Multilayer ceramic substrate and method for producing the same

Abstract

When a multilayer ceramic substrate with a cavity is reduced in thickness, a bottom wall portion defining the bottom of the cavity is reduced in thickness, thereby leading to the problem that the bottom wall portion is likely to be broken. A bottom wall portion defining a cavity of a multilayer ceramic substrate has a stack structure formed with a high thermal expansion coefficient layer sandwiched between first and second low thermal expansion coefficient layers. This configuration generates compression stress in the low thermal expansion coefficient layers during a cooling process after firing, thereby allowing the mechanical strength at the bottom wall portion to be improved.

What is claimed is: 
     
         1 . A method for producing a multilayer ceramic substrate including a cavity, comprising a peripheral wall portion including a first ceramic layer including a through hole arranged to define the cavity, and a bottom wall portion including a plurality of second ceramic layers not including a through hole, wherein the plurality of second ceramic layers of the bottom wall portion include at least two types of ceramic layers, the at least two types of ceramic layers including a high thermal expansion coefficient layer having a relatively high thermal expansion coefficient and a plurality of low thermal expansion coefficient layers having a relatively low thermal expansion coefficient, and at least a portion of the high thermal expansion coefficient layer is sandwiched between a first low thermal expansion coefficient layer and a second low thermal expansion coefficient layer of the plurality of low thermal expansion coefficient layers, the method comprising the steps of:
 preparing a first ceramic green layer including the through hole, the first ceramic green layer to be subjected to firing to form the first ceramic layer, and the first ceramic green layer including a low-temperature sintering ceramic material;   preparing, as a plurality of second ceramic green layers to be subjected to firing to form the second ceramic layers, a high thermal expansion coefficient green layer to form the high thermal expansion coefficient layer, a first low thermal expansion coefficient green layer to form the first low thermal expansion coefficient layer, and a second low thermal expansion coefficient green layer to form the second low thermal expansion coefficient layer, each of the plurality of second ceramic green layers including a low-temperature sintering ceramic material;   producing a stacked composite body including a raw stacked body formed by stacking the first ceramic green layer and the second ceramic green layer, and an outer constraining layer provided on both principal surfaces of the raw stacked body, the outer constraining layer including an inorganic material powder which is not substantially sintered at a firing condition at which the low-temperature sintering ceramic material is sintered;   firing the stacked composite body under a firing condition at which the low-temperature sintering ceramic material is sintered; and   then removing the outer constraining layers from the stacked composite body.   
     
     
         2 . The method for producing a multilayer ceramic substrate according to  claim 1 , wherein an outer surface of the bottom wall portion is defined by the first low thermal expansion coefficient layer, and a surface of the bottom wall portion arranged in contact with the peripheral wall portion is defined by the second low thermal expansion coefficient layer, the raw stacked body further includes, as the second ceramic green layer, a first constraining interlayer arranged in contact with the second low thermal expansion coefficient green layer, the first constraining interlayer includes an inorganic material powder which is not substantially sintered at a firing condition at which the low-temperature sintering ceramic material is sintered, and the inorganic material powder is solidified by permeation of the ceramic material included in the low thermal expansion coefficient green layer as a result of the firing step. 
     
     
         3 . The method for producing a multilayer ceramic substrate according to  claim 1 , wherein the raw stacked body further includes, as the first ceramic green layer, a second constraining interlayer arranged along a surface of the peripheral wall portion in contact with the bottom wall portion, the second constraining interlayer includes an inorganic material powder which is not substantially sintered at a firing condition at which a ceramic material included in the low thermal expansion coefficient layer is sintered, and the inorganic material powder is solidified by permeation of the ceramic material included in the low thermal expansion coefficient layer as a result of the firing step. 
     
     
         4 . The method for producing a multilayer ceramic substrate according to  claim 3 , wherein in the raw stacked body, the through hole included in the second constraining interlayer is made smaller than the through hole included in the first ceramic green layer of the peripheral wall portion arranged in contact with the second constraining interlayer.