductor trace into the coating layer to a greater degree.

The present invention is not limited to a method of manufacturing the same and can be realized in various other forms as long as they are covered by the scope of claims. It may also be provided a device including a circuit substrate, a manufacturing apparatus for manufacturing a circuit substrate, etc..

In the following it is described a circuit substrate and a respective method of manufacturing a circuit substrate is described. The present invention relates to a method of manufacturing a circuit substrate. The circuit substrate in itself is however helpful for the understanding of the present invention.

<FIG> is an explanatory view schematically showing a cross section of a circuit substrate <NUM>. At least a portion of a circuit for realizing a predefined function is formed on the circuit substrate <NUM>. In the present arrangement, a circuit for transmitting signals to electronic components is formed on the circuit substrate <NUM>. The circuit substrate <NUM> includes a ceramic substrate <NUM>, conductor traces <NUM>, and a coating layer <NUM>.

The ceramic substrate <NUM> of the circuit substrate <NUM> is a plate-shaped ceramic member. In the present arrangement, the ceramic substrate <NUM> is 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 another arrangement, the main component of the ceramic substrate <NUM> may be any of other types of ceramic materials such as alumina, mullite, silicon nitride, and aluminum nitride. In the present arrangement, the ceramic substrate <NUM> is a low temperature fired ceramic substrate. The ceramic substrate <NUM> has a structure in which a plurality of insulating ceramic layers (unillustrated) are laminated. In the present arrangement, the ceramic substrate <NUM> has an unillustrated conductor layer, unillustrated vias, and unillustrated through holes as conductors which form the circuit.

The ceramic substrate <NUM> has surfaces <NUM> and <NUM>. The surface <NUM> is a backside surface opposite the surface <NUM>. Conductor traces <NUM> and a coating layer <NUM> are formed on the surface <NUM>. In another arrangement, like the surface <NUM>, a conductor trace <NUM> and a coating layer <NUM> may be formed on the surface <NUM>.

The surfaces <NUM> and <NUM> are mainly formed of a ceramic. In the present arrangement, the surfaces <NUM> and <NUM> are surfaces of ceramic layers formed by firing powders of borosilicate glass and alumina (Al<NUM>O<NUM>). Borosilicate glass is mainly formed of silicon dioxide (SiO<NUM>), alumina (Al<NUM>O<NUM>), and boron oxide (B<NUM>O<NUM>).

The conductor trace <NUM> of the circuit substrate <NUM> is an electrically conductive ceramic formed on the surface <NUM> of the ceramic substrate <NUM>. The conductor trace <NUM> is mainly formed of silver (Ag). In the present arrangement, the conductor trace <NUM> contains powders of silver (Ag) and borosilicate glass, and has electrical conductivity. In the present arrangement, each conductor trace <NUM> is covered entirely by the coating layer <NUM>. In another arrangement, a portion of each conductor trace <NUM> may be exposed out of the coating layer <NUM>. In the present arrangement, each conductor trace <NUM> has a thickness of about <NUM>.

The coating layer <NUM> of the circuit substrate <NUM> is an electrically insulating ceramic formed on the surface <NUM> of the ceramic substrate <NUM>. The coating layer <NUM> contains a glass component. The coating layer <NUM> is also referred to as an overcoat glass layer. In the present arrangement, the coating layer <NUM> is mainly formed of a glass ceramic. In the present arrangement, the coating layer <NUM> is an electrically insulating ceramic formed by firing powders of borosilicate glass and alumina (Al<NUM>O<NUM>). The coating layer <NUM> covers at least a portion of each conductor trace <NUM>. In the present arrangement, the coating layer <NUM> has a thickness of about <NUM> to <NUM>.

The coating layer <NUM> includes regions <NUM> located adjacent to the conductor traces <NUM>. In the regions <NUM> located adjacent to conductor traces <NUM>, the concentration of silicon atoms (Si) and/or boron atoms (B) contained in the coating layer <NUM> increases toward the conductor traces <NUM>.

<FIG> is a flowchart showing a method of manufacturing the circuit substrate <NUM>. First, the ceramic substrate <NUM> is formed by firing (step P110).

As a preparation for forming the ceramic substrate <NUM>, a green sheet which is the material of the ceramic substrate <NUM> is prepared. The green sheet is formed by mixing a binder, a plasticizer, a solvent, etc., into powders of inorganic components and forming the resultant mixture into the shape of a thin plate (sheet). In the present arrangement, powders of borosilicate glass and alumina which are inorganic component 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 arrangement, the green sheet has a thickness of <NUM>. In the present arrangement, the green sheet is formed into a desired shape by means of punching.

After preparing the green sheet, a conductor paste is applied to the green sheet. In the present arrangement, the conductor paste is prepared by mixing a binder, a plasticizer, a solvent, etc. into powders of inorganic components; i.e., a powdery mixture of silver (Ag) and borosilicate glass. In the present arrangement, 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, a plurality of the green sheets are laminated to form a laminate. In the present arrangement, by cutting operation, the laminate is formed into a shape suitable for firing. In the present arrangement, the laminate is exposed to an atmosphere of <NUM> for <NUM> hours for debindering.

After preparing the laminate, the laminate is fired to thereby form the ceramic substrate <NUM>. In the present arrangement, the laminate is fired by being exposed to an atmosphere of <NUM> for <NUM> minutes. After these steps, the ceramic substrate <NUM> is obtained.

After forming the ceramic substrate <NUM> (step P110), a conductor paste which is the pre-firing form of the conductor traces <NUM> is prepared (step P120). The conductor paste which is the pre-firing form of the conductor traces <NUM> is prepared by adding a powder of at least one of a metal boride and a metal silicide to a powder of silver (Ag).

From the viewpoint of preventing diffusion of silver into the coating layer <NUM>, the metal boride to be added to the conductor 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 coating layer <NUM>, the metal silicide to be added to the conductor paste is preferably at least one of zirconium disilicide (ZiSi<NUM>) and tantalum disilicide (TaSi<NUM>).

From the viewpoint of sufficiently preventing diffusion of silver into the coating layer <NUM>, the total amount of the metal boride and the metal silicide with respect to the amount of the inorganic component of the conductor paste is preferably <NUM> vol. % to <NUM> vol.

In the present arrangement, for preparing the conductor paste which is the pre-firing form of the conductor traces <NUM>, a mixture of silver (Ag) powder which is an electrically conductive material and borosilicate glass powder which is common with the ceramic substrate <NUM> is prepared as an inorganic component material of the conductor paste. 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 inorganic components. The resultant material is kneaded with a triple roll mill, whereby the conductor paste is obtained.

After preparation of the conductor paste (step P120), the conductor paste is applied to the surface <NUM> of the ceramic substrate <NUM> which has been fired (step P130). In the present arrangement, the conductor paste is applied to the ceramic substrate <NUM> by means of screen printing.

After the conductor paste is applied to the ceramic substrate (step P130), a glass paste which is the pre-firing form of the coating layer <NUM> is applied to the surface <NUM> of the ceramic substrate <NUM> carrying the conductor paste applied thereto (step P150). In the present arrangement, a mixture of borosilicate glass powder and alumina powder is prepared as an inorganic component material of the glass paste. Subsequently, ethyl cellulose serving as a binder and terpineol serving as a solvent are added to the mixture of powders of inorganic components. The resultant mixture is kneaded with a triple roll mill, whereby the glass paste is obtained. In the present arrangement, the glass paste is applied to the surface <NUM> of the ceramic substrate <NUM> by means of screen printing.

After applying the glass paste to the ceramic substrate <NUM> (step P160), the conductor paste and the glass paste applied to the surface <NUM> of the ceramic substrate <NUM> are fired to thereby form the conductor traces <NUM> and the coating layer <NUM> (step P160). In the present arrangement, the ceramic substrate <NUM> carrying the conductor paste and the glass paste applied thereto is exposed to an atmosphere of <NUM> for <NUM> minutes to fire the conductor paste and the glass paste. Thus, the conductor traces <NUM> and the coating layer <NUM> are formed on the surface <NUM> of the ceramic substrate <NUM>. After these steps, the circuit substrate <NUM> is completed.

When the conductor traces <NUM> and the coating layer <NUM> are formed by firing, 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 conductor paste. As a result, oxidation of the silver component contained in the conductor paste is prevented. Thus, diffusion of the silver component into the coating layer <NUM> is prevented.

At least a portion of the additive component(s) oxidized during firing diffuses into the regions <NUM>, which are located adjacent to the conductor traces <NUM>, of the coating layer <NUM>. Therefore, in the regions <NUM> located adjacent to the coating layer <NUM>, the concentration of silicon atoms (Si) and/or boron atoms (B) contained in the coating layer <NUM> increases toward the conductor traces <NUM>.

<FIG> is a table showing the results of an evaluation test. In the evaluation test whose results are shown in <FIG>, samples S01 to S10 of the circuit substrate <NUM> were made through use of different conductor pastes and different glass pastes. In the table of <FIG>, the amount of the additive(s) in the conductor paste which is the pre-firing form of the conductor traces <NUM> is shown as the amount (volume percent) of the additive(s) with respect to the amount of the inorganic component of the conductor paste.

The method of preparing the samples S01 to S07 is the same as that described with reference to <FIG>. The method of preparing the sample S08 is the same as that described with reference to <FIG> except that the borosilicate glass powder contained in the material of each member of the circuit substrate <NUM> is substituted with a powder of Na<NUM>-ZnO-B<NUM>O<NUM> glass and that the firing temperature for the conductor paste and the glass paste is <NUM>. 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 conductor paste. The method of preparing the sample S10 is the same as that for the sample S08 except that the metal boride and the metal silicide are not added to the conductor 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 coating layer <NUM>. The concentration of silver (Ag) at the interface between each conductor trace <NUM> and the coating layer <NUM> was used as a reference concentration, and the distance between the interface and a position in the coating layer <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 coating layer <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 conductor paste which is the material of the conductor traces <NUM>.

The evaluation results of the samples S04, S05, and S09 show that diffusion of silver into the coating layer <NUM> can be prevented by adding one of zirconium disilicide (ZiSi<NUM>) and tantalum disilicide (TaSi<NUM>), which are metal silicides, to the conductor paste which is the material of the conductor traces <NUM>.

The evaluation results of the samples S01 to S07 show that diffusion of silver into the coating layer <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 conductor paste which is the material of the conductor traces <NUM> is <NUM> vol. % to <NUM> vol.

The evaluation results of the samples S01, S08, S09, and S10 show that, even if the material of the glass paste is a powder of Na<NUM>-ZnO-B<NUM>O<NUM> glass, diffusion of silver into the coating layer <NUM> can be prevented as in the case where the material of the glass paste is borosilicate glass.

According to the arrangement described above, at least one of the metal boride and the metal silicide added to the conductor paste oxidizes during firing. Therefore, diffusion of the silver component of the conductor traces <NUM> into the coating layer <NUM> can be prevented. As a result, it is possible to prevent deterioration of the electrical insulation performance of the coating layer <NUM> and change of the color of the coating layer <NUM>. Thus, the quality of the circuit substrate <NUM> can be improved.

The metal boride added to the conductor 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 traces into the coating layer.

The metal silicide added to the conductor paste may be at least one of zirconium disilicide (ZiSi<NUM>) and tantalum disilicide (TaSi<NUM>). This prevents diffusion of the silver component from the conductor traces into the coating layer.

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

Since the conductor traces <NUM> and the coating layer <NUM> are formed at the same time by firing the conductor paste and the glass paste together (step P160), the production process can be simplified compared with the case where the conductor traces <NUM> and the coating layer <NUM> are formed separately.

<FIG> is a flowchart showing a method of manufacturing the circuit substrate <NUM> according to an illustrative arrangement that is not part of the invention. The manufacturing method of the illustrative arrangement is the same as that of the first arrangement except that the steps after applying the conductor paste (step P130) are different.

In the illustrative arrangement, after the conductor paste is applied (step P130), the conductor paste is fired to form the conductor traces <NUM> (step P240). In the present arrangement, the conductor paste having been applied to the ceramic substrate <NUM> is fired by being exposed to an atmosphere of <NUM> for <NUM> minutes. Thus, the conductor traces <NUM> are formed on the surface <NUM> of the ceramic substrate <NUM>.

After the conductor traces <NUM> are formed (step P240), the glass paste which is the material of the coating layer <NUM> is applied to the surface <NUM> of the ceramic substrate <NUM> (step P250). The methods of preparing and applying the glass paste in the illustrative arrangement are the same as those of the first arrangement.

After the glass paste is applied (step P250), the glass paste applied to the surface <NUM> of the ceramic substrate <NUM> is fired to form the coating layer <NUM> (step P260). In the present arrangement, the glass paste applied to the ceramic substrate <NUM> is fired by being exposed to an atmosphere of <NUM> for <NUM> minutes. Thus, the coating layer <NUM> is formed on the surface <NUM> of the ceramic substrate <NUM>. After these steps, the circuit substrate <NUM> is completed.

According to the illustrative arrangement that is not part of the invention, as in the first arrangement, diffusion of the silver component of the conductor traces <NUM> into the coating layer <NUM> can be prevented. As a result, it is possible to prevent deterioration of the electrical insulation performance of the coating layer <NUM> and change of the color of the coating layer <NUM>. Thus, the quality of the circuit substrate <NUM> can be improved.

After the conductor traces are formed by firing the conductor paste (step P240), the glass paste is applied to the surface <NUM> of the ceramic substrate <NUM>. As a result, diffusion of the silver component from the conductor traces <NUM> into the coating layer <NUM> is prevented to a greater degree.

The present invention is as defined in the claims.

In another arrangement, when the conductor paste which is the material of the conductor traces <NUM> is prepared (step P120), before adding a binder and a solvent to a powder of silver (Ag), a powder of at least one of the metal boride and the metal silicide may be caused to adhere to the surfaces of powder particles of silver (Ag) by adding a powder of at least one of the metal boride and the metal silicide to the silver (Ag) powder. This prevents diffusion of the silver component from the conductor traces <NUM> into the coating layer <NUM> to a greater degree.

Claim 1:
A method of manufacturing a circuit substrate (<NUM>) including a ceramic substrate (<NUM>) having a surface mainly formed of a ceramic, a conductor trace (<NUM>) formed on the surface of the ceramic substrate (<NUM>) and mainly formed of silver (Ag), and a coating layer (<NUM>) which contains a glass component and which is formed on the surface of the ceramic substrate (<NUM>) and covers the conductor trace (<NUM>), comprising:
a step of firing the ceramic substrate (<NUM>);
a step of preparing a conductor paste in which a powder of at least one of a metal boride and a metal silicide is added to a powder of silver (Ag);
a step of applying the conductor paste to the surface of the fired ceramic substrate (<NUM>);
a step of applying a glass paste to the surface of the ceramic substrate after applying the conductor paste;
a step of firing the conductor paste applied to the surface so as to form the conductor trace (<NUM>); and
a step of firing the glass paste applied to the surface so as to form the coating layer (<NUM>),
wherein the conductor paste and the glass paste are fired at the same time so as to form the conductor trace (<NUM>) and the coating layer (<NUM>) at the same time,
and when the glass paste and the conductor paste are fired, oxygen near the conductor paste is consumed by oxidation of the at least one of the metal boride and the metal silicide contained in the conductor paste.