Ceramic multilayer substrate

1. A ceramic multilayer substrate formed by stacking and firing a plurality of insulating layers made of ceramic materials, wherein: the insulating layer in an uppermost surface layer includes a first ceramic layer, a second ceramic layer directly stacked on the first ceramic layer and configured to suppress contraction, and a surface-layer via conductor penetrating through the first and second ceramic layers and having a tapered shape gradually tapering toward a lower layer side, the surface-layer via conductor having an end surface that is exposed from the second ceramic layer and that is directly connected to a terminal of a component mounted to a surface of the second ceramic layer, and at least the second ceramic layer contains a material suppressing thermal contraction of the surface-layer via conductor or deforming in compliance with the thermal contraction of the surface-layer via conductor, wherein a weight ratio of alumina contained in the second ceramic layer is higher than a weight ratio of alumina contained in the first ceramic layer; wherein the surface-layer via conductor is a continuous body that includes a portion formed in the first ceramic layer and a portion formed in the second ceramic layer, and that has two tapered shapes defined by the two portions at different spreading slope angles, and a spreading slope angle of the tapered shape on the second ceramic layer side is larger than a spreading slope angle of the tapered shape on the first ceramic layer side.

2. The ceramic multilayer substrate according to claim 1 , wherein a weight ratio of a glass ingredient contained in the second ceramic layer is higher than a weight ratio of a glass ingredient contained in the first ceramic layer.

3. The ceramic multilayer substrate according to claim 1 , wherein a contraction starting temperature of the second ceramic layer during firing is lower than a contraction starting temperature of the first ceramic layer during firing.

4. The ceramic multilayer substrate according to claim 1 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one insulating layer including at least one of the first ceramic layer, at least one of the second ceramic layer, and an inner-layer via conductor penetrating through the at least one of the first ceramic layers and the at least one of the second ceramic layers of the other one insulating layer and formed in a tapered shape gradually tapering toward a lower layer side of the other one insulating layer, and

at least the one of the second ceramic layers of the other one insulating layer contains a material suppressing thermal contraction of the inner-layer via conductor or deforming in compliance with the thermal contraction of the inner-layer via conductor.

5. The ceramic multilayer substrate according to claim 4 , wherein a weight ratio of a glass ingredient contained in the surface-layer via conductor is lower than a weight ratio of a glass ingredient contained in the inner-layer via conductor.

6. The ceramic multilayer substrate according to claim 1 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one of the insulating layers including at least one of the first ceramic layers, at least one of the second ceramic layers, and an inner-layer via conductor penetrating through the at least one of the first layers and the at least one of the second ceramic layers and formed in a tapered shape that is reversed to the tapered shape of the surface-layer via conductor in a stacking direction, and

the inner-layer via conductor is overlapping with the surface-layer via conductor when viewed in a direction perpendicular to the surface of the second ceramic layer.

7. The ceramic multilayer substrate according to claim 4 , wherein a maximum sectional area of the surface-layer via conductor is smaller than a maximum sectional area of the inner-layer via conductor.

8. The ceramic multilayer substrate according to claim 1 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one insulating layer including at least one of the first ceramic layer, at least one of the second ceramic layer, and an inner-layer via conductor penetrating through the at least one of the first ceramic layers and the at least one of the second ceramic layers of the other one insulating layer and formed in a tapered shape gradually tapering toward a lower layer side of the other one insulating layer, and

at least the one of the second ceramic layers of the other one insulating layer contains a material suppressing thermal contraction of the inner-layer via conductor or deforming in compliance with the thermal contraction of the inner-layer via conductor.

9. The ceramic multilayer substrate according to claim 2 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one insulating layer including at least one of the first ceramic layer, at least one of the second ceramic layer, and an inner-layer via conductor penetrating through the at least one of the first ceramic layers and the at least one of the second ceramic layers of the other one insulating layer and formed in a tapered shape gradually tapering toward a lower layer side of the other one insulating layer, and

at least the one of the second ceramic layers of the other one insulating layer contains a material suppressing thermal contraction of the inner-layer via conductor or deforming in compliance with the thermal contraction of the inner-layer via conductor.

10. The ceramic multilayer substrate according to claim 3 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one insulating layer including at least one of the first ceramic layer, at least one of the second ceramic layer, and an inner-layer via conductor penetrating through the at least one of the first ceramic layers and the at least one of the second ceramic layers of the other one insulating layer and formed in a tapered shape gradually tapering toward a lower layer side of the other one insulating layer, and

at least the one of the second ceramic layers of the other one insulating layer contains a material suppressing thermal contraction of the inner-layer via conductor or deforming in compliance with the thermal contraction of the inner-layer via conductor.

11. The ceramic multilayer substrate according to claim 1 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one of the insulating layers including at least one of the first ceramic layers, at least one of the second ceramic layers, and an inner-layer via conductor penetrating through the at least one of the first layers and the at least one of the second ceramic layers and formed in a tapered shape that is reversed to the tapered shape of the surface-layer via conductor in a stacking direction, and

the inner-layer via conductor is overlapping with the surface-layer via conductor when viewed in a direction perpendicular to the surface of the second ceramic layer.

12. The ceramic multilayer substrate according to claim 2 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one of the insulating layers including at least one of the first ceramic layers, at least one of the second ceramic layers, and an inner-layer via conductor penetrating through the at least one of the first layers and the at least one of the second ceramic layers and formed in a tapered shape that is reversed to the tapered shape of the surface-layer via conductor in a stacking direction, and

the inner-layer via conductor is overlapping with the surface-layer via conductor when viewed in a direction perpendicular to the surface of the second ceramic layer.

13. The ceramic multilayer substrate according to claim 3 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one of the insulating layers including at least one of the first ceramic layers, at least one of the second ceramic layers, and an inner-layer via conductor penetrating through the at least one of the first layers and the at least one of the second ceramic layers and formed in a tapered shape that is reversed to the tapered shape of the surface-layer via conductor in a stacking direction, and

the inner-layer via conductor is overlapping with the surface-layer via conductor when viewed in a direction perpendicular to the surface of the second ceramic layer.

14. The ceramic multilayer substrate according to claim 4 , wherein another one of the insulating layers is disposed under the insulating layer in the uppermost surface layer, said other one of the insulating layers including at least one of the first ceramic layers, at least one of the second ceramic layers, and an inner-layer via conductor penetrating through the at least one of the first layers and the at least one of the second ceramic layers and formed in a tapered shape that is reversed to the tapered shape of the surface-layer via conductor in a stacking direction, and

the inner-layer via conductor is overlapping with the surface-layer via conductor when viewed in a direction perpendicular to the surface of the second ceramic layer.

15. The ceramic multilayer substrate according to claim 5 , wherein a maximum sectional area of the surface-layer via conductor is smaller than a maximum sectional area of the inner-layer via conductor.

16. The ceramic multilayer substrate according to claim 6 , wherein a maximum sectional area of the surface-layer via conductor is smaller than a maximum sectional area of the inner-layer via conductor.