1. Field of the Invention
The present invention relates to a ceramic multilayer substrate.
2. Description of the Related Art
Various high-frequency modules, e.g., chip antennas, delay lines, high-frequency combination switch modules, and receiving devices, are mounted in the inside of information communications apparatuses, e.g., portable terminals. Such a high-frequency module is mounted on a wiring substrate.
In general, circuit components are mounted on a multilayer substrate in such a high-frequency module. Multilayer substrates including ceramic multilayer substrates are well-known. Most of ceramic multilayer substrates are provided with ground electrodes in order to eliminate noise. This is disclosed in Japanese Unexamined Patent Application Publication No. 2002-94410 (Patent Document 1), for example.
In general, ground electrodes are provided in the inside of ceramic multilayer substrates at locations as close to the bottom surfaces as possible. This is because unnecessary impedance components, e.g., stray capacitances and stray inductances, are eliminated by arranging the ground electrodes as close to ground electrodes of wiring substrates as possible.
FIG. 8 shows an example of a known ceramic multilayer substrate. In a ceramic multilayer substrate 100, electronic components 13a, 13b, and 13c are mounted on a ceramic laminate 10 including laminated ceramic layers 11. A ground electrode 12 is provided in the vicinity of a bottom surface of the ceramic multilayer substrate 100 and is sandwiched between ceramic layers 11m and 11n. 
The ground electrode requires a wide area and, therefore, a conductor pattern having a wide area must be provided on a ceramic green sheet during production of a ceramic multilayer substrate. However, as the area of the conductor pattern is increased, the mutual contact area of the two ceramic green sheets sandwiching the conductor pattern therebetween decreases. As a result, the bondability between the ceramic green sheets is reduced.
In the ceramic multilayer substrate 100, shown in FIG. 8, the bondability between the ceramic green sheets 11m and 11n is reduced due to an increase in the area of the ground electrode 12 sandwiched by the ceramic green sheets 11m and 11n. 
Furthermore, the ceramic layers are loaded due to the difference in the amount of shrinkage between the conductor pattern and the ceramic green sheets during baking. The effect of this load increases as the area of the conductor pattern increases. Consequently, delamination and cracks in the ceramic layers often occur, particularly in the vicinity of the ground electrode of the ceramic multilayer substrate after baking.
In order to overcome this problem, the ground electrode has been provided so as to be exposed on the bottom surface of the ceramic multilayer substrate. However, in that case, another problem occurs in that a short circuit often occurs between the ground electrode and wirings on the wiring substrate.